How Many Gallons of Water Does it Take to Power your Air Conditioner?
Ever since global climate change became a chief concern on the world stage, the environmental policy debate on energy has focused chiefly on carbon emissions. While this is an important factor in considering energy options, it’s far from the only one. A recent policy paper from the World Policy Institute and EBG Capital suggests that water usage in the energy sector is also a significant concern, especially in a world facing localized water shortages. In other words, we should be asking not only is an energy source green, but is it blue as well?
The study begins by noting that energy production is water intensive: “Energy production consumes significant amounts of water; providing water, in turn, consumes energy.” The authors then note that there are three dimensions to the issue of blue energy: water consumption, water withdrawal and water quality.
Consumption refers to water that disappears or is diverted from its source, for example by evaporation, incorporation into crops or industrial processes, drinking water, etc. The source may or may not eventually be replenished. If replenished, the process could potentially take many years— decades, centuries, or longer.
Withdrawal refers to water that is essentially ‘sucked up’ for a given use, but then returned to its source. The quality of the returned water may or may not be the same as it was prior to removal.
Quality is an umbrella term that can refer to pollutants that enter the water; changes to oxygen content, salinity, and acidity; temperature changes; destruction of organisms that live in the water; and so on.
Every method of energy generation uses different amounts of water and must be judged in these terms differently. For instance, nuclear power withdraws a lot of water per unit of energy produced and returns it to the environment much warmer than when it was taken. Hydraulic fracturing (“fracking”) for natural gas production largely affects water quality. The installed base of solar-thermal, as opposed to solar photovoltaic energy, consumes a great deal of water, usually in deserts.
To properly compare different sources of energy on an apples to apples basis, water, carbon, and cost impact need to be evaluated per unit of energy produced. This adjusts for differences in the amount of energy that, say, a gallon of ethanol contains relative to a gallon of gasoline or jet fuel, and also helps to give aggregate numbers some context. For example, people often say it takes millions of gallons of water to frack a natural gas well — by taking the next step to divide that aggregate number by the amount of energy the well produces allows us to compare water use (or carbon or cost impact) of natural gas versus, say, petroleum or biofuels.
While the study offers several proposals for further research, the greatest value of the report compares various energy sources not for their carbon emissions but their usage of water. For example, nuclear power, which up until the Fukushima disaster happened, has been called a green source of energy because there is no carbon released in the fission process. However, “nuclear consumes roughly three times more water than gas-fired plants and 1½ times more than coal- or oil-fired plants.” In short, it isn’t very blue compared with fossil fuels.
The most compelling part of the entire paper is in a single table which shows how much water it takes to run an 18,000 BTU air conditioner for 12 hours a day for a week:
- Hydroelectric Minimal to 2,000 gallons
- Geothermal 700 gallons
- Solar Thermal 400 gallons
- Nuclear 300 gallons
- Thermoelectric, coal 200 gallons
- Thermoelectric, oil 200 gallons
- Thermoelectric, natural gas 100 gallons
- Coal IGCC 100 gallons
- (integrated gasification combined cycle)
- Wind Minimal
- Solar PV Minimal
Another significant discovery in this report is just how water intensive the production of the current generation of biofuel can be when it uses irrigated crops. The report deals with transportation fuels separately from electricity generation in a fairly successful attempt to avoid comparing apples to oranges. The paper looks at how much water is needed to produce the fuel required to drive from New York City to Washington, DC (a distance of roughly 200 miles, consuming about 2 million BTUs).
- Natural gas (as on land) 5 gallons
- Unconventional natural gas (shale) 30 gallons
- Oil (traditional) 28 gallons
- Oil sands (mining) 550 gallons
- Biofuels (irrigated corn) 32,000 gallons
- Biofuels (irrigated soy) 89,000 gallons
When looked at from a blue perspective rather than a green one, biofuels are simply untenable. Agriculture consumes 80 percent of all water used in the United States, and to turn 40 percent of our corn crop into biofuel just doesn’t make much water sense.
The authors are clear that there are no easy choices. “On-shore natural gas production may be less of an issue in rural areas but extremely challenging in New York City’s watershed. Water-hungry technologies might make sense where water is abundant, but concerns about building solar thermal plants in arid Southwestern deserts deserve further probing. First generation irrigated biofuels pose difficult questions.”
Because we have not asked the right questions about the water-energy nexus, there is not as much good data as we need. In general, data about water and energy are incomplete, fragmented, difficult to compare on an apples to apples basis, and often taken out of context (i.e., politized, mis-characterized,, misunderstood etc.). Even this study has barely begun to scratch the surface and had to make do with data from 2006. As a result, there are some caveats about the data and the interpretation thereof
One of the most important points is that we need to consider water as well as carbon when we choose a method of generating energy. Solar pv is both green and blue energy and would be excellent an a grand scale in Arizona, but it isn’t much value in a place like Scotland (tidal works there, but it doesn’t in Arizona). However, we are starting to ask those questions. Better data, and therefore, better energy policy, both blue and green, will follow. As the report says, “Now — as new energy policies are emerging – is the window of opportunity to add water to the agenda.”