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Energy Water Nexus
The energy water nexus is the relationship between energy and water. Energy is consumed to supply water, and water is consumed to supply energy.

Energy required to supply water
There are many steps in the water consumption cycle:
 * Extract Water from its source
 * Transport the extracted water to a treatment facility
 * Treat the water to make it suitable for use
 * Distribute the water to the end users
 * End users use the water and generate wastewater
 * Wastewater is treated and discharged

Energy is used in each of these steps.

Water Extraction
The first step in getting water to consumers is extracting it from it's natural environment. Fresh water is derived from two main sources, surface water and groundwater. Extracting groundwater uses approximately 30% more energy than surface water due to the higher requirements for pumping in groundwater systems.

Water Transportation
The amount of energy required to transport water from the source to the treatment plant varies greatly based on distance and geography. For example, in California, water pumping is the most significant user of electricity in the state, with 7% of the total energy usage. This is driven by large distance between the bulk of the demand and the source, as well as the mountain ranges that in some cases get in the way. The high mass of water means that pumping it vertically for any elevation change is extremely energy intensive.

Water Treatment
Water Treatment is the process that makes the water suitable for its end use. The energy required for water treatment depends on the treatment method, the input water source( which determines the contaminants to be removed), and the intended use (which dictates the level of quality the water must have after treatment). Treating ground water requires less energy than treating surface water because fewer contaminants are generally found in groundwater. Water used for irrigation requires little to no treatment, while water treatment for medical use would clearly need to lead to a very high level of purity.

Desalination
Desalination is a water treatment process where salt is removed from seawater or brackish water to create water suitable for drinking and domestic use. The minimum energy that can be used by this process is the difference between the free energy of the input and output. For seawater this is 0.86kWh per cubic meter of water, however typical desalination plants use 5-26 times as much energy as this. The energy required for the desalination process is affected by a number of factors, including salt concentration and water temperature. Although energy generation for desalination is currently dominated by fossil fuels, renewable energy solutions are being developed, such as using a solar boiler to drive a reverse osmosis plant.

Water Distribution
Energy for water distribution is dominated by pumping to pressurize the distribution system. Newer systems with more efficient pumps and pipes with less resistance to flow reduce this energy consumption.

Water Use
It is easy to overlook the fact that the use of water requires a substantial amount of energy. One study noted that that energy consumption associated with using water is greater than the energy consumption for the supply and treatment of that water. This includes heating water, clothes washing, clothes drying, and other activities dominated by the residential sector.

Wastewater Treatment
Similar to water treatment, the pumping required to treat wastewater can use a significant amount of energy. However, steps can be taken to generate energy from the process that exceeds the energy used to perform it. Domestic wastewater contains energy potential in several ways: energy contained in wastewater organics, avoiding the energy used to create Nitrogen and Potassium by extracting it from wastewater, and energy from the wastewaters thermal content. Technology such as microbial fuel cells can transfer that energy potential to electricity while attaining biological wastewater treatment. Methods like this could save 628 to 4940 million kWh annually in the United States.

Optimization of a water system to reduce energy use
When optimizing a water system to reduce energy usage, all of the steps must be optimized together as a water system to get the best gains. Optimizing at one step will often provide no benefit. The power to desalinate water is greater than other water treatment options, however when the total energy cost including transport to the treatment facility is included, it may be more efficient than long distance transportation of fresh water to coastal areas. Extracting groundwater may take more energy but it requires less energy to treat. Less energy is required to distribute water in a new distribution system, but the construction of this new system will use a large amount of energy.

Water required to supply energy
The energy-water nexus consider mainly consumption in the use of energy to supply water. The effects of supplying energy on water do not stop with consumption. The extraction, generation, and storage of energy can affect teh quality of water, the location of water, the temperature of the water, and availability of the water for other uses.

Coal Extraction
Coal is the predominant fuel for electrical generation. Mining of coal often contaminates drinking water supplies.

Fracking
Fracking uses pressurized fluid to fracture rock, allowing the extraction of greater quantities of natural gas and petroleum. Each well uses 2-9 million gallons of water, which is returned to streams contaminated with chemicals used to make the fluid more effective, naturally occurring metals and radioactive elements. This has a very negative impact on local water quality.

Biofuel Cultivation
The use of biofuels creates another energy water interaction, where water is used to cultivate fuel crops.

Corn Cultivation
Corn is a common feedstock in the US for the creation of ethanol. In addition to having an impact on the supply and therefore the price of corn based food, the cultivation of additional corn has a substantial impact on water consumption. Water use to generate the same amount of energy using ethanol is similar to that used for petroleum extraction.

Algae Cultivation
Microalgae are a potential feedstock for biofuels. However, as with any biofuel, the total process must be analyzed to find if the energy stored in the algae is greater than the energy used to grow the algae. In addition, algae grow in ponds of water. The aquisition, pumping, and containment of that water requires energy, which is calculated to be twice the potential energy output of the algae. Algae is not really an energy source, but an energy storage medium.

Thermo-Electric Power Generation
Water is used to cool thermoelectric power plants. Open loop cooling takes water from a river or other source, transfers heat to it, and releases it back to the source. These power plants do not actually consume water, but water is essential to their operation, and the higher water temperature can be an environmental problem.

Hydro Electric Power Generation
Hydroelectric power consumes very little water, however controlling the flow of the water can limit what is available downstream for other uses.

Data Center Cooling
Data centers use a large amount of energy to power their servers and a large amount of water to keep them cool. A 15MW data center can use 360,00 gallons of water a day, and there are nearly 3 million data centers across the US. Some data centers use treated waste water to cool their servers, avoiding stress on the water supply and reducing cost

Regional Aspects
The interaction between energy and water varies significantly across the globe, with differences in water scarcity, energy use, generation methods, proximity of population centers to water sources, and other regional factors.

China
The growth in China has caused an increase in demand for resources of all kinds. Water and energy are no exception. Energy demand for the growth of industry Agriculture remains the dominant user of water, however the usage has been shifting to industrial and residential uses. As water is increasingly used for industrial purposes, maintaining water quality becomes an issue. In addition, cultivation of corn for bioethanol begins to fight for a share of scarce water resources and cultivated land. To meet biofuel targets for 2020, 5-10% of cultivated land and 32-72 cubic km of water will be needed, challending already constrained resources.

Middle East
The Middle East is typically considered rich in energy, but poor in water. Countries in the Arabian Gulf use 5-12% of total electricity consumption for desalination. Water use is increasing. Conventional water sources are generally used for irrigation, while desalinated water fills the gap between water demanded and the supply from these conventional sources. Increasing the efficiency of irrigation could allow 22% of electricity could be saved in 2050.

Energy, water, and Food Nexus
There are some benefits to expanding the energy water nexus to include food. The trade offs between biofuel production and food consumption, and water directly used for food or energy become clear. There are significant interactions between all three resources. They all have increasing demand, constrained supply, regional variations, and interact with climate change. Pursuing biofuel production in areas with scarce water suppllies will be in direct competition with food production

Systems Approach
The energy water nexus may be considered using a systems engineering approach. This will consider all of the subsystems that impact water and consume or generate energy. The energy water nexus is precisely the type of interdisciplinary problem that systems engineering has been designed to optimize. Considering the energy water nexus using system model allows technical experts, rather than policy experts, to identify and optimize the coupling points between the two.