news / 2014-11-03

Thermochemical storage systems ensure load equalisation

The aim of the “Thermochemical energy storage unit for thermal power plants and industrial heat” research project, TcET for short, is to achieve continuous load delivery in fossil fuel-fired power plants. Scientists at TU Munich are investigating ways to operate the combustion chambers of power plants at a constant level of efficiency in the case of fluctuating electrical power output. This is because most fossil fuel-fired power plants are designed to be operated at a constant level of output and are susceptible to reduced service life as a consequence of dynamic load changes imposed by the increased use of wind and solar energy. The problem-solving approach pursued by the researchers involves the temporary storage of energy in a thermochemical storage system.

Storage is under ambient conditions; the process of reaction is at high temperatures. Photo: Institute for Energy Systems – TU Munich

The approach of the TcET project participants involves powering down the power plant only partially in the case of reduced power requirements. "Temporary storage” of the energy produced is instead envisaged. The researchers hope to investigate various storage materials in order to facilitate this process of storage. Materials looked at have in common the fact that they exist in two different variants, as a charged and uncharged material. The terminology in this context relates to chemically bound energy, in that the materials can be “charged” with energy, i.e. store energy delivered from an external source, and release this when necessary.

Power plants are able to adapt their power output flexibly to power requirements. This, however, is possible only within a defined load band. If the minimum heating capacity is fallen short of, the combustion process becomes at risk of instability. The consequence is that the power plant has to be shut down. By using energy storage systems, the boiler can continue to operate without compromising stability, with the power, however, made available for storage rather than turbine operation. In an initial stage, the surplus thermal energy is transferred to the chemical storage medium in fluidised-bed reactors at temperatures of up to 600 °C. Possible storage materials include for instance calcium or magnesium oxide. One advantage with respect to chemical storage systems is the ability to store these materials under ambient conditions.

In the case of heightened energy requirements or if volatile energies produce insufficient power due to lack of wind or daylight, the original, lower energy level of the charged storage media can be released in an exothermic reaction. The thermal energy released can be used to bolster electricity generation in the turbine. The researchers hope to be able to design the storage system such that the power plant is capable of maintaining a consistent level of output over a period of up to three days including when the level of power required by the grid is low. Conceivable degrees of efficiency in power plants with district heating extraction top 80 %; otherwise around 50 % is realistic in the researchers’ current estimation.

Energy is stored temporarily at reduced load and converted back into electricity when necessary. Photo: Institute for Energy Systems – TU Munich

In addition to enhanced consistency, a further crucial advantage results: owing to the constant heat output of the power plant, the turbines remain warm. This on the one hand increases their service life, and on the other, the power plant is able to respond more quickly to shifting conditions in the power grid, as the individual components run continuously at operating temperature. The aim is to implement the shift between storage system operation and power generation with such speed that the necessary power is made available within the minutes reserve.

The work of TU Munich involves calculating and simulating the system integration of various storage systems and the assembly of  laboratory scale storage systems.
Bavarian Center for Applied Energy Research (ZAE) is providing expertise in the area of storage systems <300 °C, if applicable to be used for a potential “cold lead”.
Alstom is providing power plant data and support in the area of dynamic improvement in particular.
E.On and RWE are providing power plant data and expert scientific input.
Märker Kalk is providing expert knowledge in the area of solids handling and equipment and is providing substance samples.
Clariant is providing support in the area of catalysis and is supplying materials.

Projects currently being funded