We wrap up our discussions of renewable energy options for the home by looking at the ground under our feet. Geothermal energy is perhaps the most consistent renewable energy resource. Just a few feet under the surface,the temperature of the ground remains fairly constant year round. This fact allows systems that harness this energy to work all day, all night and all year.
The temperature of the earth at depth ranges from 45-75 degrees depending on latitude and geological features. Geothermal systems work by digging into the surface to either partially bury the home or to run coolant lines, allowing the system to reabsorb this heat energy and put it to work in our homes.
Geothermal Heat Pumps:
Geothermal heat pumps are perhaps the most widely used strategy for the homeowner to harness the latent power of the land under their feet. A geothermal heat pump uses large holes dug deep into the earth with tubing running through those holes. These tubed holes are referred to as loops. The heating and cooling loops are then filled with water, water antifreeze mixtures or a chemical coolant. During summer months, the loops run on a cooling cycle, pulling heat from the house and transferring it into the soils. Conversely, in the winter months, the loops run on a heating cycle, absorbing heat from the earth and using that energy to heat the home.
The difference in heat pump systems lies primarily in how the heat exchange mechanism works. The oldest form of geothermal heat pump technology is the direct exchange heat pump. This type of system has only one loop that typically contains a refrigerant that will go through a phase shift (heating or cooling) as it circulates through the loop. Because of the use of a refrigerant and the risk of leakage, these systems typically use brazed copper tubing to protect from corrosion.
Closed loop systems are the most popular geothermal heat pumps and use a two loop system. The refrigerant loop is located inside the house while the heat exchanging loop, typically filled with a water and anti-freeze solution, is run below the frost line and is known as the secondary loop. The secondary loop works to exchange heat with the soils or water. Vertical loops are typical in situations where there is limited land area, but the cost is higher than other methods. Vertical loops have the benefit of tapping into higher temperatures at depth. Bores for vertical loops range from 75 to 500 feet in depth depending on the application and soil conditions. When land is available, horizontal loops are the most common. Installation is cheaper and can yield the same performance.
An open loop system is very similar to the closed loop systems, with the difference being that the secondary loop circulates ground or pond water through to the refrigerant loop rather than the water and antifreeze mixture. Open loop systems are more efficient than their closed loop cousins due to a more direct transfer of ground temperatures to the refrigerant loop. There are concerns about corrosion and fouling (build-up and clogging from sediment and limescale) in open loop systems as well as discharge concerns. Systems that draw water from the soils and then dump the water on the surface can contribute to draining aquifers.
Once the heat energy has been transferred to the refrigerant loop, there are additional option on how to distribute the energy to the home. There are two basic options here, liquid-to-air or liquid-to-liquid. The liquid-to-liquid applications are more efficient and are especially suitable for radiant floor systems. A liquid-to-air system has a benefit of being able to work well with existing forced air systems or in situations where cooling is the primary load on the house. Liquid-to-liquid systems can also be used to aid with the loads of domestic hot water needs.
All well designed, properly installed ground source heat pumps/exchangers are efficient and reduce consumption of fossil fuel electricity. The EPA is a strong proponent of the use of ground source heat exchangers, especially when used in both heating and cooling capacities.
Passive Geothermal Heating and Cooling:
While less common than the installation of a geothermal heat pump, one option to harness geothermal energy is to dig part of your home into the earth. Soil is a natural insulator and reduces the heat losses and gains that are inherent with construction above ground. The second benefit is that you are still in direct contact with the constant temperatures of the earth. Like a heat exchanger system, you are able to take advantage of the fact that the earth is cooler than the air in the summer and warmer in the winter.
There is, of course, the drawback of reducing the amount of light that is able to penetrate a home that is fully or partially embedded into the earth. This problem can be addressed with proper solar orientation of the house, the use of skylights or the use of solar tubes to bring more natural light into the deeper interior spaces. One design strategy that uses this concept is the Earthship. Among the many concepts used in the Earthship design strategy, harnessing the energy of the earth is integral to the energy efficiencies of such designs. To see more about Earthships, you can look here.
Conclusions:
Geothermal energy is a consistent and efficient source of energy for heating and cooling applications in the home. While there are limitations on the residential applications for geothermal installations, the reliability and consistency of these systems make them well worth any trade-offs. As with wind and solar power that we looked at in previously articles, geothermal systems work well in concert with other renewable resource systems. An ideal system would use all three sources to partially reduce or completely eliminate a home's need for energy from the current fossil fuel grid. Each system creates a synergy with the other, allowing systems to be downsized, saving money and resources.