Solar thermal plants
Solar thermal energy is a technology for harnessing solar power for practical applications from solar heating to electrical power generation. Solar thermal collectors, 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. 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
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 insolation onto a hollow tube running along above the trough. The whole trough tilts through the course of the day so that direct insolation 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
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.
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.
Fresnel Reflectors
A linear Fresnel reflector power plant uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear absorbers positioned above the mirrors. Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo). These systems aim to offer lower overall costs by sharing a heat-absorbing element between several mirrors (as compared with trough and dish concepts), while still using the line-focus geometry that allows reduced complexity in the tracking mechanism (as compared with central towers). The absorber is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do no need to support the absorber, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed at different absorbers at different times of day), this can allow a denser packing of mirrors on available land area.
Conversion rates from solar energy to electrical energy
Of these technologies the solar dish/ stirling has the highest energy efficiency (the current record is a conversion efficiency of 40.7% of solar energy). A single solar dish-Stirling engine installed at Sandia National Laboratories’ National Solar Thermal Test Facility produces as much as 25 kW of electricity, while its footprint is a hundred times smaller than Spain’s solar updraft tower.Solar trough plants have been built with efficiencies of about 20%.
The Concentrated Solar Power (CSP) Plant using the parabolic trough principle called the SEGS system, in California in the United States, produces 330 MW, and it is currently the largest solar thermal energy system in operation. Furthermore, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity.
The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the insolation (1 kW/m²; see Solar power for a discussion) that falls on its 4,500-acres (18.2 km²). For the 50 MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6%
