©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
In last week's blog  I took a look at some of the water conservation features in our new house, but I began the blog 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.
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.
In some parts of the world, this evaporation is a big problem. In other areas, not so much. In the arid Southwestern U.S. this evaporation is a huge issue, especially from reservoirs like Lake Mead.
Most electricity in the U.S. (about 89%) 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 generate electricity. Cooling water is then used to condense the steam back to water.
Reduce Water Use 
Depending on the type and age of the power plant, the cooling water is 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 — albeit at a higher temperature (thermal pollution can be a major problem). 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 originally drawn, but they still evaporate considerable water — in fact, typically more than once-through cooling systems — so the consumptive water is very significant.
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 of water per kWh of electricity produced, natural gas power plants consume 0.17 gallons/kWh, and nuclear plants 0.57 gallons/kWh.
With coal, according to the report, 73% (0.506 gal/kWh) of the water consumption is from evaporation, as described above, while 27% (0.186 gal/kWh) is from upstream sources (mostly mining, and transportation). Once-through cooling of coal plants results in consumptive water use (evaporation) of about 0.3 gal/kWh, while recirculating systems evaporate about 0.7 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.
While the water intensity of natural gas power generation is a lot lower than for coal and nuclear, there are significant differences depending on the type of power plant. Combined-cycle plants use much use much less water per unit of electricity output than do 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 (fracking), which results in far greater water use (typically 4-5 million gallons per well) and heavily contaminates that water. 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 sheds some light on this issue.
There are two primary ways electricity is generated from solar: utility-scale solar-thermal power plants and either utility-scale or building-scale photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. power generation. Most utility-scale solar thermal is more water-intensive than coal or nuclear power plants.
From the Burning Our Rivers report, parabolic trough systems are shown to consume about 0.80 gal/kWh, while linear Fresnel systems consume about 1.0 gal/kWh, solar power tower systems consume 0.63 gal/kWh, and dish Stirling Engine systems, which are far less common but do not use the heat to generate steam, consume only 0.020 gal/kWh.
Adding to the challenge with large-scale solar-thermal is that these systems want to be located where there is a lot of sunlight, such as the American Southwest, and those places tend to be much drier.
Photovoltaic systems use almost no water in their operation — only 0.002 gal/kWh — with most of that upstream water use for manufacturing.
Finally, wind systems consume less than 0.001 gal/kWh — the lowest of any electricity source — with most of that also upstream.
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 from a water-use standpoint are efficiency measures. Using less electricity is the place to start if the goal is to conserve water resources.
Alex is founder of BuildingGreen, Inc . and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute . To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.