Battery-powered electric buses currently face the challenges of high cost and limited range, especially in winter conditions, where interior heating is required. To face both challenges, the use of thermal energy storage based on metallic phase change materials for interior heating, also called thermal high-performance storage, is considered. By replacing the battery capacity through such an energy storage system, which is potentially lighter, smaller, and cheaper than the batteries used in buses, an overall reduction in cost and an increase of range in winter conditions could be reached. Since the use of thermal high-performance storage as a heating system in a battery-powered electric bus is a new approach, the requirements for such a system first need to be known to be able to proceed with further steps. To find these requirements, a review of the relevant state of the art of battery-powered electric buses, with a focus on heating systems, was done. Other relevant aspects were vehicle types, electric architecture, battery systems, and charging strategies. With the help of this review, requirements for thermal high-performance storage as a heating system for a battery-powered electric bus were produced. Categories for these requirements were the thermal capacity and performance, long-term stability, mass and volume, cost, electric connection, thermal connection, efficiency, maintenance, safety, adjustment, and ecology.

Latent heat storage systems are a promising technology for storing and providing thermal energy with low volume, mass and cost requirements, especially when operated at high temperatures. Metallic phase change materials are particularly advantageous for high thermal input and output, which is especially important for mobile applications. When designing a storage system, it is essential to have precise knowledge about the potential storage capacity. However, the system’s storage capacity is typically calculated from material properties determined at lab scale, although systemic boundary conditions can have a considerable influence. Systemic influences can result from thermal and reactive interfaces or from the storage design. In order to consider these influences, we propose three calorimetric procedures to thermally analyse high-temperature metallic latent energy storage systems at an application scale. We examined the procedures in a transient simulation environment, monitoring the storage capacity of the system. The procedure, based on adiabatic conditions, shows the least deviation from the simulation input parameters, but is limited to the heating process of the storage. Discharging the storage can be represented by isoperibolic conditions with controlled heat exchange. The precision of the procedures depends on the evaluation routine, the calibration routine, the heat extraction rate and the thermal inertia of the test bench.

The application of aluminium-based metallic phase change materials (mPCMs) in latent heat thermal energy storage is limited by the poor compatibility of molten alloys with conventional stainless steel containers. This work evaluates thermally sprayed oxide coatings deposited directly on 304 stainless steel as protective barriers for molten Al–Si and Al–Cu–Si phase change materials. Alumina (Al₂O₃), zirconia (ZrO₂) and Al₂O₃/ZrO₂ (60/40 wt. %) coatings were produced by atmospheric plasma spraying (APS) and high-velocity suspension flame spraying (HVSFS) and assessed through coating characterisation, long-term static compatibility tests, thermal cycling experiments and thermal contact resistance measurements. APS coatings exhibited a pronounced dependence of coating thickness on spray angle, whereas the HVSFS coating provided a more uniform thickness distribution. After two weeks of static exposure, no reaction layer formation, coating dissolution or detectable elemental interdiffusion was observed for the investigated coating systems. For the Al₂O₃/Al–12.3 wt. % Si system, no coating degradation or interfacial reaction was detected after approximately 300 thermal cycles. Furthermore, average liquid thermal contact resistances of 0.31 × 10⁻³ and 0.39 × 10⁻³ m² K W⁻¹ were measured for smooth and rough Al–Si/coating interfaces, respectively. These results demonstrate that thermally sprayed oxide coatings can effectively protect stainless steel from molten aluminium-based PCMs while providing design-relevant data for the development of high-temperature latent heat thermal energy storage systems.