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.

Contaminant Crossover in Residential Energy Recovery Ventilators: Mass Spectrometric Analysis and Introducing Remediation Measures

“…Furthermore, for RAMEEʼs they found that EATR values are insensitive to changes in airflow rates, liquid desiccant flow rates, latent effectiveness and environmental conditions. Huizing et al 7 reported EATR values observed by previous researchers in their paper. They used CONTAM software to model pollutant transfer rates under different permeabilities of the membrane material.…”

“…While ERVs offer the opportunity for energy-efficient ventilation, prior studies indicate that mass transport of contaminants from the exhaust to supply airflow of ERVs are possible for an indoor burst of pollutants due to everyday operations [5][6][7][8] . The mass transport in ERVs is mainly assessed by the factor referred to as exhaust air transfer efficiency (EATR) 5 .…”

“…Hult et al 5 performed experiments on rotary enthalpy ERV based on formaldehyde crossover to evaluate EATR value. The EATR calculation process introduced by studies [5][6][7] is related to the definition by ASHRAE. Hult et al 5 found that the EATR values range from 10-29% for a rotary enthalpy wheel, concluding that the bulk of the transfer is due to the air leakage from the wheel and 30% of the transfer process due to absorption and desorption.…”

“…In addition, Huizing et al 7 performed experiments considering a single membrane by an experimental test rig fabricated to ASTM F-739 standard and their EATR values ranged from 0-50%. Weerasekera and Laguerre 9 introduced a coupled continuum scale transport model for contaminant crossover in ERVs.…”

Spacer enhanced heat and mass transfer in membrane-based enthalpy exchangers

“…Modern building ventilation systems make use of energy recovery devices instead [6]. A technology which becomes increasingly relevant and popular is what one calls a membrane based enthalpy exchanger [7][8][9][10][11][12]. Such devices ensure an indirect contact of fresh and discharged air via vapor permeable membranes in order to recover sensible as well as latent heat.…”