Water Use Management for Evaporative Cooling Technologies

Evaporative cooling systems have a large potential to reduce both the peak electricity demand and the energy-use associated with cooling in California’s hot and dry climate. However, the technology is not widely used. Surveys conducted by the California Energy Commission in 2003 and 2009 estimated that less than 4% of California’s homes with air conditioners use evaporative cooling technology (California Energy Commission, 2009). In both residential and commercial buildings, there are a number of reasons for underutilization of evaporative cooling, with two important ones being: 1) concerns about maintenance and longevity, and 2) concerns about use of water in drought-prevalent California. Water management of evaporative cooling units is essential and particular care must be taken to reduce the effects of hard water on the system. Evaporative processes lead to the concentration of minerals in the cooling water reservoir that may precipitate out of solution, scaling the condensing coil, pumps, piping, and other surfaces. Various methods are used to address this issue, most of which include additional water to reduce mineral precipitation. One commonly used method is called water bleed by which a portion of the concentrated sump water is removed from the evaporative system and replaced with tap water. This process requires additional water (over and above that evaporated) in order to maintain the longevity of these units. Manufacturers suggest varying bleed rates for their products; however, currently there are no scientific studies providing evidence of what the optimal bleed rate is to minimize scaling on equipment. The primary goal of this investigation is to evaluate the impact of water bleed rates on mineral precipitation in Evaporatively-Cooled Condensing Units (ECCU) over a range of inlet water conditions. The second objective is to determine optimum bleed rates for these systems. The first step in this investigation was to better understand the distribution of water conditions in California. This was accomplished by obtaining data from thousands of water districts in California, followed by a statistical analysis to reduce the number of different waters to be tested. The result of this process was the selection of four water compositions to be tested experimentally, which were: 1) high mineral, high measure of hydrogen ion concentration (pH), 2) high mineral, low pH, 3) low mineral, low pH, and 4) low mineral, high pH. The lower levels were selected as the 25th percentile for calcium and magnesium concentration (Cmg), alkalinity, electrical conductivity and pH, whereas the high level was selected as the 75th percentile for these parameters. Once these water types had been chosen, four different bleed rates were tested for each water composition, yielding a total of 16 experiments. The experimental apparatus consisted of four “identical” miniature ECCU coils, each of which was set to a different bleed rate while being tested with the same supply water composition.