Sorptive Heat Storage

An important contribution to the achievement of climate protection targets is an improved utilization ratio for both fossil and regenerative primary energy sources. This is done by secondary usage of energy which was not used during its first application. A case in point is the utilization of waste heat created by combustion engines during the generation of power from biogas. The waste heat produced typically makes up over 50 percent of the energy content of the biogas.This not only shows the great potential which lies in the increase of the degree of efficiency, but also the need for temporal and spatial decoupling of secondary energy utilization.In addition, there are many more processes in commerce, energy supply and manufacturing industry which generate large amounts of waste heat. Against the backdrop that 50-60 percent of the EU energy requirements are needed for heat [EUREC, European Renewable Energy Research Centres Agency, 2009] production it becomes apparent that there is great potential for optimizing energy use.

To optimize energy efficiency of processes, there is a need for compact and flexible storage systems to decouple or compensate the supply and demand for heat in terms of location through mobility and with regard to time through minimization of heat loss. Currently available industrially manufactured thermal storage systems regularly only store sensible heat. They usually use water as a storage medium thus restraining the storage density and limiting the storage temperature level to 100 °C at the most. Latent-heat storage units which may achieve slightly better storage density values regularly lack the required flexibility due to their defined operating temperature. The disadvantage of both systems is their permanent heat loss based on the fact that the driving gradient in both systems is the temperature difference between the medium and its environment. Insulation can reduce this effect, but only to a limited extent.

Chemical and sorptive heat storage systems which are counted as thermochemical storage systems, are relatively new, promising technology approaches with considerable benefits compared to both the sensible and the latent-heat storage systems. Here storage densities can theoretically be up to 10 times above those of the medium water; i.e. these systems can store much more energy without requiring a bigger construction volume.

Further information on chemical and sorptive thermal storage methods

Systems with highly porous adsorbent agents

Fraunhofer IGB is currently working with sorption systems utilizing a physico-sorptive bond between the reaction pair adsorbent (A) – adsorptive (B) with a preferably high energy turnover. For wide investigations a closed system adsorbing water vapor in the pores of zeolites and other highly porous adsorbents was chosen and implemented in a small pilot plant.

Outlook

To translate such technologies to industrial serial production quality further development steps are still necessary. The heat storage densities and thermal output which can be achieved with the currently available technical solutions are still too low to be cost-efficient when used for industrial systems and when compared with the theoretically achievable potential. Here, further need for research exists particularly for solutions concerning heat and mass transport and system configurations. The target is to provide industrially relevant solutions, which enable for instance the operation of biogas-based power plants as fully-fledged combined heat and power plants, by using the waste heat energy via storage systems – at different locations or times.