The direct steam generation (DSG) in solar thermal power plants is an interesting option to increase the efficiency
further than the current one of state-of-the-art parabolic trough power plants using synthetic oil as primary heat
transfer fluid (HTF). The live steam temperature of the turbine hereby is increased up to 550 °C. According to [1],
DSG plants can only become economical competitive if a cost effective storage systems is available for DSG
systems.
State-of-the-art parabolic trough power plants use a sensible HTF (synthetic oil) and a sensible storage medium
(molten salt). Thus, the energy during charging and discharging can be transferred by a standard heat exchanger. If
water/steam is also used as the primary HTF, the situation becomes more complex. Potential storage configurations
are shown in schematic temperature-enthalpy diagrams in figure 1. During charging, the superheated steam from the
solar field (SF) enters the storage system and is cooled down to the according saturation temperature by transferring
its heat to the storage medium. During condensation the heat from the HTF is transferred at a constant temperature
level. Finally, the condensate is sub-cooled in the storage system. If a sensible storage medium would be used (see
figure 1 (a)), the occurring pinch-point problem causes a significant reduction of the live steam temperature and
pressure during discharging and thus a significant reduction of the power block efficiency during discharging.
The direct steam generation (DSG) in solar thermal power plants is an interesting option to increase the efficiencyfurther than the current one of state-of-the-art parabolic trough power plants using synthetic oil as primary heattransfer fluid (HTF). The live steam temperature of the turbine hereby is increased up to 550 °C. According to [1],DSG plants can only become economical competitive if a cost effective storage systems is available for DSGsystems.State-of-the-art parabolic trough power plants use a sensible HTF (synthetic oil) and a sensible storage medium(molten salt). Thus, the energy during charging and discharging can be transferred by a standard heat exchanger. Ifwater/steam is also used as the primary HTF, the situation becomes more complex. Potential storage configurationsare shown in schematic temperature-enthalpy diagrams in figure 1. During charging, the superheated steam from thesolar field (SF) enters the storage system and is cooled down to the according saturation temperature by transferringits heat to the storage medium. During condensation the heat from the HTF is transferred at a constant temperaturelevel. Finally, the condensate is sub-cooled in the storage system. If a sensible storage medium would be used (seefigure 1 (a)), the occurring pinch-point problem causes a significant reduction of the live steam temperature andpressure during discharging and thus a significant reduction of the power block efficiency during discharging.
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