By VEERA GNANESWAR GUDE, DANIEL SIMON and THOMAS COWING February 25, 2013
Increasing demand for potable water due to population growth and rapid development is a major concern nationally and globally. According to the Energy Information Administration projections, the U.S. population is expected to grow by about 70 million by 2030. The direct domestic water demand and the indirect industrial, agricultural and environmental water needs to sustain this growth are expected to place serious strains on the currently available water resources. At the same time, this growth in population is expected to increase the electricity demand by approximately 50 percent, which will place additional demands on available water.
Future demand for potable water will be much higher in the global context. According to the World Health Organization, nearly 2.8 billion people (approximately 40 percent of the world population) currently have no access to safe drinking water, and water-borne diseases account for 90 percent of all infectious diseases in the developing world. The World Resources Institute predicts that by 2025, at least 3.5 billion people will experience water shortages.
Provision of clean water inevitably requires energy, which is currently being provided essentially by nonrenewable fossil fuels. Total energy demand for providing the U.S. water needs is reported as 123×106 megawatt-hours per year. It has been estimated that production of 1 cubic meter of potable water from seawater requires the equivalent of about 0.03 tons of oil (or 1,000 gallons of potable water requires 30 gallons of oil). Extraction and refining of fossil fuels and production of energy not only places additional demands on water, but also results in pollution of water sources and air. Thus, the projected global demand for clean water supply for the future will significantly accelerate not only depletion of fossil fuel reserves but also the concomitant environmental damage and emission of greenhouse gases.
Energy Required for Clean Water Production
Clean water supply without any energy investment is almost impossible. Even if freshwater is readily accessible under the ground level, energy is required to pump the freshwater from its source. Freshwater drawn from the groundwater source requires 0.14 to 0.24 kilowatt-hours per cubic meter (kWh/m3), or 0.5 to 0.9 kilojoules per kilogram (kJ/ kg) for a pumping head of 100 to 200 feet. Conventional treatment of surface waters to potable quality requires 0.36 kWh/m3 (1.3 kJ/kg). The cost of freshwater supply through conventional treatment is around 25 cents per cubic meter.
Recently, due to excess population growth and rapid industrialization, desalination has been sought as an alternative to fill the gap between demands and supply for freshwater. Desalination is a nonconventional water-treatment technology applied to recover freshwater from surface and ground waters that have high dissolved solids concentrations. In the early 1950s, desalination was predominantly performed by thermal desalting technologies such as multi-stage flash desalination, multi-effect evaporation and mechanical vapor compression, which consumed enormous amounts of thermal energy. With the advent of reverse osmosis (RO) technology and remarkable improvements in the membrane performance and associated energy consumption, RO technology has increased its visibility comparable to thermal desalination technologies. Desalination by RO process has been accepted as a feasible option particularly in areas where transportation cost of freshwater and high living standards override the negative impacts of desalination.
Energy Needed to Clean Water
Wastewater treatment accounts for about 3 percent of the U.S. electrical energy load, similar to that in other developed countries. The energy needs for a typical domestic wastewater treatment plant employing aerobic activated sludge treatment and anaerobic sludge digestion is 0.6 kWh/m3 of wastewater treated, about half of which is for electrical energy to supply air for the aeration basins. With conventional approaches involving aerobic treatment, a quarter to half of a plant’s energy needs might be satisfied by using the CH4 biogas produced during anaerobic digestion, and other plant modifications might further reduce energy needs considerably. However, if more of the energy potential in wastewater were captured for use and even less were used for wastewater treatment, then wastewater treatment might become a net-energy producer rather than a consumer.
Using Water for Energy Production
Energy from conventional power plants
Electricity production results in one of the major uses of water in the United States and worldwide. Water for thermoelectric power is used in generating electricity with steam-driven turbine generators. In 2005, about 201,000 million gallons of water each day was drawn to produce electricity (excluding hydro-electric power). Surface water was the source for more than 99 percent of total thermoelectric-power withdrawals. In coastal areas, the use of saline water instead of freshwater expands the overall available water supply. Thermoelectric-power withdrawals accounted for 49 percent of total water use, 41 percent of total freshwater withdrawals for all categories, and 53 percent of fresh surface-water withdrawals.
One of the main uses of water in the power industry is to cool the power-producing equipment. As a result the cooling water becomes hot water and cannot be released back into the environment due to fish kill in the downstream. So, the used cooling water must first be cooled before discharging into the surface waters by using cooling towers. As water is cooled down, some evaporation occurs in the process and is lost. For this reason, large power-production facilities are often located near rivers, lakes and the ocean.
Energy from renewable energy sources
Gasoline production from fossil fuels (considered nonrenewable) involves water consumption (about 3 gallons of water per gallon of gasoline).
Water is also required for irrigation of renewable fuel feedstock such as corn, soybean and switch grass and solar-based technologies.
The amount of water required for irrigation varies significantly from one region to another.
Gallons of water required per vehicle mile traveled for different energy sources are shown.
Clean energy and water production involves utilization of environmental and land resources and their degradation due to release of air pollutants (greenhouse gases and other toxic pollutants) and waste products. However, when renewable energy sources such as solar, wind, geothermal and biomass are considered, the environ- mental impact is well reduced. Moreover, the environmental emissions and energy payback periods, for these energy resources, are very reasonable. Thus, renewable energy source utilization for clean water production can offer sustainable solutions where applicable with minimal environmental impact. Similarly, developing water-efficient (with water recycling and reuse) renewable energy sources will reduce the water footprint for energy production.
About the Authors
Veera Gnaneswar Gude is an assistant professor in the Civil and Environmental Engineering Department at Mississippi State University and chair of the ASES Clean Energy and Water Division (CEW).
Daniel Simon is president of 3D Solar Inc. and vice chair of CEW.
Thomas Cowing is the principal consultant of Thomas Cowing & Associates and serves as the CEW newsletter editor.
Learn more about ASES technical divisions.