Example of a heat pump
Concentrate heat from the environment into your home. An electric pump is used to transfer thermal energy from a low temperature source (~5ºC), the ground or air, to a higher temperature sink (40ºC), typically a hot water tank. Ground source systems in the UK typically return 3 units of energy for every unit spent by the pump. Heat pumps are most suited to integration in new-builds as they work best with under floor heating, however, they can also be a compelling alternative in areas without gas.
The earth’s surface absorbs a large proportion of the incident solar radiation, which in the UK keeps the ground, and ground water, at a stable temperature of between 11-12°C all year round. The temperature of the earth is therefore higher than the mean air temperature in winter but lower than the mean air temperature in summer.
This is a readily accessible energy source for heating dwellings in the UK. A heat pump extracts solar heat from the ground by ‘moving’ heat from one place to another and from a lower temperature to a higher temperature, to heat dwellings. It is essentially the same technology (in reverse) as a conventional domestic refrigerator.
Ground source heat pumps (GSHP) therefore utilize stored solar heat, and do not generally use deep geothermal heat generated by the earth’s core.
There are three key elements to any GSHP system:
Ground heat exchanger
Heat distribution system.
Ground heat exchanger
The ground heat exchanger is a loop or coil of pipe that is buried in the ground. The loops can also be placed in water, see later under rural applications. A fluid consisting mainly of water mixed with antifreeze circulates through the loop and increases in temperature by absorbing heat from the surrounding soil.
The majority of domestic installations in the UK use an indirect circulation closed-loop system. A sealed loop of high density polyethylene pipe is buried horizontally or vertically in the ground, and a water/antifreeze mixture is pumped directly through it.
A more efficient alternative is a direct circulation or direct expansion (DX) system. In this system, an additional heat exchanger and pump are no longer required, since the refrigerant from the heat pump is circulated through a copper ground coil. Although a shorter ground loop can be used which reduces the cost of installation – the downsides are higher cost of the pipe material, the large amount of refrigerant required, protection of the copper pipes against corrosion and the risk of leaks from the refrigerant system. This system is rare in the UK.
The heat pump
A heat pump works by driving a working fluid or refrigerant around a circuit which comprises an evaporator, compressor, condenser and an expansion valve. As heat is absorbed by the heat source, the refrigerant changes its state by evaporation from a liquid to a gas. The refrigerant is now at a low temperature and pressure; it enters the compressor where both the temperature and pressure are increased as a result of work done on the refrigerant. The gas now enters the condenser
where the heat absorbed by the collector coil is released, to be used in the dwelling via the heat distribution system. The refrigerant, still in the form of a gas, but reduced in pressure and temperature, is throttled back further in the expansion valve before recommencing the cycle by absorbing more heat from the collector.
The compressor runs on electricity and consumes typically approximately one unit of electricity for every three units of heat energy produced. However, the total amount of heat energy delivered to the dwelling is equal to the energy required to run the heat pump plus the energy extracted from the soil. Therefore GSHP systems typically achieve overall efficiencies of between 200-400 per cent depending on the operating conditions.
The heat distribution system
A distribution system is needed to transfer the heat extracted from the ground by the heat pump into the dwelling. In the UK the heat is usually in the form of hot water, and is distributed around the dwelling by radiators or a low temperature underfloor heating system.
The difference in temperature between the heat source and the distribution system is a key factor in determining the overall efficiency of the heat pump. The smaller the difference, the higher the Coefficient of Performance (COP) will be.
This is why GSHP systems, with their more constant source temperatures, are generally more efficient than other forms of heat pump such as Air Source HP’s. It is also important to use the lowest possible temperature distribution system in the dwelling to maximise the COP.
Although efficiencies are constantly improving, claims of COP’s of 5 and above should be verified through monitoring of outputs by an independent body.
Suitability to rural environments
HP systems are at their most cost-effective where high levels of insulation have been achieved and mains gas is unavailable.
Where there is a low heating demand, underfloor heating and other low temperature heating distribution systems become possible options.
The choice between vertical or horizontal system depends on the land area available, local ground conditions and excavation costs. Horizontal collectors require a relatively large area of land for the trench. For a new-build medium detached house trenches are typically 1m wide x 1m deep and 60-120m in length with the pipe laid in a series of overlapping coils (slinkies). Alternatively it can be placed vertically in a narrow trench.
Horizontal collectors are therefore more suitable to rural applications where more land is often available.
New developments suitable for limited land availability include horizontal plastic panel collectors that are laid in the ground and connected together with polyethylene pipes.
In contrast, vertical collectors are inserted into boreholes (between 50m-100m deep) and are ideally suited to locations where ground space is limited. However boreholes are more expensive to install and are consequently less cost-effective. If multiple boreholes are needed the comparative cost comes down.
Water source heat collection.
In rural environments it may be possible to use an alternative heat source such as a spring, lake, flowing water or even an existing well to permit a smaller and therefore cheaper collector to be used.
Ground loops can be placed in large bodies of still water such as lakes, large ponds or deep wells. Calculations are necessary to ensure that the volume of water is sufficient so that freezing of the water source will not occur. The loops collect heat in the same way as ground source loops but the heat transfer is usually more effective and they are cheaper as there are no trenches to dig.
Where ground water is available, ie a spring, the spring water is used as a constant heat source passing through a small chamber in the ground and only short ground loops are needed to collect the heat from the water in the chamber. This is the most effective method of collecting heat.
Running water can be used by passing water from a stream through a small chamber next to the stream with ground loops connected back to the heat pump. As the water temperature is affected by the air temperature in winter, the results are less predictable than ground loops, but can be better than air source. Problems can occur where a stream receives direct snow melt water in the winter and the water source may not be suitable in this instance.
The initial capital cost of installing a ground source heat pump system is usually higher than other conventional central heating systems; a large proportion of the outlay will be for the purchase and installation of the ground collector. See table below for indicative system costs including installation and commissioning for a 9 kW system suitable for a 3 bed house. You will need to get a quotation for an installation to suit your property and individual requirements.
NB Costs based on a system from ‘Ice-energy’ current at June 2007
|System Type||Length of ground loops||Heat pump &Ground coil & installation cost||Costs to be met locally by purchaser|
|3 trenches 1m x 1m x 43m long separated by a 2m gap||£7,500 less a £1500 grant||Excavation of trench and backfilling. Covering pipes with 100mm of sand.
Connection of HP to central heating system.
|2 trenches 2m wide x 1m deep x 20m long OR
3 trenches 2m wide x 1m deep x 14m long separated by a 2m gap
|£9,500 less a £1500 grant||Excavation of trench and backfilling. Covering panels with 100mm of sand.
Connection of HP to central heating system.
|2 no. x 75m deep||£7,500 less a £1500 grant||Drilling boreholes and fitting pipes into the boreholes.|
The figures above do not take into account the cost of installing a suitable heat distribution system in the building as this will tend to be specific to the building. However, an additional cost of between 50-75 per cent of the above system figures can be expected. By increasing the insulation to roofs, walls and floors beyond the requirements of the building regulations, the heating requirement and therefore the total system cost (including the distribution system) can be reduced significantly.
The running costs of GSHP systems arise from electricity requirement for: (1) operating the compressor in the heat pump, and (2) the pump for the circulating fluid in the ground collector. To minimise costs, a system with a good coefficient of performance (COP) should be selected and the system operated on a dual tariff (which includes off-peak or economy seven etc.).
NB Coupling the GSHP system with PV modules or a small wind turbine is problematic as they do not provide a constant or sufficient source of electricity.
GSHP systems have annual running costs comparable with the most efficient of the currently available mains natural gas condensing boilers.
Servicing and maintenance costs are lower than for other types of heaters as systems generally need no regular servicing or safety checks (similar to a conventional domestic refrigerator), although specific requirements should be checked with the equipment manufacturer.
There are only two moving parts in the heat pump: a compressor, which is a sealed unit with a lifetime in excess of 15 years, and a circulating pump which is unlikely to be guaranteed for more than one year.
Ground loops made from polyethylene pipe should last for more than 50 years.
NB: The underfloor/radiator circuit also requires a circulating pump as with any wet CH system.
Air Source Heat Pumps
Air source heat pumps work in the same manner as a GSHP except that they do not need ground loops as they extract heat from the outside air. They look like the air conditioning units that you can see on the outside of office buildings.
Some ASHP’s have had problems of freezing in the UK’s damp winter weather although solutions to this problem are now being offered. Nevertheless it is advisable to get a guarantee to cover this aspect.
ASHP’s are much easier and cheaper to purchase and install than GSHP’s but they are not as efficient.
Table: Comparison of different types of heat pump. (1 is best, 3 is worst)
|Heat Pump Type||Ease of installation||Small land requirement||Purchase and installation costs||Efficiency & running costs|
Author – C Mather – June 2007
See our page on Financial support for energy saving and generation. The sections on that page relevant to this technology are those on the Renewable Heat Incentive and on the Green Homes Grant Scheme.
Have a look at our advice on choosing an installer if you plan to install this technology.
For local case studies that use ground source heat pumps and were written after the article above follow the links below
For local case studies that use air source heat pumps and were written after the article above follow the links below
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