Thermal Solar Finally Breaks Through the Clouds

It has been obvious to most thinking persons for several years, that solar thermal is a more reasonable approach to utility-scale solar plants at this time. This is due to the need to match energy production to energy use. Photovoltaics cannot provide good matching, until utility scale electrical storage becomes affordable to utilities and energy producers. Solar thermal can provide good matching now--with existing methods of thermal storage.

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.

...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. __NYT

The particular design for large scale heat storage will probably vary with the location and utility needs. But the underlying idea of storing heat instead of electricity may just make solar electricity competitive with coal fired electrical plants, at long last.