The focus of this project was to improve the trade-off between water consumption and cooling efficiency in cooling towers used for heat rejection from various processes, including building heating, ventilation, and air-conditioning (HVAC) loads; data center cooling; power generation; etc. The overarching objective of the field demonstrations was to substantiate a performance assessment of the technology that:
The technology under evaluation was based on the use of a hygroscopic working fluid as a direct contact heat-transfer medium between a cooling water loop and the ambient air. Aside from the desiccant-based working fluid, hygroscopic cooling is schematically similar to a conventional closed-circuit water-based cooling tower. The innovative features come from the fact that, unlike pure water, the hygroscopic fluid restricts the free evaporation of moisture and results in more sensible (or dry) heat transfer to the air relative to pure water. The desiccant working fluid also enables a novel strategy for dissolved solids control to save blowdown water, and it does not require the use of a biocide for microbial control.
Data to evaluate hygroscopic cooling were gathered by installing prototype hygroscopic cooling towers at two Department of Defense facilities and monitoring their performance, along with that of existing conventional cooling towers at each site. The performance assessment showed that the water savings fraction was highest for the moderate climate site compared to the hot, arid location. However, the magnitude of potential cost savings was higher for the hot site, and this location had the more attractive payback given the reduced amount of HVAC run time in the moderate climate. Other important factors in the assessment were the cost of makeup water and sewer and the quality of the makeup water.
Currently, hygroscopic cooling appears applicable to closed-circuit cooling tower applications until further improvements are made regarding heat exchanger fouling and materials compatibility with the desiccant. The technology is not currently available commercially, but this project has demonstrated its key features and has identified attractive applications for follow-on work.