What is Water Energy?
The first in a series of articles to look at water use associated with energy production and requirements, and the water-energy nexus.
As plumbing engineers, we tend to have a pretty singular focus on water: water coming in, water going out, and water efficiency or water capture for re-use as we strive to make our plumbing systems more sustainable. Sure, water heater efficiencies and booster pumps need to be considered for energy models, but this information is often passed through to the mechanical engineer or energy modeler to account for. Even for engineers who do both plumbing and mechanical design, we still tend to keep these worlds of water efficiency (plumbing) and energy efficiency (mechanical) separated — separate drawing sheets, separate codes, separate specifications, separate LEED points. However, the reality is that these two resources are anything but separate.
In our buildings and our national infrastructure, energy use and water use are so inextricably linked that there is a relatively new area of focus described as the water-energy nexus that examines the interconnections of these two systems. In our world where changing climates and shifting populations are stressing both systems, understanding this nexus is becoming more important for design professionals in every sphere.
This is the first in a series of columns exploring the water-energy nexus. We’ll start here by looking at the water use associated with energy production, then flip it in the next article to look at the energy requirements of our water systems. In the third column, we’ll bring these topics home to look at the water-energy nexus at a building or neighborhood scale.
In the U.S., 90 percent of our electricity is generated in thermoelectric power plants, which use a heat source (most often coal, natural gas, or nuclear but occasionally biomass, solar or geothermal) to heat steam and drive a turbine. Cooling these power plants accounts for more than 40 percent of all freshwater withdrawals in the country.
At this point it’s important to clarify the difference between water “withdrawal” and “consumption.” Withdrawal is the amount of water drawn out of freshwater sources; consumption is water that is not returned (e.g. it evaporates, is incorporated into a product, is absorbed and transpired through plants, etc.)
While the withdrawal number seems staggering (because it is), the water consumption by these plants is much less at approximately 4 percent of national water consumption. This is because many of these plants use open-loop or “once-though” cooling. This draws water into the plant then returns it to the source at a higher temperature.
While the water is not consumed, these once-through cooling plants are dependent on the availability of an abundant, cool water source, which is not always reliable particularly in the drought stricken Southwest. Droughts in Texas in 2006 forced the shutdown of power plants during peak periods and resulted in rolling black-outs. Even in Connecticut, where water is more abundant, a power plant was forced to shut down during the summer of 2012 because the incoming water temperatures were too warm.
There is currently a move toward “closed loop” power plants that use cooling towers (similar to commercial buildings but much larger). These will reduce the withdrawal requirements and dependence on water temperature; however, almost all of the water withdrawn for these plants is consumed through evaporation. This means that while water withdrawal for thermoelectric plants may begin to be curbed, the consumption will increase dramatically unless there is a major shift in future technologies.
While cooling is the primary water use associated with thermoelectric plants, it’s not the only one. Obtaining the fuel source has its own water demands, whether it’s mining coal or uranium, or natural gas extraction, which is becoming increasingly water intensive with the growth of hydraulic fracturing. These industries account for 1-2 percent national water consumption.
The percentages may seem low, but when you consider that more than 80 percent of all water consumption is associated with agriculture (including livestock) for food production, then these percentages start taking on new significance. Essentially, thermoelectric power plants and their fuels account for 25-30 percent of all non-agricultural fresh water consumption.
After thermoelectric plants, hydroelectric power is the next major source of electricity in the U.S. meeting about 7 percent of the nation’s demand. Without getting into all of the social and environmental impacts of dams, we’ll focus strictly on water here. The power generation itself can be thought of as an “in stream” process — not actually withdrawing or consuming water, but just letting it pass through the turbine as it moves downstream. However, the reservoirs behind dams create huge surface areas of still water that are prone to evaporation. This evaporation can be considered consumption associated with the hydroelectric generation because it wouldn’t have occurred without the dam.
Seen through this lens, hydroelectric dams evaporate/consume up to 30-40 times more water per kilowatt-hour than thermoelectric plants do! However, this consumption doesn’t generally get factored into national usage calculations because it occurs before the water is even withdrawn from the source.
All is not lost though. As aging power plants are retired, the national plan for future power plants are beginning to take specific steps to reduce the water demands associated with them. Dry cooling systems, though more expensive and traditionally less efficient, are becoming more common in thermoelectric plants. Technical innovation within the plants are improving efficiency and reducing the overall cooling needs. Finally, the installed capacity of wind and photovoltaic power generation — both of which have minimal water requirements — continues to grow.
So, while we as the plumbing engineers continue to improve water efficiency in our individual projects, at the end of a long day spent agonizing a low-flow fixtures or just the right aerator to save every possible gallon, take a second to double-check that the light is turned off behind you on your way out of the office.
Calina Ferraro, P.E., CxA, CPD, LEED AP, is a mechanical associate principal at Randall Lamb, a consulting engineering firm specializing in for mechanical and electrical systems. She plays a key leadership role spearheading the company’s sustainable design group, managing projects and leading design teams. Ferraro’s project experience includes commercial, science and technology, health care and institutional market sectors. She is an active member in a number of industry groups including the American Society of Plumbing Engineers (ASPE), currently serving on the Women of ASPE committee; ASHRAE; and the USGBC. She can be reached at CFerraro@RandallLamb.com.