In order to minimize the energy use for heating, passive houses for residential use are constructed using heavy insulation. In addition, they have minimal air leakages and no vents in exterior walls for direct supply of fresh air, and thus, mechanical ventilation systems are a mandatory requirement in such buildings.
With the aim of reducing energy use, efficient energy recovery from used air will be of high importance. In residential buildings with several living units, centralized air handling units are regarded as the most energy efficient system. However, to prevent odours to transfer between apartments it is important to avoid carryover leakages of pollutants between the exhaust air and the supply air inside the heat exchanger.
Rotary heat exchangers (heat wheels) are very energy efficient (85 %), but have the drawback of transferring odours from exhaust air to fresh supply air. To avoid transfer of odours in apartment buildings, flat plate heat exchangers are commonly used instead. Nevertheless, the state-of-the-art flat plate heat exchangers may not handle properly water condensation and frost formation at low supply inlet temperatures. To avoid this problem, the efficiency must be reduced on cold days, causing an increase in yearly energy use for air heating.
An alternative to the flat plate heat exchanger are the so called quasi-counter flow membrane-based heat and mass recovery exchangers. In a membrane based exchanger, moisture is transferred from the humid exhaust air to the dry supply air. In this way, condensation and frosting should be avoided at the exhaust air side. In this work, a membrane energy exchanger was compared to a thin non vapour permeable plastic foil heat exchanger. The study focused on verifying condensation and freezing problems and how the membrane energy exchanger performs.
To compare the different plate materials, a test rig was built in the laboratory at the Department of Energy and Process Engineering at NTNU. The experiments showed that non permeable heat exchangers have problems with condensation and freezing during the tested conditions, such as metal plate heat exchanger experience in real conditions. For the same conditions, the membrane based exchanger did not experience the same problems. Yet, additional problems with swallowing of the membrane in high humidity conditions showed that the tested membrane type had drawbacks and needs further development to become commercially applicable.