In humid climates, a disproportionate amount of the energy of mechanical cooling is used in dehumidifying the air. A desiccant dehumidification system can often handle this latent load more efficiently, leaving the existing mechanical system to satisfy the sensible cooling load. The thermodynamic implication of this technique is that the sensible cooling of the air at the cooling coils can be done more efficiently with a warmer cooling medium than with water from a chiller. Often the return water from a cooling tower can accomplish this cooling, or even well water or waste cooling water from industrial processes. In the latter case, the chiller can be taken off-line altogether.
The mechanical configuration of this modified chilled water system is not complicated. The chiller can remain in the condenser water/chilled water loop, to be immediately accessible for peak loading conditions, morning cooling, and other special circumstances. Alternately, a flat-plate heat exchanger can be installed parallel to the chiller, with valving to reject heat from the building loop to the cooling-tower loop. The advantage of this arrangement is that it can be used during times of low humidity for “free cooling” of the facility without the need for a dehumidifier.
The savings with either setup comes from running a dehumidifier instead of the chiller, with the cooling-tower loop operating in both arrangements. The savings can be further boosted if waste process heat is available for the desiccant rejuvenation, such as from the boiler stack or condensate return system.
Another lesson from this project is that the chilled-water supply temperature to the building can be increased at a rate proportional to the building humidity. A possible scenario is to operate the chillers at full load (e.g., their most efficient operating condition) for an hour or so before opening for business, for maximum dehumidification. Then reset the temperature a few degrees higher for the remainder of the day, allowing the humidity to rise as the day goes on. This effectively shifts some of the cooling load to off-peak hours and otherwise reduces the energy used by the chiller during the peak electricity demand charge hours.
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Excerpt from Energy Conservation Projects.