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.
One possibility to reduce this problem would be to use a three-part storage system for preheating, evaporation and
superheating like proposed in [2]. To store the sensible heat, a molten salt storage system with a cold tank, a hot tank
and an intermediate tank is used. This approach would allow two different mass flows. A higher mass flow in the
lower temperature range and a smaller mass flow in the higher temperature range. According to figure 1 (b), the live
steam temperature and pressure reduction is not as large as in the previous case leading to a more moderate reduction
of the power block efficiency during discharging.
If a latent heat storage system is used to store the evaporation enthalpy of water/steam, the temperature profile in
the storage system is matched to the temperature profile of the heat transfer fluid during charging and discharging.
This approach leads to the highest live steam parameters and thus power block efficiency during discharging at the
expenses of an increased system complexity. Since the specific heat capacity of water is much higher than that the
one of steam, the gradient of the steam curves is steeper when compared to the water curves in figure 1. To
compensate the different slopes, different molten salt mass flows are utilized in the preheating and in the superheating section of the storage system. This demands the use of a three tank molten salt system for the sensible
heat. The schematic diagram of a conceivable set-up is shown in figure 2.