Closed loop systems are more common than open loop systems because they can be installed at almost any site. Closed loop systems have a continuous ground loop; the fluid inside never comes in contact with the environment.
A vertical loop system uses wells drilled one to three hundred feet into the ground as the ground loop. A u-shaped pipe is placed in each trench so that fluid can go down the well, transfer heat, and come back to the surface without ever contacting the rock or soil. These wells take up significantly less room than other configurations, but will cost more. The depth of vertical systems can also lead to consistent performance.
It is also possible to place the ground loop in a horizontal configuration, in which the piping is simply laid in one or many trenches in the ground. This is an inexpensive method of installation, and is often used in residential or light commercial applications. There are many different layouts for horizontal systems. In some cases 2-6 pipes are distributed in each trench; in some cases a ‘slinky’ formation is used. If a large amount of earth can be moved to create one very wide trench, rows of pipe can all be laid in this trench in a ‘racetrack’ formation. All have been successful heat transfer devices.
If a pond (or lake) is available on the building site, it is often convenient to simply sink the loop into the pond and transfer heat to and from the water of the pond. This can be just as efficient as a ground loop if care is taken in choosing a large enough body of water.