Green

Fixing solar's intermittency: Do SolarReserve and PG&E have the answer?

Pacific Gas & Electric has signed a contract with SolarReserve to buy power from its 150-megawatt plant. Power purchase agreements like this are pretty commonplace at this point — so what makes this one special? SolarReserve claims its technology can supply solar-generated power even when the sun’s not shining — revolutionizing the way we look at solar.

SolarReserve relies on solar thermal systems to make this possible. Using a field of reflective mirrors, it concentrates sunlight on a central receiving tower. The intense heat, topping 1,000 degrees Fahrenheit, is stored in molten salt in this tower. This salt solution is then funneled into a storage tank where it can sit there, glowing hot, for hours, before being used to generate steam to turn a turbine. The salt can retain useful amounts of energy for up to a week after it was last exposed to the sun — essentially taking care of solar’s intermittency problem.

If this works, it could be a huge boon for the solar thermal industry (makers of photovoltaic panels, not so much). The chief reason that solar hasn’t become more widespread is that the energy it produces is too inconsistent. People need to be able to trust that when they flip a switch, lights will go on — even at night. SolarReserve could help deliver “on-demand” solar.

But it didn’t accomplish this all by itself. Its system resulted from partnerships with United Technologies and its subsidiary Rocketdyne — the folks who brought us the Saturn series of rocket engines. They developed the molten salt energy storage concept as a spin-off of their rocketry programs. First proved a decade ago at the Solar Two facilities outside Barstow, Calif., the technology hasn’t been deployed commercially, until now, by SolarReserve.

Screen shot 2009-12-23 at 10.26.39 AMThe company is also working on a 100-megawatt plant in Nye County, Nev., which will sell power to Nevada Energy for the next 25 years. If this facility, and the one slated to supply PG&E (which has yet to win approval from the California Public Utilities Commission), are successful, SolarReserve could be the first to provide emissions and radiation-free solar systems that work 24 hours a day.

If solar’s intermittency problem can be fixed with thermal solutions, there may also be less need for grid-scale battery storage. Though batteries still seem to be the best storage solution to make wind a consistent source of alternative energy too.