This study presents a novel concept for a direct contact latent heat storage, which can be used for storing thermal energy from industrial waste heat or renewable energy systems. A storage medium, phase change material (PCM), and mineral oil heat transfer fluid (HTF) are placed in direct contact. The PCM and the oil are not miscible. The direct contact leads to an improved heat transfer and faster loading and unloading periods with a high storage density. Only affordable salt hydrates or mixtures of salt hydrates and additives are used as the PCM. For the scaling of the system, the heat transfer coefficient of the complex system must be known. A cylindrical channel inside the PCM is created, and the heat transfer coefficients between the oil and solid or melting PCM is investigated. Subsequently, the transmission in a real channel is described and discussed.

Introduction

Storage of thermal energy is an important issue for the upcoming energy transition in Germany, due to an increased use of renewable energy sources instead of fossil fuels. Furthermore, the use of industrial waste heat is desirable to reduce the number of primary energy sources and corresponding amount of greenhouse gases produced. Consequently, it is necessary to provide storage technology to synchronize the supply and demand of thermal energy. In general, there are three main types of thermal storage [1]:  sensible heat storage  latent heat storage  thermochemical storage Despite high storage densities of approximately 500 kWh/m³, thermochemical storage systems have the major disadvantage that the technology is complex and not mature [2]. Compared to sensible heat storage, latent heat storage systems provide the following advantages [3]:  removal and supply of thermal energy at an approximately constant temperature  high storage density that is ~50–۱۰۰ times greater than that in sensible heat storage Figure 1 shows, in principle, the difference between sensible and latent heat storage for the supply and removal of thermal energy. With sensible storage, energy supply results in a continuous nearly linear temperature increase. However, with latent heat storage, energy supply results in a temperature increase until the melting temperature of the PCM is reached. After that point, additional energy supply results in phase change without a further increase in temperature until the material is fully melted, after which the temperature again increases. It can be seen that a smaller temperature difference, Δϑ, between a loaded and unloaded latent heat storage results in a higher stored energy, ΔQlatent, compared to the stored energy, ΔQsensible, of a sensible heat storage. This is because latent heat storage makes use of the phase change enthalpy of the PCM.