Fundamental Thermo-Economic Approach to Selecting sCO2 Power Cycles for CSP Applications

Abstract

The interest in Supercritical Carbon Dioxide (sCO2) power cycles has grown exponentially in the last decade, thanks to distinctive features like the possibility to achieve high thermal efficiencies at intermediate temperature levels, small footprint and adaptability to a wide variety of energy sources. In the present work, the potential of this technology is studied for Concentrated Solar Power applications, in particular Solar Tower systems with Thermal Energy Storage. A thorough sensitivity analysis based on turbine inlet temperature and pressure ratio is done for twelve sCO2 cycles, considering their effects on thermal efficiency and specific work, along with solar share and temperature rise across the solar receiver. The most important conclusions of this section are that: a) the peak values of these thermodynamic figures of merit are obtained at different pressure ratios; b) specific work and temperature rise across the receiver seem to follow parallel trends whilst this is not the case for thermal efficiency; c) for a given turbine inlet temperature, higher pressure ratios always increase the receiver temperature rise strongly, but the effect on thermal efficiency is uncertain as this can either increase or decrease, depending on the cycle considered. A deeper analysis of thermal efficiency and receiver temperature rise is therefore mandatory, given that these parameters strongly affect the capital cost of CSP power plants. On one hand, a higher thermal efficiency implies a smaller solar field, the largest contributor to the plant capital cost; on the other, the temperature rise across the receiver is inversely proportional to the size of the thermal energy storage systems, as it is also the case for state of the art steam turbine based CSP plants. An economic analysis is developed using an in-house code and the open-source software System Advisor Model to evaluate the trade-offs between these two effects. The results obtained for the two most representative sCO2 cycles show somewhat unexpected results.