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Figure 1: Reverse cycle heat pump – heating cycle
This CPD article reports on further developments in the technology, legislation and application of heat pump systems and equipment, particularly to the domestic housing market. There have been a number of heat pumprelated CPD articles over the last three years, which indicates the rate at which the industry is changing. References to these are provided at the end.
As an introduction, recent press releases have confirmed that heat pumps are a renewable technology. There has been debate querying the contribution of heat pumps to saving energy and climate change, but it is obvious that a device that harnesses energy from a renewable source, i.e. ground, air or water, is renewable. Following debate in the EU parliament surrounding the Directive for the Promotion of Energy from Renewable Sources (RES), all of Europe is committed to including heat pumps as a major tool in the introduction of renewables to reduce CO2 emissions. So heat pumps are now seen as part of renewable deployment and not solely as energy efficiency measures.
Another major development has been the formation of the Department of Energy and Climate Change (DECC), a unit dedicated to a more coherent view of energy policy. The energy activities of the Department for Business, Enterprise and Regulatory Reform (BERR) have been transferred to DECC, as has the Standard Assessment Procedure (SAP) programme – previously run by Defra and headed by Ed Milliband, government minister for energy and climate change. Further support has come from the European Partnership for Energy and the Environment (EPEE), the voice of the building services industry in Europe. The group is delighted that the use of aerothermal energy (energy in the air) and hydrothermal energy (energy stored in water) will now be promoted as part of the new RES policy.
For domestic users and charitable institutions in the UK, an installation using either ground or air source heat pumps is available with reduced VAT at five per cent. In addition, a ground source installation can attract a maximum £1,200 grant and an air source installation a maximum grant of £900, through the Low Carbon Building Programme. For organisations wishing to invest in commercial or industrial heat pump installations, an enhanced capital allowance is available for products that satisfy the requirements of the scheme. These products may be found listed on the Carbon Trust’s energy technology list at www.eca.gov.uk.
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Figure 2: Reverse cycle heat pump – cooling cycle
Heat pump principles
There are many definitions for a heat pump. However, the European standard EN14511 defines it as an encased assembly or assemblies designed as a unit to provide delivery of heat. It includes an electrically operated refrigeration system for heating. It can have means for cooling, circulating, cleaning and dehumidifying the air. The cooling is by means of reversing the refrigeration cycle. The majority of heat pump applications are electrically driven and based on the “vapour compression cycle”. However, heat driven systems are also available based on the absorption cycle and use gas, or waste heat, or solar/geothermal heat.
The principle of the vapour compression cycle is well documented but, for a reminder, look back to the March CIBSE Journal and the cpd programme – “Refrigeration – inside the box”. This referred to the cycle operating in the cooling mode – whereas, in heat pump mode, the condenser heat exchanger is the useful “heat output” component (the heat sink), whilst the evaporator is the “heat collection” component (heat source). The majority of heat pump applications in building services use a reverse cycle system that is able to heat or cool the space. Figures 1 and 2 show how this is achieved with the introduction of a four-way reversing valve.
Referring back to the March issue, perhaps the most significant section explains the efficiency and performance characteristics of the cycle. A heat pump coefficient of performance (COPh) is:
Useful heating duty (kW)/power input to the compressor (kW)
Today, COPh average values between 3 and 4 are achievable for air source heat pumps over the heating season. This means that, for every 1kW of electricity consumed by the heat pump unit, between 3 and 4kW of heating are produced. At this point the performance characteristic of a vapour compression cycle must be appreciated, whether operating in cooling or heating mode. Basically, the efficiency is a function of the pressure difference over which the heat pump operates. The greater the pressure difference, the lower the efficiency, and the two pressures in the system are determined by the heat source and heat sink temperatures – together with the refrigerant fluid used. This means that the lower the heat source temperature, say ambient air or the ground, the lower is the COPh and therefore the heat output is reduced. The higher the heat sink temperature, say for air or water heating, the lower the COPh. So to optimise maximum efficiency for any heat pump application, the refrigerant heat exchangers must have high heat transfer rates and the condensing and evaporating temperatures must be maintained at the minimum possible temperature difference. This is why high temperature heat sources, like waste heat extract, and low temperature heating systems, like underfloor heating, can give COPh values greater than 5.
Heat pump types
This is well documented now, but basically there are a number of heat sources used by heat pumps and these, in simplest terms, can be defined as air, ground and liquid. In the case of air, the source would normally be an ambient temperature; however, more sophisticated systems may use recovered exhaust air to drive the heat pump process where this is available. The ground as a heat source normally consists of coils of piping laid in a shallow trench or deep boreholes with a U-tube inserted in each bore. In each instance the piping forms a closed loop, containing an antifreeze solution, which exchanges heat with the ground. Open systems that take water directly from the ground can also be utilised – however, the requisite permits from the Environment Agency are required to operate a system such as this. Heat pumps that use liquid as a source will generally utilise water. This may be in the form of closed loop energy transfer systems or closed loops utilising man made ponds, rivers or lakes with heat exchangers embedded in the source water. Other high-energy liquid sources that can be used, include slurries and effluent.
The heat delivery methods divide into either heated water or heated air. Either of these can be supplied at medium temperatures (say 35ºC to 45ºC) for space heating requirements such as heater batteries or underfloor heating, or at high temperature for domestic hot water heating or for process drying. The highest temperatures that can normally be achieved with vapour compression cycle heat pumps is 60ºC to 65ºC, but most applications would suggest 55 degrees C as the economic maximum. When used for heating with radiators, the size of radiator will have to be increased to give the same heat output with the lower water temperature.
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Figure 3: Comparison heating methods and operating costs
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Figure 4: Comparison of heating methods: carbon emissions
Technology constantly improves, however. Figure 3 shows that, with a COPh between 3.0 and 3.6, heat pumps outperform all other types of heating shown and can be included in low-carbon heating solutions, along with biomass, CHP and solar thermal. Figure 4 also shows that heat pumps have lower CO2 emissions than other forms of heating.
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Figure 5: Progressive domestic housing: peak heat loads
These figures are based on air source heat pumps (ASHPs), which are marketed specifically for the domestic housing market. They are emerging as a primary low-carbon replacement technology in this market – as a stand-alone solution or in combination with other technologies. Figure 5 shows how the housing peak heat load has reduced significantly since the 1970s, such that a fourbedroom detached house today has a lower heat loss than a two- bedroom flat in 1970.
A new ASHP recently marketed towards domestic housing has the refrigerant circuit completely sealed, very similar to the domestic fridge and freezer. The unit includes a plate heat exchanger for the heat sink, or condenser, heating water that is piped to the heating application, e.g. radiators, underfloor heating or hot water cylinder. The unit also includes a four-way reversing valve, enabling a cooling mode in the summer. The advantagesof the sealed refrigerant circuit are improved reliability, installation by plumbers without specialist refrigerant qualification, and easyreplacement into the existing water system.
Case study
A retrofit of a four-bedroom house in Toddington, built in 2000, saw the house fitted with a non-condensing boiler rated at 23kW, supplying radiators with water at 70°C and with an output of 8.4kW. The hot water demand was 140 litres per day. The boiler was replacedwith an ASHP based on a house heat loss of 8kW at an ambient temperature of minus 3ºC. Some radiators were upgraded and were supplied with water at 55ºC to provide heating at 8.4kW. The average COPh over the 2007/8 winter was 3.5. The savings, compared to the gas boiler, were 48% less CO2 emissions and a 38 per cent reduction in running costs.
© Terry Welch