We’ve all heard of solar and wind power generation for residential home applications, but what about geothermal heat pumps? Residential ground source heat pump (GSHP) systems harness the stable temperatures occurring underground at or below 6 meters, to either extract or reject heat.
How a Heat Pump Works
Thermodynamically speaking, heat pumps are based on the principle of taking heat from a source (heat source) and moving it to another (heat sink). This can be used in reverse for both heating and cooling functions.
Subterranean temperatures at depths below 6 meters roughly equal the mean air temperature throughout the year above that point, and depending on latitude, will generally be in the range of 10 and 16 degrees Celsius. These underground temperatures are warmer than the air above during the winter, and cooler during the summer.
Therefore, in GSHP systems, the ground acts as the heat source during the winter and the heat sink during the summer. By taking advantage of the unique properties of refrigerants, such as low boiling point and high critical temperature, we can efficiently extract and reject heat through what’s known as a vapor-compression refrigeration cycle.
Heat pumps circulate a fluid mixture — typically water and a refrigerant — through pipes buried in the ground (ground loop) extracting or rejecting heat via convection. In the case of cooling:
1. Saturated vapor passes through a compressor which superheats the vapor.
2. The superheated vapor passes through a condenser — in this case, the ground loop — which rejects a portion of the heat into the cooler ground via convection.
3. The vapor, having been cooled significantly, condenses into a saturated liquid and passes through an expansion valve which abruptly releases the pressure.
4. This sudden drop in pressure results in a flash evaporation, where a portion of the liquid turns into vapor and a temperature-reduction takes place (Adiabatic process) of the fluid to its saturation temperature (boiling point) at the current pressure. Note here that the boiling point of many common refrigerants are -30 and 40 degrees Celsius.
5. The now cooled liquid/vapor passes through a system of evaporator tubes across which a fan blows warm room-temperature air. The air is subsequently cooled and the liquid having absorbed heat from the room, vaporizes and returns to the compressor as a saturated vapor.
In the case of heating, the cycle is reversed:
1. Heat is absorbed from the warmer underground temperatures into the cold liquid passing through the ground loop (in this case, the evaporator) via convection, turning it into vapor.
2. This vapor is then superheated via compression, and passed through the condenser (in this case, condensing tubes across which a fan blows cold room-temp air), warming the room.
3. The vapor is cooled as it loses heat to the room and subsequently condenses back into a saturated liquid.
4. The liquid is then passed through the expansion valve and through the ground loop to complete the cycle.
Comparison with Air-source Heat Pumps (ASHP)
Most electric heating and cooling systems in residential homes today use an air-source heat pump due to lower initial cost. Although ASHP systems are cheaper, they aren’t nearly as effective, as air temperature varies considerably depending on latitude, elevation, and time of day and year.
Coefficient of Performance (COP)
The coefficient of performance is the ratio of heat output to energy used. For example, a conventional electric heater will generally output 1 joule of heat for 1 joule of electrical energy, a COP of 1.
Heat can still be extracted from very cold air, but the coefficient of performance (COP) goes down considerably — ranging from 2 or 3 at optimal outdoor temperatures (around 10 degrees or more) to 1 at lower temperatures (5 degrees and lower). In contrast, GSHP systems are known to have a COP of between 3 and 6 due to the more consistent underground temperatures.
Performance vs. Cost
Although the energy efficiency or COP has been proven to be extremely high in comparison with all other comparable heating systems, high initial cost has been and still is the primary bottleneck. Thus, considering cost, GSHP systems can be said to be more suited to colder climates, while an ASHP system might be sufficient for more moderate ones. Colder climates have higher heating requirements, and so will take less time to make up the initial cost of installation.