Heat Pump
Water Heaters
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Heat pump water heater (HPWH) systems mine the energy content of
air to produce hot water very efficiently. Depending on
cold-water and ambient-air temperatures and on patterns of hot
water use, heat pump water heaters do the same job as standard
electric water heaters using two to three times less electric
energy.
Heat pump water heaters use a motor to run a compressor. The
compressor draws a gaseous refrigerant through an evaporator,
raising its pressure until it liquefies in the condenser.
This familiar process heats the condenser and cools the
evaporator. In wringing the heat from air, HPWHs both cool and
dehumidify the air that passes through them, thus helping to
meet space conditioning needs during cooling seasons. Under most
scenarios, the extra costs of heat pump water heaters over
standard electric water heaters are paid back in two to three
years. In areas like Hawaii--where natural gas in unavailable,
electric energy costs are high, and the need for
dehumidification is virtually constant--paybacks of well less
than two years are routine. |
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What Are the Options?
HPWH systems are available in the U.S. in a variety of
capacities, from small residential to large commercial,
producing more than 350 gallons per hour of hot water and 6 tons
of air conditioning. Options to consider include system
configurations and efficiencies.
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Integrated Versus Add-on
Systems
With integrated systems, the
heat pump apparatus is physically connected to the hot water
tank, and the condenser is typically wrapped around the tank,
surrounded by insulation. Integrated systems are also called
"drop-in" systems for good reason. They are designed to require
no special expertise in HVAC installation or wiring; ordinary
plumbers can install them without the aid of other trades
people. More compact than add-on systems, they tend to have the
same footprint as ordinary electric hot water systems of the
same water capacity. Accordingly, integrated systems tend to be
easier to retrofit. On the other hand, the largest model
currently sold in the U.S. has a capacity of 120 gallons. This
is adequate for many light commercial applications, but not for
installations where the average demand for hot water exceeds 30
gallons per hour. |
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With add-on systems, the heat pump apparatus stands alone. Heat
is transferred from the condenser to the water tank via a heat
exchanger and a small pump, using the tank's water as a heat
exchange medium.
An add-on HPWH system has the virtue of making use of the
existing water heater that heats by electrical resistance. Thus,
the capital expense is lower and, in many cases, installation
can be more flexible, since there can be some distance between
the HPWH apparatus and the water tank. |
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HPWH SYSTEM EFFICIENCIES
The instantaneous energy efficiency of a heat pump water heater
system depends on incoming water temperature, intake air
temperature, the heat transfer characteristics of the heat pump,
and various conductive and convective losses throughout the
system. Further, although in most circumstances the hot water
output is useful throughout the year, the cold air output may
not be. Accordingly, there is no |
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simple index that accounts for both outputs and describes
overall HPWH efficiency. Instead, the HPWH industry relies on
two indexes of energy efficiency--coefficient of performance
(COP), which is favored by manufacturers of larger HPWH systems,
and energy factor (EF), routinely used by manufacturers of
domestic HPWH systems. |
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COP is a measure of the instantaneous energy output of a system
in comparison with its instantaneous energy input. Standby
losses and the interaction of changing water and air
temperatures are not reflected in measurements of COP.
Accordingly, the COP of a standard hot water system is close to
1, and the COP of a typical HPWH heater may be 3.
The EF is a more useful measure, for it reflects more realistic
circumstances likely to occur in the field. The test to
determine EF is conducted over a 24-hour period with
temperatures of incoming water and input air held constant. A
measured amount of water is pulled from the system every other
hour for the first 12 hours, and no water is drawn for the final
12 hours. Because this test reflects standby losses, the EF of a
typical hot water system is 0.86, and the EF of a typical HPWH
heater may be 2.2. This represents an efficiency improvement of
more than 250 percent, even ignoring the cooling benefit.
Buyers of commercial systems should be aware that COPs quoted by
manufacturers sometimes reflect the combination of the
production of cold air and hot water in relation to energy
input. This is helpful if full use is made of the cold air, but
not otherwise. |
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Match the technology to the application. Because
HPWHs produce cool, dry air as a by-product of heating water,
the best applications are those that take advantage of both
outputs. Accordingly, HPWHs are especially well fitted for
commercial sector applications where demand for hot water is
relatively constant and the need for cooling or dehumidification
is continuous. Commercial laundries fit this description, as do
many commercial kitchens and even fast-food restaurants,
particularly in climates where space cooling is essential. (A
tale is told of a kitchen worker who, realizing the connection
between hot water and cool air with HPWH systems, ran extra hot
water just to stay cool!) |
Table : Planning a system and estimating cost
effectiveness
If a conventional system uses about 600 gallons per day of
hot water and utility rates are $0.10/kWh, annual savings
for switching to a HPWH system should approach $3,000. If
the system costs $4,500 for parts and labor (a very high
estimate), it should pay for itself in savings in a year and
a half. |
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Hot water use (gallons/day) |
Electric cost ($/kWh) |
Electric HW annual cost ($/yr) |
HPWH annual cost ($/yr) |
Annual savings ($) |
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300 |
0.05 |
1,244 |
490 |
735 |
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300 |
0.10 |
2,244 |
980 |
1,469 |
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300 |
0.15 |
3,673 |
1,469 |
2,204 |
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600 |
0.05 |
2,449 |
980 |
665 |
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600 |
0.10 |
4,898 |
1,959 |
2,939 |
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600 |
0.15 |
7,347 |
2,939 |
4,408 |
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Assumptions: Hot water system COP = 1, EF = 0.86; HPWH
system COP = 2.5, EF = 2.15; no use of backup electric
resistance heating for the HPWH system; no credit taken for
cooling air output of HPWH system. |
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For buildings that use rooftop cooling towers or large
refrigerators, it may be worthwhile to harvest waste heat from
these units, using the HPWH system both to produce hot water and
to help meet the air conditioning load.
Pick a good location. All HPWH systems should
be installed with careful attention to the flow of air across
their evaporators. First, because air flow is a necessity
(several hundred cubic feet per minute even for smaller
systems), do not place systems in isolated, tight areas. Second,
because they produce dry, cool air, put them where their output
air will be useful, such as damp basements or spaces that need
cooling most of the year. Of course, ducts and dampers may be
employed to achieve the needs of source and output air, thus
allowing flexibility in choosing a location. Finally, as with
refrigerators, the compressor motor on a HPWH system produces
some noise. Although most users report that it is
unobjectionable, it may be wise to pick a location where the
noise can never be a nuisance.
Perform regular maintenance. Heat exchange
surfaces perform better when clean, and HPWH systems are no
exception to the rule. To maintain good energy performance, keep
the filter that protects the evaporator's heat exchange surfaces
clean. This is particularly important in kitchens and other
areas that contain airborne pollutants. |
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