Study assesses solar photovoltaic technologies

December 17, 2015 by Nancy W. Stauffer
Joel Jean (left) and Vladimir Bulović (center) of the Department of Electrical Engineering and Computer Science and Patrick Brown of the Department of Physics have, together with their collaborators, performed a rigorous assessment of today’s many commercial and emerging solar photovoltaic tech­nologies. They conclude that none should be ruled out, given the urgent need to move to a low-carbon energy future. Credit: Stuart Darsch

An MIT assessment of solar energy technologies concludes that today's widely used crystalline silicon technology is efficient and reliable and could feasibly be deployed at the large scale needed to mitigate climate change by midcentury. But novel photovoltaic (PV) technologies now being developed using specially designed nanomaterials may one day provide significant advantages. They could be easier and cheaper to manufacture; they could be made into ultrathin, lightweight, flexible solar cells that would be easy to transport and install; and they could offer unique attributes such as transparency, opening up novel applications such as integration into windows or textiles. Since no single technology—established or emerging—offers benefits on all fronts, the researchers recommend rapidly scaling up current silicon-based systems while continuing to work on other technologies to increase efficiency, decrease materials use, and reduce manufacturing complexity and cost.

One of the few renewable, low-carbon energy resources that could scale up to meet worldwide electricity demand is solar. Silicon do a good job transforming the sun's energy into electricity today, but will they be up to the task in the future, when vast solar deployment will be needed to mitigate climate change? And what role might be played by the many other PV technologies now being developed in research labs the world over?

Addressing such questions was the goal of a recent wide-ranging assessment by Vladimir Bulović, the Fariborz Maseeh (1990) Professor of Emerging Technology and MIT's associate dean for innovation; Tonio Buonassisi, associate professor of mechanical engineering; Robert Jaffe, the Jane and Otto Morningstar Professor of Physics; and graduate students Joel Jean of the Department of Electrical Engineering and Computer Science and Patrick Brown of the Department of Physics.

The solar resource

The researchers' first task was to examine their energy resource—sunlight. To no one's surprise, the assessment confirmed that is abundantly available and quite evenly distributed across the globe. It varies by only about a factor of three across densely populated areas, and it isn't highly correlated with economic wealth. In contrast, fossil fuels, uranium, and suitable sites for hydropower are heavily concentrated, creating potential tensions between the haves and have-nots. "Solar is a much more democratic resource," Jean notes.

And the world is beginning to take advantage of it. More than 1 percent of total global electricity is now provided by solar. Within the United States, solar deployment is growing at rates significantly exceeding projections made by experts just five years ago. In 2014, solar accounted for fully one-third of all new U.S. generation capacity; and as shown in Figure 1 below, residential, commercial, and (especially) utility-scale PV installations have all flourished in recent years.

About 90 percent of current solar PV deployment is based on crystalline —a technology that has been commercial for decades and is still improving. This efficient, reliable technology could achieve the needed large-scale deployment without major technological advances, says Bulović.

But it's tough to make it cheaper. In the solar PV business, costs are divided into two categories: the cost of the solar module—the panel consisting of multiple solar cells, wiring, glass, encapsulation , and frame—and the "balance of system" (BOS), which includes hardware such as inverters and wiring plus installation labor, permitting, grid interconnection, inspections, financing, and the like. Since 2008, the cost of the module has dropped by 85 percent, but the BOS cost hasn't changed much at all. Today, the solar module is responsible for just one-fifth of the total cost of a residential installation and one-third of the cost of a utility-scale installation in the United States. The rest is the cost of the BOS.

Reducing BOS costs isn't easy with silicon. Silicon isn't very good at absorbing sunlight, so a thick, brittle layer is needed to do the job, and keeping it from cracking requires mounting it on a heavy piece of glass. A silicon PV module is therefore rigid and heavy—features that raise the BOS cost. "What we need is a cell that performs just as well but is thinner, flexible, lightweight, and easier to transport and install," Bulović says.

Research teams worldwide are now on the track of making such a PV cell. They're starting not with silicon—a structurally simple material—but rather with a variety of more complicated nanomaterials that can be specially designed to capture solar energy and convert it into electricity.

Comparing and contrasting the technologies

Study assesses solar photovoltaic technologies
Figure 1. The world’s installed photovoltaic (PV) capacity exceeds 200 gigawatts (GW), accounting for more than 1 percent of global electricity generation. This chart shows annual additions to PV capacity in the United States from 2008 to 2014. Additions to utility, commercial, and residential capacity grew substantially each year, with the greatest increase occurring in the utility arena.

Evaluating the many PV technologies now in use and under development is difficult because they are all so different. At the most basic level, they employ different active materials to absorb light and collect electric charge. In general, they fall into three broad categories. Wafer-based cells include traditional crystalline silicon and alternatives such as gallium arsenide; commercial thin-film cells include amorphous (non-crystalline) silicon, cadmium telluride, and copper indium gallium (di)selenide (CIGS); and emerging thin-film technologies include perovskite, organic, and quantum dot (QD) solar cells.

Comparing the strengths and weaknesses of those and other options requires a way to organize them. The conventional classification system, established in 2001, groups solar technologies into three "generations" based on efficiency and cost. But that scheme "may not adequately describe the modern PV technology landscape," says Bulović, because many of the technologies—both old and new—don't fit well into their assigned categories. In addition, such a chronological scheme treats older technologies pejoratively. "Third generation" will always sound better than "first generation." But silicon, a first-generation technology, still offers many advantages and commands the vast majority of the solar cell market.

To help guide today's thinking, the MIT team came up with a new framework. It's based on the complexity of the light-absorbing material—a concept defined roughly as the number of atoms in the molecule or crystal unit that forms the building block for the material. The building blocks in modern PV technologies range in complexity from single silicon atoms to increasingly complicated compounds and nanomaterials—from cadmium telluride through perovskites and organics and finally to QDs (see Figure 2 above). In the new classification system, all of the technologies appear on a single scale; they don't move around over time; and one location isn't better than another. In addition, says Jean, "we find that there's some correlation between complexity and the performance measures that we're interested in."

One such measure is manufacturing complexity and cost. While silicon is structurally simple, turning it into wafers and solar cells is complicated and expensive, in part because of the need for stringent purity (>99.9999 percent) and high temperatures (>1,400 degrees Celsius). Processing more complicated-looking nanomaterials is generally easier, cheaper, and less energy-intensive. For example, preliminary chemical reactions at moderate temperatures can be used to transform starting materials into organic molecules or QDs. Those complicated building blocks can then be deposited at low temperatures through vapor or solution processing, which could make them compatible with a variety of substrates as well as with high-speed production processes such as roll-to-roll printing.

Another critical measure of PV technology is power conversion efficiency, defined as the fraction of the incoming solar energy that comes out as electrical energy. Crystalline silicon is still the technology to beat, with record cell efficiencies of up to 26 percent. Emerging nanomaterial-based technologies are currently in the 10 to 20 percent range. However, because complex nanomaterials can be engineered for maximum light absorption, they can absorb the same amount of light as silicon with orders of magnitude less material. "So while the typical silicon solar cell is more than 100 microns thick, the typical nanostructured solar cell—one that uses QDs or perovskites—can be less than 1 micron thick," Bulović says. And that active layer can be deposited on flexible substrates such as plastic and paper, with no need for mechanical support from a heavy piece of glass.

Thus far, the high efficiencies promised by such novel thin-film PV technologies have been achieved only in laboratory samples smaller than a fingernail, and long-term stability remains an issue. But with additional work, technologies based on complex materials could offer a range of valuable attributes. Such technologies could be made into lightweight, flexible, robust solar modules, which could bring down BOS costs in systems connected to the power grid. They could be used to power portable electronic devices ranging from mobile phones to small water purification systems; they could be transported and installed in remote areas; and they could be well-suited to the low-power lighting and communication requirements of the developing world. Finally, they could have unusual properties that permit novel applications. For example, some nanomaterials can be engineered to absorb ultraviolet and infrared light while letting through visible light, so they could be integrated into, say, windows, skylights, and building facades.

Materials availability

The prospect of scaling up today's solar generation—perhaps by a factor of 100—raises another issue: materials availability. Will the large-scale deployment of solar power be limited by the availability of critical materials needed to manufacture solar cells? How do the different technologies perform on this measure?

To find out, the researchers determined the materials requirements for each PV technology. They then calculated how much of those materials would be needed if that technology were used to satisfy 5, 50, or 100 percent of global electricity demand in 2050. (Using the International Energy Agency's estimates of demand in 2050, those fractions translate to installed PV capacities of 1,250, 12,500, and 25,000 gigawatts [GW] of power—all of which dwarf today's installed PV capacity of roughly 200 GW.) Finally, they checked current global production of each material and determined how many additional hours, days, or years of production at current levels would be needed to meet the selected deployment targets with the various technologies.

Study assesses solar photovoltaic technologies
Figure 2. This figure shows the researchers’ proposed scheme for classifying PV technologies based on material complexity, defined roughly as the number of atoms in a molecule or repeating crystal unit.

Figure 3 above summarizes their findings. Meeting 100 percent of 2050 global electricity demand with solar cells would require the equivalent of just six years of current silicon production. Such a scale-up of production by 2050 is certainly feasible, so materials constraints are not a major issue for silicon.

Materials requirements for PV technologies

The availability of critical materials could constrain a major scale-up of solar capacity using certain PV technologies. This figure shows how much additional time would be needed at current production rates to supply key materials to meet three levels of 2050 electricity demand—5, 50, and 100 percent—using selected PV technologies. Materials availability doesn't limit the expanded use of today's silicon-based cells or emerging PV technologies. In contrast, using commercial thin-film technologies such as cadmium telluride to supply the bulk of projected electricity demand would require hundreds of years of producing key materials at current rates. The needed growth in annual production of those materials between now and 2050 would be well beyond the realm of historical precedent.

The same can't be said of today's commercial thin-film technologies. Consider cadmium telluride. Tellurium is about a quarter as abundant as gold and is produced primarily as a byproduct of copper refining. Providing the tellurium for cells to meet all of 2050 demand would require the equivalent of 1,400 years at the current rate of mining. Indium, gallium, and selenium are also produced as byproducts of major metals, and using CIGS solar cells to fulfill all electricity needs in 2050 would require well over 100 years of current production for all three. "That isn't to say these technologies don't have a future—they could still generate hundreds of gigawatts of power," Brown says. "But materials constraints make it seem unlikely that they will be the dominant solar technology."

Study assesses solar photovoltaic technologies
Figure 3. The availability of critical materials could constrain a major scale-up of solar capacity using certain PV technologies. This figure shows how much additional time would be needed at current production rates to supply key materials to meet three levels of 2050 electricity demand — 5, 50, and 100 percent — using selected PV technologies.

In contrast, the emerging thin-film technologies use abundant primary metals that are produced in high volume. For example, meeting 100 percent of demand with QD-based solar cells would require the equivalent of only 22 days of global lead production and six hours of global sulfur production. Perovskites would require at most three years of current production of their constituent elements.

The bottom line

The researchers conclude that work should continue on all the technologies, with efforts focused on increasing conversion efficiency, decreasing materials use, and reducing manufacturing complexity and cost. Right now, no single technology promises to be best on all three measures, and predicting how each will evolve over time is difficult. For example, if emerging technologies start being used in mobile phone displays or windows or curtains, meeting that demand could help manufacturers work through production issues, perhaps enabling lower-cost, larger-scale production in the future.

The researchers also stress the time required to get a new technology developed and to market. "Today's emerging technologies are improving far faster than currently deployed technologies improved in their early years," says Bulović. "But the road to market and large-scale deployment is invariably long." In addition, PV deployment may be limited or influenced by unforeseeable technical, economic, and political factors. Given the urgency of the climate change problem, says Brown, "We need to be deploying and improving today's technology and at the same time setting the groundwork for emerging technologies that we might discover in the lab. It's critical that we push forward on both fronts."

Explore further: Study finds unprecedented production of metals needed to meet some solar energy goals

More information: Joel Jean et al. Pathways for solar photovoltaics, Energy Environ. Sci. (2015). DOI: 10.1039/C4EE04073B

The Future of Solar Energy: An Interdisciplinary MIT Study led by the MIT Energy Initiative. mitei.mit.edu/futureofsolar

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32 comments

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gkam
1.4 / 5 (10) Dec 17, 2015
Is anybody working on home coal technology? No?

Home nukes? No?

Maybe we have grown up.
WillieWard
2.3 / 5 (6) Dec 17, 2015
Is anybody working on home coal technology?
Coal technology is being essential for German Energiewende to compensate wind/solar intermittency.
"The German Energiewende has not resulted in less dependence on the burning of coal to generate electricity and will not do so anytime soon."
http://judithcurr...misstep/
aksdad
2.7 / 5 (7) Dec 18, 2015
It is extremely unlikely that solar will ever replace base load power plants. It's excellent for reducing the need for coal and gas power during the daytime, but it does nothing at night or when it's cloudy. Of current technologies, nuclear fission is the clear choice for reducing use of fossil fuels. There are no emissions, it's orders of magnitude more efficient at producing power than any existing technology, and it produces very little waste. Generation 3 and 4 plants will reduce that waste to virtually nothing and are much safer than existing plants, which already have the best safety record of all the base load power plant technologies we use today.
gkam
2 / 5 (8) Dec 18, 2015
"There are no emissions, it's orders of magnitude more efficient at producing power than any existing technology, and it produces very little waste."
-------------------------------------

Nope. Nuclear plants use three-million-degree Neutrons to boil water, and do so at the lowest thermal efficiency of any thermal plant.

If there are no emissions why the extremely-high stacks at nuclear plants? They are way too high for combustion or engine exhaust. The nasty little secret is they vent radioactive gases removed from the reactor and put them into the atmosphere for us to breathe, but they keep them up high, so they don't get any themselves.

Want the waste? We can't find a way to even store it safely, and it must be safeguarded forever in Human terms, with deadly force. Who is going to arrange that and pay for it?

And we cannot afford nuclear power, when it is at thirteen cents/kWh and wind and solar PV are less than four cents.
WillieWard
2.1 / 5 (7) Dec 18, 2015
nasty little secret is they vent radioactive gases .. and put them into the atmosphere for us to breathe
Another nasty little secret: wind/solar, rare-earth metals, "green" radioactive waste for our children to breathe
"consider that mining one ton of rare earth minerals produces about one ton of radioactive waste"
"wind industry may well have created more radioactive waste last year than our entire nuclear industry"
http://institutef...inerals/
http://www.bbc.co...on-earth
http://finance.to...age/full
http://www.mother...y-secret
Lord_jag
4.3 / 5 (6) Dec 19, 2015
As opposed to coal? Nothing radioactive in the raw dust from coal sent into the atmosphere, right?
SuperThunder
4 / 5 (8) Dec 19, 2015
Stanford scientist unveils 50-state plan to transform U.S. to renewable energy:
https://news.stan...414.html
Uncle Ira
3.7 / 5 (9) Dec 19, 2015
Stanford scientist unveils 50-state plan to transform U.S. to renewable energy:
https://news.stan...414.html


Well I for one would be glad if they started the ball rolling with replacing natural gas first. And oil too but that is not the problem in Louisiana like the gas is.
gkam
1 / 5 (7) Dec 19, 2015
" started the ball rolling with replacing natural gas first"
----------------------------------------------

Wow. That is a hard one, since it is the go-to alternative to coal. Unless it can be replaced with low-cost wind and solar PV, or some other "fuel", it will be difficult to stop.

Maybe the next time the gas sellers get together and raise prices, we will respond with better technology and stricter environmental protections.
Uncle Ira
3.8 / 5 (10) Dec 19, 2015
" started the ball rolling with replacing natural gas first"
----------------------------------------------

Wow. That is a hard one, since it is the go-to alternative to coal. Unless it can be replaced with low-cost wind and solar PV, or some other "fuel", it will be difficult to stop.

Maybe the next time the gas sellers get together and raise prices, we will respond with better technology and stricter environmental protections.


Given the pricing of gas, and the dirtiness of coal, I don't have any high hopes of seeing the drilling slacking up. I don't like the gas for personal reasons. Like you try to do with the nuclear plants, I wish we could give all the gas deposits away and let somebody else enjoy wells and the pipelines.

We lose a football field size piece of barrier wetlands every minute to the gas drilling. I would rather have the wetlands with nuclear than no wetlands with anything else. Peoples don't realize how important the wetland is.
Uncle Ira
3.8 / 5 (10) Dec 19, 2015
P.S. For you glam-Skippy. That is every minute of every day all year long and year after year. You can not restore wetlands or swamps like you do with reforestation after coal mining. The wetlands are too complicated for that.

Oh yeah, I almost forget. That is just what the problem is, the producers raising prices. They don't. Prices keep going down, and they drill more and more. I wish they would get together and raise the prices so high nobody could afford it.
gkam
2.7 / 5 (7) Dec 19, 2015
I understand wetlands. They are the most biologically-productive areas on Earth.

They are the margins, the buffer between us and the sea, between salt and fresh, aerobic and anaerobic. Much of land and sea life have parts of their life cycles in wetlands.
Estevan57
4.4 / 5 (7) Dec 20, 2015
Well I'll be hornswoggled. An actual mostly factual straight forward post from gkam.

The old dog can be taught! A sure sign of the Apocalypse is here!

ps. The ocean is both aerobic and anaerobic.

"Oxygen and aerobic life found all the way to the ocean's "basement"

http://www.gso.ur...asement/
gkam
1.6 / 5 (7) Dec 20, 2015
"Well I'll be hornswoggled."
-----------------------------------

Yes, and often.

If you had been educated, you would not have had to look it up.

TheGhostofOtto1923
4.3 / 5 (6) Dec 21, 2015
Well I'll be hornswoggled. An actual mostly factual straight forward post from gkam.

The old dog can be taught
This is because he googled it and paraphrased it from wiki.
https://en.wikipe.../Wetland

-Of course he would never admit to this because he is a lying, posturing psychopath IMO.

Georges phony MS didnt require any coursework to obtain and he apparently never got an undergrad degree.

And so his 'education' is a lie as well, apart from learning the advantages of posturing and paraphrasing.

"... they believe their own lies. After all their life is nothing but a lie, a sham, how can we possibly assume they know anything different."
gkam
1.6 / 5 (7) Dec 21, 2015
It really gets to the coward who hides behind the phony name of otto, when those he accuses of being like him turn out to be real people with real experiences, not spiteful little kids who think they have an OBE, kids who hide behind anonymity, too scared to use their real identities because of their adolescent behavior.

And no, Toots, I did not have to look it up on Wiki, and did not. I learned it in 1979 in a graduate-level course in Ecoscience.
TheGhostofOtto1923
4.3 / 5 (6) Dec 21, 2015
real people with real experiences
Sorry george I try not to engage with lying posturing psychopaths who dropped out of college, and who hide behind lies about their pasts for fun and profit.

Whats the point?
Estevan57
4 / 5 (4) Dec 23, 2015
Well Mr gkam , if you were educated you wouldn't need to be corrected.

I have enough education to know that your "facts" are usually wrong.

Was the ocean anaerobic in 1979? Graduate level, sure, but what grade school?

The link is for your benefit, not mine.

If you would spend more time listening to other people instead of compulsively posting random thoughts, you would learn that it is a common practice on Physorg.

gkam
1.7 / 5 (6) Dec 23, 2015
I wasn't corrected. They said I got it from Wiki. But I didn't.

Don't let your personal pique affect your judgment.
Whydening Gyre
5 / 5 (4) Dec 23, 2015
If you would spend more time listening to other people instead of compulsively posting random thoughts, you would learn that it is a common practice on Physorg.

Personally, I like posting IMpulsive random thoughts...:-)
Estevan57
4 / 5 (4) Dec 23, 2015
Pay attention gkam, I was talking about my correction of you, not Ottos.

Anaerobic, remember? Don't let your uh, judgement affect your judgement.

Good for you Widening G. have a good night/morning as the case may be.
gkam
1.7 / 5 (6) Dec 23, 2015
"Was the ocean anaerobic in 1979?"
--------------------------------

Of course there are anaerobic locations in the deep . Ocean life started out as anaerobic.
Uncle Ira
3.4 / 5 (5) Dec 23, 2015
Pay attention gkam, I was talking about my correction of you, not Ottos.

Anaerobic, remember? Don't let your uh, judgement affect your judgement.


What till he sneaks into the Wiki-Skippy's place and throws out obligate anaerobes and facultative anaerobes. Not that he will know the difference, but he will really love the sciencey way they sound.


Good for you Widening G. have a good night/morning as the case may be.


It was a good one Widening-Skippy. I am here hoping you are doing as fine and dandy as I am doing.
Uncle Ira
3.4 / 5 (5) Dec 23, 2015
"Was the ocean anaerobic in 1979?"
--------------------------------

Of course there are anaerobic locations in the deep . Ocean life started out as anaerobic.


Of course you would be wrong about that. There might be some anaerobic locations deep in the mud. You know, what the article is about? But in the deep, you got O2 dissolved into the water. The only anaerobes you find there are the "falcutative anaerobes" like I just mentioned to Estevan-Skippy. So no Cher, in 1979 the ocean (top or bottom) was not anaerobic.
gkam
1.7 / 5 (6) Dec 24, 2015
You have anaerobes living in your decaying swamps, giving it that rotten smell. They are all over, still in places where they can survive. The matter-fall that covers the deep seas decays into anaerobic matter.
Uncle Ira
3.7 / 5 (6) Dec 24, 2015
You have anaerobes living in your decaying swamps, giving it that rotten smell.


But the decaying stuffs is not anaerobic. There are some anaerobic microbes that can live in the aerobic environment. They are called facultative anaerobes. So one more time Cher, no part of the oceans is anaerobic. Or the swamps either. There might be some facultative anaerobes living in it. But it is still the aerobic environment. That is the finer point that Estevan-Skippy tricked you with.

The matter-fall that covers the deep seas decays into anaerobic matter.


Wrong again Cher. You did not even read the article,. did you Cher? Or are you just trying to drift away from the place where Estevan-Skippy gave you a little schooling?

Instead of pulling foolishments out of your butt, why you don't take Estevan-Skippy's really good advice. Take time off from trying show how smart you are and spend some time trying to look into things first.
gkam
1.7 / 5 (6) Dec 24, 2015
Out of my Ira?

What?
Uncle Ira
3.7 / 5 (6) Dec 24, 2015
Out of my Ira?


Well that would be better than using using your imagination all the time and getting all prickly when peoples have to correct your wrong blurts and blahs. At least ol Ira-Skippy tries to make sure what he postums is right. Don't you get tired of getting embarrassed by getting so much of the basic stuffs wrong all the time?

What?


Well here is what it is Cher. Now on those new-agey activist forums and down in front of the Starbucks you might be able to pretend to be really science smart by just making stuffs up because you "got a feeling this might be right". And get away with it, because most of the time, those new-agey types are more worried about appearances than they are about how things really are. Cher a lot of really smart peoples have tried to explain to you that a lot of real engineers, real science smart Skippys and peoples who really did go to the schools you pretend about are here. That is why you get caught looking foolish so much.
gkam
1.7 / 5 (6) Dec 24, 2015
This is a thread regarding PV technology. Got any panels?

My installation keeps being delayed because of rain. Who'd'a thunk?
Uncle Ira
4 / 5 (4) Dec 24, 2015
This is a thread regarding PV technology.


Well Cher, it was about that. Until you wanted to change the subject to anaerobes in the swamps. It's not my fault you got lost from the other article about anaerobic microbes. That's on you Skippy.

Got any panels?


I already told you in the extreme weather article postums. I got the 40kW generator with the Perkins diesel.

My installation keeps being delayed because of rain. Who'd'a thunk?


Well, I could have thunk it me. Why you didn't ask me.
gkam
1.8 / 5 (5) Dec 24, 2015
40kW?? Why would you have something so inefficient, with you using less than 10% of it's capacity?

But too much is way better than none.
Uncle Ira
4 / 5 (4) Dec 24, 2015
40kW?? Why would you have something so inefficient, with you using less than 10% of it's capacity?


Since I have had him I have only used about 0% of it's capacity. I fire him up every four or three months for 30 minutes or so just to keep the parts loose. I only get him for when we have another Katrina. I ain't going through the six weeks of no power again. Ever.

But when I do need him, I will probably use about 25 to 50% of his capacity not 10%. I have a 250 amp service from the power company, so it is not so much the over-kill. I got a pretty large house, total electric and the garage and shop out in the back. I can run it all without having to change my way of spending my day non. It's expensive yeah, but it is only for temporary times.

But too much is way better than none.


That's the way I see it too. Cutting things to close is begging for troubles and cost more in the end than starting with a little extra.

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