Microchannel Membrane-Based Absorbers using Surfactant-modified Ionic Liquids for Heat/Mass Transfer Enhancement towards Compact and Crystallization-Free Absorption Heat Pumps

Description

Technology advancement for heating, ventilating, and air-conditioning (HVAC) equipment for buildings is a critical part of mitigating global warming and creating a sustainable future. Energy consumption is the major source of greenhouse gases (GHGs) related to HVAC, where the emissions are combustion products from fuels for heating or electricity production. Building energy typically accounts for 20–40% of the total consumption in different regions, with HVAC equipment using the largest portion of this energy. The other significant source of GHGs from conventional HVAC equipment is leakage of the high global-warming potential (GWP) hydrofluorocarbon (HFC) working fluids. Therefore, both energy efficiency improvement and low-GWP refrigerant development are essential for combating global warming. Absorption heat pumps (AHPs) are a promising solution to significantly reduce both of these sources of GHGs. AHPs can utilize renewable energy/waste energy, instead of fossil fuels, for cooling, heating, dehumidification, and energy storage. Additionally, AHPs usually use environmentally-friendly working fluids (e.g., H2O/LiBr, NH3/H2O) with very low GWP values, in line with the Kigali Amendment to Montreal Protocol.However, there are two major problems preventing the H2O/LiBr AHPs from wider applications: (1) large size and thus high cost, and (2) crystallization risk and thus system blockage. Therefore, there is a growing need to develop compact AHPs with minimized space requirement, reduced capital cost, and eliminated crystallization risk. This proposal seeks to address these problems by developing a microchannel membrane-based absorber using ionic liquids (ILs). The microchannel membrane-based absorber offers a high specific surface area (surface-area-to-volume ratio) and thus allows for very high compactness. ILs are promising absorbents that can eliminate crystallization due to their low melting temperatures. The proposed research also includes analysis of surfactants to further enhance heat/mass transfer and to counteract the possible high viscosities of ILs. The project objectives include: (1) establish distributed and dynamic models of the microchannel membrane-based absorber to characterize the heat/mass transfer behaviors; (2) develop a prototype for model validation and experimental investigation; (3) develop accurate correlations for heat/mass transfer and pressure drop within the absorber; (4) optimize the membrane geometry and absorber configuration to maximize the cooling capacity per volume; and (5) screen and test potential ILs and surfactants to find those that further enhance the cooling efficiencies of AHPs. The proposed research is significant for the characterization and development of smallsize, low-cost and crystallization-free AHPs, facilitating efficient and affordable low-carbon technologies in Hong Kong and all over the world.