Liquid desiccant air conditioners

ASHRAE Journal, Oct, 2008 by John Dieckmann, Kurt Roth, James Brodrick

An evaporatively cooled absorber lowers the humidity below the desired indoor level to a level sufficient to manage internal moisture loads. The dry-bulb temperature of the OA would approach 10[degrees]F (5.5[degrees]C) above the wet-bulb temperature and provides moderate sensible cooling of the air. Although at typical design conditions the system provides no sensible cooling to the building, at lower wet bulb temperatures, the air delivery temperature decreases and also provides some sensible cooling.

Overall, unless they use waste or solar heat or triple-effect regeneration, * current liquid desiccant AC units offer little national primary energy-savings potential as a wholesale replacement for vapor compression systems.

In humid environments, however, they can save energy, notably when used as part of a dedicated outdoor air system (DOAS), primarily to dehumidify the OA. Because the liquid desiccant DOAS handles main latent source/load, this eliminates the need to overcool ventilation air to remove humidity and decreases reheat energy consumption. Furthermore, because the indoor AC system needs only to address indoor moisture sources, it requires very limited latent capacity. This allows it to operate at a higher evaporating temperature, which improves the COP relative to a conventional chiller by about 20%. (4)

When used as part of a DOAS for warm and humid OA conditions (dry bulb temperature=86[degrees]F [30[degrees]C], wet-bulb temperature=78[degrees]F [25.6[degrees]C], an advanced liquid desiccant system with a COP of 1.2 (3,5,6,7) could achieve appreciable energy cost savings relative to conventional systems using both conventional reheat and heat pipes (approximately 30% and 5% respectively). Primary energy savings would tend to be more modest relative to reheat (~15%) and negligible relative to heat pipes. (3)

The superior dehumidification performance of desiccant systems at moderate ambient, high humidity conditions also has the potential to save energy by increasing the indoor temperature setpoint, typically by 2[degrees]F to 5[degrees]F (1.1[degrees]C to 2.8[degrees]C). In some instances, occupants respond to the poor dehumidification performance of conventional systems by decreasing the indoor temperature set point to ensure that the unit runs long enough to dehumidify the space. This, in turn, increases the sensible loads because it increases the temperature difference between the outdoor and indoor air. Furthermore, under this condition, the conventional unit provides negligible sensible cooling and very inefficient latent cooling, e.g., EERs of approximately 3 to 4. (8) As a result, desiccant systems can realize significant savings under these conditions.

Liquid desiccant systems can also use lower-temperature waste heat from distributed generation (microturbines, internal combustion engines, fuel cells, etc.) sources, district heating systems, and solar thermal energy to regenerate the desiccant. If this heat is of sufficient quality, e.g., single-effect systems require temperatures of approximately 160[degrees]F - 180[degrees]F (70[degrees]C - 80[degrees]C) for single-effect and 245[degrees]F - 320[degrees]F (120[degrees]C - 160[degrees]C) for double-effect, (3,6,9) it can dramatically improve the economics and energy savings of liquid desiccant AC.