Grid Intensity

An Eccentric Anomaly: Ed Davies's Blog

There's a little group of people centred around the OpenEnergyMonitor project/products doing some very good work related to reducing carbon emissions over a wide range of scales from monitoring and control equipment that people can use in their own homes to supporting studies for CAT's Zero Carbon Britain project.

However, they do seem to me to have too much interest in the realtime carbon intensity of the UK grid (measured in kilograms of CO₂ emitted for each kilowatt hour of electricity generated). While looking at these figures can be educational, I think it's too easy for people to be mislead in a way which would actually increase emissions in the short term.

The carbon intensity of the UK grid varies as the generation mix varies. On a windy sunny day it's lower than on a still winter's night as the proportion of electricity coming from turbines and PV goes up and down.

Sometimes it's a bit difficult to account for the carbon emissions. E.g., power coming through the French interconnector predominantly comes from French nuclear generation so might be considered as low carbon. However, for every MW the UK takes, France has one less MW to export to other countries (e.g., Germany or Italy) where it would likely have displaced coal or gas generation so it can also be considered as quite high-intensity electricity.

Various sites use different rules of thumb to get an approximation to the current intensity:

I've had a number of on-line exchanges with various people about this, most recently on Twitter with Trystan Lea triggered by this tweet by Mark W Tebbutt:

@mwt2008: Thus is if I had charged my #NissanLeaf between 12 & 5pm today my Leaf would emit the equivalent to 31.5g CO2 /km!

My reply is not long but more than 140 characters.

Fundamentally, I think that what grid intensity figures you should use depends on what decisions you're making. Obviously long-term decisions like whether to install gas powered central heating or a heat pump or whether to buy an internal-combustion or a battery-electric car depend on projected average intensities over the lifetime of the devices whereas choosing when to run a dish washer or an immersion heater is more likely to be influenced by the real-time intensity.

Such decisions can also be sort-of political in that you might chose, for example, to install a heat-pump rather than a gas boiler in order that grid generation will be expanded on the assumption that this will mostly come from low-carbon sources (with the assumption that gas won't be low-carbon - not entirely certain, but likely). You might decide this even if the heat-pump appears in the short-term to produce a little more emissions than the gas boiler.

Where I think it falls down is making short-term decisions based on the real-time grid intensity in the way that, for example, the Earth Notes twitter account recommends:

@EarthOrgUK: National Grid CO2 intensity is high so don't run anything that can wait!


@EarthOrgUK: National Grid status GREEN: CO2 intensity is low so you can run your wash etc now!

The problem is that what matters is not the average intensity across the whole grid but the marginal intensity: the amount that emissions will increase as the supply is altered to match each extra kW of demand.

With the UK grid as it is now this will largely come from extra generation by gas-powered stations. Perhaps a bit of coal or biomass, perhaps a bit more through the interconnectors (but see the caveat above) but mostly gas. There can be other complications as Dominic Zapaman just tweeted:

@Zapaman: @ed_davies @mwt2008 @TrystanLea my estimate is that there may have been ~5-8% wind pwr curtailment (200-320MW) yesterday btw 12:00-16:00hrs

Assuming that's right, it's still not obvious if it's relevant. Just looking that the little graphs on Gridwatch gas generation (CCGT) started that period at just under 10 GW dropping to maybe 7 GW towards the end so any wind curtailment would have been due to local grid congestion rather than overall generation capacity. If your local distribution network connects to the grid near a curtailed wind farm (or there's such a wind farm embedded on your local distribution network) then, yes, charging your Nissan Leaf at that time would have been very low marginal intensity - essential CO₂ free.

More likely, though, the extra generation would have come from gas as it does most of the time.

The big problem arises if people are distracted by the current grid intensity into using more electricity than they need to. For example, somebody might use their dishwasher when the intensity is low even though it's not full yet thinking they're doing a good thing when actually the emissions would be lower if they left it until it was full and ran it even when the intensity is high.

Similarly, somebody might heat their hot water cylinder when the intensity is low incurring losses as heat leaks out when it would be better to wait until the water is about to be needed. If the leaked heat contributes to heating the home thereby displacing other heating (gas or oil, perhaps) this might not be so terrible but still the calculation should be done against the extra emissions from gas generation, not the instantaneous intensity.

In summary: