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.