Types of Open and Closed Geothermal Loops

The UK’s ground temperature averages between 8 and 13°C. This heat is partly due to the earth’s internal heat emanating from its core of magna, but mainly from the heat of the sun in summer producing low level geothermal energy which the ground maintains, up to 20m below the surface.

This heat can be exploited by use of a geothermal loop feeding a heat pump which transfers this heat to a refrigerant such HFC R-41-A. This refrigerant is biologically safe (older types of CFC refrigerants were harmful to the ozone layer) and flashes of to a hot gas which is used to heat domestic system.

The only energy required after installation costs is power required by the heat pumps circulating system and this can be offset by the use of photovoltaic panels to drive the pump.

We shall look first at the closed and open types of geothermal loops which are normally fabricated from tough durable plastic pipe that allows heat to pass through it efficiently. The fluid used in the loops is a mixture of biological antifreeze in case of any ingress into the surrounding environment of soil or water.

• Vertical Loop
• Horizontal Loop
• Slinky Loop
• Pond Loop
• Open loop

This is the most common type of closed loop, as it requires the least amount of ground to contain it. However it is the most expensive but most efficient as the earth’s temperature is more consistent with depth.

To install a loop system firstly a vertical bore holes are drilled 50 to 100m deep and at least 5m apart, this gap ensures that the individual loops do not encroach on the available heat energy in the soil.

When the required number of boreholes has been drilled (the contractor will have calculated the number required to suit the buildings heating requisite) the U shaped pipe typically between ¾” and 1^1/4” diameter, are then inserted down into the borehole.

An efficient heat-transferring sealing compound or grout is poured into the gap between the pipe and the soil. This is not only to ensure a good contact between the pipe and the ground, but also to prevent rainwater from penetrating into the borehole.

When all the pipes have been inserted and grouted, they are connected up to an inlet and outlet manifold which supply and return the loop circulating fluid, a mixture of water and antifreeze to and from the heat pump via the circulating pump.

Provided there is plenty of ground available, this design of ground loop is very economical, as it only requires a digger with a backhoe to excavate the required number of 2m deep trenches, over an area of ¼ to ¾ acre for a typical dwelling house which is a much cheaper option than a vertical loop.

When the required number of trenches is dug, the prefabricated U shaped pipes are laid horizontally at the bottom of the trenches and the whole area backfilled leaving the pipe tails exposed. These tails are connected to inlet and outlet manifolds, supplying the fluid to and from the heat pump via the circulating pump.

The one disadvantage of horizontal geothermal ground loops has is that it cannot be used in any location subject to thermofrost.

Relatively new, this system is becoming more popular as it can be even more economically viable and, space saving than the traditional horizontal loop system.

In this type of installation the trenches are dug as before in the horizontal loop system, only much wider. This is to accommodate overlapping pipes in the form of a flat coil which are laid in the bottom of the trenches and backfilled with soil leaving the tails exposed. As before these tails are then connected up to inlet and outlet manifolds supplying fluid to and from the heat pump.

Continue onto the next page to read about pond and open loop systems.