Ground source heat pumps (GSHPs) are not new technology – there are literally millions installed around the world (primarily in the United States and Europe), and the number is growing rapidly.
In general, GSHPs cost more than the commonly-used alternatives, because they usually require drilling holes to install the ground loops. This is offset by the fact that they use 30–50 per cent less energy, so the running costs are much lower, as well as additional advantages that make them very attractive long-term alternatives – not only in terms of energy efficiency and reducing greenhouse gas emissions, but also reduced maintenance and longer system life.
GSHP technology
GSHPs are a very flexible technology. They can be installed in any size building in virtually any climate as long as the system is designed correctly. Of course, some situations are better suited than others. As a rule, commercial buildings have shorter payback times than residential buildings due to economies of scale and the fact that they often have a higher load per unit area. The economics are also more favourable where energy costs are higher. Examples include remote areas that use diesel generators for electricity or where liquefied petroleum is used for heating.
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The cost of a GSHP system depends on two major factors – the heating and cooling requirements (or load) of the building and the sub-surface conditions, which determine the loop design. The above-ground part of the system is basically the same as what is required for any ducted heating and air-conditioning system, and the calculations done to determine the load are the same.
Loop systems
The loop design of the system depends on a number of factors. If there is a suitable body of water nearby then a pond loop system provides a very cost-effective solution because no drilling is necessary. An open loop system can also be relatively inexpensive since it requires only one or two wells, but it requires high quality ground water to minimise the chances of chemical reactions occurring that could clog the pipes or make the system less efficient.
Most GSHP systems installed are closed loop. This design has the advantage of being flexible, reliable and minimises any environmental issues related to groundwater use since there is no exchange of fluid between the system and the ground. These loops are either buried in trenches (horizontal loops) or placed in bores (vertical loops). The heat exchange that occurs between the ground and the fluid in the loop is determined by the surface area of the loop, which is proportional to the length.
GSHP drilling
Determining the appropriate loop lengths is a key design element, particularly since drilling can be the single biggest cost. If they are too short then the system won’t work properly; too long means incurring unnecessary drilling or digging costs. Given the system load requirements, the length of the loops needed is primarily determined by the thermal conductivity (TC) of the rock, or how effectively heat can be transferred through the material – the higher the TC the better.
The two main factors that determine TC are the lithology and the moisture content. For example, sand can have TC values more than three times greater than clay, and in any given soil or rock the higher the moisture content, the higher the TC. This means that horizontal loops generally need to be longer than vertical loops because they remain above the water table and therefore have a lower TC. Similarly, loops installed in clay-rich material will generally have to be longer than loops installed in sand to get the same system performance.
Practically speaking, a qualified installer can make an adequate estimate for residential systems, but for commercial systems (greater than 50 kilowatts) a TC test to measure the local conditions more precisely allows system design to be refined. This often allows the number of holes required to be reduced while ensuring the system’s effectiveness and reliability.
Drilling costs can also vary greatly depending on how difficult the holes are to drill. For example, the cost to drill a relatively deep well in hard granite can be more than three times the cost (per metre) of drilling shallow holes into soft sand.
Ultimate benefits
Where circumstances are favourable, GSHPs can pay for themselves very quickly (within less than 5 years) while in other situations it will take longer. In the meantime, they provide benefits that include reducing energy use and peak load, and a smaller carbon footprint.
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