For the environment we need to minimise carbon emissions. Burning of fossil fuel (that's coal oil or gas) will release carbon dioxide into the atmosphere. A boiler burns fuel, and we can calculate the carbon emissions. A heat pump uses electricity which is generated in a power station. Many power stations burn coal, some burn oil, some burn gas. Nuclear power stations do not burn fossil fuel, so they do not release any carbon dioxide. Renewables such as hydroelectric and wind power do not release any undesirable substances, but the proportion of power generated from these sources is very small in UK.
Gas Boiler Heating
So, lets try and understand just how much CO2 is emitted when we heat a building using gas central heating. In order to do this we need to know the amount of carbon in the gas. Gas consists mainly of Methane - its chemical formula is CH4 . When it burns in the boiler it reacts with oxygen in the air to form carbon dioxide, CO2, and water, H2O. The amount of CO2 produced from combustion of a given quantity of gas is known.
It can be expressed in terms of the heat generated. For every kWh of heat 0.19kg of CO2 is liberated. If the building requires 15kW of heat and the boiler is 85% efficient, 17.5kW must be provided by burning gas, because 2.5kW goes up the flue.
Every hour requires the equivalent of 17.5kWh of gas to heat tthe building and so it's easy to calculate that this produces 3.3kg of CO2 every hour. ( Hold your mouse over the image)Quantity of CO2 Generated = 17.5 x 0.19 = 3.3kg
To find the amount of CO2 liberated when the heat pump is operated it is necessary to consider the emissions during the power generation process. There are several different generation methods used in the UK. For example, fossil fuel fired power stations, using coal, oil or gas. Each of these methods results in a certain amount of CO2 emissions. Nuclear generation has zero emissions. Use is also made of renewable sources, such as hydro-electric and more recently wind turbines and tidal flows and these have zero emissions.
It is difficult to know which type of power generation is used to power any particular heat pump, and so an average factor is used to encompass the mix of power sources in the UK. The factor used by the Carbon Trust is 0.43kg CO2 per kWh of electricity consumed.
A heat pump delivering 15kW of heat for a dwelling needs
5kW of electricity. Take this link to see why. Every hour the heat
pump uses 5kWh of electricity, and this gives
rise to 2.2 kg CO2
Quantity of CO2 Generated = 5 x 0.43 = 2.2kg