Horizontal ground collectors are sometimes the lowest cost method of providing an energy source for a ground source heat pump. The pipes that form the collector are laid in shallow trenches which are then backfilled. The trenching work can be carried out using conventional digging and trenching equipment which is often on site anyway during a construction project. This type of collector is the preferred option in many domestic installations, particularly for self builders who frequently carry out the trenching work themselves. The main drawback is the large area of land required.

Design and area needed

During the detailed design stage Econic’s engineers will produce a drawing showing the proposed location of the collector trenches and manifolds on a site plan. The collector should not be too far away from the proposed plant room location but at the same time does not need to be immediately adjacent to the building. Sloping ground can usually be dealt with although level ground is easier from a design and installation perspective. An important stipulation is that the ground remains permeable after the installation. A big part of the ground energy re-charge happens as rain water permeates the soil around the collector piping. This means horizontal collectors can not normally be put under car parks for example.

The area needed for a horizontal collector depends on the size of the heating system and the type of ground at the site. It’s easier to extract heat from wet, loamy soil than it is from dry sandy ground. In general we use rule of thumb methods to calculate the length of trenching required and the area over which it will be installed. A typical figure would be 25W per square metre of ground depending on the soil type. This means that if we want to extract 10kW from the ground, an area of 400m2 would be needed. If a horizontal collector is undersized then the energy we are looking to remove each heating season will not be sufficiently replenished during the summer. Gradually the ground will cool around the collector and the heat pump will become highly inefficient. For larger systems the amount of ground needed often means that a horizontal collector is not practical.

Process

The trench locations are laid and according to our plan and the trenches can then be dug to a depth of about 1.2m. Two types of pipe are used for horizontal collectors. In most cases standard plastic MDPE pipe of 32mm diameter is laid in a series of narrow trenches. This pipe comes in lengths of 100m which dictates the maximum trench length. The second type of pipe used is the so-called slinky. This is simply a stretched out coil of MDPE pipe that resembles the children’s toy of the same name. This is laid in a wider trench and can reduce the amount of trenching work required overall. Econic have experience of both types of installation and can advise you on the most suitable approach for your project. Once laid the pipes are connected within a manifold pit to form the overall collector array. Usually a single flow and return pair are taken, in another trench, from the manifold pit back to the heat pump in the plant room. For servicing reasons we avoid any buried joints in this type of collector. At this point the collector is pressure tested to ensure there are no leaks anywhere. Once installed, a collector of this type can have a life in excess of 50 years.

The trenches are then backfilled using the spoil that was removed during the digging process. If the soil is very stony or contains sharp stones it may be necessary to cover the pipe with a layer of sand before backfilling to avoid damaging the pipe. Not all of the material removed will go back in the trenches so there may be a need to dispose of an amount of soil after backfilling.

As with any trenching operation like this some settlement will occur over the 18 months or so following the work. You may not want to consider digging up your bowling green or croquet lawn to form part of your ground collector!

The final stage involves the filling of the collector loops with the heat transfer fluid and the commissioning of the complete heat pump installation. Contact Econic for more information on the suitability of a horizontal collector system for your project.