Adsorption processes on porous materials currently form the core element in numerous technical processes. These include gas storage and separation, heterogeneous catalysis and particularly thermally driven heat pumps and chillers. Our research is focused on the application of a new class of materials of metal organic frameworks (MOF) to these heat conversion processes for resource-efficient generation of heating and cooling energy. We have succeeded in synthesizing various water-stable MOF compounds and developing two complementary coating processes, using which MOFs can be applied to heat exchanger structures. Our work has already reached a pre-industrial level.
At present, silica gels or zeolites are used in adsorption heat pumps. The novel, crystalline compounds of the metal organic framework (MOF) class are based on a unique chemical “building-block system” and combine chemical variability with partly enormously high interior surface areas (SBET > 4000 m²/g). Unfortunately, most representatives of this class have so far been quite unstable in water vapor.
Over the last few years, we have been able to identify and synthesize promising compounds, which feature a high water vapor capacity (up to 1.4 g/g) as well as long-term stability (more than 5000 adsorption cycles). At the same time, we were able to upscale the synthesized quantity.
With these new, high-performance adsorbents, the demands on heat and mass transport are also raised simultaneously. Particularly in cyclically operated applications, such as adsorption heat pumps and chillers, it must be guaranteed that the cooling medium (e.g. water, alcohol) has easy access to the surfaces and that good thermal contact is made so that released heat can be removed quickly. We developed two complementary coating processes – one indirect, the other direct – that meet these criteria and filed patent applications for them.
In the indirect coating process, the sorption material is deposited together with a binder onto the substrate structure. Here, flexibility with regard to adsorption and substrate material is advantageous. This allows the adsorption material, the heat exchanger material, and its respective geometrical configurations to be best adapted to the process requirements. We were able to realize coatings on different metals and ceramics. In addition, a wide range of adsorbents, such as silica gels, zeolites, silica alumino-phosphates (SAPOx), or also MOFs can be used. In the direct crystallization process, the functional layer with a thickness of up to 200 μm is deposited directly onto the substrate surface from a solution of the MOF “building blocks”. The thermal contact resistance between coating and the substrate corresponds to that of a classic soldered connection. We were able to prove that the coating is optimally accessible and simultaneously features very high mechanical stability.