Development and demonstration of frost-free Membrane Energy Exchanger and reduced ice and frost in heat wheels for Nordic weather

Defreeze MEE Now is a project that has developed solutions and simulation models for a new type of frost-free membrane-based energy exchanger (MEE). Digital models of this are included as part of a larger simulation tool which also models how to reduce ice and frost by different types of heat recovery for the Nordic climate. Modern homes have mechanically balanced ventilation with heat recovery where fresh outside air is blown in as used air is sucked out. It requires a lot of energy to heat cold outside air to indoor air temperature, which is why all newer ventilation solutions have heat recovery. Heat is transferred from the hot used air to the cold fresh air, so that it is heated before it is blown in. This avoids using heaters in the room to warm up cold incoming air. Indoor air contains more or less moisture. When it is cold outside, the surfaces in the heat recovery unit will become so cold that the water vapor in the air can condense on the surfaces in the recovery unit. If it is below zero outside, the water can freeze to ice. A widely used type of heat recovery is based on a rotating porous wheel. These wheels have little problems with freezing because they transfer the condensed water to the fresh air so that the condensed water is constantly removed. It is known that such exchangers have a certain risk of cross-contamination. The fresh air can be contaminated by the used air due to the rotational movement and unwanted leaks. Another main type of recyclers are the static ones. These separate the air flows completely but have major problems with freezing so that the efficiency is reduced over the year. In this project, work has been done on a new type of static energy exchanger which is based on the use of a membrane which, in addition to heat, can pass through water vapor from exhaust air to fresh air, so that freezing can be significantly reduced. However, the unwanted cross-contamination is reduced or prevented by the semi-permeable membrane. The research has aimed, among other things, to gain more knowledge about the way it works and how the design of the exchanger affects the transfer of heat and water vapour. Furthermore, it has been necessary to find suitable materials and a good constructive design. The project has also looked at how freezing can be limited and how ventilation can be controlled to reduce the problems as much as possible. This applies to both rotary exchangers of various types and membrane exchangers. In Nordic climates, indoor humidity can be too low in winter. Membrane exchangers will be able to contribute to better balanced humidity in the indoor air. In the project, work has been done to find suitable membrane types. Various types have been acquired which have been tested in a laboratory in Canada. Testing has also been carried out in Flexit's laboratory of membrane-based recyclers available on the market and what properties these have. A prototype of a membrane-based exchanger has been tested under realistic operating conditions in the Living Lab at NTNU in Trondheim. The Living Lab is a small, detached house where people can live during the experiments. A postdoctoral fellow started the project in October 2020. He developed numerical simulation models for different types of recyclers and how ice forms in them. This forms part of the basis for developing strategies for defrosting recyclers together with other models that have been created. Models have been created to be able to simulate both the recovery of dry heat and moisture in membrane-based and other recuperators. The models have been verified against measurements in Flexit's laboratory. Together with a detailed model for production of moisture in homes owned by Flexit, the models are now used to calculate moisture levels in different rooms in homes for different recycler types. Annual energy use, where all hours of the year are calculated, is part of the tool. All the models are collected in a common simulation tool. The project started in October 2019 and ran until autumn 2023.

Prosjektet vil ha vitenskapelige innflytelse gjennom publisering av resultater. Dette vil bidra til økt kunnskap om varmegjenvinnere og spesielt muligheter for økt masse- og varmeoverføring ved energigjenvinning fra ventilasjonsluft. For Flexit vil prosjektet føre til økt konkurransekraft og økte markedsandeler fordi deres produkter etter hvert som produket blir kommersialisert vil kunne yte bedre enn eksisterende løsninger. For Norge betyr det at vi får en mer konkurransedyktig byggesektor. Spesielt innen tekniske installasjoner blir mange av produktene produsert utenfor Norden. Når innovasjonen introduseres på markedet, vil det vises at høyere effektivitet enn krav i dagens byggeforskrifter er mulig. Dette kan føre til strengere reguleringer og redusert energibehov for byggesektoren. Det vil gjøre det lettere å realisere nesten nullenergibygg. Simuleringsverktøy som er utviklet i prosjektet vil bidra til at løsninger lettere kan tilpasses lokalt nordisk klima for et forbedret inneklima.

“Defreeze MEE Now”-Development and demonstration of frost-free Membrane Energy Exchanger (MEE) and reduced ice and frost in heat wheels for Nordic weather -will enable more energy-efficient solutions for buildings’ ventilation by developing ventilation energy recovery systems adapted to cold climates. The main objective is to develop Nordic adapted 1)air-to-air MEE that have higher heat and moisture recovery efficiency than existing solutions and 2)energy efficient defrost protocols. It should also ensure a superior indoor air quality (IAQ). This answers ENERGIX demands to develop solutions for a more sustainable, value-creating and competitive Norwegian industry and achieving nZEB. Heat wheels have the problem of air leakages from exhaust to fresh air. These leakages also transfer odors and particles to the fresh air. In multifamily houses with shared air handling unit this means odor transfer between flats. To avoid leakages, cross or counter flow exchangers can be used, but in cold climates these are vulnerable to frost. All types of frost prevention measures mean temporary reduced efficiency and increased annual energy use. These problems are removed or considerably reduced when using MEE because of the moisture transfer from used to fresh air. Thus, for shared ventilation systems, odor transfer is not expected to be a problem and frost is significantly reduced. However, we do not know how membrane exchangers react to aging or to unbalanced airflow rates. We need to prove that MEE are reliable towards these challenges. One specific problem related to the indoor environment in Nordic buildings is the extremely low indoor relative humidity (RH) during winter. Low RH is related to the perception of unsatisfactory IAQ, dryness of breathing ways and eyes and frequent allergies and asthma. MEEs are expected to help to control RH. We are seeking a higher efficient recovery system that controls RH, giving an edge regarding the control of the IAQ in Nordic climates.