Geothermal heating, in which hot underground water is used to heat a building, and geothermal power, in which steam from very hot underground rock (more than 300 degrees Fahrenheit) is used to drive an electric generator are the natural ways we can get energy, or heat from underground sources. The only problem is these hydrothermal resources are only available in a few geologically favourable areas.
A geothermal heat pump (sometimes called a ground source heat pump) can work anywhere. These systems operate based on the stability of underground temperatures: the ground a few feet below surface has a very stable temperature throughout the year, depending upon location’s annual climate.
A geothermal heat pump uses that available heat in the winter and puts heat back into the ground in the summer. A geothermal system differs from a conventional boiler by its ability to transfer heat instead of the standard method of generating heat. As energy costs continue to rise and pollution concerns grow, geothermal systems may hold a solution to both of these concerns.
They are the fastest growing source of geothermal energy, with about 20 percent annual growth globally.
If you’ve ever touched the tubes on the back of a working refrigerator, you know that it is taking the heat from the inside the fridge and then radiating it to the rest of the kitchen.
A heat pump is like a refrigerator run backwards. It pulls heat from outdoors (as if it were trying to cool the outside) and releases it indoors. In both a fridge and a heat pump, a system of tubes circulates a refrigerant fluid that becomes hot when compressed and cold when expanded.
To heat a home, the hot compressed fluid is typically passed through a heat exchanger that warms the air that feeds into a duct system. This “spent” fluid is then cooled through expansion and brought into contact with a ground source, so it can “recharge” with heat.
Although pumping the fluid requires electricity, a geothermal heat pump is more efficient than any alternative heating system. In fact, current models can produce as much as 4 kilowatts of heat for every 1 kilowatt of electricity. This is because they are not generating heat, but rather moving it from the outside where the average temperature underground is always 15 degrees centigrade.
And some heat pumps can cool as well as heat a home. A valve controls the direction of the fluid, so that heat can flow in both directions.
Single and Double Loop Systems
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.
The Difference Between Air and Geothermal Heat Pumps
Some people are familiar with heat pumps that exchange heat with the air outside. These sometimes get lukewarm reviews because they do not work well when the temperature drops below freezing — just when you need them the most.
Geothermal heat pumps overcome this problem by exchanging heat with the ground, which maintains a constant temperature between 45 and 70 degrees Fahrenheit, depending on the location.
Different Types of Geothermal Heat Pumps
The most popular type is the vertical geothermal heat pump, in which holes are drilled 150 to 200 feet (45 to 60 meters) below the surface. Pipes installed in these holes circulate water (with a dash of anti-freeze) that brings up heat to warm the refrigerant fluid.
An alternative is the horizontal heat pump, where the water-filled pipes are laid about 6 feet (2m) deep over a wide area. Although less expensive, in hardware terms, they can be more expensive in certain locations due to the amount of land required to heat a moderate-size building.
For those who live near a body of water or who have their own water well, it is possible to use that water directly as the outside heat source.
Geothermal Heat Pump Costs
The biggest drawback for geothermal heat pumps is that their initial cost can be several times that of traditional heating and cooling systems. The installation for a typical house can run from $6,000 to $13,000. But geothermal heat pumps can pay for themselves over time because of reduced utility bills. A homeowner can save 30 to 70 percent on heating and 20 to 50 percent on cooling costs over conventional systems, according to the U.S. Environmental Protection Agency.
Because of this their popularity is growing rapidly. The United States leads the way with close to a million geothermal heat pumps, mostly in the Midwest and East Coast. Another million units can be found throughout Europe and Canada.