In the last couple of articles we have looked at the various forms of electricity generation serving the UK grid, exploring the varying impacts on greenhouse emissions. One of the least understood but most important aspects of our electricity supply is the idea of marginal generation, and in particular its role in how we account for our individual and collective carbon impact.
Some of our generation requirement met by what we call baseload – this is the background level of demand that is generally satisfied by a combination of gas and nuclear generation. Nuclear power stations cannot easily be modulated so are well suited to this role; similarly, gas-fired generation generally has a minimum “always-on” output that needs to be maintained, whereas renewable energy is of course much more intermittent and in some respects more challenging to control.
Over the last 15 years or so, our overall grid carbon intensity has reduced considerably from around 500g CO2 per kWh to around 200g as we have transitioned away from coal and incorporated an increasing amount of renewable energy into the grid. However this headline figure only provides part of the story, as it does not represent the interaction of demand and supply in determining the impact of an individual action with regard to carbon emissions.
Consider the scenario where you are adding a new demand on to the grid: charging your EV, switching on your heat pump or inflating your girlfriend for an evening’s entertainment. In most parts of the country for most of the year, this extra demand will be met by turning up a gas-fired power station a little bit higher – this is because we are almost invariably already using all of the renewable energy we can get our hands on. As a result, we need to consider the carbon intensity of this so-called marginal generation rather than the grid average, and as we have seen these marginal emissions can be in the range of 350-400g/kWh for gas – around double the grid average.
Of course, there are occasional times when there is a surplus of renewable energy on the grid, and these will increase in frequency as we roll out more renewable capacity. Curtailment of excess renewable generation happens when there is insufficient available demand on the grid to soak up the available power, although at present this is more to do with the lack of capacity in the National Grid to get the surplus power (generally situated offshore or in the remote northern and western reaches of the UK) to the cities where it is most needed, rather than due to a lack of general demand. An analogy is trying to move modern rush-hour traffic around the country without motorways, the result being a complete logjam. Renewable energy generators are paid significant sums to curtail output of solar or wind farms when available demand does not match generation – money that ultimately comes from the public purse.
Smart tariffs like Octopus Agile now exist as one way of managing the mismatch in supply and demand, using variable and occasionally negative pricing (i.e. paying you to use power) to encourage consumers to take advantage of renewable energy generation that would otherwise be curtailed, although this is only likely to be partially effective for the reasons stated above. Other solutions being developed include the storage options discussed previously, however the capacity of the National Grid is generally the limiting factor at this time, and network upgrades are sorely needed as a key part of our transition to a low-carbon electricity supply.
At the moment, curtailment of renewable energy is much more the exception than the rule. The concept of marginal emissions therefore remains very important in the short- to medium-term, at least until we have a consistently available surplus of renewable energy and an effective system to balance supply and demand around the country – a situation that could be 10-15 years away. As a result we should be considering the marginal rather grid average emissions when it comes to calculating the impact of adding electrical demand to the grid. Unfortunately in many cases this is not happening – for example studies of the lifecycle impact of EVs do not always consider marginal emissions, significantly underestimating the in-use emissions, as well as potentially the manufacturing emissions which can greatly exceed those of ICE vehicles. This is often also the case with the arguments made for electrifying our heating as well. It remains true that heat pumps should have lower carbon emissions than a gas or oil boiler in all but the most inefficient installations, however many people mistakenly believe that any electric heating will be lower-carbon than gas.
Before anyone points it out (ok, maybe not!) there is also a philosophical perspective to these marginal emissions: why should I be the one to take the emissions hit when I turn on my EV charger and someone else claim the renewable energy for themselves as they “got there first”? Well that of course is one way of looking at it, however the other is that if we all share responsibility for that hit then the grid average increases incrementally with the same overall carbon output… and after all, the climate doesn’t care who emits that carbon.
The concept of marginal emissions also highlights the idea that decarbonisation is as much about the balance of supply versus demand as it is of the need to electrify our vehicles and home heating; indeed, the faster we increase the rate at which we roll out renewable energy and/or reduce our demand then the quicker we will head towards those all-important zero-carbon targets.