I don’t know about you, but the term “solar energy” has usually made me think about photovoltaics, in which light from the sun is converted into electrical energy. No more. This week I was browsing Bill Foster’s energy policy paper and read the term “thermo-solar energy” for the first time. Also called solar thermal energy, this technology is about using the sun’s heat for generating electricity.

The problem is, if you’re only using the light, you can store electricity up to a point but you can’t generate it when it’s overly overcast or dark. Also, it can be more expensive to generate electricity from photovoltaic than from other sources. Unlike light, heat can be stored for power generation during times the sun is sleeping, rendering it more reliable and cost-effective.

power tower solar dish fresnel reflector
Photo source: Wikipedia

At least four technologies have been developed when it comes to collecting the solar heat, including one hybrid that uses supplementary natural gas. Trough, power tower, dish and Fresnel reflector systems all show promise in themselves, but the latest breakthrough is in using molten salt to receive and store the heat energy.

The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. The salt melts at 430 F and is kept liquid at 550 F in an insulated cold storage tank. The salt is them [sic] pumped to the top of the tower, where concentrated sunlight heats it in a receiver to 1050 F. The receiver is a series of thin-walled stainless steel tubes. The heated salt [sic] then flow back down to a second insulated hot storage tank. The size of this tank depends on the requirements of the utility; tanks can be designed with enough capacity to power a turbine from two to twelve hours. When electricity is needed from the plant, the hot salt is pumped to a conventional steam-generating system to produce superheated steam for a turbine/generator.

The uniqueness of this solar system [heh!] is in de-coupling the collection of solar energy from producing power, electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. The tanks are well insulated and can store energy for up to a week.

Harnessing solar thermal energy is not a new idea. It was first used for heating water and a practical commercial model was patented in 1891.

In 1897, nearly 30% of the houses in Pasadena, California had solar water heaters. Solar thermal energy became popular again during the 1970′s when the oil crisis occurred. Unlike photovoltaics, which have resembled the stock markets in boom and bust, the solar thermal industry has been slowly growing since the 1970′s. Although solar thermal energy is mostly used for heating water, it can also be used to turn water into steam and produce electricity. First appearing in the early 1980′s, currently 400 peak megawatts of solar thermal electricity are available in the United States.

As noted in our story about Henry Ford and vegetable-based fuels and plastics, cheap and widely-available petroleum suppressed interest in developing solar energy technologies for many years. This is changing now that energy costs are more volatile. Among the latest developments:

  • An Israeli company has been contracted to build the world’s largest (553mW) solar power plant in the Mojave desert.
  • The first U.S. plant to manufacture solar thermal systems will begin production April 2008 in Nevada.
  • “Developing the Huge Solar Thermal Potential” is the theme for next year’s European Solar Thermal Technology Platform meeting in Brussels in January.
  • Side note: It would seem that presidential hopeful Bill Richardson was ahead of the curve in this area.