A geothermal heat pump is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode.
Geothermal heat pumps can be characterized as having one or two loops. The heat pump itself, explained more fully in the article on heat pumps, consists of a loop containing refrigerant. The refrigerant is pumped through a vapor-compression refrigeration cycle that moves heat from a cooler area to a warmer one.
In a single loop system, the copper tubing refrigerant loop actually leaves the heat pump appliance cabinet and goes out of the house and under the ground and directly exchanges heat with the ground before returning to the appliance. Hence the name "direct exchange" or DX. Copper loop DX systems are gaining acceptance due to their increased efficiency and lower installation costs but the volume of expensive refrigerant remains high. In a double loop system, the refrigerant loop exchanges heat with a secondary loop made of plastic pipe containing water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the plastic pipe goes out of the house and under the ground before returning, so the water is exchanging heat with the ground. This is known as a water-source system. In principle this need not be pressurized, so inexpensive plastic tubing could be used, but in practice the heat-exchange coil in the appliance requires pressurization to flush out air and to obtain the necessary flow.
Geothermal systems require a length of buried tubing on the property, a liquid pump pack and a water-source heat pump. Expansion tanks and pressure relief valves can be installed. The tubing can be installed horizontally as a loop field or vertically as a series of long U-shapes (see below). The purpose of the tubing is to transfer heat to and from the ground. The size of the loop field depends on the size of the building being conditioned. Typically, one loop (400 to 600 feet) has the capacity of one ton or 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts. An average house will range from 3 to 5 tons (10 to 18 kW) of capacity. The second component is a liquid pump pack, which sends the water through the tubing and the water-source heat pump. Lastly, the water-source heat pump is the unit that replaces the existing furnace or boiler. This is where the heat from the tubing is transferred for heating the structure. Heat pumps have the ability to capture heat at one temperature reservoir and transfer it to another temperature reservoir. Another example of a heat pump is a refrigerator; heat is removed from the refrigerator's compartments and transferred to the outside.
Benefits of Geothermal Heat Pumps
Geothermal systems are able to transfer heat to and from the ground with minimal use of electricity. When comparing a geothermal system to an ordinary system, a homeowner can save anywhere from 30% to 70% annually on utilities. Even with the high initial costs of purchasing a geothermal system the payback period is relatively short, typically between three and five years. Geothermal systems are recognized as one of the most efficient heating and cooling systems on the market.
The U.S. Environmental Protection Agency (EPA) has called geothermal the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. The life span of the system is longer than conventional heating and cooling systems. Most loop fields are warranted for 25 to 50 years and are expected to last at least 50 to 200 years. Geothermal systems use electricity for heating the house. The fluids used in loop fields are designed to be biodegradable, non-toxic, non-corrosive and have properties that will minimize pumping power needed.
Some electric companies will offer special rates to customers who install geothermal systems for heating/cooling their building. This is due to the fact that electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity. It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants. For the same reasons, other utility companies have started to pay for the installation of geothermal heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer. It is important to recognize that this may be ultimately less sustainable resulting in more overall energy being used by the house.
Geothermal heat pumps are especially well matched to underfloor heating systems which do not require extremely high temperatures (as compared with wall-mounted radiators). Thus they are ideal for open plan offices. Using large surfaces such as floors, as opposed to radiators, distributes the heat more uniformly and allows for a lower temperature heat transfer fluid.
Undisturbed earth below the frost line remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, so is usually 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on location. Because this temperature remains more constant than the air, geothermal heat pumps perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces, and even air-source heat pumps.
A particular advantage is that they can use electricity to heat spaces and water much more efficiently than an electric heater.
Geothermal heat pump technology is a Natural Building technique. It is also a practical heating and cooling solution that can pay for itself within a few years of installation.
Today there are more than 1,000,000 geothermal heat pump installations in the United States.
The current use of geothermal heat pump technology has resulted in the following emissions reductions:
These 1,000,000 installations have also resulted in the following energy consumption reductions:
Annual savings of nearly 8,000 GWh
Annual savings of nearly 40 trillion Btus of fossil fuels
Reduced electricity demand by more than 2.6 GW
The impact of the current use of geothermal heat pumps is equivalent to:
Taking close to 1,295,000 cars off the road
Planting more than 385 million trees
Reducing U.S. reliance on imported fuels by 21.5 million barrels (3,420,000 m3) of crude oil per year.