Ground source closed-refrigerant-loop heat pumps use the same concept as the ground coupled units. For example, in the Midwest the temperature of the earth near the surface is typically around 55°F. With ground coupled systems water is taken from the ground or surface water (pond, lake, etc.), circulated to the individual heat pumps and the returned to the ground. With refrigerant loop systems a refrigerant such as R-22 is substituted for the water.
The Refrigerant Loop or Direct Expansion system circulates refrigerant, instead of water or antifreeze. The refrigerant is circulated in copper piping buried in the ground, rather than plastic piping. These systems are potentially more efficient than water loop systems.
- Fewer feet per ton of buried piping are needed since copper is a better conductor of heat than plastic.
- No freeze protection problem since no water or antifreeze is required.
- Heat is transferred directly from the refrigerant to the earth. There is no loss of efficiency due to the lower refrigerant temperature needed to first transfer heat into the water and then into the ground.
- Many contractors have well-developed skills with copper pipe.
- No fusion techniques are required.
- No purge and flush operations are required.
- No circulating pumps are needed, and
- The system has lower operating costs.
- Refrigerant management and oil return is important. The length of the copper piping is limited in order to achieve oil return and to minimize the refrigerant pressure drop losses. This precludes its application to many commercial buildings.
- Environmental issues related to the system's use of more refrigerant than the water loop systems.
- Corrosion issues, since the copper piping needs anodic protection and can be harmed by acidic soil conditions which can develop over time.
- Operating with too low a refrigerant temperature in winter can freeze the ground and reduce heat transfer.
- Operating with too high a refrigerant temperature in summer can bake the ground and reduce heat transfer.
- Industry acceptance with comparatively few products available, no ARI standard, and few experienced contractors.
These systems are in the field development stage.