PART-LOAD OF STEAM RANKINE CYCLES FOR SOLAR SALTS-BASED CONCENTRATING SOLAR POWER PLANTS

This work proposes two ad hoc part-load control strategies for steam cycles adopted in concentrated solar power plants. The control strategies are designed to keep the molten salt temperature above the minimum allowed value set by solidification issues in the 30-100 % load range. Particularly critical is the temperature of molten salts in contact with the heat exchanger tubes, the so called skin temperature. The first control strategy adopts a turbine with controlled extraction and readmission valve while the second strategy employs a throttling valve and a feedwater preheating loop. Off-design simulations show that both strategies are capable of avoiding the molten salts solidification issue but at the cost of a non negligible penalty (up to -1.9 percentage points) in power block efficiency at low loads (30- 50%). The off-design analysis considers also the effect of ambient temperature variations and the optimization of the cooling fan rotational speed. The results are used to derive best-fit polymonials relating the power block efficiency to the ambient temperature and load.