Ground Source Heat Pumps (GSHPs) use low level heat energy created by solar gain in the near surface layers of the earth to extract energy which can be used for space and water heating. In principle, a GSHP uses a refrigeration system in reverse to extract low temperature heat from one location (the ‘source’) and deliver higher temperature heat to another location (the ‘sink’). Electricity is used to drive the pumps. This operational principle can be used to produce heating as well as cooling energy.
Heat Pumps use refrigerant gases and a compressor to absorb heat from the ground or ground water. This in turn delivers heat to the target building. A heat exchanger, called an evaporator, is used to interface between the fluid from the buried pipes and the heat pump. GSHP systems use the gained energy to heat another fluid – usually water – via another heat exchanger, called a condenser.
The efficiency or SCOP (Seasonal coefficient of performance) of a heat pump is very much dependent on the temperature of a heat source and the output temperature of the heat pump. The higher the heat source temperature and the lower the output temperature, the better efficiency. Most modern GSHP’s can achieve SCOP’s of 4 or more, with the best performing being able to achieve around 6 if used efficiently.
APPLICATIONS
GSHPs are commonly used for space heating utilising water as a distribution medium. Low temperature radiators can be used to deliver heat into the space; however, GSHP’s are particularly suitable and most efficient for low-temperature distribution systems such as under-floor heating.
The system can be designed to deliver temperatures of 45°C to 55°C for a system with low temperature radiators and 30°C to 40°C for an under-floor heating system. A significant point to note is that the temperature of the fluid in a conventional central heating system leaves the boiler at about 80ºC in and returns at about 60ºC. The lower output temperature from a GSHP system means that conventional radiators are not suitable for use with GSHP systems as they are less efficient at emitting low temperature heat. GSHP systems are most efficient when all components of the system – heat collection, heat pump and heat delivery – are designed to be compatible.
GSHP’s can also be used for water heating, via a heat exchanger to hot water appliances. It is required to supply hot water with temperatures in a range of 55 – 65°C to prevent contamination of hot water system by legionella bacteria.
SYSTEM DESIGN
GSHP’s are more efficient than air source heat pumps due to relative stability of ground temperatures all year round. However, efficiency of a GSHP system is very much dependent on a number of factors including system design and controls, heating/cooling loads, the temperature of the heat source, the output temperature and energy consumption of auxiliary equipment (e.g. fans, pumps).
Most GSHP systems have two loops: the primary loop in the appliance cabinet which exchanges heat and a secondary loop that is buried underground. The underground (secondary) loop can be classified as either open or closed, with closed loop systems typically sub-divided further into horizontal and vertical systems.