Design of a thermosyphon-based thermal valve for controlled high-temperature heat extraction

Conventional concentrated solar power (CSP) is a reliable alternative energy source that uses the sun's heat to drive a heat engine to produce electrical power. An advantage of CSP is its ability to store thermal energy for use during off-sun hours which is typically done by storing sensible heat in molten salts. Alternatively, thermal energy may be stored as latent heat in a phase-change material (PCM), which stores large quantities of thermal energy in an isothermal process. On-sun, the PCM melts, storing energy. Off-sun, the latent heat is extracted to produce dispatchable electrical power. This paper presents the design of a thermosyphon-based device with sodium working fluid that is able to extract heat from a source as demand requires. A prototype has been designed to transfer 37 kW of thermal energy from a 600 degrees C molten PCM tank to an array of 9% efficient thermoelectric generators (TEGs) to produce 3 kW of usable electrical energy for 5 h. This "thermal valve" design incorporates a funnel to collect condensate and a central shut-off valve to control condensate gravity return to the evaporator. Three circumferential tubes allow vapour transport up to the condenser. Pressure and a thermal resistance models were developed to predict the performance of the thermal valve. The pressure model predicts that the thermal valve will function as designed. The thermal resistance model predicts a 5500x difference in total thermal resistance between "on" and "off' states. The evaporator and condenser walls comprise 96% of the "on" thermal resistance, while the small parasitic heat transfer in the "off' state is primarily (77%) due to radiation losses. This simple and effective technology can have a strong impact on the feasibility, scalability, and dis-patchability of CSP latent storage. In addition, other industrial and commercial applications can benefit from this thermal valve concept. (C) 2017 Elsevier Ltd. All rights reserved.