Operating Principle of Thermal Energy Storage Systems
With the increasing expansion of renewable energy, thermal heat storage systems are becoming increasingly important. They help to balance out fluctuations in energy generation and make renewable heat more widely usable. When combined with solar thermal energy systems, heat can be provided relatively cost-effectively even at night or well into winter. Different technologies are used depending on the storage principle and the required duration of storage.
In generel, the various thermal energy storage technologies can be divided into the following categories:
- Sensible heat storage systems
- Latent heat storage systems (phase-change storage)
- Thermochemical heat storage systems
Sensible heat storage systems are among the most widely used and commercially established heat storage technologies. In sensible storage systems, the input of energy leads to a measurable increase in temperature. The amount of heat that can be stored depends on the specific heat capacity of the storage material, its mass and the temperature difference. Water, for example, has a high specific heat capacity and is therefore particularly well suited for heat storage in the low and medium temperature ranges. For higher temperatures, solids such as concrete are used. During discharge, heat is extracted from the storage medium, resulting in a noticeable decrease in its temperature.
Latent heat storage systems store not only sensible heat but also latent heat. So-called phase-change materials, such as paraffins, salt hydrates and fatty acids, are used as the storage medium. When the storage unit is charged, heat is supplied to the phase-change material. Depending on the properties of the storage material, the heat input triggers a phase transition, during which a large proportion of the supplied energy is stored as latent heat without the temperature of the material changing significantly. During discharge, heat is removed from the storage material, causing the phase transition to reverse. The previously stored latent energy is then utilised. The phase-change storage system offers the advantage of high energy density.
Thermochemical energy storage systems are based on reversible chemical reactions in which thermal energy is stored in the form of chemical bond energy. When heat is supplied, the reactants undergo a chemical reaction and are converted into reaction products. During discharging, the products are converted back into their original reactants via an exothermic reverse reaction, whereby the reaction enthalpy previously supplied is released in the form of heat. A typical example is the hydration of salts. This storage technology enables virtually loss-free heat storage with a comparatively high energy density.