Every year the sun transmits about one billion terawatt hours of energy towards earth, but so far, only a small fraction of it is ever used effectively. This is why high hopes have been placed on solar thermal power plants for converting solar radiation into electrical energy. One of the first European plants, Andasol, is located in the Spanish province of Granada, and Vega sensor technology has been there from the very beginning.
Power from the desert is no longer a vision but a reality – even in Europe. In the Andasol complex, three parabolic trough power plants concentrate enough direct solar radiation to generate the heat energy required to drive the steam turbines. Vega measuring instruments can be found in many areas across all aspects of the solar power facility.
Some applications are extreme
One application is the expansion tank for the heat transfer fluid. In solar thermal systems, a special heat transfer medium transports the heat produced by the mirror system to the steam turbine and connected generator. The heat transfer fluid is synthetic oil operating in a temperature range between 300 and 400°C. This large range is due, among other things, to heat losses in the piping system that leads to significant changes in the volume of the transfer medium. The expansion tanks, with pressures up to 12 bar and temperatures up to 400°C, compensate for this change in volume.
For level detection in the tank, the facility relies completely on radar technology. The Vegapuls 62, mounted in a standpipe in the system, is ideal for these extreme temperatures and high pressures and since they are installed with a ball valve fitting, the sensors can be exchanged when the system is operating without interrupting the process.
Monitoring the pressures through the entire piping system for the heat transfer fluid is extremely important. The pressure transmitter Vegabar 51 is deployed here; ideal thanks to its high chemical resistance high operating temperature range. Due to the harsh conditions this instrument is subjected to, a chemical seal system is implemented, but there are, however, measuring points where the pressure transmitter is installed directly on the heat transfer vessel. Since debris and buildup can contaminate the pressure diaphragm, flushing rings are used in such cases.
Planning for sustainability
One particularly interesting aspect of the solar thermal system is the function of the two salt reservoirs that store thermal energy during times of abundance, particularly in the afternoon. In times of low or no sunlight, the heat transfer oil is pumped through the salt reservoirs, allowing the power plant to provide energy even at night – electricity generation is thus continuous and can be planned. The thermal storage system is designed to supply energy for 7 to 8 hours.
The salt is stored in two large tanks. In the first tank, the temperature of the hot, liquid salt is above 400°C. In the second, the temperature is lower but must never fall below 300°C. If the temperature drops below that value, the high-grade salt reverts to a solid state and damages the pipes or containers beyond repair. Such being the case, the temperatures and salt levels of the two tanks are monitored continuously.
Process temperatures above 400°C, measuring ranges up to 30 m and not least, the corrosive salty environment all place high demands on the measuring technology. Because of its physical characteristics, the non-contact radar measuring principle lends itself especially well for level measurement in the reservoirs. Once again, Vegapuls 62 radar sensors are used.
For more information contact John Groom, Vega Instruments SA, +27 (0)11 795 3249, [email protected], www.vega.com
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