Geothermal Heating and Cooling

Geothermal (Ground Source) Heat Pumps work similar to air source heat pumps, except the energy is transferred to the ground instead of to the atmosphere. The higher efficiency that can be achieved from a geothermal heat pump is due to the relatively stable ground temperature during the heating and cooling seasons compared to the variable air temperature of air source heat pumps. While these systems are more efficient than standard air source heat pumps or air conditioning units, they are also more expensive to install and will run upwards of $5,000 to $10,000 for the installed system.

Figure 27: Schematic of a Ground Source Heat Pump

There are three types of geothermal heat pumps, open loop, closed loop, and direct exchange (DX).

Open Loop

In an open loop system, ground water is used as the heat exchange fluid between the ground and the refrigerant loop. Water is drawn out of the ground and circulated through a heat exchange tank containing the refrigerant line. This is not a very common system due to the potential problems with dirt and debris and general water quality issues that may be encountered from using the natural ground water.

Closed Loop

The closed loop system is the most common system used with geothermal heat pumps. This system uses plastic tubing that is run through either vertical or horizontal wells to transfer the energy to the ground. While similar to an open loop system in that the water is circulated through a heat exchange tank containing the refrigerant line of the heat pump, in this system the water used is not connected to the ground water, but instead run in the plastic tubing. This controls the water quality used in the system, and therefore reduces the potential for problems and maintenance. In heating climates, there is a concern for freezing of the system and therefore some form of anti-freeze will need to be added to the ground loop system.

Direct Exchange

Direct Exchange systems run the refrigerant line directly into the ground, eliminating the heat exchange fluid. Because this extra heat transfer step is eliminated from the design, the system should be more efficient. In this system, copper lines are installed into the ground and the refrigerant of the system is circulated through them. Copper, due to it’s higher thermal conductivity is better able to exchange the heat with the ground when compared to a water circulated system with plastic pipe. While more efficient, there are some considerations that need to be made.

The cost of the system is higher due to the use of copper tubing instead of plastic tubing. Depending on the number and depth of the wells required, this can create a significant cost to the system. The system also has to be site charged with refrigerant, so unlike factory built closed loop GSHP systems, the efficiency is based on the quality of the installation.

Design Considerations

In order for the system to perform properly there must be adequate heat exchange with the ground. The heat exchange is through either vertical or horizontal wells in which the heat exchange fluid is circulated. A general rule of thumb is that a 200 ft ground well is required for each ton of cooling needed. Therefore, for a 3 ton cooling load, three 200 ft wells, would be needed.

From the heat pump side, there are generally two systems currently being used on the market, a packaged system and a split system.

In the packaged system, the compressor and heat exchange fluid to the interior of the house and integrated into the air handler. The benefit of this system is that the charging of the refrigerant line is all done in the factory under controlled conditions and it is a fairly simple installation and connection to the ground loops at the site. On the other hand, the compressor is now inside the house, and issues with noise can sometimes occur.

Split systems place the compressor and heat exchange fluid on the exterior and the refrigerant line is run to the air handler as in more conventional air source heat pumps. This reduces the noise inside the house; however the refrigerant charge must now be determined on site by a mechanical contractor.

Energy Model Results

The system used in the energy model is based on the specifications of a ClimateMaster Genesis Packaged Unit. The efficiency of the system is based on the entering water temperature. Therefore the performance of the system used in the energy model was based on the expected entering (returning) water temperature in the both the heating and cooling seasons. This entering water temperature is a function of the average ground temperature and the heat transfer efficiency of the ground. This resulted in a 17 EER for cooling and a 3.0 COP for heating. The resultant incremental whole house energy consumption reduction was 5.5%.

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