Heat and Cold Storage

Fraunhofer ISE focuses on the development of new materials for sensible and latent heat and cold storage and has an extensive R&D infrastructure and many years of experience in this field. The aim of our research is in particular to increase the storage density and long-term stability of the storage materials. In addition, we analyze the interactions between tower packings, heat transfer fluid and tank or capsule materials. In addition to sensible and latent heat storage materials, we also develop and optimize sorption materials. 

Our R&D services include:

• Material screening of heat storage and insulation materials
• Characterization of thermophysical material properties such as specific heat capacity, heat of fusion and phase transition temperatures, enthalpy of reaction of sorption materials, viscosity, density, thermal diffusivity and thermal conductivity
• Investigation of thermal stability, flash point and chemical composition
• Investigation of outgassing and gas atmospheres in the storage tank
• Tests on the long-term stability and compatibility of storage materials, tower packings, working fluid and storage components
• Development of formulations for the production of PCM emulsions (PCM slurries) on a liter scale and upscaling of production to a cubic meter scale
• Determination of the crystal structure (XRD), phase analysis and conversion of sorption materials
• Thermal stabilization of PCM
• Testing and characterization of PCM and PCM composite materials for the RAL-PCM seal of approval 

Fraunhofer ISE has particularly broad expertise in the development of PCM slurries. These are high-performance heat transfer fluids that make it possible to reduce the required storage volume due to the high heat capacity of the fluid. We disperse organic PCMs in water or water-glycol mixtures to produce a heat transfer fluid that has up to four times the storage density of water in the melting range of the PCM. We are also working on oil-water emulsions for temperature ranges below 0 °C. 

 We are developing composite materials for use in buildings as well as for technical applications, e.g. for battery and electronics cooling. Microencapsulated PCM can be finely distributed in the composite material and a large surface area can be created between the composite material and the PCM, which is conducive to rapid charging and discharging. 

In the field of high-temperature storage systems, we have many years of experience in the development of sensible molten salt storage systems based on nitrate salts, which are used on an industrial scale in solar thermal power plants. We also support project partners in the development of storage systems based on water, thermal oil, metal alloys and ceramic solids. 

 We measure and evaluate the various storage technologies in our laboratories and outdoor test sites around Freiburg or in demonstration plants directly in the application.

Fraunhofer ISE develops components for the efficient use of thermal storage materials. The integration of phase change materials (PCM) in a heat and cold storage system is a particular R&D challenge. In principle, PCMs have a low thermal conductivity, which significantly limits the performance of storage systems. This limitation must be overcome, for example, by using suitable heat exchangers or encapsulations. Another task is the reduction of heat losses.  

The integration of tower packings is a challenge for sensible high-temperature storage tanks. Efficient thermal coupling between the packing and the working fluid and at the same time homogeneous flow through the storage tank are necessary in order to achieve optimum performance of the storage tank. The selection and layering of suitable packing materials enable considerable savings compared to a storage tank without packing material. 

 Our R&D Services include:

• Development of heat exchanger systems and storage concepts 
• Development of macro encapsulations for PCM 
• Integration of PCM in storage tanks that use water as a heat transfer fluid
• Integration into air systems for the air conditioning of buildings
• Development of loading devices, e.g. diffusers
• Thermal, fluidic and mechanical simulations to optimize components
• Development of mass and heat exchangers for sorption storage tanks 
• Development and testing of components: Valves, pumps, heaters, coolers, sensors 

To investigate new heat exchanger solutions, we use various test benches for performance characterization in the temperature range from -30 °C to 700 °C. For storage tanks, we develop methods and components for the optimal storage and retrieval of heat. For this purpose, we develop loading devices such as diffusers for efficient loading with optimized temperature stratification. In addition, Fraunhofer ISE uses simulation tools such as "Comsol Multiphysics" or "Modelica" to optimize heat exchanger structures, macrocapsules and arrangements of heat exchangers in the storage system.

We dimension both sensible and latent storage systems for a wide range of applications, especially in buildings or technical applications. We use our simulation tools "Comsol Multiphysics", "Modelica" and "ColSim", among others, for this purpose. Depending on the requirements, we combine the various storage technologies into hybrid systems and thus design storage systems specifically for a given load profile. 

Our R&D services include:

• Answering design and dimensioning questions in relation to specific conditions such as load and heat supply requirements 
• Storage tank optimization through simulation of its thermal and hydraulic properties 
• Design of single-tank stratified storage tanks with and without fillers
• Design of two-tank systems 
• Design of latent heat and cold storage tanks 
• Characterization of charging and discharging characteristics and heat losses
• Development of hybrid storage systems

The aim of our R&D work is to optimally integrate heat and cold storage systems into the respective overall system. To this end, we develop concepts for hydraulic coupling and operational management. We use detailed numerical simulations that enable an objective assessment of the efficiency of solutions. Examples include the integration of storage systems in systems with fluctuating renewable heat, e.g. solar thermal energy, or for waste heat utilization and increasing efficiency in industry. 

In this way, we want to contribute to the efficient use of energy, reduce installation and control effort and cut costs. 

Our R&D services include:

• Numerical simulation of the overall system 
• Hydraulic integration of the storage tank
• Integration of thermal storage tanks into existing systems
• Adaptation of components to each other
• Development of operational management concepts
• Monitoring, analysis, evaluation and optimization of storage systems
• Fault analysis
• Predictive maintenance
• Life cycle analysis
• Identification of potential / market analysis
• Simulation of the storage system in the context of the energy system 

We plan and create monitoring concepts for the energy analysis of storage systems and for the optimization of operational management strategies. Based on the data collected and user surveys, we evaluate the efficiency of storage solutions, e.g. for passive and active solutions for cooling buildings in terms of comfort and energy efficiency.