Numerical Optimization and Power Output Control of a Hot Thermal Battery with Phase Change Material

Numerical simulations were conducted to investigate the release of heat from a thermal storage unit, which we refer to as a hot thermal battery. The battery is composed of a hexagonal arrangement of parallel tubes through which a heat absorbing fluid flows, surrounded by phase change material (PCM) that fills spaces between adjacent tubes. The simulations implemented, aimed to optimize the battery such that it meets a total volume constraint while ensuring a critical power output before the phase change fronts of the PCM surrounding two adjacent heat transfer tubes merge-indicating the PCM is completely solidified-after which, only sensible heat can be released by the battery. It was found that the PCM latent heat has negligible impact on the optimal heat exchanger (HEx) design. In comparison, increasing either the flow velocity of the heat transfer fluid in the tubes or PCM thermal conductivity can significantly reduce the needed volume of heat exchanger. Additionally, a novel closed loop control modeling approach is proposed to dynamically tune the heat transfer fluid flow rate such that the thermal battery yields a constant power output. The flow tuning results indicate the optimal dynamic HTF velocity curve shape, obtained from closed-loop method, is unique and this optimal flow velocity is dependent on the location of the phase change front. Numerical results were also compared against hot battery discharge experiments, using both constant and dynamically tuned flow rates, indicating a good agreement for both cases.