There are various waste heat sources in cities. Municipal utilities, computer centres, commercial districts, sewage treatment plants or solar thermal systems often generate more heat than they need. It is often lost to the environment without being used. Storage technologies could save excess heat and release it when required. It reduces heat-related energy demands as well as carbon dioxide emissions.
The demands placed on thermal energy storages can be versatile: High recovery efficiency, low standby losses, low operating costs, low material costs, long-term or short-term heat storage, high energy density or high loading- and unloading speed. Several researchers try to improve existing technologies and develop new concepts for heat storages.
Thermochemical systems, for example, absorb and save thermal energy through the endothermic reaction. During the exothermic reaction, the opposite of an endothermic reaction, they release thermal energy. Both are chemical or physical reactions. Storage mediums can be, for example, metal hydrides, zeolites or silica gel.
The particular advantage of thermochemical storage systems compared to conventional storages like water tanks is the higher storage density. Another advantages are the wide temperature range between room temperature and 1000°C and the lossless storage for a long time.
Recently, an innovative thermochemical storage system has been brought into service by researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). They intend to store thermal heat more economically and efficiently with lime as the storage medium. The system can be used for processes with high temperatures from about 450°C. It creates calcium oxide during the endothermic reaction. Around eighty percent of the heat input can be stored as chemical energy. The other part is sensible heat. Whilst the sensible energy gets lost in the course of time, the chemical energy is storable for an unlimited amount of time until it is needed. Steam can trigger the exothermic reaction for the release of the thermal energy.
The researchers decided to use lime because of its low material costs and its very high energy density. These two factors have a strong influence on the profitability. Their aim is to create an economical heat storage systems in the future.
Marc Linder, Research Area Manager for Thermochemical Systems at the DLR Institute of Engineering Thermodynamics in Stuttgart, said: “Storing heat thermochemically with lime is an interesting alternative to the more developed technologies in the fields of power plant technology and process heat. In addition to these fields of application, we see potential for lime storage in the seasonal storage of energy, for example, so as to support the supply of heat to households.”
For a flexible storage capacity the researchers enable the burning of any desired amount of lime in a more advanced storage system. There, lime flows through a stainless steel tube. A special heat exchanger heats and burns the lime during the flow. However, if that wouldn’t be enough: It is also possible to release the thermal energy under different temperatures. The steam can be injected at differing high pressures into the burnt lime. The reaction occurs at a higher temperature if the steam pressure is increased. The reaction temperature goes down if the pressure is decreased.
“The challenge with the new lime storage facility is to optimise the interaction of continuous motion of the storage medium together with the supply of heat and the control of the steam,” says Matthias Schmidt, Project Manager at DLR’s Institute of Engineering Thermodynamics.