Membrane-based enthalpy exchangers

The comfort within residential buildings is often related to indoor climate conditions. However, sufficient air quality can only be guaranteed if ventilation rates exceed minimum standards. In most cases this is realized by window ventilation, a process accompanied by a tremendous loss of energy. Applying modern building ventilation systems instead, is a promising approach to reduce losses and enhance sustainability. The reason is that building ventilation systems make use of energy recovery devices. In air-to-air heat exchangers, discharged and fresh air get in contact via impermeable exchanger plates. Here, energy recovery is limited to sensible heat. Substituting the exchanger plates with water vapor permeable membranes makes the device also capable of recovering latent heat (in terms of water vapor). Such devices are called membrane-based enthalpy exchangers. Efficiency is typically a function of fluid dynamics, material properties and process parameters. The scope of this thesis is to describe the governing parameters, identify transport limitations and point out potential solutions to successfully tackle such limitations. First, the influence of vapor activity on membrane permeance was evaluated for different materials. Single-gas measurements proved that permeance is a strong function of both feed and permeate activity. Depending on the polymer of the selective layer, a change in activity either enhanced or reduced membrane permeance. In complementary mixed-gas measurements the overall transport resistance (including boundary layer and membrane support) was deconvoluted in detail. While the impact of the support changed with the membrane sample, the boundary layer had a similar effect regardless of the material. Performance loss due to the stagnant boundary layers was successfully minimized by application of so-called membrane spacers. Finally, a commercial software (Aspen Custom Modeler ® ) was used to model the heat and mass transfer in membrane-based enthalpy exchangers. By means of model predictions it was possible to identify the economic limits of material optimization. An optimization beyond this limit will only make sense if the impact of the stagnant layer is reduced simultaneously. A case study revealed that the actual saving potential of membrane spacers depends on multiple parameters like outer climate conditions, energy prices and the humidification technology of the corresponding building ventilation system. Even though the focus of the study is on membrane-based enthalpy exchangers, results and experimental approaches can be useful for many other applications like technical (de)hydration processes and the optimization of functional clothing. In addition upcoming material developments might boost the impact of boundary layers in gas permeation processes not related to water vapor. If so, the findings of this thesis will help to identify and overcome transport limitations of these applications, too.