Cutting Electricity Consumption has Bigger Impact on CO2 Emissions than Previously Thought

by Jamie on November 21, 2010

Some interesting research was published over the summer (and reported in the Independent) which suggests that the impact of electricity savings on carbon emissions is a lot larger than previously thought.

It’s a little bit technical, but I’ll try and summarise it as clearly as I can – if you have any questions please post in the comments below.

This paper looks at the UK electricity grid’s carbon emissions factors over an 8 year period. An emissions factor is used to calculate the emissions generated by consuming a quantity of energy. It’s a very simple calculation:

Energy (kWh) x Carbon Factor (kgCO2/kWh) = CO2 Emissions (kgCO2)

By way of example, the average house consumes about 3,900kWh of electricity each year and the UK average electricity carbon factor is 0.543kgCO2/kWh so the CO2 emissions generated by electricity consumption in the average UK home amounts to 2,100kg or 2.1 tonnes of CO2.

In the above calculation I used the Average Emissions Factor which is published by Defra and is calculated by adding up all of the emissions generated by all of our power stations and dividing by the total amount of electricity generated (less 7% to take into account the losses which you get when you distribute the electricity through the national grid).

However there is another emissions factor which is the Marginal Emissions Factor. This is a measure of the emissions which are generated in response to a change in demand on the electricity grid and it’s this factor which was calculated to be about 60% higher than previously assumed and 30% higher than the average emissions factor.

In an electricity supply system, supply has to be precisely matched to demand so there is always a group of electricity generators and power stations which are being tweaked up and down or switched on and off in order to match the variations in demand over the course of a day.

If the grid wasn’t monitored 24/7 and supply wasn’t adjusted to match demand, it would lead to instabilities. If there’s too little supply it can lead to black outs and if there is too much supply it can lead to generators tripping and shutting down. A measure of this balance between supply and demand is the grid frequency which you can see live here.

There are three main types of generators in an electricity supply network – base load, peaking and load following. A base load generator is one that churns away at a constant rate for hours or days at a time, a peaking plant will be switched on just for the peak periods to cover the higher electricity demand, and a load following plant is quickly adjusted up and down to follow short term fluctuations in demand.

So when you turn on your washing machine or switch on your lights when you get home, somewhere a generator has to work a little bit harder to deliver that electricity to your home. If enough people turn on their washing machine at the same time then a whole new generator might have to be switched on and this is the marginal supply that the Marginal Emissions Factor refers to.

These marginal generators that respond to fluctuations in demand are in general less efficient and dirtier than the base load and peaking generators and this means that they have higher emissions.

So if you want to cut your carbon emissions easily and cheaply, really hit those electricity saving behavioural measures hard (I’ve written about standby and fridge freezer temperatures so far but there are many more to come) and make sure your lighting is as efficient as possible.

Also if you can manage it without annoying your neighbours, try and defer as much electricity consumption, especially energy intensive actions like running the washing machine, to the night time as both the average and marginal emission factors are lower at night than during the day.

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