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How does a geothermal system transfer heat?
- Doug Schuster
Everyone can understand how heat is created. You light a fire, you have heat. The heat is produced by the chemical reaction that is fire. Because of this it is very easy to understand how a gas furnace or other combustion device can heat your home.
What can be more difficult for some to grasp is how you transfer heat from the ground to heat your house. If we were talking about a hot yard and a cool house it would be easy. Open the doors and windows and let the hot air in. But we are talking about extracting heat from a relatively cool yard and trying to create a warm house.
Let's start with a few basic principles that come into play. First, heat goes to cold. It's a basic principle of thermodynamics. Heat always moves to cold and the rate at which it happens depends on the temperature difference between the two. Second, far more energy is used or supplied when a substance changes state. An example would be boiling water. The water keeps getting hotter on the stove until it starts to boil. At that point, even though the stove is still adding heat, the temperature doesn't rise. The energy changes the state of the water from a liquid to a gas. Another example would be an old aerosol can like hair spray. If you would hold the spray on, the can would get very cold. This was from the propellant in the can going from a liquid to a gas very quickly.
So how does this relate to a geothermal system? A geothermal system is a large refrigeration system. Just like your refrigerator, a refrigeration system move heat from point A to point B. Your refrigerator removes heat from your ice, milk, and other food making them cold and puts that heat into your kitchen. What your geothermal system will do is transfer heat from your yard and put it into your house.
A geothermal system, like any refrigeration system, is a Freon based system. The system works because a compressor takes Freon gas and compresses it into a liquid. This change of state of the Freon causes it to get extremely hot. The extremely hot Freon passes through a heat exchanger allowing it to give off its heat to the house. In most cases this will create a 15 to 25 degree rise in the air temperature. The Freon then passes through an expansion valve causing it to flash into a gas, similar to an old aerosol can. At this point the Freon is super cold and it passes through another heat exchanger. This allows us to extract heat from the loop fluid. Now this cold loop fluid circulates through the ground loop. The earth, which is warmer than the loop fluid, gives up heat and warms the cold loop fluid. The Freon that has been warmed completes its cycle back to the compressor. The system is designed so that the Freon can recover the same amount of heat from the ground loop that was delivered to the house. Essentially, we refrigerate your backyard to heat your house.
This expert insight brought to you by Doug Schuster, owner of Schuster Heating & Pump Co., Inc., and an Enertech geothermal dealer.
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