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Low Temperature Geothermal Resources: Ground Source Heat Pumps 187

coefficienT of performance (cop) and enerGy efficiency raTio (eer)
The efficiency of the heat transfer process is measured by comparing the energy required to drive
this cycle to the amount of heat transferred. Consider the thermodynamic properties of the system
in Figure 10.4, again. To vaporize the propane from a liquid to a gaseous state takes approximately
425 kJ/kg. The heat capacity of water is about 4180 kJ/kg-K. Normally a temperature drop of about
10°C can be expected between the supply to the heat pump and the fluid exiting the heat pump.
Hence, 1 kg of geothermally heated water in the closed loop feeding the heat pump can vaporize
approximately 10 kg of propane by adding just over 4 kJ of energy to the propane. The compressor
in a heat pump is driven by a small electric motor, with power ratings on the order of 1.5 kW. The
power consumed by the motor does work on the fluid. Assuming the motor has an efficiency of 0.8,
and the flow rate for the geothermal fluid is 1 kg/s, the rate of input of energy to the working fluid
in the heat pump is

E = 4,180 J/s + 0.8 × 1500 J/s = 5,380 J/s.
Tot
A measure of the efficiency of this system can be obtained comparing the total heat input (that
is 5,380 J/s) to the amount of energy consumed in doing so in the system (that is 1.5 kW) to run the
compressor. The ratio of these values is a measure of the efficiency,

E /E consumed = (4180 J/s + 0.8 ×1500 J/s)/1500 J/s = 3.59.
Tot
For heating cycles, this measure is called the coefficient of performance or COP and is defined as

COP = delivered heat energy/compressor electrical demand.

Common values for the COP for ground source heat pumps are between 3.0 and 5.0, meaning
that 300–500% of the energy used to run the heat pump is delivered to the space to be heated. For
comparison, the most efficient gas-fueled furnaces convert 90–95% of the energy that is potentially
available in the gas to useable heat for heating and have a COP of about 0.9.
The thermodynamic significance of the COP can be understood by considering the path described
in Figure 10.4. The compression cycle increases the enthalpy of the gas from about 600 kJ/kg to
about 680 kJ/kg. This is the work done by the compressor on the gas phase and is the electricity
demand of the system. The heat delivered to the building is the heat of vaporization that is released
when the fluid condenses, which is where the long, left-pointing arrow crosses the two-phase bound-
ary at about 390 kJ/kg. Hence, the heat delivered to the room, compared to the energy consumed in
the compression, is
COP = (680 kJ/kg − 390 kJ/kg)/(680 kJ/kg − 600 kJ/kg) = 3.6.

Actual COP values depend upon the design of the heat pump components and its operating
parameters as well as the temperatures of the end points of the cycle.
The cooling efficiency is measured in units of energy efficiency ratio (EER) and is the cooling
capacity (in Btu/hour) of the unit divided by its electrical input (in watts) at standard peak rating
conditions. The EER values for ground source heat pumps are generally in the range of 15 to 25.

near-sUrFace Thermal reserVoIr

The soil and rock that makeup the top few hundred feet of the Earth act as a heat reservoir that
evolves in response to two heat sources. As previously noted in Chapter 2, heat flow from the Earth’s
interior averages 87 mW/m . The source of this heat is the slow cooling of the Earth’s interior and
2