Simscape/Simulink modelling of a cascade cycle solar power plant: a study of control strategy for typical climate conditions in Libya

Due to its inherent intermittency and fluctuation, solar energy is not friendly to the power distribution network and its connection to the grid. Steam accumulators are the only commercial solution for heat storage in direct steam generation (DSG) solar thermal power plants. Current accumulators have low storage capacity as the turbine suffers from inefficient off-design operation during heat discharge, thereby restricting the development of DSG technology. The cascade Rankine cycle solar power station equipped with thermal energy storage can effectively compensate for the instability and periodic fluctuation of solar energy, and a reasonable operation control strategy is essential for its peak-regulating operation mode. A solar thermal power generation system using cascade steam-organic Rankine cycle and steam storage has been proposed. This system offers a significantly higher heat storage capacity than a single heat storage DSG solar power system. The system consists of PTCs, high temperature tank, steam turbine and intermediate heat exchanger in the first cycle. The second coupling cycle consists of ORC turbine, valves, condenser, low temperature tank and pump. Thermodynamic performance of the system was investigated under typical climate conditions. Overall, the proposed solution improved the cost-effectiveness of the DSG system. The exhaust steam from the direct steam generation cycle was used to drive the ORC and it had a direct effect on the thermal efficiency of ORC and the power output. The results of this study indicated that a direct steam generation system could generate up to 160 kW while the maximum power output ORC was 35 kW under a clear climate condition day. The maximum thermal efficiency of the system was 0.17.