IEEE Electrification Magazine - September 2015 - 56

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Dynamic Control Signal
Water Heater Fleet Response

Normalized Output (MW)

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Figure 5. A water heater fleet charges around a set point, increasing or decreasing their load according to the frequency control needs of the PJM system.

for grid services are lower per kilowatt-hour than centralized grid storage systems.
Additionally, they are often ubiquitous devices that
offer tremendous scale. The business models and regulatory hurdles for deploying these storage resources can be
difficult, however, and introduce some added complexity
in the way of customer acquisition.
Water heaters epitomize this opportunity. There are
approximately 53 million electric water heaters already
deployed in the United States, representing a resource greater than 200 GW. Americans replace roughly four million of
these water heaters every year, equivalent to deploying
18,000 MW and 35,000 MWh of energy storage annually
[assuming an average 55-gal tank (4.5 kW/8.7 kWh per water
heater)]. If outfitted with control technology-preferably at
the point of manufacture-and two-way communication,

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Figure 6. The energy storage capacity of different tank sizes and
temperature set points. (Source: Steffes Corporation.)

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2015

these distributed thermal batteries can be a boon for grid
operators and the integration of renewables.
In PJM today, water heaters are aggregated to provide
fast-response regulation, often outscoring thermal generation on their performance. Using tank temperature as a
state-of-charge indicator, aggregators can set a fleet of water
heaters to charge at a constant load level (base point) and
adjust that load level across the fleet in proportion to the
control signal, as shown in Figure 5. In 2014, an electric resistance water heater providing 2.25 kW of regulation for 6 h
overnight would have earned approximately US$195 gross
for its contribution toward system control.
Although providing ancillary services such as regulation
is a great opportunity for water heaters, today's market size
is relatively small; 300,000 water heaters could provide the
total amount of regulation in PJM's footprint. The larger
opportunity for storage water heaters is in off-peak energy
storage and load-shifting of renewable energy. By oversizing
the storage tank or allowing internal tank temperatures to
rise above 120°F (yet controlling outlet temperatures safely
via thermostatic mixing valves), water heaters can store
enough energy to meet a customer's needs during the day
without adding load during system peak (see Figure 6).
(Note: the average household consumes 13 kWh of energy
from the water heater every day.) This strategy would
reduce the operating cost of the water heaters by using lowprice energy at night (or negative-priced energy in some circumstances) and provide vast amounts of off-peak wind
energy storage in places like PJM, the Midwest, and the
Pacific Northwest, where light load and high wind energy at
night offer challenging operations.
In places like California, water heaters could be controlled to store solar photovoltaic overproduction in the
middle of the day. Bulk energy storage and regulation are
not mutually exclusive services. PJM has demonstrated
with water heater manufactures and load aggregators that
water heaters can charge off-peak with low-cost wind energy
while following a regulation control signal.

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015