Energy Recovery with Air-to-air Membrane Energy Exchanger for Ventilation in Cold Climates

Abstract

Air-to-air membrane energy exchangers (MEEs) are attracting increasing attention as one of the most important novel trends in heat and energy recovery exchangers for heating, ventilating and air-conditioning (HVAC) systems. In MEEs, simultaneous heat and moisture transfer through the selectively permeable membrane reduces the energy consumed for heating, cooling, dehumidifying or humidifying the ventilation air. In cold climates, the moisture transfer from the warm and humid exhaust airstream to the cold and dry supply air lowers the dew point of the exhaust airstream. Consequently, condensation and frost initiates at lower outdoor air temperatures compared to sensible-only heat exchangers. Frosting may be reduced or even prevented when the exhaust air is sufficiently dried by the moisture recovery in the MEE. However, the metrics of MEEs for avoiding frosting in cold climates are less known and explained in the literature. The main objective of the thesis is to explore the feasibility of the membrane energy exchanger for cold climates with respect to frosting limits, performance of the quasi-counter-flow MEE and to further explain heat and mass transfer processes in MEEs. In order to qualitatively and quantitatively evaluate the frost-tolerant characteristic and the performance of the membrane energy exchanger in cold operating conditions, MEEs and other heat/energy recovery exchangers applied in cold climates are compared. The frosting limit models for the cross-flow and the quasi-counter-flow MEEs were theoretically developed and experimentally verified through this work. The frost-free operating conditions of the membrane energy exchanger was analyzed through parametric analyses on the frosting limit model. A quasi-counter-flow membrane energy exchanger was designed and constructed. This configuration combines the easy sealing of cross-flow headers and the high effectiveness nature of a counter-flow core. The performance under cold operating conditions of the quasi-counter-flow membrane energy exchanger filled with spacer was analyzed and tested. The conjugate heat and mass transfer through the membrane energy exchanger for various membranes in cold climates were investigated. The convective heat and mass transfer were examined under developing and fully developed flow regimes for open channel and the spacer-filled MEEs. The permeation or diffusion through the dense or the porous membranes were investigated for cold operating conditions. The frosting limits for cross-flow and quasi-counter-flow MEEs have been mathematically developed and experimentally verified. Based on the results and analyses in the thesis, it can be confirmed that the membrane energy exchanger tends to lower the outdoor air temperatures which initiate onset of frosting, compared to sensible-only heat exchangers. Both thermal and hydraulic performance of the quasi-counter-flow MEE under cold operating conditions were investigated. The quasi-counter-flow arrangement is able to provide relatively high sensible and latent effectivenesses. This flow arrangement may be an ideal alternative to the widely-used cross-flow exchangers. The heat and mass transfer in the MEE are more complex and boundary conditions may be different from sensible-only heat exchangers. However, most classic knowledge and data of heat exchangers tend to be applicable to MEEs under some conditions.