Solar power, the holy grail of renewable energy, has always faced the problem of how to store the energy captured from the sun's rays so that demand for electricity can be met at night or whenever the sun is not shining.
The difficulty is that electricity is hard to store. Batteries are not up to efficiently storing energy on a large scale. A different approach being tried by the solar power industry could eliminate the problem.
The idea is to capture the sun's heat. Heat, unlike electric current, is something that industry knows how to store cost-effectively. For example, a coffee thermos and a laptop computer's battery store about the same amount of energy, said John S O'Donnell, executive vice president of a company in the solar thermal business, Ausra. The thermos costs about $5 and the laptop battery $150, he said, and "that's why solar thermal is going to be the dominant form."
Solar thermal systems are built to gather heat from the sun, boil water into steam, spin a turbine and make power, as existing solar thermal power plants do - but not immediately. The heat would be stored for hours or even days, like water behind a dam.
A plant that could store its output could pick the time to sell the production based on expected price, as wheat farmers and cattle ranchers do. Ausra, of Palo Alto, Calif., is making components for plants to which thermal storage could be added, if the cost were justified by higher prices after sunset or for production that could be realistically promised even if the weather forecast was iffy. Ausra uses Fresnel lenses, which have a short focal length but focus light intensely, to heat miles of black-painted pipe with a fluid inside.
A competitor a step behind in signing contracts, but with major corporate backing, plans a slightly different technique in which adding storage seems almost trivial. It is a "power tower," a little bit like a water tank on stilts surrounded by hundreds of mirrors that tilt on two axes, one to follow the sun across the sky in the course of the day and the other in the course of the year. In the tower and in a tank below are tens of thousands of gallons of molten salt that can be heated to very high temperatures and not reach high pressure.
"You take the energy the sun is putting into the earth that day, store it and capture it, put it into the reservoir, and use it on demand," said Terry Murphy, president and chief executive of SolarReserve, a company backed in part by United Technologies, the Hartford, Conn., conglomerate.
Power plants are typically designed with a heat production system matched to their electric generators. Murphy sees no reason why his should. His design is for a power tower that can supply 540 megawatts of heat. At the high temperatures it could achieve, that would produce 250 megawatts of electricity, enough to run a fair-size city.
It might make more sense to produce a smaller quantity and run well into the evening or around the clock or for several days when it is cloudy, he said.
At Black & Veatch, a builder of power plants, Larry Stoddard, the manager of renewable energy consulting, said that with a molten salt design, "your turbine is totally buffered from the vagaries of the sun." By contrast, "if I've got a 50 megawatt photovoltaic plant, covering 300 acres or so, and a large cloud comes over, I lose 50 megawatts in something like 100 to 120 seconds," he said, adding, "That strikes fear into the hearts of utility dispatchers."
Thermal storage using molten salt can work in a system like Ausra's, with miles of piping, but if the salt is spread out through a serpentine pipe, rather than held in a heavily insulated tank, it has to be kept warm at night so it does not solidify, among other complications.
A tower design could also allow for operation at higher latitudes or places with less sun. Designers could simply put in bigger fields of mirrors, proponents say. A small start-up, eSolar, is pursuing that design, backed by Google, which has announced a program to try to make renewable electricity for less than the price of coal-fired power.
