Heat Pumps Factsheet
A heat pump is a device which moves heat energy from one place to another while raising it from a lower to a higher temperature. A domestic refrigerator is a heat pump. Heat is removed from the contents (the source) and discharged out the back into the surrounding air (the sink). In heating applications, heat is removed from ambient air, water, soil or bedrock and delivered to the building where it is needed. In cooling applications, the reverse happens and heat is removed from the building to be discharged to the ambient air, water, soil or rock.
There are three main parts to any heat pump system:
- A heat source and the means of extracting heat.
- The circuit of working fluid within the heat pump itself and a power source.
- 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.
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. The gas is then compressed, raising its temperature. Later in the cycle, the working fluid condenses to liquid as heat is released to where it is needed.
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 low temperature radiators. Underfloor heating is considered to be the most energy-efficient of these methods.
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.
What are the various possible heat sources?
Ambient heat from water, air, or the ground, or waste heat from industrial processes or combined heat and power units are usually used.
In air-source heat pumps (ASHP), external air at ambient temperature is passed over a finned heat exchanger, whereby the air heat is extracted into the evaporator of the heat pump. In a water source heat pump (WSHP), heat is extracted from river, lake or ground water which cools. For ground source heat pumps (GSHP) – see images below – the collector pipe is installed in either trenches or boreholes. A brine solution is pumped around this loop of plastic pipe, extracting heat from the ground. In the UK, below 10m depth, the temperature of the ground is fairly consistent throughout the year at around 11ºC.
The heat extracted from the ground or water is replaced by heat from the sun, and collectors must be sized accurately to allow for seasonal recovery.
Heat pump efficiencies
Energy is needed to activate the heat pump cycle and to compress the vapour for the production of useful heat. The efficiency of this process is expressed by the ratio of the useful heat delivered to the driving energy used by the compressor. This ratio is called the Coefficient of Performance (COP).
As environmental heat is free and available in very large quantities, it is not included in the COP. That is why the COP is always larger than 1. The COP of the current generation of heat pumps varies from 2.5 to 5. Since the COP shows performance at a steady state only, a second parameter is usually used to show the performance of the heat pump over an entire year. It is called the seasonal performance factor (SPF), which is the ratio of annually delivered useful heat over annually used driving energy. When calculating the SPF, it is common to include the annual electricity requirements of auxiliary equipment, such as circulation pumps, fans, etc.
Stoves – the traditional stove or room heater can range from 2 - 15kW, have efficiencies of up to 90% and burn either logs or pellets. They are normally utilised for individual room applications and tend to work in conjunction with conventional heating systems, although in a well-insulated house there is no reason not to utilise two or three well placed stoves for all the heating requirements. They can also be fitted with a back boiler to also provide hot water.
Therefore, the performance of a heat pump system is affected by several factors, which include:
- The climate (annual heating and cooling demand and peak loads);
- The temperature of the heat source and the heating distribution system;
- The auxiliary energy consumption;
- The heat pump control.
The performance of heat pumps should be balanced by the fact that the efficiency of electricity generation in the UK is less than 35%. That means that for every unit of electricity used, more than 2.5 units of primary energy (mix of gas, coal, nuclear etc.) have been used. Therefore, a heat pump with a COP of 4 driven by electricity generated by a thermal power plant has a primary energy efficiency of 160%.
That is already better than the 90%+ achieved by a modern gas condensing boiler operating at low temperature, but you can increase the primary energy efficiency of your heat pump, and therefore its environmental benefit, by a factor of three by driving it with green electricity from a PV array or wind turbine.
ASHPs achieve 10-30% lower SPFs than GSHPs or WSHPs. This is mainly due to the rapid fall in capacity and performance with decreasing outdoor temperatures.
Heat pumps are best utilised in buildings with a 'wet' underfloor heating system throughout. This is due to heat pumps operating at their most efficient when the heat distribution requires low grade heat with temperatures in the region of 30 - 35ºC. The better insulated the building, the lower the flow temperature required to the underfloor heating system. The lower the output flow temperature from the heat pump, the higher the efficiency of the heat pump - and the lower the running costs.
Radiators can be connected to heat pumps, but it is not recommended unless they are specific low-temperature radiators (which are normally larger than standard versions), although a ground floor underfloor system and upstairs radiator system in domestic dwellings should be fine. This is because standard radiators typically require a high flow temperature of 50 ºC or more, which will require more energy to run the heat pump and seriously undermines the economic and environmental benefits of the installation. Additionally, radiators cause heat pumps to cycle frequently, which means that they start several times per hour, reducing their life span.
Ducted air as a heat distribution system tied to a heat pump is popular in North America, and can be used effectively in the UK, especially if cooling is required. However, it is not as efficient as underfloor, and can be noisy and physically intrusive.
Many GSHPs can provide cooling as well as heating, although this option normally costs extra. It is possible to provide a small amount of cooling using an underfloor heating system, but not generally in homes because the floor area is too small and the response time too long. Cooling with air using fan coils is ideal; there are some hybrid systems that use underfloor heating in the winter and fan coil cooling in the summer, both connected to the heat pump, with a 3-port changeover valve between the two.
In large buildings, several individual heat pumps can be placed in different zones and each can be sized to meet the needs of the space it conditions. Some zones of the building may need heating at the same time as other zones need cooling. When properly integrated, a heat pump system can recover excess heat in one zone (sunny side, computer rooms, etc.) and transfer it via a water pipe loop to areas of the building requiring heating. It is therefore possible to achieve a balance between heating and cooling needs during a good part of the year.
Sizing a system
The heat pump should be sized to meet the full space-heating losses from the building, unless the building is very large in which case sizing for base load with additional capacity for peak requirements may be a better option. The better insulated the building, the smaller the heat pump required.
As an example, any building which obtained Building Regulations approval after April 1st, 2002 will have a figure of 50 watts per square metre of peak heating requirement. Therefore an estimate of the size of GSHP required for an average UK home can be roughly calculated thus - 160 sq m @ 50 watts per sq m = 8 kW). However, this is just a rule-of-thumb guide, and more accurate figures should be sought from building services professionals.
If using the ground as a heat source the collectors are currently sized as follows: 10 metres length of slinky trench (this will be longer for standard trenches) for every 1 kW of heat delivered from the heat pump, while for vertical systems, one 70 metre borehole should deliver between 3 and 5 kW of heat delivered from the heat pump.
A typical heating-only installation for a medium sized, new build detached house would have a GSHP of 8 kW and need at least two narrow trenches, each 300 mm wide, 40 metres long and 1.8 metres deep. The trenches can be straight or curved and laid in any direction to suit the site, providing they are always a minimum of 5 metres apart. A standard excavator, such as the type used to dig conventional foundations and footings, can dig the trenches and backfill them after the ground loops have been installed. Once completed, and the ground loops pressure tested and buried, the system can basically be forgotten, although its location does need to be recorded to avoid accidentally digging it up!
Operation & maintenance
GSHPs and WSHPs should be able to provide heating throughout the coldest parts of the year. An ASHP may require some back up heating for the infrequent very cold periods occasionally felt in the UK.
It is also important to be aware that underfloor heating systems are not as responsive as traditional radiator based systems and take longer to bring a space up to the temperature required. In most instances this should not be a problem if adequate controls are specified as part of the installation package.
There are no annual maintenance requirements for a heat pump, which is one of the advantages over boiler-based systems. Heat pumps tend to have life expectancies of 20 - 25 years, compared to 15 years for gas condensing boilers. The ground loops tend to have 50 - 100 year lifespan.
The installed cost of a GSHP for a professional installation ranges from about £1000-£1,600 per kW of peak heat output, excluding the cost of the distribution system. Trench systems tend to be at the lower end of this range. Installed cost of a typical 8kW (domestic) system would be around £10,000 plus the cost of the distribution system. It is however site dependent and would increase significantly if a borehole is required.
Air Source Heat pumps (ASHPs) have no collector system and are the cheapest option. A 5 kW ASHP will cost around £5,250 and a 12 kW version about £6,000 + 5% VAT installed. This does not include the cost of a heat distribution system.
WSHPs tend to be slightly cheaper as no trenching or boreholing is required; the collectors will be mounted onto a rack and dropped into the water.
ASHPs (see image below) have no collector system and are the cheapest option. A 6 kW ASHP will cost around £3,500 and a 12 kW version about £6,000 + 5% VAT installed. This does not include the cost of a heat distribution system.
The above are merely general examples. Larger systems will need to more dedicated design work, making it more difficult to provide generic cost data.
In terms of running costs, heat pumps compete with all conventional fuels and are particularly good against electric heating, LPG and oil.
Planning permission is rarely required, unless an open loop (which involves taking water direct from a river, lake or underground store) or borehole system is specified, in which case the Environment Agency (EA) should be consulted regarding ground water contamination (but the statutory status of this is still unclear).
- UK Heat Pump Network - www.heatpumpnet.org.uk - aims to assist the UK industry to develop according to best practice on environmental and economic grounds.
- The Heat Pump Association (HPA) - www.feta.co.uk/hpa/ - A leading authority on the use and benefits of heat pump technology and includes many of the country's leading manufacturers of heat pumps, components and associated equipment.