Quantifying the energy embedded in the US water system

January 8th, 2013

Kelly T. Sanders & Dr. Michael E. Webber, University of Texas, U.S.A.

Connecticut River High Water in Bellows Falls Vermont 1888 © French & Keene

Water and energy share an interdependency commonly referred to as the “Energy-Water Nexus” since it takes energy to treat, pump, and prepare water for end-use, and water to produce electricity and liquid fuels. Although the energy-water nexus has gained attention in the literature in the past decade, there exist a number of data gaps that make it difficult to quantify the energy consumed for water, and the water used for energy at national or global levels.1,2 In a recent article published in Environmental Research Letters we address one of these data gaps by quantifying total water-related energy use in the United States.3

To conduct this study, we first defined three categories of water-related energy use, since we use water in very different ways in the United States. The first, Direct Water Services, refers to the energy consumed for moving, treating, heating, cooling, pressurizing, or evaporating water. The second category, Direct Steam Use, quantifies the energy consumed to make steam that is used directly in processes (e.g. steam for sterilization, cleaning, food preparation, steam stripping, paper and pulp manufacturing, etc.). The third category, Indirect Steam Use, refers to steam that is used indirectly in processes. This includes steam generated to produce electricity by spinning a steam turbine or to be used for space or process heating through the indirect transfer of heat through boiler walls or other mediums.

Overall, we found that 8.2 quads of 2010 primary energy (1 quadrillion BTU = 1 quad) was consumed for Direct Water Services (Figure 1). A large portion of this energy (44%) was consumed for residential and commercial water heating.3

Energy consumed for Direct Steam Use represented about half of that consumed in the Direct Water Services category (4.1 quads). The majority of this energy use was consumed at large industrial facilities such as chemical manufacturing facilities, oil refineries, and paper and pulp factories, which all use large quantities of steam as feedstocks in processes.3

The Indirect Steam Use category consumed much more energy than either of the other categories, since it includes all steam-driven power generation in the US. Although thermoelectric power production is often thought of as of a sector that represents “water for energy” (i.e. power plant cooling generally uses a lot of water) rather than “energy for water”, it is instructive to include this energy use here since thermoelectric power production typically requires that water be converted into steam to generate electricity.3

Figure 1 illustrates the flows of energy for water in the United States for each of the three categories discussed here.

In addition to determining the absolute quantity of energy that was consumed for water in the United States, we were also interested in the flows and conversions of energy delivered for water services. Figure 2 illustrates the primary energy sources consumed for water in 2010 on the left hand side of the figure and how those energy sources were consumed to provide water to the end-use sectors on the right hand side of the figure. (This figure only includes the energy consumed for the Direct Water Services and Direct Steam Use categories.) It is interesting to note that 58% of the total energy consumed for water is rejected as waste heat pointing to the inherent inefficiency in energy conversions and end use appliances.3

Thermoelectric power generation is particularly inefficient when compared to the direct energy consumed at the point of use. For example, using natural gas directly in a natural gas water heater is much more efficient than using natural gas to generate electricity that is transferred through power lines to be used in electric water heaters. Efficiency ratings on appliances in the United States are often misleading in this regard, since they apply only to end use, and therefore, can misinform consumers as to the lifecycle energy consumption of certain devices. Electric water heaters have energy efficiency ratings of ?=0.90, which is much higher than standard natural gas water tank heaters that are rated around ?=0.60. In reality a more appropriate measure of the electric water heater’s efficiency would be to multiply end-use efficiency (0.90) by the efficiency of the regional electricity mix (38.5% is the US average) to derive an efficiency rating closer to 0.32% (just over half of the natural gas water heater). Including losses during power transmission and distribution would reduce this estimate of efficiency even more.

This study concludes that 12.6% of 2010 annual primary energy consumption in the US was dedicated to direct water and steam use, which is equivalent to the energy consumed by roughly 40 million Americans. (The US consumed 98 quads of primary energy in 2010). Approximately 5.4 quads of this energy were used to generate electricity for water pumping, treatment, heating, cooling, and pressurizing water. This energy use is 25% more than that consumed for lighting in the commercial and residential sector, but receives far less attention from policy makers.3 The results of this study suggest that the water sector might be a meaningful area to target for energy-efficiency policy.

References:

1. King, C.W., et al., Coherence Between Water and Energy Policies. Natural Resources Journal, 2012. in press.

2. Webber, M.E., Energy versus Water: Solving Both Crises Together. Scientific American, 2008. October 2008.

3. Sanders, K.T. and M.E. Webber, Evaluating the energy consumed for water use in the United States Environmental Research Letters, 2012. 7(3).

Kelly Twomey Sanders received her Bachelor of Science in Bioengineering from The Pennsylvania State University in 2007 and is currently pursuing an MS and PhD at the University of Texas at Austin in the Cockrell School of Engineering. A recipient of a National Science Foundation fellowship, Sanders conducts research within the Webber Energy Group under the supervision of Dr. Michael Webber, whose focus is to analyze energy problems at the intersection of science, engineering, and public policy. Her research interests include the nexus of energy, food and water. The full paper and podcast can be downloaded here.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.