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• Energy Factor
The emphasis was mostly on the operation, as the test lab lacked the environmental control needed to test
within the tolerances needed for producing a performance rating. Of secondary importance was to
determine how system performance is affected by the operating conditions of ambient temperature and
humidity and tank temperature setpoint.
METHODOLOGY
Thermodynamics and Terminology
A heat pump uses the familiar refrigeration cycle to move thermal energy “uphill” from a low temperature
source to a high temperature sink. For an air conditioner, the cool space where energy is absorbed would
be the interior of a building or car, and the warm space where the energy is discharged is the ambient air.
For a heat pump water heater, the warm space for energy discharge is the water in the tank, while the
cooled space for absorbing energy is the room air around the water heater or from some other source
(such as a water loop used in a ground-source heat pump).
All refrigeration cycles contain four basic components: an evaporator to absorb energy, a condenser to
reject energy, a compressor to raise the pressure and create flow, and an expansion device to control the
flow. The system works with a fluid, or refrigerant, that evaporates (transitions from a liquid to a vapor)
at a low pressure and temperature, and condenses (transitions from a vapor back to a liquid) at a
reasonably high pressure and temperature. The majority of the energy consumed is the input to the
compressor, with smaller contributions from devices that move the heat absorption or rejection medium
through the evaporator or condenser (such as air or water). The energy rejected at the condenser is the
sum of the energy absorbed by the evaporator and the energy consumed by the compressor. Since heat
pump efficiency is measured as the heat rejected divided by the total input energy (compressor and fans
or pumps), values in excess of 100% are possible, with numbers approaching 300-400% achievable in
some cases.
Expansion Valve
Figure 1: Basic Refrigeration Cycle
Heat Absorbed
(from ambient air)
As a thermodynamic heat engine in reverse, the heat transfer capacity, the amount of compression energy
required, and thus the system efficiency, are all a function of the evaporating and condensing temperature
difference. As either the condensing temperature rises or the evaporator temperature drops, energy
consumption increases, and capacity and efficiency both decrease. When applied to a heat pump water
heater, in order to keep the electric energy input low and efficiency high, the evaporator temperature
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Condenser
Evaporator
Heat Rejected
(to the water in
a HPWH tank)
Compressor
Electric
Energy