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PROFESSOR: And I think we're
about ready to get started.

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00:00:29,060 --> 00:00:30,920
So welcome, folks,
for lecture number two

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of Fundamentals of Photovoltaics
focused on the solar resource.

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00:00:35,080 --> 00:00:36,580
What I wanted to
do to get everybody

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00:00:36,580 --> 00:00:38,220
in the mood of
thinking about the sun

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00:00:38,220 --> 00:00:39,940
is pass around a few balls.

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00:00:39,940 --> 00:00:44,420
So this is really a, to limber
you all up, but b-- whoop.

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00:00:44,420 --> 00:00:44,920
[LAUGHTER]

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00:00:44,920 --> 00:00:46,660
There we go.

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00:00:46,660 --> 00:00:47,200
All right.

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00:00:47,200 --> 00:00:49,119
Don't let it fall.

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00:00:49,119 --> 00:00:49,660
There you go.

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00:00:49,660 --> 00:00:51,230
I know Ashley's got solid hands.

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00:00:51,230 --> 00:00:51,730
All right.

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00:00:51,730 --> 00:00:52,460
Here's one.

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00:00:52,460 --> 00:00:53,870
Pass that around as well.

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00:00:53,870 --> 00:00:58,210
We'll get a few
more there, there,

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00:00:58,210 --> 00:01:01,040
and lastly, right up the middle.

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00:01:01,040 --> 00:01:01,870
There you go.

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00:01:01,870 --> 00:01:02,490
OK.

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00:01:02,490 --> 00:01:06,400
So today's lecture is really
about the solar resource.

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00:01:06,400 --> 00:01:08,820
And as we go through,
it kind of helps

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00:01:08,820 --> 00:01:12,100
to have a sphere in your
hands since oftentimes we

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00:01:12,100 --> 00:01:16,760
perceive the world as being
flat-- no fault of our own.

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00:01:16,760 --> 00:01:19,710
Locally, one can approximate
it as a flat body.

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00:01:19,710 --> 00:01:22,430
That's certainly a
good possibility.

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00:01:22,430 --> 00:01:23,625
You wouldn't mind passing these
out to your friends as well?

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00:01:23,625 --> 00:01:24,280
AUDIENCE: Sure.

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00:01:24,280 --> 00:01:25,520
PROFESSOR: Thanks.

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00:01:25,520 --> 00:01:27,110
But in reality,
if we really want

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00:01:27,110 --> 00:01:28,620
to understand the
solar resource,

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00:01:28,620 --> 00:01:30,710
we really have to begin
understanding or thinking

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00:01:30,710 --> 00:01:35,180
in terms of spheres and in terms
of circles or, in most cases,

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00:01:35,180 --> 00:01:36,370
ellipses.

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00:01:36,370 --> 00:01:38,742
And so we're going to dive
into the solar resource.

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00:01:38,742 --> 00:01:41,200
Before we really dive in in
detail into the solar resource,

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00:01:41,200 --> 00:01:43,710
I wanted to give you
feedback to your surveys.

45
00:01:43,710 --> 00:01:46,020
So you did a background
assessment survey, a census,

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00:01:46,020 --> 00:01:48,930
if you will, and filled out a
number of questions last class

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00:01:48,930 --> 00:01:50,040
about your backgrounds.

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00:01:50,040 --> 00:01:51,910
And I wanted to provide
you the feedback,

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00:01:51,910 --> 00:01:54,560
the consolidated information,
because it's really telling

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00:01:54,560 --> 00:01:56,630
about who your colleagues are.

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00:01:56,630 --> 00:02:00,810
This right here is a little
bit of a snapshot of expertise

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00:02:00,810 --> 00:02:03,410
and current career trajectory.

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00:02:03,410 --> 00:02:05,410
So the self-defined
expertise in the bottom left

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00:02:05,410 --> 00:02:08,150
is really, I think, the
most telling parameter.

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00:02:08,150 --> 00:02:10,229
For the undergrads here,
you may define yourself

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00:02:10,229 --> 00:02:12,260
by your major today, but
when you graduate and go on

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00:02:12,260 --> 00:02:14,790
to grad school, you might, say,
for example, do your undergrad

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00:02:14,790 --> 00:02:16,225
in physics and then do
your graduate school

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00:02:16,225 --> 00:02:17,725
in mechanical
engineering, but still

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00:02:17,725 --> 00:02:19,680
consider yourself a
physicist at heart.

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00:02:19,680 --> 00:02:21,690
And so that's why I asked
this question here--

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00:02:21,690 --> 00:02:23,500
what is your self-defined
expertise-- because there

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00:02:23,500 --> 00:02:25,870
are several graduate students
who have changed fields,

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00:02:25,870 --> 00:02:28,060
if you will, from undergrad
to graduate school.

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00:02:28,060 --> 00:02:30,530
Most people in the
audience, by and large,

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00:02:30,530 --> 00:02:33,380
consider themselves engineers--
either material science

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00:02:33,380 --> 00:02:35,900
engineers or
mechanical engineers.

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00:02:35,900 --> 00:02:38,500
Chemistry is strong as well.

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And then we have about 10
different departments here

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00:02:42,280 --> 00:02:44,080
represented.

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And that's really cool.

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It's going to manifest
itself in the class projects.

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And you'll see the diversity
of different inputs

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00:02:50,420 --> 00:02:53,300
and perspectives
from your colleagues.

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00:02:53,300 --> 00:02:55,580
The degree in progress--
undergrad/grad

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00:02:55,580 --> 00:03:00,670
is about split 1/3 2/3
undergrad and grad.

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00:03:00,670 --> 00:03:02,460
ASP is Advanced Studies
Program, so these

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00:03:02,460 --> 00:03:04,460
are folks coming in from
industry who are actually here

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00:03:04,460 --> 00:03:05,080
in the classroom.

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00:03:05,080 --> 00:03:06,579
Some of your
colleagues in the class

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are folks who are in
industry and perhaps

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00:03:08,640 --> 00:03:11,134
have real world PV experience.

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Several of the people
here in the class,

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as well, have
gained-- how do we say

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00:03:17,180 --> 00:03:19,670
it-- have gained expertise
in solar with their hobbies,

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00:03:19,670 --> 00:03:20,610
with their work.

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00:03:20,610 --> 00:03:22,079
Some have installed
solar panels.

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00:03:22,079 --> 00:03:24,620
Other ones have done research
or are doing research in solar.

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So it's a pretty diverse group.

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00:03:25,930 --> 00:03:27,888
And some are, as well,
members of the solar car

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00:03:27,888 --> 00:03:31,190
team, which is rolling out its
new model in a few days' time.

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00:03:31,190 --> 00:03:33,450
In terms of learning
methods, it was pretty well

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00:03:33,450 --> 00:03:36,020
split between hands-on
labs, field trips,

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00:03:36,020 --> 00:03:37,560
and guest lectures.

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00:03:37,560 --> 00:03:40,550
And I'll get back to
that in a few slides.

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00:03:40,550 --> 00:03:42,060
In terms of class
project interest,

97
00:03:42,060 --> 00:03:44,260
there was a strong
interest in working

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00:03:44,260 --> 00:03:47,340
with pre-established projects,
so we've listened to that.

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00:03:47,340 --> 00:03:49,310
This is pretty consistent
with previous years.

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00:03:49,310 --> 00:03:51,620
And so we have several
pre-prepared class projects

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00:03:51,620 --> 00:03:52,960
ready for you.

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00:03:52,960 --> 00:03:55,840
And a few of you had an interest
in the self-design project.

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00:03:55,840 --> 00:03:57,090
I'd like to talk to you.

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00:03:57,090 --> 00:03:59,500
I'd like to begin developing
those ideas as soon

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00:03:59,500 --> 00:04:03,380
as possible so that when
we start assembling teams,

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00:04:03,380 --> 00:04:05,625
if you do have a strong
idea for class project,

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00:04:05,625 --> 00:04:08,000
we can begin crafting that
and molding that starting now.

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00:04:08,000 --> 00:04:09,708
So please come up and
have a chat with me

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00:04:09,708 --> 00:04:13,470
after class or during office
hours or during recitation.

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00:04:13,470 --> 00:04:17,240
These are your learning
objectives defined by you.

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00:04:17,240 --> 00:04:21,190
And they range-- I tried to
give it some continuum spectrum

112
00:04:21,190 --> 00:04:23,150
from natural sciences
to social sciences

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00:04:23,150 --> 00:04:24,860
and engineering in the middle.

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00:04:24,860 --> 00:04:29,620
And obviously, this is more
of a loop than a linear line,

115
00:04:29,620 --> 00:04:31,597
but bear with me.

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00:04:31,597 --> 00:04:33,430
There was a strong
interest in fundamentals.

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00:04:33,430 --> 00:04:35,810
And that certainly, I think,
what the core of the class

118
00:04:35,810 --> 00:04:38,550
is about or at least the
first third of the class.

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00:04:38,550 --> 00:04:40,350
Going in terms of
size of the bubble,

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00:04:40,350 --> 00:04:41,766
these are the
number of people who

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00:04:41,766 --> 00:04:44,770
listed a particular topic as
of great interest to them.

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00:04:44,770 --> 00:04:48,385
Economics and market, systems
and grid, current technologies,

123
00:04:48,385 --> 00:04:50,210
and emerging technologies.

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00:04:50,210 --> 00:04:52,267
And so listening to
all of this, we have,

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00:04:52,267 --> 00:04:54,600
or we are in the process of
preparing for you some guest

126
00:04:54,600 --> 00:04:57,640
lectures and field trips based
on this feedback right here.

127
00:04:57,640 --> 00:04:59,630
We have already
lined up a field trip

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00:04:59,630 --> 00:05:02,830
to a local PV research
laboratory that

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00:05:02,830 --> 00:05:06,150
produces modules like this
one right here only much, much

130
00:05:06,150 --> 00:05:09,350
bigger and has strong
collaboration with existing

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00:05:09,350 --> 00:05:11,130
companies, startup
companies, in the area

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00:05:11,130 --> 00:05:12,950
as well as more
established companies.

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00:05:12,950 --> 00:05:15,034
So that'll be a lot of fun.

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00:05:15,034 --> 00:05:16,450
And we're currently
in the process

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00:05:16,450 --> 00:05:18,116
of arranging other
field trips and guest

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00:05:18,116 --> 00:05:21,450
lectures to match this feedback
right here, so thank you.

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00:05:21,450 --> 00:05:26,740
We'll mold the course, shape it,
craft it to fit your interests.

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00:05:26,740 --> 00:05:28,830
So to hop into
the solar resource

139
00:05:28,830 --> 00:05:31,726
and without further ado,
the subject of today

140
00:05:31,726 --> 00:05:33,350
and the motivation
for wearing this tie

141
00:05:33,350 --> 00:05:36,074
is really the solar
resource, the sun.

142
00:05:36,074 --> 00:05:37,240
This is where it all starts.

143
00:05:37,240 --> 00:05:39,260
If we're to understand
PV, photovoltaics,

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00:05:39,260 --> 00:05:41,100
the conversion of
sunlight into electricity,

145
00:05:41,100 --> 00:05:42,360
it starts from the sun.

146
00:05:42,360 --> 00:05:44,680
And so spending some good
time thinking about the sun

147
00:05:44,680 --> 00:05:46,510
is really, really important.

148
00:05:46,510 --> 00:05:49,050
And it will avoid the
embarrassing situation--

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00:05:49,050 --> 00:05:52,070
how many of you have been at a
shopping center, walking along,

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00:05:52,070 --> 00:05:54,820
and a little child is
asking his parent, Dad,

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00:05:54,820 --> 00:05:56,420
why is the sky blue?

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00:05:56,420 --> 00:05:57,850
Or why is the such and such?

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00:05:57,850 --> 00:05:59,580
And the answers
you'll hear just make

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00:05:59,580 --> 00:06:02,910
you want to tear your ears
out, say, my goodness.

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00:06:02,910 --> 00:06:04,997
And so part of this is
just general knowledge.

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00:06:04,997 --> 00:06:07,080
It's getting a feel for
the world and the universe

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00:06:07,080 --> 00:06:09,663
and asking those questions again
that the little children will

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00:06:09,663 --> 00:06:12,990
ask but we forget to ask as we
move on with our lives, right?

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00:06:12,990 --> 00:06:13,490
OK.

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00:06:13,490 --> 00:06:17,160
So moving forward, the
learning objectives for today

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00:06:17,160 --> 00:06:18,510
are these right here.

162
00:06:18,510 --> 00:06:20,260
By the end of the
lecture-- and hopefully,

163
00:06:20,260 --> 00:06:22,330
you already have a good sense
of this based on your readings

164
00:06:22,330 --> 00:06:22,829
already.

165
00:06:22,829 --> 00:06:25,832
I'll quiz you on that
second-- verbally.

166
00:06:25,832 --> 00:06:28,290
We want to be able to quantify
the available solar resource

167
00:06:28,290 --> 00:06:31,880
relative to human energy
needs and other fuel sources.

168
00:06:31,880 --> 00:06:35,200
We want to recognize
and plot air mass zero

169
00:06:35,200 --> 00:06:38,220
and air mass 1.5 solar
spectra and describe

170
00:06:38,220 --> 00:06:39,950
the physical origins.

171
00:06:39,950 --> 00:06:42,910
We want to describe how
solar insulation maps--

172
00:06:42,910 --> 00:06:45,390
these are solar resource
maps, in other words, how

173
00:06:45,390 --> 00:06:47,270
much sunlight is available.

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00:06:47,270 --> 00:06:51,080
And we want to be able to
estimate a solar resource

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00:06:51,080 --> 00:06:54,870
amount locally at a
specific spot on the planet.

176
00:06:54,870 --> 00:06:57,710
We want to list the causes of
variation and intermittency

177
00:06:57,710 --> 00:07:00,270
of the solar resource
and quantify their time

178
00:07:00,270 --> 00:07:01,355
constant in magnitude.

179
00:07:01,355 --> 00:07:03,230
In other words, we want
to be able to discern

180
00:07:03,230 --> 00:07:05,354
what are the big effects
and what are the ones that

181
00:07:05,354 --> 00:07:06,216
don't really matter.

182
00:07:06,216 --> 00:07:08,340
We want to be able to
estimate the land area needed

183
00:07:08,340 --> 00:07:11,100
to provide sufficient solar
resource for a project,

184
00:07:11,100 --> 00:07:13,940
whether it's a house, a car,
a village, a country, a world.

185
00:07:13,940 --> 00:07:16,190
And a lot of this will be
on your homework assignment,

186
00:07:16,190 --> 00:07:18,480
so we'll give you the
tools here, but then ask

187
00:07:18,480 --> 00:07:20,290
you to address those questions.

188
00:07:20,290 --> 00:07:21,831
And for those of
you who have already

189
00:07:21,831 --> 00:07:24,630
picked up your p-set number one,
you'll see relevant questions.

190
00:07:24,630 --> 00:07:25,213
Where is this?

191
00:07:28,501 --> 00:07:33,250
Does anybody recognize
this right here?

192
00:07:33,250 --> 00:07:37,780
If I start rambling
off names, what city

193
00:07:37,780 --> 00:07:40,239
has Pennsylvania Avenue,
Independence Ave?

194
00:07:40,239 --> 00:07:41,280
AUDIENCE: Washington, DC.

195
00:07:41,280 --> 00:07:42,363
PROFESSOR: Washington, DC.

196
00:07:42,363 --> 00:07:45,140
This is right outside of the
National Air and Space Museum.

197
00:07:45,140 --> 00:07:49,160
What this little girl here
is pointing to is the sun.

198
00:07:49,160 --> 00:07:52,940
And then we have Mercury, Venus,
Earth, Mars, and so forth.

199
00:07:52,940 --> 00:07:55,100
So it's essentially a
solar system to scale.

200
00:07:55,100 --> 00:07:59,460
As you walk out of the
Air and Space Museum

201
00:07:59,460 --> 00:08:02,160
and walk down the street, you'll
be passing the different bodies

202
00:08:02,160 --> 00:08:03,610
in our solar system.

203
00:08:03,610 --> 00:08:05,560
And so just as a
quick little review

204
00:08:05,560 --> 00:08:08,220
to kind of get us situated
and to ask the questions

205
00:08:08,220 --> 00:08:10,240
that little kiddies
might ask us,

206
00:08:10,240 --> 00:08:14,434
how far is it from
the earth to the sun?

207
00:08:14,434 --> 00:08:15,790
AUDIENCE: 93 million miles.

208
00:08:15,790 --> 00:08:16,850
PROFESSOR: About
100 million miles.

209
00:08:16,850 --> 00:08:17,350
Yeah.

210
00:08:17,350 --> 00:08:19,420
93 million miles.

211
00:08:19,420 --> 00:08:22,580
Plus or minus somewhere in
the range of maybe a percent

212
00:08:22,580 --> 00:08:26,224
or so, a few percent
depending on what time of year

213
00:08:26,224 --> 00:08:28,640
we are since we're in a little
bit of an elliptical orbit.

214
00:08:28,640 --> 00:08:29,140
Good.

215
00:08:29,140 --> 00:08:32,140
So that's the
distance to the sun.

216
00:08:32,140 --> 00:08:34,960
It would be about 150
million kilometers.

217
00:08:34,960 --> 00:08:37,427
How long does it take light
to travel that distance?

218
00:08:37,427 --> 00:08:38,564
AUDIENCE: Eight minutes.

219
00:08:38,564 --> 00:08:40,980
PROFESSOR: About eight minutes,
eight and a third minutes,

220
00:08:40,980 --> 00:08:42,159
right?

221
00:08:42,159 --> 00:08:44,330
So it takes a little bit
for the light to reach us.

222
00:08:44,330 --> 00:08:45,300
Good.

223
00:08:45,300 --> 00:08:48,840
Some more questions-- how
far are the other planets

224
00:08:48,840 --> 00:08:52,980
in our solar system to the sun,
going in order from Mercury

225
00:08:52,980 --> 00:08:54,360
out?

226
00:08:54,360 --> 00:08:56,400
It's to get us situated here.

227
00:08:56,400 --> 00:08:57,830
If we are, at any
point, planning

228
00:08:57,830 --> 00:08:59,580
on throwing up satellites
and sending them

229
00:08:59,580 --> 00:09:01,246
with other planets,
this is a good thing

230
00:09:01,246 --> 00:09:03,500
to kind of keep in
the back of our minds.

231
00:09:03,500 --> 00:09:06,080
So if we define an
astronomical unit--

232
00:09:06,080 --> 00:09:07,580
not in terms of
our national debt,

233
00:09:07,580 --> 00:09:10,440
but in terms of the distance
from the earth to the sun--

234
00:09:10,440 --> 00:09:12,860
that's an astronomical unit.

235
00:09:12,860 --> 00:09:15,510
Mercury would be
somewhere around 0.4.

236
00:09:15,510 --> 00:09:16,640
Venus 0.7.

237
00:09:16,640 --> 00:09:18,060
Mars 1.5.

238
00:09:18,060 --> 00:09:20,160
So that's all kind of in
our neighborhood, right?

239
00:09:20,160 --> 00:09:22,201
And then from Mars to
Jupiter is a bit of a jump.

240
00:09:22,201 --> 00:09:23,615
It goes from 1.5 to 5.

241
00:09:23,615 --> 00:09:26,550
Then from Jupiter to
Saturn is 10-- well, sorry.

242
00:09:26,550 --> 00:09:28,620
10 is the distance
from Saturn to the Sun.

243
00:09:28,620 --> 00:09:31,580
And then 30 and then 40.

244
00:09:31,580 --> 00:09:33,080
Sorry, 10, 20, 30, 40.

245
00:09:33,080 --> 00:09:36,600
So it goes-- Jupiter's
5, Saturn 10.

246
00:09:36,600 --> 00:09:38,000
Uranus would be 20.

247
00:09:38,000 --> 00:09:38,920
Neptune 30.

248
00:09:38,920 --> 00:09:39,730
Pluto 40.

249
00:09:39,730 --> 00:09:42,010
Pluto, planet, sort of.

250
00:09:42,010 --> 00:09:44,705
So it's easy to remember those
numbers because it goes 5, 10,

251
00:09:44,705 --> 00:09:46,913
20-- that's just essentially
a sequence of doubling--

252
00:09:46,913 --> 00:09:48,894
and then 10, 20, 30, 40.

253
00:09:48,894 --> 00:09:50,560
I'm giving you
approximate numbers here,

254
00:09:50,560 --> 00:09:53,530
but that's something just
to keep in mind all in terms

255
00:09:53,530 --> 00:09:54,520
of astronomical units.

256
00:09:54,520 --> 00:09:56,436
So in case a little kid
comes up and asks you,

257
00:09:56,436 --> 00:09:59,780
you can spit out the answer.

258
00:09:59,780 --> 00:10:02,170
Let's talk about the sun.

259
00:10:02,170 --> 00:10:04,940
This is just a review of
our readings right here.

260
00:10:04,940 --> 00:10:07,090
This was a
representation of the sun

261
00:10:07,090 --> 00:10:08,800
and the Earth moving around.

262
00:10:08,800 --> 00:10:11,602
And what is solstice
and equinox?

263
00:10:11,602 --> 00:10:12,560
What do those refer to?

264
00:10:17,250 --> 00:10:20,574
What is the equinox?

265
00:10:20,574 --> 00:10:22,860
AUDIENCE: Equal amounts
of light and dark.

266
00:10:22,860 --> 00:10:24,860
PROFESSOR: Yeah, equal
amounts of light and dark

267
00:10:24,860 --> 00:10:26,610
throughout pretty
much all the world

268
00:10:26,610 --> 00:10:29,370
except if you're really
standing on the tippy top

269
00:10:29,370 --> 00:10:30,870
and the bottom.

270
00:10:30,870 --> 00:10:32,587
So equal amount
of light and dark

271
00:10:32,587 --> 00:10:34,420
throughout the world
on that particular day.

272
00:10:34,420 --> 00:10:39,380
So the day and the night have
the same amount of length.

273
00:10:39,380 --> 00:10:41,560
If you move over to
this region right here,

274
00:10:41,560 --> 00:10:44,350
this would be a region of our
northern hemisphere summer,

275
00:10:44,350 --> 00:10:45,750
southern hemisphere, winter.

276
00:10:45,750 --> 00:10:47,390
Over here, vice versa.

277
00:10:47,390 --> 00:10:49,855
And the solstice would be?

278
00:10:49,855 --> 00:10:51,764
AUDIENCE: The shortest
day of the year.

279
00:10:51,764 --> 00:10:52,430
PROFESSOR: Yeah.

280
00:10:52,430 --> 00:10:55,500
So the shortest or the
longest day of the year,

281
00:10:55,500 --> 00:10:57,810
depending on what
side you're on.

282
00:10:57,810 --> 00:11:01,010
So in the northern
hemisphere, the June solstice

283
00:11:01,010 --> 00:11:02,860
would be the summer solstice.

284
00:11:02,860 --> 00:11:04,692
For us, it would
be the longest day.

285
00:11:04,692 --> 00:11:06,400
And if you're in the
southern hemisphere,

286
00:11:06,400 --> 00:11:07,580
it would be the shortest day.

287
00:11:07,580 --> 00:11:09,163
Depending on what
time zone you're in,

288
00:11:09,163 --> 00:11:12,910
there might be a variation
of one day hither to.

289
00:11:12,910 --> 00:11:13,440
Good.

290
00:11:13,440 --> 00:11:14,130
OK.

291
00:11:14,130 --> 00:11:17,009
And what is this?

292
00:11:17,009 --> 00:11:18,300
What are the seasons caused by?

293
00:11:18,300 --> 00:11:20,550
What is this kind
of tilt right here?

294
00:11:20,550 --> 00:11:21,854
What is that called?

295
00:11:21,854 --> 00:11:23,020
AUDIENCE: Declination angle.

296
00:11:23,020 --> 00:11:24,228
PROFESSOR: Declination angle.

297
00:11:24,228 --> 00:11:26,026
And approximately
how much is that?

298
00:11:26,026 --> 00:11:26,900
AUDIENCE: 23 and 1/2.

299
00:11:26,900 --> 00:11:27,970
PROFESSOR: 23 and 1/2.

300
00:11:27,970 --> 00:11:28,520
23.45.

301
00:11:28,520 --> 00:11:29,970
Yeah, so 23 1/2 degrees.

302
00:11:29,970 --> 00:11:30,620
Good.

303
00:11:30,620 --> 00:11:31,120
OK.

304
00:11:31,120 --> 00:11:34,740
And we can visualize all
of this on the PV CD-ROM

305
00:11:34,740 --> 00:11:35,940
on the website.

306
00:11:35,940 --> 00:11:37,730
So this was part of
your assigned reading.

307
00:11:37,730 --> 00:11:41,070
And here's the earth
going around the sun

308
00:11:41,070 --> 00:11:42,592
in this representation.

309
00:11:42,592 --> 00:11:44,550
Likewise, if you want to
take one of your balls

310
00:11:44,550 --> 00:11:48,270
and just kind of imagine being
on one of those surfaces,

311
00:11:48,270 --> 00:11:49,860
you see the diurnal
rotation here.

312
00:11:49,860 --> 00:11:51,470
It's spinning around its axis.

313
00:11:51,470 --> 00:11:54,380
And as well, the
seasonal variation as it

314
00:11:54,380 --> 00:11:56,310
spins around the sun.

315
00:11:56,310 --> 00:11:58,900
That's important for a
number of reasons, right?

316
00:11:58,900 --> 00:12:02,020
That will determine how
much sunlight is normally

317
00:12:02,020 --> 00:12:04,080
incident on the planet.

318
00:12:04,080 --> 00:12:07,300
If you are normally incident,
if you're at this exact spot

319
00:12:07,300 --> 00:12:09,720
right here, you're receiving
the sunlight full on.

320
00:12:09,720 --> 00:12:13,180
But if you're up here
somewhere, your surface normal

321
00:12:13,180 --> 00:12:15,680
is some vector
pointing out like that.

322
00:12:15,680 --> 00:12:18,400
You're only going to be
receiving the cosine theta

323
00:12:18,400 --> 00:12:20,690
of that amount of sun.

324
00:12:20,690 --> 00:12:24,074
So if you're an extreme
example, if you're right here,

325
00:12:24,074 --> 00:12:25,240
you're not going to get any.

326
00:12:25,240 --> 00:12:26,823
But if you're in
this part right here,

327
00:12:26,823 --> 00:12:29,970
you're going to get cosine
of 0, which would be 1.

328
00:12:29,970 --> 00:12:31,500
So you get the
full amount of sun.

329
00:12:31,500 --> 00:12:33,900
And likewise, as you move
through the angles here.

330
00:12:33,900 --> 00:12:36,510
So it's important to understand
what the relative angle is

331
00:12:36,510 --> 00:12:40,710
between our surface normal and
the vector pointing at the sun.

332
00:12:40,710 --> 00:12:42,480
That varies as a
function of season.

333
00:12:42,480 --> 00:12:44,890
It varies as a function
of time of day.

334
00:12:44,890 --> 00:12:47,740
And obviously, over
the entire earth,

335
00:12:47,740 --> 00:12:51,147
you can define the
precise amount of sunlight

336
00:12:51,147 --> 00:12:52,730
coming in, the precise
solar resource,

337
00:12:52,730 --> 00:12:54,390
by a series of trig formula.

338
00:12:54,390 --> 00:12:55,587
It gets pretty complex.

339
00:12:55,587 --> 00:12:57,670
And this website will
actually walk you through it

340
00:12:57,670 --> 00:13:00,000
if you're so interested.

341
00:13:00,000 --> 00:13:02,250
Now this is all review
since folks have all done

342
00:13:02,250 --> 00:13:03,677
the background ready, right?

343
00:13:03,677 --> 00:13:05,010
All done the background reading.

344
00:13:05,010 --> 00:13:07,140
I expect you to before class.

345
00:13:07,140 --> 00:13:09,310
Let me ask you a few
trickier questions just

346
00:13:09,310 --> 00:13:11,650
to see if our creative
juices are really moving

347
00:13:11,650 --> 00:13:13,930
at this early time of day.

348
00:13:13,930 --> 00:13:17,060
When would be the
shortest day of the year?

349
00:13:17,060 --> 00:13:17,810
Let's start there.

350
00:13:17,810 --> 00:13:19,955
The shortest day of the year
is approximately December 22,

351
00:13:19,955 --> 00:13:20,510
right?

352
00:13:20,510 --> 00:13:21,860
In the solstice.

353
00:13:21,860 --> 00:13:29,215
When is the latest sunrise and
when is the earliest sunset?

354
00:13:32,566 --> 00:13:34,315
You might need to pick
up your little ball

355
00:13:34,315 --> 00:13:38,014
and rotate it around.

356
00:13:38,014 --> 00:13:40,590
Does anybody have
even just a gut sense?

357
00:13:40,590 --> 00:13:42,580
Would it be on the solstice?

358
00:13:42,580 --> 00:13:45,260
How many people think it's going
to be exactly on the solstice?

359
00:13:45,260 --> 00:13:45,760
Let's see.

360
00:13:45,760 --> 00:13:47,590
Earliest sunset--
how many people

361
00:13:47,590 --> 00:13:49,173
think that the
earliest sunset's going

362
00:13:49,173 --> 00:13:51,270
to be a little bit
before the solstice?

363
00:13:51,270 --> 00:13:53,520
How many people think a
little bit after the solstice?

364
00:13:53,520 --> 00:13:55,570
I know people don't really know.

365
00:13:55,570 --> 00:13:59,680
The reality is that the earliest
sunset would be a little bit

366
00:13:59,680 --> 00:14:04,070
before the solstice here,
and the latest sunrise

367
00:14:04,070 --> 00:14:06,790
would be a little bit
after the solstice.

368
00:14:06,790 --> 00:14:09,030
And it flips in summertime.

369
00:14:09,030 --> 00:14:15,510
It would be, let's see,
the earliest sunrise

370
00:14:15,510 --> 00:14:18,780
and the latest sunset before
and after the solstice

371
00:14:18,780 --> 00:14:20,380
respectively.

372
00:14:20,380 --> 00:14:20,880
OK.

373
00:14:20,880 --> 00:14:23,030
And you can think
about that in terms

374
00:14:23,030 --> 00:14:26,050
of what is the solar noon.

375
00:14:26,050 --> 00:14:29,830
The solar noon is when the
sun this is directly overhead

376
00:14:29,830 --> 00:14:32,200
relative to our
chronological noon, which

377
00:14:32,200 --> 00:14:36,350
is, I would say, less
dependent on the specific angle

378
00:14:36,350 --> 00:14:38,320
of the earth relative
to the sun as it

379
00:14:38,320 --> 00:14:41,126
moves around this trajectory.

380
00:14:41,126 --> 00:14:42,750
Let's get more into
that in recitation.

381
00:14:42,750 --> 00:14:44,750
I sense since there wasn't
much traction there I

382
00:14:44,750 --> 00:14:47,310
don't want to dwell.

383
00:14:47,310 --> 00:14:49,370
OK.

384
00:14:49,370 --> 00:14:51,230
Good, good, good.

385
00:14:51,230 --> 00:14:53,290
OK.

386
00:14:53,290 --> 00:14:55,230
Let's think a little
bit more in terms

387
00:14:55,230 --> 00:15:00,270
of the trajectory
of the sun later on

388
00:15:00,270 --> 00:15:02,920
as we move through some of
the introductory material.

389
00:15:02,920 --> 00:15:04,599
I don't want to dwell too much.

390
00:15:04,599 --> 00:15:06,140
I want to give a
little bit of review

391
00:15:06,140 --> 00:15:09,870
since I'm sensing that not
everybody did the readings.

392
00:15:09,870 --> 00:15:11,600
I expect you to do
the readings, folks.

393
00:15:11,600 --> 00:15:13,250
So let's keep with me here.

394
00:15:13,250 --> 00:15:16,440
So a touch of history,
since that was asked for.

395
00:15:16,440 --> 00:15:17,530
It was requested.

396
00:15:17,530 --> 00:15:20,560
I decided to launch a
little bit into a history

397
00:15:20,560 --> 00:15:22,660
of the study of the sun.

398
00:15:22,660 --> 00:15:27,500
Philosophers, going back to what
I suppose most would consider

399
00:15:27,500 --> 00:15:29,980
early India, studied the sun.

400
00:15:29,980 --> 00:15:34,910
There were some writings of
some of the earlier philosophers

401
00:15:34,910 --> 00:15:36,340
that were recorded.

402
00:15:36,340 --> 00:15:37,890
Some have interpreted
these writings

403
00:15:37,890 --> 00:15:41,570
as being indicative of,
perhaps, heliocentric models.

404
00:15:41,570 --> 00:15:43,900
These are poetry, folks.

405
00:15:43,900 --> 00:15:47,280
It's a very different
style of communication

406
00:15:47,280 --> 00:15:49,279
than what we have today
of technical writing,

407
00:15:49,279 --> 00:15:51,320
so it's difficult for us
to discern, or difficult

408
00:15:51,320 --> 00:15:54,820
for me, at least, to discern
when I read the lines verbatim

409
00:15:54,820 --> 00:15:58,490
if this is really
somebody thinking

410
00:15:58,490 --> 00:16:00,490
about the heliocentric
model or whether this

411
00:16:00,490 --> 00:16:02,594
is somebody just
describing the universe

412
00:16:02,594 --> 00:16:03,885
in the best of their abilities.

413
00:16:07,410 --> 00:16:10,850
I would say the real beginnings
of heliocentric models

414
00:16:10,850 --> 00:16:14,730
began in the third century
Before the Common Era

415
00:16:14,730 --> 00:16:17,750
and the notions of
interstellar distances

416
00:16:17,750 --> 00:16:20,030
estimated in a similar
manner to what we just

417
00:16:20,030 --> 00:16:21,280
walked through today, right?

418
00:16:21,280 --> 00:16:23,640
Estimating the distance
between the earth and the sun

419
00:16:23,640 --> 00:16:26,950
and then using that as a
measure or yardstick with which

420
00:16:26,950 --> 00:16:29,250
to measure the distances
to the other planets

421
00:16:29,250 --> 00:16:32,100
began somewhere
during that time.

422
00:16:32,100 --> 00:16:35,220
Likewise, these writings
of old made their way

423
00:16:35,220 --> 00:16:36,090
to the Middle East.

424
00:16:36,090 --> 00:16:38,690
And in the 10th
and 11th centuries

425
00:16:38,690 --> 00:16:41,190
of the Common Era,
the Middle East,

426
00:16:41,190 --> 00:16:46,450
the Arab world is really where
science and technology was at.

427
00:16:46,450 --> 00:16:49,080
And these days, I
mean, Europe was still

428
00:16:49,080 --> 00:16:51,780
largely mired in
the Middle Ages,

429
00:16:51,780 --> 00:16:53,620
starting to emerge
in a few places.

430
00:16:53,620 --> 00:16:57,330
But by and large, the
carriers of civilization

431
00:16:57,330 --> 00:17:00,750
in the Western world
were really centered

432
00:17:00,750 --> 00:17:03,020
and in some of the Arab
cities in the Middle East.

433
00:17:03,020 --> 00:17:07,420
And al-Biruni, in
particular, was very avid

434
00:17:07,420 --> 00:17:11,400
at applying methods of
astronomical observation,

435
00:17:11,400 --> 00:17:15,230
in particular to aid travel,
but in the process, discovering

436
00:17:15,230 --> 00:17:19,250
a thing or two about
our known universe.

437
00:17:19,250 --> 00:17:22,329
And of course, finally,
Johannes Kepler in Europe,

438
00:17:22,329 --> 00:17:25,410
once it starts to emerge
from the Middle Ages,

439
00:17:25,410 --> 00:17:30,680
with really taking observation
some other scientists, who

440
00:17:30,680 --> 00:17:32,990
very carefully plotted
out the position

441
00:17:32,990 --> 00:17:34,912
of the different
bodies, he came up

442
00:17:34,912 --> 00:17:36,620
with some of the
mathematical models that

443
00:17:36,620 --> 00:17:40,250
describe the motion of the
planets through the skies

444
00:17:40,250 --> 00:17:42,390
and is largely credited
with developing

445
00:17:42,390 --> 00:17:46,180
a series of laws that define
interplanetary motion.

446
00:17:46,180 --> 00:17:48,412
So a couple of
interesting things to note

447
00:17:48,412 --> 00:17:49,870
is that international
collaboration

448
00:17:49,870 --> 00:17:50,940
was really essential.

449
00:17:50,940 --> 00:17:53,520
These ideas didn't
develop in isolation.

450
00:17:53,520 --> 00:17:56,530
They were flowing
throughout the world.

451
00:17:56,530 --> 00:17:58,710
It's important to know
that many of the scientists

452
00:17:58,710 --> 00:18:00,430
were well-traveled
polyglots, meaning

453
00:18:00,430 --> 00:18:01,780
they spoke different languages.

454
00:18:01,780 --> 00:18:03,946
And that's how they were
able to interpret the texts

455
00:18:03,946 --> 00:18:05,970
and readings of other people.

456
00:18:05,970 --> 00:18:08,180
And it's also to note
that parallel astronomical

457
00:18:08,180 --> 00:18:11,570
developments were happening in
other regions of the world--

458
00:18:11,570 --> 00:18:14,490
the Far East, Mesoamerica,
and so forth, right?

459
00:18:14,490 --> 00:18:19,570
And so obviously, there may
have been some communication--

460
00:18:19,570 --> 00:18:22,770
I would say rather
sparse-- between especially

461
00:18:22,770 --> 00:18:23,820
with the Far East.

462
00:18:23,820 --> 00:18:26,230
But there was a fair
amount of communication

463
00:18:26,230 --> 00:18:28,860
between these regions here.

464
00:18:28,860 --> 00:18:31,660
And you can imagine writings
or ideas traveling from word

465
00:18:31,660 --> 00:18:34,450
of mouth along trade routes.

466
00:18:34,450 --> 00:18:36,922
So there was some communication.

467
00:18:36,922 --> 00:18:38,380
So back to our
learning objectives.

468
00:18:38,380 --> 00:18:41,040
Today, we're about to quantify
the available solar resource

469
00:18:41,040 --> 00:18:43,710
relative to human energies
and other fuel sources.

470
00:18:43,710 --> 00:18:44,440
So let's do that.

471
00:18:44,440 --> 00:18:46,065
We'll jump right into
one of the slides

472
00:18:46,065 --> 00:18:47,260
that I showed you last time.

473
00:18:47,260 --> 00:18:49,430
This is in terms
of terawatts av.

474
00:18:49,430 --> 00:18:52,780
Terawatts is a unit of
power, a very big one.

475
00:18:52,780 --> 00:18:55,240
"Tera" is 10 to the 12.

476
00:18:55,240 --> 00:18:57,610
And you can see
here the resource

477
00:18:57,610 --> 00:19:00,310
of the sun relative to the
wind energy resource base

478
00:19:00,310 --> 00:19:01,930
relative to human energy needs.

479
00:19:01,930 --> 00:19:05,470
If we just consider the resource
falling on the earth's surface,

480
00:19:05,470 --> 00:19:07,910
as opposed to that falling
on the outer atmosphere,

481
00:19:07,910 --> 00:19:09,368
there's a little
bit of a discount.

482
00:19:09,368 --> 00:19:12,660
But we're still very large
compared to human energy use.

483
00:19:12,660 --> 00:19:15,250
And if we redefine our
units from terawatts

484
00:19:15,250 --> 00:19:18,900
into HECs, which are Human
Energy Consumptions, defined

485
00:19:18,900 --> 00:19:23,220
in, say, 2050, where
one HEC is the average,

486
00:19:23,220 --> 00:19:28,880
let's say, energy
burn rate of 2050,

487
00:19:28,880 --> 00:19:30,460
you can see here
that these numbers

488
00:19:30,460 --> 00:19:33,060
are a few orders of
magnitude larger than what

489
00:19:33,060 --> 00:19:34,250
our human needs are.

490
00:19:34,250 --> 00:19:37,200
So if you were able
to capture only 1%

491
00:19:37,200 --> 00:19:40,500
of all of the solar resource
falling on the earth's crust,

492
00:19:40,500 --> 00:19:44,130
we would be actually
in pretty good shape.

493
00:19:44,130 --> 00:19:48,264
And 1% of the total land
area on the earth's crust--

494
00:19:48,264 --> 00:19:50,680
I believe somewhere between
1% and 2% of the United States

495
00:19:50,680 --> 00:19:53,530
is covered in asphalt
right now for roads.

496
00:19:53,530 --> 00:19:56,640
So this could be on houses,
on rooftops, on buildings,

497
00:19:56,640 --> 00:19:57,500
and so forth.

498
00:19:57,500 --> 00:20:01,530
We wouldn't necessarily
have to exclusively repave

499
00:20:01,530 --> 00:20:05,520
virgin farmland
with solar panels.

500
00:20:05,520 --> 00:20:09,020
So going back to quantifying the
solar power, we have our sun.

501
00:20:09,020 --> 00:20:10,700
We're going to
start by quantifying

502
00:20:10,700 --> 00:20:13,970
the solar resource by assuming
that the sun is a black body.

503
00:20:13,970 --> 00:20:17,210
The same way that hot
objects emit light-- say,

504
00:20:17,210 --> 00:20:18,880
for example, when you
turn up your stove

505
00:20:18,880 --> 00:20:23,200
and you have a very warm
glow coming out of it--

506
00:20:23,200 --> 00:20:26,500
the sun is as well a hot
body, a black body, sorry.

507
00:20:26,500 --> 00:20:28,850
Very hot as well, somewhere
around 6,000 Kelvin.

508
00:20:28,850 --> 00:20:33,140
And the total radiated power is
given by Stefan Boltzmann's law

509
00:20:33,140 --> 00:20:35,940
here in the following way
where we have temperature

510
00:20:35,940 --> 00:20:38,135
to the fourth and
the temperature

511
00:20:38,135 --> 00:20:40,460
is somewhere around
6,000 Kelvin.

512
00:20:40,460 --> 00:20:42,980
And so we have this
power being radiated out

513
00:20:42,980 --> 00:20:44,280
at the surface of the sun.

514
00:20:44,280 --> 00:20:48,550
And then, as it travels outward,
it becomes, you could say,

515
00:20:48,550 --> 00:20:49,530
diluted in effect.

516
00:20:49,530 --> 00:20:53,700
Because the total surface area
of that sphere is increasing,

517
00:20:53,700 --> 00:20:55,060
obviously as r squared.

518
00:20:55,060 --> 00:20:57,530
And so by the time that that
light reaches the earth,

519
00:20:57,530 --> 00:20:59,660
it's only a very
small cross section,

520
00:20:59,660 --> 00:21:02,500
or very small solid
angle to be more precise,

521
00:21:02,500 --> 00:21:06,090
of the sun's surface that is
radiating directly at the earth

522
00:21:06,090 --> 00:21:08,947
right here because this, in the
absence of scattering centers

523
00:21:08,947 --> 00:21:10,780
in the universe that
might reflect or bounce

524
00:21:10,780 --> 00:21:12,774
the light back toward the earth.

525
00:21:12,774 --> 00:21:14,690
And you can calculate
the total power incident

526
00:21:14,690 --> 00:21:17,210
on the earth by that
simple formula right there.

527
00:21:17,210 --> 00:21:20,070
What is the radius of the earth?

528
00:21:20,070 --> 00:21:22,130
Again, one of these
simple numbers

529
00:21:22,130 --> 00:21:23,630
you should kind of
having your head.

530
00:21:23,630 --> 00:21:27,010
6,370 kilometers, somewhere
in that order, right?

531
00:21:27,010 --> 00:21:31,390
And so you can begin
estimating here the total power

532
00:21:31,390 --> 00:21:32,850
and the order of magnitude.

533
00:21:32,850 --> 00:21:35,250
It's going to be tiny
compared to the total power

534
00:21:35,250 --> 00:21:37,110
that the sun is
radiating thankfully,

535
00:21:37,110 --> 00:21:39,220
or else we'd be
pretty hot right now.

536
00:21:39,220 --> 00:21:43,910
So the average power
coming from the sun

537
00:21:43,910 --> 00:21:46,680
on the surface of
the outer atmosphere

538
00:21:46,680 --> 00:21:49,860
is around 1,366 watts
per square meter.

539
00:21:49,860 --> 00:21:51,525
Who's heard of 1,366 before?

540
00:21:51,525 --> 00:21:52,150
AUDIENCE: Yeah.

541
00:21:52,150 --> 00:21:52,310
PROFESSOR: Yeah?

542
00:21:52,310 --> 00:21:53,010
You've heard about it?

543
00:21:53,010 --> 00:21:53,509
All right.

544
00:21:53,509 --> 00:21:55,220
You know where the
number comes from now.

545
00:21:55,220 --> 00:21:56,880
It's pretty wonky.

546
00:21:56,880 --> 00:21:59,390
1366 is a startup
company, a spin-off

547
00:21:59,390 --> 00:22:02,251
of MIT focused on solar energy.

548
00:22:02,251 --> 00:22:02,750
OK.

549
00:22:02,750 --> 00:22:06,640
And so that's at the equinox,
which we just learned

550
00:22:06,640 --> 00:22:09,320
is occurring somewhere
around March 21,

551
00:22:09,320 --> 00:22:11,750
September 21-- coming up soon.

552
00:22:11,750 --> 00:22:13,440
Celebration.

553
00:22:13,440 --> 00:22:15,620
The ratio of the surface
areas of the spheres

554
00:22:15,620 --> 00:22:17,970
is really something to
keep in mind right there.

555
00:22:17,970 --> 00:22:18,980
OK.

556
00:22:18,980 --> 00:22:21,690
So we've quantified
the available resource

557
00:22:21,690 --> 00:22:23,300
relative to human energy needs.

558
00:22:23,300 --> 00:22:26,420
Now we have to come
up with some language

559
00:22:26,420 --> 00:22:28,340
that we use to
describe the sunlight

560
00:22:28,340 --> 00:22:30,460
moving through the
atmosphere of the earth

561
00:22:30,460 --> 00:22:32,700
and reaching the surface
of the earth, right?

562
00:22:32,700 --> 00:22:34,860
So we're going to use
what's called the air mass

563
00:22:34,860 --> 00:22:36,660
convention or AM convention.

564
00:22:36,660 --> 00:22:39,226
AM stands for Air Mass.

565
00:22:39,226 --> 00:22:41,600
Even without knowing much
about how the light is absorbed

566
00:22:41,600 --> 00:22:43,270
or how to quantify
it mathematically,

567
00:22:43,270 --> 00:22:46,980
we can assume that our
atmosphere contains molecules.

568
00:22:46,980 --> 00:22:48,490
It contains particulate matter.

569
00:22:48,490 --> 00:22:50,380
And that's going to interact
with the light in some ways.

570
00:22:50,380 --> 00:22:52,437
Probably either going
to absorb or scatter it.

571
00:22:52,437 --> 00:22:54,520
And so as the light passes
through our atmosphere,

572
00:22:54,520 --> 00:22:56,220
there's going to
be some absorption.

573
00:22:56,220 --> 00:22:58,792
And the greater the distance,
the greater the optical path

574
00:22:58,792 --> 00:23:01,250
length through the atmosphere,
the more absorption and more

575
00:23:01,250 --> 00:23:02,520
scattering there will be.

576
00:23:02,520 --> 00:23:05,070
And so we use air
mass, or AM convention,

577
00:23:05,070 --> 00:23:07,520
to define the path length
or the path distance

578
00:23:07,520 --> 00:23:08,570
through the atmosphere.

579
00:23:08,570 --> 00:23:11,160
AM0 would mean the
outer atmosphere.

580
00:23:11,160 --> 00:23:13,950
AM1 would be essentially
just going straight through,

581
00:23:13,950 --> 00:23:17,060
normal incidence so that the
direction of the trajectory

582
00:23:17,060 --> 00:23:19,970
of the light is parallel
to the surface normal

583
00:23:19,970 --> 00:23:21,580
of the earth at that location.

584
00:23:21,580 --> 00:23:23,200
And then air mass
1.5 and so forth

585
00:23:23,200 --> 00:23:26,039
is as we increase the
angle of the entry of light

586
00:23:26,039 --> 00:23:27,330
relative to the surface normal.

587
00:23:27,330 --> 00:23:28,967
So in other words,
as we go further

588
00:23:28,967 --> 00:23:30,550
from the equator to
northern latitudes

589
00:23:30,550 --> 00:23:32,770
that air mass number is
going to go up, up, up.

590
00:23:32,770 --> 00:23:33,270
OK.

591
00:23:33,270 --> 00:23:34,740
We'll explain it
with a few graphs

592
00:23:34,740 --> 00:23:36,710
and figures and a few slides.

593
00:23:36,710 --> 00:23:39,350
So we have our
atmospheric absorption.

594
00:23:39,350 --> 00:23:42,480
When we just glance at the earth
in these beautiful pictures

595
00:23:42,480 --> 00:23:46,290
taken from outer space, we can
see very obviously the clouds

596
00:23:46,290 --> 00:23:47,180
present.

597
00:23:47,180 --> 00:23:49,850
And more importantly,
if we were to zoom

598
00:23:49,850 --> 00:23:51,940
in on one of these
regions right here,

599
00:23:51,940 --> 00:23:56,980
we would see this
bluish hue coming

600
00:23:56,980 --> 00:24:00,460
from our atmosphere, which
is scattering preferentially

601
00:24:00,460 --> 00:24:02,410
the shorter
wavelengths of light.

602
00:24:02,410 --> 00:24:05,090
And more importantly,
this radius right here

603
00:24:05,090 --> 00:24:07,870
is somewhere on the order
of 6,370 kilometers.

604
00:24:07,870 --> 00:24:10,572
And this thin atmospheric
shell in on the order of 30.

605
00:24:10,572 --> 00:24:12,020
Right?

606
00:24:12,020 --> 00:24:14,200
So that's why you
don't really see

607
00:24:14,200 --> 00:24:18,340
too much of a ring around the
planet from this distance.

608
00:24:18,340 --> 00:24:20,910
So the atmospheric effects,
let's try to bend them

609
00:24:20,910 --> 00:24:22,880
into discrete buckets.

610
00:24:22,880 --> 00:24:25,120
This is a simplification,
but it helps us gain

611
00:24:25,120 --> 00:24:26,310
a foothold in understanding.

612
00:24:26,310 --> 00:24:28,520
And then from there, we
can make our understanding

613
00:24:28,520 --> 00:24:30,190
a bit more complex.

614
00:24:30,190 --> 00:24:33,180
So we have incoming solar
radiation coming from here.

615
00:24:33,180 --> 00:24:37,480
We have a number now that's
342 watts per square meter.

616
00:24:37,480 --> 00:24:40,584
Why is that number so
much lower than the 1,366

617
00:24:40,584 --> 00:24:42,480
that we were just talking about?

618
00:24:49,786 --> 00:24:51,619
AUDIENCE: Particles in
the air and pollution

619
00:24:51,619 --> 00:24:52,894
in the atmosphere.

620
00:24:52,894 --> 00:24:53,560
PROFESSOR: Yeah.

621
00:24:53,560 --> 00:24:56,070
So this is meant
to be an average

622
00:24:56,070 --> 00:24:59,160
over the entire day at one fixed
point along the ground, right?

623
00:24:59,160 --> 00:25:01,860
So as we're rotating around, we
have at least half of the day

624
00:25:01,860 --> 00:25:03,510
normally when we
don't have sunlight.

625
00:25:03,510 --> 00:25:06,950
And then there's, I would
say, the cosine theta angle

626
00:25:06,950 --> 00:25:09,160
is not 1 at all times.

627
00:25:09,160 --> 00:25:11,220
It's very rarely 1.

628
00:25:11,220 --> 00:25:13,980
And so this is a bit of a
discounted incoming solar

629
00:25:13,980 --> 00:25:16,460
radiation, essentially a
time-averaged solar radiation

630
00:25:16,460 --> 00:25:20,490
for a given patch of the
planet over a typical day.

631
00:25:20,490 --> 00:25:22,890
And so we have a variety
of processes here.

632
00:25:22,890 --> 00:25:25,140
We have reflection
off of clouds.

633
00:25:25,140 --> 00:25:26,840
That's pretty clear
to see from here.

634
00:25:26,840 --> 00:25:29,240
It looks nice and white.

635
00:25:29,240 --> 00:25:31,380
We have some absorbed
by the atmosphere,

636
00:25:31,380 --> 00:25:35,570
typically in a rotational or
vibrational modes of molecules

637
00:25:35,570 --> 00:25:38,100
up in the outer atmosphere,
sometimes by particulate matter

638
00:25:38,100 --> 00:25:39,370
as well.

639
00:25:39,370 --> 00:25:42,120
And then we have
the amount that's

640
00:25:42,120 --> 00:25:45,350
absorbed here by the surface,
of course, reflected as well.

641
00:25:45,350 --> 00:25:47,720
And so the amount that's
incident on the surface

642
00:25:47,720 --> 00:25:49,760
or coming down to us
is what we can actually

643
00:25:49,760 --> 00:25:51,980
use to make solar energy.

644
00:25:51,980 --> 00:25:53,350
And this is an average, right?

645
00:25:53,350 --> 00:25:55,890
Because sometimes the cloud
cover is a lot greater.

646
00:25:55,890 --> 00:25:58,090
And for that
particular day, we will

647
00:25:58,090 --> 00:26:00,460
have a lot less resource
striking the ground

648
00:26:00,460 --> 00:26:02,110
at a given time.

649
00:26:02,110 --> 00:26:05,740
This over here, this is
all mostly infrared, right?

650
00:26:05,740 --> 00:26:08,830
Where this is visible coming
in, once the light gets absorbed

651
00:26:08,830 --> 00:26:10,860
and then gets
re-emitted, it usually

652
00:26:10,860 --> 00:26:12,740
gets re-emitted in
the longer wavelength

653
00:26:12,740 --> 00:26:14,620
light or the infrared light.

654
00:26:14,620 --> 00:26:18,610
And this is the stuff that
gets blocked by or absorbed

655
00:26:18,610 --> 00:26:21,350
by greenhouse gases and then
re-emitted equiangularly.

656
00:26:21,350 --> 00:26:24,690
And some of it makes its way
back to the earth, right?

657
00:26:24,690 --> 00:26:25,190
OK.

658
00:26:25,190 --> 00:26:28,240
So air mass, let's
define that so

659
00:26:28,240 --> 00:26:30,070
that we have a common
language that we

660
00:26:30,070 --> 00:26:33,020
can use to describe the solar
resource from place to place.

661
00:26:33,020 --> 00:26:34,700
So again, this is the sun.

662
00:26:34,700 --> 00:26:37,310
This is the surface right here.

663
00:26:37,310 --> 00:26:41,760
And let's imagine that the angle
between the incident sunlight

664
00:26:41,760 --> 00:26:45,240
and the surface normal is 0
such that the cosine theta

665
00:26:45,240 --> 00:26:47,040
term is 1.

666
00:26:47,040 --> 00:26:49,470
Air mass here at this
point, at this point

667
00:26:49,470 --> 00:26:53,270
on the surface of the
earth, is going to be AM 1.

668
00:26:53,270 --> 00:26:56,640
So we call it air mass 1 if
the sun is literally directly

669
00:26:56,640 --> 00:26:59,570
overhead.

670
00:26:59,570 --> 00:27:02,100
What did we learn about the
declination angle of the earth?

671
00:27:02,100 --> 00:27:04,890
It's about 23 and
1/2 degrees, right?

672
00:27:04,890 --> 00:27:06,430
How far north are we?

673
00:27:06,430 --> 00:27:09,389
What is our latitude
here in Boston?

674
00:27:09,389 --> 00:27:09,930
AUDIENCE: 41.

675
00:27:09,930 --> 00:27:11,720
PROFESSOR: 41, 42ish, right?

676
00:27:11,720 --> 00:27:22,980
So let's for simplicity say that
here in Boston, our latitude

677
00:27:22,980 --> 00:27:28,320
of Boston Logan Airport is
approximately 41 degrees north.

678
00:27:28,320 --> 00:27:36,720
And let's say that the
declination of our planet

679
00:27:36,720 --> 00:27:41,150
is approximately
23 and 1/2 degrees.

680
00:27:41,150 --> 00:27:46,270
So what would be the angle
of the sun in the sky

681
00:27:46,270 --> 00:27:49,730
if you were to lie on your back
in the middle of the summer

682
00:27:49,730 --> 00:27:52,709
solstice and the middle
of the winter solstice

683
00:27:52,709 --> 00:27:54,750
and you're lying straight
on your back looking up

684
00:27:54,750 --> 00:27:57,970
at solar noon, what would the
angle of the sun in the sky

685
00:27:57,970 --> 00:28:01,070
be relative to the
surface normal?

686
00:28:01,070 --> 00:28:03,750
How far south would the sun be?

687
00:28:03,750 --> 00:28:07,200
Imagine that this is 0
degrees and that's 90, right?

688
00:28:07,200 --> 00:28:09,370
So relative to this
angle right here,

689
00:28:09,370 --> 00:28:11,620
where would the
sun be in the sky?

690
00:28:11,620 --> 00:28:15,090
Why don't you turn to your
neighbor right now and discuss?

691
00:28:15,090 --> 00:28:18,360
On the summer solstice,
the winter solstice,

692
00:28:18,360 --> 00:28:20,040
come up with some
set of angles there.

693
00:28:33,212 --> 00:28:33,920
All right, folks.

694
00:28:33,920 --> 00:28:36,050
What do we come up with?

695
00:28:36,050 --> 00:28:39,890
This is our little human
being, you, in Boston.

696
00:28:39,890 --> 00:28:42,260
This is south, and that's north.

697
00:28:42,260 --> 00:28:45,070
And we're at the solar
noon in the winter solstice

698
00:28:45,070 --> 00:28:46,300
in the summer solstice.

699
00:28:46,300 --> 00:28:48,130
So here's you.

700
00:28:48,130 --> 00:28:50,069
This is directly above.

701
00:28:50,069 --> 00:28:51,860
So I would say if you're
lying on your back

702
00:28:51,860 --> 00:28:53,651
and looking straight
up, that's the surface

703
00:28:53,651 --> 00:28:55,380
normal of the earth.

704
00:28:55,380 --> 00:28:57,730
In the summer, at
solar noon, the sun

705
00:28:57,730 --> 00:29:00,170
would be at what angle
relative to the surface

706
00:29:00,170 --> 00:29:01,206
normal of the earth?

707
00:29:01,206 --> 00:29:02,424
AUDIENCE: 17 and 1/2.

708
00:29:02,424 --> 00:29:04,560
PROFESSOR: 17 and 1/2,
somewhere around there.

709
00:29:04,560 --> 00:29:06,680
And how did you get that number?

710
00:29:06,680 --> 00:29:08,060
Subtract those two, right?

711
00:29:08,060 --> 00:29:09,930
So you get 41 minus 23 and 1/2.

712
00:29:09,930 --> 00:29:14,250
You're somewhere around 17,
18, somewhere around there.

713
00:29:14,250 --> 00:29:16,940
So we'll call it 18
degrees in summer,

714
00:29:16,940 --> 00:29:19,210
again, relative to
the surface normal.

715
00:29:19,210 --> 00:29:24,595
And in wintertime, what
does that work out to be?

716
00:29:24,595 --> 00:29:25,470
AUDIENCE: 64 and 1/2.

717
00:29:25,470 --> 00:29:26,480
PROFESSOR: 64 and 1/2.

718
00:29:26,480 --> 00:29:27,440
Similar logic, right?

719
00:29:27,440 --> 00:29:30,370
So we'll call it 65
degrees in winter.

720
00:29:30,370 --> 00:29:31,000
Good.

721
00:29:31,000 --> 00:29:34,340
So what does that work out
to be in terms of air mass

722
00:29:34,340 --> 00:29:36,040
for winter and summer?

723
00:29:38,460 --> 00:29:39,960
Quick engineering
approximation, I'm

724
00:29:39,960 --> 00:29:42,240
going to say that, in summer,
it's approximate air mass 1,

725
00:29:42,240 --> 00:29:43,500
but you can calculate
it real quick.

726
00:29:43,500 --> 00:29:45,584
Somebody with a calculator
want to plug those in?

727
00:29:45,584 --> 00:29:47,280
AUDIENCE: And
roughly 2 in winter.

728
00:29:47,280 --> 00:29:48,738
PROFESSOR: And
roughly 2 in winter.

729
00:29:48,738 --> 00:29:51,250
All right?

730
00:29:51,250 --> 00:29:52,320
OK.

731
00:29:52,320 --> 00:29:54,850
We're going to get into
scattering of light

732
00:29:54,850 --> 00:29:56,920
in next lecture actually.

733
00:29:56,920 --> 00:29:59,375
And we'll see why that
matters in terms of especially

734
00:29:59,375 --> 00:30:01,400
of getting sunburned
since the shorter

735
00:30:01,400 --> 00:30:04,620
wavelengths, the ultraviolet,
are more sensitive to the path

736
00:30:04,620 --> 00:30:05,900
length.

737
00:30:05,900 --> 00:30:08,220
OK.

738
00:30:08,220 --> 00:30:08,790
Good.

739
00:30:08,790 --> 00:30:09,290
Very good.

740
00:30:09,290 --> 00:30:12,170
So we have pretty
much a gut sense

741
00:30:12,170 --> 00:30:14,310
now of where the
sun is in the sky.

742
00:30:14,310 --> 00:30:17,870
And as it moves from summer to
winter, from our perspective,

743
00:30:17,870 --> 00:30:19,846
it follows a little
bit of a, I would

744
00:30:19,846 --> 00:30:21,410
say, sinusoidal path, right?

745
00:30:21,410 --> 00:30:24,700
It stays in summer for a
long period of time up here.

746
00:30:24,700 --> 00:30:27,250
And then it moves pretty quickly
down here and stays here.

747
00:30:27,250 --> 00:30:30,555
So versus time, the angle
of the sun in the sky

748
00:30:30,555 --> 00:30:33,556
is following a sine curve,
or cosine curve if you will.

749
00:30:33,556 --> 00:30:34,250
Right?

750
00:30:34,250 --> 00:30:38,500
And so right now, the sun is
actually close to the middle.

751
00:30:38,500 --> 00:30:40,140
We're in September 13.

752
00:30:40,140 --> 00:30:42,930
The solstice is coming
up in a week's time.

753
00:30:42,930 --> 00:30:45,200
And so the slope
of that sine curve

754
00:30:45,200 --> 00:30:47,570
is at a maximum right about now.

755
00:30:47,570 --> 00:30:51,070
And so that the amount of
time that the day will change

756
00:30:51,070 --> 00:30:53,820
in length is changing
at its greatest point

757
00:30:53,820 --> 00:30:55,315
right now in the year.

758
00:30:55,315 --> 00:30:56,850
And so you really
begin to notice it

759
00:30:56,850 --> 00:30:58,610
if you start paying
attention or if you

760
00:30:58,610 --> 00:31:00,810
go to weather.com
and start looking up

761
00:31:00,810 --> 00:31:03,690
how long is today's
day going to last,

762
00:31:03,690 --> 00:31:05,337
when is the sunset tomorrow.

763
00:31:05,337 --> 00:31:07,420
I don't know about you
folks, but I like to cycle.

764
00:31:07,420 --> 00:31:09,790
And when I'm doing
my evening rides,

765
00:31:09,790 --> 00:31:12,220
I'm noticing it now
that I have to start

766
00:31:12,220 --> 00:31:14,849
earlier and earlier if I want
to put in, say, 30 or 40 miles.

767
00:31:14,849 --> 00:31:16,890
I'm not going to be able
to make it home in time.

768
00:31:16,890 --> 00:31:20,550
So that's the sun
and how it relates

769
00:31:20,550 --> 00:31:23,170
to you in your daily lives.

770
00:31:23,170 --> 00:31:25,145
We're going to get back
to this in a minute,

771
00:31:25,145 --> 00:31:26,020
so keep this in mind.

772
00:31:26,020 --> 00:31:28,550
Don't let it out of your RAM.

773
00:31:28,550 --> 00:31:31,900
Let's talk about the actual
solar spectrum for a minute.

774
00:31:31,900 --> 00:31:35,770
This is the sunlight intensity
in some very real units.

775
00:31:35,770 --> 00:31:37,020
We'll get to that in a minute.

776
00:31:37,020 --> 00:31:39,750
But think of this in terms
sort of like the total amount

777
00:31:39,750 --> 00:31:41,730
of power in a given bandwidth.

778
00:31:41,730 --> 00:31:44,732
So the wavelength right
here-- or the power density

779
00:31:44,732 --> 00:31:45,440
in the bandwidth.

780
00:31:45,440 --> 00:31:47,610
The wavelength is the
wavelength of light.

781
00:31:47,610 --> 00:31:50,140
Shown for your convenience
here is the visible spectrum.

782
00:31:50,140 --> 00:31:51,820
That's what our
eye-- mostly what

783
00:31:51,820 --> 00:31:54,530
our eye-- is able to
detect in this wavelength

784
00:31:54,530 --> 00:31:55,490
range right here.

785
00:31:55,490 --> 00:31:58,740
And the sun is emitting
over a much broader range

786
00:31:58,740 --> 00:31:59,540
of wavelengths.

787
00:31:59,540 --> 00:32:03,060
It's emitting following
a black body emission

788
00:32:03,060 --> 00:32:05,035
source at 6,000 Kelvin.

789
00:32:05,035 --> 00:32:06,410
And that's in this
very difficult

790
00:32:06,410 --> 00:32:08,790
to see green line right there.

791
00:32:08,790 --> 00:32:13,604
Air mass 0 spectrum looks like
this, this red line right here.

792
00:32:13,604 --> 00:32:15,520
And again, let me remind
you that the air mass

793
00:32:15,520 --> 00:32:18,410
0 is the light that's falling
on the outer atmosphere.

794
00:32:18,410 --> 00:32:21,390
There is no earth
atmospheric absorption yet.

795
00:32:21,390 --> 00:32:24,670
Why do we have these
little lines here?

796
00:32:24,670 --> 00:32:26,920
Do you see it's not
a perfect black body.

797
00:32:26,920 --> 00:32:30,480
We have some-- I'll give you
a hint-- absorption lines.

798
00:32:30,480 --> 00:32:32,340
Where is that light
being absorbed?

799
00:32:32,340 --> 00:32:35,620
Is it the ether between
the earth and the sun?

800
00:32:35,620 --> 00:32:36,120
No.

801
00:32:36,120 --> 00:32:37,120
There's no ether between
the earth and sun.

802
00:32:37,120 --> 00:32:38,578
Where is that light
being absorbed?

803
00:32:38,578 --> 00:32:40,130
AUDIENCE: Hydrogen
ions and stuff?

804
00:32:40,130 --> 00:32:41,850
PROFESSOR: In the
sun itself, right?

805
00:32:41,850 --> 00:32:44,100
So these are absorption
events occurring

806
00:32:44,100 --> 00:32:46,070
in the solar atmosphere.

807
00:32:46,070 --> 00:32:49,880
And now, if we do
air mass 1 or 1.5--

808
00:32:49,880 --> 00:32:53,180
let's push it up a little
bit from air mass 0--

809
00:32:53,180 --> 00:32:55,255
this is now passing
through an angle

810
00:32:55,255 --> 00:32:57,590
of somewhere around 60 degrees.

811
00:32:57,590 --> 00:32:59,420
Now what do we have?

812
00:32:59,420 --> 00:33:03,320
We have several absorption
lines occurring, right?

813
00:33:03,320 --> 00:33:05,920
And these correspond to
absorption events where?

814
00:33:05,920 --> 00:33:07,420
AUDIENCE: In the
earth's atmosphere.

815
00:33:07,420 --> 00:33:08,280
PROFESSOR: In the
earth's atmosphere.

816
00:33:08,280 --> 00:33:09,090
Exactly.

817
00:33:09,090 --> 00:33:11,722
And so we can attribute each of
these little absorption lines

818
00:33:11,722 --> 00:33:13,180
here to a particular--
usually it's

819
00:33:13,180 --> 00:33:16,120
a molecule in the
earth's atmosphere.

820
00:33:16,120 --> 00:33:18,890
Note the sensitivity of
the human in black right

821
00:33:18,890 --> 00:33:21,310
here and how well-matched
it is to the air mass

822
00:33:21,310 --> 00:33:23,500
1, air mass 1.5 spectrum.

823
00:33:23,500 --> 00:33:25,330
That's pretty cool.

824
00:33:25,330 --> 00:33:26,680
That's pretty cool.

825
00:33:26,680 --> 00:33:27,960
OK.

826
00:33:27,960 --> 00:33:30,050
So there you have the spectrum.

827
00:33:30,050 --> 00:33:34,460
Let's get to these units of
power density per bandwidth

828
00:33:34,460 --> 00:33:35,480
for a second.

829
00:33:35,480 --> 00:33:36,930
The way to think
about those units

830
00:33:36,930 --> 00:33:39,270
is as follows-- kilowatts
per meter squared.

831
00:33:39,270 --> 00:33:40,030
OK, I get that.

832
00:33:40,030 --> 00:33:43,340
It's the amount of power
falling on a unit area.

833
00:33:43,340 --> 00:33:46,220
Per micron, the reason
it's normalized per micron

834
00:33:46,220 --> 00:33:48,846
is because the wavelength
units right here is in microns.

835
00:33:48,846 --> 00:33:50,470
And if you take the
product of the two,

836
00:33:50,470 --> 00:33:52,460
it makes it pretty easy to
calculate the total power

837
00:33:52,460 --> 00:33:53,190
density, right?

838
00:33:53,190 --> 00:33:57,050
So if you want to calculate the
power density, the total watts

839
00:33:57,050 --> 00:33:59,440
per square meter,
falling on the earth,

840
00:33:59,440 --> 00:34:01,600
say, between 0.5
and 1 micron, you

841
00:34:01,600 --> 00:34:03,650
can calculate the
total amount of power

842
00:34:03,650 --> 00:34:05,204
by multiplying one
versus the other.

843
00:34:05,204 --> 00:34:07,370
So that's why they're in
this weird unit right here.

844
00:34:07,370 --> 00:34:09,212
It's to help you
perform calculations

845
00:34:09,212 --> 00:34:10,920
like the ones you'll
do in your homework,

846
00:34:10,920 --> 00:34:13,440
like the ones you'll do for
your class project and so forth.

847
00:34:13,440 --> 00:34:16,760
And it strikes a little bit odd
the first time you look at it,

848
00:34:16,760 --> 00:34:18,110
but it begins making sense.

849
00:34:18,110 --> 00:34:20,540
And you're appreciative
of it after a while.

850
00:34:20,540 --> 00:34:22,400
There are standard spectra.

851
00:34:22,400 --> 00:34:23,989
They're standard
reference spectra.

852
00:34:23,989 --> 00:34:26,880
Sure, you can go
outside and using

853
00:34:26,880 --> 00:34:28,230
some form of spectrophotometer.

854
00:34:28,230 --> 00:34:31,639
You can measure the incident
solar radiation and map

855
00:34:31,639 --> 00:34:35,739
out the spectral irradiance
as a function of wavelength.

856
00:34:35,739 --> 00:34:39,540
In terms of the planning
or communicating

857
00:34:39,540 --> 00:34:42,760
with other scientists, we
typically refer to standards.

858
00:34:42,760 --> 00:34:45,090
We use common yardsticks,
common metrics.

859
00:34:45,090 --> 00:34:46,830
And that facilitates
communication,

860
00:34:46,830 --> 00:34:49,550
avoids ambiguity,
avoids misunderstanding.

861
00:34:49,550 --> 00:34:52,860
And so these standards
right here, these ASTMs,

862
00:34:52,860 --> 00:34:55,370
refer to the
particular standards

863
00:34:55,370 --> 00:34:57,226
that are used for
those solar spectra.

864
00:34:57,226 --> 00:34:58,850
And in your supporting
online material,

865
00:34:58,850 --> 00:35:01,660
at the very end of the
lecture slides online,

866
00:35:01,660 --> 00:35:04,650
we go into some more
detail regarding that

867
00:35:04,650 --> 00:35:06,980
for those who are interested.

868
00:35:06,980 --> 00:35:09,130
So again, these little
absorption lines

869
00:35:09,130 --> 00:35:12,260
here correspond to specific
atmospheric events,

870
00:35:12,260 --> 00:35:15,100
interactions of that
particular wavelength of light

871
00:35:15,100 --> 00:35:19,650
with some molecule
usually in the atmosphere.

872
00:35:19,650 --> 00:35:22,400
We can, as well, have
generalized attenuation due

873
00:35:22,400 --> 00:35:24,140
to other scattering mechanisms.

874
00:35:24,140 --> 00:35:28,500
Notice right here in the short
wavelengths what's happening.

875
00:35:28,500 --> 00:35:31,300
From the red to the
blue, this light

876
00:35:31,300 --> 00:35:33,010
is particularly effective.

877
00:35:33,010 --> 00:35:35,520
We have a sharp drop in
the shorter wavelengths.

878
00:35:35,520 --> 00:35:39,450
And it grows sharper the
shorter in wavelength you go.

879
00:35:39,450 --> 00:35:43,500
So the attenuation due to
passing through the atmosphere

880
00:35:43,500 --> 00:35:47,310
grows or increases the
shorter in wavelength you go.

881
00:35:47,310 --> 00:35:52,420
And this is a process generally
called Rayleigh scattering.

882
00:35:52,420 --> 00:35:54,920
And there's a wavelength
to the fourth dependence.

883
00:35:54,920 --> 00:35:58,310
So as you go shorter and
shorter in wavelength,

884
00:35:58,310 --> 00:36:00,850
the likelihood or probability
of scattering will increase.

885
00:36:00,850 --> 00:36:02,380
Why is that pertinent to us?

886
00:36:02,380 --> 00:36:04,170
Well, now you can
answer that little child

887
00:36:04,170 --> 00:36:05,930
who walks up to you in the
shopping mall and says,

888
00:36:05,930 --> 00:36:06,890
why is the sky blue?

889
00:36:06,890 --> 00:36:11,130
You say, well, there is this
elastic scattering mechanism

890
00:36:11,130 --> 00:36:14,030
of electromagnetic
radiation whereby

891
00:36:14,030 --> 00:36:16,520
in a broad spectral event,
such as the sun, black body

892
00:36:16,520 --> 00:36:18,660
emission, we have the
shorter wavelengths that

893
00:36:18,660 --> 00:36:19,694
are scattered more.

894
00:36:19,694 --> 00:36:21,610
And that's why when we
look away from the sun,

895
00:36:21,610 --> 00:36:24,185
in the other direction, we see
those shorter wavelengths that

896
00:36:24,185 --> 00:36:25,770
are scattered back to us.

897
00:36:25,770 --> 00:36:27,270
That is pertinent
for two reasons.

898
00:36:27,270 --> 00:36:28,970
A, it makes the sky look blue.

899
00:36:28,970 --> 00:36:32,314
Secondly, it's not only the
short wavelengths in blue light

900
00:36:32,314 --> 00:36:34,855
that we're worried about, but
also the ultraviolet radiation.

901
00:36:34,855 --> 00:36:35,650
Right?

902
00:36:35,650 --> 00:36:40,750
So even on a cloudy day, if
there are a few open patches

903
00:36:40,750 --> 00:36:43,230
and you can get scattered
light coming in,

904
00:36:43,230 --> 00:36:44,790
you can still get sunburned.

905
00:36:44,790 --> 00:36:48,140
And secondly, there is
a very strong dependence

906
00:36:48,140 --> 00:36:51,400
on the path length, the optical
path length, the air mass,

907
00:36:51,400 --> 00:36:51,900
right?

908
00:36:51,900 --> 00:36:54,690
So if you go further north
in latitudes, where your air

909
00:36:54,690 --> 00:36:57,240
mass increases,
right-- because now,

910
00:36:57,240 --> 00:36:59,050
if you think about
the atmosphere

911
00:36:59,050 --> 00:37:00,137
as being kind of a flat.

912
00:37:00,137 --> 00:37:01,970
Just giving you an
approximation for minute.

913
00:37:01,970 --> 00:37:03,210
If you think of the
earth being flat

914
00:37:03,210 --> 00:37:04,585
and the atmosphere
of being flat,

915
00:37:04,585 --> 00:37:07,480
now the path length in
winter is much, much greater

916
00:37:07,480 --> 00:37:09,120
than the path length in summer.

917
00:37:09,120 --> 00:37:11,480
Same sun, just
different path length.

918
00:37:11,480 --> 00:37:14,880
You're much more likely to
get sunburned in the summer

919
00:37:14,880 --> 00:37:18,180
than you are in winter
because the path length is

920
00:37:18,180 --> 00:37:21,919
a lot shorter and the amount
of short wavelength ultraviolet

921
00:37:21,919 --> 00:37:23,460
radiation that will
be scattered away

922
00:37:23,460 --> 00:37:26,040
is going to be less in
the summer than in winter.

923
00:37:26,040 --> 00:37:28,460
That's also why if you go
south latitude, for example,

924
00:37:28,460 --> 00:37:31,160
from here to, say, Miami, your
incidence of getting sunburned

925
00:37:31,160 --> 00:37:35,085
is a lot greater, a lot
more than the total increase

926
00:37:35,085 --> 00:37:36,710
of the visible portion
of the spectrum.

927
00:37:36,710 --> 00:37:39,120
So the sun might not look
that different to you,

928
00:37:39,120 --> 00:37:42,850
but your incidence of sunburn
events goes up quite a bit.

929
00:37:42,850 --> 00:37:45,050
And that is due to
Rayleigh scattering.

930
00:37:45,050 --> 00:37:47,240
And now, by this
point, the little child

931
00:37:47,240 --> 00:37:49,310
has already run crying
back to the parent.

932
00:37:49,310 --> 00:37:51,530
But you have a full
satisfaction of knowing

933
00:37:51,530 --> 00:37:53,820
how the universe around
you is put together.

934
00:37:53,820 --> 00:37:56,620
And that actually was
a pretty deep problem.

935
00:37:56,620 --> 00:38:00,810
It took a long time for at
least European scientists

936
00:38:00,810 --> 00:38:04,810
to crack that nut and figure
out what was going on.

937
00:38:04,810 --> 00:38:07,430
Describe how solar
insulation maps are made

938
00:38:07,430 --> 00:38:10,230
and use them to estimate
the local solar resource.

939
00:38:10,230 --> 00:38:12,184
So we have metrology.

940
00:38:12,184 --> 00:38:13,600
We have techniques
that we can use

941
00:38:13,600 --> 00:38:16,380
to measure the amount of
sunlight that is out there.

942
00:38:16,380 --> 00:38:19,880
And now we want to apply those
in some systematic fashion

943
00:38:19,880 --> 00:38:23,680
to measure the average solar
resource around the planet,

944
00:38:23,680 --> 00:38:26,820
including the oceans, and
then use that information

945
00:38:26,820 --> 00:38:30,580
to estimate later on the
land area needed, or the size

946
00:38:30,580 --> 00:38:32,250
of the array, or
how many cells we're

947
00:38:32,250 --> 00:38:34,880
going to have to string together
based on the solar resource

948
00:38:34,880 --> 00:38:35,380
locally.

949
00:38:35,380 --> 00:38:37,390
So how are these
insulation maps made,

950
00:38:37,390 --> 00:38:40,880
these maps that we'll use as
engineers to size our systems?

951
00:38:40,880 --> 00:38:43,150
First off, let me
define insolation.

952
00:38:43,150 --> 00:38:45,737
This is not insulation, as in
stuff you put around the house

953
00:38:45,737 --> 00:38:47,070
to keep the heat from going out.

954
00:38:47,070 --> 00:38:49,980
This is insolation with
an "o," a shorthand

955
00:38:49,980 --> 00:38:51,820
for incoming solar radiation.

956
00:38:51,820 --> 00:38:53,360
Insolation at the top there.

957
00:38:53,360 --> 00:38:56,130
It's typically given in units
of energy per unit area per unit

958
00:38:56,130 --> 00:38:59,650
time, so kilowatt
hours-- that's energy--

959
00:38:59,650 --> 00:39:01,376
per meter squared per day.

960
00:39:01,376 --> 00:39:04,000
And it's helpful when designing
or projecting these PV systems.

961
00:39:04,000 --> 00:39:06,290
And it's affected by a bunch
of stuff, which we'll get

962
00:39:06,290 --> 00:39:07,510
to over the next few slides.

963
00:39:07,510 --> 00:39:11,180
So we can measure
insolation from the ground.

964
00:39:11,180 --> 00:39:13,700
That's a surefire way to do it.

965
00:39:13,700 --> 00:39:16,930
This right here
is a pyranometer.

966
00:39:16,930 --> 00:39:20,130
Pyro, fire, sun.

967
00:39:20,130 --> 00:39:21,990
Ano, on top of.

968
00:39:21,990 --> 00:39:23,890
So anode/cathode.

969
00:39:23,890 --> 00:39:25,130
Cata, under.

970
00:39:25,130 --> 00:39:26,530
Ano, above.

971
00:39:26,530 --> 00:39:27,591
Catatonic, under.

972
00:39:27,591 --> 00:39:28,090
Right?

973
00:39:28,090 --> 00:39:28,890
OK.

974
00:39:28,890 --> 00:39:29,780
So pyranometer.

975
00:39:29,780 --> 00:39:32,600
So it's basically measuring
the sun above, right?

976
00:39:32,600 --> 00:39:34,120
Measuring the sunlight above.

977
00:39:34,120 --> 00:39:37,837
This is a full hemisphere
measuring the sunlight

978
00:39:37,837 --> 00:39:38,920
coming in from all angles.

979
00:39:38,920 --> 00:39:40,628
There's a small little
sensor right here.

980
00:39:40,628 --> 00:39:41,470
It's lying flat.

981
00:39:41,470 --> 00:39:43,510
And that glass is essentially
allowing the light

982
00:39:43,510 --> 00:39:46,270
from different angles
to get into the sensor.

983
00:39:46,270 --> 00:39:49,410
And this is a very narrow,
solid angle of the sky.

984
00:39:49,410 --> 00:39:51,420
It's probably just
looking at the sun,

985
00:39:51,420 --> 00:39:54,300
or in a particular direction
rather, plus or minus 2 and 1/2

986
00:39:54,300 --> 00:39:56,480
degrees in either direction.

987
00:39:56,480 --> 00:39:59,830
So it's a very limited
solid angle of the sky.

988
00:39:59,830 --> 00:40:02,130
This one over here would
be more appropriate, say,

989
00:40:02,130 --> 00:40:04,380
for a flat panel that's
receiving scattered light

990
00:40:04,380 --> 00:40:06,030
coming in at all angles.

991
00:40:06,030 --> 00:40:08,700
This one over here would be
more appropriate for a tracking

992
00:40:08,700 --> 00:40:11,460
system, especially a
concentrator system that

993
00:40:11,460 --> 00:40:13,850
has optics that only
accept light in from a very

994
00:40:13,850 --> 00:40:15,100
limited solid angle.

995
00:40:15,100 --> 00:40:17,830
So imagine you have a lens that
has to have like incident to it

996
00:40:17,830 --> 00:40:19,560
to focus it on the right spot.

997
00:40:19,560 --> 00:40:21,927
And if the sun moves in the
wrong spot or the-- put it

998
00:40:21,927 --> 00:40:24,010
another way-- if the lens
is in the wrong position

999
00:40:24,010 --> 00:40:26,270
relative to the incident
solar radiation,

1000
00:40:26,270 --> 00:40:28,600
the light is being focused
off of the solar cell.

1001
00:40:28,600 --> 00:40:30,260
And it doesn't
produce any power.

1002
00:40:30,260 --> 00:40:35,110
So this is a system
that's used for measuring

1003
00:40:35,110 --> 00:40:36,820
the direct solar
spectrum, which will

1004
00:40:36,820 --> 00:40:39,920
be useful for calculating the
total output from concentrator

1005
00:40:39,920 --> 00:40:40,940
systems.

1006
00:40:40,940 --> 00:40:42,570
And this pyranometer
over here is

1007
00:40:42,570 --> 00:40:46,870
useful for flat panel systems.

1008
00:40:46,870 --> 00:40:49,587
And we can also measure
the total amount

1009
00:40:49,587 --> 00:40:51,920
of incident solar radiation,
total amount of insolation,

1010
00:40:51,920 --> 00:40:54,690
from the sky using
satellite imagery.

1011
00:40:54,690 --> 00:40:57,640
This is an example
of a measurement.

1012
00:40:57,640 --> 00:41:00,460
And this right here is
insolation, average insolation,

1013
00:41:00,460 --> 00:41:04,520
from 0 to 550 watts
per square meter taken

1014
00:41:04,520 --> 00:41:08,560
from a NASA satellite with
the NASA Earth Observatory.

1015
00:41:08,560 --> 00:41:10,950
Very cool website, great
place to spend a Friday night

1016
00:41:10,950 --> 00:41:12,390
if you don't have plans.

1017
00:41:12,390 --> 00:41:15,740
Just log on here, and
bunches of maps from snow

1018
00:41:15,740 --> 00:41:18,630
cover, to population density,
to CO2 being emitted,

1019
00:41:18,630 --> 00:41:20,651
to wildfires around
the planet-- anything

1020
00:41:20,651 --> 00:41:22,650
that a satellite can
measure, they're measuring.

1021
00:41:22,650 --> 00:41:26,180
And the insolation
value is one of them.

1022
00:41:26,180 --> 00:41:29,900
And so we have data
from various points.

1023
00:41:29,900 --> 00:41:32,540
This is the
insolation in January.

1024
00:41:32,540 --> 00:41:36,980
So in January, it's the
Southern Hemisphere's summer,

1025
00:41:36,980 --> 00:41:38,710
the Northern
Hemisphere's winter.

1026
00:41:38,710 --> 00:41:42,530
And as a result, we have
less insolation up north.

1027
00:41:42,530 --> 00:41:44,050
We're in the blues.

1028
00:41:44,050 --> 00:41:46,600
And the Southern Hemisphere
is more in the reds.

1029
00:41:46,600 --> 00:41:49,170
And of course, the
tide turns in July.

1030
00:41:49,170 --> 00:41:53,300
We have our summer and
the Southern Hemisphere

1031
00:41:53,300 --> 00:41:54,400
has their winter.

1032
00:41:54,400 --> 00:41:56,730
And the poor folks here in
Antarctica have nothing.

1033
00:41:56,730 --> 00:41:59,610
So a couple of things
to note just already

1034
00:41:59,610 --> 00:42:01,560
straight off the
bat, we're noticing

1035
00:42:01,560 --> 00:42:05,050
that there's, in general,
higher insolation

1036
00:42:05,050 --> 00:42:07,320
near the equator, the
equator passing right

1037
00:42:07,320 --> 00:42:10,461
through here approximately.

1038
00:42:10,461 --> 00:42:12,710
Fun fact-- small city up
there in the north of Brazil,

1039
00:42:12,710 --> 00:42:14,709
there's a soccer field
that is half in the north

1040
00:42:14,709 --> 00:42:16,101
and half in the south.

1041
00:42:16,101 --> 00:42:17,350
That's neither here nor there.

1042
00:42:17,350 --> 00:42:20,800
We have rainforests up here
in the north of Brazil,

1043
00:42:20,800 --> 00:42:23,520
Central Africa, and
here in Southeast Asia.

1044
00:42:23,520 --> 00:42:25,320
And even when the sun
is directly overhead,

1045
00:42:25,320 --> 00:42:27,670
those clouds are preventing
some of the sunlight

1046
00:42:27,670 --> 00:42:28,450
from getting in.

1047
00:42:28,450 --> 00:42:31,040
And that's why right
at the equator itself,

1048
00:42:31,040 --> 00:42:32,850
we typically have
less insolation

1049
00:42:32,850 --> 00:42:34,860
than we do in the
tropics, say, Tropic

1050
00:42:34,860 --> 00:42:36,960
of Capricorn, Tropic of Cancer.

1051
00:42:36,960 --> 00:42:39,149
Tropic of Capricorn
running straight

1052
00:42:39,149 --> 00:42:40,940
through Sao Paulo,
Brazil, Tropic of Cancer

1053
00:42:40,940 --> 00:42:42,320
running through Key West.

1054
00:42:42,320 --> 00:42:43,740
Just to situate yourselves.

1055
00:42:43,740 --> 00:42:46,801
And the Tropics are how
far away from the equator?

1056
00:42:46,801 --> 00:42:47,300
Right.

1057
00:42:47,300 --> 00:42:47,799
OK.

1058
00:42:47,799 --> 00:42:48,390
23 and 1/2.

1059
00:42:48,390 --> 00:42:50,380
Good guess.

1060
00:42:50,380 --> 00:42:51,130
Good.

1061
00:42:51,130 --> 00:42:54,736
So what we're going
to do is now launch

1062
00:42:54,736 --> 00:42:56,110
into our next
learning objective,

1063
00:42:56,110 --> 00:43:00,210
which is to list the causes
of variation and intermittency

1064
00:43:00,210 --> 00:43:02,250
of the solar resource
and quantify the time

1065
00:43:02,250 --> 00:43:03,550
constants in magnitudes.

1066
00:43:03,550 --> 00:43:06,897
This is really, really,
really, really important.

1067
00:43:06,897 --> 00:43:08,980
The other stuff is very
useful from an engineering

1068
00:43:08,980 --> 00:43:10,896
point of view from
answering certain questions

1069
00:43:10,896 --> 00:43:12,580
in your homework.

1070
00:43:12,580 --> 00:43:17,280
This right here is the singular
reason-- one of the singular

1071
00:43:17,280 --> 00:43:19,080
reasons-- why solar
doesn't behave

1072
00:43:19,080 --> 00:43:22,460
like a regular fossil fuel
source, why solar does not

1073
00:43:22,460 --> 00:43:24,330
produce power all the time.

1074
00:43:24,330 --> 00:43:26,510
It is variable in terms
of its power output.

1075
00:43:26,510 --> 00:43:28,750
Variability generally
refers to the fact

1076
00:43:28,750 --> 00:43:30,730
that we can predict it's coming.

1077
00:43:30,730 --> 00:43:33,460
It's going to vary, but at least
we can predict it's coming.

1078
00:43:33,460 --> 00:43:37,390
Intermittency, while
not a strict definition,

1079
00:43:37,390 --> 00:43:40,220
the understanding when
somebody says "intermittency"

1080
00:43:40,220 --> 00:43:43,340
or "intermittent power source,"
the impression that it gives

1081
00:43:43,340 --> 00:43:45,980
is that it's
unpredictable in terms

1082
00:43:45,980 --> 00:43:48,300
of its variability
and its variation.

1083
00:43:48,300 --> 00:43:50,390
So we've talked a little
bit about the variation

1084
00:43:50,390 --> 00:43:52,590
so far and about the
predictable nature of the sun.

1085
00:43:52,590 --> 00:43:55,080
We've talked about how the
sunlight, the solar resource,

1086
00:43:55,080 --> 00:43:57,170
varies from summer to winter.

1087
00:43:57,170 --> 00:44:01,710
We've talked about how
the solar resource varies

1088
00:44:01,710 --> 00:44:04,190
as a function of
latitude, right?

1089
00:44:04,190 --> 00:44:07,780
But now, we're going to talk not
only in a little bit more depth

1090
00:44:07,780 --> 00:44:10,870
about that and have a few
fun in-class exercises

1091
00:44:10,870 --> 00:44:14,500
to get us really grasping
that concept in its entirety,

1092
00:44:14,500 --> 00:44:18,660
but also talk about some
sources of intermittency, which

1093
00:44:18,660 --> 00:44:21,230
if you have a large amount of
solar contributing to the grid

1094
00:44:21,230 --> 00:44:23,480
and it is intermittent, and
you have no way of dealing

1095
00:44:23,480 --> 00:44:26,860
with that, you're going to have
fluctuations of energy level

1096
00:44:26,860 --> 00:44:30,810
on the grid or power levels
here as a function of time.

1097
00:44:30,810 --> 00:44:33,250
And that's not going to be good.

1098
00:44:33,250 --> 00:44:35,130
So in terms of the
seasonal variations,

1099
00:44:35,130 --> 00:44:38,780
in terms of predicting
the amount coming

1100
00:44:38,780 --> 00:44:40,350
from the sun at
a given point-- I

1101
00:44:40,350 --> 00:44:44,100
told you it looks a little bit
like a sine, a cosine wave.

1102
00:44:44,100 --> 00:44:45,190
And indeed, it does.

1103
00:44:45,190 --> 00:44:47,990
You can calculate those values
based on this website right

1104
00:44:47,990 --> 00:44:48,690
here.

1105
00:44:48,690 --> 00:44:50,790
Just to show you how
nifty and cool it is,

1106
00:44:50,790 --> 00:44:53,140
our friends at Arizona State
University, Stuart Bowden

1107
00:44:53,140 --> 00:44:55,306
and Christiana Honsberg,
really put in a lot of time

1108
00:44:55,306 --> 00:44:56,170
to make this.

1109
00:44:56,170 --> 00:44:58,380
You can vary the time
or the day of year

1110
00:44:58,380 --> 00:44:59,846
right here for instance.

1111
00:44:59,846 --> 00:45:00,540
Right?

1112
00:45:00,540 --> 00:45:03,520
And you can see how
the solar resource--

1113
00:45:03,520 --> 00:45:07,690
this is the direct radiation,
kilowatts per meter squared.

1114
00:45:07,690 --> 00:45:09,040
And this is the time.

1115
00:45:09,040 --> 00:45:11,709
So if you take the integral
of the curve right here,

1116
00:45:11,709 --> 00:45:12,750
you're going to get what?

1117
00:45:12,750 --> 00:45:16,044
Units of--

1118
00:45:16,044 --> 00:45:16,710
AUDIENCE: Power.

1119
00:45:16,710 --> 00:45:17,418
PROFESSOR: Power.

1120
00:45:17,418 --> 00:45:18,850
Power times time is?

1121
00:45:18,850 --> 00:45:19,580
AUDIENCE: Energy.

1122
00:45:19,580 --> 00:45:20,710
PROFESSOR: Energy.

1123
00:45:20,710 --> 00:45:22,010
Energy per unit area, right?

1124
00:45:22,010 --> 00:45:23,120
So you're going to
be able to calculate

1125
00:45:23,120 --> 00:45:24,994
the total amount of
energy falling on a given

1126
00:45:24,994 --> 00:45:28,360
area per day, let's say, right?

1127
00:45:28,360 --> 00:45:32,730
So if we look at the size of
this little curve in winter,

1128
00:45:32,730 --> 00:45:35,130
the total area under this
is going to be very small.

1129
00:45:35,130 --> 00:45:37,370
And that's because the solar
resource is very small.

1130
00:45:37,370 --> 00:45:42,000
And the sun rises late
and sets early in winter.

1131
00:45:42,000 --> 00:45:45,770
And as we move towards
summer, obviously,

1132
00:45:45,770 --> 00:45:48,470
the total amount of the
solar resource increases.

1133
00:45:48,470 --> 00:45:51,200
Not only it increases
because of this

1134
00:45:51,200 --> 00:45:53,550
that we have at solar noon.

1135
00:45:53,550 --> 00:45:56,080
We have less of a path
through the atmosphere.

1136
00:45:56,080 --> 00:45:58,190
We have more sunlight
reaching the earth.

1137
00:45:58,190 --> 00:46:02,580
We have a total increase of
the amount reaching the earth.

1138
00:46:02,580 --> 00:46:05,480
We also have that
cosine theta term here

1139
00:46:05,480 --> 00:46:09,610
dictating the
cross-section incident

1140
00:46:09,610 --> 00:46:12,110
to that sunlight
coming in increasing.

1141
00:46:12,110 --> 00:46:13,990
And so that's driving
this going up.

1142
00:46:13,990 --> 00:46:17,302
And we also have a
second fact that the time

1143
00:46:17,302 --> 00:46:19,510
of the day, the total duration
of the day, increases,

1144
00:46:19,510 --> 00:46:22,710
at least in northern
latitudes here at around 40,

1145
00:46:22,710 --> 00:46:25,820
let's say, 41 degrees
north, here in Boston.

1146
00:46:25,820 --> 00:46:30,830
And we have because these two
effects a much larger area

1147
00:46:30,830 --> 00:46:33,170
underneath that curve.

1148
00:46:33,170 --> 00:46:38,050
And so as we go through summer
and now finally to September 13

1149
00:46:38,050 --> 00:46:40,690
and back to winter, our solar
resource goes back down again.

1150
00:46:40,690 --> 00:46:41,689
So you can calculate it.

1151
00:46:41,689 --> 00:46:42,940
You can visualize it.

1152
00:46:42,940 --> 00:46:45,900
That's cool.

1153
00:46:45,900 --> 00:46:49,660
And we can plot the
total amount of energy

1154
00:46:49,660 --> 00:46:51,890
per unit area per day,
essentially the integral

1155
00:46:51,890 --> 00:46:55,260
under that curve, as a function
of location around the US,

1156
00:46:55,260 --> 00:46:58,520
around the world per
month let's say, right?

1157
00:46:58,520 --> 00:46:59,769
So this is January.

1158
00:46:59,769 --> 00:47:01,810
This is kilowatt hours
per meter squared per day.

1159
00:47:01,810 --> 00:47:07,660
So it's just taking the
integral of the curves measured.

1160
00:47:07,660 --> 00:47:10,290
So it's accounting for
cloudy days, which kind of

1161
00:47:10,290 --> 00:47:11,321
has a depressive effect.

1162
00:47:11,321 --> 00:47:13,070
This is an envelope
function, if you will,

1163
00:47:13,070 --> 00:47:14,360
the maximum you could get.

1164
00:47:14,360 --> 00:47:16,151
And then, of course,
local weather patterns

1165
00:47:16,151 --> 00:47:17,590
will suppress that,
drive it down.

1166
00:47:17,590 --> 00:47:21,230
So this is the real map
of the United States.

1167
00:47:21,230 --> 00:47:24,630
And you can see in sunnier
areas that are less cloudy, over

1168
00:47:24,630 --> 00:47:27,120
here, for example, in
Arizona and New Mexico,

1169
00:47:27,120 --> 00:47:29,820
there's a large solar
resource even in January.

1170
00:47:29,820 --> 00:47:33,410
Atlanta, which has half of the
number of sunny days per year

1171
00:47:33,410 --> 00:47:37,160
as Phoenix does, even as
it's at the same latitude,

1172
00:47:37,160 --> 00:47:39,324
is getting about half
the solar resource.

1173
00:47:39,324 --> 00:47:40,740
They got a short
end of the stick.

1174
00:47:40,740 --> 00:47:44,600
Again, this curve right here is
the envelope function, right?

1175
00:47:44,600 --> 00:47:47,972
And off of that, you
can only go down.

1176
00:47:47,972 --> 00:47:50,430
You can only decrease the amount
of solar resource actually

1177
00:47:50,430 --> 00:47:53,310
arriving at our feet here.

1178
00:47:53,310 --> 00:47:54,470
And so this is in January.

1179
00:47:54,470 --> 00:47:56,560
And this is all in
the same color scale

1180
00:47:56,560 --> 00:47:58,390
here as we move
through the months.

1181
00:47:58,390 --> 00:48:03,620
So we'll move from January, to
February, to March, April, May,

1182
00:48:03,620 --> 00:48:10,230
June, July, August, September,
October, November, December.

1183
00:48:10,230 --> 00:48:12,460
So you can see across
the United States

1184
00:48:12,460 --> 00:48:14,830
how the resource is
distributed geographically.

1185
00:48:14,830 --> 00:48:17,590
The general trend that as
you go from south to north

1186
00:48:17,590 --> 00:48:19,657
you have a decreasing
solar resource holds.

1187
00:48:19,657 --> 00:48:21,990
You can also see the influence
of local weather patterns

1188
00:48:21,990 --> 00:48:25,280
as well for the same latitude.

1189
00:48:25,280 --> 00:48:26,450
So that's pretty nifty.

1190
00:48:26,450 --> 00:48:29,570
And another nifty fact, if
you look at the year average

1191
00:48:29,570 --> 00:48:32,330
value, annual-- this is the
annual average value-- here

1192
00:48:32,330 --> 00:48:35,300
in Boston, we're averaging
around 4.5 kilowatt hours

1193
00:48:35,300 --> 00:48:36,870
per meter squared per day.

1194
00:48:36,870 --> 00:48:40,190
Phoenix, Arizona can be
upwards of 6 somewhere

1195
00:48:40,190 --> 00:48:42,090
in the outskirts.

1196
00:48:42,090 --> 00:48:43,840
It's not that bad.

1197
00:48:43,840 --> 00:48:46,650
It's only a few
tenths of percent.

1198
00:48:46,650 --> 00:48:48,680
It's not that bad,
I tell myself.

1199
00:48:48,680 --> 00:48:50,830
I don't believe
it myself either,

1200
00:48:50,830 --> 00:48:53,490
but I try to convince myself
of that during winter.

1201
00:48:53,490 --> 00:48:53,990
All right.

1202
00:48:53,990 --> 00:48:58,780
Let me show you the seasonal
and diurnal variations.

1203
00:48:58,780 --> 00:49:00,620
We're increasing the
level of sophistication

1204
00:49:00,620 --> 00:49:01,554
as we go along, right?

1205
00:49:01,554 --> 00:49:03,220
We've assumed you've
done your readings.

1206
00:49:03,220 --> 00:49:04,960
We've started with
some simple examples,

1207
00:49:04,960 --> 00:49:07,320
and now we're really
taking it one step further,

1208
00:49:07,320 --> 00:49:09,420
which is to introduce
the full 3D model.

1209
00:49:09,420 --> 00:49:13,730
And I'm going to do that by use
of this really cool app that's

1210
00:49:13,730 --> 00:49:16,960
available here.

1211
00:49:16,960 --> 00:49:17,940
Right here.

1212
00:49:17,940 --> 00:49:20,920
This is you standing
on the earth.

1213
00:49:20,920 --> 00:49:22,900
And you can drag and
pull this around.

1214
00:49:22,900 --> 00:49:25,130
You can see there's north,
south, east, and west.

1215
00:49:25,130 --> 00:49:26,755
So I'm going to pull
it up a little bit

1216
00:49:26,755 --> 00:49:28,890
just to give us a little
bit of perspective.

1217
00:49:28,890 --> 00:49:30,480
Still, south is
facing toward us.

1218
00:49:30,480 --> 00:49:31,620
North is away.

1219
00:49:31,620 --> 00:49:34,310
The sun will rise in the
east and set in the west.

1220
00:49:34,310 --> 00:49:41,490
Now, let's say I pull the
date back to September.

1221
00:49:41,490 --> 00:49:45,340
So this little tool is so cool
because it recognizes your IP

1222
00:49:45,340 --> 00:49:47,647
address and situates you
at the proper latitude,

1223
00:49:47,647 --> 00:49:49,230
so we don't have to
touch that at all.

1224
00:49:49,230 --> 00:49:50,360
It's approximately right.

1225
00:49:50,360 --> 00:49:52,860
We're at 40, yeah,
about 41 degrees.

1226
00:49:52,860 --> 00:49:54,490
We're right here in September.

1227
00:49:54,490 --> 00:49:57,250
And in terms of time of
day, we can pretty much just

1228
00:49:57,250 --> 00:50:00,080
cycle through the time
of day if we like.

1229
00:50:00,080 --> 00:50:03,910
We could, for example,
start animation.

1230
00:50:03,910 --> 00:50:06,780
Let's see, this is going
very fast right now.

1231
00:50:06,780 --> 00:50:10,590
I'm going to slow it down so you
can see the time of day moving

1232
00:50:10,590 --> 00:50:11,810
right over here.

1233
00:50:11,810 --> 00:50:15,100
And you can see that
relative to our vantage

1234
00:50:15,100 --> 00:50:16,960
point on the surface
of the earth,

1235
00:50:16,960 --> 00:50:20,440
this little yellow dot here
and this yellow line is tracing

1236
00:50:20,440 --> 00:50:24,780
through the path of the sun in
the sky from our perspective.

1237
00:50:24,780 --> 00:50:26,691
And so as we go
through the seasons,

1238
00:50:26,691 --> 00:50:28,440
I'm going to speed it
up just a little bit

1239
00:50:28,440 --> 00:50:31,810
so that we pay more
attention to the position

1240
00:50:31,810 --> 00:50:34,150
of this yellow line
and less attention

1241
00:50:34,150 --> 00:50:35,240
to the diurnal variations.

1242
00:50:35,240 --> 00:50:37,350
We're paying more attention
to the seasonal variations.

1243
00:50:37,350 --> 00:50:39,183
I'm going to vary the
seasons by force here.

1244
00:50:39,183 --> 00:50:43,470
I'm going to go back to
July or June if I may.

1245
00:50:43,470 --> 00:50:44,329
Here we go.

1246
00:50:44,329 --> 00:50:45,620
AUDIENCE: What's the blue line?

1247
00:50:45,620 --> 00:50:46,090
So you can see it?

1248
00:50:46,090 --> 00:50:46,756
PROFESSOR: Yeah.

1249
00:50:46,756 --> 00:50:48,840
So there's a number of
other lines right here,

1250
00:50:48,840 --> 00:50:51,190
and they're all
explained very carefully.

1251
00:50:51,190 --> 00:50:54,700
There is the hour
of ascension, which

1252
00:50:54,700 --> 00:50:59,560
would be prime hour circle.

1253
00:50:59,560 --> 00:51:00,980
Yes.

1254
00:51:00,980 --> 00:51:03,040
I'd have to go back and
double-check all of this,

1255
00:51:03,040 --> 00:51:06,252
but I believe they relate
to would be the sunrise

1256
00:51:06,252 --> 00:51:07,460
and sunset of that given day.

1257
00:51:07,460 --> 00:51:09,140
Let's see if our
hypothesis is correct.

1258
00:51:09,140 --> 00:51:11,730
No, it is not.

1259
00:51:11,730 --> 00:51:14,540
That would have to be,
since it is varying

1260
00:51:14,540 --> 00:51:18,400
in a systematic way from
January through the summer

1261
00:51:18,400 --> 00:51:20,480
and then back to the
winter, I'm imagining

1262
00:51:20,480 --> 00:51:23,711
this has something to do
with the direction of the sun

1263
00:51:23,711 --> 00:51:25,460
relative to the earth,
right, as it traces

1264
00:51:25,460 --> 00:51:27,567
that ellipse through the sky.

1265
00:51:27,567 --> 00:51:29,900
So let's pay attention to
that yellow line for a minute.

1266
00:51:29,900 --> 00:51:32,480
That's the one I want to attract
everybody's attention to.

1267
00:51:32,480 --> 00:51:34,832
Now we're in June, so
in the height of summer.

1268
00:51:34,832 --> 00:51:36,540
And relative to this
observer right here,

1269
00:51:36,540 --> 00:51:38,970
the sun is further
up in the sky just

1270
00:51:38,970 --> 00:51:40,800
like we traced out right there.

1271
00:51:40,800 --> 00:51:43,460
And now, as we go
to winter, that line

1272
00:51:43,460 --> 00:51:45,572
drops close to the horizon.

1273
00:51:45,572 --> 00:51:46,780
So a couple of things happen.

1274
00:51:46,780 --> 00:51:49,020
If we look like
this for instance,

1275
00:51:49,020 --> 00:51:51,440
now we're looking straight
down on the observer.

1276
00:51:51,440 --> 00:51:55,380
In wintertime, the sun
will rise in the southeast,

1277
00:51:55,380 --> 00:51:57,810
and it will trace this
arc through the sky

1278
00:51:57,810 --> 00:52:00,220
and set in the
southwest over here.

1279
00:52:00,220 --> 00:52:05,930
In the summertime,
the sun will rise

1280
00:52:05,930 --> 00:52:10,780
almost in the northeast,
slightly north of east, just

1281
00:52:10,780 --> 00:52:12,130
slightly north of east.

1282
00:52:12,130 --> 00:52:14,789
And that's why if you
have a north-facing window

1283
00:52:14,789 --> 00:52:16,830
and you put your little
plant on the window sill,

1284
00:52:16,830 --> 00:52:19,413
it'll get a little bit of direct
sunlight early in the morning

1285
00:52:19,413 --> 00:52:20,370
and late at night.

1286
00:52:20,370 --> 00:52:23,370
Because when the sun is tracing
this part or that part through,

1287
00:52:23,370 --> 00:52:25,370
it's tracing the sky.

1288
00:52:25,370 --> 00:52:28,270
So it's worth sitting down
with one of these plots,

1289
00:52:28,270 --> 00:52:30,610
toying around with it,
getting accustomed to it,

1290
00:52:30,610 --> 00:52:33,089
and understanding really
how the sun traces its arc

1291
00:52:33,089 --> 00:52:35,130
throughout the sky relative
to our position right

1292
00:52:35,130 --> 00:52:36,150
here on the earth.

1293
00:52:36,150 --> 00:52:38,760
If we shift this
further up north,

1294
00:52:38,760 --> 00:52:40,540
really interesting
things begin to happen.

1295
00:52:40,540 --> 00:52:42,550
So for example, my
wife is in Sweden.

1296
00:52:42,550 --> 00:52:45,970
If we go to her hometown right
here in the middle summertime

1297
00:52:45,970 --> 00:52:48,940
at the solstice,
you can see the sun

1298
00:52:48,940 --> 00:52:51,390
traces this awesome
route from north

1299
00:52:51,390 --> 00:52:54,200
to north barely
leaving the horizon.

1300
00:52:54,200 --> 00:52:56,880
If we look at, again,
from the perspective

1301
00:52:56,880 --> 00:53:00,540
of the little creature
here, that yellow arc

1302
00:53:00,540 --> 00:53:02,800
is really close to the horizon.

1303
00:53:02,800 --> 00:53:04,710
Maybe it goes up
about that high,

1304
00:53:04,710 --> 00:53:07,080
but it continues going
all the way to the north.

1305
00:53:07,080 --> 00:53:09,130
And if you keep going
north, it will never

1306
00:53:09,130 --> 00:53:11,220
set during the middle of summer.

1307
00:53:11,220 --> 00:53:13,304
It'll just be
light all the time.

1308
00:53:13,304 --> 00:53:15,470
And you can see here it
just traces that orbit right

1309
00:53:15,470 --> 00:53:16,610
around there.

1310
00:53:16,610 --> 00:53:17,860
It's all trig, folks.

1311
00:53:17,860 --> 00:53:18,420
We can do it.

1312
00:53:18,420 --> 00:53:21,920
We can sit down, and we can work
through the equations by hand.

1313
00:53:21,920 --> 00:53:22,740
I did that once.

1314
00:53:22,740 --> 00:53:23,698
It took me a long time.

1315
00:53:23,698 --> 00:53:25,130
I didn't learn that much.

1316
00:53:25,130 --> 00:53:29,410
I would instead advise you to
go to one of these simulations

1317
00:53:29,410 --> 00:53:31,860
right here, but to understand
all the inputs into it,

1318
00:53:31,860 --> 00:53:34,410
all the different
components, the fact

1319
00:53:34,410 --> 00:53:37,110
that the earth is moving
around the sun as a declination

1320
00:53:37,110 --> 00:53:40,380
angle, seasonal
variations, and so forth.

1321
00:53:40,380 --> 00:53:41,370
Very useful tool.

1322
00:53:41,370 --> 00:53:44,840
You have the website
link right here.

1323
00:53:44,840 --> 00:53:45,340
And yeah.

1324
00:53:48,000 --> 00:53:52,170
So from this tool-- actually,
one last tiny, tiny thing.

1325
00:53:52,170 --> 00:53:57,290
From this tool right here, we
can understand why-- OK, this

1326
00:53:57,290 --> 00:53:58,810
is a real stretch.

1327
00:53:58,810 --> 00:54:02,500
And forgive me, social
scientists in the room,

1328
00:54:02,500 --> 00:54:05,970
for doing this, but I have to
project a little bit of science

1329
00:54:05,970 --> 00:54:08,320
onto human behavior.

1330
00:54:08,320 --> 00:54:13,980
How far west is Madrid from GMT?

1331
00:54:13,980 --> 00:54:15,730
Madrid, Spain?

1332
00:54:15,730 --> 00:54:19,420
It's 3 degrees west
of the Great Meridian.

1333
00:54:19,420 --> 00:54:21,960
So the line that divides the
East and the West Hemispheres

1334
00:54:21,960 --> 00:54:23,830
is 3 degrees west.

1335
00:54:23,830 --> 00:54:29,200
But it is one time zone
earlier than London,

1336
00:54:29,200 --> 00:54:32,100
so it's in the same
time zone as Germany

1337
00:54:32,100 --> 00:54:37,320
and all the other cities
that are east of London.

1338
00:54:37,320 --> 00:54:39,265
And this is just for
convenience factor.

1339
00:54:39,265 --> 00:54:41,640
If you're traveling from one
continental European country

1340
00:54:41,640 --> 00:54:43,250
to the other, it
just makes sense

1341
00:54:43,250 --> 00:54:44,690
to have everything be
on the same time zone.

1342
00:54:44,690 --> 00:54:46,398
You get to work at
the same time sort of.

1343
00:54:46,398 --> 00:54:48,160
Pick up the phone,
call somebody,

1344
00:54:48,160 --> 00:54:49,510
you're doing business.

1345
00:54:49,510 --> 00:54:53,570
Now relative to
everybody else in Europe,

1346
00:54:53,570 --> 00:54:57,650
though, is the sun
setting later or earlier

1347
00:54:57,650 --> 00:54:59,820
if you're that far
west in your time zone?

1348
00:54:59,820 --> 00:55:00,680
AUDIENCE: Later.

1349
00:55:00,680 --> 00:55:01,679
PROFESSOR: Later, right?

1350
00:55:01,679 --> 00:55:04,080
So the sun is setting
later if you're there.

1351
00:55:04,080 --> 00:55:06,790
So if you're eating according to
the sun, not according to what

1352
00:55:06,790 --> 00:55:09,770
your watch is saying, but if
you're choosing to eat dinner

1353
00:55:09,770 --> 00:55:12,100
when the sun is
setting, when will your

1354
00:55:12,100 --> 00:55:14,940
watch say, oh my
goodness, it's really late

1355
00:55:14,940 --> 00:55:16,899
when you're in Berlin or
when you're in Madrid?

1356
00:55:16,899 --> 00:55:17,606
AUDIENCE: Madrid.

1357
00:55:17,606 --> 00:55:19,280
PROFESSOR: When you're
in Madrid, right?

1358
00:55:19,280 --> 00:55:21,890
So again, I'm not
saying that this

1359
00:55:21,890 --> 00:55:25,007
is the sole reason for
social behavior being

1360
00:55:25,007 --> 00:55:26,840
a little different on
the Iberian Peninsula,

1361
00:55:26,840 --> 00:55:28,340
since Portugal
also eats very late

1362
00:55:28,340 --> 00:55:30,173
and they're in the same
time zone as London,

1363
00:55:30,173 --> 00:55:32,380
but it could be a
contributing factor.

1364
00:55:32,380 --> 00:55:36,280
The sun is still up in the sky
when it's 5:00 PM in winter,

1365
00:55:36,280 --> 00:55:40,350
let's say, where in Germany,
it's set a long time ago.

1366
00:55:40,350 --> 00:55:42,640
So these are just little
things to keep in mind.

1367
00:55:42,640 --> 00:55:46,550
An easy way to calculate,
when is the solar noon,

1368
00:55:46,550 --> 00:55:48,090
you look at the
earth more or less

1369
00:55:48,090 --> 00:55:49,740
like we're looking
at this right now.

1370
00:55:49,740 --> 00:55:51,460
We have 360 degrees.

1371
00:55:51,460 --> 00:55:54,940
We divide that
into 24 time zones.

1372
00:55:54,940 --> 00:55:58,240
And then we say, OK,
about 15 degrees each.

1373
00:55:58,240 --> 00:56:00,260
And then we can begin
counting from there.

1374
00:56:00,260 --> 00:56:02,740
If in Boston, were
41 degrees north,

1375
00:56:02,740 --> 00:56:05,720
but we're 71 degrees
west, we can say,

1376
00:56:05,720 --> 00:56:08,010
OK we should be for GMT minus 5.

1377
00:56:08,010 --> 00:56:10,550
We should be at
around 75 degrees.

1378
00:56:10,550 --> 00:56:12,620
And so we're a little
earlier, so we do things

1379
00:56:12,620 --> 00:56:15,119
a little earlier around here
than what the solar noon should

1380
00:56:15,119 --> 00:56:17,510
be telling us to do things--
wake up a little earlier,

1381
00:56:17,510 --> 00:56:18,350
go to bed a little earlier.

1382
00:56:18,350 --> 00:56:19,974
And that's why students
are like, dang,

1383
00:56:19,974 --> 00:56:21,540
there's no night
life around here.

1384
00:56:21,540 --> 00:56:23,123
I'm not saying that's
the only reason,

1385
00:56:23,123 --> 00:56:25,380
but it could be a
contributing factor.

1386
00:56:25,380 --> 00:56:27,950
Whereas the opposite
happens when you're

1387
00:56:27,950 --> 00:56:30,161
far west in your time zone.

1388
00:56:30,161 --> 00:56:30,660
OK.

1389
00:56:30,660 --> 00:56:32,760
So again, just trying
to wrap our heads

1390
00:56:32,760 --> 00:56:36,079
around the solar resource and
around the world around us

1391
00:56:36,079 --> 00:56:38,370
so that we can answer that
little child in the shopping

1392
00:56:38,370 --> 00:56:40,080
mall when they come
with questions.

1393
00:56:40,080 --> 00:56:42,004
What are these?

1394
00:56:42,004 --> 00:56:43,662
AUDIENCE: [INAUDIBLE]

1395
00:56:43,662 --> 00:56:45,370
PROFESSOR: Solar trash
compactors, right?

1396
00:56:45,370 --> 00:56:46,180
AUDIENCE: At the Student Center.

1397
00:56:46,180 --> 00:56:47,555
PROFESSOR: At the
Student Center.

1398
00:56:47,555 --> 00:56:49,089
Anna's Taqueria is
right over there.

1399
00:56:49,089 --> 00:56:50,380
Dunkin' Donuts is there, right?

1400
00:56:50,380 --> 00:56:53,880
So these are solar panels
mounted on the tops of those.

1401
00:56:53,880 --> 00:56:57,704
And what the solar is doing
is charging a battery inside.

1402
00:56:57,704 --> 00:56:59,370
Once the trash reaches
a critical lever,

1403
00:56:59,370 --> 00:57:00,670
a sensor is triggered.

1404
00:57:00,670 --> 00:57:05,010
It stops you from opening this
bin, and it compacts the trash

1405
00:57:05,010 --> 00:57:08,179
and then releases and allows you
to open and put more stuff in.

1406
00:57:08,179 --> 00:57:09,970
And what it does is it
minimizes the number

1407
00:57:09,970 --> 00:57:11,890
of times between trash pickups.

1408
00:57:11,890 --> 00:57:17,060
If labor is a large portion of
the cost of trash management,

1409
00:57:17,060 --> 00:57:21,390
of refuse management, then it
eliminates some of the labor,

1410
00:57:21,390 --> 00:57:23,610
transferring it instead
to the technology.

1411
00:57:23,610 --> 00:57:27,110
And so installing these
at the Student Center,

1412
00:57:27,110 --> 00:57:28,830
I had a little bit
of a pet peeve.

1413
00:57:28,830 --> 00:57:31,120
[LAUGHTER]

1414
00:57:31,120 --> 00:57:36,300
So you have angle
of the sun right

1415
00:57:36,300 --> 00:57:39,030
here, which we just walked
through, 18 degrees in summer.

1416
00:57:39,030 --> 00:57:42,760
And it just so happened
to work out that--

1417
00:57:42,760 --> 00:57:44,224
and these numbers
were approximate.

1418
00:57:44,224 --> 00:57:46,390
This was me going with my
kind of engineering sense.

1419
00:57:46,390 --> 00:57:47,560
That's about 45 degrees.

1420
00:57:47,560 --> 00:57:49,830
Count the number of paces.

1421
00:57:49,830 --> 00:57:51,030
Equilateral triangle.

1422
00:57:51,030 --> 00:57:52,640
Estimate the height of there.

1423
00:57:52,640 --> 00:57:56,400
But in the middle of summertime,
when you are at the solstice,

1424
00:57:56,400 --> 00:58:00,120
there is no so direct sunlight
hitting this trash can because

1425
00:58:00,120 --> 00:58:01,950
of that overhang way up there.

1426
00:58:01,950 --> 00:58:04,700
And it's not a problem for this
particular trash collector,

1427
00:58:04,700 --> 00:58:06,214
since those panels
are way oversized

1428
00:58:06,214 --> 00:58:08,630
for the amount of energy that
the trash collector actually

1429
00:58:08,630 --> 00:58:09,860
needs.

1430
00:58:09,860 --> 00:58:11,510
And there is a
fair amount of what

1431
00:58:11,510 --> 00:58:13,590
we call diffuse sunlight,
meaning sunlight being

1432
00:58:13,590 --> 00:58:14,840
scattered off of other things.

1433
00:58:14,840 --> 00:58:17,324
That's why this portion
of the image looks white.

1434
00:58:17,324 --> 00:58:19,615
It wouldn't look white if
there was no diffuse scatter.

1435
00:58:19,615 --> 00:58:21,780
It'd look black, pitch
black, if there was nothing

1436
00:58:21,780 --> 00:58:22,970
to scatter the light off.

1437
00:58:22,970 --> 00:58:25,345
Like an outer space, there's
nothing to scatter this way.

1438
00:58:25,345 --> 00:58:27,920
It looks like a black night.

1439
00:58:27,920 --> 00:58:30,170
But instead, there's a large
amount of diffused light.

1440
00:58:30,170 --> 00:58:31,628
There is some
sunlight reaching it.

1441
00:58:31,628 --> 00:58:33,340
And since the panels
are way oversized

1442
00:58:33,340 --> 00:58:35,540
and the system is
over-engineered,

1443
00:58:35,540 --> 00:58:37,770
it still manages to
acquire enough energy

1444
00:58:37,770 --> 00:58:38,710
to compact the trash.

1445
00:58:38,710 --> 00:58:40,560
And it doesn't have
a catastrophic stop.

1446
00:58:40,560 --> 00:58:42,700
But it was an
example of somebody

1447
00:58:42,700 --> 00:58:46,810
not really thinking much about
the direction or the angle

1448
00:58:46,810 --> 00:58:47,970
of the sun in the sky.

1449
00:58:47,970 --> 00:58:50,040
They probably installed
it sometime around March

1450
00:58:50,040 --> 00:58:52,490
when the snow started to melt
and the sun was right around

1451
00:58:52,490 --> 00:58:54,090
here.

1452
00:58:54,090 --> 00:58:57,660
And the angle was around
there, and it made it

1453
00:58:57,660 --> 00:58:59,663
into the trash collectors.

1454
00:58:59,663 --> 00:59:03,490
But as the summer came
along, it got shaded.

1455
00:59:03,490 --> 00:59:05,730
So it's something
to keep in mind when

1456
00:59:05,730 --> 00:59:07,010
doing a solar installation.

1457
00:59:07,010 --> 00:59:09,370
It is important to
calculate these things.

1458
00:59:09,370 --> 00:59:11,114
And I just pick out
once more example.

1459
00:59:11,114 --> 00:59:12,530
I'd encourage you
to walk through.

1460
00:59:12,530 --> 00:59:15,340
I was going to have that be a
small little in-class example,

1461
00:59:15,340 --> 00:59:18,860
but since we're
running short on time,

1462
00:59:18,860 --> 00:59:20,650
I'll just give it
to you like that.

1463
00:59:20,650 --> 00:59:21,150
OK.

1464
00:59:21,150 --> 00:59:23,190
Fixed versus tracking systems.

1465
00:59:23,190 --> 00:59:27,230
So if the sun is moving
as a function of season

1466
00:59:27,230 --> 00:59:30,570
and as a function of time
of day throughout the sky--

1467
00:59:30,570 --> 00:59:32,960
and we can see that
very nicely, again,

1468
00:59:32,960 --> 00:59:38,860
through our demo right here-- so
if the sun is actually moving,

1469
00:59:38,860 --> 00:59:41,364
one embodiment would
say, OK, I know

1470
00:59:41,364 --> 00:59:42,780
more or less what
the sun is going

1471
00:59:42,780 --> 00:59:44,196
to do as a function of season.

1472
00:59:44,196 --> 00:59:45,820
Forget the diurnal
variations, but just

1473
00:59:45,820 --> 00:59:46,847
the seasonal variations.

1474
00:59:46,847 --> 00:59:49,430
I know that the sun is going to
be on average somewhere around

1475
00:59:49,430 --> 00:59:52,060
here, somewhere
around my latitude.

1476
00:59:52,060 --> 00:59:56,910
So if I point my panel
at latitude tilt,

1477
00:59:56,910 --> 01:00:01,410
since this angle was--
what was it-- 41 plus 23.

1478
01:00:01,410 --> 01:00:03,510
This angle was 41
minus 23, so this would

1479
01:00:03,510 --> 01:00:05,990
be right around 41 latitude.

1480
01:00:05,990 --> 01:00:09,600
So if I aim my solar
panels at latitude tilt,

1481
01:00:09,600 --> 01:00:12,540
then I'm going to get, on
average, some pretty decent

1482
01:00:12,540 --> 01:00:14,320
power throughout the year.

1483
01:00:14,320 --> 01:00:15,820
I'll have a little
lesson in winter,

1484
01:00:15,820 --> 01:00:19,489
a little more in summer
because, well, just because

1485
01:00:19,489 --> 01:00:21,280
of the amount of solar
resources available.

1486
01:00:21,280 --> 01:00:24,320
But all in all,
I'll be all right.

1487
01:00:24,320 --> 01:00:28,900
At most, I'll be off
by 23 and 1/2 degrees.

1488
01:00:28,900 --> 01:00:29,750
You can do that.

1489
01:00:29,750 --> 01:00:31,790
And that's called
fixed latitude tilt.

1490
01:00:31,790 --> 01:00:34,680
And typically, you'll face the
panels south approximately.

1491
01:00:34,680 --> 01:00:35,930
We'll get to that in a minute.

1492
01:00:35,930 --> 01:00:37,500
It depends on local
weather patterns.

1493
01:00:37,500 --> 01:00:38,730
If you have fog in the
morning, for instance,

1494
01:00:38,730 --> 01:00:40,970
you want to face them
a little to the west.

1495
01:00:40,970 --> 01:00:44,920
But we'll face them south
and at latitude tilt.

1496
01:00:44,920 --> 01:00:47,130
Or we can decide,
no, let's actually

1497
01:00:47,130 --> 01:00:49,955
track the sun throughout
the sky throughout the day.

1498
01:00:49,955 --> 01:00:51,830
And so we'll start in
the east in the morning

1499
01:00:51,830 --> 01:00:54,010
and have it rotating
through on one-axis tracker

1500
01:00:54,010 --> 01:00:54,890
throughout the day.

1501
01:00:54,890 --> 01:00:56,795
Or we can have a
two-axis tracker where

1502
01:00:56,795 --> 01:00:59,590
it rotates to follow the sun
throughout the seasons as well,

1503
01:00:59,590 --> 01:01:01,300
a kind of a north/south tilt.

1504
01:01:01,300 --> 01:01:03,450
And that's what's
plotted right here.

1505
01:01:03,450 --> 01:01:09,810
This is a quick approximation
of the fixed one-axis

1506
01:01:09,810 --> 01:01:11,330
and two-axis
trackers for a given

1507
01:01:11,330 --> 01:01:14,870
system in Boston using a
simulation tool called PVWatts.

1508
01:01:14,870 --> 01:01:16,420
There's a link to that.

1509
01:01:16,420 --> 01:01:18,920
It's based on the National
Renewable Energy Laboratory

1510
01:01:18,920 --> 01:01:19,590
website.

1511
01:01:19,590 --> 01:01:21,990
There's a link to that
at the end of the slides.

1512
01:01:21,990 --> 01:01:23,850
But this shows you
the total system

1513
01:01:23,850 --> 01:01:26,200
output in terms of
kilowatt hours in terms

1514
01:01:26,200 --> 01:01:28,820
of hours per day.

1515
01:01:28,820 --> 01:01:31,304
I think, yeah, it's a little
bit of an odd units there

1516
01:01:31,304 --> 01:01:31,970
on the one-axis.

1517
01:01:31,970 --> 01:01:35,082
But it shows you
the relative gain

1518
01:01:35,082 --> 01:01:36,790
that you would get by
going to a one-axis

1519
01:01:36,790 --> 01:01:38,570
and then, finally,
a two-axis tracker.

1520
01:01:38,570 --> 01:01:40,250
So in many places,
it makes sense

1521
01:01:40,250 --> 01:01:43,430
to go to one-axis tracker,
since especially you broaden out

1522
01:01:43,430 --> 01:01:45,860
the peak near the
peak hours of the day.

1523
01:01:45,860 --> 01:01:48,030
And that's really good.

1524
01:01:48,030 --> 01:01:50,114
But not always does it
make sense financially

1525
01:01:50,114 --> 01:01:51,280
to go to a two-axis tracker.

1526
01:01:51,280 --> 01:01:53,113
You're adding another
motor onto that thing.

1527
01:01:53,113 --> 01:01:55,390
It's really not
gaining you that much.

1528
01:01:55,390 --> 01:01:57,630
Obviously, you have to
calculate it out yourself

1529
01:01:57,630 --> 01:01:58,900
for that specific location.

1530
01:01:58,900 --> 01:02:01,380
But by and large, a
generality, one-axis tracker

1531
01:02:01,380 --> 01:02:03,320
makes sense for a flat panel.

1532
01:02:03,320 --> 01:02:05,716
Now if you have a
concentrator lens in the front

1533
01:02:05,716 --> 01:02:07,590
and it has to be looking
directly at the sun,

1534
01:02:07,590 --> 01:02:10,680
then you're kind of forced
to go to two-axis tracker.

1535
01:02:10,680 --> 01:02:11,180
OK.

1536
01:02:11,180 --> 01:02:13,013
And there you have your
total system output,

1537
01:02:13,013 --> 01:02:17,790
which is just the integral under
the curve over the entire year.

1538
01:02:17,790 --> 01:02:19,890
So definitions-- I should
have done this before.

1539
01:02:19,890 --> 01:02:22,190
But direct sunlight,
looking directly at the sun,

1540
01:02:22,190 --> 01:02:24,810
and then diffuse sunlight or
scattered light coming off

1541
01:02:24,810 --> 01:02:25,830
of other things.

1542
01:02:25,830 --> 01:02:30,340
So diffuse sunlight would be
coming from the other angles

1543
01:02:30,340 --> 01:02:32,199
in other directions in the sky.

1544
01:02:32,199 --> 01:02:33,740
The direct sunlight
would pretty much

1545
01:02:33,740 --> 01:02:37,325
be at the sun plus or
minus a few degrees.

1546
01:02:37,325 --> 01:02:38,950
And we have different
ways of measuring

1547
01:02:38,950 --> 01:02:42,492
the-- this would be flat plate.

1548
01:02:42,492 --> 01:02:44,700
Up there on the upper left
it says, flat plate facing

1549
01:02:44,700 --> 01:02:47,550
south latitude tilt. Just like
we said, it's facing south.

1550
01:02:47,550 --> 01:02:49,520
In the United
States, that's good.

1551
01:02:49,520 --> 01:02:51,710
Latitude tilt, meaning it's
tilted at our latitude.

1552
01:02:51,710 --> 01:02:54,350
So if we go from the southern
tip of Florida and Texas

1553
01:02:54,350 --> 01:02:56,490
up to the northern
tip of Minnesota,

1554
01:02:56,490 --> 01:02:59,750
we'll be tilting
it more and more

1555
01:02:59,750 --> 01:03:03,560
toward the south like this,
going from Texas to Minnesota.

1556
01:03:03,560 --> 01:03:05,570
From Texas to Minnesota.

1557
01:03:05,570 --> 01:03:09,880
And so latitude tilt
facing south flat plate.

1558
01:03:09,880 --> 01:03:13,095
This is essentially accumulating
all of the sunlight, the direct

1559
01:03:13,095 --> 01:03:15,470
and the scattered light,
because it's all being collected

1560
01:03:15,470 --> 01:03:17,240
by the flat plate there.

1561
01:03:17,240 --> 01:03:20,990
This map, on the other hand,
is for a two-axis tracker.

1562
01:03:20,990 --> 01:03:24,400
And it's looking at the sun
plus or minus 2 and 1/2 degrees.

1563
01:03:24,400 --> 01:03:27,120
And so it's really only
picking up this right

1564
01:03:27,120 --> 01:03:28,760
here out of the sky.

1565
01:03:28,760 --> 01:03:31,130
And when it's sunny all
the time, you're golden.

1566
01:03:31,130 --> 01:03:32,130
You're tracking the sun.

1567
01:03:32,130 --> 01:03:34,270
You're actually
getting more energy

1568
01:03:34,270 --> 01:03:36,180
than you would if you
just at a flat plate

1569
01:03:36,180 --> 01:03:38,721
because the cosine theta angle
is changing throughout the day

1570
01:03:38,721 --> 01:03:39,820
if you have a flat plate.

1571
01:03:39,820 --> 01:03:41,320
When the sun is in
the morning time,

1572
01:03:41,320 --> 01:03:42,810
you have a flat plate like this.

1573
01:03:42,810 --> 01:03:44,980
So if my sunlight is
coming in like this

1574
01:03:44,980 --> 01:03:46,771
and I do cosine
theta of the angle,

1575
01:03:46,771 --> 01:03:48,270
I'm only getting a
very small amount

1576
01:03:48,270 --> 01:03:51,510
of the incident sunlight
projected onto this plate right

1577
01:03:51,510 --> 01:03:52,010
here.

1578
01:03:52,010 --> 01:03:54,960
If I was in the middle of the
day, now cosine theta is 1,

1579
01:03:54,960 --> 01:03:57,010
I get the full sunlight.

1580
01:03:57,010 --> 01:03:59,700
But if I have, for
instance, a tracker,

1581
01:03:59,700 --> 01:04:02,579
I would be able to face
this panel due east

1582
01:04:02,579 --> 01:04:04,120
and then track it
throughout the day.

1583
01:04:04,120 --> 01:04:07,240
And that's where I'd get this
additional energy boost here

1584
01:04:07,240 --> 01:04:11,760
in the mornings right there
from the tracking system.

1585
01:04:11,760 --> 01:04:14,790
So if I have a tracker,
I get a big gain

1586
01:04:14,790 --> 01:04:17,466
in the places that have
a lot of direct sun.

1587
01:04:17,466 --> 01:04:19,590
And if I'm only looking at
a very small solid angle

1588
01:04:19,590 --> 01:04:22,090
of the sky, if I'm only looking
at, say, this little portion

1589
01:04:22,090 --> 01:04:25,010
the sun, on cloudy days,
most of the sunlight

1590
01:04:25,010 --> 01:04:28,340
is coming off of the diffused
light, not from the sun.

1591
01:04:28,340 --> 01:04:29,920
If I have a flat plate, I win.

1592
01:04:29,920 --> 01:04:33,479
If I have a concentrator,
I lose on a cloudy day.

1593
01:04:33,479 --> 01:04:35,770
And so that's why, in some
of the regions of the United

1594
01:04:35,770 --> 01:04:39,030
States that are notoriously
cloudy-- I won't point

1595
01:04:39,030 --> 01:04:42,290
to any one in
particular-- it's actually

1596
01:04:42,290 --> 01:04:44,940
better for non-concentrating
solar in terms of total energy

1597
01:04:44,940 --> 01:04:45,612
output.

1598
01:04:45,612 --> 01:04:47,320
And in other regions
of the United States

1599
01:04:47,320 --> 01:04:50,020
where we have a
lot of sunny days,

1600
01:04:50,020 --> 01:04:52,667
you start having two-axis
trackers making sense.

1601
01:04:52,667 --> 01:04:54,750
And it's just really the
ratio of these two maps--

1602
01:04:54,750 --> 01:04:56,920
whoopsie-- this one,
which is flat plate,

1603
01:04:56,920 --> 01:04:58,560
and that one, which
is concentrator.

1604
01:04:58,560 --> 01:05:03,020
You see over here, we win if
we go for the concentrator.

1605
01:05:03,020 --> 01:05:06,230
And over here, we
get more energy out--

1606
01:05:06,230 --> 01:05:10,050
oopsie-- more energy out
if we use the flat plate.

1607
01:05:10,050 --> 01:05:11,620
Now that's just energy.

1608
01:05:11,620 --> 01:05:14,190
Obviously, cost and
economics factors

1609
01:05:14,190 --> 01:05:17,890
what it takes to install it.

1610
01:05:17,890 --> 01:05:19,370
OK.

1611
01:05:19,370 --> 01:05:23,630
And weather patterns, I
promised I'd get back to this.

1612
01:05:23,630 --> 01:05:26,330
Interesting to note that
for right near the equator,

1613
01:05:26,330 --> 01:05:30,290
we have this drop
of the insolation.

1614
01:05:30,290 --> 01:05:34,650
And there are these beautiful
maps put out by, again,

1615
01:05:34,650 --> 01:05:37,660
NASA Earth Observatory that
show you the cloud fraction

1616
01:05:37,660 --> 01:05:40,155
coverage of particular
spots around the planet.

1617
01:05:40,155 --> 01:05:42,030
And you can see that
right near the equators,

1618
01:05:42,030 --> 01:05:45,310
typically we have these
beautiful tropical forests.

1619
01:05:48,320 --> 01:05:50,080
The high cloud cover
in those regions

1620
01:05:50,080 --> 01:05:51,886
is blocking out some of the sun.

1621
01:05:51,886 --> 01:05:53,510
And likewise, you
can see the dichotomy

1622
01:05:53,510 --> 01:05:54,960
between Phoenix and Atlanta.

1623
01:05:54,960 --> 01:05:58,200
Same latitude again, but Atlanta
having a higher cloud coverage

1624
01:05:58,200 --> 01:06:01,450
than Phoenix and, hence, a
lower solar resource, a lower

1625
01:06:01,450 --> 01:06:03,340
insolation.

1626
01:06:03,340 --> 01:06:05,260
So you how all this
kind of ties together.

1627
01:06:05,260 --> 01:06:07,180
That's all predictable
in a sense.

1628
01:06:07,180 --> 01:06:11,820
This is unpredictable at a
local level for one system.

1629
01:06:11,820 --> 01:06:14,500
If the sun is tracing its
route through the sky,

1630
01:06:14,500 --> 01:06:16,002
and you have that
envelope function

1631
01:06:16,002 --> 01:06:18,210
that you've just spent so
much effort calculating out

1632
01:06:18,210 --> 01:06:20,543
with all your trig functions
in the computer simulation,

1633
01:06:20,543 --> 01:06:22,720
the code that you've
just been given here.

1634
01:06:22,720 --> 01:06:24,780
And now, a cloud comes
over, some random cloud

1635
01:06:24,780 --> 01:06:26,110
that was very hard to predict.

1636
01:06:26,110 --> 01:06:29,030
And you just have one panel,
one tiny little thing like this.

1637
01:06:29,030 --> 01:06:30,200
And the cloud goes over it.

1638
01:06:30,200 --> 01:06:31,741
Boom, all of a
sudden, you get a drop

1639
01:06:31,741 --> 01:06:34,390
in your instantaneous
power output.

1640
01:06:34,390 --> 01:06:35,330
Boom, drops again.

1641
01:06:35,330 --> 01:06:36,800
Another cloud, drops again.

1642
01:06:36,800 --> 01:06:39,830
And then a thunderstorm
in the afternoon.

1643
01:06:39,830 --> 01:06:41,810
So a meteorologist
could have told you

1644
01:06:41,810 --> 01:06:43,980
that this thunderstorm
was coming,

1645
01:06:43,980 --> 01:06:45,760
but a meteorologist
would be hard pressed

1646
01:06:45,760 --> 01:06:48,120
to be able to
predict the evolution

1647
01:06:48,120 --> 01:06:50,730
of a tiny little cloud
over your system.

1648
01:06:50,730 --> 01:06:54,810
And so the question is, to what
degree are these local weather

1649
01:06:54,810 --> 01:06:56,610
patterns predictable
and unpredictable?

1650
01:06:56,610 --> 01:06:59,850
And hopefully, some of
you, over this class,

1651
01:06:59,850 --> 01:07:03,150
will be able to help answer
that question by analyzing data

1652
01:07:03,150 --> 01:07:05,150
from tens of thousands
of systems that have been

1653
01:07:05,150 --> 01:07:09,349
installed throughout California
in local geographical systems,

1654
01:07:09,349 --> 01:07:11,890
for example, in the Los Angeles
region, San Francisco region,

1655
01:07:11,890 --> 01:07:13,080
and so forth.

1656
01:07:13,080 --> 01:07:15,700
If you start averaging
the curves, the energy

1657
01:07:15,700 --> 01:07:18,580
outputs as a function of time,
from a variety of systems

1658
01:07:18,580 --> 01:07:21,020
throughout a neighborhood,
you can probably average out

1659
01:07:21,020 --> 01:07:22,370
the small tiny clouds.

1660
01:07:22,370 --> 01:07:24,442
You probably can't average
out the thunderstorm.

1661
01:07:24,442 --> 01:07:26,900
But you probably could have
predicted that the thunderstorm

1662
01:07:26,900 --> 01:07:28,940
would've come along.

1663
01:07:28,940 --> 01:07:32,290
And so this is a hot area of
solar research at the systems

1664
01:07:32,290 --> 01:07:32,790
level.

1665
01:07:32,790 --> 01:07:34,720
It's trying to
understand, to what degree

1666
01:07:34,720 --> 01:07:36,680
are PV systems predictable?

1667
01:07:36,680 --> 01:07:38,840
To what degree can
the power output

1668
01:07:38,840 --> 01:07:42,070
of a PV system or an
ensemble of PV systems

1669
01:07:42,070 --> 01:07:45,550
be predicted in advance so we
don't wind up in a situation

1670
01:07:45,550 --> 01:07:48,300
where all of a sudden we
have this catastrophic drop

1671
01:07:48,300 --> 01:07:51,979
of a cumulative PV system
output of, say, 30%

1672
01:07:51,979 --> 01:07:53,520
and meanwhile, it's
a hot summer day,

1673
01:07:53,520 --> 01:07:55,080
everybody's air
conditions are going,

1674
01:07:55,080 --> 01:07:57,050
and we cause failure
of the power grid?

1675
01:07:57,050 --> 01:08:00,000
Kind of worst case scenario.

1676
01:08:00,000 --> 01:08:02,850
So this boils down to
intermittency with a short time

1677
01:08:02,850 --> 01:08:05,760
constants tend to
be less predictable.

1678
01:08:05,760 --> 01:08:06,650
Cloud cover.

1679
01:08:06,650 --> 01:08:09,840
And it's relevant for predicting
power supply reliability.

1680
01:08:09,840 --> 01:08:12,450
And the longer time constants
tend to be more predictable.

1681
01:08:12,450 --> 01:08:15,100
The diurnal or seasonal
variations-- and these

1682
01:08:15,100 --> 01:08:18,710
are relevant to calculating
total energy output annually.

1683
01:08:18,710 --> 01:08:20,859
And oftentimes, when
we just work off

1684
01:08:20,859 --> 01:08:23,380
of these long time constant
variability issues,

1685
01:08:23,380 --> 01:08:26,920
we're assuming we have
access to easy storage.

1686
01:08:26,920 --> 01:08:29,330
Right now, the solar
panels on top of my roof,

1687
01:08:29,330 --> 01:08:30,789
they're producing
in excess of what

1688
01:08:30,789 --> 01:08:33,038
we're consuming right now
because neither my wife or I

1689
01:08:33,038 --> 01:08:33,550
are at home.

1690
01:08:33,550 --> 01:08:35,100
And they're injecting
that power into the grid,

1691
01:08:35,100 --> 01:08:37,120
and the grid is serving
as our big battery,

1692
01:08:37,120 --> 01:08:39,319
as our storage unit.

1693
01:08:39,319 --> 01:08:41,120
And I'm riding free.

1694
01:08:41,120 --> 01:08:42,460
I'm a free rider right now.

1695
01:08:42,460 --> 01:08:44,550
I don't pay for that
service necessarily.

1696
01:08:44,550 --> 01:08:46,490
I do to NSTAR.

1697
01:08:46,490 --> 01:08:52,135
But they're not charging
extra for the service of using

1698
01:08:52,135 --> 01:08:53,260
the grid as my big battery.

1699
01:08:55,770 --> 01:08:57,770
Another thing to keep
in mind is that we're

1700
01:08:57,770 --> 01:09:01,710
calculating all this on
the basis of engineering

1701
01:09:01,710 --> 01:09:03,010
and scientific principles.

1702
01:09:03,010 --> 01:09:04,580
We're calculating
the solar resource,

1703
01:09:04,580 --> 01:09:07,350
which is a good,
important first step.

1704
01:09:07,350 --> 01:09:09,649
Now if we look at the
actual solar installations

1705
01:09:09,649 --> 01:09:13,050
by year and by country--
this is a big table,

1706
01:09:13,050 --> 01:09:16,520
but follow me on the two
underlined red lines here,

1707
01:09:16,520 --> 01:09:21,260
DEU, that stands for
Deutschland, Germany, and USA.

1708
01:09:21,260 --> 01:09:23,479
We'll see that Germany
has about seven times

1709
01:09:23,479 --> 01:09:25,830
more solar installed
cumulatively

1710
01:09:25,830 --> 01:09:27,710
than the United States does.

1711
01:09:27,710 --> 01:09:31,000
And yet, if we look at the
solar resource in Germany

1712
01:09:31,000 --> 01:09:34,830
relative to the US-- this is
average annual, same scale,

1713
01:09:34,830 --> 01:09:38,920
going from 900 to 1,200 kilowatt
hours per kilowatt per year--

1714
01:09:38,920 --> 01:09:41,439
we can see that there's a lot
more sun in the United States

1715
01:09:41,439 --> 01:09:42,740
than there is in Germany.

1716
01:09:42,740 --> 01:09:44,540
So there's something
else going on

1717
01:09:44,540 --> 01:09:46,470
than just the solar resource.

1718
01:09:46,470 --> 01:09:47,738
And that's economics.

1719
01:09:47,738 --> 01:09:49,779
That's why we talk about
the policy and economics

1720
01:09:49,779 --> 01:09:50,970
later on in class.

1721
01:09:50,970 --> 01:09:53,399
That's why we dedicate
a sizable portion later

1722
01:09:53,399 --> 01:09:55,230
on talking about that.

1723
01:09:55,230 --> 01:09:57,580
Lastly, our final
learning objectives

1724
01:09:57,580 --> 01:10:02,199
before we halt for questions
and for comments, we

1725
01:10:02,199 --> 01:10:03,740
need to estimate
the land area needed

1726
01:10:03,740 --> 01:10:06,131
to provide sufficient solar
resource for a project.

1727
01:10:06,131 --> 01:10:08,130
And this is really where
your homework picks up,

1728
01:10:08,130 --> 01:10:09,755
and we'll spend some
time in recitation

1729
01:10:09,755 --> 01:10:11,470
walking through this.

1730
01:10:11,470 --> 01:10:14,730
But it's important to
get your units right.

1731
01:10:14,730 --> 01:10:18,480
And so I want to do a
quick quiz right now.

1732
01:10:18,480 --> 01:10:23,320
Think in your minds
which of these properties

1733
01:10:23,320 --> 01:10:25,720
corresponds to which units
or are described by which

1734
01:10:25,720 --> 01:10:27,140
units over here on the right.

1735
01:10:27,140 --> 01:10:29,752
So do a kind of a linkage
in your mind one to one,

1736
01:10:29,752 --> 01:10:31,210
connect, connect,
connect, connect,

1737
01:10:31,210 --> 01:10:34,670
without looking at your slides.

1738
01:10:34,670 --> 01:10:40,720
And then we'll do it
in three, two, one.

1739
01:10:40,720 --> 01:10:42,090
Those are your answers.

1740
01:10:42,090 --> 01:10:44,530
So the ones that
usually get confused

1741
01:10:44,530 --> 01:10:49,382
are power and energy,
kilowatts and kilowatt hours.

1742
01:10:49,382 --> 01:10:51,590
I'm not saying it's easy,
but the way to remember it,

1743
01:10:51,590 --> 01:10:53,131
the easiest way to
remember it, would

1744
01:10:53,131 --> 01:10:55,210
be to remember
that the power time

1745
01:10:55,210 --> 01:10:57,270
product is equal to energy.

1746
01:10:57,270 --> 01:10:58,960
So if you have
power instantaneous

1747
01:10:58,960 --> 01:11:01,337
energy burn rate versus
time, some plot that

1748
01:11:01,337 --> 01:11:03,920
looks like this as you take the
integral of that curve, that's

1749
01:11:03,920 --> 01:11:06,260
your total energy.

1750
01:11:06,260 --> 01:11:11,220
So in terms of units, current
voltage power and energy,

1751
01:11:11,220 --> 01:11:13,370
a hair dryer versus
a fridge, which

1752
01:11:13,370 --> 01:11:15,310
is more likely to
blow a fuse and which

1753
01:11:15,310 --> 01:11:17,906
is more likely to
blow your budget?

1754
01:11:17,906 --> 01:11:20,120
AUDIENCE: A hair dryer is
more likely to blow a fuse.

1755
01:11:20,120 --> 01:11:22,369
PROFESSOR: A hair dryer is
more likely to blow a fuse.

1756
01:11:22,369 --> 01:11:23,252
Why is that?

1757
01:11:23,252 --> 01:11:24,805
AUDIENCE: High voltage.

1758
01:11:24,805 --> 01:11:26,055
PROFESSOR: Yeah, high voltage.

1759
01:11:26,055 --> 01:11:27,180
Well, high current, really.

1760
01:11:27,180 --> 01:11:28,570
Because everything's
running 120.

1761
01:11:28,570 --> 01:11:29,610
It's a higher current.

1762
01:11:29,610 --> 01:11:32,305
It's pushing the wattage
up to around 1.5,

1763
01:11:32,305 --> 01:11:36,930
1.7, 1,500, 1,700 watts.

1764
01:11:36,930 --> 01:11:40,490
And so it's running close
to the 15 amp limit.

1765
01:11:40,490 --> 01:11:41,910
And the fridge,
on the other hand,

1766
01:11:41,910 --> 01:11:44,336
is probably an order
of magnitude lower.

1767
01:11:44,336 --> 01:11:45,960
But the fridge is
running all the time,

1768
01:11:45,960 --> 01:11:48,790
and your hair dryer is only
running a few minutes a day.

1769
01:11:48,790 --> 01:11:51,820
So in terms of blowing a fuse,
it uses a large amount of power

1770
01:11:51,820 --> 01:11:53,610
for a very short amount of time.

1771
01:11:53,610 --> 01:11:55,760
Whereas, the fridge uses
a small amount of power

1772
01:11:55,760 --> 01:11:57,170
for a long amount of time.

1773
01:11:57,170 --> 01:11:59,890
And the integral under that
curve winds up being more,

1774
01:11:59,890 --> 01:12:01,970
typically, then your hair dryer.

1775
01:12:01,970 --> 01:12:03,740
That's the total
amount of energy.

1776
01:12:03,740 --> 01:12:05,070
And you pay by the energy.

1777
01:12:05,070 --> 01:12:07,538
You pay by the kilowatt hour
to your utility company,

1778
01:12:07,538 --> 01:12:09,246
and that's why it
would blow your budget.

1779
01:12:09,246 --> 01:12:10,760
Yeah.

1780
01:12:10,760 --> 01:12:11,260
Oopsie.

1781
01:12:11,260 --> 01:12:13,135
So the numbers work out
to somewhere around 1

1782
01:12:13,135 --> 01:12:15,270
kilowatt hour per day for
the fridge and about 1/2

1783
01:12:15,270 --> 01:12:17,150
a kilowatt hour
for the hair dryer.

1784
01:12:17,150 --> 01:12:19,780
A kilowatt hour is
how much in here?

1785
01:12:19,780 --> 01:12:20,910
$0.18?

1786
01:12:20,910 --> 01:12:23,070
Most people pay about
$0.18 per kilowatt hour.

1787
01:12:23,070 --> 01:12:24,444
And so you can
calculate how much

1788
01:12:24,444 --> 01:12:26,780
it costs to keep things
running in your house.

1789
01:12:26,780 --> 01:12:30,057
Most of us don't usually
think about that.

1790
01:12:30,057 --> 01:12:32,140
And last, last, last, last
point, in your homework

1791
01:12:32,140 --> 01:12:34,960
assignments, you're going to
be asked to size out systems,

1792
01:12:34,960 --> 01:12:37,530
PV systems, photovoltaic
systems, in different parts

1793
01:12:37,530 --> 01:12:38,550
of the world.

1794
01:12:38,550 --> 01:12:41,910
And the easiest way to do
that is if we take a panel

1795
01:12:41,910 --> 01:12:43,510
and we say that this
panel right here

1796
01:12:43,510 --> 01:12:46,910
is rated at a certain
amount of power,

1797
01:12:46,910 --> 01:12:49,710
so under peak
illumination conditions.

1798
01:12:49,710 --> 01:12:52,290
When the panel is seated
at incident sunlight,

1799
01:12:52,290 --> 01:12:57,740
so the cosine theta term is 1,
and the incident solar resource

1800
01:12:57,740 --> 01:13:02,320
is 1,000 watts per square meter,
so around AM 1.5 conditions,

1801
01:13:02,320 --> 01:13:05,830
this panel right here would be
producing-- this one is tiny.

1802
01:13:05,830 --> 01:13:07,400
It would be producing
6 watts peak.

1803
01:13:07,400 --> 01:13:08,850
But most of these
panels over here

1804
01:13:08,850 --> 01:13:12,820
are producing somewhere around
a few hundred watts peak.

1805
01:13:12,820 --> 01:13:15,810
And so the panels are
rated in terms of watt peak

1806
01:13:15,810 --> 01:13:17,860
because the panel manufacture
doesn't know where

1807
01:13:17,860 --> 01:13:19,350
you're going to install them.

1808
01:13:19,350 --> 01:13:20,550
You could decide that
you're going to install it

1809
01:13:20,550 --> 01:13:22,910
in Alaska, in which case, it's
going to produce about half

1810
01:13:22,910 --> 01:13:24,368
the energy than if
you installed it

1811
01:13:24,368 --> 01:13:27,430
down south in the continental
US in, say, Arizona.

1812
01:13:27,430 --> 01:13:29,750
Maybe a third of the energy.

1813
01:13:29,750 --> 01:13:31,531
And so it doesn't
want to rate the panel

1814
01:13:31,531 --> 01:13:32,780
in terms of the energy output.

1815
01:13:32,780 --> 01:13:33,990
It's not going to
guarantee that you're

1816
01:13:33,990 --> 01:13:36,421
going to get a certain energy
output off of the panel.

1817
01:13:36,421 --> 01:13:37,920
But it will guarantee
that the panel

1818
01:13:37,920 --> 01:13:40,320
was rated at maximum
power of such and such

1819
01:13:40,320 --> 01:13:42,454
under standard
testing conditions.

1820
01:13:42,454 --> 01:13:44,370
Now that's good because
you can generalize it,

1821
01:13:44,370 --> 01:13:46,650
which you'll do
for your homework.

1822
01:13:46,650 --> 01:13:49,830
But it also has a downside
because standard testing

1823
01:13:49,830 --> 01:13:51,780
conditions aren't
real world conditions.

1824
01:13:51,780 --> 01:13:54,900
The panel isn't always
operating at 25 degrees Celsius.

1825
01:13:54,900 --> 01:13:57,400
The panel isn't operating
always at incident sunlight

1826
01:13:57,400 --> 01:13:58,500
normal, right?

1827
01:13:58,500 --> 01:14:02,480
And so the gap between
actually knowing

1828
01:14:02,480 --> 01:14:04,540
how much the panel is
going to output in terms

1829
01:14:04,540 --> 01:14:06,920
of its energy for
a specific location

1830
01:14:06,920 --> 01:14:09,600
and what our calculations,
the back of the envelope

1831
01:14:09,600 --> 01:14:14,640
calculations, will give us, that
gap is, from an economics point

1832
01:14:14,640 --> 01:14:16,790
of view, there's a lot of
money to be made there.

1833
01:14:16,790 --> 01:14:20,140
If you understand really well
how much of a given panel

1834
01:14:20,140 --> 01:14:24,050
will output, you can then
predict to a much finer degree

1835
01:14:24,050 --> 01:14:26,900
what you should be charging your
customer for installing panels

1836
01:14:26,900 --> 01:14:29,145
in a particular location.

1837
01:14:29,145 --> 01:14:31,270
So there's a lot of work
being done right now, more

1838
01:14:31,270 --> 01:14:34,180
on the business side
of pulling the systems

1839
01:14:34,180 --> 01:14:35,770
engineers along and
trying to increase

1840
01:14:35,770 --> 01:14:37,530
the accuracy of predictions.

1841
01:14:37,530 --> 01:14:41,750
And so with that, I will
leave off on this slide.

1842
01:14:41,750 --> 01:14:44,200
We can get to it during
recitation tomorrow at 4:00.

1843
01:14:44,200 --> 01:14:46,658
I welcome your questions here
at the front if you have any.

1844
01:14:46,658 --> 01:14:48,860
Otherwise, I'll
see you tomorrow.