Receiver Heat Transfer Model
Professor Luke Venstrom
We are developing a solar process for producing magnesium (Mg) from magnesium oxide (MgO) via electrolysis in a molten salt. Magensium is a valuable commodity with an increasing demand currently produced in fossil fuel and electric energy intensive processes that significantly contribute to global climate change. Our process reduces the electric energy required by operating at high temperature and also displaces electric energy with solar thermal energy, mitigating greenhouse gas emissions. In order to effectively deliver the solar thermal energy to the MgO electrolysis process, we propose the Active Reflux Sodium Heat Pipe Receiver. In this concept, sodium is evaporated by high-flux solar radiation at the top of a power tower. The sodium vapor then flows to the ground to the electrolytic cells where it condenses, liberating the heat of vaporaziation to heat the cells. The condensed sodium is then pumped back to the tower and reused for continued heating.
We are testing the concept of the Active Reflux Sodium Heat Pipe Receiver using a laboratory-scale 5 kW receiver and the VALPO solar furnace. Our tests focus on developing the heat pipe-electrolytic cell interface. We thus are using conventional sodium heat pipes in which the sodium flows by gravity to avoid the need to actively pump the sodium. In this study, we develop a numerical model for the heat transfer in the solar reciver. The model uses a finite volume method to solve for the distribution of temperature throughout the receiver.The purpose of this model is to determine (1) the time it takes the heat pipes to reach operating temperature of 1400 K, (2) the rate of heat transfer through the heat pipes, and (3) the thermal efficiency of the receiver.
The model predicts a heat up time of three minutes for the heat pipes to reach 1400 K. The resulting rate of heat transfer through the heat pipes to the outside air is 298 W. The thermal efficiency of the receiver, defined as the fraction of solar thermal input that is transferred through the heat pipe to the electrolysis cell is low while the receiver heats up as most of the solar energy is utilized for heating. At steady-state, the receiver efficiency is greater than 15%. The thermal efficiency is expected to increase with additional heat pipes.
Sondag, Nicholas and Yager, Jacob, "Receiver Heat Transfer Model" (2015). Symposium on Undergraduate Research and Creative Expression (SOURCE). 482.