A heat pump is a device, which moves heat energy from one place to another and from a lower to a higher temperature. A domestic refrigerator is a heat pump. Heat is removed from the contents (the source) and discharged elsewhere (the sink). In heating applications, heat is removed from ambient air, or water, soil or bedrock) and delivered to where it is needed. In cooling applications, the reverse happens and heat is removed, to be discharged to the ambient air, water, soil or rock.
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Heat pumps use a little energy (usually in the form of electricity) to move available energy as heat from A to B. For every unit of energy purchased as electricity, several units of heat are delivered. So, relating the energy purchased to the energy delivered, heat pumps can be 300% or 400% efficient.
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There are three main parts of any heat pump system:
1. A heat source and the means of extracting heat,
2. The circuit of working fluid within the heat pump itself and a power source,
3. A distribution system to deliver the energy in the required form.
The heat source can be the ambient air, water, soil or rock. The outside heat exchanger (the collector) transfers energy as heat to the circuit of working fluid within the heat pump itself. It is preferable, in terms of maximising efficiency, to have constant temperature differences between (a) the source and the working fluid and (b) the working fluid and the sink, but this often not possible in some heat pump models.
The distribution system takes the heat from the heat pump (often as hot water) and delivers it to the end-use. Heat can be distributed within a building using underfloor pipes, fan coil units, an air handling system, or wall-mounted radiators.
It is necessary to include a power supply to the compressor to drive the heat pump. It is usually an electric motor which drives the compressor, but a gas engine is used in some designs.
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A heat pump works by driving a working fluid around a refrigeration circuit containing four elements; (1) evaporator, (2) compressor, (3) condenser and (4) expansion valve.
The working fluid changes from liquid to gas (evaporates) as heat is absorbed from the heat source. Later in the cycle, the working fluid condenses to liquid as heat is released to where it is needed.
A heat pump can be used for cooling with the addition of a reversing valve that reverses the direction of the working fluid and so the direction of the heat transfer. The central component of the heat pump is the compressor. This is usually driven by an electric motor, although gas engine driven compressors are also available.
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A heat pump can be used where there is a low temperature source of heat. For example, heat can be transferred from a source at 5ºC and delivered as heated water at 45 to 50ºC.
Applications include space heating and cooling, pre-heating domestic hot water, heat recovery and dehumidification in both domestic and industrial sectors.
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Yes, the source is usually renewable energy from an ambient heat source or waste energy. For example, with a 3:1 performance ratio, for every three units of heat delivered, two units can be from the renewable heat source and one from the electrical power supply. A heat pump, operating on a ‘green electricity’ supply from an accredited renewable source, offers emissions-free heating and then three units of renewable energy would be delivered for every one unit of energy purchased as ‘green electricity’.
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Ambient heat from water, air, or the ground; or waste heat from industrial processes or combined heat and power units are often used.
In air-source heat pumps, external air at ambient temperature is cooled by passing over a finned heat exchanger, thus heat is extracted into the evaporator of the heat pump. In a water source heat pump, river, lake or ground water is cooled as heat is extracted.
In the case of ground source heat pumps, the collector pipe is installed in one or more trenches or boreholes. A water/anti-freeze solution is pumped around this loop of plastic pipe, extracting heat from the ground.
The heat extracted from the ground or lake or river water is replaced by heat from the atmosphere.
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Energy Efficient: Heat pumps are very energy efficient as they consume a little energy (usually as electricity) in order to deliver three or four times as much energy as heat. The ratio of energy-out to energy-in depends on the operating conditions and typically varies between 2 to 1 and 5 to 1. The overall average is called the seasonal performance factor.
Environmental: Heat pumps access renewable or waste energy and so displace consumption of conventional fossil fuels (gas, oil, coal). As electricity generation technologies improve, the emissions performance from the combustion of fossil fuels and renewable electricity generating capacity increases, so the greenhouse gas emissions associated with electricity consumption are reducing – making heat pumps even more environmentally beneficial.
Physical: Different types of heat pumps have different benefits. Ground source heat pumps have -
• No visible external equipment
• No external fans
• Quiet running
• Very little risk of accidental damage
• Adaptable to many locations.
Economics of heating houses: The running costs of a heat pump can be less than those of a traditional gas boiler heating system and definitely less than some other forms of heating (LPG, oil, electricity). The initial capital cost is usually higher than other conventional heating systems. The ‘whole-life’ cost, combining the capital and running costs, can be favourable for heat pumps compared to fossil fuelled systems – and, especially, compared with other forms of electric heating.
In addition to providing domestic heating, heat pump systems are used in commercial premises (offices, hotels, supermarkets) – often providing heating and cooling – and also in some industrial processes and applications.
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Typical costs of a Ground Source Heat Pump for an owner-occupier in a small, new house in the UK:
Gas Heating system
| Capital
cost Gas consumption per year Gas consumption per year CO2 (at 0.1876 kg/kWh) |
£2,500 |
Ground Heat Pump (incl. auxiliary and DHW heating)*
| Capital
cost Electricity consumption/year Electricity consumption/year CO2 (at 0.43 kg/kWh) |
£4,500 |
All-electric heating (storage heaters and panel heaters)*
| Capital
cost Electricity consumption/year Electricity consumption/year CO2 (at 0.43 kg/kWh) |
£2,500 |
* based on detailed monitoring (1997/99); DHW is Domestic Hot Water.
In areas with no mains gas, where liquefied petroleum gas (LPG) and oil are options, the capital cost would be higher to include an on-site fuel tank and the running costs and CO2 emissions would also be higher than using mains gas.
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Heat pump systems consume energy in moving heat from one place to another. The principal use of energy in a heat pump is to generate the motive power to drive the compressor. The associated emissions depend on the power source driving the compressor. In the UK the heat pump will typically create around 55% of the greenhouse gas emissions from the most energy efficient domestic gas heating system (where the heat pump is providing space and domestic hot water heating). This is a considerable improvement.
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Technological improvements in the reliability of heat pump components have addressed the operating problems of the early heat pump designs. The ground loop pipes, or collectors, use materials and jointing developed in the oil, water and gas industries. Ground collectors are now believed to have a reliable operating life of 40+ years.
Similarly, the technology to install ground collectors has improved. This, coupled with better understanding of heating, cooling and heat recovery, has made ground-source heat pumps an environmentally attractive and practical option.
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Ground-source heat pumps can be used with collector pipes in horizontal trenches or in boreholes. The area of available open ground adjacent to the property may limit the use of trenching. Collector pipes in boreholes require much less open ground than trenching for the same size of heat pump; however, drilling is more expensive than trenching. In either installation, there is nothing visible above ground. Air-source heat pumps can be located in the roof space.
If the heat pump is to be part of an industrial process, e.g. for heat recovery, then any restrictions might be site specific and would require consideration in the system design. Industrial heat pumps tend to be of bespoke design for specific applications.
If the difference in temperature between the source and the sink is too great, then this will limit the operational performance of the heat pump. The smaller the temperature difference, the better is the performance.
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No; the UK Heat Pump Network is a technical interest group for anyone interested in using heat pumps. For heat pump suppliers in the UK, you should look in our on-line directory of providers of products and services. Look for the Directory on the Home Page.
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Yes; look elsewhere on this website for links, case study materials and references.
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Sorry, you will have to contact your supplier or one of the experts listed in our Directory. Look for the Directory on the Home Page.
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You may find what you seek in the case study publications listed elsewhere on this site. Alternatively, there is a lot of complementary material on the IEA Heat Pump Centre website. www.heatpumpcentre.org
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The UK Heat Pump Network produces periodic newsletters. Also other related heat pump publications and reports are publicised by the Network when these become available. You can register free on-line as a member of the Network. Also you should visit the IEA Heat Pump Centre website for international news, research reports and technical articles. www.heatpumpcentre.org
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Yes; if you have an interest in heat pumps as a potential user, installer, researcher or manufacturer, then you can register free on-line as a member of the UK Heat Pump Network.
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