In fact, the most commonly applied geothermal system utilises ground temperatures that are roughly equivalent to the average annual air temperature for a particular location, and boreholes that are less than 150 metres (m) deep. These low temperature geothermal systems do not generate power and instead are used for heating, cooling and hot water.
The intention of this article is to provide an overview of two technologies at the low temperature end of the geothermal spectrum. In the process, it is hoped that the article will remove some of the general confusion evident amongst the general public, as well as industry, when it comes to defining low temperature geothermal applications. The first low temperature geothermal application to be addressed is commonly referred to as direct-use geothermal: the second to be addressed is a low temperature geothermal process that is coming to be widely labelled ‘geoexchange’.
Direct-use geothermal
Direct-use geothermal utilises temperatures from 30–150°C in the form of heated water or a saturated steam. It is not as geologically limited as geothermal electricity generation and does not necessarily require the same deep drilling depths, with borehole depths less than 1000 m common.
Most people are familiar with this form of geothermal through tourist destinations such as the Peninsula Hot Springs in Victoria, the Artesian Baths at Lightning Ridge in NSW, Dalhousie Springs in South Australia, Katherine Hot Springs in the Northern Territory and many others.
In these instances, geothermally heated water with temperatures greater than 30°C are present at the earth’s surface and are easily accessible. Traditional peoples across the world have utilised, and revered, these areas for thousands of years and these were the first human uses of geothermal systems.
The advent of drilling technologies has enabled access to these waters in areas where they do not naturally reach the surface. It is now possible to drill boreholes to access and extract from hot sedimentary aquifers (HSAs) – tracts of heated water.
The Mantra Blue Resort at Warrnambool in Victoria is one example where 45°C water extracted from a 770 m deep borehole is providing both space heating and hot water for the resort. In Perth, Western Australia, a number of public and private swimming pools are utilising heated water from HSAs located beneath the Perth Metropolitan area. The Challenger Stadium Pool utilises 41°C water extracted from an 800 m deep borehole to heat its five swimming pools.
The University of Western Australia is developing a direct-use system using HSAs for direct heating and hot water generation as well as cooling via sorption chillers. This cooling component is similar to absorption chillers used in trigeneration facilities and is a comparatively new application of geothermally heated waters and brings direct-use geothermal applications more in line with geoexchange, the second technology to be discussed.
Geoexchange
The second technology to be addressed, 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 the systems operate through a heat exchange process with stable temperatures located at depths of just a few metres 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, harbour or lake.
Open loop system
Open loop systems utilise a local body of water such as a lake, stream, groundwater aquifer or wastewater to provide almost constant temperature water to the GSHP. Once utilised 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
It has been the intention of this article to provide some differentiation of technologies within the low temperature geothermal spectrum. Geothermal technologies have the capacity to provide electricity with higher temperatures, or heating and cooling with lower temperatures, although there will always be some overlap of technologies and applications.
Direct-use geothermal involves the utilisation of geothermally warmed waters for direct space heating and hot water, while the advent of cooling is a new aspect of this technology.
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 utilise 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.
