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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.