The water-energy nexus is the relationship between the water used for energy production,Spang, E. S., Moomaw, W. R., Gallagher, K. S., Kirshen, P. H., and Marks, D. H. (2014). "The water consumption of energy production: an international comparison." Environmental Research Letters, 9(10), 105002. including both electricity and sources of fuel such as oil and natural gas, and the energy consumed to extract, purify, deliver, heat/cool, treat and dispose of water (and wastewater) sometimes referred to as the energy intensity (EI). Energy is needed in every stage of the water cycle from producing, moving, treating and heating water to collecting and treating wastewater. The relationship is not truly a closed loop as the water used for energy production need not be the same water that is processed using that energy, but all forms of energy production require some input of water making the relationship inextricable.
Among the first studies to evaluate the water and energy relationship was a life-cycle analysis conducted by Peter Gleick in 1994 that highlighted the interdependence and initiated the joint study of water and energy.Gleick, P. H. (1994). "Water and Energy." Annual Review of Energy and the Environment, 19(1), 267–299. In 2014 the US Department of Energy (DOE) released their report on the water-energy nexus citing the need for joint water-energy policies and better understanding of the nexus and its susceptibility to climate change as a matter of national security.Bauer, D., Philbrick, M., and Vallario, B. (2014). "The Water-Energy Nexus: Challenges and Opportunities." U.S. Department of Energy. The hybrid Sankey diagram in the DOE's 2014 water-energy nexus report summarizes water and energy flows in the US by sector, demonstrating interdependence as well as singling out thermoelectric power as the single largest user of water, used mainly for cooling.
Water used in the energy sector
All types of power generation consume water either to process the raw materials used in the facility, constructing and maintaining the plant, or to just generate the electricity itself. Renewable power sources such as photovoltaic solar and wind power, which require little water to produce energy, require water in processing the raw materials to build. Water can either be used or consumed, and can be categorized as fresh, ground, surface, blue, grey or green among others. Water is considered used if it does not reduce the supply of water to downstream users, i.e. water that is taken and returned to the same source (instream use), such as in thermoelectric plants that use water for cooling and are by far the largest users of water. While used water is returned to the system for downstream uses, it has usually been degraded in some way, mainly due to thermal or chemical pollution, and the natural flow has been altered which does not factor into an assessment if only the quantity of water is considered. Water is consumed when it is removed completely from the system, such as by evaporation or consumption by crops or humans. When assessing water use all these factors must be considered as well as spatiotemporal considerations making precise determination of water use very difficult. According to the International Energy Agency (IEA), water stress also poses risks to the transport of fuels and materials. In 2022, droughts and severe heatwaves led to low water levels in key European rivers such as the Rhine, limiting barge transport of coal, chemicals and other materials.IEA (2023), Clean energy can help to ease the water crisis, IEA, Paris https://www.iea.org/commentaries/clean-energy-can-help-to-ease-the-water-crisis, License: CC BY 4.0
Spang et al. (2014) conducted a study looking at the water consumption for electricity production (WCEP) internationally that both showed the variation in energy types produced across countries as well as the vast differences in efficiency of power production per unit of water use (Figure 1). Operations of water distribution systems and power distribution systems under emergency conditions of limited power and water availability is an important consideration for improving the overall resilience of the water – energy nexus. Khatavkar and Mays (2017a) present a methodology for control of water distribution and power distribution systems under emergency conditions of drought and limited power availability to ascertain at least minimal supply of cooling water to the power plants.Khatavkar, P., & Mays, L. W. (2017 a) Model for the Real-Time Operation of Water Distribution Systems under Limited Power Availability. In World Environmental and Water Resources Congress 2017 (pp. 171–183). Khatavkar and Mays (2017) applied an optimization model for water – energy nexus system for a hypothetical regional level system which showed an improved resilience for several contingency scenarios.Khatavkar, P., & Mays, L. W. (2017). Testing an Optimization/Simulation Model for the Real-Time Operations of Water Distribution Systems under Limited Power Availability. In Congress on Technical Advancement 2017 (pp. 1–9).
Increasingly controversial has been the use of water resources for hydraulic fracturing of shale gas and tight oil reserves. Many environmentalists are deeply concerned about the potential for such operations to exacerbate local water scarcity (since the water volumes required are large) and to produce considerable volumes of polluted water (both directly through pollution of fracking water, and indirectly through contamination of groundwater). With rising energy prices in North America and Europe in the 2020s it is likely that government and industry interest in hydraulic fracturing will grow.
Energy intensity The operation of urban water systems requires substantial energy support. Key processes such as water transfer, consumption, and wastewater treatment consume significant amounts of energy, sparking discussions about the energy intensity and carbon emissions of water systems.
US (California) In 2001, operating water systems in the US consumed approximately 3% of the total annual electricity (~75 TWh).Cohen, R., Nelson, B., and Wolff, G. (2004). "Energy Down The Drain: The Hidden Costs of California's Water Supply." E. Cousins, ed., Natural Resources Defense Council The California's State Water Project (SWP) and Central Valley Project (CVP) are together the largest water system in the world with the highest water lift, over 2000 ft. across the Tehachapi Mountains, delivering water from the wetter and relatively rural north of the state, to the agriculturally intensive central valley, and finally to the arid and heavily populated south. Consequently, the SWP and CVP are the single largest consumers of electricity in California consuming approximately 5 TWh of electricity each per year. In 2001, 19% of the state's total electricity use (~48 TWh/year) was used in processing water, including end uses, with the urban sector accounting for 65% of this.Klein, G., Krebs, M., Hall, V., O'Brien, T., and Blevins, B. B. (2005). "California's Water – Energy Relationship." California Energy Commission, Sacramento, California. In addition to electricity, 30% of California's natural gas consumption was due to water-related processes, mainly residential water heating, and 88 million gallons of diesel was consumed by groundwater pumps for agriculture. The residential sector alone accounted for 48% of the total combined electricity and natural gas consumed for water-related processes in the state.
According to the California Public Utilities Commission (CPUC) Energy Division's Embedded Energy in Water Studies report:"'Energy Intensity' refers to the average amount of energy needed to transport or treat water or wastewater on a per unit basis." Energy intensity is sometimes used synonymous with embedded or embodied energy. In 2005, water deliveries to Southern California were assessed to have an average EI of 12.7 MWh/MG, nearly two-thirds of which was due to transportation. Following the findings that a fifth of California's electricity is consumed in water-related processes including end-use, the CPUC responded by authorising a statewide study into the relationship between energy and water that was conducted by the California Institute for Energy and Environment (CIEE), and developed programs to save energy through water conservation.Bennett, B., and Park, L. (2010). "Embedded Energy in Water Studies Study 1: Statewide and Regional Water-Energy Relationship." California Public Utilities Commission Energy Division.Bennett, B., and Park, L. (2010). "Embedded Energy in Water Studies Study 2: Water Agency and Function Component Study and Embedded Energy- Water Load Profiles." California Public Utilities Commission Energy Division.
Arab region According to the World Energy Outlook 2016, in the Middle East, the water sector's share of total electricity consumption is expected to increase from 9% in 2015 to 16% by 2040, because of a rise in desalination capacity. The Arab region which includes the following countries: Kuwait, Lebanon, Libya, Mauretania, Morocco, Oman, Palestinian Territories, Algeria, Bahrain, Egypt, Iraq, Jordan, Qatar, Sudan, Saudi Arabia, Syria, Tunisia, the United Arab Emirates, and Yemen. Some general characteristics of the Arab region is that it is one of the most water stressed regions of the world, rain fall is mostly rare, or it rains in an unpredictable
pattern.UNDP (2013) Water governance in the Arab Region: managing scarcity and securing the future. UNDP, New York. The cumulative area of the Arab region is approximately 10.2% of the world's area, but the region only receives 2.1% of the world's average annual precipitation. Further, the region accommodates 0.3% of the world's annual renewable water resources (ACSAD 1997). Consequently, the region has experienced a declining fresh water supply per capita, roughly a shortage of 42 cubic kilometers of water demand.Devlin J (2014) Is water scarcity dampening growth prospects in the Middle East and North Africa? Brookings Institution, 24 June 2014 This shortage is expected to grow three times by 2030, and four times by 2050.World Bank (2012) Renewable energy desalination: an emerging solution to close the water gap in the Middle East and North Africa. World Bank, Washington, DC. This is crucially alarming given the world's economic stability highly depends on the Arab region.