Fraunhofer Institute for Solar Energy Systems ISE
Fraunhofer ISE is developing, optimizing and testing heat storage systems for the high-temperature range up to 1400 °C. These storage systems are used wherever a temporal decoupling between heat supply and heat utilization is desired: In the industrial environment, e.g. for heat recovery as well as for power-to-heat applications, or in solar thermal power plants, if stored solar heat is to be used at night for power generation. The technical possibility to temporarily store large amounts of renewable energy at a high temperature level at low cost has a wide range of applications and contributes significantly to the decarbonization of processes. Thermal storage can also be used advantageously for the provision of industrial cooling. While high-temperature storage tends to be used in power plants and industry, heat and cold storage at lower temperatures is more relevant in the building sector.
The R&D services of Fraunhofer ISE range from material and component development to optimization of a storage system for the corresponding application and integration into the respective overall system. For all development steps, we have extensive practical experience from existing systems. For validation of numerical simulations and testing of components and systems, we operate a dedicated high-temperature lab.
Fraunhofer ISE researches and develops solutions for various storage technologies:
Sensible heat storage
In sensible heat accumulators, the supply or withdrawal of thermal energy is perceptibly (sensibly) indicated by a change in the temperature of the storage material. Depending on the desired temperature range, water (up to 100 °C), thermal oil (up to 400 °C), molten salts (up to 700 °C), as well as packed or solid-state storage systems, e.g. with air as the heat transfer fluid (up to 1400 °C), are used. With our experience in the different storage technologies, we are able to select and adapt the optimal storage technology for a wide temperature range and for the respective application.
Sensible heat storage systems are either designed as single-tank liquid storage systems with thermoclines, in which there is a vertical separation between the cold and hot storage media. Alternatively, two-tank systems are built in which the storage medium is pumped from one tank to the other during charge and discharge. Sensible heat storage systems based on molten nitrate salt are used in solar thermal power plants or CSP/PV hybrid power plants, where they enable demand-driven power generation long after sunset by buffering large amounts of energy.
R&D questions exist, for example, with regard to the choice of materials, the improvement of the separation of hot and cold zones, and the development of optimized operating strategies.
Latent heat storage
Latent heat storage systems primarily use the change in the aggregate state of a phase change material for storage and change their temperature only slightly during the charging and discharging process. In the case of a solid-liquid phase transition, the latent heat corresponds to the heat of fusion or crystallization of the storage material. The constant temperature during dischargin and a potentially high energy density are among the advantages of latent heat storage systems over purely sensible storage systems.
Depending on the desired temperature level, organic substances, salts or metal alloys with phase transitions between -30 °C and 1400 °C are used as storage materials. Possible designs of latent heat storage systems include micro-encapsulation, macro-encapsulation or embedding of a heat exchanger.
The development, characterization and selection of suitable phase change materials as well as research into efficient methods for heat transfer are the focus of Fraunhofer ISE's activities in this field.
Sorption storage
Sorption storage systems are heat storage systems based on the physical interaction between two materials, the sorbent and the sorbate. The adsorption of gases on porous materials such as silica gels, zeolites or metal organic frameworks (MOF) is often used, but the absorption of a gas in a liquid is also used. In thermochemical storage, the chemical bond between two substances is used to store heat. An example of this is the hydration (water bonding) of salts such as CaCl2, MgCl2 or MgSO4. Both concepts offer a quasi lossless way to store heat. Compared to sensible storage, sorption and thermochemical storage potentially offer higher storage densities. However, their use requires more complex systems and tends to involve higher investment costs compared to purely sensible storages. Therefore, a comprehensive analysis of the system, boundary conditions, material, reactor design and thus heat production costs is essential to determine the advantages and disadvantages of different storage solutions.
