The next step was to factor in the cooling-tower water and water-treatment costs for each water-cooled chiller. Table 3 shows the most recent City of Houston commercial water rates, with volume water and sewer charges.

A comparison of water rates in Houston and other major U.S. cities proved challenging because of differences in municipal water-rate structures attributed to various peak, off-peak, service, commodity, and block consumption charges. However, one major Pacific Northwest city published rates between $4.50 and $6.03 per thousand gallons, while a large Midwestern city showed a simple rate structure of $3.50 per thousand gallons (sewer charges not included). While those cities were comparable, it is important to determine the water-rate structure specific to the location of any project.

A 2012 report2 shows users pay 75 percent more for water today than they did in 2000. The report predicts water rates will increase by 5 percent to 15 percent per year, outpaced only by heating-oil rates. City of Houston water rates are shown in Table 3, with a volume charge of $3.74 per thousand gallons for commercial users. Unless the customer is metering tower makeup water separately to account for waste, volume sewer charges (an additional $5.30 per thousand gallons) also may apply. In Houston, a separate water meter must be purchased from the utility company, a cost not included in this analysis.

To determine cooling-tower water consumption and the resulting cost impact, a number of variables were considered: ambient wet-bulb temperature, tower loading, tower turndown, chiller/tower control strategy, type of chemical treatment used, cycles of concentration allowed, and type of tower installed. Table 4 shows estimated tower water consumption for the water-cooled chillers. Consumption was considered to be waste based on evaporation, drift, and blowdown, which are lost to the surrounding atmosphere. Using City of Houston rates, water consumption would result in a cost of $19,097 per year.

Other cost considerations are maintenance (not included in this analysis) and chemicals or water treatment. A nominal cost of $3 per thousand gallons was added for water treatment based on input from facility managers in the health system who maintain large central chilled-water plants. Note costs associated with water treatment can vary considerably based on the technology applied. When water and chemical costs are combined, the total reaches $34,409 per year. With water costs added to the chiller-only electrical costs for each water-cooled chiller, a more realistic evaluation can be made. Table 5 ranks each chiller based on total annual operating cost, including electricity and water.