November 2012
Key Facts
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Comprehensive environmental management programs will yield optimal protection when used in choosing electric generation facilities and cooling systems.
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When considering water resources as part of this holistic approach, power generation accounts for 3.3 percent of freshwater consumption.
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Among cooling technologies for coal, natural gas and nuclear power plants, “once-through” systems actually consume only 1 percent of the water they withdraw. Wet cooling tower systems at these facilities, on average, consume twice as much water as once-through systems.
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Power plant cooling systems do not have an adverse impact on aquatic life, according to scientific studies.
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Compared to other energy sources used for electricity production, nuclear power plants use moderate amounts of water and minimal land per amount of electricity produced.
Holistic Environmental Management
Electricity production and the availability of water are interdependent. While electricity generation requires water for cooling, usable water production requires substantial amounts of electricity for pumping and purification.
To manage this interdependence responsibly, a range of issues should be taken into account. Within the energy sector, these issues include the potential environmental impacts of power plant cooling systems, the environmental footprint of various energy sources, and their reliability and economics when generating electricity.
A holistic approach to environmental stewardship requires balancing the relationships among all aspects of the environment and making responsible trade-offs appropriate to the unique characteristics of each ecosystem where an electricity generation facility exists or will be developed.
This integrated approach will yield optimal results for all issues, including water use. Local ecosystem considerations include water quantity and quality, aquatic life, wildlife, land use (habitat), and air quality. These issues, in turn, impact the broader environmental issues of sustainable development, climate change mitigation/adaptation and drought alleviation.
All low-carbon energy sources—including nuclear energy and renewables—will be required for sustainable development. These energy sources must be developed in an environmentally responsible manner, taking into account environmental benefits and potential impacts, economics, and differing electricity generation attributes.
Water Use Definitions
Water use consists of two processes that can occur separately or in sequence: withdrawal and consumption. Water is withdrawn when it is removed from a water body. Withdrawn water is not necessarily lost water, as it may be returned to its original source in a manner that complies with environmental law and regulations. Water is consumed when it either ceases to exist as a liquid through evaporation or is not fit to be returned directly to its original source.
Water Consumption by Economic Sector
According to the U.S. Geological Survey, electric generation accounts for 3.3 percent of freshwater consumption, about the same percentage as the industrial sector (3.4 percent) and raising livestock (3.2 percent). The residential sector consumes 6.7 percent of freshwater, while the commercial and mining sectors consume the least, at 1.3 percent and 0.8 percent, respectively. The largest consumption of freshwater is for irrigation, at 81.3 percent.
To put residential and electric power water consumption in perspective, a typical nuclear power plant supplies 740,000 homes with all of the electricity they use while consuming 13 gallons of water per day per household in a once-through cooling system, and 23 gallons per day per household in a wet cooling tower system. By comparison, the average U.S. household of three people consumes about 94 gallons of water per day.
The distinction between withdrawal and consumption is important when evaluating potential ecosystem impacts. According to the U.S. Geological Survey, power plants withdrew the second-largest amount of freshwater of all economic sectors in 1995—131 billion gallons per day. But they returned 98 percent of that water back to its natural sources.
By contrast, withdrawals for irrigation were 134 billion gallons per day, more than the electric power sector, and only 20 percent of that water was returned, making irrigation by far the single largest consumer of water resources. Residential withdrawals were 26 billion gallons per day, but only 74 percent was returned, resulting in consumption about twice that of power plants.
Electric Power Plant Cooling Systems
Power plants heat water to steam, which turns a turbine to produce electricity. Cooling systems are required to turn the steam back to water so the cycle can continue.
Most power plants use one of two types of cooling water systems. Once-through cooling withdraws water from a water body and circulates it within the plant to condense the steam from the turbine into water through heat absorption. In a wet cooling tower system, circulating water from the plant moves through the tower and is cooled by evaporation.
Water Consumption by Energy Source
For energy sources, water consumption is calculated in terms of the amount of water consumed to pro- duce a standard measure of electricity generated—a megawatt-hour (MWh).
Nuclear energy consumes 400 gallons/MWh with once-through cooling and 720 gallons/MWh with wet cooling towers. Coal consumes less, ranging from about 300 gallons/MWh for plants with minimal pollution controls and once-through cooling to 714 gallons/MWh for plants with advanced pollution control system and wet cooling towers. Natural gas-fueled power plants consume even less, at 100 gallons/MWh for once- through, 370 gallons/MWh for combined-cycle plants with cooling towers, and none for dry cooling.
Hydropower’s typical water consumption is the most significant at 4,500 gallons/MWh, due in large part to evaporation from reservoirs. Renewable energy sources such as geothermal and solar thermal consume two to four times more water than nuclear power plants.
Local Ecosystem Considerations
An examination of the specific environmental and economic issues involved in electricity generation will demonstrate the value of the holistic environmental management approach for selecting various types of electricity production and cooling system options.
Water Quantity
Considering cooling technologies for coal, natural gas and nuclear power plants, once-through systems consume 1 percent of the water they withdraw. Wet cooling tower systems, on average, consume twice as much water as once-through systems.
Water Quality
To ensure water quality, the U.S. Environmental Protection Agency or the states regulate discharge water temperature and impurity concentrations from power plant cooling systems. States may set a discharge temperature limit. Plant operators may apply for a higher temperature limit for their facility by demonstrating through scientific evidence that the additional temperature will not adversely affect fish and wildlife. The higher temperature of water discharged by a once-through system is reduced by using cooling canals or after-bays before discharge to the main water body.
Aquatic Life (Impact Studies)
Energy companies take significant steps to reduce the number of fish trapped on (impinged) or drawn through cooling water systems (entrained). Scientific studies demonstrate that aquatic life mortality at the cooling intake structure does not have an adverse impact on aquatic life populations in the water body. This is because this mortality is a very small percentage of the overall population (roughly 1 percent) and prolific reproduction quickly replaces those individuals lost.
Aquatic Life (Mitigation Technologies)
To minimize the impact on aquatic life, power plant operators install and maintain proven fish protection technologies at cooling water intake structures. The use of a particular technology depends on the species in the habitat, the geographic location of the plant, the kind of water body, and the design and operating characteristics of the plant.
Some of these measures include physical barriers that prevent fish from entering the intake structure, such as stationary screens. These barriers are often equipped with flushing structures to free the fish if necessary and passageways for fish to return fish to their habitat. Collecting systems, such as traveling screens, gather smaller aquatic life and mature fish and transfer them to baskets for return transport. Diversion systems direct fish away from the intake structure and include angled screens or louver systems that alter flow direction and velocity. Behavioral deterrents involving light or sound are also effective.
Wildlife and Habitat
According to a study commissioned and supervised by the New York State Energy Research and Development Authority, wind power and nuclear energy have the lowest potential life-cycle impact on wildlife of all energy sources.
Land use is an indicator of the impairment or loss of habitat for wildlife and plant life as well as potential impacts on our communities. Nuclear power plants are the most eco-efficient of any energy source, producing more electricity per amount of land required for operation.
For example, to generate the electricity equivalent of a 1,000-megawatt nuclear plant, a wind farm would have to cover about 150,000 to 180,000 acres and a solar photovoltaic park 54,000 acres. Considering actual plants in operation, the Peach Bottom nuclear plant in Pennsylvania generates twice as much electricity on 400 acres and the Millstone nuclear plant in Connecticut twice as much on 220 acres. In other words, a nuclear power plant requires about one-third of a percent of the land required by wind power to produce the same amount of electricity.
Air Quality
Nuclear power plants produce no air emissions during operations. Several studies have demonstrated that the life-cycle CO2 emissions of nuclear power plants are comparable to or lower than renewable energy sources. In addition, during operations nuclear plants produce no nitrogen oxides (NOx), which contributes to ground-level ozone formation, a cause of respiratory ailments, or sulfur dioxide (SO2), which contributes to acid rain formation, harmful to the natural and man-made environments.
Broader Environmental Issues
Along with these local considerations, broader environmental issues also are a critical part of the decision- making process that constitutes holistic environmental management.
Climate Change Mitigation
Critical for climate change mitigation, nuclear energy generates 72 percent of U.S. emission-free electricity. All credible domestic and international carbon-reduction proposals call for the expansion of nuclear energy.
Climate Change Adaptation
Clean energy sources, by mitigating climate change, alleviate the water shortages that may be caused by climate change. Nuclear power plants can contribute to climate change adaptation by providing reliable, emission-free electricity for desalination and a low-carbon transportation sector.
Sustainable Development
According to the United Nations, sustainable development is simultaneous environmental preservation and economic progress. Looking at the sustainability from an environmental viewpoint, power plants return 98 percent of the water they withdraw. They represent half of residential consumption of U.S. freshwater, while irrigation consumes by far the most freshwater.
On the economic side of sustainable development, our standard of living depends upon the availability of usable water and electricity. Fully 90 percent of U.S. electricity is generated by power plants. Conversely,
80 percent of municipal water processing and distribution costs are for electricity.
Nuclear Energy in a Water-Constrained World
Despite moderate water use, nuclear power plants are a valuable option for water constrained ecosystems because they can provide large-scale, economical electricity for desalination. The Palo Verde nuclear plant, located in the Arizona desert, uses reclaimed municipal waste water from Phoenix for cooling. It produces more electricity than any other U.S. power plant. Two proposed reactors at the Turkey Point site in Florida plan to use municipal waste water for cooling systems.
Locating nuclear plants on the ocean provides ample water supply for cooling and desalination. For example, Diablo Canyon and San Onofre nuclear power plants are located on the Pacific coast in water-strapped southern California.
Industry Sponsoring Cooling System R&D
Extensive cooling system research and development work is being conducted by the Electric Power Research Institute, often in partnership with commercial enterprises or the Department of Energy’s national laboratories. For instance, for once-through cooling, technology advances are focused on minimizing the impingement and entrainment of aquatic life. For cooling tower systems, the emphasis is on reducing water consumption and the adverse effects on equipment when using recycled water for cooling.
Two other cooling systems are the focus of technology development. With dry cooling, circulating water from the turbine is cooled by heat transfer to the ambient air. For large plants, dry cooling is three times as expensive to build and 10 times as expensive to operate. It is more viable for smaller plants. R&D is focusing on improving the air-cooled condenser to reduce cost.
Hybrid cooling uses cooling towers during those seasons of the year when ambient air temperatures are relatively high, and dry cooling is used during those seasons when temperatures are relatively low. The goal of R&D for hybrid cooling is to optimize the system’s operating performance. The proposed reactor at the North Anna nuclear plant in Virginia will use hybrid cooling.