Design and performance testing of counter-cross-flow run-around membrane energy exchanger system
Abstract
In this study, a novel counter-cross-flow run-around membrane energy exchanger (RAMEE) system was designed and tested in the laboratory. The RAMEE system consists of two (2) counter-cross-flow Liquid-to-Air Membrane Energy Exchangers (LAMEEs) to be located in the supply and exhaust air streams in the building Heating Ventilation and Air-Conditioning (HVAC) system. Inside each exchanger, a micro-porous membrane separates the air and liquid streams and allows transfer of the sensible and latent energy from the air stream to the liquid stream or vice-versa. The system exchanges sensible and latent energy between supply and exhaust air streams using a desiccant solution loop. The supply and exhaust air streams in the RAMEE can be located far apart from each other or adjacent to each other. The flexibility of non-adjacent ducting makes the RAMEE system a better alternative compared to available energy recovery systems for the retrofit of HVAC systems.
Two counter-cross-flow exchangers for the RAMEE system were designed based on an industry recommended standard which is to obtain a target overall system effectiveness of 65% for the RAMEE system at a face velocity of 2 m/s. The exchanger design was based on heat exchanger theory and counter-cross-flow design approach. An exchanger membrane surface aspect ratio (ratio of exchanger membrane surface height to exchanger length) of 1/9 and the desiccant solution entrance ratio (ratio of desiccant solution entrance length to exchanger length) of 1/24 were employed. Based on different heat transfer case studies, the energy transfer size of each exchanger was determined as 1800 mm x 200 mm x 86 mm. Propore™ was used as the membrane material and Magnesium-Chloride solution was employed as the desiccant solution.
The RAMEE performance (sensible, latent and total effectiveness) was evaluated by testing the system in a run-around membrane energy exchanger test apparatus by varying the air stream and liquid solution-flow rates at standard summer and winter operating conditions. From the test data, the RAMEE effectiveness values were found to be sensitive to the air and solution flow rates. Maximum total effectiveness of 45% (summer condition) and 50% (winter condition) were measured at a face velocity ≈ 2 m/s. A comparison between the experimental and numerical results from the literature showed an average absolute discrepancy of 3% to 8% for the overall total system effectiveness. At a low number of heat transfer units, i.e. NTU = 4, the numerical and experimental results show agreement within 3% and at NTU = 12 the experimental data were 8% lower than the simulations. The counter-cross-flow RAMEE total system effectiveness were found to be 10% to 20% higher than those reported for a cross-flow RAMEE system by another researcher.
It is thought that discrepancies between experimental and predicted results (design and numerical effectiveness) may be due to the mal-distributed desiccant solution-flow, desiccant solution leakage, lower than expected water vapor permeability of the membrane, uncertainties in membrane properties (thickness and water vapor permeability) and heat loss/gain effects. Future research is needed to determine the exact cause of the discrepancies.