Geothermal heat pump or ground Source heat pump works in the same manner as air source heat pump but instead of the outdoor condensing unit (outdoor coil) you see with air source heat pumps, it uses different types of heat exchangers to discharge or extract heat from the ground surrounding a home.
Air source heat pumps are exactly that “air source”. They use the air outside a home as their source for heating and cooling. Outdoor air temperatures vary with time and at the extremes negatively impact the capacity and efficiency of any air source heat pump. In general when the temperature outdoor fell below 40°F (4°C) the air source heat pump , doesn’t work very well .Ground source heat pumps take advantage of the fact that ground temperatures remain relatively constant throughout the year once you go 3 feet below ground or deeper. The animation to the left symbolizes what concept happens during both the heating and cooling seasons. The relatively stable temperature source for heat transfer contributes to the ground source heat pump’s more stable operating capacity and increased efficiencies. Think of how well an air source heat pump operates in heating mode when it is below 40°F outside or in cooling mode when it is above 95°F. They work but not very well, especially in the heating mode. This is not the case with ground source heat pumps.
Another advantage of Geothermal heat pump or ground source heat pumps is all the equipment is in the building and below ground. There is no outdoor condensing unit. This can improve the look of a house and eliminates the noise of compressor and fan outside a home.
Over the years there have been several different methods for installing and operating ground source heat pumps. Below we will discuss each method.
I. Direct Exchange
The first method and probably the oldest is the direct exchange heat pump. The concept of using the ground as a source for cooling/heating with heat pumps has been around as long as heat pumps have existed. In the original systems the refrigerant piping was direct buried in the ground and a compressor was sized to pump refrigerant through the loop.
These systems worked and worked well but had one significant draw back; the extensive amount of refrigerant pipe required meant a large amount of refrigerant had to be used. In the earlier years of zero regulations on refrigerant use this was not an issue. These days there are significant restrictions on the types and amounts of refrigerants that can be used due to their ozone depletion potential. These systems were also susceptible to leaks with the amount of piping that had to be routed below grade. Leaks occurred both from faulty joints and from incidental damage during digging. Because of this direct exchange systems are rarely seen in residential and commercial systems.
II. Open and Closed Loop Systems
While the next two systems are technically two different types we will discuss them together since they basically operate on the same principle. Unlike direct exchange geothermal heat pump, these two types use a use a coaxial heat exchanger within the heat pump to transfer heat from the refrigerant to water circulated in the ground. This is beneficial compared to the direct exchange systems because it reduces the overall amount of refrigerant in the system and the probability of leaks.
The coaxial heat exchanger replaces the traditional outdoor coil and fan with a tube-in-tube type heat exchanger to separate the water from the refrigerant. Below is an example of a typical coaxial heat exchanger and a cutaway showing the tube-in-tube design. Water flows through the inner tube while refrigerant is passes between the inner tube and the outer tube. The spiraled inner tube creates turbulent flow and increases surface area to improve heat transfer between the two liquids.
Figures 1a and 1b below show a geothermal heat pump system in both the cooling and heating modes. All of the components (compressor, expansion valve, heat exchangers) shown are located within the cabinet of the heat exchanger. The only connections needed are power and water for the cooling loop. Note the desuperheater in the diagram. Not all geothermal heat pumps have them but they are a good option for homeowners who want to preheat their domestic water and further reduce their energy costs.
The first type of these systems, and the one most commonly used today, is the closed loop systems. Water is pumped by a small circulator pump through the coaxial heat exchanger, through the geothermal piping, and then back to the geothermal heat pump. There are no breaks in the piping and water is kept separated from the ground water. Below are the three types of installations used, (Vertical loop, Horizontal Loop, Pond Loop)
Notice in all cases the water flows from the geothermal heat pump, through the loop and then back to the heat pump. In the heating mode the water extracts heat from the ground surrounding the piping and transfers the heat to the refrigerant in the coaxial heat exchanger. In the cooling mode the refrigerant transfers heat to the water in the coaxial heat exchanger and then the water discharges the heat to the ground. The pond loop uses a large body of water instead of the ground but operates in the same manner.below is a typical geothermal cabinet including all its components.
In all types of closed loop systems the water within the loops heats up in the cooling season and then cools down in the heating season. Water temperatures typically run around 50°F in heating and 90°F in cooling. Just like we discussed the temperature of the air impacting the performance of air source heat pumps; the temperature of the water also impacts a ground source heat pump. The amount of loop piping is carefully selected by a designer to ensure the heat pump operates efficiently.Below is installation of geothermal heat pump pipes 3 feet underground.
Open loop systems pump ground or well water through the coaxial heat exchanger and then discharge the water to some type of a receptor. This can be seen in the below animation. These types of geothermal heat pump systems have the advantage of a constant temperature of water flowing through the heat exchanger in both cooling and heating modes. Depending upon location it will typically fall within a range of 50°F to 70°F which is ideal for a ground source heat pump to operate most efficiently in both heating and cooling
The disadvantage to open loop systems is the water itself. The water running through the coaxial heat exchanger needs to be high quality and have little sediment or it can foul the heat exchanger and shutdown the heat pump. The well must also have enough consistent water flow ranging from 2.5 to 3.0 gpm per ton. The other challenge is what to do with the water once if flows through the heat exchanger. Most jurisdictions limit returning the water to the aquifer it came from to prevent ground water contamination so the water must be deposited on the surface. All these issues present significant challenges and as a result open loop systems are only used in a few small cases were all these conditions are satisfied.
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