Solar Cells for Cheap

Not everyone gets a solar cell named after them: but Michael Gratzel did. He says his novel technology, which promises electricity-generating windows and low manufacturing costs, is ready for the market.

Michael Grätzel, chemistry professor at the Ecoles Polytechniques Fédérales de Lausanne in Switzerland, is most famous for inventing a new type of solar cell that could cost much less than conventional photovoltaics. Now, 15 years after the first prototypes, what he calls the dye-sensitized cell (and everyone else calls the Grätzel cell) is in limited production by Konarka, a company based in Lowell, MA, and will soon be more widely available.

Grätzel is now working on taking advantage of the ability of nanocrystals to dramatically increase the efficiency of solar cells.

Technology Review asked him about the challenges to making cheap solar cells, and why new technologies like his, which take much less energy to manufacture than conventional solar cells, are so important.

Technology Review: Why has it been so difficult to make efficient, yet inexpensive solar cells that could compete with fossil fuels as sources of electricity?

Michael Grätzel: It's perhaps just the way things evolved. Silicon cells were first made for [outer] space, and there was a lot of money available so the technology that was first developed was an expensive technology. The cell we have been developing on the other hand is closer to photosynthesis.

MG: That has to do with the absorption of light. Light generates electrons and positive carriers and they have to be transported. In a semiconductor silicon cell, silicon material absorbs light, but it also conducts the negative and positive charge carriers. An electric field has to be there to separate those charges. All of this has to be done by one material--silicon has to perform at least three functions. To do that, you need very pure materials, and that brings the price up.

On the other hand, the dye cell uses a molecule to absorb light. It's like chlorophyll in photosynthesis, a molecule that absorbs light. But the chlorophyll's not involved in charge transport. It just absorbs light and generates a charge, and then those charges are conducted by some well-established mechanisms. That's exactly what our system does.

The real breakthrough came with the nanoscopic particles. You have hundreds of particles stacked on top of each other in our light harvesting system.

TR: The dye absorbs the light, and the electron is transferred to the nanoparticles?

TR: The image of solar cells is changing. They used to be ugly boxes added to roofs as an afterthought. But now we are starting to see more attractive packaging, and even solar shingles (see "Beyond the Solar Panel"). Will dye-sensitized cells contribute to this evolution?

MG: Actually, that's one of our main advantages. It's a commonly accepted fact that the photovoltaic community thinks that the "building integrated" photovoltaics, that's where we have to go. Putting, as you say, those "ugly" scaffolds on the roof--this is not going to be appealing, and it's also expensive. That support structure costs a lot of money in addition to the cells, and so it's absolutely essential to make cells that are an integral part.

[With our cells] the normal configuration has glass on both sides, and can be made to look like a colored glass. This could be used as a power-producing window or skylights or building facades. The wall or window itself is photovoltaicly active.

TR: The cells can also be made on a flexible foil. Could we see them on tents, or built into clothing to charge iPods?

MG: Absolutely. Konarka has a program with the military to have cells built into uniforms. You can imagine why. The soldier has so much electrical gear and so they want to boost their batteries. Batteries are a huge problem--the weight--and batteries cost a huge amount of money.

Konarka has just announced a 20-megawatt facility for a foil-backed, dye-sensitized solar cell. This would still be for roofs. But there is a military application for tents, and Konarka is participating in that program.

MG: I expect in the next couple of years. The production equipment is already there. Konarka has a production line that can make up to one megawatt [of photovoltaic capacity per year].

TR: How does the efficiency of these production cells compare with conventional silicon?

MG: With regard to the dye-cells, silicon has a much higher efficiency; it's about twice [as much]. But when it comes to real pickup of solar power, our cell has two advantages: it picks up [light] earlier in the morning and later in the evening. And also the temperature effect isn't there--our cell is as efficient at 65 degrees [Celsius] as it is at 25 degrees, and silicon loses about 20 percent, at least.

If you put all of this together, silicon still has an advantage, but maybe a 20 or 30 percent advantage, not a factor of two.

MG: A factor of 4 or 5 [lower cost than silicon] is realistic. If it's building integrated, you get additional advantages because, say you have glass, and replace it [with our cells], you would have had the glass cost anyway.

MG: People say you should be down to 50 cents per peak watt. Our cost could be a little bit less than one dollar manufactured in China. But it depends on where you put your solar cells. If you put them in regions where you have a lot of sunshine, then the equation becomes different: you get faster payback.

TR: Silicon cells have a head-start ramping up production levels. This continues to raise the bar for new technologies, which don't yet have economies of scale. Can a brand-new type of cell catch up to silicon?

MG: A very reputable journal [Photon Consulting] just published predictions for module prices for silicon for the next 10 years, and they go up the first few years. In 10 years, they still will be above three dollars, and that's not competitive.

Yes, people are trying to make silicon in a different way, but there's another issue: energy payback. It takes a lot of energy to make silicon out of sand, because sand is very stable. If you want to sustain growth at 40-50 percent, and it takes four or five years to pay all of the energy back [from the solar cells], then all of the energy the silicon cells produce, and more, will be used to fuel the growth.

And mankind doesn't gain anything. Actually, there's a negative balance. If the technology needs a long payback, then it will deplete the world of energy resources. Unless you can bring that payback time down to where it is with dye-cells and thin-film cells, then you cannot sustain that big growth. And if you cannot sustain that growth, then the whole technology cannot make a contribution.

MG: The silicon people need to make silicon out of silicon oxide. We use an oxide that is already existing: titanium oxide. We don't need to make titanium out of titanium oxide.

TR: An exciting area of basic research now is using nanocrystals, also called quantum dots, to help get past theoretical limits to solar-cell efficiency. Can dye-sensitized cells play a role in the development of this approach?

MG: When you go to quantum dots, you get a chance to actually harvest several electrons with one photon. So how do you collect those? The quantum dots could be used instead of a [dye] sensitizer in solar cells. When you put those on the titanium dioxide support, the quantum dot transfers an electron very rapidly. And we have shown that to happen.

TR: You are campaigning for increased solar-cell research funding, and not just for Grätzel cells.

I am excited that the United States is taking a genuine interest in solar right now, after the complete neglect for 20 years. The Carter administration supported solar, but then during the Reagan administration, it all dropped down by a factor of 10. And labs like NREL [National Renewable Energy Laboratory in Golden, CO] had a hard time surviving. But I think there is going to be more funding.

Which is to say you would have added power (a very small intermittent and expensive amount) but not capacity to the grid.

So what? You said, ""If Silicon cells were *FREE* they would NOT be a viable alternative for our energy needs." The thing is that most people at least aren't looking at solar energy as an alternative to all our energy needs, only part. If I could get a solar electric system set up at my house on the cheap and significantly reduce my electric bills, I'd be all for that, as would most people.

You still require that real power plants capable of on demand generation be built.

If some breakthrough comes along that makes home solar electric systems affordable it will still take years for that technology to proliferate. A lot of plants are already perfectly capable of demand generation already, and new plants will have to be built and old plants will have to be upgraded in the future anyway, so having to build or modify generating facilities isn't that big of a deal. A lot of the problem could be solved just by energy storage systems and getting creative in how we manage our existing electricity generating facilities. For instance, we have a lake house on a lake with a dam that generates electricity. Because of a boom in population density and hot and dry conditions in recent years, the lake is getting really shallow in the summer time as people try to beat the heat running their air conditioners. Even though our dock is set up such that it can handle a good deal of fluctuation in the water levels we've had to disconnect our dock from the permanent moorings on the creek bank and let it float farther out in the creek in the summers in recent years or it would have ended up laying on the hillside. If people could set up cheap solar electric systems on their homes in the area and reduce the amount of electricity they are drawing from the grid during the hot summer days, we wouldn't have the problem of the lake getting too shallow. Not only that, but as more people move into the area and energy needs grow, if enough were supplementing their grid power with solar power to cover most of their daytime needs in the summer, all we'd have to do is generate from the dam at night when the solar panels aren't getting any sun rather than during the day. We wouldn't have to build some new coal or natural gas fired power plant to keep up with growing demand for electricity.

Uh, if the energy density was higher you could get by with a single small panel.

Again, so what? If I could get a 160 watt panel for $800 or two 80 watt panels that together take up twice the space of one 160 watt panel, but I could get those two 80 watt panels for a total of $200, it would be a good deal. I'd be paying $1.25 per rated peak watt as opposed to $5.00 per watt. I could only produce half as much electricity with a roof full of those as I could with a roof full of the expensive 160 watt panels, but my up front costs would be considerably less, and I'd be able to pay off my initial investment a lot faster with the money I'd save on my electric bill and every bit of savings after that would just be an increase of disposable income for my family.

So again, when it comes to important matters, like not freezing to death, you choose an energy source with a much, much higher energy density.

Why is it that we have to pick one single energy source and rely solely on that? I'd use gas for heat because it is a lot more efficient than electricity when it comes to heating your home, and because that's what I already have and it would be stupid to spend the money to replace my gas furnace with less efficient. I'd keep my gas dryer too, and my gas water heater.

An advancing civilization requires increased energy consumption.

There has to be a point when energy consumption does not need to increase for societies to advance. Third world type countries are going to have to use a lot more energy to catch up with the big industrial nations, but our country already uses something like 26% of the energy consumed in the world even though we have less than 5% of the world's population. We waste an awful lot of the energy we consume too. You know, looking into solar power has been a learning experience for me. I had no idea how much energy I waste. You can find sites all over the web where people are chronicling their experiences with powering their homes with solar and/or wind power, and one thing you'll see in common with all of these people is that they realize that before going to solar and/or wind power they were wasting an enormous amount of energy. Those going off the grid really have to cut down on their energy consumption and those with grid-tie systems end up really cutting down on their electricity consumption too because they're trying to pay off the huge costs of their new systems. They insulate their homes better, turn of lights and other appliances when they aren't using them. They analyze their energy consumption and look for where they are wasting electricity. They replace all the incandescent light bulbs they can with compact fluorescent bulbs. They turn off lights they don't need on, disconnect all those appliances and wall warts and other things we have in our homes that draw power even when not being used. They learn to live with a little warmer temperatures in the summer and a little cooler temperatures in the winter, and they don't leave their heaters and air conditioners on all day when they are at work like so many of us do. They become watt misers and often end up cutting their energy consumption by half or more just doing things not so hard or expensive to do.

One thing I think would happen if a lot more people started using solar power or some other means of generating electricity at home is that collectively we would all start thinking a lot more about how much energy we waste and we would reduce our consumption. If you invest a lot of money to generate your own electricity you're probably going to be paying a lot more attention to how that power is getting used. Those who manufacture things that use electricity will see this and pay a lot more attention to making products that are energy efficient. New home builders would build much more energy efficient homes and homes that are not energy efficient would sell for a discount. In time we might actually reduce our per capita energy consumption, and be as productive or more productive than ever.

Would building a lot of nuclear power plants really reduce our need on foreign oil? Most of our electricity is generated with coal or natural gas.

I like nuclear energy too (as long as it's not in my backyard). It is good clean energy, but what nuclear waste does come from it is extremely toxic and it remains highly toxic and dangerous for thousands of years, and man made or natural disasters at nuclear plants can lead to some pretty horrible results. The quantity of toxic waste produced is relatively small though and nuclear plants have become very safe in this country. We can handle it. I'm not so much worried about us. What worries me are the other guys. If we start building a whole lot of new nuclear plants everyone else is going to want to build them too and we're going to have a hard time stopping all these little countries out there from going nuclear. Some of them will enrich their uranium. That's inevitable, and we'll have more nuclear weapons to worry about. Also, we don't know how good their safety track records will be and how well they're going to handle their nuclear waste. What's going to happen when their governments fail and crazies take over or when they have wars and nuclear plants everywhere start getting targeted? There will be a lot more nuclear plants in the world and there will be a lot less control over them. That scares me just a little.

I was in the Army in Germany when the Chernobyl incident occurred and several in my unit went out and checked radiation levels in the days and weeks after the accident. I personally saw levels much higher than were being reported in the news. That really kind of scared me, already being one who had grown up during the Cold War and having put more than a little thought into the prospect of nuclear war, radiation poisoning, and that sort of thing. I personally wouldn't want to live anywhere near a nuclear plant, or anywhere near a big city that would be a target for nuclear attacks for that matter. I'm sure nuclear plants are just as safe as can be in this country but I don't want one near my house, and I'd really hate it if I was living near the Mexican border and there was a Mexican nuclear plant not far from me. Think about that.

It amusing that you mention being nervous about living near a nuke plant. We took our kids on a field trip up to Seabrook Nuke plant in NH with a group of homeschoolers. One of the questions asked was were people worried about living nearby. The man showed us aerial photos of the beach area where the inflow-outflow pipes of the plants were located. When the plant was built, about 20 years ago, the beach was almost empty. Now there are a scad of high end homes within a few blocks of the pipes. Obviously, those folks are not worried. Having seen how the plants are constructed, I'd have no worries about living near one.

Chernobyl was nothing like anything in this country in its style of contruction, so there are no concerns about that type of accident. Seabrook was phenomenal in the level of construction for security purposes. The reinforcing steel within the concrete walls and dome are 4" thick! There's no way a plane crashed into it would do any damage to the structure; the plane would simply disentegrate. Even with the accidents that have happened, there hasn't been any danger to the general public. Three Mile Island didn't release any real levels of radiation outside the plant, regardless of breathless reporting to the contrary. As a bumper sticker reads: "More people have died in Teddy Kenndey's car than in any nuclear power plant accident in the US."

The biggest reason nuclear hasn't gone further than it has is because folks just don't understand the process, and were easily manipulated and frightened by the rhetoric of the anti-nuke crowd.

From what I've read there is space to store the spent nuke fuels safely, again it's just an education process.

If Silicon cells were *FREE* they would NOT be a viable alternative for our energy needs.

There is no single viable alternative. Twenty years from now, I believe our power generation will be more scattered and diverse, with more and smaller plants using a variety of sources; still fossil fuels, mostly, but with solar, wind, tidal, biomass, and stuff we haven't thought of yet or I'm not thinking of just now.

Photovoltaics are useless on cloudy days or at night, and even under ideal circumstances they're not efficient and don't do much. But make them cheap and durable enough to use as roof shingles, and you've got something.

Fire up Google Earth, skim over any city or town, and check out all that rooftop space with sunlight beaming down on it. That's free energy that's either reflected back or absorbed as heat, which the A/C has to combat. Solar doesn't have to be very efficient to be useful on a scale that huge.

All roofing materials will eventually have to be replaced. The next time I have to replace the roof, if I could spend $800 for new asphalt shingles or $1200 for solar shingles, it's worth the extra money even if it only knocks 10% off my energy usage. Much more so in desert climates where they get a lot of sun and use a lot of power in daylight hours, and even more near the poles, where they get sunlight for most of the day half of the year.

The one and only problem with solar energy, under current conditions, is price. The energy itself is free. Make the cells cheap enough and durable enough, and it will become cost-effective, because there's enough available surface area to reap a lot of energy even with poor efficiency.

The average household would need hundreds of square meters of cells

My house has hundreds of square meters of roof with the sun beating down on it in the daytime. You know what that does? It makes my attic real damn hot. Make solar cells cheaper, with the input energy free, and that changes the math.

I live in the mountains and have modest power needs. I've run the numbers on solar and quickly concluded that even the battery banks required for modest power were cost prohibitive.

Battery banks? You only need that if you're trying to go completely off the grid. If you can get electricity for free 12 hours a day and pay for it the other 12, isn't that better than paying all 24?

You're right that energy storage is a legitimate issue with solar, but there are plenty of ways to store energy other than electrical batteries.

If your solar panels are generating more power than you need in the daytime, which is an unlikely occurrence, apply the excess power to pump water uphill; after the sun goes down, it can flow back downhill and drive a turbine.

Someone more clever than I could probably come up with something using coiled springs, pendulums and counterweights -- in a word, clockwork -- to capture mechanical energy and release it over time. Excess energy winds it up when the sun is out, and the clock releases it when it's dark.

Or the excess electricity could be used for electrolysis, and the H2 and O2 recombined later to run a generator, create heat or drive a vehicle.

The biggest mental shift in thinking about energy is going to have to be that we stop looking for one silver bullet. Until and unless we can build cheap and safe fusion reactors all over the place, it just ain't out there.

We've got to think more flexibly, be more nimble, and be able to take what's available where it is -- whether it's sunlight, wind, tides, coal, petroleum, methane, biomass, fission or fast-moving rivers -- and ramp one up and the others down as the economics shift.

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