One of the biggest hurdles to clear when talking to others about geothermal heat pumps (GHPs) is to erase the common misperception that this equipment is somehow connected to hot rocks and steam generation of electricity from deep under the earth. That’s not it! GHPs leverage electricity to efficiently work with the earth’s thermal battery.
WE’RE NOT HOT ROCKS
GHPs use 40-75°F temperature underground at depths from six to 600 feet, using a fluid loop either horizontally or vertically in contact with undisturbed earth. We call these loops geothermal heat exchangers (GHEXs). They harvest thermal energy from or export it to the underground formation. This is much easier than trying to do the same using widely varying outdoor air temperatures. GHEXs carry water or treated water and this exchange medium is over 3,000 times as dense as air, providing improved heat exchange over what air-sourced heat pumps can do.
LEVERAGE AND EFFICIENCY
Heat exchange leverage and low power consumption combine forces to make GHPs the most efficient equipment on the market. They move the greatest amount of thermal energy with the least amount of electricity consumed. In the chart at right, you can see how one unit of electricity in a heat pump system delivers four-to-six times that amount of thermal energy to occupied space. In cooling, you know these performance benchmarks by the acronym EER (energy efficiency ratio). In heating, the benchmark is COP (coefficient of performance) where the thermal output of one Kwh of electricity is compared to that same Kwh if it had been run through a GHP.
The leveraging of electricity in this way is possible by the nature of the refrigerant compression cycle. Heat pumps concentrate and move energy with this cycle and can provide you with 92°F interior heat even though it originated from 45°F dirt and can provide cool 55°F air that originated at 78°F inside your house. If this seems illogical, consider that refrigeration compression creates 38° and 0°F space in your refrigerator while being parked in a 75° kitchen. Such is the power of refrigerant compression.
HEATING, COOLING, AND HOT WATER
One compressor, one condenser and one evaporator built into a GHP— and you’ve got all your heating and cooling covered. Most residential GHPs carry supplemental de-superheaters that pre-heat hot water, too. One GHP can cover three big household loads. In summer cooling, the hot water pre-heat is free.
—Bill Martin, President of California Geothermal Heat Pump Association - CaliforniaGEO