Optimal concentration and temperatures of solar thermal power plants

Abstract

Using simple, finite-time, thermodynamic models of solar thermal power plants, the existence of an optimal solar receiver temperature has previously been demonstrated in literature. Scant attention has been paid, however, to the presence of an optimal level of solar concentration at which solar-to-electric efficiency is maximised. This paper addresses that gap. A simple mathematical model is developed to assess the influence of key system temperatures, thermal conductances and the receiver irradiance (radiation striking unit receiver area) upon the optimal receiver temperature and solar-to-electric efficiency of Rankine-cycle, solar-trough and solar-tower, direct-steam and molten-salts, power plants. The analysis shows that, as the thermal resistance of the solar receiver and condenser increases, the optimal receiver temperature increases whilst the optimal receiver irradiance decreases. The optimal level of receiver irradiance, for solar thermal plants employing a service fluid of molten salts, is found to occur within a range of values achievable using current solar tower technologies. The efficiency trade-off involved in using molten salts rather than direct steam is investigated. The optimal receiver temperatures calculated suggest the use of sub-critical Rankine cycles for solar trough plants, but super-critical Rankine cycles for solar tower plants, if the objective is to maximise solar-to-electric efficiency.