Sorption-based Heat Transformation
At the moment about 40 percent of end energy consumption accounts for provision of low temperature heat and hot water, an additional 17 percent for process heat (source: Federal Statistical Office). One promising way to reduce that amount consists in the use of microporous and mesoporous materials like zeolites, SAPOs (silico-aluminophosphates), AlPOs (aluminophosphates) or MOFs (metal-organic frameworks) for adsorption-based heat pumps and chillers. Especially, the adsorption of water, methanol and ethanol is gaining more and more attention.
In Figure 1 an adsorption chiller in a closed design is shown. The process starts with the dry adsorbent adsorbing the working fluid (shown at the right side). The energy needed for the evaporation of the working fluid is delivered by the low temperature loop (blue). During that process, heat of adsorption is released by a cooling loop (green). When the adsorbent is saturated (shown at the left side), a high temperature loop (red) is used to desorb the working fluid, which is collected by condensation at a cooler.
Metal-organic frameworks (MOFs) for heat transformation
Metal-organic frameworks (MOFs) are microporous networks build from metal nodes that are linked by organic molecules. Usually, for the reaction a metal salt and an organic acid were mixed in a solvent and heated under solvothermal conditions. For instance one of the first published MOFs is the well-examined copper trimesate called HKUST-1 (for Hong Kong University of Science and Technology), shown in Figure 2. At the moment mainly MOFs based on aluminum, iron, copper or chromium are examined for heat transfer application.
Besides adsorption capacity of different working fluids like water, methanol and ethanol, the stability over many thousand cycles even at high desorption temperatures as well as sorption kinetics are most interesting. Fast sorption speed results in short cycle times and therefor higher power density, higher efficiency and smaller dimensions of the apparatus. Cycle times are not only influenced by the applied material but by the diffusion of the working fluid and the heat transfer also. In state-of-the-art adsorbers the pelletised material is loosely filled around a heat exchanger, a setup that lacks of good heat and mass transfer. To improve these, the adsorbent can be coated directly on the heat exchanger.
Coating of Heat Exchangers
The performance of thermally-driven heat pumps and chillers depends not only on intrinsic material properties, also, it is a matter, how fast the heat of adsorption resulting from the process can be dissipated and how fast the adsorbent can be reached by the working fluid. To improve heat and mass transfer the heat exchanger can be coated for instance by direct crystallisation, meaning that the adsorbent directly grows on the surface of the heat exchanger. To produce a coating by this technique, a structure is set in a reaction solution that is cooled from the outside and subsequently heated. The heating and cooling results in a thermal gradient that is guiding the reaction to these areas of the surface, that show the highest heat flux.
The process of the thermal gradient deposition is investigated in the experimental setup shown in figure 3. The substrate (coloured red) is fixed at the heating device and immersed in the solution that is cooled from the outside (coloured blue). In this setup synthesis conditions like solvents, reactants, temperature, temperature gradient and different surfaces can be tested. Following on those experiments the procedure is transferred to 3D-structures like fibres and real heat exchanger respectively. Products of these three steps are show in figure 4.
