Ground source heat pumps are likely to work at their optimal under the following conditions:
• Where the pump is powered by electricity from renewable sources.
• Where there is a low temperature differential between the ground and the output (eg underfloor heating)
• Where mains gas is unavailable.
• Where high levels of insulation have already been installed.
Appraising the possible value of installing a heat pump is difficult. At source is the confusion surrounding the veracity of the fundamental claim that heat pumps are a carbon-saving technology.
Heat pumps work by ‘multiplying’ the electrical energy input into heat energy output. The difference between the input and the output is called the ‘coefficient of performance’ or ‘COP’ commonly expressed as a single digit eg ‘3’ or ‘4’.
Taken in isolation the argument for converting electrical energy into multiples of heat energy stands up; But problems begin when the heat pump’s performance is measured in the context of other heat generators and energy-efficiency measures in terms of both carbon-saving and capital investment.
A likely scenario will be the comparison between providing heat either from a mains gas powered boiler or from a ground source heat pump. The simplest argument will be that carbon emissions will result from burning gas in the boiler whereas there will be no emissions resulting from ‘pumping’ heat from the ground. However, the more detailed argument will be that, yes, emissions will occur using mains gas heating, but they will also result from generating the electricity at the (probably) fossil-fuelled power station needed to power the heat pump. It is argued that in generating one unit of energy, a gas boiler emits around a third of carbon than the equivalent electricity grid unit – a ratio of 1:3. Since most heat pumps operate in an overall like-for-like COP range of 2 – 3, the current likelihood is that mains gas fuelled condensing boilers will produce a smaller carbon footprint.
Under different scenarios where the building is beyond gas mains and where boilers might be fuelled by LPG or oil and the heat pumps powered all or in-part by renewable electricity (eg solar PV), the balance of the argument will shift towards heat pumps becoming more viable in terms of their carbon footprint.
Future power supply scenarios are not considered here, but it is probable that if the UK progresses with a planned increase in the proportion of non-fossil fuel generated grid electricity, the carbon emissions balance between heat pumps and m
Space and water heating and COP
A further factor involved in the comparison equation is that of the relative efficiencies heat pumps usually achieve when applied to space and domestic hot water heating.
Heat pumps work best at producing large amounts of low grade energy rather than small amounts of high grade energy. The COP resulting from generating high temperature water sufficient to supply radiators or DHW can fall to as little as 2 - 2.5, whereas the COP usually achieved in producing the low flow temperature outputs associated with underfloor space heating is likely to be in excess of 3 and sometimes in excess of 4.
Heat pumps are not cheap, coming-in at around £1,000 per kW capacity. Compared with mains gas fuelled boiler running a radiator and DHW system, a GSHP combined with underfloor heating system can be around three times more expensive. If a designer is working from a strictly controlled budget, it might be considered that the potential difference in costs would be better employed in upgrading the building fabric to higher grades of thermal efficiency.
Ground Source Heat Pump (GSHP) technology
The more usual ‘closed loop’ GSHP installation comprises of plastic piping buried in the ground and connected to a heat pump. A water or water-antifreeze mixture is passed around the looped pipe where it absorbs heat from the ground. The fluid flows into an electrically powered heat pump, comprising a compressor and a pair of heat exchangers before discharging back to the underground loop.
Heated water from the compressor heat exchanger can be :
• used directly to produce space heating in an underfloor system,
• or used indirectly via the main hot water storage tank as part of a ‘Combisystem’.
Locating the loop
A key consideration in planning the installation of a ground source heat pump is the location and format of the loop. The most cost efficient format is the ground loop which involves the excavation of shallow trenches and the laying of either straight or ‘slinky’ pipes at a depth of between 1.5 and 2m. Ground loops will require space around the building, but where this is unavailable, the alternative might be to consider boreholes. Boreholes are dug to a depth of around 150m and the loop lowered into it. Though more expensive to execute, the borehole format provides a more stable temperature at the greater depths than the ground loop method.
• The design should be prepared by or in conjunction with a specialist engineer.
• Feasibility: site survey – check that there is their sufficient space to accommodate a GSHP – both external loop space (typcially around 350m2 for an average house using a horizontal ground loop) and internal equipment and storage space.
• Loop design
• Confirmation of suitable ground conditions (Lithology)
• Determine whether GSHP is to provide space heating and / or DHW
• If the GSHP is to be used in conjunction with underfloor heating, ensure that the heating system is designed to use low water temperatures.
• If the GSHP is to be used in conjunction with DHW, ensure that the heat-exchange coil in the hot water cylinder is adequately sized. Be aware that ‘Thermal stores’ might be incompatible due to their higher storage temperature requirement.
• Calculate heat demand based on heat loss, occupancy and occupation patterns.
• If GSHP is the main form of heating, consider a back-up boiler provision in the event of extreme temperatures.
• Heat pumps can be noisy – design accordingly.
• Be aware of capital and maintenance costs
• Ensure that the manufacturer / installer provides a long-term warranty and that insurance can be made available.
• When storing water, ensure that it will be maintained at a temperature of at least 65o to kill legionella bacteria.
• Heat pump manufacturers are very prone to exaggerated COP claims. When considering a system, insist on verifiable COP expectations.
Air Source Heat Pumps (ASHP)
• Air source heat pumps are less common than their ground source cousins. Though they work in a similar fashion, their heat source is the outside air rather than the ground. They are also much cheaper.
• There are two types of air-source heating systems. Air-to-air systems provide warm air, which is circulated to heat the building. Air-to-water systems heat water to provide heating to a space heating system and or a DHW system.
• Though similar in operation, an ASHP is likely to have a lower COP than the ground source equivalent, particularly in winter when the heat input is most needed. The air temperature in winter is much lower than a ground temperature that fluctuates little during the year (typically between 8 – 12o in England and Wales). The winter air temperature results in a considerable differential between the source temperature and the output temperature which means that the pump becomes far less efficient.
• 'Geothermal Heat Pumps: A Guide for Planning and Installing', Karl Ochsner.
• Domestic Heating Design Guide, CIBSE, 2007
Ground Source Heat Pumps
• BS EN 15450:2007. Heating systems in buildings. Design of heat pump heating systems
• BS EN 814:1997 (3 parts) Air conditioners and heat pumps with electrically driven compressors.
• BS EN 14511:2007 (3 parts). Air conditioners, liquid chilling packages and heat pumps with electrical driven compressors for space heating and cooling.
• BS EN 378:2000 (4 parts) Specification for refrigerating systems and heat pumps – Safety and environmental requirements
Other useful standards
• BS 8211-1:1988. Energy efficiency in housing. Code of practice for energy efficient refurbishment of housing
• BS EN 832:2000. Thermal performance of buildings. Calculation of energy use for heating. Residential buildings
• BS EN ISO 13790:2008. Energy performance of buildings. Calculation of energy use for space heating and cooling
• BS EN 15316-1:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. General
• BS EN 15316-2-1:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-3:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-4:2007 (6 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15459:2007. Energy performance of buildings. Economic evaluation procedure for energy systems in buildings
• BS EN 15377-3:2007. Heating systems in buildings. Design of embedded water based surface heating and cooling systems. Optimizing for use of renewable energy sources.
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