Before the installation of a Geothermal Heat Pump (GHP) system a number of design factors need to be considered. One of the most critical design factors is the type the loop of the GHP system. There are 2 types of GHP systems namely the closed loop systems and the open loop systems.
A closed loop system is the most common type of a GHP system. They typically consist of two different loops: the primary refrigerant loop and the secondary water loop and heat pumps placed on each loop. The primary loop is part of the main system and is installed inside the building. The secondary water loop is buried under the ground and it consists of pipes that are placed in the ground. The pipes contain water with anti-freeze liquid (Monopropylene glycol). The secondary loop pipes are place underground in a depth at which the temperature is more constant. The closed loop system works by exchanging heat between the primary and the secondary loop. As the water inside the secondary loop flows through the pipes inside ground it is heated (by exchanging heat with the ground) and then returns into the building where it exchanges heat with the primary loop. Then the water returns back into the ground to start the process all over again. There are three major types of closed loop systems:
- Vertical Loop System. In a vertically system the pipes are laid out vertically into the ground. This is very useful in places where there is limited area of land available. However installation is difficult in rocky grounds where it is difficult to dig vertical holes into the ground. The holes in which the pipes are laid out are 4 inches (10,16 cm) in diameter and between 100 – 500 feet (30 – 150 meters) deep, depending on ground characteristics. The holes are separated by 20 feet (6 meters) from each other.
- Horizontal Loop System. Horizontal Loop systems are more effective than the vertical type of installations but require a large area of land to be installed. They are particularly good for new constructions where this area of land is usually available. The pipes are buried at about 3,5 – 6,5 feet (1 – 2 meters) deep and they can be placed either in two levels (one lower and the other higher) or side by side. If there is not enough space to properly lay out a horizontal loop the slinky closed loop (or coiled loop) can be used (although not recommended), where the pipes are laid out like a coil. The drawback of horizontal loops is that if the secondary loop is placed closely to the ground (at about 3,5 feet) it affected by the outside weather conditions, which is not good particularly for locations with long and cold winters.
- Pond/Lake Loop System. In this type of systems the pipes are laid out in the water at the bottom of a pond or a lake. The pipes are laid out in a coil fashion similar to the slinky method described above. This type of systems is not commonly used because open loop systems are preferred at places where there is a water source. A Pond/Lake loop system may be used at places where the quality of the water is not very good and an open loop system cannot be used.
In an open loop system the water from a water source is pumped into the pipes and directed towards the heat pump. The heat is then exchanged with the primary loop in a similar way to the closed loop system. The water is then returned to a different water body using different pipes (this is important in order to ensure that the temperature of the water source is not affected). These types of systems are also known as groundwater heat pumps.