A liquid to air membrane energy exchanger (LAMEE) transfers heat and moisture between a liquid desiccant and an air stream in order to condition the temperature and humidity of the air flowing through the LAMEE. LAMEEs can be employed in a number of different applications, including, for example, Heating Ventilation and Air Conditioning (HVAC) systems, dehumidification systems, evaporative cooling, industrial applications requiring treated air, etc. In another application, the exchanger transfers heat and moisture between the desiccant and air in order to condition the temperature and concentration of the desiccant by both releasing or gaining heat and releasing or gaining moisture in the desiccant in any combination.
Existing LAMEEs use micro-porous hydrophobic membranes to contain a liquid desiccant, including, for example, a halide salt. In these membranes a micro-porous structure is created in a thin film of a low surface energy polymer such as PTFE, polypropylene or polyethylene. The hydrophobic membrane resists penetration by the liquid due to surface tension, while freely allowing the transfer of gases, including water vapor, through the membrane pores. In the LAMEE application, the membranes are typically about 20 microns thick with a mean pore size of 0.1-0.2 micron. Micro-porous membranes however, are subject to two potential problems which can directly affect their function. Firstly, the membrane pores could become blocked by mineral deposition, dust accumulation, etc., which would degrade the moisture vapor transfer rate of the membrane and require cleaning to restore performance. Secondly, the membrane pores could become contaminated by a surfactant, oil, or other compound, which lowers the surface tension of the desiccant and allows penetration of the liquid through the pores. The susceptibility of micro-porous membranes to surfactants may require prevention or inhibition of environmental exposure to these compounds.
Liquid desiccants used in HVAC and drying applications are typically solutions of halide salts (such as lithium chloride, magnesium chloride, calcium chloride, or lithium bromide) and water. These solutions have two important properties: they are strongly hygroscopic and the salt is non-volatile. The hygroscopic property allows the solution to either release or absorb water vapor from an air stream depending on the water vapor pressure of the solution compared to that of the air. High concentration salt solutions (for example a 40% LiCl solution) can have very low vapor pressures, which produces a large potential for dehumidification of the air stream encountering the solution. Traditionally, liquid desiccant systems designed for industrial and commercial drying applications have used direct contact exchangers, in which a media wetted with desiccant (such as a cellulose honeycomb matrix) is exposed directly to an air stream. These exchangers require a salt that is non-volatile under the temperatures and pressures used for air treatment so that the salt does not evaporate into the air. The salt charge remains in the system and does not have to be replenished over time. Membrane exchangers with micro-porous membranes have the same requirement, since a gas phase can freely move through the membrane.
The main disadvantage of salt-based desiccants is that they are very corrosive to metals. For example, lithium chloride (LiCl) causes rapid corrosion of most ferrous and nonferrous metals with example exceptions including titanium and some copper-nickel alloys. Direct contact exchangers have not been widely used in HVAC applications because of the potential for desiccant droplet carryover into the air stream and corrosion of downstream metal ducting, fans and other air handling equipment. The membrane exchanger provides separation of the desiccant from the air, preventing droplet carryover; however, in the event of a membrane failure or liquid circuit leak, some local corrosion may occur if the desiccant is not detected and cleaned up. The corrosive desiccant also contributes to an increased cost for the exchanger and liquid pumping circuit because corrosion resistant materials may need to be used in portions of the circuit that may encounter the desiccant. Corrosion resistant sensors, pumps, heat exchangers, etc. may all contribute to increased cost for the circuit vs. components mass produced from non-corrosion resistant metals. In addition, secure containment of the desiccant is required in the event of a circuit leak or spill to prevent migration of corrosive desiccant into the air handling unit cabinet, mechanical room or building structure. This can further add to system cost and complexity. The lithium salts can also be relatively expensive, and costs may continue to rise due to pressure on the global lithium supply from lithium-ion battery production.
Glycols are another type of liquid desiccant that have been used in some drying applications. Glycols, however, are volatile and will steadily evaporate into an air stream when used in a direct contact exchanger or a membrane exchanger with micro-porous membranes. The steady consumption of glycol and the impact on air quality has made this desiccant unacceptable for most HVAC and dehumidification applications. Other potential strong hygroscopic fluids that are low cost, non-toxic and non-corrosive also may not be used in existing liquid desiccant systems because of either volatility, reactivity with air or air pollutants or production of odors. Therefore, current liquid desiccant systems are sometimes restricted to using halide salts with their inherent drawbacks.