Contaminant transport in membrane based energy recovery ventilators

Abstract

Ventilation systems are used to exhaust stale air and bring fresh air into sealed buildings. To maintain a comfortable indoor environment, this incoming air must be heated or cooled, which consumes energy. Consequently, membrane-based plate-type energy recovery ventilators are a component used in many energy-efficient ventilation systems. In these air-to-air energy recovery ventilator exchangers, incoming and outgoing air streams are passed over opposing sides of a membrane through which heat and moisture are transferred. This decreases the energy use of buildings by using the exhaust air to heat/cool and humidify/dehumidify the incoming air depending on the season. Ideally, membranes for these devices should have high water vapor permeation rates and be selective for water vapor over the transport of other gases and volatile organic compounds, including formaldehyde, odors, and contaminants, that may be present in the outgoing indoor air stream. Current certification and standards for contaminant crossover in North America focus on the measurement of the exhaust air transfer ratio based on tracer gas tests. This study demonstrates that although this test may be appropriate for measuring defects and leakage in exchangers, it may not account for all sorption and permeation phenomena that may be observed in polymeric membrane systems. Results are reported for the transport of water vapor, carbon dioxide, oxygen, and volatile organic compounds through a number of energy recovery ventilator membranes based on different polymers. Building ventilation systems are modeled using CONTAM software to demonstrate the effect of crossover through the energy recovery ventilator on indoor air quality under intermittent release (i.e., cooking odors, smoking, cleaning, etc.) and consistent release (i.e., off-gassing of volatile organic compounds or carbon dioxide associated with building materials or occupancy) of contaminants. It is shown that moderate crossover leakage in energy recovery ventilators should have minimal overall impact on indoor air quality in ventilated buildings.