Thermodynamic and economic assessment of a new generation of subcritical and supercritical solar power towers

Abstract

The feasibility of using more efficient Rankine power blocks in solar power towers (SPTs) with molten salt as the heat transfer fluid has been studied as a method for increasing the global efficiency of these power plants. The temperature and pressure of the main steam and the reheating pressure affect the temperature of the molten salt in the receiver; for temperature increase decreasing the receiver efficiency and increasing the power block efficiency. Therefore, a detailed study of these SPTs has been conducted to determine whether the proposed changes increase the global efficiency of the SPTs.

A total of eight different subcritical and supercritical SPTs have been investigated. To set the most important cost of the SPT, the same heliostat field has been used. The receiver geometry has been optimised for each SPT to maximise the heliostat-receiver efficiency, fulfilling the material limitations.

It has been observed that the pressure at the inlet of the turbine increases the SPT efficiency even more than the temperature. However, special attention has to be paid to the reheating pressure, which is the most influential factor on the SPT efficiency. A high reheating pressure considerably decreases the SPT efficiency. Therefore, the best efficiencies have been obtained for the supercritical SPTs with a low reheating pressure and high temperature. It is closely followed by subcritical SPTs at high pressure and temperature.

The investment cost of the different SPTs also increases with the pressure and the temperature of the PB, with subcritical SPTs being less expensive than supercritical SPTs. However, the cost increase is balanced by the increase in the efficiency. The same cost per kWe is found in subcritical SPTs working at 16 MPa and in supercritical SPTs with low reheating pressure.