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that of the source so that heat can be absorbed. A compressor pressurises the vapour which passes to a heat exchanger (condenser) where it condenses at the higher temperature, releasing heat into the medium being heated. The machine effectively “pumps” heat from a low temperature to a higher temperature where it can be employed for heating purposes. It can be seen that by using work energy in the compressor the heat pump reverses the normal principle that heat flows from a high to a lower temperature.

As noted above the fluid must be compressed to provide the lift in temperature and this requires energy input to the compressor such as electricity for an electric motor drive. The energy delivered into the building or process comprises the free energy extracted from the low temperature source plus the majority of the energy input during the compression process. In fact a greater proportion of the amount of heat delivered is the free heat “pumped” to the useful temperature which means the heat pump has the benefit that it delivers more energy than the compressor input.

The most important performance characteristic of any heat pump is the ratio of the useful energy output to the energy supplied to the compressor. This is referred to as the Coefficient of Performance (CoP).

Heat pumps have been used for heating in buildings and in process applications for many years. The first

practical heat pump for building heating in the UK was installed during the 1930s, while the basic principle of the vapour compression heat pump cycle was first established as far back as the early 19th century.

Over the more recent past the market for comfort cooling units, which is essentially the same technology, has matured and technical development has improved performance and reliability, as well as reducing capital cost. Current interest in heat pumps is growing because the technology is regarded as a low carbon heating option and could play a part in the Government’s legal commitment to meet 15 per cent of the UK’s energy demand from renewable sources by 2020.

A report1 published 12 months ago by the ‘Low carbon Innovation Co-ordination Group’ concluded that air and ground source heat pumps are one of a number of technologies, in association with heat networks and storage, that could potentially be required to meet the demand for low carbon heat over the coming decades. The same report stated that although heat pumps can be regarded as a relatively mature and cost-effective technology they are not yet proven to be ready for broad adoption in the UK context.

A heat pump can be described as a machine which employs high-grade energy to extract low-grade or free heat from a relatively low temperature source. It then delivers it at a higher temperature. A heat pump is thermodynamically identical to the common refrigerator we use in our homes today. The principle difference

Heat Pump TechnologyBy Chris Burgess, director Alpha Management Solutions Ltd

ENERGY IN BUILDINGS & INDUSTRY 29

widely written about elsewhere (reference sources are suggested at the end of the module). However, if the basic principle of the heat pump is understood, the key characteristics and performance limitations of the technology become clear and this is best explained using the vapour compression heat pump as an example.

The starting point is the internal “working” fluid which circulates within the machine. When heat is added to the fluid it reaches a temperature at which it turns into a vapour. The amount of heat absorbed to evaporate the fluid (latent heat) is high compared to the amount required to raise its temperate to that point (sensible heat). If the pressure of the vapour is increased and then let to cool it condenses back to a liquid, releasing the latent heat, but because of the higher pressure it condenses at a higher temperature.

In any heat pump the fluid has to evaporate at a temperature below

Fundamentalsmodule 5 : heats pumps

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between the heat pump and the refrigerator is the role they play as far as the user is concerned. Refrigerators provide useful cooling, whereas the heat pump provides useful heat.

There are two key benefits of heat pumps as a technology. First, they can extract low-grade heat (i.e. low temperature and consequently considered to be free heat) and upgrade it to a temperature where it becomes useful for heating. Second, and more important, the amount of energy output is greater than the energy used to power the heat pump.

There are a number of different types of heat pump including those operating on the absorption principle and other designs such as the carbon dioxide cycle, but the vapour compression cycle is the most common.

Vapour compressionThis CPD module will not describe the detailed design of the various types of heat pump since this is

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35 45 55-15 2.2-7 2.652 3.84 3.287 4.39 3.69 3.18

Ambient temperature (DegC)

Heating flow temperature (DegC)

Table 1: Manufacturer's specified COP values at different ambient and central heating flow temperatures for a typical domestic air-source heat pump

To be precise it should be called the Coefficient of Performance heating (CoPh) because the performance of refrigeration plant is also defined by its CoP but this relates to the ratio of cooling out to energy in. As an example a vapour compression heat pump with a CoPh of 3.2 (at standard test conditions) and a 10kW motor drive will deliver 32kW of heat.

An important factor in the performance of any heat pump is the temperature lift because the CoP is heavily dependent upon it. The higher the temperature difference between the evaporator (source temperature) and the condenser (output temperature) the greater the pressure difference required, which means more energy is needed to compress the working fluid. This means for any given amount of heat delivered the energy input increases with increasing temperature lift, resulting in a reduction in CoP.

Heat pumps are available in a wide range of types and are often categorised by the heat source and delivery media. An air-to-air heat pump for instance, extracts heat from an air source and delivers heated air into the building.

Air to air electric drive vapour compression units are common for comfort cooling and in many cases the operation of these units can be reversed so that heating is available. In the heating mode, heat is extracted from ambient air drawn across the outdoor heat exchanger or evaporator.

Ground source heat pumps use a closed pipe-work loop of antifreeze solution buried in the ground, either vertically via boreholes or horizontally in trenches about one metre below the surface.

Closed or open loopHeat pumps can also be installed in either a closed or an open loop configuration extracting heat from an open water source, river or ground aquifer, but generally these are less popular. In effect ground source heat pumps can also be categorised as water source heat pumps.

A gas engine can be used to replace the electric motor drive for the compressor. The heat output is supplemented by that recovered from the engine cooling water and exhaust

ambient temperature as the heating flow temperature increases.

The COP measured in the laboratory is useful but in reality the source temperature, for example with an air source heat pump, varies as the ambient temperature varies and so the machine operates over a range of temperature lifts. This means a measure of the seasonal COP, called the Seasonal Performance Factor, is more useful but in looking at such data it is important to understand what electricity inputs and heat outputs are included in the figure.

In recent years the seasonal CoP performance of electric motor vapour compression heat pumps has been improved through the application of inverter drives to the compressor resulting in improved CoPs under part load conditions.

A report2 published in May 2013 by the Energy Saving Trust provides comprehensive results from Phase II of detailed monitoring of a large sample domestic heat pump (electric vapour compression) installations. For example the average measured space heating efficiency was 2.73 for air-to-air heat pumps and 3.21 for ground source heat pumps. Space heating efficiency is defined in the report as the ratio of heat output to electricity input, taking into account the electricity consumption of the heat pump unit and the necessary fans and pumps.

Monitoring showed that on average the electricity consumed by the compressor accounted for 82% of the total electricity demand for each installation with the remaining 18% for fans, pumps and auxiliary space and water heating. The report also showed that the energy consumed to defrost the evaporator coils of air source heat pumps could be significant and that minor modification to plant configurations and controls can result in significant improvements in season CoP.

The underlying consideration in the application of heat pumps is to minimise the temperature difference between the heat source and the medium being heated. The best performance is, therefore, achieved using the warmest available source and the lowest distribution temperature.

Air source heat pumps are the most

gases to increase overall heat output and efficiency.

Heat pumps based on the absorption principle are also available. These comprise a sealed thermodynamic circuit, normally containing an ammonia water solution that acts as the working fluid, with ammonia being the refrigerant and water the absorbent. This type of heat pump is driven by the input of thermal energy, either waste heat or that derived from natural gas or LPG. The input energy source has a lower cost than electricity but the CoP is much lower.

The majority of vapour compression heat pumps use organic working fluids. Modern electric heat pumps typically use new fluorinated refrigerants (such as the HFCs R407C and R410A) that, although less damaging than the previously used CFCs, still have a significant global warming potential (GWP). GWP is a measure of the impact of the refrigerant compared with carbon dioxide in the context of climate change.

Alternative working fluids such as propane, ammonia and carbon dioxide have low ozone depletion potential and zero or low GWP. Manufacturers are developing new heat pump technology to use these less damaging fluids. A key benefit of absorption heat pumps is, therefore, that they use fluids which are more environmentally friendly but capital costs are higher than electric driven heat pumps.

As already noted the coefficient of performance is a critical performance measure for heat pumps. It is determined by laboratory testing at defined source and delivery temperatures such as 7oC ambient and 45oC flow temperature (BS EN 14511-2:2011 sets out test conditions for heat pumps).

Unless the temperatures at which the CoP is measured are quoted, the value is meaningless. Table 1 shows the CoP values for a typical air to water domestic heat pump – note how at a 45oC flow temperature the CoP drops away as the ambient temperature gets colder or at 7oC

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Air source Ground source Nat gas Gas oilEnergy input per unit heat output (kWh) 0.37 0.31 1.14 1.18Cost per unit heat output (p/kWh) 4.40 3.74 3.41 6.94CO2e emission kg/kWh of heat output 0.16 0.14 0.21 0.32

Heat pump BoilerFactorTable 2: Comparison of heat pump and boiler performance

Assumptions:Heat pump seasonal efficiency: air source = 2.73, ground source = 3.21Boiler seasonal efficiency: gas condensing = 88%, oil = 85%Energy costs: electricity 12p/pWh, natural gas 3.0 p/kWh, oil 5.9p/kWh Carbon dioxide equivalent emission factor: electricity 0.4455kg/kWh, natural gas 0.184kg/kWh, oil 0.2718kg/kWh

common and tend to be the cheapest and easiest to install. However, ambient air as a heat source is not ideal because as the temperature falls the heat pump CoP reduces; resulting in reduced heat output at a time when the building heat demand is increasing. The need to defrost the external evaporator also adversely impacts upon overall efficiency.

Ground source heat pumps are becoming more popular because of the stable source temperature, around 10°C, just below the ground surface which means the temperature lift is lower and the COP higher compared to air source units. However, the seasonal performance of ground source heat pumps is dependent upon the type of soil and the moisture content which affects the heat transfer to the coils.

Specialised heat sourcesThere are also many specialised heat sources that can be used for heat pumps, for example industrial process waste warm water or extract ventilation air. As an example; heat pumps have been popular in swimming pool buildings where air recirculation coupled with the dehumidification achieved by the heat pump significantly reduces energy demand.

In general heat pumps are not ideally suited to conventional radiator heating because of the high flow temperature required but low temperature convectors and under floor heating are good options, especially for new building installations. Air-to-air units and heat pumps integrated into air heating systems have an advantage in this respect but the condenser still needs to operate at around 50oC to make

provides two key financial incentive schemes:

Heat pumps that meet high performance criteria are listed on the Energy Technology List (ETL). Any company paying corporation tax can receive a tax benefit under the Enhanced Capital Allowance Scheme if it installs a heat pump (or other energy saying product) from the ETL.

The Renewable Heat Incentive (RHI) is another financial incentive scheme available for the non-domestic sector. It rewards the use of renewable energy to heat buildings. There are two phases: one for the non-domestic sector (industrial, commercial, public sector and community organisations) and the second, to be introduced in 2014, to include residential homes. The RHI offers quarterly ‘tariff’ payments over seven years for eligible renewable heating projects.

Ground and water source heat pumps are currently eligible for the RHI provided that they meet a minimum standard seasonal COP. Air source heat pumps are not currently eligible but it looks like they will be from Phase II to be launched in 2014.

With increasing pressure being applied to the UK heating sector to deliver solutions with higher efficiencies and a lower carbon footprint, heat pumps have the potential to provide part of the solution.

References:Technology Innovation Needs Assessment (TINA)- Heat Summary Report available at http://www.carbontrust.com/media/190042/tina-heat-summary-report.pdf

Energy Saving Trust - Detailed analysis from the second phase of the Energy Saving Trust’s heat pump field trial available at www.gov.uk/government/publications/analysis-from-the-first-phase-of-the-energy-saving-trust-s-heat-pump-field-trial

Further reading:Carbon Trust - How to implement guide on air source heat pumps available at http://www.carbontrust.com/media/147466/j8058_ctl151_how_to_implement_guide_on_air_source_heat_pumps_aw.pdf

Carbon Trust - How to implement guide on ground source heat pumps available at http://www.carbontrust.com/media/147462/j8057_ctl150_how_to_implement_guide_on_ground_source_heat_pumps_aw__interactive.pdf

Heat Pump Association at www.heatpumps.org.uk

ENERGY IN BUILDINGS & INDUSTRY 31

air heating effective. For a given heating duty and

when correctly applied, a heat pump should consume less energy and produce lower carbon dioxide emissions than a conventional natural gas or oil heating arrangement, as shown in Table 2. It can be seen that the heat pump has a much lower emissions level than very efficient gas or oil-fired heating.

However, the economic benefits of heat pumps depend on the relative price of electricity compared with fossil fuels. The price of energy varies widely between different consumer types but those below are a realistic comparison.

Heat pumps have a high capital cost compared with a standard boiler and radiator installation, particularly ground source units because of the additional cost of the ground works. However, the following circumstances can make heat pumps

more economically attractive:• relatively high price for fuel

compared with electricity• new buildings where low

temperature heating systems such as under-floor heating can be applied

• buildings where air heating can be applied

• buildings where summer cooling is required making the reverse cycle heat pump more viable.

In view of the relatively high investment cost of heat pumps it is essential that the heat demand in the building is minimised first in order to reduce the size and capital cost of heat pump needed.

The UK Government recognises the contribution that heat pumps can make to meeting CO2 reduction targets and is aware of the barriers to the wider implementation of the technology. To support the installation of heat pumps in the non-domestic sector the Government

10.2013 EIBIFundamentalsmodule 5 : heat pumps

Series 11

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The Education Department, Energy in Buildings & Industry,P. O. Box 825, GuIlDFORD, Gu4 8WQ

Energy in Buildings and Industry and the Energy Institute are delighted to have teamed up to bring you this Continuing Professional Development initiative.

This is the fifth module in the eleventh series and focuses on heat pumps. It is accompanied by a set of multiple-choice questions. To qualify for a

CPD certificate readers must submit at least eight of the ten sets of questions from this series of modules to Energy in Buildings and Industry for the Energy Institute to mark. Anyone achieving at least eight out of ten correct answers on eight separate articles qualifies for an Energy Institute CPD certificate. This can be obtained, on successful completion of the course and notification by the Energy Institute, for a fee of £15 (for members) or £25 (for non-members).

The articles, written by a qualified member of the Energy Institute, will appeal to those new to energy management and those with more experience of the subject.

The five modules to come in this series will focus on: thermal imaging; boilers and burners; data centre management; utility purchasing; and monitoring and targeting.

The first four modules from this series (examining the Renewable Heat Incentive, the Green Deal, underfloor heating, and voltage optimisation) as well as previous modules are available free of charge on CD. Send your request to mark.thrower@btinternet.com. Alternatively, they can be downloaded from the EiBI website:www.energyzine.co.uk.

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Please mark your answers on the sheet below by placing a cross in the box next to the correct answer. Only mark one box for each question. You may find it helpful to mark the answers in pencil first before filling in the final answers in ink. Once you have completed the answer sheet in ink, return it to the address below. Photocopies are acceptable.

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heat pumpssErIEs 11 module 5: quesTions

32 ENERGY IN BUILDINGS & INDUSTRY

1. Coefficient of Performance is a measure of? nThe heat pumps electricity consumption nThe heat output relative to the heat transferred from the source nThe total heat output relative to the compressor energy input nThe heat transferred from the source relative to the energy input

2. The Government is promoting heat pumps because: nThe technology is little used in the UK nThey use electricity rather than fossil fuels nThey have low carbon emissions nThey help to conserve gas supplies

3. Under test conditions an electric heat pump has a COP of 3.5 and delivers 31.5kW of heat. What is the energy input in kW ?

n3.5 n9.0 n15.7 n31.5

4. Which is not one of the four basic components of a vapour compression heat pump? nEvaporator nAir movement fan nCondenser nCompressor

5. Heat pumps provide low cost heat because: nThere is a small difference between the source and output temperatures nElectrical equipment is efficient nOperating temperatures are low nThe heat source has no economic value.

6. The ‘free’ energy input to a heat pump is extracted by the: nCondenser nExpansion valve nEvaporator nCompressor

7. A key benefit of an absorption heat pump is? nHigh COP nLow capital cost compared with air-to-air heat pumps nThe working fluid is more environmentally friendly than the new fluorinated refrigerants nThey use electricity as the power input.

8. An electric motor heat pump will have the best return on the investment when: nGas and electricity prices are high nGas price is low and the electricity price is high nGas and electricity prices are low nGas price is high and the electricity price is low

9. An essential feature of heat pump system design to maximise efficiency is? nThe temperature of the heat demand must be less than about 60°C nThere must be a nearby source of water nA high grade energy source is required nThe difference between source and output temperatures should be minimised.

10. The seasonal performance of motor driven heat pumps has improved due to: nImproved working fluids nDevelopment of absorption heat pumps nA warming climate nThe application of inverter drives.

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10.2013 EIBIFundamentalsmodule 5 : heat pumps

Series 11

Taking the first steps in energyThe Energy Institute is now offering energy managers the chance to obtain an introductory energy management qualification online.

level 1: Certificate in Energy Management Essentials: E-learning includes ten modules, each taking between five and eight hours each, concluding with assessment through exam.

Delegates can complete at their own pace with support from expert tutors. Modules can either be completed as part of the full course or individually. Successful delegates will receive the Energy Institute Level 1 - Certificate in Energy Management Essentials.

Full course will be available from July, but the introduction module, “Introduction to energy management – building an energy management process” is available now free of charge.

Please visit www.energyinst.org/e-learning for more details or to register for the free introduction or contact the training team on:

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