The impacts of outdoor air conditions and non-uniform exchanger channels on a run around membrane energy exchanger

Abstract

This thesis contains the numerically investigations of the performance of a run-around membrane energy exchanger (RAMEE) at different outdoor air conditions and the effects of non-uniform exchanger channels. The RAMEE is a new type of building ventilation air energy recovery system that allows heat and moisture to be transferred between isolated supply and exhaust air streams. Two liquid-to-air membrane energy exchangers (LAMEEs) are placed in the supply and exhaust air ducts and transfer heat and moisture between air and a circulating liquid desiccant that couples the two LAMEEs together. The ability of the system to transfer heat and moisture between isolated supply and exhaust ducts makes it appropriate for numerous HVAC applications (e.g., hospitals and building energy retrofits).

The performance of the RAMEE at different outdoor air conditions is shown to be highly variable due to the coupling of the heat and moisture transfer by the desiccant. This coupling allows the humidity ratio between the indoor and outdoor air to influence the heat transfer and the moisture transfer is influenced by the difference between the indoor and outdoor air temperatures. The coupling produces some complex RAMEE performance characteristics at some outdoor air conditions where the effectiveness values (i.e., sensible, latent, and total) were shown to be less than 0% or greater than 100%. Effectiveness and operating correlations are developed to describe these complex behaviours because existing correlations do not account for the coupling effects. The correlations can serve as design and operation tools for the RAMEE which do not require the use of an iterative computational numerical model.

Non-uniform exchanger channels are present in the RAMEE because of pressure differences between the air and solution channels which deform the membrane into the air channel. The non-uniform channels are analytically shown to create maldistributed fluid flows and variable heat and mass transfer coefficients. The combined effects of these two changes lead to a reduction in the RAMEE effectiveness, which increases as the size of the membrane deformation increases. The reduction in total effectiveness for an exchanger where the membrane has a peak deflection of 10% of the nominal air channel thickness operating at a NTU of 12 was shown to be 12.5%. These results of non-uniform exchanger channels agree with previously conducted experimental results.