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. This can be used to distribute the heat (at ~40-65ºC) in a building and return it to the heat pump at about 35ºC.

Heat Pumps How They Work

The efficiency or SCOP (Seasonal coefficient of performance) 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 of a heat pump, 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.  Radiators can be used to deliver heat into the space; however, they 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 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 the heat delivery system of a conventional central heating system leaves the boiler at about 80ºC and returns at about 60ºC. The lower output temperature from the GSHP system means that heat delivery systems designed for conventional central heating systems are not suitable for use with GSHP systems. 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.

Open loop GSHPs

Open loop GSHP’s either use ground water (from a borehole) or surface water of lakes / rivers as a heat source. A large open loop system design may require an abstraction licence even if the water is returned to the source. The abstraction and recharge (return) of water should be separated by as great a distance as feasible within the site constraints to minimise the impacts of possible temperature pollution. The implications of mixing warmer and cooler bodies can encompass environmental concerns but would also reduce the temperature differential over time and thereby reduce the efficiency of the system.

Closed-loop systems are often cheaper to install than their open-loop counterpart. However, open-loop systems can achieve greater efficiencies and also work well with new build properties and those being converted.