Selection of the liquid desiccant in a run-around membrane energy exchanger

Abstract

In this thesis, several possible liquid desiccants (aqueous solutions of LiCl, LiBr, MgCl2 and CaCl2) are investigated to find the most appropriate working fluid to be used in a run-around membrane energy exchanger (RAMEE). The liquid desiccant is one of the main components of the RAMEE and indirectly conditions the outdoor ventilation air by using the energy of the exhaust air, significantly reducing the building energy consumption.

Numerical simulations, in this thesis, show that the total effectiveness of the RAMEE changes less than 0.5% when different salt solutions are used. However, the capital and operational costs of the RAMEE are significantly different for different desiccants. MgCl2 is the most inexpensive among the selected salt solutions and is followed by CaCl2, LiBr and LiCl. The price of a LiCl solution in the RAMEE is almost 20 times more than the price of MgCl2 solution. Different thermo-physical properties of the salt solutions result in different pumping energy consumptions for each specific salt solution. For example, the pumping energy consumption for a MgCl2 solution is 3.5 times more than for a LiBr solution in the RAMEE. The change in the volume of the liquid desiccant throughout a year is another characteristic which depends on the thermo-physical properties of the salt solution. Solutions with larger volume expansion require larger storage tanks and will experience longer transient delays. The difference between the volume expansions of different salt solutions is less than 5% of the total solution volume. MgCl2 solution expands more than 17% throughout a yearly operation of the system in Saskatoon.

Crystallization of the salt solution is another important parameter in the selection of the liquid desiccant. Simulations show that, for a specific indoor and outdoor operating condition the risk of crystallization is greatest for MgCl2, followed by CaCl2, LiCl and LiBr. The risk increases as the supply or exhaust airstreams become dryer. For a cross flow RAMEE with a total effectiveness of 55% (NTU=10 and Cr*=3) operating in a building with indoor RH of 50%, the critical outdoor humidity below which crystallization will begin to occur is 28% RH for MgCl2, 20% for CaCl2 and 0%RH for LiCl and LiBr. According to the simulations, all four investigated salt solutions can be used in North America (except the states of Nevada, Arizona, New Mexico and parts of Texas) with no risk of crystallization when the indoor humidity is 50% RH. However, with indoor humidity of 30% MgCl2 and CaCl2 solutions will have risk of crystallization for a large number of hours in a year in most of the central western United States. A mixture of 50% LiCl and 50% MgCl2 solution is suggested to be used when the cost-effective MgCl2 solution cannot be used due to crystallization issues. The price of this newly suggested mixture is 30% less than that of a pure LiCl solution and can be used in all North American climates with very small risk of crystallization.