Geothermal Ground Source Heat Pumps

Schematic shown is in heating mode; cooling mode would be the reverse.

A ground source heating system is a special kind of geothermal technology that extracts solar energy stored in the earth's crust. At depths of six to eight feet, the ground temperature remains roughly constant year round and this results in a reliable temperature difference between the surface air and the soil. During the winter, ground source heat pump (GSHP) systems can be used for heating by utilizing the relative warm temperature of the soil. This process can be reversed during the summer months, since the soil is then cooler than the air and can be utilized for air conditioning systems instead (see left). A GSHP system consists of closed polyethylene (HDPE) piping which is buried under the earth's surface. In heating mode, a fluid is passed through the underground piping and is able to absorb thermal energy as it moves through the ground. This fluid is a water and alcohol mixture (commonly methanol or propylene glycol) giving it a large heat capacity and low freezing temperature. Once the fluid reaches the surface again, a heat pump removes the energy and transfers it to either air that is blown through heating coils (see below) or to water where it can be used for low temperature radiant heating. There are two current methods for placing the piping. The first uses vertical loops which are placed in wells bored deep (between 200 and 400 feet) straight downward while the second method places the pipes in a horizontal plane at a depth of eight to ten feet (see bottom left diagram). A horizontal system requires less drilling but needs a large cleared area while a vertical field is just the opposite; it requires significant drilling but needs a much smaller area and is thus good when space is a concern. The choice of which system to use will vary depending on the job site at hand.

Fluid temperatures above represent lower 48 conditions. Southeast Alaska fluid temperatures would be approximately 38 degrees in from loop field and 30 degrees out to loop field. (Diagram by Climate Master, Inc.)




The largest advantage of a heat pump system is the efficiency. Such a system is capable of producing 3 kWh of heat for every 1 kWh of electricity that it uses; thus, providing all 4 kWh as heat into the surrounding area. This means that the system has an efficiency approaching 400%. By comparison, the best fossil fuel systems provide less than 100% efficiency and with steady increases in oil prices, efficiency is becoming a major concern. In addition to this, the system is able to recycle energy introduced by outside bodies within the structure, such as solar radiation through windows, body heat, lights and electronic equipment. As the alcohol-water based fluid passes through an overheated room, the energy can be absorbed by the GSHP system and passed on accordingly.

The main disadvantage of a heat pump system is the initial cost for the ground coupling whether it be vertical bore hole drilling or horizontal excavation. Although, despite the high construction expense, life cycle cost analysis for local commercial projects have shown conceptual paybacks between 7-14 years.


Examples of vertical and horizontal piping configurations.



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