Descrizione

Semi-permeable membrane-based enthalpy exchangers (MEE) are considered as a key component in the new generation of heating, ventilation and air conditioning systems. MEE allow simultaneous heat and moisture exchange between two flowing fluids. They are first and foremost applied in ventilation systems to reduce or even avoid a humidification or dehumidification demand. It not only provides an environmentally sustainable, energy-efficient solution for the building sector, but also facilitates fine-tuning of humidity control and thus plays a relevant role in the health sector. Furthermore, there are additional implementations in development that take advantage of MEE, such as frost prevention in air-sourced heat pumps or the avoidance of undesirable dehumidification in cooling applications. In this context, new research challenges and opportunities are arising, especially related to the operation of MEE under condensation and freezing conditions, which require further investigation to bring the enthalpy exchangers' technology to its full potential. 

Thus, we state following research questions: 

1. How can the properties of membranes be accurately described, modelled, and simulated in the event of imminent ice formation? 
2. How does the ice accumulate on the surface and in the interior of the membrane under changing environmental conditions. How can this process be adequately modelled by means of numerical methods? 
3. How does the shape and the so-called spacers of the MME influence the heat and moisture transfer efficiency and how can they be successfully applied to increase the exchanger’s efficiency without adapting the membrane properties? 

Responding the above questions two table-top experiments located in climate chambers are planned. The first experiment is designed to measure the moisture permeability of membranes. For this purpose, a high-accurate setup is engaged in which the membrane is exposed on one side to a pressurized air flow with controlled temperature and relative humidity. The dew point in the air before and after the membrane is measured using dew point mirrors. On the other side of the membrane sits a vessel (wet cup) containing stagnant air and a salt solution ensuring constant relative humidity, therefore moisture diffusion through the membrane is induced. For the investigation under condensation and frosting condition, the experiment is modified by replacing the wet cup with a secondary air channel. The condensate or frost layer thickness on the membrane surface, as well as the moisture content in the membrane are measured via a single-sided NMR-device. The process is investigated under laminar and turbulent flow conditions. In addition, a CFD-model allowing simulation of condensate and frost formation is developed and validated against the experimental data. The validated model is subsequently engaged to explore promising flow configurations, spacers’ and partitions’ geometries as well as different membrane typologies.

Partner

Lead Partner Fraunhofer Italia, Partner Università di Innsbruck