New ways to store solar energy for nighttime and cloudy days

[hari]

Worried about frequent power cuts? 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 dollar 5 and the laptop battery dollar 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, California, 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.

"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 Solar Reserve, a company backed in part by United Technologies, the Hartford conglomerate.

Power plants are typically designed with a heat production system matched to their electric generators. Mr Murphy's 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.

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, e-Solar, 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.