SolarReserve

As I reported in Tuesday’s paper, some companies think that one form of renewable energy is going to open a window for another — that solar photovoltaic panels, which convert sunlight directly into electricity, are going to increase the grid’s need for a type of solar energy that can be stored.

That would be solar thermal power, which harnesses heat from the sun and converts it to steam to make electricity as the need arises, especially when the sun has disappeared behind a cloud or dropped below the horizon.

Electricity is unique among major commodities in that it must be produced and consumed simultaneously. It can be stored in a battery, of course, but for now, that technology’s costs are so high that batteries are used mostly to smooth out production from renewable sources, not to save it for later.

The economics of a plant that can store bulk amounts of energy are a bit arcane. At the simplest level, the idea is to gather the sun’s heat when it is available and save it until prices for electricity reach a peak. At the moment, though, prices peak when the sun is high in the sky, because that is when the demand for power, mostly for air-conditioning, is highest. Some experts think it will be years before the power system is so saturated with solar photovoltaics that thermal storage becomes worthwhile.



“As the world exists now, what you’re doing with storage is taking high-priced peak potential generation and moving it to off-peak,” said George Sterzinger, director of the Renewable Energy Policy Project, a nonprofit group in Washington.

Yet some solar thermal plants with storage are already in service. in Europe — for example, a plant in Seville, Spain, built by Torresol Energy. The plant is small, just under 20 megawatts. And four are in construction or on the drawing boards in the American Southwest, as I explained in my article.

Their backers are betting that photovoltaics will get cheap and will drive down the price of electricity in daylight hours. But there are other reasons that energy storage might be a good deal from a financial point of view.

One is that the two biggest forms of renewable energy, wind and solar, have a tendency to gear up and then fall off very quickly, at least by the standards of conventional generators. If the rest of the system has to respond, then a lot of plants running on coal or natural gas would have to increase their output or cut it very quickly.

If the fraction of energy derived from renewable sources is small, that’s not a big problem; if solar makes up, say, 2 percent of production, and if it falls by half in a few minutes, the rest of the system can compensate. But if solar makes up 20 percent, the potential problem gets bigger.

Paul Denholm, a researcher at the National Renewable Energy Laboratory, in Boulder, Colo., recently estimated that 5 percent of annual photovoltaic production might have to be shut off because it came at the wrong time or introduced too much instability into the system. Adding storage, he said, could be worth 0.3 cents per kilowatt-hour. (That sounds small, but it’s an appreciable fraction of the national average retail price of a kilowatt-hour, which is around 11 cents.)

Worth even more is the value of a source that can be counted on to produce when needed, as opposed to when the sun is shining; that’s worth 0.7 cents to 2 cents, he calculated.

There are other ways to store electricity, but all of them incur costs, both for equipment and the energy. The “round-trip efficiency” of a solar thermal system – that is, the ratio of energy recovered compared with the energy invested – is in the range of 95 percent. That’s far higher than the ratio for the biggest conventional form of storage, pumped hydro, which involves pumping water up a hill and letting it turn a turbine to make electricity on the way down later.

Another technique is storing energy by compressing air. But with either of these, the energy being stored might have come from a coal-fired plant, which will not help the environment or help a utility meet its quota for renewable energy.



An earlier version of this post referred incompletely to the adoption of solar thermal plants with storage capability. Several such plants have opened in Spain; a plant near Seville is not the only one.