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Enthalpy – Universität Innsbruck

Enthalpy - Behaviour of semi-permeable, membrane-based enthalpy exchangers under icing conditions

Project leader overall project: Fraunhofer Italia

Project leader: Michele Bianchi Janetti

Project collaborator: Jakob Plesner

Project partner: Fraunhofer Italia

Funding body: Province of South Tyrol-Alto Adige

Duration: 1 June 2024 - 31 May 2027

Project website: https://innovationforschung.provinz.bz.it/de/projekte/membrane-frosting-conditions

Summary

Semi-permeable, membrane-based enthalpy exchangers (MEE) are considered a key element of the new generation of heating, ventilation and air conditioning systems. MEEs enable simultaneous heat and moisture exchange between two flowing fluids. They are primarily used in ventilation systems to reduce or even eliminate the need for humidification or dehumidification. This not only provides an environmentally friendly and energy-efficient solution for the building sector, but also enables precise control of humidity, which is particularly important in the healthcare sector. In addition, other areas of application are being developed that utilise the advantages of MEE, such as frost prevention in air heat pumps or the avoidance of undesirable dehumidification in cooling applications.

In this context, new research challenges and opportunities arise, particularly with regard to the operation of MEE under condensation and icing conditions. These aspects require further investigation in order to realise the full potential of enthalpy exchanger technology.

Research questions:

  • How can the properties of membranes be precisely described, modelled and simulated in the event of impending ice formation?
  • How does ice accumulate on the surface and inside the membrane under changing environmental conditions? How can this process be adequately modelled using numerical methods?
  • How do the shape and the so-called spacers of the MEE influence the heat and moisture transfer efficiency, and how can these be successfully used to increase the efficiency of the heat exchanger without adjusting the membrane properties?

To answer these questions, two table-top experiments in climate chambers are planned.

The first experiment is designed to measure the moisture permeability of membranes. For this purpose, a high-precision experimental set-up is used in which the membrane is exposed on one side to a pressurised air flow with controlled temperature and relative humidity. The dew point of the air before and after the membrane is measured using dew point mirrors. On the opposite side of the membrane is a vessel (wet cup) containing stagnant air and a salt solution that ensures a constant relative humidity. This induces a moisture diffusion process through the membrane.

For the investigations under condensation and freezing conditions, the experiment is modified by replacing the wet cup with a secondary air channel. The condensate or frost layer thickness on the membrane surface and the moisture content in the membrane are measured using a single-sided NMR device. The process is analysed under both laminar and turbulent flow conditions.

In addition, a CFD model will be developed to simulate condensate and frost formation and validated using the experimental data. The validated model will then be used to investigate promising flow configurations, geometries of the spacers and partitions as well as different membrane typologies.

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