Fraunhofer Institute for Solar Energy Systems ISE
More than 40 percent of the final energy demand in Germany is used for the provision of heat and cold in buildings. Consumption is subject to strong seasonal, daily and weekly fluctuations. Thermal storage is therefore a key technology for ensuring the flexible provision of heating and cooling in buildings. By using thermal and also electrical storage solutions, the generation and consumption of renewable energy can be decoupled from each other.
With the energy transformation and the greater availability of renewable energy, the requirements on the dimensioning and control of supply systems in buildings increases. In addition, global warming increases the need for building cooling. Building developers, property developers of large installations, technical building equipment suppliers as well as component and material manufacturers are increasingly faced with the challenge of providing heating and cooling in their buildings and systems in a demand-oriented, cost-effective and efficient manner.
Fraunhofer ISE develops and optimizes heat and cold storage systems for residential and office buildings as well as for power plants and industrial applications. In the building segment, we work on storage solutions for temperatures ranging from 0 to around 100 degrees Celsius, storage capacities of up to 500 kWh and a storage duration of a few hours up to several days. We cover the entire value chain and support our customers in the selection of suitable technologies, the development of new materials and components, the optimization of storage systems as well as in analysis, monitoring and operational optimization.
Fraunhofer ISE researches and develops solutions for various storage technologies:
Latent heat storage
Phase change materials do not change their temperature during charging and discharging. The change in the state of aggregation is used for storage. In the case of a solid-liquid phase transition, the latent heat corresponds to the heat of melting or crystallization of the storage material. Within a defined, narrow temperature range, phase change materials offer significantly higher storage densities than conventional sensible storage materials. Phase change materials can be used in many applications in buildings and industry. They are also very well suited for temperature buffering in batteries, electronic components or household appliances. The development, characterization and selection of suitable phase change materials as well as the research of efficient methods for heat transfer are the main focus of Fraunhofer ISE's activities.
Sensible storage/water storage tanks
Sensible heat storage using water as a heat transfer medium is already being used on a large scale in buildings for domestic hot water and space heating, e.g. as decentralized storage tanks in the home or as central storage systems for buildings and real estate, including residential areas. In these storage systems, the supply and extraction of thermal energy manifests itself noticeably by a temperature change. Water is very well suited as a heat transfer medium because it has a comparatively high specific heat capacity, is widely available and safe for the environment. Nevertheless, there are still R&D issues, e.g. with regard to reducing heat losses through better insulation or the development of optimized charging and discharging devices.
Sorptive storage
Sorptive systems store heat based on the physical interaction between two substances, the sorbent and the sorbate. Often the adsorption of gases on porous materials (adsorption) such as silica gels, zeolites or Metal Organic Frameworks (MOFs) is used, but liquids can and are also used for gas absorption. In thermochemical storage, the chemical bond between two substances is used for heat storage. The hydration (binding of water) of salts such as CaCl2, MgCl2 or MgSO4 is an example here. Both concepts offer a virtually loss-free possibility to store heat. Compared to sensible storage systems, sorption and thermochemical storage systems offer higher storage densities. They are used in the building sector in air conditioning and ventilation systems, for example. However, these systems require more complex systems and generally higher investment costs compared to purely sensible storage systems. Therefore, a comprehensive analysis of the system, the boundary conditions, the material, the reactor design and thus the costs of heat generation is essential in order to determine the advantages and disadvantages of different storage solutions.
