METODOLOGÍA. ORIENTACIONES DIDÁCTICAS

Improved operation of cooling systems is the second largest opportunity identified by Gleick, et al. among indoor water C&I uses. Although water is used for a variety of cooling needs, in the C&I sector it is primarily used as part of the heating, ventilation and air conditioning (HVAC) system that provides space cooling. When water is used for this purpose in a commercial

building, it is typically the largest use of water in the building. Water may also be used to provide cooling for refrigeration as well as industrial processes.

Water use for cooling purposes is sufficiently important to warrant providing some detail on it here. Water provides cooling by acting as a sink for excess heat. By flowing water through a heat exchanger, heat is transferred from its source to the water. As a result, the water temperature increases. Examples of equipment that use cooling water include:

• air conditioners that transfer heat from inside the building to the cooling water;

• refrigeration systems that transfer heat from inside the refrigerated space to the cooling water; and

• compressors that transfer heat from the compressed air or other fluid to the cooling water.30

After the water is heated, it may be discharged. However, this “once-through” cooling is considered to be particularly wasteful. To save water, it is preferred to recirculate the cooling

Total C&I use is about 14,400 mgd x 70 percent (from Exhibit 6-1) = 10,080 mgd. Of this use, restroom use is about 15 to 30 percent, or 1,500 to 3,000 mgd. A 50 percent savings would be 750 to 1,500 mgd.

30

water so that it is used multiple times. Before it can be reused, however, the cooling water itself must be cooled, which is typically accomplished using a cooling tower.

The cooling tower is critical to the recirculation of the cooling water. After it flows through the heat exchanger, the heated water is pumped to the cooling tower where it is sprayed downward, forming small droplets. As the droplets fall, air is blown up through the tower, so that the

droplets contact the air. Some of the water droplets evaporate, causing the air to absorb heat, and thereby reducing the temperature of the remaining water. The evaporation of the water in the cooling tower is the primary mechanism by which the water is cooled prior to its recirculation to the heat exchanger. The evaporation also reduces the quantity of the cooling water, so that additional cooling water must be added. Some droplets also drift out of the cooling tower without evaporating, creating an additional loss of water.

The proper design, operation, and maintenance of cooling towers and cooling water systems have received considerable attention because of the impact they have on both water and energy use. If the cooling water system is not functioning properly, the efficiency of the air conditioner or refrigeration system may suffer, resulting in increased energy costs. Additionally, a poorly operating cooling water system can waste substantial quantities of water.

Of note is that water-cooled air conditioning systems are more energy efficient than air-cooled systems. Residential air conditions, and small commercial air conditioners, do not typically include water cooling. For larger systems, however, the energy savings from increased energy efficiency more than offset the cost of installing water cooling equipment. Consequently, shifting from water cooling to air cooling is not recommended for larger systems as a means of saving water. In fact, in California newly installed systems above 300 tons are required to be water cooled systems.31

Recognizing the opportunity to improve cooling tower operations, particularly on commercial buildings, the San Jose Environmental Services Department (SJESD) developed guidelines for managing water in cooling systems. The objective of their guidelines is to reduce water use and discharge to the San Jose/Santa Clara Water Pollution Control Plant, one of the largest

advanced wastewater treatment facilities in California (SJESD, 2002, p. iv).

The most effective method of reducing the amount of water used in a cooling system is to ensure that the water is recirculated as many times as possible. The number of recirculation cycles that can be used is limited by the build up of dissolved solids and salts in the water. Because a portion of the water evaporates in the cooling tower, the concentration of solids and salts increases in the remaining water. When the concentration reaches a level that can

damage the cooling system or cause scale to build up on system components, the water must be discharged and replaced with freshwater.

Exhibit 6-8 displays the amount of water used as a function of the number of cycles of concentration of the cooling water (cycles of concentration refers to the number of times the water is recirculated). As shown in the exhibit, the water requirements per ton-hour of air conditioning are reduced with increased cycles of concentration. It is not uncommon for cooling towers to be operated in the range of two to four cycles of concentration. Substantial reductions in water use can be achieved by increasing the cycles of concentration to the range of four to six. This can typically be accomplished through the use of chemical treatments for the water, as well as through better cooling water quality monitoring and water discharge control. Gleick, et al. estimated that overall a 39 percent improvement in cooling water efficiency is possible in

“Tons” is used to describe the amount of cooling provided by an air conditioning system. One ton of cooling is the ability to remove 12,000 Btu of heat per hour.

California, which includes replacing once-through cooling with recirculated cooling in some applications.

Although no national data are available that describe water use in cooling towers, an

approximate value can be developed for water use in air conditioning of commercial buildings using data from CBECS (1999).

• Buildings with Cooling Towers: Buildings over 50,000 square feet with central chillers or district chilled water are assumed to use cooling towers. The total inventory of these buildings was 14,256 million square feet in 1999 (CBECS, Table B7).

• Tons of Air Conditioning: As an approximation, we assume one ton of cooling per 350 square feet of space.

• Water Use: Recognizing that the water consumption varies with the manner in which the cooling tower is operated, we adopt assumptions of water circulation of 3 gallons per minute per ton of cooling and a water use rate of 1.5 percent. These values imply 2.7 gallons of water use per ton-hour of cooling, which represents about three cycles of concentration (see Exhibit 6-8).

• Cooling: On average, the air conditioning system is assumed to run 26 weeks per year, 5.5 days per week, 12 hours per day at half load on average, yielding 858 hours of equivalent full load operation.

Exhibit 6-8: Water Requirements for Cooling Towers as a Function of Cycles of Concentration 0.0 1.0 2.0 3.0 4.0 5.0 6.0 0 2 4 6 8 10 12 Cycles of Concentration Ga ll on s pe r To n H o u r 1.25 kW/ton 1.00 kW/ton 0.75 kW/ton 0.50 kW/ton

Note: kW/ton of cooling values represent a range of air conditioner efficiencies. More efficient systems (lower kW/ton) require less cooling system water. A “Ton Hour” is one ton of cooling operating for one hour, which is equal to 12,000 Btu of cooling.

Using these assumptions, total national cooling water use for air conditioning is roughly 260 mgd. While Gleick, et al. estimate a 39 percent reduction in cooling water use, we can assume conservatively a 10 percent reduction, implying a savings of 26 mgd. To put this figure into context, we recall that retrofitting a home with ULF toilets saves about 10 gallons per capita per day (gpcd). This savings of 26 mgd is equivalent to having 2.6 million people’s homes retrofitted with ULF toilets.