Ground Source Glossary
What is ground source?Ground source systems use the ground as a means of absorbing and releasing heat. The heat pump system is extremely efficient and saves energy because the ground is at a constant temperature rather than the ambient which varies. The system is used for heating in cold weather and cooling in warm weather.
What is ground source cooling?Ground source cooling uses the ground as a means of rejecting heat given off by the process. This Is then re-used for the heating cycle and thereby improving seasonal efficiency.
What is ground source heating?Ground source heating uses the ground as a means of rejecting the cooling process which is given off from the system. The heat is later recycled in cooler weather in order to improve the season efficiency.
What are ground source heat pumps?They are the integral component of the ground source system. Ground source heat pumps (GSHPs) are used to generate the cooling or heating medium. They are extremely efficient. GSHPs can give COP’s of between 4.5 and 6. Therefore for every one Kilowatt of energy used to generate heating or cooling they give in return 4.5-6kw of heating or cooling as a useable load. On the other hand a gas boiler only returns 0.9 kilowatts of useable load for every one kilowatt of energy used.
What are bore holes?These are the means of rejecting heat or cooling from the heat pumps. They are drilled to depths of between 70m and 300m in order to obtain the load required for the specific building. The bore holes are drilled using large rotary rigs to ensure that the stability of the bore hole is maintained until the ground loop is installed.
What are irrigation holes?These are used to abstract up to 20 cubic meters of water per day and do not necessitate obtaining a licence or permission for the work involved. Potable water is used as an alternative to or alongside rain water harvesting systems in order to improve water quantity and quality. The water can be used when filtered to flush toilets and also to provide water for showers. The quality of the water can be as good as drinking water in some locations depending on the geology of the area being drilled.
What are geothermal systems?Also known as ground source systems. However, geothermal systems are drilled to a much greater depth in order to capture high temperature water. Ground source systems tend to limit the drilling to 300m due to the operational pressure drop on the system.
What is drilling?Is the process by which rotary drilling rigs are used to drill the bore holes down to the required depth for the designed system? Specialist rigs using mud puppies are used to ensure that a quick and clean drilling operation is achieved.
What are HDPE loops for ground source?Once the bore hole has been drilled HDPE pipe work is installed as a means of transferring the rejected heat or cooling into the ground. The diameter varies to suit each application 32mm, 40mm or 50mm. The HDPE ground loops are very robust and have a 25 year life expectancy. They are configured in a flow and return system leading up to a chamber where they are isolated on one side and a flow measurement fitted to the other.
Vertical ground source system
A vertical closed loop field is composed of pipes that run vertically in the ground. A 150mm diameter hole is bored into the ground, typically between 50–150 m deep. The pipe flow and return in the hole are joined by fusion welding with a U-shaped cross connector at the bottom of the hole. The borehole is then filled fully with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer, Thermally enhanced grouts are available to improve this heat transfer. Grout also protects the groundwater from contamination, from the surface down, and prevents artesian water from rising and being contaminated. Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced at least 6-9 m apart and the depth depends on ground and building load profile which is a critical part of the bore field design process.
Horizontal ground source system
A Horizontal closed loop field is composed of HDPE pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is excavated and pipework or slinky coils are placed horizontally inside the same trench. Excavation for horizontal loop fields is cheaper than the cost of vertical drilling, so this system is utilized wherever there is adequate land available. A slinky closed loop field is a type of horizontal closed loop where the pipes overlay each other varying the diameter and pitch determines the amount of heat that is rejected into the trench. A slinky loop field is used if there is not adequate room for a true straight horizontal system, but it still allows for an easy installation. Straight pipe horizontal systems can be installed in a number of configurations 1,2,3,4, pipes in a single trench side by side or arranged vertically ensuring that the spacing is apart 700mm so they are able to reject/absorb heat.
Depending on soil, climate slinky coil trenches can be shorter than the horizontal straight pipe installation although this is very dependent also on the design model software capabilities. Slinky coil ground loops are essentially a more economic and space-efficient version of a horizontal ground loop.
Lake / Canal / Pond
Depending on the depth of the body of water and the proximity of the water a closed pond loop system can be employed using a slinky loop or a more highly technological stainless steel Geoplate. Slinky loops positioned on a raft is ideal for very shallow water systems and can be a very good solution from a cost and economic point of view. Geoplate are generally used for very high load systems 100kw and above but that’s not to say they cannot be used on smaller systems should the water depth be suitable. They are very reliable and due to their design offer high COP’s on the heat pumps. The main benefit of these types of water-based systems over direct lake pump systems is that there are no contaminates within the condenser loop. The system is sealed thus preventing silting of the condenser tubes or plate heat exchanger prolonging the life of the system maintaining maximum operating efficiency.
Open loop ground source system
The open loop system is monitored very closely by the Environment Agency. All systems have to be discussed in great detail prior to commencement in order to check the viability of the system. Failure to do so will lead to the system be shut down and heavy financial penalties being applied.
In an open loop system, the secondary loop pumps natural water from a well or body of water into a heat exchanger. This separates the borehole side from the load side. On the load side heat is either extracted or added by the primary refrigerant loop within the ground source heat pump and the water is returned to the plate. On the borehole side, the water is abstracted from the well through the plate heat exchanger where it is heated or cooled and then returned via an injection well, sewer or a body of water. The supply and return boreholes must be placed far enough apart to ensure that thermal recharge of the source do not interfere with the abstraction temperatures. Since the water chemistry is not controlled, the plate heat may from time to time require periodic cleaning. This is much more of a problem with cooling systems than heating systems. If the water contains high levels of salt, minerals, iron bacteria or hydrogen sulphide, or if low volumes of water are not available then a closed-loop system is usually preferable.