6. The Water-Energy Nexus
Water and energy in buildings are intertwined, not only at the point of consumption, but also at the point of generation. Four percent of electricity use in the U.S., or 75 billion kWh annually, is attributable to the supply, conveyance, and treatment of water and wastewater. Moving water from place to place accounts for nearly 80% of that electricity use.¹
Water weighs more than eight pounds a gallon, which is why moving water primarily with electric pumps consumes large quantities of electricity every year. The amount of electricity used by the nation’s 54,000 water utilities varies depending on the size and design of the water system and the elevation and distance needed to pump water, but none of that transportation is cheap in electricity terms.
Utilities also withdraw large amounts of water from rivers and other sources to create energy—although only about 3% of that water is actually consumed in the process. Even though there are several different methods to produce electricity, all require lots of water for cooling. Eighty-nine percent of electricity created in power plants for use by buildings in the U.S. is produced with thermally driven, water-cooled energy conversion cycles, which evaporate water during the cooling of condenser water. Hydroelectric power represents around 9% of the total power generated in the U.S.²
|Respondents to both the Nonresidential and Residential Surveys came out pretty much the same—dead center—on the question of the energy compo-nent of water supply and treatment (Residential mean: 3.12, Nonresidential mean: 3.04). Yet nearly a third of Nonresidential Survey respondents (32%) saw a “signi• cant” or “very signi• cant” connection between water supply and treatment and energy use, while 28% of Residential Survey respondents also saw the nexus between water and energy.|
Once at its destination buildings, water is also used heavily in managing heating and cooling loads. Most green buildings utilize evaporative cooling for air conditioning. With direct evaporative cooling, outside air is blown through a water-saturated medium (usually cellulose) and cooled by evaporation. The cooled air is circulated by a blower. Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation, so water is a key coolant in the process.
With indirect evaporative cooling, a secondary air stream is cooled by water. The cooled secondary air stream goes through a heat exchanger, where it cools the primary air stream, which is then circulated by a blower. Indirect evaporative cooling does not add moisture to the primary air stream. Unfortunately, there is not sufficient data available to gauge the water savings available through technologies such as indirect cooling. There is, however, ample evidence to show reducing water use reduces energy use, which has a direct impact on reducing greenhouse gas emissions, improves waterways by leaving more water in their river systems, and creates a more reliable water supply for agriculture, people, and wildlife.5
Water consumption for energy generation
As has been noted, most building energy use does not directly impact water, yet the water impact of energy production is substantial due to the cooling requirements of generating plants. Thermoelectric power withdrawals from bodies of water accounted for 48% of total water use, 39% of total freshwater withdrawals, and 52% of fresh surface-water withdrawals.³
Electricity demand is expected to follow Census Bureau population growth projections of 50% by 2050, with the exception of irrigation and industrial uses, which are expected to triple over that period.¹ So while energy demands should remain relatively flat (a 0.5% increase over current generation demand), water managers in 36 states in 2003 said they anticipate water shortages in the next decade under “average water conditions.”4 All 45 state water managers who responded to the GAO predicted water shortages could be accompanied by “severe economic, environmental, and social impacts.” Moreover, median decreases in annual water supply from runoff are estimated at 67-96% loss of water in all of the western states by 2050.5
|Energy consumption for public water supply is expected to rise steadily through the year 2050. This tracks Census Bureau population growth projections of 50% over the same period.|
Constraints on the ability to withdraw water have a negative impact on utility operations and restrain energy production. Several nuclear power plants in the Southeast were threatened by drought conditions in 2008. In 2007, the reactor at a nuclear plant in Brown’s Ferry, Ala., had to be temporarily shut down because of high Tennessee River water temperatures, even as a heat wave increased the demand for electricity.
All this begins to illustrate the water-energy nexus. Reducing energy consumption in buildings can have a positive impact in reducing the size and number of electricity-generating power plants, which themselves require huge amounts of water.
One way to look at the potential for significant reductions of building energy use—and therefore total water use—is through modeling. The Pacific Northwest National Laboratory’s software tool, Building Energy Analysis and Modeling System (BEAMS), provides estimates of future energy costs and emissions savings resulting from lighting and equipment upgrades, improvements to the building envelope, and energy-oriented design of the whole building.
Using BEAMS, PNNL researchers estimated that avoided water consumption from the widespread use of green building technology could total 25 billion gallons by 2015. The DOE researchers estimated 78 billion gallons of water from building upgrades and new green construction could be saved by 2025. The avoided freshwater consumption indicated by BEAMS in terms of a single person’s daily water use (annual domestic freshwater use in 2000 was 33,600 gallons per person) could offset 517,521 persons by 2015 and 1.9 million persons by 2030.
But achieving the BEAMS estimates is not easy, either. To do so would require nearly 1.7 million older toilets to be replaced with 1.6 gpf toilets by 2015 and 5.3 million toilet retrofits by 2025, as well as more than six million top-loading clothes washers. To meet the goals of the BEAMS projection, $950 million would have to be spent by 2030 on efficient toilets alone.6
Energy cost of transporting and cleaning water
Whether it’s surface water held in reservoirs or bringing up groundwater from aquifers, moving water is neither inexpensive nor particularly efficient in energy terms at delivering all the necessary water. Supplying water from groundwater sources requires 30% more electricity than supplying water from surface sources, due to the energy needed to lift raw water out of aquifers. Surface water, while cheaper to move than groundwater, can experience a high rate of evaporation before reaching its destination; this is especially true in arid regions such as the West, where it’s needed the most. Add to that the inevitable loss of stored water through evaporation from surface water reservoirs. There is currently little guidance regarding the allowance for evaporation losses during reservoir planning.
As has been noted throughout this report, California, one of the states most dependent on outside water sources, devotes about 19% of all its electricity to the distribution and treatment of water. Pumping water over 300 miles from northern to southern California and raising it 2,000 feet over the Tehachapi Mountains alone uses 2-3% of all the electricity consumed in the state. Surface water utilities typically use 1.8 kWh per 1,000 gallons of electricity produced.7 California also annually consumes four million acre-feet of the Colorado River’s water.
“The most cost-effective way to save energy in California would be to reduce water use, because they wouldn’t have to pump the water,” says Doug Elliott, research economist in the Portland, Ore., office of the U.S. Department of Energy’s Pacific Northwest National Laboratory.
Desalination also provides the Golden State with 50,000 acre-feet of water annually, or 10% of its water needs. However, desalination costs as much or more than transporting freshwater, and while technologies such as reverse osmosis are making it cleaner and less energy intensive, the cost of desalination is still prohibitive under most conditions—unless that’s the only way you can get drinking water.
There are a couple of reasons why it is difficult to estimate the cost to maintain a desalination facility. First, the choice of desalination method is a crucial factor. Reverse osmosis uses fine membranes and pressure to separate salts from water, whereas multistage flash and multi-effect distillation— processes where condensed steam is used to evaporate freshwater from seawater—use thermal and electrical energy. Multistage flash and multi-effect distillation plants lag quite a bit behind reverse osmosis in terms of energy efficiency. The other problem with making generalizations about the cost of desalination is that regional energy prices are not easy to compare.
A more promising option for desalination of salt water, known as co-location, has been recognized as a versatile, effective solution by the American Academy of Environmental Engineers. Co-location partners desalination plants with power plants, which then share energy and water. Approximately 18% of desalinated water is already used by power plants. Desalination could be a viable alternative to damming and rerouting rivers to provide water if innovations in technology can bring the cost of building and maintaining facilities down.8
Nonetheless, the better path to conserving the output of overstretched bodies of water like the Colorado River is to reduce demand through good water design strategies in green buildings.
Conserving water in buildings
For commercial buildings, the biggest opportunity to conserve water supply is in the mechanical systems. High-efficiency plumbing fixtures such as high-efficiency toilets can only do so much, and they can be defeated by inappropriate consumer use. Electric chillers represent the single largest electrical load in most commercial, institutional, and industrial buildings, accounting for 35-50% of a building’s annual electricity use. Retrofitting existing buildings and new ones with efficient chillers, boilers, and other HVAC equipment will have the greatest effect on conserving water.
The most dramatic improvement in operating efficiency can be achieved by replacing an older chiller with a new high-efficiency unit. Centrifugal chillers that are 15-20 years old had a peak efficiency of 0.75-0.85 kW/ton when new, while those that are 10-15 years old had a peak efficiency of 0.60-0.70 kW/ton out of the carton. Newer centrifugal chillers offer peak efficiencies of 0.50 kW/ton or higher. When coupled with variable-frequency drives, they can deliver higher efficiency over a wide range of cooling loads.
Another approach that can help is energy modeling of new buildings. Software programs such as Integrated Environmental Systems’ Virtual Environment-GAIA, Autodesk’s Ecotect, Graphisoft’s EcoDesigner, and Bentley’s Hevacomp give Building Teams an unprecedented capability to design for whole building energy efficiency for new designs. Done properly, this can result in significant downsizing of buildings in the early stages of design, opening up the opportunity to downsize the project’s chiller capacity—and its water consumption.
To get a better view of its water and energy consumption, the General Services Administration, the property manager of almost all federal office buildings and courthouses, last year asked the Department of Energy to investigate 12 GSA buildings and compare the performance of its green buildings to industry standard performance of energy, water, and other criteria. Eight of the 12 buildings were LEED-certified. All were designed with some LEED points available, even when the GSA had not pursued certification for them. Facility managers provided utility bills and maintenance budgets, and DOE researchers conducted an occupant survey for key data points. Twelve consecutive months of data were collected for each performance metric and normalized using building and site characteristics.
On average, the office buildings in the study performed 29% better on energy use than the Commercial Buildings Energy Consumption Survey national, regional, and GSA national averages. They performed 14% better than GSA’s national goal for energy performance.9
“Some of the observations confirmed 'common beliefs,’ such as [the belief that] buildings that intentionally incorporate energy considerations into design have better energy performance,” wrote Kim Fowler and Emily Rauch, the two analysts from the Pacific Northwest National Laboratory who conducted the study. “The data show that half the change in the Energy Star score [of a building] can be explained by the change in the LEED Energy and Atmosphere 'Optimize Energy Performance’ credits (EAc1). That is, buildings that received more EAc1 points tended to receive higher Energy Star scores.”
Determining water use per occupant in the buildings, though, was not as clear-cut. The water use data provided for eight of the buildings included process and irrigation water, so the domestic use of water in those buildings had to be estimated. For all the buildings domestic water was estimated as the base water load revealed from monthly water use data.
In the end, the researchers found that the average water use of the buildings in the study was only 3% less than the calculated water use indices baseline. Not surprisingly, courthouses and office buildings in the arid and semi-arid West used more water than their counterparts back east.
The authors recommended sub-metering and more detailed information about each of the buildings’ water use before water use could be compared to a relevant baseline.
What emerges from these various data points is that one of the best opportunities for conserving water is to use less energy. That’s how the water-energy nexus plays out.