TODAY’S STUDY: THE WATER USED FOR ENERGY
Evaluating the energy consumed for water use in the United States
Kelly T Sanders and Michael E Webber, 20 September 2012 (IOP Publishing/Environmental Research Letters)
This letter consists of a first-order analysis of the primary energy embedded in water in the United States. Using a combination of top-down sectoral assessments of energy use together with a bottom-up allocation of energy-for-water on a component-wise and service-specific level, our analysis concludes that energy use in the residential, commercial, industrial and power sectors for direct water and steam services was approximately 12:3 _ 0:3 quadrillion BTUs or 12.6% of the 2010 annual primary energy consumption in the United States. Additional energy was used to generate steam for indirect process heating, space heating and electricity generation.
The relationship between energy and water, commonly referred to as the energy–water nexus, has received increasing attention in recent years in light of growing water and energy resource demand in the United States (US). The US water system is comprised of many stages of collection, treatment, conveyance, distribution, end-use preparation, reconditioning and release, each of which has important energy implications.
National water-related energy use is expected to increase as water-stressed states such as Texas, Florida, Arizona and California shift toward more energy-intensive technologies such as desalination plants and interbasin water pipelines to address current and future water-scarcity concerns. Although these shifts toward more energy-intensive water are likely to have an appreciable impact on future energy demand, very little analysis has been done to quantify water-related energy use at the national-level to establish a benchmark for today’s conditions. Thus, there is a knowledge gap about the energy needs of the water system. This analysis serves to fill that gap by quantifying a baseline estimate of 2010 water-related energy use in the US…
Results and discussion
Our analysis indicates that direct water-related energy consumption (i.e. energy considered in the direct water services and the direct steam use categories) was 12.6% (12.3 _ 0.346 quads) of 2010 national primary energy consumption. (Total primary energy consumption was 98.0 quads for all sectors (including transportation) in 2010 .) Approximately 8.2 quads of energy was consumed for direct water services (see equation (1)) and about 4.1 quads were consumed in the direct steam use category (see equation (2)). An additional 34.1 quads of energy was consumed for indirect steam use. Figure 2 summarizes the energy used in each of these three categories.
Table 3 details the water-related energy consumption in each of the end-use sectors analyzed. Although water-related energy in the transportation sector was not included in the analysis, the majority of the energy consumed in this sector is for petroleum-based transportation fuels, which would not be considered within the scope of the analysis. An exception would be fuel consumed for the transportation of water products, but this energy consumption is not likely to be large.
Figure 3 summarizes the 12.3 quads of water-related energy flows in the US for the direct water services and direct steam use categories (Note that the indirect steam use category is not included). Primary fuels (on the left) are used directly and indirectly via retail electricity generation for the three end-use sectors (on the middle-right). The thickness of the flows is proportional to the amount of energy consumed. In order to visualize primary retail electricity used in the Residential, Commercial and Industrial (which includes Power) sectors, primary electricity data from the EIA were proportioned to reflect the distribution of primary fuels consumed to generate net US electricity in 2010 as reported in the EIA’s Annual Energy Review . Losses at the point of electricity generation were calculated using a normalized average national 2010 net heat rate of HRavg D 8830 BTU kWh1 . (Heat rate is weighted based on 2010 heat rates for fossil-fuel and nuclear generators.)
Approximately 56% (6955 trillion BTUs) of primary energy was burned directly for water; the remaining proportion (5364 trillion BTUs) was converted into electricity for retail sale and then used for water. As figure 3 indicates, much of the primary energy used in retail electricity production is lost as waste heat. National electricity production in 2010 was 38.5% efficient based on the aforementioned average national heat rate. Of the useful electricity generation, an additional 6%–8% is lost during transmission and distribution , but these losses are considered in figure 3 at the point-of-use, rather than at the point of electricity generation.
Heating water consumed nearly three-fourths of the Residential sector’s and approximately one-third (35%) of the Commercial sector’s direct water-related energy, respectively. (Note that the proportions highlighted in the blue boxes of figure 3 reflect energy consumption at the point-of-use and do not include energy losses at the power plant. See the supporting information (available at stacks.iop.org/ERL/7/034034/mmedia) for details regarding the total primary energy use for each energy-consuming activity.) On-site water pumping was relatively low in the Residential sector, in comparison to the Industrial and Commercial sectors, as housing units tend to be smaller. Residential water systems often operate off the prevailing pressure of the water distribution network, so often times pumps are not needed at all. Large industrial facilities and high-rise buildings, by contrast, tend to require large quantities of energy to move water around on-site.
Determining the average efficiency of each end-use sector required additional engineering assumptions as national data sets do not detail specific water-related processes and technologies when they report energy consumption data. We assume that electric power losses between the point of power generation and final end-use average 18% when average electric device end-use efficiencies are also considered. (This estimate assumes average transmission and distribution losses and 10%–12% losses at end-use based on [36, 37].) For on-site primary energy consumption, we estimated efficiencies based on known, commercial-scale technologies.
For example, according to the American Council for an Energy-Efficient Economy (ACEEE), average residential electric and natural gas water heaters are 90% and 60% efficient, respectively; those fueled by petroleum by-products (namely fuel oil and liquid petroleum gas (LPG)) are about 55% efficient . The efficiency rating of a particular water heater varies based on the effective transfer of thermal energy from the heating element to the water, energy losses during storage, and the energy consumed by the device by switching between active and idle modes and does not include power plant losses or distribution losses. Additional energy losses occur during the conveyance of water from the water heater to the point-of-use at a particular appliance within the home or facility. However, these losses vary a great deal depending on piping network characteristics such as total pipe length, geometry and insulation properties, and the ambient temperature around the pipe. Commercial water heating efficiency varies considerably depending on the facility. Some highly efficient commercial facilities have natural gas water heaters approaching 75%, while less-efficient facilities are comparable to average residential water heaters.
For the purpose of this analysis, we assume that the average end-use efficiency of non-electric energy consumption in the Residential, Commercial and Industrial/Power sectors were 55%, 65% and 45%, respectively. We base these assumptions on the premise that Residential and Commercial sector water-related energy consumption is dominated by water heating (discussed above), while the energy consumed in the Industrial and Power sectors is mainly in boilers to make steam and generate electricity. Although, non-steam processes and devices in the Industrial and Power sectors tend to be more efficient than in the Residential and Commercial sectors due to economies of scale, these processes consume much less energy than industrial steam boilers.
The efficiency of any boiler is sensitive to its size, age and fuel type. New boilers typically fall in the range of 60%–85% efficient [27, 38]; however, two-thirds of large, industrial boilers are greater than 30 yr old and have much lower efficiencies . Efficiencies for electricity generation technologies in the industrial sector vary by technology but are generally in the range of 15% (for simple-cycle wood boilers) to 51% efficient for combined-cycle applications . Based on the literature [26, 27, 33, 38], we chose an average end-use efficiency for the Industrial/Power sector of 45% as a conservative estimate.
Energy losses at the point of electricity generation, transmission and distribution, and end-use are represented by the quantity ‘rejected energy’ in figure 3. This quantity represents 58% of the total primary energy that was consumed for water-related purposes in 2010. It is important to note that this quantity reflects broad estimates about the average efficiency of each sector’s water-related energy processes, which are extremely diverse, and is therefore subject to uncertainty.
Useful observations can be derived from these general trends. Firstly, economies of scale, such as those in the Industrial sector and large commercial facilities, are typically more efficient than those that are smaller in scale, such as individual households. Secondly, when considering end-to-end efficiency, it is much less energy-intensive to heat water by direct use of natural gas on-site, rather than by using that natural gas to first make electricity that is used to heat water because of the large conversion losses at power plants . From the perspective of displacing fossil-fuel use, solar thermal water heater systems are even more advantageous.
This analysis is the first to quantify water-related energy consumption in the US Residential, Commercial, Industrial and Power sectors, that differentiates consistently between primary and secondary uses of energy-for-water, incorporates the relative efficiencies for power plants and direct use, integrates the most recent primary data and statistics collected by relevant agencies, and allocates embedded energy from a broad range of relevant appliances and functions.
Results indicate that the energy embedded in the US water system represents 12.3 _ 0.346 quads (12.6%) of national primary energy consumption in 2010. To put this result in context, 12.3 quads of energy is the equivalent annual energy consumption of roughly 40 million Americans .
We estimate that 5.4 quads of this primary energy (611 billion kWh delivered) were used to generate electricity for pumping, treating, heating, cooling and pressurizing water in the US, which is approximately 25% more energy than is used for lighting in the Residential and Commercial sectors . (Despite this equivalency, much more policy attention has been invested in energy-efficiency for lighting, rather than reducing hot water consumption or investing in energy-efficient water heating methods, even though the latter might have just as much impact.)
Future analyses will assess the opportunities for carbon and energy reductions by water-conservation efforts, efficiency improvements, and new technologies. They will also include the increasing role of the bottled water industry which has large energy and carbon implications that were not explored in this analysis . Additionally, future work will aim to identify a general framework for characterizing the energy and carbon intensities of water systems based on regional variability in geography, climate and policy frameworks. This extension of the analysis will become increasingly significant as population growth, water-scarcity and increasing drinking water quality standards force regional planners to identify solutions for ensuring adequate drinking water to the US population without exacerbating energy and carbon expenditures.