Geothermal Heat Pumps
Last updated 9/13/2019
There are many different technologies available to heat and cool your home. Ground source heat pumps (also known as geothermal heat pumps, GHPs, or GSHPs) use heat from the earth to warm or cool air for your property. Like air source heat pumps (ASHPs), ground source heat pumps take advantage of naturally occurring temperature differences to provide warm or cold air in an energy-efficient manner. GSHPs differ from traditional heating technologies that run off gas and oil in the sense that they concentrate and move heat instead of producing it through combustion.
How does a ground source heat pump work?
Simply put, a ground source heat pump heats your home or business by transferring heat from underground to inside your building. This happens through a four-step process:
1. A mixture of water and antifreeze is circulated through a pipe buried underground
This piping is known as a ground loop. It can be installed horizontally or vertically underground, depending upon how much space you have available and the ease of installation.
2. The ground loop absorbs heat from the ground and transfers it into the piping system.
As the ground loop circulates the antifreeze mixture through the earth, heat in the ground is captured by the fluid. This occurs because the constant temperature of the earth is warmer than the fluid, and heat naturally flows from warmer to colder areas.
3. The system’s heat exchanger concentrates the collected heat
Once the fluid collects heat from the ground, that heat needs to be concentrated to provide a comfortable climate in your home. In a typical GSHP system, fluid that has been cycled through a ground loop passes through a heat exchanger, where the captured heat is transferred to a refrigerant fluid cycling through a separate looped system. This fluid is pumped through a compressor that concentrates the absorbed heat to much higher temperatures in vapor form.
4. The heat is moved into your home.
The concentrated hot vapor passes through a second heat exchanger, which supplies the heat to your house when air is blown across this second heat exchanger, absorbing heat and moving throughout your home. As your home warms up, the compressed vapor cools and is pumped back through the first heat exchanger to collect more heat energy from the outside ground. This happens continually to maintain your indoor air temperature.
In warmer months, this process simply runs in the opposite direction to cool your home. Instead of transferring the heat from the ground to your home, the refrigerant is pumped through a heat exchanger inside your home first, where it absorbs heat energy and moves it down through the ground loop to disperse heat into the earth.
Geothermal energy is heat that exists within the earth. It is produced by the radioactive decay of rocks and minerals deep underground, a process that is constantly occurring and has been ongoing since Earth’s formation. Geothermal energy is a primary driver of volcanic eruptions. It’s also responsible for keeping the ground under our feet at a constant, warm temperature (between 50 and 60 degrees F).
Types of ground source heat pumps
There are four main types of GSHP systems that differ based on their ground loop setup. Three of these types are classified as “closed-loop” systems, while the fourth type is an “open-loop” system. Closed-loop geothermal heat pumps are much more common for residential installations than open-loop systems.
Closed-loop geothermal heat pump systems
Closed-loop ground source heat pump systems are characterized by the fact that they all circulate an antifreeze solution through a closed-loop of piping underground, usually made of plastic tubing. The heat exchanger in closed-loop systems transfers heat between the antifreeze solution in the closed-loop and the refrigerant in the actual heat pump. Closed-loop heat pumps make up the majority of GSHP installations in the United States.
There are three types of closed-loop systems: horizontal, vertical, and pond/lake. A horizontal closed-loop system, which is typically best for residential installations, involves laying a ground loop horizontally underground, usually at a depth of between four and six feet. A vertical closed-loop system is better for large commercial buildings looking to use a geothermal heat pump setup, or those looking to minimize alteration to the existing landscape above ground. In these systems, the ground loop runs straight down for between 100 and 400 feet (depending on the local soil and geology) and then returns to the surface. Vertical ground loop systems are also suitable for areas where the soil is too shallow and/or difficult to trench.
Lastly, a pond/lake closed-loop system involves laying a ground loop under a nearby water source at least eight feet below the surface. This is often the lowest cost option, but is only possible if there is a suitable body of water nearby.
One variation on closed-loop systems is called direct exchange. This approach has no heat exchanger and instead involves pumping refrigerant directly through a ground loop (usually made of copper). Direct exchange systems use a larger compressor and work best in moist soils, but you may run into environmental regulation issues due to the fact that a direct exchange system cycles refrigerant through the ground instead of a water mixture.
Open-loop geothermal heat pump systems
A less-common option for a geothermal heat pump system is an open-loop setup. Open-loop systems circulate well- or surface-water through the heat pump instead of using a closed ground loop with an antifreeze fluid inside. Open-loop systems must comply with all local codes for groundwater discharge, as the water is released back into the environment once it circulates through the heat exchanger.