Ground Source Heat Pumps
To meet Renewable Heating targets, and to avoid calamitous impacts of climate change, the UK Government must pursue a massive increase in the electrification of the nation’s heating, primarily through the use of heat pumps. The ‘carbon intensity’ of electricity has reduced radically as renewable power sources replace coal. Many heat pumps will use heat from air, but perhaps 20% will use heat from the ground or bodies of water.
Carbon Zero Consulting firmly believes, based on many years experience, that ground source heating and cooling provides the most efficient, robust and long-lasting renewable heating and cooling technology.
The temperature of the soil and upper few metres of soil and rock are at a remarkably constant temperature, equal to the average annual air temperature. In southern England this average is about 12°C and in northern Scotland about 9°C. The sun provides the majority of this heat energy with only a very small amount coming from the Earth’s interior. Of course, the ground temperature is not enough to heat a building directly. That’s where a ground source heat pump comes into the picture…see our explanation of how a heat pump works below. But first a brief explanation of the most common methods for a heat pump to take heat from the ground: For a detailed step by step guide to installation see here
Two types of Ground Source Heat Pumps
Heat is absorbed from the ground (initially at about 12°C in the south of England) via plastic pipes filled with a thermal transfer fluid. The pipes can be installed horizontally in trenches, in boreholes or in a lake or river.
The length of pipe required to provide sufficient ground-heat exchange capacity depends on the building’s seasonal heating/cooling characteristics and the properties of the soil/rock or water body.
The temperature of the fluid in the pipework, and in the ground, decreases during a heating season. Careful design is required to ensure the ground source is of sufficient capacity to prevent the temperature dropping too low.
As there is no pumping of groundwater in a closed loop system, there are no requirements for a system to be located on a productive aquifer – and (normally) no involvement from a Regulator.
The plastic pipe utilised in a system is extremely long-lasting (100 years or more) and so can be considered as ‘infrastructure’ to access renewable heat.
In areas of suitable geology, water is pumped from a borehole to a heat exchanger – linked to a heat pump. Water from a borehole is at a near-constant temperature (about 12°C in southern England). Thermal energy is extracted from the water within the heat pump cycle. The resulting cooled water is then re-injected to the ground or returned to the environment.
Ground source heating and cooling systems provide heating for small houses requiring just a few kilowatts (kW), to large commercial buildings requiring megawatts (MW) of heat and cool. Closed and open loop systems can provide cooling as well as heating. Open loop ground source is well-suited for passive cooling as groundwater is already at a perfect temperature.
The science behind ground heat transport is directly analogous to groundwater flow. Design and assessment of GSHC systems is an extension of our long standing expertise in hydrogeology (or thermogeology).
Carbon Zero Consulting has unrivalled experience of the thermogeological properties, with involvement in many of the UK’s larger and most successful systems. John Findlay was a recent Chair of the Ground Source Heat Pump Association, and closely involved with the development of industry standards.
Open loop systems require suitable geology (an aquifer) and compliance with the Environment Agency, Natural Resources Wales or the Scottish Environmental Protection Agency.
Together with our specialist associates, we provide support to your GSHC project from concept development to design, project management, installation and regulatory permitting.
How A Heat Pump Works
A heat pump ‘moves’ heat from a low-temperature external source (the ground) to a higher-temperature interior space (e.g. your house or business).
Anyone who has a refrigerator has witnessed the operation of a heat pump.
A refrigerator extracts heat from your bottle(s) of wine and ‘pumps’ the heat into your kitchen via a heat exchanger (the condenser) located on the back surface of the fridge.
The heat flows from the inside of the fridge to your kitchen until the contents of the fridge are at the desired, lower, temperature.
A heat pump’s job of elevating low-grade heat to 35°C or higher involves a cycle of evaporation, compression, condensation and expansion. A refrigerant is used as the heat-transfer medium that circulates within the heat pump.
- The cycle starts as the cold, liquid refrigerant passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (liquid from the ground loop). The refrigerant evaporates into a gas as heat is absorbed.
- The gaseous refrigerant then passes through a compressor where the refrigerant is pressurised, raising its temperature
- The hot gas then circulates through a second heat exchanger (condensor) where heat is removed and pumped into the building at up to 55°C, although 35 – 40°C provides the greatest level of efficiency and is sufficient for underfloor and specialist radiator heating applications.
- Following transfer of some heat within the condenser, the refrigerant changes back to a liquid. The liquid is rapidly cooled as it passes through an expansion valve and begins the process again.