Performance of membrane based enthalpy recovery units
A membrane based enthalpy recovery unit may contribute to a reduction of 10-20 % of the energy demand for ventilation in comparison to conventional ventilation systems. However, practical problems such as deformation of the membrane, leaks and frost formation have not been sufficiently studied. The project will uncover pros and cons of membrane recovery units.
The aim of the project is to analyze the energy performance of membrane-based enthalpy recovery units and to identify the advantages and disadvantages of such units and describe the possibilities and limitations of applications.
Studies have shown that the energy demand in a ventilation system with a membranebased enthalpy recovery unit may be 10-20 % lower than that of a comparable system with a conventional heat recovery unit.
In conventional heat recovery units, such as crossflow heat exchangers and counterflow heat exchangers only sensible heat due to differences in air temperatures are transferred. In membrane-based enthalpy recovery units both sensible heat and latent heat are transfered. Transfer of latent heat means that the humidity of the indoor air is not significantly reduced. Low humidity is a common problem in new buildings.
Studies have shown that the risk that there is transfer of contaminants through the membrane is negligible. In contrast, problems e.g. due to deformation of the membrane, leaks and frost formation has not been sufficiently studied.
The project will be carried out as a postdoctoral course over a period of two years.
Improvement in the energy saving performance in the ventilation system and to ensure a satisfactory indoor climate has been a challenging task in Denmark as well as in other cold countries. Usually rotary wheels enthalpy exchangers are used in energy recovery ventilation system. These exchangers recuperate above 80 % enthalpy energy. However, cross contamination, ice formation, size, and extra motor for operation are the drawbacks of using the rotary wheels. Therefore, it was relevant to study the performance of rather new air-to-air enthalpy exchangers under cold climate. These exchangers were stationary and no extra motor was required for the operation. Short-term performance analyses of air-to-air enthalpy exchangers can be found in the literature but longterm performance analyses has not been done yet for all I know. There was also deficiency of the knowledge regarding ice formation in the membranes, deformation in the membranes and gas transfer through the membranes.
In the present research project, performance of one air-to-air heat and four air-to-air enthalpy exchangers was analysed in terms of energy saving potential, ice accretion on the surface of the membranes and deformation in the membranes of the enthalpy exchangers. Performance was also analysed in terms of gas transfer through polymer and paper membranes of enthalpy exchangers. A test rig was built for laboratory tests and experiments were conducted at different outdoor air temperatures and airflows in order to analyse short and longterm performance.
The results show that the polymer membrane based enthalpy exchanger recuperated enthalpy energy 15 % more than treated paper membrane and 32 % more than paper membrane based enthalpy exchanger. About 79 % total energy was recuperated using polymer membrane based enthalpy exchanger. The ratio of the amount of latent energy transferred by the polymer membrane based enthalpy exchanger was 5 % to 12 % of the total energy, which is usually wasted in the sensible heat exchanger.
The outdoor air temperature had no influence on sensible effectiveness. However, latent effectiveness increased 3 % - 4 % and the total effectiveness decreased 7 % - 9 % with the increase in the outdoor air temperature from -16 ⁰C to 5 ⁰C.
Sensible, latent and total effectiveness were functions of face velocity. There was inverse relation between face velocity and the effectiveness.
No visible ice accretion and deformation in the paper, treated paper and polymer membranes was observed during short term and longterm studies.
There was no influence of outdoor air temperature and concentration of the gas on the transfer ratio through the paper and polymer membranes. The ratio of the amount of N2O gas transferred through polymer membrane and treated paper membrane was 5 % and 13 % respectively.
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Aalborg Universitet (Fredrik Bajers Vej) | ||
| System Air A/S |
Contact
Statens Byggeforskningsinstitut, Afdeling for Energi og Miljø, A. C. Meyers Vænge 15, 2450 København SV.
