For solar, the future is flexible

When you picture a solar cell, you probably think of the familiar blue and black panels framed with aluminum. These are made from silicon. They represent about 94% of all solar cell production worldwide.

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Silicon solar cells are extremely cheap to mass-manufacture, have moderate efficiencies (15-20%), and work excellently when installed on large, industrial rooftops – take, for example, the 30-acre installation Amazon turn on earlier this year.

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But silicon leaves something to be desired. Panels can be bulky and inconvenient, and the market structure around silicon doesn’t always lend the technology to flexible applications where solar energy has unique advantages over other energy sources.

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You may be surprised to learn there are actually many different kinds of solar cells technologies apart from silicon. Typically, each technology emphasizes one of two things: efficiency or flexibility.

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For example, 15-20% for silicon is okay if space isn’t a limitation, but you can get more power per square foot if you’re willing to spend more on superior materials or manufacturing techniques. Some examples include monocrystalline silicon, multi-junction, and gallium-arsenide technology.

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But maybe you don’t care as much about efficiency; instead, you’re looking for something cheaper and a bit more dynamic. A good example of this is thin film technology, such amorphous silicon, cadmium-telluride and copper-indium-gallium-diselinide (CIGS) solar cells.

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Then there is the really cool stuff: solar paint, solar fabric, and the like. These materials wouldn’t need to be very efficient at all if we had the ability to mass-manufacture them at low cost and high quality. We could put them on every exposed surface and integrate them into existing structures. The sun is shining down on us all the time – so we might as well take advantage of it wherever we can, right? Not to mention there would be no moving parts, no noise, and no emissions during operations.

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Thus, while the commercial industry remains dominated by silicon, the solar R&D space offers some interesting possibilities.

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In this blog, I want to examine emerging, innovative solar technologies apart from silicon in the context of the current solar market structure, and what advantages they might offer to the industry going forward. I also want to discuss how we can get transformative solar technologies faster to market.

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So, to begin: what’s going on in the solar industry today?

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The U.S. photovoltaic solar industry

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I’ll keep the focus of this article on photovoltaic (PV) solar, which converts light to electricity, as opposed to concentrated solar, which converts light to heat. PV solar currently accounts for 0.9% of all power production in the United States. The industry has grown rapidly from ten years ago, when solar accounted for only 0.1% of total power.

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That growth has happened thanks in part to policy incentives geared at encouraging solar technology investment, but also largely due to the sharp decrease in the price of silicon and advances in manufacturing.

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The main area of growth so far and going forward will be utility-scale installations – also known as “solar farms” – due to the price advantages of economies of scale. Let’s say we have a 1 MW solar farm, which takes up about 5 acres of land. The solar panels are made from silicon, and assuming they aren’t connected to a battery backup system, they would be collectively operating at 15-20% efficiency and producing power on an unreliable schedule, depending on available sunlight.

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The trouble with a 1 MW solar farm is it serves essentially the same purpose as a gas, coal, or nuclear which take up comparable amounts of space, but provide hundreds of times the power at a much higher efficiency and reliability.

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This makes me think: why would I even bother with solar?

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At least in the market for utility-scale installations, solar needs to be much, much cheaper than these other sources in order to compete – period.

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But that’s exactly what the solar industry has managed to do in recent years, as manufacturing and installation costs took a nosedive.

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If we look at where things stand now, however, costs are levelling out – and so is solar’s growth. Using current technology, and without cheap battery technology and other updates to the grid to solve reliability issues, solar is unlikely to ever exceed, say, 5-10% of total U.S. power production. For those who envision a world with higher penetration of renewables from solar power, silicon solar has a difficult path to carve.

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One avenue to expanding solar penetration is to move beyond utility-scale installations – and instead let solar do what it’s good at; and arguably, what it’s meant for.

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Solar is unique among energy sources

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Where solar has totally unique advantages over other sources of energy is in scalable and flexible applications. This could happen (and in some places, is happening) in a couple of different ways.

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In terms of being scalable, the obvious one is backup or off-grid power. If I’m out in the boonies without power – but I don’t actually need that much power – solar is perfect. I’m not capable of constructing a gas, coal, or nuclear plant on my own, but solar enables me to take my power needs into my own hands. Existing silicon technology is pretty good for that, too; I just need cheap, efficient batteries to store the power for the time I need it. These, too, are in the R&D pipeline, and are already economic enough to replace diesel backup generators in some markets.

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Still, in the United States and developed countries where the grid is generally pretty reliable, solar doesn’t have a huge market.

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But solar has even greater potential. And this finally brings me to the fun stuff.

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What if we had a type of solar cell that didn’t look like those clunky silicon at all – something truly transformative? Something that really only solar can do?

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Emerging PV emphasizes flexibility

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Transformative solar technology will not look anything like today’s clunky silicon cells, and applications will likewise be completely different. These innovative materials will go into versatile applications like solar paint that could go on your house, and solar fabric that could be woven into military fabric, or even handbags. Or maybe they won’t necessarily be physically flexible -- but will have the ability to adapt to their environment, like electrochromic windows.

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The National Renewable Energy Laboratory loosely categorizes this group of solar technologies as “Emerging PV”. Emerging PV materials come in a variety of forms. Most are not particularly efficient, but they aren’t meant to be. Instead, they are meant to be flexible. Most can be made using simple solution-based benchtop chemistry – as opposed to the energy-intensive manufacturing techniques used for silicon. Many emerging PV materials also display the rather interesting characteristic of being tunable to absorb different energies of sunlight.

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Apart from low efficiencies and lack of mass manufacturing experience to bring down costs, their main disadvantage is instability. They tend to fall apart when exposed to the elements.

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Below I’ve briefly described a few leading types of emerging PV:

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Dye-Sensitized Solar Cells (DSSCs)

• What are they? DSSCs use organic dyes absorb different colors of light.

• Commercial status: advanced demonstration, some commercial products for sale

• Pros: They are very easy to make. If you don’t believe me, watch this video to learn how to make one from a powder donut and some Starbucks passion fruit tea. DSSCs also have relatively high efficiencies of up to 11%. Plus, they look pretty.

• Cons: Their composition involves use of a liquid, which can freeze in low temperatures and expand in high temperatures, making them unstable and prone to leakage. They are limited to absorbing light in the visible spectrum.

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Quantum Dot Solar Cells (QDSCs)

• What are they? Born out of DSSC research, these replace dyes with “quantum dots” - particles of matter only a few nanometers in size with tunable optical properties.

• Commercial status: under development, no commercial products

• Pros: They’re easy to make, can absorb light across the entire spectrum, and can be tuned to different sizes to absorb different wavelengths of light. They have very high theoretical efficiencies, upwards of 60%.

• Cons: The best laboratory efficiencies are still prohibitively low, around 4-5%. Also, the best QDSCs use toxic materials, like cadmium. They often use liquids like DSSCs and display similar stability problems.

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Perovskite Solar Cells

• What are they? Materials that use perovskite crystal structure to absorb light.

• Commercial status: under development, no products

• Pros: Perovskite efficiency has grown very rapidly, from only 3% in 2009 to over 22% today, garnering them significant attention. They are cheap and easy to produce, and don’t involve liquids like QDSCs and DSSCs.

• Cons: The cells break down easily, especially when exposed to heat, humidity, and other environmental factors. They contain a small amount of lead.

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These technologies all show promise. But we still have a ways to go before we can bring them widely to market.

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So, what’s the next step? Lots of R&D is going on, but where’s the tech transfer? How can we get these technologies to the point where they can completely transform the way we use energy?

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The technology transfer disconnect

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Since many emerging PV applications are a far cry from silicon, conventional solar manufacturing and installation companies aren’t necessarily looking at these technologies. Instead, their attention tends to be focused elsewhere: on addressing manufacturing and balance of system costs.

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Moreover, in the policy space, while tax credits for solar cell manufacturing and installation have helped conventional silicon, I would argue they haven’t been nearly as helpful for emerging PV and non-traditional solar applications.

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It’s hard to know what kinds of policies would help emerging PV, since their applications are so broad. Moreover, most of the challenges are technical. This means, simply, that more R&D is needed. According to a report my MIT, R&D focused on cheap, scalable applications and storage technology is crucial to attaining PV penetration at a large scale.

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And while university R&D is great, if the end goal is commercialization, there needs to be emphasis on technology transfer – which means a breakthrough will most likely come from the right company. Good policy would incentivize commercial R&D and corporate-university partnerships.

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As for which companies will do this, large, innovative materials companies, such as Dow Corning might be able to. Then of course there is Tesla, with its new line of solar shingles as a teaser. There are also countless of startups that have tried and failed to commercialize emerging PV – some of them with good ideas, and some with bad.

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It’s also possible, but less likely, a breakthrough could occur through research in historically high-end solar applications. For example, Lockheed Martin and others are looking at emerging PV for satellites. Cost may not be as important to them, but they could still solve major technical problems that benefit broader emerging PV applications.

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Conclusion

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Many solar advocates continue to bet on silicon and cheap storage to take the solar industry to the next level. Expanding existing solar tax credits to focus more on storage, for example, might help incentivize cheap storage that could complement cheap solar cells for conventional applications and help solar to maintain its impressive trend of growth.

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But if you haven’t guessed already, I personally don’t find solar farms and the like very interesting. I think I can do the same thing better with other power sources, and I’m not convinced energy-intensive silicon and battery manufacturing is “green”.

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Plus, I have reason to believe there is some serious untapped demand for emerging PV. After all, who doesn’t want solar clothes and a solar house made from donuts and passion fruit tea?

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In all seriousness, though, I think emerging PV has a shot, but it will take more than getting excited about flashy solar articles in the news. Instead, we should focus our attention on encouraging research to solve the right technical problems and addressing barriers to technology transfer. It would be a shame not to take advantage of all that free fuel.

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A special thanks to my friend Tim Siegler who works on perovskite R&D at the University of Texas and helped me with this article (although he may not agree with all my views).