Solar thermal energy

Solar thermal energy


Solar thermal energy is a technology for harnessing solar energy for practical applications from solar heating to electrical power generation. Solar thermal oncentrated solar pocollectors, such as solar hot water panels, are commonly used to generate solar hot water for domestic and light industrial applications. Solar thermal energy is used in architecture and building design to control heating and ventilation in both active solar and passive solar designs. This article is devoted primarily to solar thermal electric power plants; that is, solar power plants that generate electricity by converting solar energy to heat, to drive a thermal power plant. These plants include the Solar Energy Generating Systems, Nevada Solar One, and Solar Tres. The article on photovoltaics reviews solar power generation by means of solar electric panels.

Concentrated solar power (CSP) plants

Where temperatures below about 95°C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. The fluid-filled pipes can reach temperatures of 150 to 220 degrees Celsius when the fluid is not circulating.

A concentrating collector intercepts the same amount of solar radiation as a flat-plate collector of the same area, but contains a parabolic reflector that focuses the energy onto the surface of an absorber of much smaller area. This concentration of energy heats the absorber to a much higher temperature than that produced in the flat-plate type. Whilst the amount of energy remains the same, the higher temperature enables the system to generate electrical or mechanical energy more efficiently. This is because the maximum theoretical efficiency of any heat engine increases as the temperature of its heat source increases.

Parabolic trough designs


Sketch of a Parabolic Trough Collector

Parabolic trough power plants are the most successful and cost-effective CSP system design at present. They use a curved trough which reflects the direct solar radiation onto a hollow tube running along above the trough. The whole trough tilts through the course of the day so that direct radiation remains focused on the hollow tube for as long as the sun shines. A fluid, normally thermal oil, passes through the tube and becomes hot. Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land. A solar thermal system using this principle is in operation in California in the United States, called the SEGS system.At 350 MW, it is currently not only the largest operational solar thermal energy system, but the largest solar power system of any kind. SEGS uses oil to take the heat away: the oil then passes through a heat exchanger, creating steam which runs a steam turbine. The 64MW Nevada Solar One plant also uses this design. Other parabolic trough systems, which create steam directly in the tubes, are under development; this concept is thought to lead to cheaper overall designs, but the concept is yet to be commercialized.

Power tower designs


Solar Two, a concentrating solar power plant.

Power towers (also know as 'central tower' power plants or 'heliostat' power plants) use an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One (later called Solar Two), and the 15 MW Solar Tres plants. Neither of these are currently used for active energy generation. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².Another design is a pyramid shaped structure - solar pyramid - which works by drawing in air, heating it with solar energy and moving it through turbines to generate electricity. Currently India is buiiding such pyramids.

Dish designs

A dish system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a thermal collector is used to capture the heat and transform it into a useful form. Dish systems, like power towers, can achieve much higher temperatures due to the higher concentration of light which they receive. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator.