Minimum system entropy production for the figure of merit of high temperature heat transfer fluid properties

A high temperature heat transfer fluid in a solar thermal power plant is used to receive and take the heat from the solar concentrator and then give to another fluid (such as water or air) in a thermal power cycle. With regard to their thermal and transport properties, high temperature heat transfer fluids such as synthetic oil, various molten salts, and liquid metals are different. Therefore, a general criterion is needed for evaluation of the merit of fluids regarding their transport properties. The present work considers the system entropy production as the criterion to define figure of merit (FOM). For the required amount of heat transfer rate in a heat transfer system, the fluid with properties that make the system entropy production the minimum is considered to be a winner. The system entropy production includes two parts, due to heat transfer (addition and removal) and pressure loss in the heat exchangers and fluid circulation pipes. Analysis and relevant equations are presented in this work. Several heat transfer fluids known well-known in solar thermal industry including synthetic oils, molten nitrate salts, and halide salts are compared using this standard.