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Solar Dawn

Pressure is rising in the effort to bring photovoltaics (PV) into cost parity with other power sources.

In the intensifying search for sustainable power options, solar energy is emerging as a viable solution. Many utility companies are investing in utility-scale solar plants, and the growth outlook looks strong -- especially for PV plants, which promise rapid and steady improvements in price and performance.

Solar generation capacity currently represents less than 1 percent of the nation’s power assets, but as PV costs drop, solar will inevitably become a bigger part of the mix. PV, of course, is not a single technology, but an ever-growing cluster of methods to capture solar energy, and each comes with its own set of variables, pros and cons. National Renewable Energy Laboratory (NREL) analyst Michael Woodhouse spoke with Fortnightly’s Green Utility about the current state of PV research and the ongoing effort to drive down cost.

GU: How many different photovoltaic technologies are you currently exploring at NREL?

MW: NREL is a spawning ground for a lot of PV technologies -- everything from incumbent technologies that you see in the field now, to next generation materials and systems that aren’t commercially available yet. Most products that a consumer would currently buy are made out of crystalline silicon. We have research groups working on that system. We also have groups looking at new materials for the thin-film systems, including cadmium telluride and copper indium gallium selenide, or CIGS. We have organic PV research groups. There’s one for next generation concepts, such as quantum-dot solar cells. There’s a dye-sensitized solar cell research group. So we’re looking at the whole gamut of PV technologies that you could think of, and in a lot of cases companies have sprung out of that research.

GU: Don’t some to the materials you just mentioned present sourcing problems, either because of their scarcity or geopolitical issues?

MW: Crystalline silicon, which currently accounts for 88 percent of the market, is certainly earth-abundant. All you need to make that is high quality quartz or sand. When it comes to the thin-film technologies, there are concerns about cadmium telluride, because tellurium that goes into making that presents a potential supply problem in the long term. The indium that goes into making CIGS is potential issue to look at as well. That’s not to say that materials availability is always going to be a problem for PV, because there are always alternative materials being developed. If we run into a supply problem with CIGS, for example, a good replacement technology may be copper zinc tin sulfide, or CZTS. That said, it’s still too early to say whether materials availability will be a significant problem or not.

GU: What about toxicity?

MW: Usually the one that people talk about there is cadmium telluride, because they have concerns about cadmium. A couple of points are worth noting about that: First of all, there’s more cadmium in a NiCad battery, which are all over the place, than there is in a cadmium telluride solar panel. The cadmium telluride in a solar cell is really thin -- usually something along the order of three micrometers, which is a fraction of the thickness of a human hair. So there’s actually not much material there to begin with. Also, it’s in a solid phase, in a compound phase with tellurium, so it’s not a liquid that leaches. There doesn’t seem to be a lot of reason to think that it poses a significant problem.

GU: Of course the biggest question is cost. We’ve seen a consistent reduction in PV cost per Watt as manufacturing capacity increases. Is there any reason to expect that relationship to plateau?

MW: Heck no. We’re far from reaching the bottom of cost reduction on a dollars-to-Watt basis. It’s going to continue to go down for quite some time. Ultimately the question that we’re kind of poking around here is: When do solar energy costs become the same as costs from fossil fuels? The answer to that is really regionally specific. Siting is one issue. The price of electricity in that region is another, as is the quality of the solar resource in that region. When you have a place that has a combination of expensive traditional electricity, and good solar resources, that’s where PV makes the best sense. In Europe the market that clearly seems to fit that description is Italy. Italy pays a lot for electricity, and they have a good solar resource. PV would compete on a price basis there, even if there were no subsidies. In the U.S., places like Hawaii and much of California are the best bet right now, but that will spread. Solar costs will continue to come down, while fossil fuel costs continue to rise, which means that the number of markets that reach that grid parity is only going to increase over time.

GU: Are you saying that grid parity already exists in Southern California?

MW: It depends on how you define that. In a lot of California they have a tiered pricing structure for their electricity; a base rate around 12 cents a kilowatt hour, then depending on how much energy a business or home consumes, that can spike up to around 40 cents a kilowatt hour. PV is something on the order of 20 cents a kilowatt hour, give or take five cents. So if you’re installing a PV system that can produce electricity at that rate, there are a lot of cases where it’s arguably competing with fossil fuels. And that’s even taking subsidies out of the equation.

GU: Do issues such as storage and intermittency mean PV is not really a viable base-load source?

MW: Well, with the right storage solution it actually could provide base load power. Right now pumped hydro is the cheapest on the storage scale. That’s another plus in the utility column -- the storage costs are considerably lower. Pumped hydro or pumped air storage -- another possibility -- are cheaper storage technologies. [Editor’s note: In the video above, Woodhouse discusses the prospect of using PV to generate hydrogen for purposes of fuel production and energy storage.]


GU: Recently we’ve seen many announcements for utility-scale solar PV plants. This is a notable change from the past, when most PV projects involved residential and commercial rooftops. Do you expect that trend to continue?

MW: It’s a tricky question. There are a lot of factors, and I don’t think that there’s a clear case yet which one necessarily makes the most sense. Each system has its pluses and minuses. For instance, utility scale PV costs are lower than a residential installation. It’s quicker to install the modules, you’re buying a lot of modules in volume, so you get a bulk discount. However, when you’re doing a utility plant, you’re not competing with residential rates. You’re competing with wholesale electricity rates. So by the very nature of it, utility costs would have to be half the cost of residential, because wholesale electricity rates are roughly half the rate of residential. The other thing is that utility operations sometimes have siting issues, or face additional expenses like building transmission lines. Some other pros in the utility column is that for PV, you can utilize trackers, which track the sun, which will help to improve the capacity factor of the PV system by 30 percent, roughly, depending on the location. The other advantage with the tracking system, which you can only do at a utility scale, is you can actually better match the peak power loads using tracking than you can with fixed tilt solar.

GU: Aren’t some of the newer PV technologies more efficient at oblique angles than the existing ones?

MW: Yes, there is that difference. Trackers are used on more of the higher efficiency crystalline silicon modules, and those benefit more from the direct radiation that the tracker is meant to provide. But in terms of what makes sense on an economic basis ... trackers do cost money. In the case of some of the thin film technologies, it actually makes sense to just keep them fixed in place. When you calculate it on a dollars-per-kilowatt hours basis -- comparing a higher-efficiency crystalline silicon module with a tracker versus a fixed-plate thin-film technology -- it seems to be dependent on location.

This is just the stuff we’re thinking of in 2011. Who knows what’s going to come along in the future? The sun has been the energy source forever -- even fossil fuels were initially provided by the sun. There’s not an energy problem we can solve without the sun. It’s just a question of cost, and we work constantly to drive down cost.