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4 Heat Pumps
66. A heat pump moves heat from one place to another. Heat flows naturally from a higher to a lower temperature. Heat pumps, however, are able to reverse the natural flow of heat and force the heat flow in the other direction, using a relatively small amount of drive energy. Heat pumps can transfer heat from natural heat sources such as the air, ground or water, to a building.
67. Heat pumps are named after their source of heat:
- ground-source heat pumps get their heat from ground below the frost line;
- air-source heat pumps get heat from the outdoor air; and
- water-source heat pumps get heat from water, usually well water.
68. Even at temperatures considered to be cold, air, ground and water contain useful heat that is continuously replenished by the sun. By applying a little more energy, a heat pump can raise the temperature of this heat energy to the level required.
69. Most heat pumps have two main parts; the outdoor unit and the indoor unit. The outdoor unit includes the outdoor heat exchanger, the compressor and a fan. This is where the heat from the air outside is picked up during the heating season, and where the heat from inside the house is rejected during the cooling season. The indoor unit contains the indoor heat exchanger and the fan that distributes heated or cooled air to the distribution system of the house. Some systems have a second indoor cabinet that contains the compressor. By reversing the heat pump it can also be used for cooling.
Ground Source Heat Pumps
70. The average ground temperature just below the surface, in the UK is between 8ºC and 13ºC, this temperature remains constant throughout the year. Ground source heat pumps ( GSHP) are a means of tapping into and utilising this resource. GSHP were invented more than 50 years ago, and continuous development has greatly improved their efficiency and reliability. It is now a proven, cost-effective, safe and environmentally friendly alternative to fossil fuels, that is cost-effective for certain commercial and domestic applications, particularly where mains gas is not available.
71. The market for GSHP is currently small but growing - they are currently more common in the USA and the rest of Europe. The total number of existing installations in the UK is estimated at around 600-700 units. The principal market for GSHP are domestic housing, commercial properties not connected to the natural gas network and commercial industrial properties with stable heat demand. It is estimated that there is the potential for the number of installations to increase.
72.GSHP are most likely to be an option where there is no access to natural gas and so the alternative may be oil or direct electric heating (storage heaters). Heat pumps' ground loops can be laid in the ground or in water such as rivers, lakes or ponds.
How Do They Work?
73. To access thermal energy, coils or loops of special grade pipe need to be buried in the ground either in horizontal trenches or vertical boreholes. Horizontal trenches are a cheaper option and generally used where there is sufficient space. Where there is not enough land to do horizontal trenches, vertical boreholes can be used, these normally require to go down at least 60 meters and are the more expensive option, but will provide higher efficiencies since the temperature of the earth is higher at greater depths, and less power is needed to pump the fluid around the circuit. The length and size of ground loops is designed to match the heating needs of the property. The trenches or boreholes required for the ground loops can be dug and backfilled by a standard earth excavator.
74. Systems operate by circulating water (or another fluid) through pipes buried in the ground. The water in the pipes is lower than the surrounding ground and so it warms up slightly. This low grade heat is transferred to a heat pump, which raises the temperature to around 50°C. The heat pumps typically providing 4 units of energy from 1 unit of electricity.
75. The building plot will need sufficient land available for installation of the ground works. The dimensions of trenches or boreholes will vary between manufacturers. The ground above where heat pipes are installed can be used for open space or covered over with hard materials. Where there are existing lakes or ponds or where it is proposed to install Sustainable Urban Drainage Systems ( SUDS), the opportunity to install ground source heat pumps beneath the surface of the water should be considered. Similarly in larger developments with open space requirements, ground source heat pumps could be laid beneath greeenspaces.
Planning Issues
76. The definition of development includes 'engineering operations' examples of activities held to fall within the definition of engineering operations include drilling of exploratory bore holes. Applicants should check whether planning permission is required with the relevant planning authority. Although, it should be noted that following the drilling and installation of heat pumps the ground can be returned to the previous state.
Archaeology
77. As the installation of ground source heat pumps will require the excavation of trenches or deep boreholes it is important to consider in advance whether archaeological remains exist on the development site and what the implications of the development might be. The needs of archaeology and development can usually be reconciled, and much potential conflict reduced. Further details on Archaeology can be found in PAN 42. Information on the location of scheduled monuments, listed buildings and other known archaeological sites is available on www.pastmap.org.uk
Contamination
78. Applicants should be aware that the construction or extraction of a borehole or well for the purpose of abstraction, or the abstraction or discharge to the water environment may require an authorisation from SEPA. Applicants should be advised to contact their local SEPA office for further details. Care should be taken when constructing boreholes to prevent contamination of the borehole itself and of the groundwater resource in general.
Water Source Heat Pumps
79. Water source heat pumps operate in a similar way to ground source heat pumps. The loop is submerged in water, typically a river or lake. They take heat from the water and convert it into a gas. The gas is condensed in a process which releases heat. This heat can be transferred to the heat distribution system in the building. The use of a water source such as a river or lake is likely to provide lower efficiencies due to the temperature of the source being more affected by the weather, but the advantage is the relatively cheaper installation cost achieved by avoiding any ground works.
Air Source Heat Pumps
80. Air source heat pumps, are often used in moderate climates, they use the difference in outdoor and indoor air temperatures to cool and heat the building. Air source heat pumps extract the heat in air and use a fan to draw air over coils that extract energy. This energy is then transferred to a home or building and used as part of a heating supply.
81. Although they are less efficient than ground source heat pumps, and likely to be more variable because air temperatures fluctuate both daily and seasonally. Even when the outside temperature drops, air source heat pumps can still produce 2-3 times as much energy as they use to run. However in cold weather the evaporator coil is likely to need defrosting. The air source heat pump does have advantages in terms of lower installation costs and the fact that no ground loop negates the need for trenching.
82. Air source heat pumps can be used for a wide variety of applications such as cooling for lofts, restaurant kitchens and hotel plant rooms where the hot water can easily be used for other applications. They can provide hot water using waste heat in the air. By using waste heat, they can also remove heat from an area, such as a loft space, where it is not needed.
83. Air source heat pumps can be located in the roof space or on the side of the building. They look like air conditioning boxes. Where these are proposed for listed buildings or in conservation areas, it will be important that they are sensitively sited.
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