1. Introduction

1. Introduction

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visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: Ladies and gentlemen,
thanks for coming today. I’d like to formally start
the course, The Fundamentals of Photovoltaics. That’s 2.626/2.627. Why don’t we dive quickly
into the syllabus, and then, a few slides of
motivation, why we’re here, why we’re studying photovoltaics. Hopefully, get you
excited for the course. The syllabus that
you have before you should outline the course
objectives and the course learning objectives. At the end, during the
background assessment survey, we’ll take the last
10 minutes of class for you to provide your
feedback to us, the teaching staff, to make sure that
we’re addressing your needs and your interests. So take a quick moment to read
over that while I describe the overall flow of the course. The course roadmap, this
little diagram right here, is essentially a
three step component. We first instill
the fundamentals of how light is absorbed
into a material, how charge is excited, how then
charge is separated and a voltage
created, and finally, how a charge is collected. And that is the essence
of a photovoltaic device. In 30 years time,
photovoltaic devices probably will still be
using that combination of physical processes. So understanding
these fundamentals will arm you– will give
you the information needed to be able to assess any
photovoltaic technology that might be presented to you. Then, in the second
component of the course, we’ll discuss the technologies,
the specific technologies that are out there in
the market today, and those that are up and
coming that have the potential to replace them. And as a third
part of the course, we’ll be discussing
cross cutting themes. These include the policy,
economics, and social aspects of photovoltaics that, of
course, are of general interest and are particularly interesting
for scientists and engineers, who spend most of
their time thinking about the fundamentals, to
take a step back and look at the broader picture. A note on the fundamentals. I recognize that many of you
come from diverse backgrounds, some from nontechnical
backgrounds, many from mechanical
engineering who never really have looked into semiconductors
or semiconductor devices. Not to worry, as you’ll
see on page number– page number 2, meeting times, class
recitation, and office hours. We provide a number of
opportunities for you to get more closely engaged
with us, the teaching staff, and to work through
some of the fundamentals as you might
experience difficulties in the learning process. Let’s take a quick look
at the course schedule just to situate ourselves. So the course schedule
follows that three step process very closely. The first component of the
course, the first third, roughly, is focused
on the fundamentals. So we’ll learn about light
absorption, charge excitation, charge separation,
and charge collection. And the recitation
times will be used to discuss those fundamentals
because, for many of you, this is the first time you’re
working with this material. The second third of the
course, on PV technologies– when we discuss the industry
that’s out there today, how it’s evolving, how the
different technologies are evolving, this is when
we get to experience some of the industry pain
upfront, up close and personal. We’ll be making solar cells. And as part of your take home
quiz number two– as you’ll notice, take home
quiz number two is distributed right
in the beginning of October– middle of October. And then it’s due in
the middle of November. So it’s almost a month. And the reason it’s
a month long take home quiz is because,
during the recitation times, we will be making
solar cells with you. And it will be a little
bit of a challenge. It’s not only to make the
most efficient solar cell, but the most cost
effective solar cell. And so we’ll be making
technology choices as we go along, processing
our solar cells, deciding whether we do
process A or process B. We’ll be doing the
calculations that we learned how to do during the
fundamentals section to predict what the
efficiency gains should be. And it will have
costs associated with each of the different
process steps as well. So it will be a little bit of
a game, a little competition within the group,
as well, to see who can make the most cost
effective solar cell in terms of dollars per
unit power output. And finally, in the last
third of the course, this is really when the
projects kick off in earnest. We have several really
interesting projects lined up as well as we’re open to
hearing your own project ideas. This is when you form teams
of three, four, perhaps five, but hopefully three or four. And you will be addressing some
of the most important questions of the day, obviously, in a
very bound, well-defined way. And some of the projects
that we have lined up include looking at actual
photovoltaic installer data coming from houses
with temporal resolution on the order of five minutes. So you can obtain
a huge database of maybe 10,000, 15,000 homes
distributed geographically, and determine to what
degree is the ensemble of photovoltaic
systems predictable. Obviously, if a cloud
goes over one home, power output drops
pretty dramatically. But if you begin averaging over
several homes, how predictable is the solar power
output of that ensemble? And that’s going to be very
important as photovoltaics scales up and assumes a greater
percentage of the total grid. Another interesting
project we have lined up is with the World Bank. This is with folks
in Washington DC who are looking into a project
called Lighting Africa. And they’re installing
PV on small little lights and distributing those to
folks in sub-Saharan Africa. And their big question
to the MIT audience is, with some of the newer
up and coming technologies out there, how will this
impact their technology? How will this impact
their lighting? And so the
deliverable at the end will be a technology
perspectus– one page. A lot of thought
has to go into it. That will be
delivered to companies that will be selling their
products in Africa to guide them and to inform them about
some of the up and coming technologies and how their
markets will be impacted. Like those two projects,
we have several others. And we’re open to
your ideas as well. So if you’re really jazzed
about one particular topic, there will be opportunities
to let us know, specifically on homework number
2, when there will be a specific question
there, are you interested in a particular
topic of your own. We’ll assemble– begin
creating teams early on so that there’s some bonding going
on, especially during the cell fabrication part during the
second third of the course when we make the
actual solar cells. But then, the third
part of the course will be really focusing on
the class projects themselves. So that’s the lay of the land. And I want to give you some
motivation as to why we’re here and why this is
really a special time in the field of photovoltaics. This is not your parents’
solar energy anymore. Things have changed quite a bit. And hopefully, over the
course of these slides, I’ll be able to convey that
message loud and clear. We’ll go ahead and get started. So first question is why
photovoltaics, or why solar. Photovoltaics is one particular
embodiment of solar energy where we convert sunlight
into electricity. And in most
photovoltaic panels– I’ll definitely let you guys
come up and have a look at it afterwards. In most photovoltaic
panels, you have two leads coming out, basically,
the equivalent of a positive and a negative. And you have a
bunch of cells here that are converting the
sunlight into electricity. It’s different than, let’s
say, solar thermal, which is converting
sunlight into heat, or solar to fuels, which
is converting sunlight into chemical energy. And the reason we’re
studying photovoltaics as a starting point is
because PV, photovoltaics– PV for short– is the most
widespread technology, widespread solar conversion
technology out there today. So the big question is
why solar in general. Why are we at all
interested in this? Can anybody tell me what
this is a picture of? It’s obviously not
from the United States. Does anybody recognize
the language here written on the side of the boat? It’s very small. [SPEAKING PORTUGUESE] AUDIENCE: Portuguese. PROFESSOR: It’s Portuguese. It’s from Brazil. It’s form the
northeast of Brazil. It’s a small island
called Morro de Sao Paulo. It’s located about an hour
south of Salvador in Bahia. These are folks arriving at
the island with gas cylinders. There is no underwater
cable linking the island with the mainland. So they’re arriving by
boat with gas cylinders. They’re tossing them
into the salt water. They’re pushing
them onto the beach, rolling them on the
beach, until they get to the little sandy
roads– of course, getting grains of sand
embedded inside of the nozzle and so forth. This illustrates to
me the great risks that we go through to supply
ourselves with energy. It’s just one, what might
be considered by our safety standards here, extreme
example of associated risk with supplying of energy
and effort, of course. But if you look at our energy
supply to the United States, it’s no less heroic. It just has
different dimensions. And so the energy
that we use today is often produced in some
faraway land, not always, but often, transported,
sometimes over thousands of miles, and brought home at
significant risk and peril. And the question is, why
do we go to such extremes. And second question is,
is there a better way. So to answer the
first question, here, why we go to such extremes, if
you look at the world at night, and then look at our human
development map, which I use Facebook– what better
indicator of human development is there than Facebook? This map right here shows
you the number of linkages between people on Facebook. And of course, the density
of the bright lights there is representing
the number of users. And you can see that the
two maps, the electricity consumption and the
technology adoption map very closely,
one on to another. And it’s almost down
to the specific region once in a specific country. This is especially noticeable
in some of the developing world where you see these pockets of
high concentration of people, essentially capital cities. You have Lagos, Nairobi,
and so forth– Jakarta. And you have this
huge concentration of people that, of course,
are using electricity. And more and more people
flock to those cities, especially in
developing countries, because the standard of
living tends to be higher. There is a certain indicator,
called Human Development Index, that was put together
by the World Bank, which pulls together a
number of factors, including expectation of
life, infant mortality, and so forth– education levels. So in some hand
wavy way, comes up with a metric that indicates
quality of life, roughly. And on the x-axis here, we have
annual per capita electricity use– not energy, but
electricity specifically. And we see some form of
correlation between the two. So one could naturally
conclude from this that energy is
fueling development, and energy is also fueling per
capita income, as a result. This little bubble chart
hear, courtesy of UC Berkeley, is showing you the size
of the bubble here, indicating the size of the
population, and of course, the position on this
graph indicating the per capita energy use
and per capita income. The reality is that many of the
up incoming energy consumers aren’t quite there yet in
terms of their energy use. There will be a drastically
increasing demand as several regions
of the world turn on as they begin plugging in and
demanding more electricity. So somehow we have to
satisfy that growing demand. So to put things in
perspective as well, here we have the world
somewhat at night. World population in millions. And so we have somewhere around
10 billion approaching by 2050. And you can see that the
majority of the growth, what’s driving world
population, is Asia and Africa. Those are the two lines. My apologies for the small text. But that’s the yellow
line right here. And the black line right here. They’re the two largest
bars in that Pareto chart. And the projected
human energy use is only going to
go up as a result. So again, we look at
the world at night. Now instead of looking
at the bright areas, we’re going to focus on
the dark areas instead. The regions of the world where
we do have high population densities– some of the regions,
not the deserts, obviously. Some of the regions we do have
high population densities, like sub-Saharan Africa,
but don’t have a whole lot of electricity use right now. Then we’ll take another map,
which is the solar resource. Again here, the
red is indicating a lot of solar resource. And the blue is
indicating not so much. But still, it’s pretty
amazing that the entire world is falling within
about a factor of two, maybe a factor of three. So even if you
compare Scandinavia against– let’s say,
Scandinavia against Kenya, you’re still looking at about a
variation of a factor of three, right? So the solar resource
is pretty well matched with the regions
of the world that don’t have electricity right
now, where the demand will be coming online. And to put that into
another nice chart, I don’t think this
is very common yet. You’ve seen the HDI
versus per capita income. But this is HDI
versus insulation, showing that those regions of
the world that are ranked lower on the HDI scale are
precisely those regions that have higher insulation,
that have greater access to that solar resource. Now the big question is, is
that solar resource big enough to supply necessary
energy needs. And this is a quick intro to
next lecture, where we discuss the solar resource in detail. But the short answer
is absolutely, yes, by orders of magnitude. The volumes of these
cubes represent the volume of either energy
resource or energy need. Energy need here,
on the far right– that little blue cube
represents the human energy use. Some are very small compared
to the solar resource on the Earth’s surface. This obviously is including
the ocean as well. If we’re to be realistic,
instead of calling this planet Earth, we should probably
call it ocean or water since oceans do
comprise about 2/3. But even if we discount
this for usable land area, we’re still an
order of magnitude greater than total
human energy use. So the resource base is there. It’s available. It’s up to us to figure
out how to use it, up to us scientists and engineers. So the potential
for solar energy is represented on this chart. I’m not a huge
fan of this chart, and I’ll explain
why in a minute. But there is something very
valuable to be taken away from here. These black dots, one, two,
three, four, five, six, represent around 18
terawatt equivalent, which is total human energy
use in a few years time. And you can see the total land
area there is not astronomical. The reason I don’t
like this chart so much is because we’re not going to
cover up vast swaths of Nevada, for instance, with solar
panels for the benefit of the rest of the country. We’re going to distribute those
solar panels over larger areas. But this is just meant
to emphasize the point that the land area usage
does work out in our favor. So the way we distribute
solar panels typically is either on residential
installations, like this one, or in large field installations. This one, the Sarnia
Solar Farm in Ontario is currently the largest
solar farm in the world. We call it a solar farm because
it’s just a massive land area comprised of solar panels. This is the covering half
of Nevada scenario, right? This here on the left hand
side, on the other hand, is a residential neighborhood
in California indicating the more distributed variety. And both have their distinct
strengths and weaknesses. So solar isn’t about those
small, little, rinky dinky, 20 or 30 watt panels
that are sitting on a remote thatched hut. Solar is really growing up to
be a grid tied, grid integrated, renewable energy source that
is now probably skirting a $100 billion industry worldwide. So it’s growing up, and
certainly professionalizing quite a bit. Historical perspective. It’s time to take a
look back and trace through some of the technical
history of how solar cells came into being. And that really
will inform why it is we’re at where we are today,
why the industry has some of the biases it
has today, and what are some of the
intangible barriers that could be needed to
be overcome if we are to develop new technologies. So aside from just
general knowledge and general edification, this
has an important technical aspect as well. So historical perspective. We credit the discovery
of the photovoltaic to this gentleman here,
Edmond Becquerel, shown here in his more mature years. When he wrote this
article, right here– I’ll probably butcher it, but it’s
“sur les effets electriques produit sous l’influence
des rayons solaires.” Basically, the
electrical effects produced by the
influence of solar rays on a contraption that
looked very similar to this. He noticed a current
flowing, essentially a photovoltaic, a photon
induced, a light induced effect current. And he was very smart to
decouple the effect of heat from the light. So his experiment
involved selective filters that prevented massive amounts
of heat from getting through. And he essentially produced
what is a spectral response. Varying the filter color,
he was able to trace out the response of this
apparatus to the solar light as a function of wavelength. This was a clever experiment. He wrote it up. It’s more of an
electrochemical device rather than the solid
state photovoltaic device, like the one we know now. But nevertheless, it
earned him the credit of being the discoverer of
the photovoltaic effect. Does anybody happen to know
how old he was in 1839, when he discovered this or
when he published this work? It’s a rather nice article. Very eloquent, very detailed. He was 19. He was born in 1820. Anyway, small aside. The field evolved
from 1839, when that first article came out. Folks began refining
and– well, first of all, discovering new elements during
that period in the 1800s, refining them and then
testing their properties. And this was before
we really understood what semiconductors were. They were a little bit of
a black box, a big mystery. Their physical,
electrical properties were all over the map. We’ll explain why over the
course of the next 10 lectures. And they began refining
these materials and putting them in various
contraptions testing them with light. And lo and behold, they would
get the photovoltaic effect again, maybe photoelectric
effect first, and then, the photovoltaic
effect, finally, when they set up the
experiment properly. And selenium was a popular
material at the beginning. So was cuprous oxide, Cu2O. That was a very common material. And I love pointing this out. This is a little
contraption, a vice. To hold the contact
onto the device. And as Joe can tell you,
contacting a solar cell is not the easiest
thing in the world. So it’s a pretty funny
picture, especially in light of our current
difficulties in 2011 on resolving some contact
issues, especially with new materials. But that gives you a little bit
of a historical perspective. And the references are there. In 1954, the first
embodiment of what we consider the modern
solar cell came into being. This was driven by the
purification, crystallisation, and growth of silicon, which
is the second most abundant element on the Earth’s crust. It was noted to be
superior to germanium for electronic devices because
of its larger band gap, less leakage current. We’ll get to that
in a few lectures. It had superior properties. And it was engineered into,
I would say, the first what we call a, homojunction
p-n junction based solar cell device in Bell Labs by
those three gentleman there, on the upper left. And in 1954, the paper
came out in the Journal of Applied Physics. And that really spurred a
lot of interest in the field. Why? Because 6% efficiency
was about a factor 15 higher than anything
that had come before it. And now, people could see the
potential of this technology to drive things. At the time, within a few
years, within a decade or so, folks were more interested in
sending satellites into space than they were, perhaps,
powering terrestrial objects. But we’ll get to
that in a second. But some of the
first examples here, in Bell Labs in
New Jersey– they had a small little
radio communicating with this little
device, over here. And the solar cell was
powering the gadget. And it’s interesting
to note here, the New York Times
article from that time, “with this modern version
of Apollo’s Chariot, the Bell scientists
have harnessed enough of the
sun’s rays to power the transmission of voices
over telephone wires.” And they speculate that
at some point– obviously this was written
in the 1950s, keep that social context in mind. “But eventually leading
to the realization of one of mankind’s most
cherished dreams, the harnessing of
the almost limitless energy of the sun for the
uses of civilization.” They saw the opportunity there. It was not lost to them. But of course, a
lot of development had to come under the bridge. A lot of water had to
go under the bridge before they were able to
make solar really cost effective from 1954 at
almost 60 years later. The way that basic
solar cell device worked– I’m going to introduce
you to the full picture now. And I will begin dissecting
it piece by piece, over the next lectures,
so that we really understand each component
of how the solar cell works. And we’ll put it all
back together again. We’ll actually
make it, literally. So the sunlight comes
into this device. This is a cross section
of a solar cell device. And today’s modern solar
cells are about four times the thickness of your hair. So if you can imagine
200 microns in thickness, that’s the thickness
here, the cross section of this solar cell device. Light comes inside,
excites bound charge, and makes it mobile, so it
can move around the material. There’s a built
in electric field, which serves to separate that
charge and create the voltage. And so one of the
charges goes here. The other charge
goes to the back. So you have a voltage or
a potential difference across these two terminals,
across the front terminal and across the back one. And then, if they’re connected
by an external circuit to an external
load, current will flow through that external
load to complete the circuit. And that’s essentially how
the solar cell device works. So three basic steps,
there’s charge generation. So light is exciting
charge within the material. The second important step,
up there in the upper right, is charge separation. Somehow, you have to
induce a voltage inside of your material. And the third very
important step is somehow you have to collect
the charge coming out of it. That’s why those folks
in the earlier days had that big vice over here. They were trying
to really make sure that the metal was in good
contact with the material so they could
extract the charge. And so that’s essentially it. The advantages of a
solar cell devices is that there are no moving
parts and no pollution created at the site of use. There is, obviously,
the manufacturing of the module itself. And we’ll get into
detail about that, and begin quantifying
the amount of energy, the cost to manufacture it. Bottom line is that the CO2
production per unit energy output from the solar panel is
on the order of 10 times less than coal, 5 times
less natural gas, so significantly less
than fossil fuel. It is not a zero energy system. The reason why the majority–
where the majority of that CO2 comes from is
actually the energy used to produce the solar panel. So as we transition
to solar panels made from other solar panels, as
the solar industry ramps up, obviously the carbon intensity
of producing the solar panels will go down, as well. Likewise, it matters where
you produce the panels. There’s some active
research going on at MIT to decide where
in the world it’s optimal to produce
the solar panels and where it’s optimal
to actually install them. The disadvantages,
which really embody why we haven’t seen a mass
of adoption of solar to date and why there are technical and
nontechnical challenges for you here to resolve
is because there’s no power output at night. In other words, when
the sun’s not shining, it’s not producing electricity. And there’s lower output
when weather’s unfavorable. And thirdly, today
there’s a high cost. We’ll get to that in
a few slides as well. So it’s not economically
competitive in most markets. In some, there are. In 1.5 out of the 50
states here in US, solar is cost
competitive, today. But in the remainder, it’s not. So this is the really fun part. This is why when you
pick up your phone, and text your parents, and
say I’m in a PV course. And they write
back, ah, PV, I’ve heard about that for decades. That’s an old hat. That’s not going anywhere. You can write back and say
it’s very different today than it was then. And here’s the reason why. In the 1970s, when
PV really started to take off for civilian
purposes– obviously, they had put satellites
up into space. They had proven that it worked. It was robust. In microwave relay stations up
in remote locations, that they didn’t want to service, they
also would place PV panels. But in terms of civilian
purposes on houses and so forth, really
late 1970s, early 1980s were where things were
beginning to take off. And driven by the oil crisis. The OPEC oil crisis of 1970s. This is a New York
Times article describing the state of the art of solar. This is taking a look some
20 years later at solar and saying how far have we come. And one of the interesting
things of note in this article, right here, is
that it cost upward of $10 a watt for the solar
panels, in that day in 1979. Meaning it would
take, roughly, $12,000 to run an ordinary
household toaster. So that was the
impression that folks had of solar in the 1970s. And for good reason. This is the cost of electricity
produced of solar versus time. In reality, the x-axis,
if you look closely, it’s cumulative PV
electricity production. That means for each
new panel we make and for each new unit of energy
that that panel is producing, the cost of electricity
is coming down. That’s because we learned
how to make panels better. We learned how to make cheaper
panels faster, with less cost. So the cost of the electricity
reproduced, over here, is showing going down with time. And this is a
little bit of apples to oranges
comparison, that’s why they’re two different colors
for the two different dots. The black dots represent the
average retail electricity prices. Not costs, but price. This is going to be
a repeating theme throughout the entire course. I’m going to emphasize it now. Can somebody tell me
what the difference between cost and price is? AUDIENCE: Price is going
to be more than cost because the company wants to
make a profit on the product. PROFESSOR: Yeah. So let’s see I make a gizmo–
this is a great example. I make a gizmo that costs a
certain amount, x, let’s say. And now, I sell it for 3x. And I make 2x profit. So the price would be 3x. The cost would be x. And so the cost of solar is
shown here in the white dots. And the price of retail
electricity price is in black. Why is this comparison
made right here? Why would somebody do
that sort of apples to oranges comparison? What point are they
trying to make? AUDIENCE: Because we
need to bring down how much we need to
put into PV to be at to compete with the price
that electricity is at, as opposed to cost. PROFESSOR: Exactly. This is a substitution
play, right? You’re looking at
PV substituting what is, in that case, the
base load and peaking price of electricity–
probably more driven by the peaking price
of electricity. And so what they’re
doing here is they’re saying,
OK, how much does it cost to manufacture
this panel, and how does that
compare against the grid if I were to plug into
the wall over there and extract electricity
from the grid. How much would that cost me? How much would I have to
pay for that electricity? And that’s really the
comparison that they’re trying to drive right here. AUDIENCE: Does that
adjust for inflation? PROFESSOR: Yes. The details are in
this paper, right here. Again, you can access all
this information online. But it is adjusted. These are, I believe,
in 2002 prices. I can’t remember
the exact– yeah. AUDIENCE: What are some of the
assumptions used to compute the cost of PV and electricity? PROFESSOR: Great question. So the higher density of
data points, over here, is in part because they
get closer together. It becomes harder to
drive the cost down. And of course, we were
looking at it in a log scale. But also, the
quality of the data is much better in recent
years because we had access to– greater number
of companies were able to average values
coming from multiple sources. Some of the earlier
data, especially 1957– those were some of the
first solar cells produced. If they had access
to good primary data, those numbers would
be highly accurate because it would be
one company making it. And that’s it. Very little error bar. But if they were
making guesstimates based on material
cost of the day, then there would be some error
bar associated with that data point. These curves are very
difficult to produce when you’re in academia. But I can say that when
we were in industry, we did this for our company
just for hahas one day. And it fell on a
very similar slope– with a similar slope
and a similar value. So somehow they were
getting the numbers right. AUDIENCE: In terms
of insulation, what numbers are you using to
assume– like you said, do you use values for
Nevada or do you take an average of the summation
of the entire US– US average for the retail
electricity prices. PROFESSOR: And so the
retail electricity prices in the United
States vary quite a bit. You have some coal rich
states, like Wyoming, that get $0.05 per kilowatt hour. You have states
like Massachusetts at the end of the
energy pipeline. If you look at the
natural gas pipeline, for example, we’re
at the very end. We get some of our natural
gas even shipped in by boat. $0.18 per kilowatt hour
is residential prices. And in California, which
has a tiered structure, if you’re one of the highest
consumers of electricity, you’re going to be paying
somewhere around $0.30 per kilowatt hour compared to some
of the lower use folks down around $0.12. And so, it varies quite a bit. Typically, when you’re looking
at these sorts of charts, if the chart is produced,
say, by the USDOE or some solar
promoter, let’s say, they will typically be choosing
a rosy scenario of the American Southwest because
that is– well, not only do the numbers look
better but, more importantly, that’s where a lot of the
solar is being installed, today, but not all of the solar. Because it is a substitution
economics situation, two parameters are
really of interest that drive the cost
competitiveness of the solar installation. One is the retail
price of electricity. How much are you
paying out of the wall? What are you
substituting it with? And the second is how much
sunlight you get locally. So our break even point in
the state of Massachusetts is not too far off from
Arizona because they have a lot cheaper
price of electricity even though we have
a lot less sun. So I wanted to emphasize
a couple more points. So when Gregory Nemet
put together this chart, it was within the context of
a really interesting paper in which he attempted to
decouple the effects of scale from innovation. Let me emphasize that. So if you are making
a widget– let’s imagine a razor, like Gillette
does here in South Boston, or if you’re making
some other high tech product– razors by the
way are very high tech. How many times have you cut
yourself by a defective razor? I certainly haven’t, and I’ve
probably used tens of thousands in my lifetime of
individual blades. And that’s because
they’re examined using laser technology. They’re really manufactured
in a high tech way. And they get better and
better every time they produce one razor blade. And so they follow
a learning curve, just like photovoltaics does. With cumulative production, the
cost of producing one widget goes down. And likewise, microwave ovens
and other high tech products. And so the big question is,
how much of this learning curve cost reduction is
driven by innovation and how much is driven by
scaling– just learning how to do incremental
improvements, tweaking the manufacturing line
to make it a little bit more efficient. So Gregorian Nemet, the author
of this paper right here, in which this figure appears,
looked into that question and came up with some answers. Some of those learnings
were incorporated onto this beautiful chat
here produced by 1366, a spinoff from MIT focused on
commercializing really cool next-gen PV product. They took that learning curve
from Gregory Nemet’s paper, plotted in a slightly
different scale, and showed several of the
technology innovations that drove down that learning
curve for crystalline silicon. And so those, in
the fine text there, represent specific technologies. And we’ll be getting
to know some of them over the course
of the PV course. And so, we’re approaching
this very interesting point. If you haven’t noticed
from this chart right here, this ended in 2003. And boy, these two are getting
very close to one another. We’re entering a very
interesting point where the cost of
producing PV electricity is rapidly intersecting with the
US retail electricity prices. And that is represented in
a very broad brush strokes by this DOE chart that was
produced in approximately 2006 with the Solar America
Initiative, where you have the system price
range for PV systems. Again, broad range now,
instead of finite data points. Residential and commercial
rates and utility generation. For those who have already
dealt with electricity markets, the residential
commercial rates– this is the price or
the retail price. And the utility generation,
this is more the wholesale price over here for utility scale. So again, just showing you
the range of substitutions that could be going on. And we’re entering
the regime now where, finally, solar is starting
to be cost competitive. And when you start having
this sort of interaction, you can imagine two Gaussian
curves, one curve representing the price of solar and the
other the price of electricity. And as they begin overlapping,
as the price of our electricity goes up– it went up
15% over the 2000s here in Massachusetts,
the retail price of electricity in residential. And as the price of
solar comes down, those two Gaussian curves begin
moving against each other. And at the edge
of a Gaussian, you can model that using
an exponential. And so you have two
exponential curves overlapping. You have, effectively, an
exponentially growing market penetration. In other words, the solar
adoption on the grid is following a
hockey stick curve. And that’s why you hear a lot
of interest in solar these days. We had a solar system
installed in our house in 2007. And now, our neighbors
put them up last year. The folks across are putting
them up, actually, just last week. And there’s another
family down the road. So our little neighborhood is
representing this little hockey stick, right here, as is
Cambridge as a whole and some of the places in
the United States where it does make
economic sense. You’re beginning to
see that take off. And that’s why it’s such
an exciting time right now. This is a much busier chart. There’s a lot going on. But to sensitize you, this
is the PV residential. In other words, it’s either
the cost or the price to install PV on a
residential home. In other words, it’s
a smaller system. So there’s a larger
overhead per system. The architect needs
to spend more time per unit energy produced to
design your system because it’s a smaller one. A lot of people go out there
per panel to install it. Whereas PV utility, those large
fields filled with PV panels, it’s cheaper per unit
panel to install. One architect can sit down
and design the whole thing– maybe a team of architects. But the overhead
costs are lower. And you can bargain with
the module manufacturers to get a better rate
on your modules. So get a better price. And as a result, the PV
utility costs and prices tend to be lower
than PV residential. And the blue and the
red, here, just represent the wholesale and retail
electricity costs– what they’re substituting
here in the United States. So a bit more
detailed chart, again, showing the grid penetration
down here at the bottom. Also, in terms of percent. So back here, a few
years ago, the 0.2% of total worldwide electricity
was generated by photovoltaics. And projections are that by
2020, we’ll be at around 1%, by 2030 around 5% using these
just two overlapping Gaussian curves. And it’s interesting to
note that this is global. On a local level,
Germany has already well surpassed 2% in Bavaria. I think it might be up 3%
or 4% now in photovoltaics, in the southeastern
region of Germany. There’s a small island in
the Hawaiian chain that has, I believe in peak days,
around 40% of its electricity produced from solar. So there are regions
that already, you have a very large percentage
of the total electricity being produced by solar because of
that distribution of prices. And lastly, this is a
really exciting chart. This is the convergence between
PV and conventional energy– essentially, what
this chart over here was attempting to capture
in its percentages. This is explicitly
laid out, now. And I took data going
back to the 1970s, and plotted the average
terawatts installed of new PV installations
versus total primary energy– new primary energy
installations. So for those energy wonks
here in the audience, what is the primary energy
burn rate in the world, right now, in terms
of terawatts average? Around 15, right? And so the average
new energy installed each year is represented here. It’s somewhere between 100
gigawatts and a terawatt, typically. And this is the new PV installs. You can see that we’re within
about two orders of magnitude, now, of total new
energy installs. This is primary energy. For electricity, it
looks even rosier. And so we’re rapidly
approaching the point where substitution will begin. We’re going to
start replacing, not only we’re going
to take a larger share of total new
installed energy, but we may even start
putting some existing power plants out of business. And we’ll get into that
in the economic section in the third part of our course. Interesting to note, these
three distinct phases of growth of the industry over time. Phase one was right
at the beginning, when we had the OPEC
oil crises, when people were really interested in
solar, but it was really a boutique thing. And solar cute, great PR,
but not really impactful. In this regime, right here,
where most of you were born, solar went kind of
through a down cycle. So while the price of
oil was really high, right back here, it
crashed in the early 1980s. And symbolically,
Ronald Reagan ended up taking down the solar
panels from the White House some time in ’86, ’87. And big oil companies
were the ones who kept the solar industry
going, interestingly enough. It was Mobil-Tyco. It was BP Solar. The largest companies that
were producing solar panels in the world were ones that were
small divisions of larger oil companies, which viewed
themselves as energy companies. And then finally,
this phase three, this really rapid growth here. Again, a cumulative
annual growth rate somewhere between
40% and 50% average. That took off when generous
government subsidies, whether it be for
the manufacturing or the installation. In the case of
the United States, it’s mostly been on
the installation side. In the case of
China’s, it’s mostly been on the manufacturing side. Japan and Germany
had a bit of both, but more heavily toward
the installation. And we saw a massive
growth of the PV industry because, now, the government’s
realized, well, wait, the cost is coming down, and
we will need new electricity coming on board. And our oil supply is
a little unreliable. So let’s invest in
this new technology and see where it takes us. And I think the Germans,
now, are paying somewhere on the order of a euro,
maybe a little over euro, per month on their
electricity bills as a result of having financed
a lot of this growth right here in the PV industry, which
allowed the costs to come down for the entire world. So it was a successful program. And as a result, many
pure play companies saw the financial opportunities. The case Q Cells, which
is highlighted down here, is not unusual in those days. In the late 1990s, a group
of executives at McKinsey got together and
said, wow, the numbers look really promising
in the solar business. Why don’t we form
our own company and only do solar
instead of being part of a much larger
one where they have their interests dispersed among
many different product lines and technologies? Let’s focus
exclusively on solar, burn our bridges behind
us, and just go for it. And they went for it. And for a while,
for a few months Q Cells was the largest solar
cell producer in the world. It was, I would
say, a poster child of this new generation
of PV companies coming in this third phase here. And as we’ll learn over the
course of this semester’s course, many of the
leading solar producers today are now located in China. So this is, basically, the
history of PV development. And the important thing to
note is this closing gap, right here. So when folks are saying solar,
it’s the same old, same old. It’s been gimmicky. It’s been around
for a long time, but it’s not going anywhere. You can point to some of
this data and, say, no. Actually, it’s on the cusp. It really is
beginning to take off. And these are some
of the data you can point to if you care to do so. Let me spend a few minutes
talking about the broader picture beyond just
solar photovoltaics into some of the other
solar technologies. We won’t be addressing
too many of these over the course of the lecture
because we have to focus and we have to become
very good at something, otherwise we spread
ourselves to thin. But I did want to give
you a sense of what’s out there so that you can
situate solar photovoltaics within a broader context. And so this is a solar
energy technology framework that encompasses all conversion
technologies from sunlight into energy. And so first off, I start with
a rationale for framework. Why invest the time to
come up a the framework? I’ll explain why. There are several hundreds
of technologies out there that can convert
sunlight into energy. And to make sense of
the technology space and to provide some meaningful
technology assessments, there have to be some
performance driven, technology neutral
performance metrics that you can use to evaluate
one technology against another. And that’s why coming up
with some sort of framework is very helpful. So the three criteria
that I chose together with Vladimir Bulovic
when we put the together, to design a technology
framework was an exhaustive categorisation. In other words, our framework
had to encompass more than 90% of all technologies out there. The 30 years challenge. Again, in 30 years,
the PV technologies should be able to fit
into this framework still. And it should be a
useful analysis tool. It should be able
to give insight into the complex space
that’s out there, and allow folks, like
yourselves, to make sense of it, whether you’re trying
to develop cost models or if you’re trying to
develop technology prospectus. This should allow you to
gain a foothold in it. So we have solar energy
conversion technology. And we chose an output
oriented rationale for dividing the solar
energy conversion space. So the output would be either
electricity, heat, or heat which is then used to power,
say, a turbine which generates electricity, or fuels. And those are the four primary
outputs of solar energy, today. Yes, there are
technologies out there, for example, that convert
sunlight and store it in some way and convert
light on the other end. But we’re not including
those in here because, again, the 90% rule. We’re focusing on
the major ones. And then, we do a
further subdivision between the non-tracking
and tracking. Tracking means if
the sun is moving through the sky over
the course of the day, your apparatus is
following the sun so as to maximize the cross section
between the incoming rays in your device. The reason we chose
tracking non-tracking is because tracking
requires motors, which will add cost and
reliability questions to your system considerations. And that’s why we chose this
further division right here. So on to the assessment. Let’s look at the technologies
that are out there and try to bin them. Solar to electricity. There are a few embodiments. There’s the photovoltaic
device, these ones. There’s the
thermoelectric device as well, which convert
solar energy into heat, really, and then heat
into electricity. So maybe it should have
been in the other category. But it is a device that converts
solar energy into electricity. So we’ve seen a
solar cell device. We’ve learned the three steps,
charge generation, charge separation, charge collection. And we look at the
existing technologies that are out there, today,
and say, all right, let’s start to bin them. We have non-tracking systems
that can be non-concentrating, like these panels right here. Essentially, they’re
just flat panels that are receiving the sun’s rays. Or, you can have cheap,
mirror-like materials that bounce the sunlight off
of them into the solar panels and concentrate sunlight. So let’s imagine we
put a set of mirrors on either side of this
panel, right here. And when the sunlight
bounced into the mirror, it would reflect
back into the panel. That would be a concentrating,
but non-tracking, system. And these are common on barriers
along the highway in Germany. They’re sound barriers. They’re preventing
the people who live on the other
side of that barrier from hearing the noise of the
cars going by on the Audubon. They’re not meant to
be crash barriers. Those are separate,
closer to the actual road. But these are examples of
concentrating and non-tracking photovoltaics. There are ground mounted
and roof mounted systems. So again, another
way to split the pie. In the concentrating
non-tracking system, there aren’t only
these types, there are a variety of other
species of concentrating non-tracking devices as well. There are so-called
sliver cells– which the light comes in,
bounces around a little bit, and then eventually gets
absorbed by the device. And that even happens, to some
degree, in these modules, too. Because the light
comes in– make sure I don’t reflect
this into your face. There we go. Point it up. The light can come in
sometimes and reflect off of this white back skin. If the light is coming in
at an oblique enough angle, total internal
reflection by the glass. It’ll get a second bounce
and go into the device. We’ll talk about how that
works in a couple of lectures. So internal reflections. And this is particularly timely. Does anybody know–
does the word Solyndra ring a bell for anybody? Yeah. What about Solyndra? AUDIENCE: It went bust. PROFESSOR: It went bust. So it’s one of the
three photovoltaics start up companies
in the United States that went bust over the past few
months over this past summer. And that’s a really
interesting market dynamic, which we’ll get to in the
third part of this course. And we’ll discuss that
head on because it’s an interesting,
and very important dynamic in the evolution
of the solar industry. We have some technologies
under development at MIT. Marc Baldo’s lab and
Vladimir Bulovic and others are working on devices that
absorb sunlight, reemit the light at a
different wavelength, trap it inside of some high
index medium, like glass, and then, ultimately,
concentrate it on to the cells that
are on the corners. So you can imagine a
window that absorbs some of the incoming
light, bounces light off, and eventually concentrates
the light in the corners where you have your
solar cell devices. The advantages, or the
potential advantage, here is that you can have
a very high efficiency, expensive device, but a
very small area of it. Instead of covering this
entire area right here, you’ve now reduced
the total area. And then, if this is a
very small percentage of the total system
cost, you can just switch it right out when a new
and better technology comes along, almost like you
switch out your computer. So if a better solar
cell device comes along, you can take this one out
and put the next one in. It’s almost like an
upgradable system because the majority
of the embedded cost is in the concentrator and not
the solar cell device itself. Again, just really drinking out
of the fire hose this morning. We’re drilling you
with data, but it’s meant to begin to sensitize
you to some of the terms and some of the ways of
thinking here in the field. Tracking. So when we’re talking
about tracking, there’s a rise in the
number of tracking systems in the United States. It is shown with high
efficiency modules that it can be more
cost competitive if you have a large
field installation to do one axis tracking. One axis tracking and
two axis tracking. Why would you want one
or two axis tracking? What are you tracking? One axis tracking. What would make sense to
track with a one axis? If you had one axis to choose,
would you rotate east west? Would you rotate north south? Would you rotate
northwest to southeast? Where would you go? AUDIENCE: East to west. PROFESSOR: East to west. Why is that? [CLASS MURMURS] PROFESSOR: Because
you’re tracking the sun over the course of the day. And you’re tracking, pretty
much, every day of the year. So 365 tracks per year. The two axis tracking,
what is this other axis? Presumably, it’s orthogonal
to the east west. In other words, north south. Why would you want
to track north south? Seasons, right? Yeah so from winter to
summer, you’re tracking. So you would always want your
solar panels facing south, I guess, right? AUDIENCE: In the
northern hemisphere. PROFESSOR: In the
northern hemisphere. Exactly. So if you’re in
Australia or in Brazil, your solar panels
are facing north. So let’s accustomize
ourselves with that. And the two axis
tracking, of course, would allow for that adjustment. The reason one axis
tracking is taking off as the most common field
installation tracking system is because the
seasonal adjustment, if it really needs to be done–
it’s not a huge energy benefit, but if it really
needs to be done, you can probably
just crank by hand instead of using a machine or
a motor that can break down. And the adjustments still
need to be made very often. Non-concentrating
and concentrating PV. Tracking. So these are one axis
trackers, right here, tracking over the course of
the day, but not concentrating. In other words, they’re
flat panels like this, but just mounted
a one axis tracker that follows the sun over
the course of the day. The system over here
is a two axis tracker that includes little lenses
that are focusing the sunlight onto tiny little cells. And again, very similar
idea that the solar cell itself is high efficiency,
but it is a low percentage of the total system cost. Non-concentrating and tracking. Again, several examples of that. You have fancy systems, two axis
trackers, again, most common. Can anybody guess what this
little gizmo is, right here? We’re going to get to
that in next lecture but– AUDIENCE: A solar sensor that
finds the position of the sun? PROFESSOR: Exactly. Somehow, you have to
have a measuring device if you have a tracker. It has to tell you
where the sun is. So this little
gizmo, right here, is just making sure
that the panels are facing the right way. Awesome. So concentrating and tracking. Here’s a closer look at
some of the Frenel lenses that are used to
concentrate the light down. On some cheap microscope– or
sorry, cheap magnifying glasses they also use Frenel lenses. And so this is an example
of a low cost apparatus here to concentrate the sunlight
onto your high efficiency cell. Solar to heat electricity. We’re not going to talk too much
about this during the course. But just to sensitize
you– that there are technologies out there
and some pretty exciting once. There are heat engines. In other words,
sunlight heats a fluid, which moves a turbine or
a piston, either directly or by heat exchanger. Heat exchangers. Thermoelectrics. Long wavelengths photovoltaics. These are devices
that convert the heat portion of the solar
spectrum into usable energy. And there are hybrids that
are possible with these. So if you heat up a fluid, say,
a salt or a glycol solution, then you can store the
energy in terms of heat. And if the stored
energy begins to decay with time because
of poor insulation, you can augment that
heat with natural gas or with some other fossil fuel. So you get these hybrid,
renewable solar and natural gas power plants that are
possible with the solar to heat electricity. And there are some really
fancy designs out there. And I’m happy to dive
into these in more detail. The most common one
are sunlight coming into some sort of reflector, and
then concentrating the sunlight into a thin tube that contains
your high heat capacity material, liquid
usually– so a glycol based liquid or even
a salt, sometimes. It has to have a
high heat capacity. In other words, it has to be
able to absorb a lot of heat and retain it. But it also has
to have, ideally, a minimum amount of corrosion so
that the longevity of the parts is sustained. And you can see,
here, these tubes that are running along here and going
down these fields of collectors all the way to the other side. And somewhere off in the
distance is the heat exchanger. So that’s solar thermal for you. We have parabolic dishes
concentrating sunlight into Stirling engines. That’s kind of neat. And so your T high is basically
that of generated by the sun. And you T low is the ambient. So typical mechanical
engineering there. And you also have
solar power towers. There’s some work being done at
MIT in this as well with Alex Slocum and others that are
using fields of mirrors to concentrate the sunlight
into a tiny little spot, right here, in a big tower. Say, for example, that spot
right there, it’s dark. It’s not in operation. But if it were, the sun
would be concentrated onto that little spot. It’d be really, really
bright, indicative of it’s very high
temperature, on the order of a couple thousand Kelvin. And then the sunlight would
either be absorbed up here, with some molten salt, or
reflected down underground to a heat reservoir. And that would be your T
high running your engine. So your Carnot engine. And then the T low would
be the ambient temperature. Solar to heat. This is really important
in developing countries. Not to be overlooked, the very
simple, low tech conversion of sunlight into heat. You can heat water. This is very, very
common on rooftops all throughout the
sunbelt of the planet. You’ll see these on the
roof, painted in black. They contain potable water,
typically used for either, say, for example,
showers or kitchen use. And the fancier
versions that are really marvels of engineering. These materials all
have to be coefficient of thermal expansion matched. As it heats up, the
glass tubing has to match the expansion
of the metal around it. So it is quite a
feat of engineering that they make these so well. There are a few
companies in Germany that really pioneered
this effort right here. Of course, you have tracking
versions, like solar ovens. Not too common. You typically find more still
in developing countries. Unfortunately, you find a
lot of wood burning, which isn’t good for the cook,
which, unfortunately, most often is female. And so this illustrates some
of these societal questions that solar involves. It’s not just the technology. This involves gender equality. This involves
societal development. This is a much broader topic
than just the fundamentals of the physics of how the
solar cell device works or how sunlight is
converted into energy. And that’s why we have the
three segments of the course. Lastly, solar to fuels. The way I’ve traditionally
broken it down– it’s a little bit wishy washy–
is into enthalpy and entropy in the sense that,
in enthalpy, you’re storing the sunlight in
bonds– in chemical bonds. The bonds are
forming– more complex, higher energy molecules
are being created. So you’re taking water and
splitting into the gases. Or you’re taking CO2 and
water and converting it into hydrocarbons. And those can be used
to store the fuel and, ultimately, release it in
the form of burning the fuel. So it’s a closed loop cycle. And what I refer to
as entropy, which I get some flack from the
folks in chemistry for, is the separation of phases,
in other words, desalination. If you separate your
salts from your water, then you’re increasing
the energy of your system. You’re doing a
physical separation. And it is a form
of energy storage. So this right here is the
example of the renewable fuel cycle where you
have sunlight coming into your starting compounds. Using some catalyst,
typically, you’re creating the
intermediate compound, which is a solar fuel. Then you burn your solar fuel. Then you have your
final compounds. In the ideal world, 5 equals 1. The final compounds are
identical to the beginning compounds. And you have a closed loop
cycle, a renewable cycle. And so a lot of work is
going on here at MIT. This is a recent paper
we published together with Dan Nocera. His group is looking to
special types of catalysts. Our group makes solar cells. So we work together to make
these nifty little devices that convert sunlight
into storable fuels. What you see here are
little bundles coming up from the water in which the
solar cell device is embedded. The water is near pH neutral. Then it’s converting
that sunlight into gas, into hydrogen and oxygen,
which can then be stored. On one side of the
device, you could be creating oxygen. On the other
side, hydrogen, for instance, if you have a
physical separator, you’d be able to store
that electricity. This is an example– a
very simple example– of desalination driven by solar. There are much fancier
examples, as well. But that gives you an idea. You have contaminated
or salty water. And you’re
evaporating the water. It dribbles down into this
little collector over here, and finally out into your
collecting pot, leaving the salty, brine behind. And then in the
broader perspective, we have many other issues beside
just the conversion technology itself. We have how do we
use the electricity and how do we store it. Is the solar power generation
centralized and all the users distributed, similar to
how we produce power today? Do we have one big solar field
that’s producing electricity for all of Cambridge,
or do we have the individual solar panels
in each of our houses that are producing
the power locally, and they’re all interconnected? In case a cloud goes over
one region of Cambridge, there’s still coverage. That’s a really big question. And the economics are what’s
driving this right now. These large field
installations give you a sense. This is a road right here. These little green
specs are trees. These are huge field
Installations of solar. The economics are
driving it right now. But there are opportunities
with commercial buildings. This is the Moscone Center
in downtown San Francisco. It’s like the Heinz Convention
Center equivalent there. This is an example of a
house in Rochester, New York. That housing development in
Rancho Cordova in California. So you have examples of
residential installations as well. Are we just going to let
economics drive this? Is there going to be
some policy involved? Is there a smarter way to
do it, not only from a cost perspective, but from a societal
perspective or an energy grid robustness point of view? What are the right choices here? There’s a lot of open questions
right now in the field. And what about energy storage? Are we going to store it in
terms of batteries and fuel? Centralized storage? Are we just going dump it into
the grid and be free riders? Let the grid handle it, somehow. Hope that the grid a stable
enough that when a lot of solar is being produced and when
no solar is being produced, it’ll just be able
to accommodate. I guess the resistance in the
turbines of the fossil fuel plants will either
increase or decrease depending on how much energy
we’re pumping into the grid. And so at the end of the day,
we have this very complex space of conversion technologies. The solar electricity,
solar to heat, and so forth. And the system itself, whether
we have centralized generation of electricity
distributed generation, and whether the storage is
centralized or distributed, whether you have storage inside
of our house on the inverter, let’s say, or in the
basement, or rather the storage is some
centralized storage facility in the
center of Cambridge that serves as a buffer. And we have all of
this space to play in. We’re going to be focusing
on solar to electricity. So we’ll be focusing on
these two columns right here. And specifically,
the technologies during the first two thirds, and
then, the broader, system level impacts in the
third of the course. So that puts it
all in perspective, I’m not going to get
too much into this. I’m just going to say one
quick word about CO2, energy, and climate change. You hear a lot of
talk about, at least from the political sector, that
scientists are, shall we say, in a lot of debate whether
climate change exists or not. That is patently false. The majority of
scientists, upwards of 96%, believe that there
is strong evidence to support the fact that human
energy consumption, especially the high CO2 intensity of
our energy consumption, is driving some form
of climate change. What the magnitude is and what
the impact is– obviously, that is still under discussion. But the reality
that our emission of energy– our emission of
CO2 as a result of energy use, our fossil fuel energy use,
is driving some form of climate change that there is widespread
consensus among the established scientists in the field. Now if you want to do some back
of the envelope calculations just to convince yourself
that we, tiny, puny, little human beings are having
an impact on our world, do this for me. Take the total energy
consumption rate. This is the energy burn rate. So it’s the average
power– average rate of electricity use. Look at just the fossil
fuel based energy sources. Or if you prefer, take
the average CO2 intensity of our energy mix, which
somewhere around 600 or maybe 800 grams of CO2
per kilowatt hour. And then look at that
amount of CO2 emitted. You can calculate how much
CO2 is emitted per unit time from our energy mix knowing the
carbon intensity of our energy mix. Then do a quick back of
the envelope calculation. Assume that our atmosphere
is 30 kilometers thick. It’s a generous assumption. The density of the atmosphere
dwindles pretty quickly above 10 kilometers. But assume it’s 30
kilometers thick. And then dissolve
all of that carbon that we’re creating
from this energy mix into that thin shell
surrounding our earth. Our earth is on the order of
6,370 kilometers in radius. And it’s only 30 kilometers
thick, the atmosphere. That’s why those beautiful
photos from the space missions, when you see that thin
blue shell on the planet, right– that’s the atmosphere. It’s really, really thin. Just do a quick carbon
density analysis. And you’ll see that we’re adding
hi tens of parts per million of CO2 to the atmosphere. And then you look at the total
CO2 in the atmosphere, which is in the order of
400 parts per million, and you’ll see that we’re adding
an appreciable amount, just given the carbon intensity
of our energy mix and the total volume of
atmosphere into which we’re dumping that carbon. And so the question
of whether or not we are adding carbon to
the atmosphere, I think, is indisputable, based
on some quick back of the envelope
calculations and, of course, the more in-depth models. The only place where you can
have some wiggle room to argue is whether or not CO2 actually
influences the climate. And for that, there
are a number of studies discussing that point. I would refer you, specifically,
to these here, published in Science in 2005, that
discuss historical correlations over the last 600,000
years, correlating CO2 and mean global temperatures
based on oxygen isotope ratios containing gas bubbles,
for example, in ice cores. So I would say if you’re
arguing whether or not we’re having an influence
on our atmosphere, I would say that is a
difficult position to take. The only room that I would
give you some room to maneuver would be if you
said, well, you know, CO2 really isn’t that
bad in the atmosphere, despite what our
infrared absorption data seems to indicate,
that it really does absorb infrared
light and reemit it. So that’s what I have to
say about the climate, which is a huge motivator for a lot
of people taking the course. And you’re welcome to talk
about that in more detail, but I’d really love to keep
this focus on the technology, by and large. And for that, I’d like to hand
out these background assessment quizzes for each of you. Please take a few moments to
fill these out– just pass them back– so we can learn
more about your interests. And what I’ll also
do is pass around this little solar
module, right here, so you can get a sense of what
a small little solar cell looks like up close and personal. Once you’re done, feel
free to come up and take a look at the solar module,
right here, as well. And thanks.


    I Love his style when he refer to other scientific concepts seems new and hard for a person to cover at all. Thanks is a small word for MIT and this great teacher.

    Very exciting lecture. A good speaker should be like this never boring or annoying. Love to watch the whole video.

    Here it is now 2019. Where has the world advanced to with Solar cells?
    100 Watts Flexible Solar Panel, 12V RV/Boat + Charge Controller, 25 year warranty, selling on ebay for $100. 150 Watt Inverter portable generator & Solar Charging station for 100 watt solar panel selling on ebay for $100. Now finding Climate change was a hoax perpetrated on the people by the top world elites. It's all about scamming the public for excess funding to their wasteful gov't projects. Now finding that Big media CNN, ABC, CBS & NBC are the propaganda arm of the DNC. Half of Americans are now split between allowing open borders for more illegal alien invaders coming through our Southern borders. Democrat Governors have many large 'sanctuary cities' to protect illegal aliens for their votes to keep them in power. The Dems are giving illegal's free medical, housing, food, insurance, & licenses to drive anywhere for their votes. Dems and the Big media are hell-bent on protecting these criminal alien invaders. The year 2019, Dems are trying to impeach the U.S. President because they hate him & lost the election. The Dems will lose again in 2020 because of their support to open Southern border allowing illegal invaders to come in.

    wonderful lecture by this professor. very informative and interesting. important thing is it was motivating. Highly eloquent and interesting way of speaking as well.

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