Solar Energy: What To Do When the Sun Sets

A big solar developer makes a major move toward storing electricity

BrightSource Energy

Solar-thermal plants use mirrors or "heliostats" to focus sunlight on a tower receptor that produces steam to generate electricity.

A major barrier for solar power has always been that it doesn’t work at night (Duh). A few years ago, developers of big “utility-scale” solar projects were able to shrug this off to some degree. But Oakland-based BrightSource Energy has reversed field and decided to add to several projects the ability to store electricity for distribution after dark.

BrightSource managers say times have changed. Where utilities once wanted all the renewable capacity they could get, to meet state requirements, the priority has since shifted to having those renewable electrons available when they’re needed.

“The challenges of integrating photovoltaics and wind into the grid have driven a much deeper appreciation for those that can be highly reliable,” BrightSource CEO John Woolard told me in a phone interview.

But another driver is — well — us. When I interviewed Woolard a couple of years ago, I asked him why his company wasn’t including storage technology in its California projects. He said it wasn’t needed in California, which had a different pattern of electrical use than, say, Arizona.

That’s changing. Woolard says peak demand, which has traditionally hit around 4 p.m., has been shifting to later in the day, and by the end of this decade, will probably happen around 6 p.m. He says changing lifestyles are behind the shift, such as when people arrive home and fire up their air conditioners and other appliances.

BrightSource says it will use a molten-salt technology to store the power, rather than huge banks of batteries or experimental technologies such as flywheels. “That’s a solution for 2050 or 2060,” Woolard told me, “depending on whether you’re an optimist or a pessimist.”

Molten_salt_system

Here’s a good summary of how it works, from The Energy Blog:

“The molten salt, with properties like water at temperatures above its 240oC (464oF) melting point, is pumped from a large storage tank to the receiver, where it is heated in tubes to temperatures of 565oC (1049oF). The salt is then returned to a second large storage tank, where it remains until needed by the utility for power generation. At that time, the salt is pumped through a steam generator to produce the steam to power a conventional, high-efficiency steam turbine to produce electricity. The salt at 285oC (545oF) then returns to the first storage tank to be used in the cycle again.”

BrightSource will add molten-salt units to three of its projects in Riverside and San Bernardino Counties, but not to its Ivanpah Valley project, already under construction near the Nevada border. Spokesman Keely Wachs says that the added cost will be “fairly nominal,” and that expanding the plants’ operating hours will, in effect, reduce the price of energy from those plants.

CEO Woolard says that despite plunging prices for conventional photovoltaic solar panels, BrightSource will not be joining a trend among developers to convert some of their giant solar arrays to PV. He says PV output tends to peak at noon (PV power is a function of light, not heat), so their output is falling just as demand is rising. He says that by adding storage capacity to solar-thermal plants such as his,”You can extend when you deliver power and and you’re delivering more of it when the real system peak is.”