Solar Thermal Energy
Solar Thermal Energy
Solar thermal energy (STE) is a technology for harnessing solar energy for thermal energy (heat). Solar thermal collectors are defined by the USA Energy Information Administration as low-, medium- , or high-temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. This is different from solar photovoltaics, which convert solar energy directly into electricity.
Low-temperature collectors
Of the 21,000,000 square feet (2,000,000 m2) of solar thermal collectors produced in the United States in 2006, 16,000,000 square feet (1,500,000 m2) were of the low-temperature variety. Low- temperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the medium to transfer the heat to its destination.
Medium-temperature collectors
These collectors could be used to produce approximately 50% of the hot water needed for residential and commercial use in the United States. In the United States, a typical system costs $5000-$6000 and 50% of the system qualifies for a tax credit. With this incentive, the payback time for a typical household is nine years. A crew of one plumber and two assistants with minimal training can install two systems per week. The typical installation has negligible maintenance costs and reduces a households' operating costs by $6 per person per month. Solar water heating can reduce CO2 emissions by 1 ton/year (if replacing natural gas for hot water heating) or 3 ton/year (if replacing electric hot water heating). Medium-temperature installations can use any of several designs: common designs are pressurized glycol, drain back, and batch systems.
High-temperature collectors
Concentrated solar power plant using parabolic trough design.
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. This temperature is too low for efficient conversion to electricity.
The efficiency of heat engines increases with the temperature of the heat source. To achieve this in solar thermal energy plants, solar radiation is concentrated by mirrors or lenses to obtain higher temperatures — a technique called Concentrated Solar Power (CSP). The practical effect of high efficiencies is to reduce the plant's collector size and total land use per unit power generated, reducing the environmental impacts of a power plant as well as its expense.
As the temperature increases, different forms of conversion become practical. Up to 600°C, steam turbines, standard technology, have an efficiency up to 41%. Above this, gas turbines can be more efficient. Higher temperatures are problematic because different materials and techniques are needed. One proposal for very high temperatures is to use liquid fluoride salts operating above 1100°C, using multi-stage turbine systems to achieve 60% thermal efficiencies. The higher operating temperatures permit the plant to use higher- temperature dry heat exchangers for its thermal exhaust, reducing the plant's water use — critical in the deserts where large solar plants are practical. High temperatures also make heat storage more efficient, because more watt-hours are stored per kilo of fluid.
Since the CSP plant generates heat first of all, it can store the heat before conversion to electricity. With current technology, storage of heat is much cheaper and more efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the CSP plant becomes a reliable power plant. Reliability can further be improved by installing a back-up system that uses fossil energy. The back-up system can reuse most of the CSP plant, which decreases the cost of the back-up system.
With reliability, unused desert, no pollution and no fuel costs, the only obstacle for large deployment for CSP is cost. Although only a small percentage of the desert is necessary to meet global electricity demand, still a large area must be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.