Big-picture issues with water
Last week I took a look at some of the water conservation features in our new house, but I began the column by addressing the relationship between water and energy. That got me curious, so I’ve been digging deeper into this water-energy nexus, examining the water-intensity of our different electricity sources.
Some of this information is drawn from a 2012 report that I only recently came across: "Burning Our Rivers: The Water Footprint of Electricity," by the River Network in Portland, Oregon.
Evaporative losses from hydropower plants
Nearly all of our methods for generating electricity involve at least some water consumption, but the differences are huge. Producing electricity with hydropower is the most water-intensive, owing to evaporation from reservoirs. Nationwide, electricity from hydropower plants consumes about 9 gallons of water per kilowatt-hour (kWh) of electricity produced.
Water use for thermoelectric power plants
Most electricity in the U.S. (about 89 percent) is produced using thermoelectric power plants. These use a heat source (most commonly coal, natural gas, or nuclear fission) to boil water, creating superheated, high-pressure steam. This steam spins a steam turbine connected to a generator to produce electricity. Cooling water is then used to condense the steam back to water.
Depending on the type and age of the power plant, the cooling water is either once-through (pulled from a river, for example and then returned to the river at a higher temperature), provided by a cooling pond, or recirculating. The once-through systems use tremendous quantities of water, but the vast majority returns to the water source from whence it was drawn. Some evaporates, however, and is not returned to the river; this is the consumptive use.
Recirculating cooling systems in power plants use far less water and they don’t add thermal pollution to the body of water from which the water was drawn, but they still evaporate considerable water -- in fact, typically more than once-through cooling systems -- so the consumptive water is very significant.
Comparing coal, natural gas and nuclear relative to water use
Of the three primary fuels used in thermoelectric power plants, natural gas power plants have the lowest water intensity. According to "Burning Our Rivers," coal power plants consume 0.69 gallons per kWh, natural gas power plants consume 0.17 gallons per kWh generated, and nuclear plants consume 0.57 gallons/kWh.
With coal, according to the same report, 73 percent (0.506 gal/kWh) of the water consumption associated with coal-fired power plants is from evaporation, as described above, while 27 percent (0.186 gal/kWh) is from "upstream" sources (mostly mining, and transportation). There are big differences on water consumption, depending on the type of cooling. Once-through cooling requires huge quantities of water, but the evaporation (consumptive use) is fairly low, about 0.3 gal/kWh -- which is less than half as much as with recirculating systems (700 gal/kWh).
Water consumption from nuclear plants is similar to that of coal though the spread between once-through and recirculating systems is even greater: 0.27 gal/kWh for once-through cooling vs. 0.76 for recirculating systems.
With natural gas, the water intensity of power generation is a lot lower than for coal and nuclear, but there are significant differences depending on the type of power plant. Combined-cycle plants are nearly two-and-a-half times as water-efficient as single-cycle power plants.
The "Burning Our Rivers" report shows very low upstream water consumption for natural gas power plants, but the report did not consider hydraulic fracturing (fraking). An October 2013 report on the water intensity of natural gas extraction from Marcellus Shale in Pennsylvania and West Virginia by researchers at Downstream Strategies and San José University points out that the water consumption per thousand cubic feet (Mcf) of natural gas extracted is fairly low (0.677 gal/Mcf), or 0.0053 gal/kWh of power produced. But because chemicals used in fraking contaminate the water used in this process, a far larger amount of essentially taken out of the usable water cycle.
Solar and wind power
There are two primary ways electricity is generated from solar: utility-scale solar-thermal power plants and either utility-scale or building-scale photovoltaic power generation. The former (solar thermal) is more water-intensive than coal and nuclear power.
From the same report, parabolic trough systems are shown to consume about 0.80 gal/kWh, vs. 1.0 gal/kWh for linear Fresnel systems, 0.63 for power towers systems, and 0.020 for dish Stirling Engine systems (which do not use the heat to generate steam, but which are still very uncommon). Adding to the challenge with large-scale solar-thermal is that these systems want to be located where there is a lot of sunlight, and these places then to be much drier -- such as the American Southwest.
Photovoltaic systems use almost no water in their operation: only 0.002 gal/kWh., with most of that upstream water use (manufacturing equipment).
Finally, wind systems consume less than 0.001 gal/kWh -- the lowest of any electricity source -- with most of that also upstream.
Low global warming
and low water use
It is worth pointing out that the renewable energy technologies for power generation that are growing the quickest in implementation (photovoltaics and wind) are the least water-intensive.
The only measures that do even better are efficiency measures. Using less electricity is the place to start if the goal is to conserve water resources.
Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to email@example.com.
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