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©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 9, September 2017

By Omar Hawit, P.E., Member ASHRAE; Trevor Jaffe, P.E., Member ASHRAE

About the Authors
Omar Hawit, P.E., is a principal at DLR Group, Washington, D.C. Trevor Jaffe, P.E., is a mechanical engineer at DLR Group, Washington, D.C.

Most building design projects involve choosing between water-cooled and air-cooled HVAC systems. While water-cooled condensers have historically been considered more efficient, air-cooled condensers are not far behind with advances in manufacturing. On the other hand, codes and standards still push for water-based cooling systems. For example, the California Energy Code Title 24 includes a prescriptive requirement for water-cooled chillers when the plant cooling capacity is greater than 300 tons (1055 kW). ASHRAE Standards 90.1 and 189.1 use a water-cooled chiller system baseline in a performance-based compliance for buildings greater than 150,000 ft2 (13 935 m2) or buildings greater than five stories. While energy modeling can be used to justify the use of air-cooled systems through performance-based compliance, codes and standards that are based on a specific technology can mislead designers.

To effectively compare water-cooled and air-cooled systems, it is necessary to also understand the impact of climate on the system. Cooling towers are most effective in climates with relatively low humidity levels. Geographical location also affects the availability of freshwater as a natural resource and the intensity of freshwater withdrawal by power plants in the area. The water demand of various types of thermoelectric power plants varies based on how the power plants use the water, whether they are recirculating, once-through, or dry-cooling. Nuclear fission uses the most water followed by coal and natural gas. However, when accounting for water evaporation at the reservoir surface, hydroelectric power generation consumes the most water. For the purposes of this analysis, surface water evaporation has been ignored. This is due to the variability in surface water evaporation based on the type of hydroelectric plant and the fact that the water evaporated per unit of electricity generated is so high compared to other electric generation plants, that the average rates in the analysis would be greatly skewed. Another consideration is the changing fuel mix of plants used to generate electricity. Reduced cost of renewable energy is making wind and solar technologies cost-competitive with more traditional generation technologies. This parametric study identifies regional electricity production from power plants and respective fresh water consumption, and then conducts a holistic evaluation of water and energy consumption of air- and water-cooled chiller plant types in six different climate zones.


Water-Energy Interrelationship

The complexity of the interrelationship between water and energy cannot be understated. For example, power plants consume water for cooling systems and water plants consume power for treatment and distribution. This interrelationship is often referred to as the water-energy or energy-water nexus. Figure 1 illustrates a few of the interdependencies.

The average energy intensity of water in cities nationally is estimated to be 3.3 – 3.6 kWh per kgal (0.87 – 0.96 kWh/kL) delivered and treated. However, this ranges from 2.7 kWh/kgal (0.71 kWh/kL) in New York to 12.7 kWh/kgal (3.36 kWh/kL) in southern California. The energy intensity of water transport and treatment in southern California is high due to the volume of water pumped, lifted, and transported, the distance from points of collection to points of need, and the path, which is often through mountains. The State Water Project (SWP) is the largest single user of energy in California, and the Edmonston Pumping Plant alone uses 40% of that energy to lift water 1,926 ft (587 m) over the Tehachapi Mountains with (14) 80,000 hp (60 MW) pumps consuming 5 billion kWh/yr, accounting for 2% to 3% of all electricity consumed in California.


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