Welcome to the Ground Arrays module.
As we have seen in the Heat Pump Basics module, the ground arrays are the part of a heat pump system that harvest low-grade heat from the ground (or from water).
In this section we will examine the different types of ground array, and consider the pros and cons of each type so that you can select the right kind for your project.
We'll go over sizing arrays in order to get the best performance and to comply with MCS guidelines.
We will look at how the arrays are installed in the ground, and illustrate a few important points to remember.
We've seen that a heat pump uses pipe buried in the ground, filled with heat transfer fluid, to absorb energy from the surrounding earth.
The amount of pipework that's needed will depend on the thermal properties of the ground that they sit in.
Straight pipe is just that! In this case, the plastic pipe is usually 32mm or 40mm in diameter. It is available on reels, in various lengths depending on your supplier.
The pipe is buried just over a metre deep, in long trenches 1m wide. If the soil is stony, the trenches are lined with sand to prevent stones from damaging the pipe and causing leaks. A 1 metre gap must be left between trenches (centre to centre). MIS3005 actually stipulates 0.75m, but that's for 25mm pipe, so stick to 1m.
If you pack too much pipe into a small space, you will extract too much heat from that area of ground. It's possible to freeze the ground, because the heat transference in soil just doesn't happen fast enough to make up for the amount of heat being taken out.
On a small scale this can damage grass laid over the top of the trenches. At its worst, perma-frosted ground will buckle and crack, and when there is no more heat to be taken from the ground, the heat pump will stop working. It can even damage the heat pump beyond repair.
This is why the seperation distances are defined by the MCS guidelines.
The heat in the soil isn't "geothermal". It's not that deep! The energy we are trying to use is warmth from the sun and from rainwater.
Anyway, straight pipe is laid in a meandering pattern up and down. This means it's easy to avoid obstacles like trees. Remember though that the ground-side pump in the heat pump has a limited amount of head, so you will probably have to install straight pipe in loops of no more than 300m each. You will use a manifold to link the loops together.
A borehole is a very deep, drilled hole that contains a U shaped length of pipe, filled with heat transfer fluid. The pipe runs from the top of the hole down to the bottom, and back up again. This pipe is known as the "probe" and is the equivalent to straight pipework, just running vertically.
Boreholes are drilled by specialist drillers using drilling rigs. They are backfilled with grout which helps the thermal conductivity between pipe and soil.
Boreholes should not be placed too close to building foundations (4 metres) and should be seperated from each other by 8 metres. MIS3005 stipulates 6m; Kensa are always cautious.
Because the ground temperature gets warmer the further down you go, boreholes are considered to be the best way to extract heat, metre for metre. The hitch is that they aren't cheap in comparison to other methods - depending on your driller, you could be looking at £40 to £50 per metre drilled.
Roughly speaking you need 20 metres of borehole per kilowatt of heatpump output, so for an 8kW heat pump you'd need around 160 metres drilled. You can see how this would stack up, especially for larger installations!
You can link multiple boreholes togethe, but to ensure balanced flow rates they should be kept to the same depth - or flow balancing valves will be needed.
Boreholes are really useful where you are pushed for space to fit straight pipe or slinkies. You just need to ensure there is space for a drilling rig to get in - normally this means HGV access for unloading of the rig, but some drillers have small tracked rigs that can get through gates.
We are occasionally asked "I have a borehole for drinking water, can't we just use that?". For closed loop, this would involve simply dropping the probe down the borehole. It sounds very neat, but there are a couple of catches.
1) Contamination. This is pretty unlikely, but it's possible - if that probe pipe is damaged, then you may end up with glycol contaminating your water supply. Although the glycol Kensa supply is "detoxified", that doesn't mean you would want to drink it - and if the borehole is your only source of drinking water, what happens?
2) Potable water boreholes are generally not deep - 10m to 40m depending on where the water table is. Boreholes for a heat pump need to be much longer - even given the added thermal advantages of having some of the probe pipe in water, you lose out if some of it is in air, and you can't add grout in this situation.
A pond mat is a 40 metre slinky (so that's 250m of pipe!), attached to a metal frame, that is submerged in water.
Why would you do this?
Because water is a much better conductor of heat than the ground is. If your project is close to a lake or pond, then making use of this source of energy could be well worthwhile! Remember that the better the incoming temperature, the better the heat pump's performance - well, this is exactly what we meant.
It can also be a lot less work that digging. You will need to dig a header trench, running pipework from the heat pump to the lake, but there's no digging for slinkies or straight pipe.
The pond mats are floated out into position, and as they are filled with heat transfer fluid, they sink, eventually resting on the lakebed. The rigid frames ensure that they don't move around, and breeze blocks secured to the frame help to sink the mats where you want them.
It's possibly to secure the pond mats to the side of a harbour wall (you need permission!), or even to a jetty. Take a look at this video to see a clever pond mat installation completed by a Kensa Partner.