Geoexchange: The Term
Geoexchange systems are also referred to as geothermal heating and cooling, ground heat exchangers, earth-coupled, ground source heat pumps, low temperature geothermal heating and more. The term 'Geoexchange' has been widely adopted in Australia and elsewhere for two reasons.
The first is that it is a technically more accurate description of a technology whose function is based upon heat exchange with the ground or water bodies that have been warmed by solar radiation and thus not related to geothermal heat or hot rocks.
Second, the emergence of geothermal as a form of energy generation and the widespread use of this term in the public domain has resulted in some misinterpretations and public confusion, including among industry professionals. Hence, Geoexchange has been increasingly adopted as the main term for the description of this technology.
Geoexchange: The Technology
The absorption of solar radiation / energy (47 % of the sun’s energy that reaches the Earth) by the ground is the renewable energy source of Geoexchange systems. In most localities across the globe, depths of greater than two to three metres provide a stable temperature environment that is the approximate equivalent of average annual air temperature for that location.
One is only required to enter an underground cave or wine cellar to experience this temperature stability first hand. These stable temperatures are typically present in the top 100 m or more of the ground. The rare exceptions to the rule are unique geological regions such as those present in Rotorua, New Zealand and elsewhere where geothermal heat is present at shallow temperatures.
For example, the ground temperature in Perth, Adelaide, Melbourne, Sydney and Canberra is approximately 16-18C while in Brisbane it is 22-23C. In contrast, Darwin has a ground temperature of approximately 31C and Hobart approximately 14C.
In essence, Geoexchange systems utilise this solar energy stored within the earth for heating buildings in winter (earth as a heat source) and as heat rejection in summer (earth as a heat sink). Both internationally and in Australia, the technology has been applied at the residential, commercial and district scales.
Heating
During the heating cycle, a geoexchange system uses the earth loop to extract heat from the ground. As the system pulls heat from the loop it distributes it through a conventional duct system as warm air (water to air). The same heat energy can also be used for a radiant floor system, swimming pools and spas (water to water).
Cooling
In the cooling mode, a geoexchange system air conditions your home by reversing the heating process. Instead of extracting heat from the ground, it is extracted from your home and either moved back into the earth loop, or used to preheat the water in your hot water tank. Once the heat is removed from the air by being passed through the loop, it is distributed through the duct system in your home.
Geoexchange: System Components
The two main components of a Geoexchange system are the ground loop or ground heat exchanger and the Ground Source Heat Pump (or geothermal heat pump), which is typically located within the building or plant room. The ground loop provides the passive component of the system whereas the Ground Source Heat Pump provides the mechanical component.
Ground loops can be classified as open or closed and in some instances can be coupled to conventional chillers/boilers as a hybrid system. Closed ground loops are constructed of Polyethylene (PE) pipe in a vertical (borehole) or horizontal (trench) configuration and water is used as the heat transfer fluid. The heat transfer fluid may also include small percentages (<30 %) of additives for water treatment or anti-freeze in colder regions.
The loop fluid is simply circulated through the ground loop and returned to the Ground Source Heat Pump. In accordance with the Zeroth Law of Thermodynamics, the loop fluid and the ground will endeavour to reach a thermal equilibrium.
Thus during its passage through the ground, the loop fluid is equilibrating with the stable ground temperatures either by extracting heat from the ground (winter) or rejecting heat to the ground (summer). The extent of this equilibration is determined by the residence time in the ground, the temperature differential between the loop fluid and the ground and other ground properties such as thermal conductivity and thermal diffusivity.
Open loops utilise ground or surface water directly, pass it through a heat exchanger and then return the water to either its origin or a secondary application such as irrigation, industrial water etc. Open loop sources are varied and include groundwater, rivers, oceans, dams and treated effluent.
The second component of the Geoexchange system is the Ground Source Heat Pump (or geothermal heat pump). It is the Ground Source Heat Pump which receives the water returning from the loop and transfers it to either hot/cold air via ducts (water to air) or as hot/cold water for hydronic heating, chilled beams, pools, spas etc (water to water). It achieves this through conventional (water source) heat pump technology. The main difference is that a Ground Source Heat Pump can operate on a conventional condenser water loop, whereas a conventional water source heat pump can not operate off a ground loop. This is due to the higher temperature ranges present in a ground loop.
The Ground Source Heat Pump is typically controlled by a thermostat and can be connected to a Building Management System (BMS).
An optional component of a Ground Source Heat Pump is the desuperheater. The desuperheater is a simple connection between the Ground Source Heat Pump and a domestic hot water storage tank. It provides pre-heating of domestic hot water while the Geoexchange system is operational. Depending on system usage, the desuperheater provides between 20 and 80 % of the domestic hot water requirements of a typical home.
The desuperheater option is adopted on approximately 80 % of residential installations. On a commercial installation, the desuperheater option can be used or the hot water supply can be integrated into the Geoexchange system.