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Liquid metals have been proposed in the past as high temperature heat transfer media in concentrating solar power (CSP) systems. Until the mid 80s test facilities were operated with liquid sodium-cooled central receivers. After a period of reduced interest in that approach, several new efforts have been reported recently, particularly from the US, South Africa and Australia. In addition, several recent publications have highlighted the attractive properties of liquid metals for CSP applications. A new contribution to this topic has been launched by Karlsruhe Institute of Technology (KIT) and the Solar Institute of the German Aerospace Center (DLR), combining their experience in CSP and liquid metal technology. The overall goals of this project are planning, design, construction and operation of a small concentrating solar power system in the 10 kW thermal range (named SOMMER) using liquid metal as heat transfer fluid for re-gaining operation experience and validating design methodology and providing a complete design concept for a large pilot CSP plant based on liquid metal technology, up to evaluation of O&M cost and levelized cost of electricity. This paper describes the current status of the work on the design and setup of SOMMER, the research goals of this facility, first results of numerical activities in view of the liquid metal cooled receiver design and the connection to the design activities for the pilot plant.

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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.