High-temperature energy storage
High-temperature energy storage systems, or more precisely high-temperature heat storage systems, are a subgroup of heat storage systems. In the literature, they are usually referred to as HTS (High Temperature Storage) or HTES (High Temperature Energy Storage). Starting from a temperature of 400°C, storage systems are referred to as high temperature. The operating range of heat storage systems extends from room temperature to 1300°C and is basically divided into two types. On the one hand, there is the class of latent heat storage, which includes salt storage or PCM storage (phase changing materials), and on the other hand, there is the class of sensible heat storage, which works exclusively according to the principle of temperature change of a storage medium.
Latent heat storage systems are classically constructed as salt storage systems. Here, the choice of storage salt (e.g. mixtures of potassium and sodium nitrate / carbonate salts) determines the minimum and maximum working temperature of the storage. A mixture of potassium and sodium salts is most commonly used. The minimum temperature is around 290°C (melting point of the salt), the maximum temperature of the salt is around 590 - 600°C. Above this temperature, salt shearing begins, where the molten salt begins to become chemically unstable. The molten salt mixture is used, for example, as a heat transfer medium in solar thermal power plants. Storage salt mixtures have a high specific heat capacity, which makes them so interesting for the use as a storage medium. Another advantage is that the storage salt can act both as a storage medium and as a heat transfer medium. By adding other salts, the melting point can be lowered significantly. The addition of sodium nitrite to the mixture of potassium and sodium nitrate, for example, leads to a lowering of the melting point to 140°C.
Basically, salt storage facilities have a number of problems: First of all, the corrosive properties of the molten salt and the resulting need for high-quality and thus expensive plant components must be mentioned. An increased risk to humans and the environment from damaged storage facilities cannot be excluded either. Furthermore, the storage tank temperature must be kept well above the solidification point of the salt mixture, otherwise the storage tank as well as the pumps and pipes will “freeze”. If no thermal energy is available from the heat source, the temperature in the storage tank must be kept at the minimum level using primary energy sources.
Storage systems that work with sensible storage materials have a somewhat lower heat capacity than latent systems, but are not subject to their limitations. The theoretical working range here extends from room temperature to 1200°C (in the case of minerals, e.g. magnesium rocks) or, in the case of synthetically produced rocks, up to 1300°C. In contrast to latent systems, the heat transfer medium can be changed much more easily. Here, air or gases can be used directly or, by means of an intermediate heat exchanger, also molten salts or thermal oils. Due to the possibility of storing and releasing high temperatures, sensible storage systems enable the use of more efficient technologies when it comes to utilising the stored heat. For example, from a temperature of around 500°C, the use of modern steam turbines with a theoretical efficiency of 53% (actually in the range of 37-40%) is possible.
In principle, high-temperature storage systems are a future-proof building block to significantly improve the conversion of energy generation in terms of sustainability and environmental friendliness. The storage of surplus energy (in the case of renewable energies) and the (more efficient) use of process heat make it possible to significantly accelerate the energy transition on two fronts. The possibility of absorbing energy from discontinuous processes and making it usefully available or the physical and geographical separation of heat source and heat sink are also aspects that make this area so interesting and adaptable. In the future, research into innovative material mixtures will make it possible to significantly improve the heat capacity and thus the system conduction of the sensible storage units.