What are the pros and cons of using direct exchange (DX) ground source heat
pump systems versus other geothermal systems?
The resources of the International Ground Source Heat Pump Association and the
Geothermal Heat Pump Consortium may be useful for you.
Advantages of DX Systems over Other Geothermal Systems
Direct expansion (DX) ground source heat pumps (GSHP) have the heat pump refrigerant run through the earth coil which acts like a large condenser, eliminating the need for the water/antifreeze solution, the extra pump, and the heat exchanger. These DX systems use ground coils made of copper tubing and tend to be more efficient, since there is no heat exchanger and
because copper provides better conductivity than the plastic pipe used in most other systems. Heat is transferred directly between the refrigerant and the ground, and the amount of piping can be reduced. This type of system is ideal for situations where the amount of space for the piping loop is very limited.
The copper loops contain a refrigerant as opposed to the intermediate transfer fluid of plastic systems, which also require a circulating pump. Generally DX system earth loops require a much smaller borehole for vertical and diagonal loop installations. System start-up does not require a purging cart, the addition of antifreeze, or other tasks associated with water loop systems. On the refrigerant side, factory-assembled earth loops are usually brazed to pre-fabricated manifolds, further simplifying installation.
Disadvantages of DX Systems
One of the disadvantages is the higher cost of copper piping, although savings
from shorter loops, etc., may help offset this. Other disadvantages are
the durability of copper in some soil types and the difficulty of repairing refrigerant leaks in buried pipes. Generally, leaking DX ground loops must be abandoned and refrigerant reclaimed.
Significant problems were reported with early installations of DX systems (Energy Design Update,
September 1994), showing the importance of evaluating any product's track record in the field.