Battery energy storage systems are on track to save 1.4 million tonnes of CO2 in 2024. This offsets total power sector carbon emissions by 4%, double the figure from 2023.
If you want to read about where these emissions savings have come from and how they’ve changed over the last few years, you can read the 2024 ‘Carbon benefit of batteries’ article here. This article covers how these savings were calculated and the assumptions made.
How do batteries reduce emissions?
Batteries save carbon through three different sources:
- Energy actions - batteries save emissions directly through their energy actions, by importing low-carbon energy and exporting it when demand is high. These are the only savings that come directly from the energy batteries are importing and exporting.
- Frequency response services—They perform dynamic frequency response services, which help keep the grid frequency at 50Hz. This means fewer carbon-emitting plants are used for Mandatory Frequency Response (MFR), a service with the same purpose.
- Inertia management savings - Batteries provide fast-acting post-fault frequency response which means the grid is less reliant on gas generation to maintain high inertia, which also helps maintain grid frequency.
So let’s run through the underlying methodology behind how these carbon savings are calculated.
Direct energy actions using a marginal carbon intensity
The National Electricity System Operator (NESO) publishes the average carbon intensity of the grid for each settlement period. When we published our previous ‘Carbon benefit of batteries’ article, we used this data to calculate the direct impact of battery energy actions on emissions. However, this method can result in an underestimate of how much carbon batteries save.
This is because when a battery exports power, it replaces the marginally priced power plant, rather than an average of all generators. This is often a carbon-emitting plant, like a gas turbine. These plants will only run if the wholesale price is above their short-run marginal cost (SRMC). This means we can use the wholesale price as an approximation of the carbon intensity of the marginal power plant.
We base this marginal carbon intensity on three different power generators:
- Open Cycle Gas Turbines (OCGTs) - carbon intensity 651gCO2/kWh, and the most expensive to run.
- Combined Cycle Gas Turbines (CCGTs) - carbon intensity 391gCO2/kWh, less expensive to run.
- Renewable generators - zero carbon intensity, assumed to be always running.
An example of the calculation is shown below. At 6 p.m. the wholesale price is above the run cost of a typical CCGT and below that of an OCGT. So we interpolate between these two technologies based on the power price to return a carbon intensity of 507gCO2/kWh.
This results in a carbon intensity shape that mirrors the wholesale price and is more pronounced than the average carbon intensity.
We can then multiply this marginal carbon intensity by the physical output of batteries in Great Britain. This gives us the half-hourly carbon saved or emitted by batteries.
Emissions savings from reduced Mandatory Frequency Response
Batteries performing dynamic frequency response allows NESO to procure less Mandatory Frequency Response. This service is slower than dynamic frequency response services, therefore, it’s less efficient in providing the same response.
So NESO needs to procure more Mandatory Frequency Response to have the same effect as Dynamic Frequency Response. For example, Dynamic Containment Low can displace up to three times the equivalent volume of Mandatory Frequency Response.
MFR emissions avoided are split into two parts - delivery and efficiency
MFR delivery savings come from batteries performing low-frequency response. This means batteries increase output to maintain frequency rather than CCGTs, which would emit carbon. While batteries perform high-frequency response, too, which has the inverse effect, these don't quite cancel out. This is because the highest-value service—Dynamic Containment Low—is also the largest service. This means it offsets 50kg of carbon for every MWh procured.
MWh of frequency response doesn't refer to actual energy throughput in each service. It means if a battery provides 1 MW of Dynamic Containment Low for an hour, this saves 50kg of carbon.
Another consequence of CCGTs providing Mandatory Frequency Response is a reduction in overall operating efficiency. Often, plants delivering the service have to reduce their power output to create headroom for providing low-frequency MFR, which lowers their fuel efficiency.
To balance energy in the system, another plant must increase its generation to compensate (often a less efficient CCGT). This results in a reduction in average efficiency across these two plants - and an increase in the carbon intensity of their total generation (which is greater than the volume of MFR provided).
This is assumed to only happen for generators providing low-frequency MFR. To provide high-frequency MFR generators don’t need to move from their desired output. This then means high-frequency services performed by batteries have no efficiency benefit.
As a result, when accounting for both the direct impact of delivery and the knock-on impact on efficiency, this gives the total carbon savings (or cost) per MWof each dynamic frequency response service.
Inertia management carbon savings
Inertia helps keep grid frequency stable when a power plant trips offline. NESO has to maintain a minimum level of inertia on the grid to ensure system stability. Grid inertia provided by the market can sometimes fall below this point.
The control room then has to turn down wind or interconnectors (which do not provide inertia) and turn up CCGTs to compensate - this has the effect of increasing grid emissions.
From 2022 to 2024, NESO have lowered their minimum inertia requirements from 140 to 120 GVAs. Grid inertia has spent much more time below 140 GVAs (the level CCGTs would usually increase inertia to) in the last two years.
By performing Dynamic Containment, batteries reduce the need for CCGTs to provide inertia. According to NESO, 30% of avoided inertia management is due to batteries performing the service.
To calculate the carbon emissions saved, we’ve taken the settlement periods where grid inertia has been below 140 GVAs, and assumed 35 MWh of CCGTs would have been needed to increase inertia by 1 GVAs.
This means we can calculate the carbon emissions that would have been produced to bring inertia up to 140 GVAs by using the carbon intensity of CCGTs. We then attribute 30% of this saving to battery energy storage.
