Geothermal Energy and Geo-exchange Systems

Geo-exchange, (ground source) is a cost and energy efficient air conditioning/heating/cooling technology that uses solar energy naturally stored in the ground and the constant thermal energy storage properties of the ground. The technology enables:

  • Forced air central heating and cooling
  • Radiant hydronic floor or panel heating or water cooling
  • Domestic or commercial hot water heating
  • Heating of swimming pools

The core of the concept is based on:

  • The relatively constant temperature of the ground at below a depth of a few metres, with actual temperatures determined by solar radiation. For example, in Melbourne the temperature below a few metres is around 15 °C through the year, winter and summer, while air temperatures vary from less than 5° C to over 40° C. Data from North America is shown in the Figure.
  • The fact that the ground temperatures are naturally close to the target temperatures, means the thermal conditioning system is working against a small temperature difference.

Average ground temperatures in Australia are typically:

  • The energy storage capacity (thermal inertia) of the ground.

Geo-exchange systems draw heat from relatively warm ground in winter, or reject heat into the relatively cool ground in summer.

Geo-exchange packages comprise three generic components:

  • The ground side to exchange heat with the ground. This is based on sealed polyethylene or copper tubes embedded in trenches or boreholes, with down-flow lines and up-flow lines joined at the bottom to form a continuous “U” tube. Layouts range from vertical to inclined bore holes to 30 to 100 m deep to trenches usually around 2 m deep.
  • A heat pump converts ground heat to a suitable temperature level. This includes a compressor and associated equipment, comparable visually to the external units installed with reverse cycle air-source heat pumps, but without the fans. In large projects, this plant may be distributed around the building or concentrated in a plant room. Without fans noise pollution is excluded and as there is no exhaust flu requirement the Geo-exchange heat pumps may be located internally or externally.
  • Replace with inside pic from PICAC Ground-source heat pumps are particularly energy efficient and enable Geo-exchange systems to be typically 2 to 3 times more energy efficient than alternative systems such as air-source heat pumps, i.e. they use less than half the electrical energy.
  • The temperature distribution is typically inside the building, transferring the heat or cold into the occupied areas. This may range from coils embedded in a concrete slab or similar hydronic systems, to conventional ducted air systems. The technology can also heat water for process or domestic use, with water hot water storage located internally or externally.

The technology is most often used for air conditioning, but it is also used efficiently to supply hot water, from commercial operations to swimming pools.

There are two generic versions of the technology:

Direct Exchange

Direct exchange, where a refrigerant is circulated as the working fluid throughout the total system, i.e. through the ground loops to the final air handling unit. This technology provided by EarthLinked, can be up to 20% more energy efficient and cheaper than water based Geo-exchange systems. It is based on boreholes up to 30 m deep and 75 mm diameter with copper tubes. Modules range in capacity from 5 kW to 21 kW, and it is common for several to be distributed throughout larger projects, especially projects such as schools and commercial/industrial facilities.

Water Based Solutions

Water based systems using polyethylene tubing, with boreholes typically 100 m deep and 150 mm dia. A heat exchanger is required to transfer thermal energy from the water working fluid in the ground system to a refrigerant in the above ground circuits. This is the technology supplied by Bosch and Calorex. Packages range in size from 5 kW to 200 kW per unit and are usually concentrated in plant rooms.

There are several options for water based ground loops. For example:

  • Laid in trenches or in bodies of water such as dams/large ponds
  • Installed in piled vertical foundations
  • “Open”, where the hot part of the loop is separated from cold side by several meters, relying on ground water flow or convection for heat flow.