There are 4 main types of pipe installation:
Horizontal piping: this is where the piping is laid flat, often in a loop pattern, at a depth of at least 1 meter and is the most cost effective. However, a lot of land is required for excavation. In general, 40mm piping is used. This system provides around 15W of energy per meter of collector used (although for systems that are running all day this can be as low as 12W per meter).
If more than one pipe run is being buried and run into the heat pump, each run of piping MUST be the same length and each trench must be 5 meters apart.
When sizing the amount of piping required we use the following calculation method: let’s assume we are using a 10kWp system which has a CoP (Coefficient of Performance) of 3.0.
Thus, the energy provided by the ground source needs to be: 10,000 / 3.0 = 3,333W.
If the ground gives us 15W per square meter we therefore know that we need 3,333W / 15W = 222.22 meters of piping required. The piping comes in strips up to 450m (which is looped and thus takes up less space than 222.22m) so 222m isn’t a problem to have in one run.
If the pipes were placed in sandy soil they need to be 25% longer, and in water they can be 25% shorter (as water is a better conductor of heat).
Compact Collectors:
Compact Collectors are a new way of installing horizontal piping and take up a lot less space. They are essentially panels of piping that are laid in the ground in the same manner as the loop piping:
The collectors should be installed so that there is a fall from the top to the bottom of at least 300mm.
There is approximately 250W of energy recovered for every panel that is installed. Thus for a 10kWp system that requires 3,333W of ground source energy, you need 13.3 panels installed. With each panel measuring 2m by 1.5m this gives an area of 40 square meters required. A maximum of 11 panels can be installed in any single row.
Vertical piping: the piping is dug deeper into the ground and installed vertically, with the top section 1 meter under the surface. This is in case there is not enough ground surface for horizontal piping and is often used for commercial buildings where the heat requirement means the horizontal piping system is not feasible.
Bore holes: for areas with minimal space two vertical holes can be drilled. The type of earth you are drilling into will determine how far down you will need to drill. Some materials, such as granite, will have a high heat extraction rate such as 65 Watts per meter (65 W/m). Granite has a high thermal conductivity, which means you can extract more heat out of a smaller area than in other materials. Others, such as simple dirt, will have a low heat extraction rate of just 15 Watts per meter (15W/m), meaning you will have to dig a lot deeper to get the desired heat output. How far down you need to dig will depend on both the amount of heat you require (i.e. the GSHP system size for the property), and the material you are drilling into. A typical small scale system of 5kWp will require a depth of 100 meters.
The bore-hole width would be just 150mm with the pipe that carried the heat transfer mixture being around 40mm in width. Once the fluid pipe is inserted into the bore hole, it is filled with a Grout that helps keep the shape of the hole. This grout also helps the fluid in the pipes extract heat from the ground more efficiently.
Sizing a borehole:
For a 10kWp GSHP with a CoP of 3 we require 3,333kW of energy from the ground. In a bore hole we know that each meter of depth give 40W. Thus 3,333 / 40W means that we need a hole of 83.3meters for the piping. This must be the depth the pipe reaches – not the hole.
The cost of drilling a bore hole will depend on what material is being drilled through (i.e. granite or soil) and the location of the bore hole (i.e. in a built up area or field). Typically the costs of bore-holes will range between £35 and £70 per meter drilled. This does not include the cost of the grout, piping, pressurizing and capping of the hole, which could in total add another £20 to £25 per linear meter.
Rivers/Ponds/Lakes: pipes can also be laid at the bottom of river/ponds/lake beds where the surface temperature is the same as that under the earths’ surface. Water is a better conductor of heat than the ground, making the heat transfer more efficient. Also, the lack of need for digging/drilling, means reduced costs. However, there will be very few locations where the situations are ideal for such an installation as you must have a property that is very close to a deep water source. The water depth must be AT LEAST 1m, ideally more.
Home pages of interest in this section are:
- Ground Source Heat Pumps
Pages of interest in this section:
- What is a Ground Source Heat Pump?
- How do GSHP’s work?
- Types of GSHP
- GSHP Efficiencies
- Sizing a GSHP
- How much does a GSHP cost
- GSHP Planning Permission
- GSHP’s versus ASHP’s
- GSHP’s in conjunction with Solar PV
- GSHP’s and ASHP’s in Practice
