Geoexchange

Geoexchange has been previously identified by a broad range of terms, which has undoubtedly led to confusion with other geothermal technologies. The range of terms include geothermal or ground-source heat pumps, geothermal heating and cooling, ground heat exchangers, ground-coupled low temperature geothermal and more.

The term 'geoexchange' is being increasingly adopted worldwide as systems that operate through a heat exchange process with stable temperatures located at depths of just a few meters below the surface of the ground or a water body. The stable temperatures present at these depths are the result of solar radiation, the 47 per cent of the sun's energy that reaches the Earth, and not geothermal activity. Thus, geoexchange systems are not a true 'geothermal' technology, which further complicates its description and definition.

Geoexchange systems are installed to depths ranging from 1.5 m in horizontal loops to 150 m in vertical loops – although depths of 50–120 m are the most common – and a ground-source heat pump (GSHP) is used to regulate the temperature at the surface site. Australian ground temperatures present at these depths range from 10–12°C in Tasmania, to 30–34°C in the far north. As a general rule, the temperatures encountered are the approximate equivalent of average annual air temperature plus 2–4°C for that location.

The simplest method of directly experiencing this temperature stability first hand is to enter an underground cave, basement or wine cellar. Thus, geoexchange systems can be located almost anywhere across the globe and are not reliant on unique geological features.

Recent estimates indicate the presence of over 2 million geoexchange GSHP systems worldwide providing over 15gigawatts of thermal capacity. The majority of installations are in North America and Europe, although there has been rapid uptake of the technology in countries such as China and Korea with systems also present throughout Asia, Australia, the Middle East, Africa and South America. Of these, perhaps the most famous buildings are the Birds Nest (Olympic) Stadium in Beijing, China and Buckingham Palace in the UK.

Components of a geoexchange system

Ground and water heat exchangers/loops

The ground heat exchanger (GHX) or ground/water loop is the part of the geoexchange system, which provides the passive heat exchange process with the ground or water body. They can be classified as either closed or open systems.

Closed loop system
Closed ground loop systems use a polyethylene (PE) pipe circuit to circulate water through the ground or a water body. Closed water loops submerge the PE pipe or a plate heat exchanger in a body of water such as a river, harbor or lake.

Open loop system
Open loop systems utilize a local body of water such as a lake, stream, groundwater aquifer or wastewater to provide almost constant temperature water to the GSHP. Once utilized the water is returned to its source or used for a secondary application.

Ground-source heat pumps

The second component of the system is the ground-source heat pump (GSHP). The GSHP receives the water returning from the loop and transfers it to either hot/cold air via ducts (water to air GSHP) or as hot/cold water for hydronic heating chilled beams, pools, spas etc. (water to water GSHP). This is the active or mechanical component of the system and is controlled by a thermostat.

Summary

Geoexchange heating and cooling systems are a function of solar radiation not geothermal activity and thus are not limited to areas with suitable geology to provide heating and cooling to a building. They utilize a ground loop to provide a passive heat exchange process with stable ground temperatures while the GSHP provides the mechanical control to regulate the internal environment.