Solar power plants. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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The total amount of solar energy reaching the Earth's surface in a week exceeds the energy of all the world's reserves of oil, gas, coal and uranium. Solar heat can be stored in many ways. Modern technologies include parabolic concentrators, solar parabolic mirrors and solar power towers. They can be combined with fossil fuel combustion plants and in some cases adapted for heat storage. The main advantage of such hybridization and thermal storage is that such technology can provide scheduling of electricity production (that is, electricity generation can be performed at times when it is needed). Hybridization and heat storage can increase the economic value of electricity produced and reduce its average cost.

Solar parabolic concentrators

These installations use parabolic mirrors (trays) that concentrate sunlight on receiving tubes containing a heat transfer fluid. This liquid is heated to almost 400°C and pumped through a series of heat exchangers; this produces superheated steam, which drives a conventional turbine generator to produce electricity. To reduce heat loss, the receiving tube may be surrounded by a transparent glass tube placed along the focal line of the cylinder. As a rule, such installations include uniaxial or biaxial solar tracking systems. In rare cases, they are stationary.

Built in the 80s in the Southern California desert by Luz International, nine of these systems make up the world's largest solar thermal power plant today. These power plants supply electricity to the Southern California public electricity grid. Back in 1984, Luz International installed a 13,8 MW Solar Electric Generating System I (or SEGS I) in Deggett, Southern California. In the receiving tubes, the oil was heated to a temperature of 343°C and steam was generated to generate electricity. The "SEGS I" design provided for 6 hours of heat storage. It used natural gas ovens, which were used in the absence of solar radiation. The same company built similar power plants "SEGS II - VII" with a capacity of 30 MW. In 1990, "SEGS VIII and IX" were built at Harper Lake, each with a capacity of 80 MW.

Estimates of the technology show that it is more expensive than tower and dish type solar power plants (see below), mainly due to lower concentration of solar radiation, and therefore lower temperatures and, accordingly, efficiency. However, with more operating experience, improved technology and reduced operating costs, parabolic concentrators may be the least expensive and most reliable technology of the near future.

Solar plate type

This type of solar plant is a stack of parabolic dish mirrors (similar in shape to a satellite dish) that focus solar energy onto receivers located at the focal point of each dish. The liquid in the receiver is heated up to 1000 degrees and is directly used to generate electricity in a small engine and generator connected to the receiver.

Stirling and Brayton engines are currently under development. Several pilot systems ranging from 7 kW to 25 kW are in operation in the United States. High optical efficiency and low initial cost make mirror/motor systems the most efficient of all solar technologies. The Stirling engine and parabolic mirror system holds the world record for the most efficient conversion of solar energy into electricity. In 1984, the Rancho Mirage in California achieved a practical efficiency of 29%.

In addition, thanks to their modular design, these systems represent the best option to meet the power needs of both stand-alone consumers (in the kilowatt range) and hybrid (in the megawatt range) connected to the utility grids.

This technology has been successfully implemented in a number of projects. One of them is the STEP (Solar Total Energy Project) project in the US state of Georgia. This is a large system of parabolic mirrors that worked in 1982-1989. in the Shenandoah. It consisted of 114 mirrors, each 7 meters in diameter. The system produced high pressure steam for power generation, medium pressure steam for the knitting industry, and low pressure steam for the air conditioning system in the same knitting factory.

Other companies also became interested in sharing parabolic mirrors and Stirling engines. For example, Stirling Technology, Stirling Thermal Motors, and Detroit Diesel, together with Science Applications International Corporation, formed a $36 million joint venture to develop a 25-kilowatt system based on the Stirling engine.

Solar power towers with a central receiver

These systems use a rotating field of heliostat reflectors. They focus sunlight onto a central receiver built on top of the tower, which absorbs heat energy and drives a turbine generator. A computer-controlled biaxial tracking system positions the heliostats so that the reflected sun rays are stationary and always fall on the receiver. The liquid circulating in the receiver transfers heat to the heat accumulator in the form of steam. The steam drives a turbine to generate electricity or is directly used in industrial processes. Receiver temperatures range from 538 to 1482°C.

The first tower plant, called "Solar One" near Barstow, Southern California, successfully demonstrated the application of this technology to power generation. The company operated in the mid-1980s. It used a water-steam system with a capacity of 10 MW. In 1992, a consortium of US energy companies decided to upgrade Solar One to demonstrate a molten salt receiver and thermal storage system. Thanks to heat storage, tower power plants have become a unique solar technology that allows electricity dispatching at a load factor of up to 65%. In such a system, molten salt is pumped from a "cold" tank at 288°C and passed through a receiver where it is heated to 565°C and then returned to the "hot" tank. Now hot salt can be used to generate electricity as needed. In modern models of such installations, heat is stored for 3 to 13 hours.

Solar Two, a 10 MW power tower in California, is the prototype of large industrial power plants. It provided electricity for the first time in April 1996, marking the start of a 3-year period of testing, evaluation and pilot power generation to demonstrate the molten salt technology. Solar heat is stored in molten salt at a temperature of 550°C, thanks to which the station can generate electricity day and night, in any weather. The successful completion of the "Solar Two" project should facilitate the construction of such towers on an industrial basis in the capacity range from 30 to 200 MW.

Comparison of specifications

Towers and parabolic-cylindrical concentrators work optimally as part of large, grid-connected power plants with a capacity of 30-200 MW, while disk-type systems consist of modules and can be used both in stand-alone installations and in groups with a total capacity of several megawatts. Parabolic-cylindrical installations are by far the most developed of solar energy technologies and they are likely to be used in the near future. Tower-type power plants, due to their efficient heat storage capacity, can also become solar power plants in the near future.

The modular nature of the "trays" allows them to be used in smaller installations. Towers and "dishes" make it possible to achieve higher efficiency values ​​for converting solar energy into electrical energy at a lower cost than parabolic concentrators. However, it remains unclear whether these technologies will be able to achieve the required reduction in capital costs. Parabolic concentrators are now a proven technology, waiting for their chance to improve. Tower power plants need to demonstrate the efficiency and operational reliability of molten salt technology using inexpensive heliostats. For poppet-type systems, it is necessary to create at least one commercial engine and develop an inexpensive concentrator.

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