The parabolic trough collector consists of large curved mirrors, which concentrate the sunlight by a factor of 80 or more to a focal line. Parallel collectors build up a 300–600 metre long collector row, and a multitude of parallel rows form the solar collector field. The one-axis tracked collectors follow the sun.
The collector field can also be formed of very long rows of parallel Fresnel collectors. In the focal line of these is a metal absorber tube, which is usually embedded into an evacuated glass tube that reduces heat losses. A special high-temperature, resistive selective coating additionally reduces radiation heat losses.
In the Californian systems, thermo oil flows through the absorber tube. This tube heats up the oil to nearly 400°C, and a heat exchanger transfers the heat of the thermal oil to a water steam cycle (also called Rankine cycle). A feedwater pump then puts the water under pressure. Finally, an economizer, vaporizer and superheater together produce superheated steam. This steam expands in a two-stage turbine; between the high-pressure and low-pressure parts of this turbine is a reheater, which heats the steam again. The turbine itself drives an electrical generator that converts the mechanical energy into electrical energy; the condenser behind the turbine condenses the steam back to water, which closes the cycle at the feedwater pump.
It is also possible to produce superheated steam directly using solar collectors. This makes the thermo oil unnecessary, and also reduces costs because the relatively expensive thermo oil and the heat exchangers are no longer needed. However, direct solar steam generation is still in the prototype stage.
Guaranteed Capacity
In contrast to photovoltaic systems, solar thermal power plants can guarantee capacity (see Figure 2). During periods of bad weather or during the night, a parallel, fossil fuel burner can produce steam; this parallel burner can also be fired by climate-compatible fuels such as biomass, or hydrogen produced by renewables. With thermal storage, the solar thermal power plant can also generate electricity even if there is no solar energy available.
FIGURE 2. Typical output of a solar thermal power plant with two-hour thermal storage and backup heater to guarantee capacity
A proven form of storage system operates with two tanks. The storage medium for high-temperature heat storage is molten salt. The excess heat of the solar collector field heats up the molten salt, which is pumped from the cold to the hot tank. If the solar collector field cannot produce enough heat to drive the turbine, the molten salt is pumped back from the hot to the cold tank, and heats up the heat transfer fluid. Figure 3 shows the principle of the parabolic trough power plant with thermal storage.
FIGURE 3. Schematic of a concentrated solar thermal trough power plant with thermal storage
Trough Power Plant Efficiencies
The efficiency of a solar thermal power plant is the product of the collector efficiency, field efficiency and steam-cycle efficiency. The collector efficiency depends on the angle of incidence of the sunlight and the temperature in the absorber tube, and can reach values up to 75%. Field losses are usually below 10%. Altogether, solar thermal trough power plants can reach annual efficiencies of about 15%; the steam-cycle efficiency of about 35% has the most significant influence. Central receiver systems such as solar thermal tower plants can reach higher temperatures and therefore achieve higher efficiencies.
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