Why Geothermal Heating and Cooling

Geothermal systems efficiently heat and cool buildings using sustainable geothermal energy accessed via Ground Heat Exchangers (GHEs). In closed loop systems, GHEs comprise pipes embedded in specifically drilled boreholes or trenches or even built into foundations, all within a few tens of metres from the surface. The geothermal systems are just starting to be generally known in Australia with relatively few, but highly varying and diverse installations to date offering a potentially economically viable and environmentally friendly method for heating and cooling of buildings.

However, they require excavation or drilling to bury the GHEs which lead to higher capital costs for installation in comparison with conventional systems. The design of GHEs in Australia is mostly achieved with simple rules of thumb, and simple software that has been developed but not validated for the GHE design.

The high capital cost of GHE installation is one of the main causes preventing wide adoption of direct geothermal systems in Australia. It is therefore imperative that GHEs should be designed as efficiently as possible to minimize the extent and cost of GHE installation.

At GeoFlow Australia, we are using the current design methods together with the design methodology achieved by our head of design team, Dr Amir Kivi through 4 years of research at The University of Melbourne. This is to ensure that the GHE is sized to the needs of the house and to avid under-sizing and over-sizing the heating and cooling system.

Principal Elements of a Geothermal System

The principal elements of a direct geothermal system are shown in figure below

• The heating and cooling demand of the building,
• The Ground Source Heat Pump (GSHP) which causes heat to flow “uphill” from lower temperature to higher temperature,
• The Ground Heat Exchanger (GHE) pipes buried within a few tens of metres of surface as a heat source in winter and heat sink in summer.
A heat transfer fluid (typically water) is circulated through GHE pipes and exchanges heat with the surrounding ground. If the fluid is cooler than the ground, the ground will heat it and if the fluid is hotter than the ground, it will be cooled. GSHPs efficiently upgrade the heat extraction/rejection process. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor heat delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. Read more

The key to the direct geothermal system is that for each kilowatt of electrical energy put into a direct geothermal system, depending on several parameters, about 4 kilowatts of energy is developed for the purposes of heating and cooling. This means that direct geothermal systems could reduce electricity demand for heating and cooling by 75%.  Furthermore, as much of the electrical power in Victoria is generated with brown coal, replacing 75% of the energy used with a clean and free renewable energy source, these systems will have the potential to significantly cut Australia’s carbon footprint.

It is estimated that there are over 3 million direct geothermal systems installed around the world, with the total installed capacity approximately doubling every 5 years since 2000 (Lund et al., 2010). Figure below shows the growth rate of the installed capacity and annual utilization of all forms of geothermal energy for heating and cooling applications (Lund, 2010). In the following, each of the elements of a direct geothermal system is discussed in more detail.

Types of GHE configuration

Lund (2002), Banks (2008) and Self et al. (2012) provide an excellent overview of different forms of GHEs. Based on their distinctive characteristics, different GHE configurations are classified in Figure ‎below.

 

Closed loop GHEs comprise pipes placed in the ground or water through which a fluid circulates and the heat exchange occurs by conduction through the walls of the pipes. Therefore, the fluid remains sealed in the pipes and does not come into contact with the energy storage medium. There are several advantages to this system. One of the main advantages is that there is no problem of contamination either from the loop water entering the ground, or perhaps more critically, from the ground water contaminating the workings of the pumps. Read more

Where there is a confined surface area or minimum disruption of the landscape is desired, GHEs in form of vertical boreholes (of varying diameter) comprising one or more “U-loops” of high density polyethylene (HDPE) pipe are installed in a borehole backfilled with cement/bentonite grout.

Horizontal trenches are usually the most cost-effective when adequate area is available around the building and trenches are easy to dig. Figure ‎below shows different configurations of horizontal GHE that can be constructed. The horizontal GHE has become increasingly popular due to its low cost and ease of installation. For instance in Canada, about 55% of direct geothermal installations use horizontal GHEs (CGC, 2011). Nevertheless a horizontal GHE requires a large area of ground to lay the pipe network. This problem can be alleviated to some extent by employing a slinky loop arrangement of the pipes. Slinky arrangements are coils of overlapping piping, which are spread out and laid either horizontally or vertically. The first recorded slinky GHE was developed by Bose (1992). This GHE’s ability to focus the area of heat transfer into small volume reduces the length of the trenches by 20-30% of those for single pipe configuration (Wu et al., 2010). The slinky coils can have different lengths per unit length of trench depending on the pitch spacing of successive coils. The performance of slinky coils is similar to straight pipes with an equivalent total length (IGSHPA 2011).

In an urban environment, the ground immediately below a city can be used as a low grade energy storage reservoir. Geotechnical structures such as piles, tunnels, sewers, retaining walls and ground slabs can be regarded as thermo-active structures by simply embedding GHE pipes in them. The literature includes several studies of these thermo-active structures (Adam and Markiewicz, 2009; Xia et al., 2012). For example, if the building is a large commercial or industrial building with significant foundations including large diameter piles, then it is almost certain that these elements will provide the location for the GHE. 

Single and stacked multiple pipe
Serpentine type
Slinky (one diameter pitch)
Slinky (half diameter pitch)

Vertical Ground Heat Exchanger

Where there is a confined surface area or minimum disruption of the landscape is desired, Ground Heat Exchangers (GHEs) in form of vertical boreholes (of varying diameter) comprising one or more “U-loops” of high density polyethylene (HDPE) pipe are installed in a borehole backfilled with cement/bentonite grout.

Water Loop Heat Exchanger

Where there is a body of water in the form of a dam or lake within a reasonable distance from a building, these surface water bodies can also be considered as an energy source. Water loops are gaining popularity because they require no drilling or excavation. These systems, due to an efficient heat exchange interface, potentially require less piping than other GHE configurations. In closed water loops, HDPE coils are attached to a frame and submerged in a water body. The coils are typically supported 0.5m above the lake bottom to allow for convective flow around the piping. Normally the coils should have at least 1.8m of water above them (Self et al., 2012). It is necessary to assure sufficient thermal mass is maintained during low water conditions and prolonged draughts. The CGC (2010) also mentions that in cold climates, if a water body depth is less than 3m, lakes and dams destratify and offer no advantage. Due to flooding and draughts as well as hazards due to moving debris that can damage the GHE, rivers are not ideal for this application.

 
 
 

Open Loop Heat Sources

An open water loop system involves water being removed from the ground or body of water and returned after heat is extracted or added. Clearly, considerable care must be directed at the location of the return system so that the discharge water does not affect the intake temperatures. One of the major advantages of these systems is that relatively large volumes of water can be handled leading to large quantities of heat exchange.

 
 
 

Energy Pile Ground Heat Exchanger

In an urban environment, the ground immediately below a city can be used as a low grade energy storage reservoir. Geotechnical structures such as piles, tunnels, sewers, retaining walls and ground slabs can be regarded as thermo-active structures by simply embedding GHE pipes in them. For example, if the building is a large commercial or industrial building with significant foundations including large diameter piles, then it is almost certain that these elements will provide the location for the GHE.

Ground source Heat Pump (GSHP)

According to the Second Law of Thermodynamics, heat cannot by itself move from a lower temperature to a higher temperature (just as water cannot by itself flow uphill). Heat pumps on the other hand, can move heat from lower temperature to higher temperature regions (just as water pumps can move water uphill). Heat pumps are typically used to transfer the ground’s energy to heat or cool buildings. The principle of a heat pump is illustrated in Figure below for the cooling mode of operation of a water to air heat pump.


The operation of a Ground Source Heat Pump (GSHP) in cooling mode of operation (IGSHPA 1988)

As shown in Figure ‎above, in cooling mode of operation, water from the ground loops passes through the primary circuit heat exchanger (the condenser) of the heat pump. Here, this water comes into indirect contact with the warmer gas refrigerant. Heat passes from the warmer gaseous refrigerant to the cooler water. As a result of the removal of heat from the refrigerant, it condenses to a liquid phase at a relatively high temperature. When this liquid then passes through an expansion valve, the temperature drops considerably ready to accept heat from the conditioned space. Heat passes from the warmer air from the conditioned space to the liquid refrigerant to cause it to evaporate. The gaseous refrigerant then passes into a compressor where the gas is compressed to significantly increase not only its pressure but also its temperature. The hot gas then passes through the condenser again and the refrigeration cycle continues. Read more

In the cooling mode of operation, the heat generated by the compressor is not useful and together with the heat extracted from the building, should be rejected to ground. In the heating mode of operation, the waste heat generated by the compressor electricity consumption is useful and is rejected to the building to assist in heating. The heat flux delivered at the condenser of a heat pump to the building is the sum of heat flux extracted from the evaporator/GHE and the electric power consumed by the compressor.

The “efficiency” of a heat pump is expressed as a coefficient of performance (CoP). This is defined as the ratio of heat delivered to the building( Qb in kW )

  to electricity consumption of heat pump(QE in kW) : QE in

Why Geothermal?

The key to the geothermal system is that for each kilowatt of electrical energy put into a geothermal system, depending on several parameters, about 4 to 5 kilowatts of energy is developed for the purposes of heating and cooling. This means that geothermal systems could reduce electricity demand for heating and cooling by 75%. Furthermore, as much of the electrical power in Victoria is generated with brown coal, replacing 75% of the energy used with a clean and free renewable energy source, these systems will have the potential to significantly cut Australia’s carbon footprint.