Below the surface of the ground, the temperature stays fairly constant all year. That’s how caves served as housing in prehistoric times. Depending on latitude, ground temperatures can be anywhere from 45-75º F. Geothermal energy takes advantage of this constant temperature.

Geothermal energy works either at utility scale or for heating single buildings, including homes. As long as the earth’s core generates heat, it will make an inexhaustible supply of renewable energy.

It’s also clean. A utility-scale geothermal plant emits only about 17% as much carbon dioxide as a natural gas plant and only about 3% as much as a coal-fired plant.

Compared to solar and wind farms, utility-scale geothermal is not dependent on sunlight or weather. It also requires much less land. But it can produce earthquakes and cause air and groundwater pollution.

Geothermal energy for single buildings uses heat pumps. They work differently from utility-scale geothermal. They have a different set of pros and cons.

What is a heat pump?

Heat pumps move heat from one place to another. They don’t rely on combustion to generate heat, although they require electricity to work. Solar panels or wind turbines with battery backup can supply the electricity, but then they add to the installation costs.

Because they only move heat, they replace both a conventional furnace and air conditioner. Therefore, they cost much less to operate than conventional heating and air conditioning. Heat pumps also dehumidify better than air conditioners.

They come in two varieties: air-to-air and geothermal. Air-to-air heat pumps, the most common kind, transfer heat from outside to inside during heating season and from inside to outside during cooling season. And they do it for about half the cost of an electric furnace or baseboards.

Until recently, heat pumps have not worked well in areas with extended periods of sub-freezing temperatures. They have required supplemental heating systems during the winter in those places. Technology keeps advancing, though. Now, even air-to-air hear pumps work in harsh winters.

Still, geothermal heat pumps work better in extreme climates than air-to-air heat pumps.

What is a geothermal heat pump?

Three main ground loop configurations. Wikimedia Commons

Geothermal heat pumps can heat and cool using one of five ground heat exchangers: horizontal, vertical, diagonal, pond/lake, or open loop.

The horizontal loop requires digging trenches for polyethylene pipe at least four feet deep. Some systems use two pipes with either one four feet deep and the other six feet or two pipes side-by-side five feet deep. Others loop one pipe in a coil, which requires less ground space. It is usually the best option for residential use.

The vertical loop requires holes about 100-400 feet deep and about 20 feet apart. Each hole contains two pipes joined at the bottom by a U-bend. Each pair of pipes is joined to a horizontal pipe at the top. This kind of system works well in locations with not enough land area tor a horizontal system. Commercial buildings and schools especially use it.

The diagonal system in the diagram combines features of the horizontal and vertical designs.

It the site has a sizeable pond that is at least eight feet deep, the pond can be the heat source rather than the ground. The piping is installed underwater.

An open loop system also uses water for geothermal energy, but this time, instead of pumping a heat exchange fluid through a closed loop, the water itself acts as the fluid. Once it has done its work, the system returns it to the pond or other source.

Each of these systems attaches to a heat pump inside the building, which accomplishes the heat exchange. It works in much the same way as a refrigerator.

The pros and cons of geothermal heat pumps

Inside of a geothermal heat pump.
Wikimedia Commons

Geothermal heat pumps have advantages and disadvantages.

They have a higher installation cost than just about any other heating and cooling option. The horizontal loop systems require a large land area. The land must have little rock for at least four feet beneath the surface.

Retrofitting an existing house with geothermal heat pump costs much more than installing one with new construction.

As a relatively uncommon method of heating and cooling, not many qualified contractors can install or service a geothermal heat pump. Less competition can mean higher prices. And if a tree root or gopher damages a pipe, it requires digging up the whole yard to find and repair the damage.

On the plus side, you have no large unit outside the home to distract from curb appeal or limit your landscaping choices. And if a geothermal heat pump is much more expensive to install, it has much lower operating costs than other heating/cooling systems. The whole system makes no more noise than a refrigerator. And at least in the summer, a geothermal heat pump can also heat the home’s water much more efficiently than standard water heaters.

If you’re building a new house, by all means carefully consider the advantages and disadvantages of geothermal energy among your alternatives. If you want to retrofit an old house with a heat pump, an air-to-air heat pump designed for your climate is a better choice.

Sources:

Geothermal energy pros and cons / Kiah Treece, Treehugger. February 10, 2021
Heat pump systems / US Department of Energy
The pros and cons of geothermal heat pumps / Dan Stout, Family Handyman. March 20, 2019