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Heat pumps have traditionally been used for space conditioning, but are now also being used as stand-alone units for water heating. They are usually three times more efficient than electric resistance water heaters-meaning that they can produce the same amount of hot water for one-third the amount of electricity.
There are four basic types of stand-alone HPWH. The heat pump can be integrated with or separate from the hot water storage tank, In Australia Heat Pumps are generally located outside although in cooler climates they are usually located in a garage or basement.
A typical residential HPWH can heat 67.5 litres of water per hour by 80°F, with a final storage temperature between 120°F and 140°F. At the same time, a HPWH provides some room cooling. During the heating season, this incidental cooling increases space heating needs.
A typical residential HPWH draws less than one-third the power of a standard electric resistance heater.
Operation
A heat pump works by transferring heat not by converting electrical energy into heat. A heat pump water heater removes energy from a low temperature source (ambient air or waste water) and moves it to a high temperature hot water tank.
Electricity is used to upgrade the quality (temperature) of heat energy not to generate heat energy
Similar products
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Reverse cycle air conditioner
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Refrigerator.
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Efficiency of a heat pump water heater
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Efficiency of electric heating element = 100%
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Efficiency of heat pump water heater = 250% to 400%
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A heat pump seems to provide something for nothing!
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(almost a perpetual motion machine).
Electrical demand
Heat pump peak electrical power demand = 500 to 1000W
Electric water heater peak power demand = 2400 to 4800W
How does a heat pump water heater work ?
- Liquid (refrigerant) boils at a low temperature in an evaporator. Output is low temperature and low pressure vapour.
- Pressure and temperature of vapour increased in the compressor. Electric motor used to drive vapour compressor. Output is high temperature and pressure vapour.
- Heat exchanger in water tank. Output is heat transferred to water. On refrigerant side output is warm liquid refrigerant
- Liquid is returned to the evaporator after passing through a partially open valve (TX valve or capillary tube). Output is low pressure cold liquid ready to be evaporated again.
Where does the energy come from?
A heat pump water heater collects “free energy”, just like a solar water heater. Energy is extracted from the air or from water condensing out of the air onto the evaporator.
A small amount of electricity required to operate the compressor. A Quantity of heat delivered to the water tank will be 3 to 5 times the quantity of electricity used.
Alternative Evaporators
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Air source with fan
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Standard air conditioner coil.
Air source without fan
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Large plate with natural circulation air movement
Solar boosted system
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with evaporator on the roof.
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Air source systems with enclosed fan
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and evaporator on top of the tank.
Alternative Condensers
Condenser tubing wrapped around the water tank.
• Advantages – No water pump, good heat transfer to the tank.
• Disadvantage – Tank and condenser are the one unit and must be replaced together.
Alternative Condensers
Condenser inside the water tank.
• Advantage - Good heat transfer, no pump.
• Disadvantage - Water side of condenser becomes coated with sludge.
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Alternative Systems
Air source system with passive evaporator wrapped around the tank.
Air source systems with enclosed fan
and evaporator on top of the tank.
Solar boosted system
with evaporator on the roof
Solar or Heat Pump Water Heater?
Solar
- Totally free energy in summer,
- “Feel good factor”.
- No moving parts
- No polluting refrigerants.
- Electric utilities do not like high power
- loading on rainy days.
- No noise (unless it boils).
Heat Pump
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Electricity required to make it work
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Mechanical compressor and water pump
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required.
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Low ozone impact refrigerant (Possibly propane based refrigerant).
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More expensive than solar.
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Electric utilities will promote heat pumps due to low power loading on the grid.
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Possible noise from the compressor
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“Feel good factor”.
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No moving parts
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No polluting refrigerants.
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Electric utilities do not like high powe loading on rainy days.
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No noise (unless it boils).
Why would a customer choose a heat pump water heater in preference to a solar water heater?
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Easier installation.
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Heat pump will work at night.
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Heat pump can deliver more hot water over the day (continuous recovery).
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For a four person household the annual electricity use is similar to existing electric boosted solar water heaters.
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In a commercial installation the air duct can be connected to output of air conditioning system to give improved performance.
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Heat pump works well in humid climates. (Northern coastal Australia or Tropical parts of Asia)
Heat Pump Performance on Rainy Days
Heat pump works well on cloudy humid days.
Best Applications for a Heat Pump
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In place of electric water heaters.
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In residences occupied by 4 or more people where hot water use is high.
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In warm climates where cooling may be also be of benefit (difficult to configure).
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Where the evaporator can be combined with the outlet of an air conditioning duct.
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In commercial applications where heat can be extracted from a hot plant room.
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In locations with humid conditions.
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Outdoors adjacent to a wall facing the equator.
Where Not to Install a Heat Pump
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In situations where hot water demand is low.
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Unventilated cupboards or small rooms, unless a ducted air source is available.
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In section of house that will be heated in winter.
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Outdoors in climates that experience extreme freezing conditions.
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Where noise from the compressor may be a problem.
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