In the twentieth century, global energy use grew ten-fold and global water use grew six-fold. Population growth and rising affluence are expected to drive continued increases in the demand for energy and water. The water-energy nexus refers to the fact that the production and consumption of water and energy are closely interconnected. All forms of energy require water for their extraction, processing, and distribution. Extracting, treating, and distributing water requires energy inputs, as does treating and disposing wastewater.
Water withdrawal refers to water removed from the ground or diverted from a surface water source for use. Consumptive use (water consumption) refers to the part of water withdrawn that is evaporated, transpired, incorporated into products or crops, consumed by humans or livestock, or otherwise not available for immediate use.1
Electricity is the fastest-growing form of energy, so using water for alternative forms of electricity generation is a vital statistic typically measured as liters per megawatt-hour (L/MWh). Accurate comparison requires a comprehensive lifecycle assessment of water consumption at every stage of the supply chain.
The water intensity of electricity generation varies by three orders of magnitude across the spectrum of technologies. There also is variation within a technology due to region-specific conditions such as geography, climate, and technology supply chains. Water use in thermal power generation (oil, coal, natural gas, nuclear, biomass) is strongly influenced by the type of cooling system used in a power plant.
Wind and solar PV have the lowest levels of water use compared to thermal power generation do in large power because they require no cooling system. Their water needs lie upstream in the supply chain in the mining and processing of materials that are used to manufacture wind turbines and solar panels. Solar PV may require water to clean the panels.
Note the high average water demands of hydroelectric plants. Water flowing through the turbines in hydroelectric plants and back into the river is not considered consumptive because it is still immediately available for other uses. But many hydroelectric plants have large dams that create correspondingly large reservoirs. The increased surface area of the reservoir, when compared to the free-flowing stream, results in additional water evaporation from the surface which is a form of consumptive use.2
The average water consumption from hydropower should be viewed with caution. Estimates exhibit an extensive range due to differences in methodology and the very site-specific nature of hydropower projects. There are also scientific challenges such as allocation from multipurpose reservoirs (irrigation and electricity generation) and spatial assignments in river basins with several hydropower plants.3
Electricity from biomass consumes substantial quantities of water for several reasons. Growing the crops or plants used as biomass feedstock often requires irrigation which leads to significant evaporation. Plants also lose water through transpiration, the movement of water from the soil, and eventual release into the air from their leaves. The processing and conversion of biomass can consume additional large quantities of water. Like hydropower, estimates for water use in the generation of electricity from biomass exhibit a wide range due to the site-specific nature of crop production, the type of crop grown, and the type of technology used on the farm and in the processing plant. For example, a rain-fed crop uses dramatically less energy than the same crop grown with irrigation.
Thermal power plants have a unique water need: they require a cooling system to condense steam leaving a turbine. The water requirements for cooling depend on the type of cooling system employed. Throughout much of the twentieth century thermal power plants used a method known as once-through cooling. In this system, water is drawn from a natural body like a lake or river, is passed through a plant a single time, and is then returned to the same (or sometimes different) body of water.4 Once through cooling has a major drawback: the water released back to a lake or stream is a higher temperature than the water body. This can lead to so-called thermal pollution that reduces the oxygen supply in the water and severely disrupts aquatic ecosystems.
Concern about thermal pollution led to the so-called wet cooling or recirculating system in which the water is piped into cooling towers where the heat it has absorbed from the steam dissipates through evaporation. The remaining cooling water is then recirculated through the condensers. Water lost through evaporation must be replaced from an outside source. A dry cooling system uses no water, relying instead on heat loss in the cooling tower by conduction, convection, and radiation. Dry cooling is very water efficient but is more expensive and less efficient compared to wet cooling.
1 U.S. Geological Survey, “Water-Use Terminology,” accessed December 26, 2023, Link
2 P. Torcellini, N. Long, and R. Judkoff, “Consumptive Water Use for U.S. Power Production,” National Renewable Energy Laboratory, NREL/TP-550-33905, December 2003, https://doi.org/10.2172/15005918
3 Bakken, Tor Haakon, Ånund Killingtveit, and Knut Alfredsen. “The Water Footprint of Hydropower Production—State of the Art and Methodological Challenges.” Global Challenges 1, no. 5 (2017): 1600018. https://doi.org/10.1002/gch2.201600018.
4 U.S. Energy Information Administration, “Many newer power plants have cooling systems that reuse water,” February 11, 2014, ttps://www.eia.gov/todayinenergy/detail.php?id=1497Link