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Water Heaters Find Key Role

According to the U.S. Department of Energy, water heating accounts for 14 percent to 25 percent of home energy usage, behind only space heating and cooling among major sources of home energy expenditures. Energy cost economics is breathing new life into long-standing alternatives to conventional water heating units and bringing increased attention to the potential for dynamic response methods for automated load management through the smart grid.

According to the DOE’s Residential Energy Consumption Survey, 110 million households spent nearly $32 billion on fuel for water heating in 2005. Of this total, electricity accounted for over $11 billion, natural gas for nearly $15.6 billion, fuel oil for $2 billion, and liquefied petroleum gas (LPG) for nearly $3.3 billion. Facing continually rising fuel costs, consumer interest in alternative water heating sources continues to climb rapidly. Not surprisingly, concerns about the rising cost of conventional energy sources have triggered renewed interest among consumers, water heater manufacturers, and energy agencies in an array of alternative methods including heat pump and solar heating.

For example, the GE Hybrid Water Heater uses heat pump technology to pull heat from the surrounding air and transfer it to the tank, producing the same amount of hot water as a conventional water heater, but using 62 percent less energy, according to GE. By using electricity to move heat from one place to another rather than to generate heat through resistance elements, heat pump water heaters can be two to three times more energy efficient than conventional electric resistance water heaters.

Solar water heaters can draw on a variety of collector methods including integrated collection-storage systems, flat-plate collectors, and evacuated tube collectors that drive heat into direct, indirect, active or passive systems. Using DOE Energy Star-qualified solar heating units to heat or preheat water can reduce water heating costs by 50 percent.

Beyond their impact on household energy costs, however, water heaters might be the killer app for the smart grid. While battery charging for electric vehicles has been a key rationale for deploying the smart grid, the installed base of water heaters presents both a significant challenge for electric utilities and a significant existing asset for the smart grid.

With over 43 million households using electric water heaters, the impact of these systems extends beyond the individual home owner. Hot water usage presents a predictable but significant load on the grid (see Figure 1). Many utility companies already employ smart-grid methods to manage the load of water heaters on the grid.

Figure 1: Water heater average monthly load by hour of day.

For individual consumers, the thermal storage properties of water heaters offer a practical application of low-cost off-peak electricity use. Hourly electric rates largely track daylight hours and not surprisingly match the profile of hourly load (see Figure 2). Another energy source shares similar characteristic: Wind energy is typically higher during night time and goes unused without high-capacity batteries for energy storage (see Figure 3). In fact, Paul Steffes of Steffes Corp. views water heaters as thermal batteries that are already available to manage load on the smart grid.

Figure 2: Average hourly rate measured over three years.

Figure 3: Potential hourly wind generation as a percentage of total load measured over three years.

“While much has been written about the potential for using electric vehicle battery charging for energy storage, it is not well known that a domestic electric water heater has approximately the same storage capability as an electric vehicle battery, and there are literally tens of millions of them already in service,” said Steffes in a statement to the DOE. “In terms of storage, a 100-gallon water heater can store a nominal 26 kWh of storage, or approximately a two day supply of hot water for an average family and the full equivalent of typical electric vehicle battery storage capability.”

Steffes’ company manufactures an interactive water heater control designed to turn a conventional electric water heater into an enhanced energy storage and power management device. With this approach, the control increases water temperature during off-peak hours or when wind energy is available and would otherwise go unused.

In fact, Steffes sees the energy storage potential of water heaters playing a significant role in managing the smart grid itself. Such a “grid-interactive” water heater would not only utilize off-peak energy, but also help improve grid reliability. Here, the reservoir of water heaters would respond in seconds to Automatic Generation Control (AGC) signals received from power plants through the smart grid, balancing grid load with generated power to help reduce generator fuel consumption and associated emissions. With over 43 million U.S. households already owning a significant “thermal battery,” the smart grid could tap into a truly deep energy reservoir.