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

Table of Contents for the Digital Edition of ASHRAE Journal - May 2011

ASHRAE Journal - May 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Seismic Restraint
Plug Load Design Factors
Technology Award Case Studies:
State-of-Art School
Green School Lab
Eco-Friendly, Affordable, School
Building Sciences
Special Section
InfoCenter
Solar NZEB Project
Emerging Technologies
IAQ Applications
Washington Report
International Column
Products
People
Classified Advertising
Advertisers Index
ASHRAE Journal - May 2011 - Intro
ASHRAE Journal - May 2011 - ASHRAE Journal - May 2011
ASHRAE Journal - May 2011 - Cover2
ASHRAE Journal - May 2011 - 1
ASHRAE Journal - May 2011 - 2
ASHRAE Journal - May 2011 - Contents
ASHRAE Journal - May 2011 - Commentary
ASHRAE Journal - May 2011 - 5
ASHRAE Journal - May 2011 - Industry News
ASHRAE Journal - May 2011 - 7
ASHRAE Journal - May 2011 - 8
ASHRAE Journal - May 2011 - 9
ASHRAE Journal - May 2011 - 10
ASHRAE Journal - May 2011 - 11
ASHRAE Journal - May 2011 - 12
ASHRAE Journal - May 2011 - 13
ASHRAE Journal - May 2011 - Letters
ASHRAE Journal - May 2011 - 15
ASHRAE Journal - May 2011 - 16
ASHRAE Journal - May 2011 - 16a
ASHRAE Journal - May 2011 - 16b
ASHRAE Journal - May 2011 - 17
ASHRAE Journal - May 2011 - Meetings and Shows
ASHRAE Journal - May 2011 - 19
ASHRAE Journal - May 2011 - Seismic Restraint
ASHRAE Journal - May 2011 - 21
ASHRAE Journal - May 2011 - 22
ASHRAE Journal - May 2011 - 23
ASHRAE Journal - May 2011 - 24
ASHRAE Journal - May 2011 - 25
ASHRAE Journal - May 2011 - 26
ASHRAE Journal - May 2011 - 27
ASHRAE Journal - May 2011 - 28
ASHRAE Journal - May 2011 - 29
ASHRAE Journal - May 2011 - Plug Load Design Factors
ASHRAE Journal - May 2011 - 31
ASHRAE Journal - May 2011 - 32
ASHRAE Journal - May 2011 - 33
ASHRAE Journal - May 2011 - 34
ASHRAE Journal - May 2011 - 35
ASHRAE Journal - May 2011 - State-of-Art School
ASHRAE Journal - May 2011 - 37
ASHRAE Journal - May 2011 - 38
ASHRAE Journal - May 2011 - 39
ASHRAE Journal - May 2011 - 40
ASHRAE Journal - May 2011 - 41
ASHRAE Journal - May 2011 - 42
ASHRAE Journal - May 2011 - 43
ASHRAE Journal - May 2011 - Green School Lab
ASHRAE Journal - May 2011 - 45
ASHRAE Journal - May 2011 - 46
ASHRAE Journal - May 2011 - 47
ASHRAE Journal - May 2011 - 48
ASHRAE Journal - May 2011 - 49
ASHRAE Journal - May 2011 - 50
ASHRAE Journal - May 2011 - 51
ASHRAE Journal - May 2011 - Eco-Friendly, Affordable, School
ASHRAE Journal - May 2011 - 53
ASHRAE Journal - May 2011 - 54
ASHRAE Journal - May 2011 - 55
ASHRAE Journal - May 2011 - 56
ASHRAE Journal - May 2011 - 57
ASHRAE Journal - May 2011 - Building Sciences
ASHRAE Journal - May 2011 - 59
ASHRAE Journal - May 2011 - 60
ASHRAE Journal - May 2011 - 61
ASHRAE Journal - May 2011 - 62
ASHRAE Journal - May 2011 - 63
ASHRAE Journal - May 2011 - 64
ASHRAE Journal - May 2011 - 65
ASHRAE Journal - May 2011 - 66
ASHRAE Journal - May 2011 - 67
ASHRAE Journal - May 2011 - InfoCenter
ASHRAE Journal - May 2011 - 69
ASHRAE Journal - May 2011 - 70
ASHRAE Journal - May 2011 - 71
ASHRAE Journal - May 2011 - 72
ASHRAE Journal - May 2011 - 73
ASHRAE Journal - May 2011 - 74
ASHRAE Journal - May 2011 - 75
ASHRAE Journal - May 2011 - 76
ASHRAE Journal - May 2011 - 77
ASHRAE Journal - May 2011 - Solar NZEB Project
ASHRAE Journal - May 2011 - 79
ASHRAE Journal - May 2011 - 80
ASHRAE Journal - May 2011 - 81
ASHRAE Journal - May 2011 - 82
ASHRAE Journal - May 2011 - 83
ASHRAE Journal - May 2011 - 84
ASHRAE Journal - May 2011 - 85
ASHRAE Journal - May 2011 - 86
ASHRAE Journal - May 2011 - 87
ASHRAE Journal - May 2011 - 88
ASHRAE Journal - May 2011 - 89
ASHRAE Journal - May 2011 - Emerging Technologies
ASHRAE Journal - May 2011 - 91
ASHRAE Journal - May 2011 - 92
ASHRAE Journal - May 2011 - 93
ASHRAE Journal - May 2011 - 94
ASHRAE Journal - May 2011 - 95
ASHRAE Journal - May 2011 - IAQ Applications
ASHRAE Journal - May 2011 - 97
ASHRAE Journal - May 2011 - 98
ASHRAE Journal - May 2011 - 99
ASHRAE Journal - May 2011 - 100
ASHRAE Journal - May 2011 - 101
ASHRAE Journal - May 2011 - Washington Report
ASHRAE Journal - May 2011 - 103
ASHRAE Journal - May 2011 - International Column
ASHRAE Journal - May 2011 - 105
ASHRAE Journal - May 2011 - 106
ASHRAE Journal - May 2011 - Products
ASHRAE Journal - May 2011 - 108
ASHRAE Journal - May 2011 - People
ASHRAE Journal - May 2011 - Classified Advertising
ASHRAE Journal - May 2011 - 111
ASHRAE Journal - May 2011 - Advertisers Index
ASHRAE Journal - May 2011 - Cover3
ASHRAE Journal - May 2011 - Cover4
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