Utilizing a Unit Commitment and Dispatch Model to Temporally Resolve Water Use Data in the Western United States' Power Sector

Abstract

The power sector is responsible for approximately one-half of the United States’ annual water withdrawals and approximately 3% of its annual water consumption. The majority of this water is devoted to cooling thermoelectric power plants by means of once-through or recirculating cooled systems. Despite the large water requirements of the power sector, water conservation strategies are typically concentrated in the urban and agricultural sectors. Although national agencies such as the United States Geological Survey and the Energy Information Administration do publish data regarding the water use requirements of power plants, these data are often incomplete, inaccurate, and are published infrequently. Consequently, data resolving the temporal variability of water use by the power sector are largely non-existent, making conservation strategies difficult to construct. This analysis utilizes a unit commitment and dispatch (UCD however, water consumption data (i.e., the subset of water withdrawals that is evaporated) has not been reported since 1995. The US Energy Information Administration (EIA) reports cooling water consumption and withdrawal data annually for thermoelectric power generators through its EIA-860 form based on flow rate, but these data are often missing or erroneous, and lack temporal fidelity. There are two types of cooling technologies that are used by the majority of EGUs in the US. Once-through (OT) cooling technologies withdraw large amounts of water from a river or water reservoir, use the water once to cool the hot steam exiting the turbine during power production, and then discharge the warm cooling water back to the original water reservoir. During this process, very little water is lost to evaporation since the water is only used once for cooling. Recirculating cooling systems, by contrast withdraw smaller volumes of water by recycling the water within cooling towers. Since water is cycled multiple times, most of the water that is withdrawn from the original water source is ultimately lost to evaporation, that is, it is consumed. Unlike OT cooling systems, RC cooling systems do not discharge large quantities of water back into the native water reservoirs. Because of these differences in operational water requirements and varying impacts on the cooling source water reservoir, there are tradeoffs between these two technologies. OT cooling systems withdraw large volumes of water, but consume very little, which is therefore good for water availability compared to high water consumption technologies. RC systems require smaller volumes of water, but very little (if any) of the water that is extracted from a reservoir is ultimately returned to the water shed after generation. RC systems, therefore, can be less prone to generation disruptions in areas where water is constrained, but have larger evaporative losses than OT systems. The distribution of OT and RC cooled EGUs varies regionally. Thermoelectric power generators in the western US use a larger fraction of RC cooling systems since the region is generally drier than the comparatively water-rich Eastern US. Eighty percent of the units of electricity generated in the eight most Western US (including Oregon, Washington, Idaho, Montana, Arizona, Nevada, New Mexico, and California) was produced in RC cooled facilities in 2005. (The majority of electricity generated in OT cooled facilities was produced along the coast and using saline cooling water.) By contrast, 54% and 46% of the electricity generated in the remainder of the Eastern US states is derived from OT and RC cooled facilities, respectively. This trend reflects the fact that the Eastern US generally has more water available for withdrawal-intensive cooling facilities. (Table 1 details 2005 water use for thermoelectric electricity production in Eastern and Western US states.) The Western Electricity Coordinating Council (WECC) is the regional entity within the North American Electric Reliability Corporation that coordinates and promotes reliable bulk power transmission across the western United States (i.e. Washington, Oregon, California, Idaho, Nevada, Utah, Arizona, Colorado, Wyoming, as well as portions of Montana, South Dakota, New Mexico, and Texas), Baja California, Mexico, and western Canada. The WECC power grid provides a valuable case study for this research because it is the largest regional power coordination entity in North America, serving approximately 82 million people across 1.8 million square miles and includes the majority of North American regions that have experienced “extreme” or “exceptional” drought in the past decade.