When a geothermal heat pump (GHP) is in the heating mode, the ground-coupled heat exchanger (the "ground loop") is pulling low-grade energy (Btu’s) from the earth for concentration by a compressor for higher-temperature delivery into conditioned space.
With every unit of energy (electricity) consumed to power the mechanical components of the heat pump (circulating pump, fan, blower and compressor), a GHP extracts additional units of energy (thermal) from the earth to satisfy the heating and hot water needs of a residence or building.
The measure of efficiency for a GHP in heating mode is called "Coefficient of Performance" (COP). Simply put, COP is the ratio of the amount of useful energy relative to the amount of energy used in the process. COP is calculated by dividing Heating Capacity in Btu/hr. by Power Input in Btu/hr. GHPs typically move 3 to 4 times more energy to or from the earth than is used to operate them, resulting in COP values of 3.0 to 4.0.
A GHP with a COP of 3.0 uses one unit of energy (electricity) to produce two equivalent units of renewable energy (thermal); a GHP with a COP of 4.0 uses one unit of energy (electricity) to produce three equivalent units of renewable energy (thermal).
Innovation of variable speed compressors in the past few years (the last internal component of geothermal heat pumps to be made adjustable) has significantly lowered the amount of energy used to power the units, increasing COP to 5.0 or even higher.
In cooling mode, the process is reversed in the GHP, and the earth becomes a heat “sink” for absorbing heat rather than a source of heat. The measure used for GHPs in the cooling mode is called the Energy Efficiency Rating (EER), calculated by dividing Cooling Capacity in Btu/hr. by Power Input in Watts.
The higher a GHP's cooling mode EER, the higher the amount of heat absorbed by the earth and/or the lower the energy needed to power the unit. Again, the use of variable speed compressors significantly increases the EER of GHPs.
Based on the amount of electricity (kWh) used to satisfy the thermal load (Btu's) of a building, the renewable component of applied energy can be calculated for buildings and residences, using the COP (heating mode) or EER (cooling mode).
Devices are available that measure the Btus coming out or into the earth in real time. They are standard equipment for commercial and institutional applications. Similar meters for residential measurements are on their way to the marketplace.
President and CEOGEO – The Geothermal Exchange Organization