Impact of transient storage temperature on heat transport system design for a thermal energy storage unit in vehicle applications

Abstract

Thermal energy storage units based on metallic Phase Change Materials (mPCM) are a promising potential solution to improve the thermal management of battery electric vehicles. In case thermal energy is needed by a consumer, the storage of thermal energy is potentially cheaper, smaller and lighter than the storage of electric energy. Furthermore, the usage of electric energy from the traction battery for heating goes along with a reduction of vehicle range. Important properties of thermal energy storages in vehicle applications are a high energy and power density, in order to realize a compact design and fast in- and output of heat. Therefore, latent as well as sensible heat of the mPCM is utilized to maximize system performance, leading to a dropping storage temperature over time in heat extraction mode. In this study, the impact of the transient storage temperature on heat transport system design is investigated based on theoretical considerations. The storage material considered in this study is the mPCM Al-12wt%Si (melting point 577 °C). Operation storage temperature range investigated is 100 - 600 °C. Resulting heat flux and wall temperature as a function of transient storage temperature, heat extraction rate and overall thermal resistance is studied based on a simple 1-D model. The results show a big variation of potential heat fluxes from the storage to the heat transport system, indicating the importance of a control concept. High wall temperatures of the heat transport system are found for most of the operation time taking a constant heat extraction rate of 15 W/cm² as a basis. For storage temperatures < 200 °C, the reduction of the thermal resistance plays a vital role in order to achieve the required heat output. It is shown that the transient storage temperature is challenging for an evaporation process because of temporarily high wall overheating and the risk of film and transition boiling. Single phase liquid heat transport is challenging because of the risk of evaporation and high fluid viscosities at lower temperatures in case thermal oils are used. Single phase systems with a gaseous medium show the highest flexibility to a variation in storage temperature, however, are only effective when the process medium pressure is increased and the storage temperature is sufficiently high. It is concluded that the transient storage temperature is a critical boundary condition for heat transport system design. The main challenge is that there is not only a single operation point, but a wide range of operating temperature where the system has to be functional.