Just as we can pursue water-lean energy solutions, we can also pursue energy-lean water systems. Water’s energy intensity depends on functions such as pumping, treating, and heating. While pumps have improved significantly in several thousand years of use, large pumps for raising water over mountains require nearly 2 megawatts (MW) of electrical power, which is the same amount of power that a massive wind turbine can generate.1Europump and the Hydraulic Institute, Variable Speed Pumping: A Guide to Successful Applications, (Washington: U.S. Department of Energy, 2004), Technical Report DOE/GO-102004-1913. A pump that size could move water at a rate of more than 200,000 liters (50,000 gallons) per minute. Approximately 75% of pumps are oversized, or bigger than needed for the task at hand, which means they consume more energy than necessary. Incorporating variable speed drives (VSD) and variable frequency drives (VFD) allows for tighter control over pump operation, including the capability to tweak the actual flow rates to match the desired conditions. Using VSD or VFD to dial back on pump output to match what is actually needed, rather than running at maximum power continually, achieves much higher efficiency. Some reports indicate energy savings of between 30% and 50%.
One reason desalination is so energy intensive is because high pressures are needed to push water through membranes that separate freshwater from salts. Improving the membranes would reduce the power requirements and therefore the energy consumption for the water pumps. Membranes designed with more advanced materials and more sophisticated structures will be better able to resist fouling for a longer operational lifetime.
While electric water heaters are very efficient at the point of use, sometimes exceeding 90% efficiency, the power plants that produce electricity operate at 30% to 40% efficiency. Multiplying the efficiency of the power plant and the water heater together makes the life-cycle efficiency for conventional electric water heating between 27% and 36% efficient overall. By contrast, natural gas water heaters with 60% efficiency pale in comparison to the efficiency of electric heaters, but because they avoid the inefficient power plant, they are much better in end-to-end efficiency. Since nearly 4% of our annual energy consumption is just for heating water in our homes and businesses, improved water heating technologies represent a substantial opportunity to save energy.2K.T. Sanders and M.E. Webber, “Evaluating the Energy and Carbon Dioxide Emissions Impacts of Shifts in Residential Water Heating in the United States,” Energy 81 (2015), 317-327.
Point-of-use tankless water heaters heat water on demand, usually with natural gas, which is in contrast with traditional electric tank systems that heat water around the clock. Another advantage of the tankless water heaters is that their smaller physical size means they can be installed next to showers and sinks. Distance between the faucet and the heater produces a delay, during which users waste water while waiting for water with the desired temperature to arrive at the faucet or showerhead. Tankless heaters mitigate this phenomenon because the delay for hot water to travel from the tankless heater to the showerhead or faucet can be quite brief. Smaller systems also save space within the home. The downsides are their higher up-front cost—a traditional tank water heater can cost $500 whereas a tankless one might cost $1,500.
Low-grade temperature sources such as solar energy or waste heat present a more resourceful option for water heating. Solar thermal systems for rooftop water heating are cost-effective and work efficiently. Waste heat recovery devices can harness heat from ovens, toasters, dryers, air conditioners, and refrigerators to heat water, though inefficiently. Integrated plumbing systems can co-locate sources of waste heat for water heating. The water pipe bringing water to the shower could pass by the back of the refrigerator, oven, air conditioner, or furnace for preheating before a tankless water heater in the bathroom heats the water to the desired temperature. This integration requires less energy for water heating and absorbs the waste heat from the house, thereby reducing the need for air conditioning. However, such a configuration can be difficult to retrofit to existing homes.