A Sankey diagram showing an old traditional ‘inefficient’ thermal power station coupled to a heat pump.
ENVIRONMENTAL ISSUES
Heat pumps have for a long time been presented as an environmental method of heating. They are classed as a low-carbon technology and are often considered as a ‘renewable’ form of energy. Amongst environmentalists, their credentials have not always been viewed positively; indeed, in Denmark during the 1980s they were considered undesirable by many, since their adoption would increase electricity demand and this could shift the country towards nuclear power. Whereas their neighbours in Sweden embraced the technology due to their plentiful hydro-electricity, which was far better utilized powering heat pumps than the normal electric fire.
Unfortunately, we live in a commercial climate that likes to gloss-up the case for its particular product. This is far from a new phenomenon in our society. You don’t have to look far to find an advert claiming ‘free energy’ from a heat pump. This is sort of true, but only the same way that ‘buy one, get one free’ operates in a supermarket because you have to pay before you get your free offer. The hype also stimulates sceptics, so that personal opinions about it can vary from ‘it’s brilliant’ to ‘it’s a scam’.
In reality, the application it is used in dictates how good it is, and the relative benefits of heat pumps can be assessed by considering and comparing them with various other heating methods. That said, since the CO2 pollution from electricity is currently halving every ten years, then the environmental case for heat pumps strengthens yearly.
Judging the net worth of heat pumps has always seemed an intangible topic. For the immediate future, one might weigh up the immediate environmental impact with respect to CO2 emissions. But in the long-term, and if we are to get our act together, we must inevitably find alternatives to fossil fuels and develop low-carbon methods of power generation. Electricity is bound to be the energy source of the future. We therefore need to develop and improve heat-pump technology so that it is ready to take its place as one of the many methods to help satisfy our energy-hungry ways of life.
Opinions of the long-term future can vary greatly, but for now, carbon dioxide seems to be the most important environmental factor to consider, and heat pumps are almost invariably driven by electricity, which can potentially be ‘dirty’ to produce. This must be considered if a balanced view is to be achieved.
The ‘Sankey’ diagram is a very useful way of illustrating energy flows. The width of the thick lines relates to the quantity of energy.
A Sankey diagram showing an old traditional ‘inefficient’ thermal power station coupled to a heat pump.
A Sankey diagram showing efficient fossil fuel generators with heat recovery, coupled to heat pumps.
The simple Sankey diagram shows both the power station and a heat pump. As can be seen, in this simple example, only one-third of the primary fuel is converted to electricity and two-thirds of the energy is lost up the cooling tower. A heat pump, however (with a COP of 3), uses electricity to capture ambient energy. This can give a useful heat output that is only equal to the primary calorific value of the power station’s fuel.
The values used in this old example are, in many ways, not very impressive, since using simpler technology, we may be able to burn the primary fuel directly (in a household boiler) at an efficiency of up to 90 per cent, so the heat-pump example is only slightly better than burning the fuel directly in your home.
Obviously the efficiencies of both the power station and the heat pump can vary, so let us look at a best-case scenario (opposite). This illustrates advanced combined-cycle electricity generation and a heat pump with a COP of 4. Furthermore, some of the heat from the power station is recovered directly for district heating. The total useful heat in this example is over twice that of the calorific value of the primary fuel.
The good news is that from 2011 to 2020, the generation of UK electricity changed a lot. Use of coal has all but gone, and solar and wind are now supplying as much as we dared hope. All in all, the CO2 pollution produced has halved in these nine years. This means that we are now closer to the second diagram than the first older one.
The value for carbon dioxide produced by electricity generation varies over the 24 hour period and also over the seasons. The real-time ‘grid carbon mix’ or ‘carbon intensity’ can be viewed by an internet search. It is interesting to note that the carbon intensity is often lower at night. This is in part due to the fact that the nuclear base load (which cannot easily be turned down) becomes the bigger proportion of the total.
The UK grid mix around 2020. (Ofgem.)
The current average value is somewhere around 0.25 kg CO2/kWh. This figure will vary greatly depending on the amount of solar and wind, and also on the nation’s usage. It is tempting to focus on best-case days, but in reality, your heat pump will do most of its running at times when the carbon intensity is not at its best. The government’s standard assessment procedure (SAP) for energy rating of dwellings updates its figures as it tries to keep up with the changes. This should be a representative figure to work from.
All predictions suggest that the CO2 emissions caused by electricity generation will decline considerably in the future, so it may be more rational to assume a lower figure, as it will no doubt be even ‘cleaner’ in years to come. Any equipment installed today should be running well into the future.
The topic of ‘green’ tariffs can be bewildering. The notion of ‘bestowing’ your electricity payment to a ‘clean’ provider could be questionable and arguably could result in others having to buy a greater percentage of ‘dirty’ electricity. On the other hand, if you purchase electricity from a venture that only supports renewables, then it is easy to argue that such ventures would not happen without your money. Anyhow, it seems reasonable to think that ‘green’ tariffs are beneficial, but clearly some are better than others.
As time passes, we know that fossil fuels will become scarcer and the percentage of electricity provided by renewable generation methods will increase. It is now certain that electricity will be the dominant energy of the future, at which point heat pumps will come into their own. Even heat pumps with a low COP would still be advantageous.
As already discussed, the carbon intensity varies over the 24-hour period and tends to be lowest between around midnight and 4am. Maintaining sufficient capacity to meet the nation’s need is a challenge for those managing the grid, so any levelling-off of the peaks and troughs is welcome. For this reason, cheaper night-time tariffs were introduced to encourage night use and possibly to reduce the use of peak-time electric heaters. The Economy 7 tariff gives a seven-hour period at night at reduced rate and this is typically used in heavy storage heaters that accumulate heat in bricks during the night and emit this heat during the day.
One of the disadvantages of this system is that the storage is rather crude and, inevitably, more heat is given out at night than is needed. Furthermore, the storage bricks run out of heat by the evening, so in reality, your house follows a 24-hour temperature profile that does not exactly match your needs. This can result in an overly warm breakfast time, if the house is to be warm enough for the evening. Obviously, the type of house is an important factor here.
There are systems that store heat more effectively, and types using large water tanks have been around for many years. Heat can be drawn from these, as and when needed.
If a heat pump is to use off-peak electricity, it would need to be large enough to provide a whole day’s heat in only 7 hours so it would need to be significantly bigger and hence more costly to install.
Furthermore, if heat is to be stored, it would need to be at a considerably higher temperature than normal, therefore reducing the efficiency (COP). If stored in a water tank, the tank would need to be very big, so it would at first seem an impractical proposition to use off-peak electricity with a heat pump.
However, rather than using only the off-peak period, it would be possible to simply shift as much running-time as possible to the night period. Remember, for much of the year, the heat requirements are only a fraction of the winter peak.
Part-peak/off-peak use can often stack-up on cost-saving grounds, since off-peak electricity is significantly cheaper (be mindful that peak units may be more expensive than normal flat-rate tariffs).
The 7-hour tariff is not best suited to heat pumps; however, there are other periods of the day that could be considered ‘low-peak’. For this reason, some 10-hour tariffs are available that are split into three times: night, mid-afternoon and mid-evening. Whilst this ‘off-peak’ rate may not be as favourable as the night-only tariffs, these tariffs may be more compatible with a heat-pump system.
Operating these tariffs can require a higher level of control sophistication and possibly some user adaptability, so it is not always plain-sailing. One has to be aware of what is coming on when. Also, judgements may be needed relating to the necessity of operating during more expensive ‘peak times’. Such issues may be seen as an opportunity to some, or a hassle to others.
Unlike electric storage heating, the off-peak heat-pump is storing some heat in the fabric of the building. Solid, stone walls and under-floor heating have much greater storage capacity, whereas rooms with internal insulation have little ‘mass’ to hold the heat (buffer cylinders store only a very small amount of heat). It is interesting to think that all the little bits of heat being stored in millions of houses during off-peak times could mitigate the need for a power station or two.
The off-peak issue will no doubt be tackled with increased sophistication in the future with smart metering. There are various methods that could be adopted; for example, increasing the tariff as the national load increases, or automatically dropping-off loads (like fridges and heat pumps) that would not mind a brief ‘enforced’ rest at peak times. These sophisticated methods make economic sense to the supply companies, since this can reduce the maximum demand on the national grid.
Fuels can be rated by the CO2 emissions emitted whilst in use. The graph below shows values for the delivered fuel and also the net value as it’s used. The difference between the figures relates to the efficiency of the appliance.
Pollution of various fuels.
Electric heaters are 100 per cent efficient at point of use, since all electricity ends up as useful heat. Modern boilers are rated at about 90 per cent efficient (SEDBUK database). However, according to a Carbon Trust study, the value could be 4 to 5 per cent less when in actual use. (A similar reduction could be expected with heat pumps and this may account for the difference between appliance COP ratings and measured results.)
It is worth mentioning hot-water production at this point, since losses from pipe runs and cylinders can be very considerable. Boiler or heat pump, the DHW function can be less efficient than expected. Point-of-use electric heaters do not suffer this drawback
Out of all of the piped fossil fuels, mains gas is the best option. Methane (CH4) is a relatively clean-burning fuel, being the most ‘hydrogen rich’ and ‘carbon light’ of the hydrocarbon fuels. There are also other atmospheric pollutants that burning fuel produces, but these are much less for gas than those of oil and wood.
Whilst the figures given for liquid and gas fuels will be fairly constant, it is more difficult to give a figure for wood. If wood is gathered locally in rural areas, it could be seen as ‘carbon neutral’. On the other hand, does one need to re-grow a tree to claim that accolade? Wood chip and wood pellets involve transportation and processing techniques that affect the figures dramatically. Our figures reflect this more recent and realistic thinking. This has shifted opinion, suggesting that wood may not be ideal for certain situations, such as use in urban areas.
A question frequently asked is: what SPF (SCOP or average annual COP) must be achieved to make a worthwhile environmental benefit? It is relatively easy to work out where the two values are equal (break-even), but if a saving is required, then this is more a matter of judgement to say by what magnitude the improvement should be.
Table of pollution of fuels taken from BRE – SAP.
The graph below is very useful for comparing heat pumps with conventional heating systems. The left-hand vertical scale shows figures for electric heaters (equivalent to COP1). It also shows values for gas and oil heating. The horizontal scale shows heat pumps with progressively better SPF; a SPF of 2 simply halves the electricity values, SPF 4 quarters it and so on.
As an example: compared to the current electricity figures of 0.25, a heat pump would need to have an SPF of less than 1.5 to break-even with gas, so one might consider that a SPF or SCOP of 3 would show a very significant benefit.
As can be seen, electricity is currently ‘cleaner’ than oil, so replacing an oil boiler for a heat pump gives a very big improvement.
If we look into the future, we expect at some time be looking at an electricity figure of 0.1. This makes a heat pump look far more favourable, and will surely bolster the viability of heat pumps for almost any heating situation.
Every heat pump (and fridge) contains a fluid to make it work. To be efficient, this fluid (the refrigerant) must have certain physical properties. Back in the 1980s, most heat pumps used CFC or HCFC refrigerants as the heat-transfer fluid. These are now known to cause serious damage to the ozone layer if released, due to the fact that they linger for many years in the upper atmosphere. Some very early heat-pumps could have caused more environmental damage than they saved, since they were notoriously ‘leaky’ of their working fluid.
Bottle of HFC refrigerant and recovery pump.
Things have progressed dramatically since then. Systems rarely leak throughout their lifespan since joints are all welded. Gaskets are a thing of the past. Furthermore, current refrigerants no longer affect the ozone layer.
Unfortunately, the most common and the safest refrigerants have another property that is now in the forefront of our concerns. These gasses have a ‘global warming potential’ (GWP). This is a factor compared to CO2’s rating of 1. Many commonly used refrigerants have GWPs of several thousand times that of CO2. That fact sounds frightening, but heat pumps only hold about one litre or so of the stuff, so this is small compared to the tonnes of CO2 that are produced when heating the building.
The chart above lists the environmental information for refrigerants. As can be seen, the global warming potential (GWP) for CO2 is rated at 1. The ozone depletion potential (ODP) for obsolete refrigerant R12 is rated at 1.
Let us consider a typical modern household scenario to assess the carbon savings by using a heat pump and to balance this with the risk of a refrigerant leak. As can be seen, one loss of refrigerant would do the same harm as operating the heat pump for around one year.
The graph below attempts to evaluate the CO2 savings if a heat pump is fitted to replace a conventional heating method. The time period considered is ten years and the SPF assumed to be 3. If one leak were to occur during this period, the CO2 saving would be reduced by the amount as shown by the difference between the two blocks. For gas, the CO2 saving would be halved due to one leak over this period.
Graph comparing loss of refrigerant over ten-year period.
Refrigerant leaks are very rare in a modern heat-pump and few heat pumps would need major repair work during their life. However, if this were necessary, most of the refrigerant would be recovered (it is a legal requirement and refrigerant is very expensive). Most of the refrigerant would also be recovered when a unit is scrapped.
So, for many situations, the harm caused by potential leaks would be small compared to the total CO2 numbers at stake.
Having seen that the potential impact of refrigerant loss is relatively small, it is not ideal for the planet having tonnes of refrigerants contained within heat pumps (or fridges for that matter). As can be seen on the refrigerant chart, there are alternative refrigerants, which are often referred to as ‘natural’ refrigerants.
Hydrocarbons, like propane, are one such contender and are used in a few smaller units and some that live out in the open. Flammability may be an issue, but such issues need not be insurmountable. Carbon dioxide itself is a potential up-and-coming refrigerant. Ironically it is the very substance that it is trying to reduce. The amount sealed into a system is tiny and, if lost, would cause insignificant harm to the atmosphere. Vaillant are releasing an ASHP with Hydrocarbon R290 in 2020, and Mitsubishi have been supplying CO2 refrigerant units for a few years. Hopefully other manufacturers will join them in coming years.
The total equivalent warming impact (TEWI) is an established methodology used for evaluating the environmental impact for different refrigerants. The GWP values used are debatable since they are based on an arbitrary time horizon of 100 years. None the less, whatever figures are used, they illustrate that the impact due to the occasional refrigerant leak is relatively small.
