Types of Geothermal Ground Loops for use in Geothermal Energy Home Heating
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 as geothermal energy home heating.
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.
Different Types of Loop Systems
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
Vertical Geothermal Ground 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 150’ to 300’ deep and at least 15’ 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) U-shaped pipes, typically between 3/4" and 11/4" 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.
A sketch of a typical vertical loop system follows.
Horizontal Geothermal Ground Loop
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 6’ 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.
A typical horizontal ground loop sketch is shown below
Slinky Coil Ground Loops
Although relatively new, this system is becoming more popular as it can be even more economically viable as well as space saving compared to 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.
A sketch of a typical slinky coil ground loop system is shown below.
Geothermal Loops in Water Sources
So far we’ve covered the geothermal loops that are placed in soil. These next two types use heat from a pond, lake, or other open water source. A pond geothermal loop is somewhat similar to a slinky loop system. The open loop system doesn’t use a large pipe system to recycle water. Instead, water is removed from the pond or lake and pumped to a heat pump. The water then is pumped back into the pond.
Pond Loop
This system utilizes the heat contained by the water at the bottom of a pond. Slinky coils are normally used to extract this heat, through the circulation of a mixture of water and antifreeze through the coils.
The coils are installed and supported vertically and securely anchored to the bottom of the pond, the pipe tails led to the surface.
From here they are buried underground and run to the dwelling and into a heat pump.
There are a few issues with this type of system, the main one being environmental concern in case of a leak of antifreeze into the pond water. However this can be largely overcome by using biological antifreeze in the loops circulating water.
A sketch of a typical pond loop follows
Open Loop
This system uses the heat contained at the bottom half of a pond or well, as opposed to the preceding types of ground loops which circulation water and antifreeze through closed loops.
It has been stated that this is the most economical of all the geothermal loop systems, also having the shortest pay-back period.
To install this system you either need a large pond, a lake or a well; however in this article we shall examine the installation and process of a pond loop system.
The water from the pond is pumped through a heat pump which extracts the heat and discharges the water at a cooler temperature back into the pond.
One serous drawback of this method is that the water should not be acidic, or pipes can become eroded, likewise, filters should be incorporated in the system to remove organic matter. Minerals in the water can also cause build-up inside the pipes, and it is imperative that tests should be carried out to determine the acidity, organic matter, and mineral content.
A sketch of an open loop system being used in a pond follows.
References
- groundloop: Geothermal Explained
- loop-group: Loop System Installation
- enertran: Geothermal Loops
This post is part of the series: Geothermal Energy - Energy from the Earth
This is a series about geothermal energy. It begins by examining how the heat is produced under the rocks and continues with itsextraction to tthe different types of geothermal power stations and how they operate. It continues with the different types of heat pumps and the loops that supply them
