Thermal Energy Storage, TES

In the field of energy storage, the following classes are distinguished: mechanical and thermomechanical energy storage (e.g. pumped storage power plants, compressed air storage, etc.), electrical energy storage (superconducting magnetic energy storage (SMES), capacitors, etc.), electrochemical energy storage (lithium-ion and lithium-polymer batteries, nickel-cadmium batteries, lead-acid batteries, etc.), chemical energy storage (metal hydride storage, liquid organic hydrogen carriers, etc.) and thermal energy storage (sensible heat storage, latent heat storage, thermochemical storage [sorption storage]).

For thermal energy storage, we would like to take a closer look at the aspects of energy use and components in more detail.

Power-to-Heat

Power-to-heat (electrical energy to heat) stands for the generation of heat from electrical energy by means of resistance elements. In the context of energy storage, power-to-heat systems in combination with thermal storage systems offer significant advantages over battery storage systems. For example, the conversion of the electrical energy takes place with an efficiency of 99.9% (in contrast, the efficiency of power-to-gas is around 54%) and the resulting heat can be stored well in latent or sensible storage systems. In the case of a battery, there is no change in the state of the energy, but the construction of a battery is significantly more raw material-intensive, more polluting and more expensive than that of sensible thermal energy storage systems. A thermal storage system is also much more tolerant of grid fluctuations (voltage fluctuations) than a purely electrical system.

Heat-to-Power

Heat-to-power systems involve the conversion of thermal energy into electrical energy. One system that has been used for decades is the steam turbine (first developed in 1888 by Carl Gustaf Patrik de Laval) with an efficiency of almost 30%. Modern high-pressure steam turbines sometimes achieve efficiencies close to 50%. This type of energy conversion requires temperatures of around 550°C. In recent years, the ORC (Organic Rankine Cycle) has found a way to make the principle of the steam turbine usable at lower temperatures as well. The operating range here is between 90° and 150°C (efficiency usually 6-10%), as well as around 300°C (efficiency 20- max. 24%).

Another approach is that of the thermoelectric generator, which requires the highest possible temperature gradient between a hot and cold side. The physical effect underlying the thermoelectric generator has been known for a very long time. As early as 1821, the German-Baltic physicist Thomas Seebeck observed that a temperature gradient in a metal can cause a current to flow. However, since the effect is not very efficient, such generators were and are only used in niche markets.

Heat-to-Heat

The direct use of thermal energy is the most sensible-one from an efficiency point of view. If generated heat is used directly, e.g. to heat living space or to heat industrial plants (process heat) and buildings, the available energy can be used without losses. Currently, this is particularly interesting in heating networks. The problem, however, is that the heat source and heat sink are usually not connected locally and the laying of district heating pipes is complex and expensive. The use of mobile heat storage systems can contribute to a significant flexibilisation of waste heat utilisation and is being promoted by corresponding companies.

Heat-to-Steam

The use of steam as an energy carrier has been known for a very long time in industry but also in private households (e.g. with pressure cookers). On an industrial scale, steam (usually water vapour) is used for a wide variety of processes, e.g. to provide heat for cooking, evaporation or distillation processes, to provide heat for endothermic reactions or for drying processes. The particular advantage of (water) steam compared to liquid heat transfer media or thermal oils is that most of the energy is latent in the enthalpy of vaporization (heat of condensation in water approx. 0.6 kWh / 1kg). Due to the manifold uses of steam, the heat-to-steam principle is interesting for all sectors of the economy. On a large industrial scale, steam is essentially produced by two different types of boiler, the shell boiler and the water tube boiler, whereby both types (as a rule) use fossil primary energy sources to produce steam. Even though the use of these fuels is now being made much more efficient by so-called combined heat and power systems, the approach of generating steam by means of “green” energy or process waste heat is to be regarded as a sensible and important building block for the decarbonisation of industry.