dc.description.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. | nb_NO |