The interdependence of water and energy is known as the energy-water nexus, in which water is required to produce energy, and energy is necessary to treat and transport water. In this report, two commonly used methods of procuring fresh water are compared in terms of energy demands.
Water importation is the first means to be discussed, and is used to transport water from regions with large freshwater reserves to regions lacking or exceeding their local freshwater resources. In California, the State Water Project delivers 5 billion m3 of water from Northern to Southern California at a cost of 12.4 x 106 MWh of energy per year.1 The high energy requirement is mostly from pumping. The highest pump lift is 1,925 feet with a flow rate of 127 m3/s. While hydroelectric power is able to recover some energy, the pumping energy required still far exceeds the power generated.2 The energy required per cubic meter to import water to Southern California is thus 8.9 MJ/m3.
In Texas, the Texas Water Plan proposes to deliver 15 km3 from the lower Mississippi River to Texas with a pump lift of 2,952 feet and a pumping energy cost of 40 x 106 MWh of energy per year. 1 The energy required per cubic meter to import water to Texas is thus 9.6 MJ/m3.
In China, a proposed plan to import 14 km3 of water from the Chang Jiang River to the North China Plain would require 5 x 106 MWh of energy per year for pumping.1 The energy required per cubic meter to import water to the North China Plain is thus 1.28 MJ/m3.
Only 3% of the earth’s water is freshwater, while 97% is contained in the sea.3 Desalination provides 0.001% of all freshwater use, and is especially popular in the Middle East in countries like Saudi Arabia, Kuwait, and the United Arab Emirates. Even though the theoretical minimum amount of energy necessary to take out salt from seawater is 2.8 MJ/m3, desalination plants use far more energy.1 Of the various desalination processes, “the cost of energy is the main production expense in desalination plants… and the process of reverse osmosis (RO) is the most efficient desalination process both in terms of energy and costs”.4 RO works by applying an eternal pressure in order to force solvent to flow through a semi-permeable membrane. Energy is required to power the high pressure pump, although 30-40% of this energy can be recovered when the discharged brine goes through a turbine. 4
In 1994, Gleick asserts that the energy required for reverse osmosis desalination is 90 MJ/m3, and 150 MJ/m3 using electrodialysis. Interestingly, the energy requirements for treating brackish water using reverse osmosis are far less than desalination at 14 MJ/m3.4 Writing in 2002, Einav et al. reports an energy cost of 3.5-4.5 kWh/m3, or 12.6-16.2 MJ/m3 of water for reverse osmosis.4 Benzarti, in 2007 reports the same energy cost range that Einav’s group reported. 5
Benzarti also discusses the negative externalities of power generation, including the “health effects of pollution of air, water and soil, ecological disturbances and species loss, and landscape damages”.5 These externalities are not unique to desalination as they are present in any process that is energy intensive, including water importation that requires pumping.
Zhou compares the cost of desalinating water and transporting water, and concluded that “one needs to lift the water by 2000 m, or transport it over more than 1600 km to get the transport costs equal to the desalination costs”.6 For this reason, water importation is impractical for areas located far from the ocean, or with a large vertical displacement from sea level. The energy requirements for water importation and desalination are summarized in Table 1. While the energy necessary for desalination is higher, the advancement of more energy efficient desalination techniques has led to a rapid decline in energy costs. Ultimately, the choice narrows down to economics, which include capital costs as well as energy costs.
|Water Procurement Source||Energy required per cubic meter (MJ/m3)|
|Importing water: California||8.9|
|Importing water: Texas||9.6|
|Importing water: China||1.28|
|Desalination: Reverse Osmosis (1994)||90|
|Desalination: Electrodialysis (1994)||150|
|Desalination: Reverse Osmosis (2007)||12.6-16.2|
- Gleick, Peter H. “Water and Energy.” Annu. Rev. Energy Environ. 19 (1994): 267-69. Print.
- “California State Water Project Today.” California Department of Water Resources. Web. 15 Oct. 2010. <http://www.water.ca.gov/swp/swptoday.cfm>.
- Kalogirou, Soteris A. “Seawater Desalination Using Renewable Energy Sources.” Progress in Energy and Combustion Science 31 (2005): 242-81. Print.
- Einav, Rachel, Kobi Harussi, and Dan Perry. “The Footprint of the Desalination Processes on the Environment.” Desalination 152 (2002): 141-54. Print.
- Nisan, S., and N. Benzarti. “A Comprehensive Economic Evaluation of Integrated Desalination Systems Using Fossil Fuelled and Nuclear Energies and including Their Environmental Costs.” Desalination229 (2008): 125-46. Print.
- Zhou, Yuan, and Richard Tol. “Evaluating the Costs of Desalination and Water Transport.” Water Resources Research 4 (2005): 1-10. Print.
Here are some more facts about energy and water:
Thermoelectric power generation accounts for 39% of all fresh water withdrawals in the US.
Management of water and energy is separate, but needs to be integrated.
Agriculture is 84% of total freshwater consumption.
3% of water is used for thermoelectric power generation.
5% of electricity in the US is used for water conveyance and treatment.
Thermoelectric power generation withdraws 135 billion gallons/day, and consumes 3.3 billion gallons/day.
Population grows by 70 million in next 25 years, and electricity demand grows by 50%.