There is another large scale way to use the sun’s energy, an alternative to the solar cell or photovoltaic systems now sprouting on home roofs or in larger ground installations. Sometimes called the solar power tower system, this large scale utility like solar application found its birth in the late 1970s/early 1980s. A tall tower [probably 400-600 feet or so] supports a high temperature steam boiler, which is heated by the automated positioning of solar mirrors [heliostats] that intensely focus the sun’s light. High temperature steam generated in the boiler is then used to run traditional utility-grade, turbo-electric generating equipment, much like a central station power plant of today.

Photo of Solar Power Tower (source: Internet)

Photo of heliostats: polished, sun-tracking mirrors (source: Internet)

The key difference in this design from its solar cell-photovoltaic alternative is the use of a high temperature storage medium to run the plant when the sun is not shining. In effect, this solar alternative is a 24-hour system, providing all-day, clean electricity that can be dispatched by a utility. Today, solar power tower systems have been built in the 5-20 MW size range with plans for 100 MW systems, usually located in desert areas where sunshine is plentiful, and the skies are very clear.

Key to this plant’s operation and economics is the storage medium, usually a “eutectic salt”, a compound that absorbs and releases quantities of heat as it changes from a liquid to a solid. You know those blue compounds in plastic containers you chuck into the freezer to solidify that you use the next day to keep things cold for your picnic..that’s the idea…only super-size the magic liquid up into many, many gallons. Like a modern day Thomas Edison, when he went through 15,000-20,000 experiments to find the right electrolyte for his storage battery, a company called Halotechnics [http://www.halotechnics.com/] is searching for just the right kind of eutectic salt. So far, over 14,000 chemical combinations have been evaluated.

An early promising storage medium known as Saltstream™ 565 is now being offered by the company for commercial use. It is 20% cheaper than existing storage compounds, using less expensive salts. Saltstream™ materials utilize eutectic mixtures with drastically reduced melting points, reducing the risk of freezing and expanding the operating range of the fluid. It would allow solar plants to operate at greater temperatures and store more heat in the chemical structure of the salt. The materials they have devised, which include new mixtures of salts as well as new forms of molten glass materials, could be key to making solar-thermal power plants cheap and reliable enough to compete with fossil fuels on a large scale.

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