RENEWABLE ENERGY
temperature ranges up to 620°C for Solar Tower II at the DLR’s Institute of Solar Research in Jülich. On a surface area of some 10 hectares, 2,000 mirrors collimate the sunlight in Jülich and direct it on to the external receiver located at a height of 55m. Conventional molten-salt receivers operate at only 565°C. MAN Energy Solutions is planning to use the innovative high-temperature receiver to test new temperature ranges and the limitations of the use of molten salt. Tis will also open up new potentials for everyday business using molten-salt reactors in, for example, the chemicals industry. For this reason, the control system of the test facility at Jülich has been designed by MAN Energy Solutions in such a way that it is as comparable as possible to classical power plants and will permit scaling-up of this prototype to industrial dimensions. Only the otherwise customary turbine has been omitted, since the test facility will not generate any electricity. Analogously to a classical power plant, the receiver features an inlet tank and an outlet tank. Te inlet tank also functions as a receiver vessel, to permit rapid reaction in case of any problems. Molten salt cools the receiver by one MWth per m2
. Failure of the flow
of molten-salt would cause immediate and permanent damage to the receiver. In case of operational problems, the inlet tank for this reason continues to feed salt into the receiver until the mirrors (heliostats) are aligned in such a way that no further solar energy strikes the receiver.
HIGH
DEMANDS MADE ON THE STEELS USED
Te attention of the MAN Energy Solutions designers in the case of CSP solar-power plants of this type is focused not only on optimisation of output and efficiency, but also on the materials needed for the plant’s highly stressed components. “Te steels needed for the construction of vessels of 50m diameter and 60mm plate thickness are exposed to high thermal, mechanical and corrosive loads”, explains departmental manager, Christian Schuhbauer. Te use of AISI 347H (1.4961) stainless steel is routine practice for the heat-storage tanks of solar-tower power plants. Termal oil, the working fluid of a parabolic-trough plant, is normally used at up to 400°C, since it is degraded at higher temperatures. For this reason, the molten salt used in this type of system also has only an analogous temperature. Dillinger supplies for such tanks the carbon steel SA 204 C in plate thicknesses of 100mm, a width of 5,200mm and an optimised weldability. Tis high-temperature fine-grained structural steel has a high yield strength at
Internal view of the commercial receivers
elevated temperatures and can be used up to service temperatures of 450°C. Te cold-salt tanks, in which the salt cooled down to 290 to 300°C in the steam generator is stored, are fabricated in carbon steel SA 204 B. Tis high-temperature steel is characterised by a slightly higher yield strength compared to SA 204 C. Te high-temperature molybdenum- alloyed structural steel 16Mo3 has been selected for the steam generators and heat exchangers, in wall thicknesses of up to 110mm, for this reference power plant. A significantly higher-performance alternative is provided by Dillinger in the form of the high-strength quenched and tempered fine-grained SA533 B2 steel, a material already tried and proven in many comparable generators at thicknesses of 20 to 200mm. Its mechanical strength is, at a yield strength of 485 MPa, significantly higher than that of 16Mo3 (275 MPa). Tis permits lower component wall thicknesses and thus allows designers a lighter overall structure. SA533 B2 is thus also an interesting option for the salt collector of the receiver panels up to a temperature of 450°C. Tis collector connects the pipes through which the salt flows back and forth in a meander pattern for heating up. Each of these panels in a 700 MWth CSP receiver will be up to 25m high and have a width of 3.5m. Up to 16 panels featuring up to 50 tubes are required in such a plant and form a
Salt tanks at the Andasol CSP solar-power plant in Spain
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