Global Warming Effects
Vast amounts of heat energy are used for heating our houses, and our initial focus should be to minimise the heat requirements by normal conservation measures like insulation. This is certainly the best and simplest first-step. Sadly, we cannot all live in super-insulated eco-houses, so significant amounts of heat energy are still required for many building. That said, never lose sight of the advantages of minimising your energy needs.
In the UK the vast majority of homes are heated by natural gas (e.g. North Sea) since this has been the cheapest method. Remote locations where mains gas is unavailable are commonly served with Oil or LPG (Liquefied petroleum gas). Electricity may also be used for direct heating, but generally, even off-peak electric heating is an expensive method to use.
Electricity is generated nationally by a ‘mix’ of methods, namely; Gas, coal, nuclear and renewables. Each country will have a different mix. A figure should be available to gauge the net-pollution of electricity at any one time. This is available from various sites e.g. this Ecotricity page
The current average 2017 UK value is about 0.4 kg CO2 produced per kWh of electricity.
The process of generating electricity is inefficient. It is more energy-efficienct to burn the fuel directly in a high-efficiency boiler. Electricity is therefore not a good form of energy for heating. However, the energy-advantages of heat pumps can, in most cases, more than compensate for this, and makes them score well with respect to CO2 emissions.
Note: The assumption that you can sign up to ‘green’ electricity and become ‘carbon neutral’ has not been made here since electricity all comes from the same ‘pot’. However, it is clearly an advantage to buy electricity from a genuine supporter of lower-carbon generated electricity.
The nation’s mix of electricity-generation methods results in different levels of CO2 pollution. Coal power stations are not good, modern gas power stations are much better, hydro and large scale wind power produce almost no CO2 (even when all the energy to build them is accounted for).
The official average figure used for UK electricity generation has varied quite a bit. It has been as low as 0.422kg CO2 released per kWh of electricity generated. It then rise to just over 0.5 . However, our use of coal has dropped and renewables are now having an effect. I believe that we are back to a figure of about 0.42 kg CO2 /kWhe again. However, it is predicted that electricity will get cleaner in the future, and these figures will reduce over the lifetime of a heat pump installed now.
The graph above compares the carbon dioxide emissions of common heating systems (gas, oil and electricity) to that of heat pump systems. The COP (efficiency) for a typical Ground Source heat pump in an older building with radiators is around 3, but in a well-designed underfloor new-build the efficiency can be 4. As can be seen, currently, a heat pump with COP of 4 is twice as good as a gas system. The figures for wood burning seems to be a very contentious issue. At best, locally sourced scavenged wood in rural areas should be in the region of 0.2 kg CO2/kWhe, and are an ideal companion to a heat pump system.
Notes and assumptions.
- Electricity generated at 0.42 kg CO2/ kWhe.
- Gas condensing boiler efficiency 88%
- Oil boiler efficiency 87%
- LPG condensing boiler efficiency 88%
Electricity 100% efficient at point of use. Off-peak use is less efficient due to warmer average house temperature, however, electricity is generated ‘cleaner’ at night.
As can be seen, the carbon dioxide contribution from a good heat pump system can be 1/2 that of oil or LPG. Direct electric heating, especially storage heaters, scores very badly with respect to global warming.
Wood burning is likely to show the lowest CO2 figures but this depends on how it is burnt, and where the wood is sourced from. Locally collected logs could score fairly well, but processed pellets and chips require energy to make. Currently some wood fuels are imported.
Even though the UK’s MCS Heat Pump Standard does not allow a design to use electric ‘back-up’, some heat pumps incorporate such a heater to cope with the coldest periods in the winter. This is more likely the case with air-source systems since the period of highest heat-demand also corresponds with the time when there is minimum heat available in the air. Therefore, on the coldest day the electricity consumption for some heat pumps could increase many-fold, putting a strain on the electricity supply grid. It would be better to use boilers, or ideally, wood stoves as a back-up. It should however be noted that the total annual contribution by top-up heaters can be surprisingly small if it is used and set up correctly.
The COP (coefficient of performance) of a heat pump is the ratio of input to output.
Fuel figures SAP 2009.
The Graph below might be useful when assessing the benefit of running a heat pump. (it’s not as complicated as it looks!)
The vertical Y scale on the left shows CO2 pollution per kWh of useful heat (allowing for losses). The two horizontal lines show figures for mains gas and oil heating. The curves are for heat pumps with COP’s up to 5. Direct electric heating has a COP of 1, and this is shown on the left.
The UK government figure for CO2 pollution caused by electricity generation has been 0.422 (kg co2/kWh). It rose around SAP 2009 at 0.517 (kg co2/kWh). The actual present 2017 figure is around 0.42, and may tend to be high in mid-winter when coal power stations are brought on-line to meet increased electricity demand, and lower at night when the percentage provided by nuclear is high.
Given current figures of 0.42, it is necessary to have a COP of only about 1.5 to equal the pollution caused by oil heating. A COP of say 3 gives a worthwhile advantage.
You can use this graph to decide if it’s better to run your air source heat pump or your gas boiler depending on your systems COP as it drops in mid winter.
The blue line represents the future as the electric grid becomes more de-carbonised. At this point, heat pumps will come into their own.
Click below for some amazingly useful tools that gives actual UK generation CO2 figure