ASHRAE Journal - May 2011 - 92

emerging technologies
effect, which removes the remainder of the moisture from the National Renewable Energy Laboratory (NREL) is currently desiccant solution. investigating the potential of liquid desiccant enhanced indirect Liquid or solid desiccant dehumidifiers can be applied to evaporative cooling. building air-conditioning systems. The basic approach is to deTheir system uses a membrane contactor between the liquid humidify air to a low dew point with the desiccant dehumidifier, desiccant and airstream. The air is dehumidified as liquid apply available sensible cooling (often via heat transfer with desiccant flows counter to it, with the membrane contactor return air from the conditioned space), then evaporatively cool preventing desiccant droplet formation and entrapment in to reach a lower temperature with the humidity still low enough the airstream, simplifying the downstream processing needs for space conditioning. The resulting system is a thermally of the system. driven air-conditioning system. An indirect evaporative cooling section is downstream of This basic arrangement has many variations. For example, the desiccant section. A small portion of the dehumidified indirect evaporative cooling can be used for the evapo- and cooled air is directed through the evaporative cooling rative cooling step. side of the indirect Dry Process Air Hot, Humid Purge Air There also are a vaevaporative cooling riety of heat transfer section, resulting in configurations. a low enough air deSuccessful applicalivery temperature to tions have tended to meet sensible cooing be niche applications needs.1 where there is a preEnergy savings Mass Mass mium on humidity are realized by using Transfer Transfer control or the ability active desiccant deMedia Media to maintain lower inhumidification only door relative humidwhen the outdoor air Heating Humid Purge Coil ity levels than normal. Process Air humidity is too high Air An example is confor indirect evaporaSump Sump ditioning the air near tive cooling to proDehumidification Column Regeneration Column frozen food cases in vide the necessary supermarkets. Lower Figure 1: Liquid desiccant dehumidification system. cooling. RH significantly reT h e p e r m e a bl e duces the rate of frost formation on the glass doors and evapo- membrane contactor separating the desiccant and the air is rator coils of the frozen food cases. an offshoot of polymer electrolyte membrane (PEM) fuel-cell A major obstacle to adopting liquid desiccant dehumidifica- membrane technology.2 To obtain the selective permeability tion systems for general air conditioning applications is the needed—passing water vapor, but not liquid or other gases— practical impact of direct contact between the liquid desic- the pore sizes are carefully controlled to approximately 0.1 cant and the airstream. It is difficult to prevent entrainment micron. To allow free passage of water vapor through the of desiccant solution droplets in airstreams and dust in the air membrane, the membrane thickness is limited to approxigradually accumulates in the desiccant solution, resulting in mately 25 microns.3 higher maintenance requirements than with conventional airNREL has conducted extensive design studies and analysis conditioning equipment. of the liquid desiccant enhanced indirect evaporative cooling At the thermal COP level of 0.7, the energy use in terms of system. They plan to fabricate and test a one-ton nominal caeither primary energy or energy cost is comparable to vapor pacity prototype in the near future.3 cycle air conditioners. Permeable membrane-based liquid desiccant dehumidification energy savings eliminates direct contact of the desiccant solution with the airMembrane-based liquid desiccant coolers can potentially streams, keeping the initial liquid desiccant charge contained in the provide sensible and latent cooling in the proportions needed, system for an extended period of time. The permeable membrane with the majority of the energy input being thermal energy contains the liquid desiccant (or water for evaporative cooling) rather than electric power. The thermal energy is needed to while passing water vapor to or from the passing airstream. regenerate the saturated liquid desiccant. This can come from By combining membrane-based liquid desiccant dehumidi- natural gas combustion, waste heat, solar thermal collectors or fication with indirect evaporative cooling, the potential exists other renewable energy sources. to significantly reduce air-conditioning energy consumption. Based on NREL’s extensive modeling of the liquid desicTo date, permeable membrane based desiccant cooling cant enhanced indirect evaporative cooling system in differsystems are in the research and development phase. The ent buildings and climate areas, this technology potentially
92 ASHRAE Journal
Heat Exchanger

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May 2011

ASHRAE Journal - May 2011

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