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Last edited 25 Aug 2023
Ground coupling, thermal stores and earth bermed buildings
 What is ground or earth coupling?
The ground coupling effect or earth coupling, in terms of the design and construction of buildings, is any mechanism that makes use of the natural effect of the earth's constant ground temperature. Between about three and twelve metres below ground surface, the temperature of the earth remains relatively constant throughout the year, so in summer it is cooler than the outside temperature and in winter it is warmer than the outside temperature. The effect lessens gradually as depth decreases.
The earth’s internal heat was originally produced gradually through accretion. Since then heat has been produced by the radioactive decay of elements such as uranium, thorium and potassium. Due to its high enthalpy, this type of heat is often harvested in volcanic areas for electricity production and large district heating. However the majority of heat stored within the Earth’s surface comes from the sun. This heat is widely available, and because of its low enthalpy, meaning it is generally exothermic releasing more heat than is absorbed, it can be beneficial to energy systems and buildings.
Ground source heat pumps can pump heat from the ground into a building to provide space heating and domestic hot water. They do so either using a liquid medium to transfer the heat within a closed system of component parts or in an open system making use of natural ground water. In general for every unit of electricity used to pump the the medium containing the heat, 3-4 units of heat can be produced.
- A ground loop or ground heat exchanger (GHE), normally made up of lengths of plastic pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is generally a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground. In the case of open systems a heat pump is connected directly to the ground water which extracts the warmth or coolth directly.
- A heat pump. Heat pumps are used in fridges and air conditioners, they work by using the evaporation and condensing of a refrigerant to move heat from one place to another. In this case, the evaporator, takes heat from the water in the ground loop; the condenser gives up heat to a hot water tank which feeds the distribution system.
- A compressor. This uses electricity to move the refrigerant around the heat pump, compressing the gaseous refrigerant to increase the temperature at which it condenses to that needed for the distribution circuit.
- A heat distribution system. This consists of under floor heating or radiators for space heating and water storage for hot water supply. Some systems can also be used for cooling in the summer.
There are essentially two different design characteristics to consider which create four basic options in the set up of ground source heat pumps, that being closed or open loop systems and vertical or horizontal network.
- Horizontal – closed loop heat pump
- Vertical (probe) – closed loop heat pump
- Vertical – open loop heat pump
- Horizontal surface water open heat pump.
Inter-seasonal thermal energy storage also called seasonal thermal energy storage (STES) works in a very similar way to ground source heat pumps with the main difference being that rather than extracting the existing heat from the ground via the coupling effect, it actively stores heat in the ground. Solar collectors increase the amount of heat collected during warmer summer months, this heat is then pumped into the ground loop system for longer periods of time, or seasonally. The ground loops or thermal banks are specifically designed to be able to take increased levels of heat and store this over longer periods of time in a storage medium or what is known as salt in matrix (SIM). These systems still benefit from the ground coupling effect but actively store heat, increasing temperatures and cycles of storage and extraction.
An earth-to-air heat exchangers work in a similar way to ground source heat pumps but are more direct because the medium used to transport the heat is air, which is directly warmed or cooled. The systems draw ventilation supply air through ducts or tubes buried in the ground. This substantially reduces the fluctuations in ambient air temperature, which can provide conditioned air throughout the year, with the incoming air being heated in the winter and cooled in the summer by means of earth coupling.
These types of systems might be driven by natural stack ventilation, which draws the air through the system from a thermal pressure difference at the highest point. However because of design restrictions many systems use low level mechanical ventilation at one or both ends to create the required circulation. In some cases air is circulated via air handling units, allowing filtering and supplementary heating or cooling where a simple controller can be used to monitor inlet and outlet temperatures, as well as indoor air temperatures. Other more simplified systems might use direct air supply either drawn through the system by the stack effect or by mechanical drivers. The ducts or tubes that draw the air though the ground can be of plastic, concrete or clay. The material choice is of little consequence thermally due to the high thermal resistance of the ground.
Earth-to-air heat exchangers are suited to mechanically ventilated buildings with a moderate cooling demand, located in climates with a large temperature differential between summer and winter, and between day and night. Location of the duct sin sand or gravel below the water level, where there is moving ground water, gives the best performance, however, the presence of ground water involves extensive sealing precautions.
Earth bermed buildings, often called earth shelters or earth sheltered buildings, are a design approach that connects a building more significantly to the ground and landscape around it than simply sitting upon it. This typology is normally built slightly below ground or at ground level with an earth berm around that covers most of the external walls and potentially also the roof, creating a sunken or earth sheltered appearance.
Depending on the detail design approach, the building will, to differing degrees, benefit from an earth coupling effect whereby the thermal mass, insulative and protective qualities of the surrounding earth benefit the thermal performance of the building. An earth coupled building does not insulate between the internal space and the earth and as such benefits from the decreased fluctuation of the earths temperature, it will however have insulation and normally high internal mass or glazing on any walls exposed externally.
One well known example of an earth bermed housing scheme is the Hockerton Housing Project, which is a linear cluster of five self-sufficient houses built in Nottinghamshire in 1997 by Brenda Vale. Theses houses are earth coupled and as such require zero to minimal heating and have lower-than-normal energy consumption, which is supplied by onsite renewable energy generation from two 6 kW turbines and 7.6 kW solar panels.
- Coefficient of Performance CoP.
- Dynamic thermal modelling of closed loop geothermal heat pump systems.
- Earth berm.
- Earth bermed buildings.
- Earth to air heat exchangers.
- Geothermal energy.
- Geothermal pile foundations.
- Ground energy options
- Ground preconditioning of supply air.
- Ground source heat pumps.
- Renewable energy sources: how they work and what they deliver: Part 3: Electrically driven heat pumps DG 532 3.
- Thermal labyrinths.
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