A large number of industrial processes generate waste heat. With rising energy prices, the storage and use of waste heat as a substitute for fossil fuels is becoming increasingly attractive. In the "Fenopthes" project funded by the German Federal Ministry for Economic Affairs and Climate Protection, the Fraunhofer Institute for Solar Energy Systems ISE has worked with industrial partners to develop and optimize cost-effective tower packings for thermal storage. They replace expensive high-temperature fluids. The technology has now been tested on an industrial scale.
Thermal oils and molten salts are typically used as working fluids in high-temperature storage systems that operate in the 250 to 600 °C range. If tower packings are used as a storage medium in thermal storage tanks, the filling quantity of these expensive storage media can be reduced. In addition, other cost-effective gaseous working fluids such as air can be used, which allow temperatures of up to 1000 °C and more. When using air as a heat transfer fluid, which itself has a very low heat capacity, the energy is stored in the storage mass of the packing.
"In the project, we investigated what shape, size and properties, such as density and thermal conductivity, such fillers should have. The aim was to achieve the best possible heat transfer and the most compact packing possible in the storage tank," explains project manager Julius Weiss from Fraunhofer ISE.
High cost-saving potential possible
The project investigated ceramic tower packings developed by industrial partner Kraftblock GmbH. They consist of a low-cost recycled material, phosphate binder and additives. Replacing expensive fluids with fillers with the same or, in the best case, a higher thermal capacity results in a potential cost saving of around 30%.
The tower packings were produced in different shapes in order to investigate the influence of the geometry on the thermal efficiency of the storage tank. First, the compatibility of the packing with different high-temperature media (thermal oils, molten salt) was investigated by means of ageing tests. The packing and fluids were then chemically analyzed and the mechanical stability of the packing was tested. In order to characterize the different packing configurations, water was flowed around them in a test stand, which behaves physically like a molten salt under certain boundary conditions. The project team observed that different packing configurations exhibit different temperature profiles and time curves during cyclical charging and discharging of the storage tanks. A selected configuration of the packing was also analyzed experimentally in a molten salt storage tank at Fraunhofer ISE.
Demonstration of high-temperature storage in industrial operation
A specific application in an industrial company shows that the energy efficiency of industrial processes can be improved with storage technology through the temporally decoupled utilization of recovered heat. At Comet Schleifscheiben GmbH, a demonstrator of an air storage unit with tower packings was tested. This stores the waste heat from the firing of ceramic grinding wheels before it is reintegrated into the process. The heat was recovered by a heat exchanger integrated into a chimney. One challenge here was the temperature level of the waste heat: the exhaust air is traditionally "diluted" with ambient air and thus cooled before entering the chimney, which makes it difficult to achieve high temperatures.
According to the researchers, the decision as to which storage system is the best solution depends on the specific conditions of the production process (temperature levels, volume flows, flexibility in reintegrating the heat, predicted number of cycles, available space).
Wide range of industrial applications
The project team sees many possible applications for the intermediate storage and use of industrial (waste) heat: Processes in a wide temperature range between 150 and 900 °C are suitable for this, from the paper, food or chemical industries to steam generation. "It's not just the recovery of heat that is exciting for industry. The topic of Power2Heat is also becoming increasingly important, as more and more processes are being electrified," explains Dr. Thomas Fluri, Group Manager Climate Neutral Industrial Processes and High Temperature Storage. In this way, heat can be generated at times when electricity prices are low and made available throughout the day. This allows loads to be shifted from peaks to off-peak times, saving companies money and relieving the strain on electricity grids. www.ise.fraunhofer.de
