Solar power plants
The full quantity of the solar energy arriving on the Earth's surface in the week exceeds energy of all world reserves of oil, gas, coal and uranium. The solar heat can be preserved in the different ways. Modern technologies include parabolic concentrators, solar parabolic mirrors and heliopower stations of tower type. They can be combined with the installations burning fossil fuel, and to adapt in certain cases for accumulation of heat.
The main advantage of such hybridization and heataccumulation are that such technology can provide scheduling of production of electricity (that is electricity production can be made during the periods when in it there is the need). Hybridization and heat storage can increase the economic value of the produced electricity and reduce its average cost.
Solar parabolic concentrators
In these installations parabolic mirrors (trays) which concentrate sunlight on the receiving tubes containing heat-transfer liquid are used. This liquid heats up almost to 400 C and is pumped over via the number of heat exchangers; at the same time the superheated steam setting in motion the normal turbine generator for production of electricity is developed. For decrease in thermal losses the transparent glass tube placed along the focal line of the cylinder can surround the receiving tube. As a rule, such installations include the uniaxial or biaxial systems of tracking the Sun. In rare instances they are stationary.
The constructed in the eighties in the southern Californian desert by Luz International, nine such systems form the enterprise largest today for production of solar thermal electricity.
These power plants deliver electricity in the utility power supply network of South California. In 1984 "Luz International" established in Deggetta (South California) the solar electrogenerating Solar Electric Generating System I system (or SEGS I) 13,8 MW.
In receiving tubes oil heated up to temperature of 343 oc and steam for production of electricity was produced. The design "SEGS I" provided 6 hours of heat storage. In it furnaces on natural gas which were used in case of lack of solar radiation were used. The same company constructed the similar power plants "SEGS II-VII" 30 MW. In 1990 in the Harper Leyk also IX were constructed "SEGS VIII", each 80 MW.
Estimates of technology show its higher cost, than at solar power plants of tower and dish type (see below), generally because of lower concentration of solar radiation, so, lower temperatures and, respectively, efficiency.
However, on condition of accumulation of operating experience, improvement of technology and decrease in operational costs parabolic concentrators can be the least expensive and most reliable technology of the near future.
Solar installation of dish type
This type of the solar power plant represents the battery of parabolic dish mirrors (similar the form with the satellite plate) which focus solar energy on the receivers located in the focal point of each plate. Liquid in the receiver heats up to 1000 degrees and is directly applied to production of electricity in the small engine and the generator connected to the receiver.
Now in development there are Stirling engines and Brighton. Several pilot systems with power from 7 to 25 kW work in the United States. High optical efficiency and small initial expenses do the systems of mirrors/engines by the most effective of all heliotechnologies. The system from the Stirling engine and the parabolic mirror possesses the world record by efficiency of transformation of solar energy into electricity. In 1984 to Rancho the Mirage in the State of California was succeeded to achieve with a practical efficiency of 29%.
In addition to it, thanks to modular design, such systems represent the optimal variant for satisfaction of the electricity demand as for independent consumers (in the kilowatt range), and for hybrid (in megawatt), the utility companies connected to power supply networks.
This technology is successfully implemented in a number of projects. One of them - the STEP project (Solar Total Energy Project) in the American State of Georgia. It is the large system of parabolic mirrors working in 1982-1989 in Shenandoah. It consisted of 114 mirrors, each 7 meters in the diameter. The system produced high-pressure vapor for production of electricity, pairs of average pressure for knitted production and also low-pressure vapor for the air conditioning system at the same knitting factory.
Also other companies became interested in sharing of parabolic mirrors and Stirling engines. So, Stirling Technology, Stirling Thermal Motors and Detroit Diesel together with Science Applications International Corporation created joint venture with the capital of 36 million dollars for the purpose of development of 25-kilowatt system on the basis of the Stirling engine.
Solar power plants of tower type with the central receiver
In these systems the rotary field of reflectors heliostats is used. They focus sunlight on the central receiver built on tower top which absorbs heat energy and puts the turbine generator in action. The biaxial system of tracking managed by the computer establishes heliostats so that the reflected sunshine were not mobile and always fell on the receiver. The liquid circulating in the receiver transfers heat to the heat accumulator in the form of steam. Steam rotates the turbine for electricity production, or is directly used in industrial processes. Temperatures on the receiver reach from 538 to 1482 of C.
The first tower power plant under the name "Solar One" near Barstow (South California) with success showed use of this technology for production of electricity. The enterprise worked in the mid-eighties. On it the 10 water and steam system МВтэ was used. In 1992 the consortium of the energy companies of the USA made the decision to modernize "Solar One" for demonstration of the receiver on fused salts and heat-retaining system.
Thanks to heat storage tower power plants became the unique heliotechnology allowing scheduling of the electric power at the demand factor up to 65%. In such system fused salt is pumped from the "cold" tank at the temperature of 288 of C and passes through the receiver where heats up to 565 C, and then returns to the "hot" tank. Now hot salt as required can be used for production of electricity. Heat is kept in modern models of such installations for 3 - 13 hours.
"Solar Two" - the 10 MW tower power plant in California is the prototype of large commercial power stations. It for the first time gave electricity in April, 1996 that was the beginning of the 3-year period of tests, assessment and pilot electricity production for demonstration of technology of fused salts. The solar heat remains in fused salt at the temperature of 550 of C thanks to what the station can produce electricity day and night, in any weather. The successful completion of the Solar Two project has to promote construction of such towers on the industrial basis within power from 30 to 200 MW.
Comparison of technical characteristics
Towers and parabolic-cylindrical concentrators optimum work as a part of the large, connected to network power plants with a power of 30-200 MW whereas the systems of dish type consist of modules and can be used as in autonomous installations, and groups by total power in several megawatt. Parabolotsilindrichesky installations - the most developed from solar energy technologies and they will probably be used for today in the short term.
Power plants of tower type, thanks to the effective thermal storage capacity, can also become solar power plants of the near future. The modular character of "plates" allows to use them in small installations. Towers and "plates" allow to reach more efficiency high values of transformation of solar energy in electric at the smaller cost, than at parabolic concentrators. However, remains not clear whether will be able to reach these technologies of necessary decrease in capital expenditure.
Parabolic concentrators now - already approved technology expecting the chance of improvement. Tower power plants need demonstration of efficiency and operational reliability of technology of fused salts when using inexpensive heliostats. The systems of dish type require creation at least of one commercial engine and development cheap of the concentrator.Top