Harvesting Geothermal Energy

Geothermal Heat Pumps

Geothermal heat pumps, also known as ground-source or water-source heat pumps, can heat, cool, and even supply hot water by transferring heat to or from the ground (or nearby water source), according to Energy.gov. Geothermal pumps cost more to install than other types of pumps but have lower operating costs since they take advantage of relatively constant ground or water temperatures.


Capturing the Energy

To obtain the temperature from inside the earth, a series of pipes called loops is placed horizontally or vertically. In closed-loop systems which are more common than open-loop systems, a liquid antifreeze/water solution circulates inside these loops and is used with a heat exchanger. The heat exchanger concentrates the heat and releases warm air inside the home during cool months. In the summer, the process works in reverse. A geothermal system not only provides hot air in the winter and cool air in the summer, but it is also an efficient way to heat water. The efficiency is due to the heat coming from the earth rather than being created.

Typically, ground loop designs fall into two categories, closed loop and open loop. There are three types of closed-loop geothermal heat pump systems: vertical, horizontal and pond/lake.

Horizontal ground source heat pump system for heating home with geothermal energy.
Closed-loop systems

A horizontal system is installed over a wide area of ground and requires plenty of space. Trenches can run hundreds of feet long and 6 to 10 feet deep.

A vertical ground loop is installed via one or more boreholes approximately 200 to 500 feet into the ground. Vertical loops are used when space is limited, where minimum disruption to the landscaping is requested or when other limitations, like rock formations close to the earth’s surface, present themselves. 

The third type of closed loop system, pond/lake loops, requires a water source so the pump can be submerged.

illustration of horizontal pond-lake type of geothermal loop
Closed Pond-Lake Geothermal Loop System. Illustration: Safe Electricity
Horizontal Ground and Vertical Borehole Closed Loop Systems

The most common type of geothermal energy in use in the US are horizontal ground and vertical borehole closed loop heat exchangers. As the name implies, the systems utilized a thermal transfer fluid circulating through a closed loop to move the energy in the form of heat. Heat is absorbed or dissipated through the portion of the loop underground. The energy (heat or cooling) is then connected to an application such as a central air unit, hot water heating system, or radiant floor unit.

illustration of horizontal geothermal closed-loop
Closed Horizontal Geothermal Loop System. Illustration: Safe Electricity

Horizontal closed loop systems have been utilized for many years.  They are easy to install in open trenches, excavations and directional boring.  This helps keep the installation cost down.  Cons: They require sufficient surface area to bury the loop. Horizontal systems tend to be less efficient than vertical systems.  The seasonal changes in soil temperature and moisture content impact the thermal energy transfer efficiency between the soil and fluid in the loop.

illustration of closed vertical geothermal loop
Closed Vertical Geothermal Loop System. Illustration: Safe Electricity

Vertical closed loop systems consist of one or a series of boreholes with pipe loops down each hole.  These are becoming more common due to better thermal performance. Installation costs have been reduced by new technologies for boring holes and other aspects.  New research data has helped reduce costs through better estimates of heat transfer characteristics based on ground geology and location.

illustration open loop geothermal unit
Open-loop Geothermal System. Illustration: Safe Electricity
Open-loop systems

Open-loop geothermal systems pump water directly out of a pond or well. The water that is drawn circulates directly through the heat pump system to use it for the heat exchange process. The water is then returned to the earth by running it on the ground and letting it seep into the aquifer, or it can be returned to a well, pond or drainage ditch, depending on local codes and regulations. If there is an abundant source of relatively clean water, an open loop system can be a cost-effective way to harness geothermal energy.

Regardless of the type, the ground loop is the core of geothermal systems.