Conception and layout of a measuring cell to determine heat transfer coefficients in molten salt

Kurzfassung

Charging molten salt-based sensible heat storage systems by using electrical resistance fluid heaters is a promising technology with the potential to be a key component on the global pathway towards a renewable energy supply. It can be applied either to complement existing thermal energy storage systems of concentrated solar power plants or as stand-alone, large scale energy storage systems. The forced convective heat transfer process between electrical resistance fluid heaters and molten solar salt needs to be known in detail in order to be able to design efficient and cost-effective fluid heaters for this application. As empirical correlations for forced convective heat transfer processes have been found to be invalid for molten salt at high Reynolds and Nusselt numbers, detailed investigations of the heat transfer processes in flow conditions characteristic for electrical fluid heaters are needed. This thesis develops a layout of a measuring cell, that allows the determination of heat transfer coefficients in molten solar salt, locally resolved around the circumference of a heating rod. This is achieved by deducing a characteristic flow configuration typical for fluid heaters and designing a heater cell geometry that allows measurements in this conditions. Additionally are applicable measuring methods and equipment evaluated and suggested, especially to be able to measure the wall temperature of a heating rod in a molten salt flow. The investigation has shown that a cross-flow configuration of a single cylinder is representative for typical flow conditions in an electrical resistance fluid heater. A geometry including an electrical cartridge heater leading to these conditions has then been designed and optimized using numerical methods and by developing approaches to quantify how well the wanted conditions are met. Numerical simulations of the resulting geometry, including a representation of the cartridge heater, have then shown that the placement of a metal tube with a low thermal conductivity around the cartridge heater is needed to allow a locally resolved measurement of the heater‘s surface temperature, which is needed to evaluate the wanted heat transfer coefficients. Analyzing the error propagation of the potential measurement errors of all quantities needed to discuss the heat transfer have shown that the main contributor to uncertainties in the Nusselt number are caused by large uncertainties in material properties.