Entropy Generation Minimization Analysis of Solar Salt, Sodium, and Lead-Bismuth Eutectic as High Temperature Heat Transfer Fluids

The amount of entropy generation in heat transfer devices impacts their operation economy and should therefore be minimized during the design phase. Entropy generation also depends on the individual thermophysical properties of the heat transfer fluid (HTF). An entropy generation minimization analysis of three different liquid coolants, namely, solar salt (SS), sodium, and lead-bismuth eutectic (LBE) is thus performed for fully turbulent flow in a circular tube under circumferentially uniform heat flux by considering a heat rate, inlet and outlet temperatures and heat flux densities typical of a concentrated solar tower plant. The Reynolds number is determined at which the proper combination of Nusselt number and friction factor minimizing the entropy generation, and consequently the exergy loss, is obtained as well as the best thermodynamically performing coolant fluid over the operating range of Reynolds numbers. Sodium can operate at 60% lower entropy generation than solar salt while providing a smaller wall-to-bulk temperature difference. Despite its high thermal conductivity, LBE performs similarly to solar salt. However, it can be advantageous compared to solar salt if operated at higher temperatures that cannot be achieved by the latter due its thermal stability limit.