Minimum system entropy production as the FOM of high temperature heat transfer fluids for CSP systems

High temperature heat transfer fluids are used in concentrated solar thermal power (CSP) plants to receive heat from a solar concentrator and then transfer it to a working fluid (such as water, air, or super critical CO₂) in a thermal power system. For various high temperature heat transfer fluids, such as, synthetic oils, various molten salts, and liquid metals, a general criterion is proposed in this work to evaluate the merit of fluids regarding their transport properties. For the goal of transferring a desired amount of heat, a fluid that causes less entropy production is believed to have better figure of merit (FOM). This is due to the fact that entropy production is associated with the destruction of exergy or useful energy. The entropy production in a heat transfer system in a solar thermal power plant includes the part due to the processes of heat addition and removal and the other part due to pressure losses in the flow in heat exchangers and pipes. Theoretical analysis and relevant equations for total entropy production are derived. As an example, the FOM for several heat transfer fluids used in CSP industry are compared for the goal of heat transport in the range of 50 MW_th to 600 MW_th. The modeling provides a fundamental approach for the comparison of various heat transport systems which may have different designs and using different heat transfer fluids/media (gas, liquid, or solid particles) in CSP systems.