The basic principle of a solar thermal plant is to convert primary solar energy into electricity for homes, business and industry by means of a collector field, steam turbine and electric generator. The solar field consists of parallel loops of parabolic trough collectors. The new parabolic SKAL-ET collectors, developed and qualified by the Solar Millennium group and its partners for the AndaSol projects, track the sun from East to West in order to reflect and concentrate the direct solar radiation about eighty times onto the absorber tubes installed in the focal line of the reflecting surface. An absorber tube consists of a stainless steel tube with a selective coating that is covered by a glass envelope tube to reduce thermal losses. The annular space between absorber tube and glass tube is evacuated. Through the absorber tubes circulates a heat transfer fluid (HTF), normally synthetic oil, which is heated by the concentrated solar radiation up to a temperature of almost 400 °C.

In the direct operation mode, the HTF is circulated through the solar field (see Figure 2) where it is heated and supplied through a main header to the heat exchangers located in the power block, where superheated steam is produced at a temperature of 370 °C and a pressure of 100 bar. After passing through the HTF side of the heat exchangers, the cooled HTF is then re-circulated through the solar field to repeat the process. In this way, the HTF fluid acts as the heat transfer medium between the solar field and the power block of the steam cycle, heating up in the solar collectors and cooling down while producing steam for the steam generator. The superheated steam is then fed to the high-pressure
casing of a conventional steam reheat turbine. The steam is reheated before being fed to the low-pressure casing. The exiting steam from the turbine is condensed in a conventional steam condenser and returned to the heat exchangers via condensate and feed-water pumps to be transformed back into steam. With this process, the collected and concentrated solar radiation from the solar field is converted into electricity and afterwards fed to the general power supply.

To extend the operation of the AndaSol solar power plant beyond sunshine hours a thermal energy storage will be integrated into the plant design. Solar energy collected by the solar field during the day will be stored in the storage system and can be dispatched after sunset. The medium, which stores the thermal energy, is a molten salt of similar type like used as agricultural fertilizers. To charge the storage the salt will be heated up to about 385°C. To discharge the system the salt is cooled down again to about 295°C. At both temperatures the salt is still liquid. Cold and hot salt are stored in separate tanks.

In the AndaSol configuration, HTF from the solar field is diverted through a heat exchanger that is used to charge the thermal storage system. For charging, cold salt is pumped from the cold tank to the hot tank passing the salt-to-oil heat exchanger. In the heat exchanger the salt is heated up to the hot tank temperature. During night, the storage is discharged through the same oil-to-salt heat exchanger by changing the flow direction of the salt. The salt is pumped from the hot to the cold tank and the stored heat is transferred to the oil via the heat exchanger. The cooled salt is returned to the cold storage tank. The heated oil is fed to the steam generator to generate high-pressure steam. The storage system of the first two AndaSol projects is designed to store approximately 880 MWht each, of thermal energy. This is enough to operate the plants for 6 hours at full load during non-sunshine hours.

The principle of the AndaSol solar field layout is illustrated in Figure 3: Two SKAL-ET150 collectors with a length of 150 m are connected ign series to form a row running in a north — south direction. Two adjacent rows are connected via a cross over pipe to a loop, consisting of four SKAL-ET150 collectors. The cold heat transfer fluid is going from the power block to the cold header pipe and then entering the parallel loops. The fluid is heated up in the loops and then going back to the power block through the hot header. Each AndaSol plant has a reflecting surface of approximately 500.000 m2 made up by 624 SKAL-ET 150 collectors in 156 loops. Altogether, the solar field of one AndaSol project covers approximately 200 hectars of flat land; 1500 m in east-west direction and 1300 m in north-south direction. All SKAL-ET collectors of the AndaSol-1 & 2 solar fields are controlled from the control room by a central computer. It starts up the solar field in the morning according to weather and availability conditions and stows it for the night and during strong winds.