Electricity storage / Flexibility requirements in low-carbon power systems. 

The system value of electricity storage originates from its ability to increase the utilisation of power system assets like intermittent or inflexible generators and increase the penetration of low-carbon electricity. 


Figure 1 compares the findings of 28 studies across the US, EU, Germany and Great Britain regarding the required electricity storage power capacity in low-carbon power systems with increasing variable renewable energy. The capacity requirement is displayed relative to peak power demand. Most studies appear to agree that up to 70% VRE penetration can be achieved with storage capacity of 25% relative to peak demand. For up to 90% VRE penetration, results vary more strongly from just over 25% to up to 100% storage capacity relative to peak demand.


Figure 1 - Results from 28 studies of the future electricity system, comparing the amount of storage that gets built in the coming decades against the amount of energy from variable renewables. Installed storage capacities are divided by the peak demand for electricity in each region to account for the relative size of countries. Data from multiple sources as cited in the legend, in part compiled by Zerrahn et al., 2018 in "On the economics of electrical storage for variable renewable energy sources."

Figure 2 displays electricity storage requirements as a function of wind, solar and nuclear penetration as modelled by various studies for the future GB power system. The required absolute power capacity in a system with up to 90% low-carbon generation capacity could remain at the 3 GW installed in 2017 or increase to 35 GW. Similar for energy capacity as a function of low-carbon electricity penetration. At 90%, required capacity could be as low as today at 30 or up to 140 GWh.


The requirement range becomes more defined when accounting for peak and annual demand assumptions across the studies. While the current power system at 42% penetration of wind, solar and nuclear power has 5% electricity storage power capacity relative to peak demand, it could range from 5-20% at 60% and 5-40% at 80% penetration. These values are equivalent to 2.7 to 10 GW (60%) and 2.7 to 21 GW (80%) electricity storage power capacity at current peak demand.


Figure 2 - Electricity storage capacity requirements as a function of wind, solar and nuclear penetration in the GB power system. Top: Absolute electricity storage power (left) and energy capacity (right) as a function of wind, solar and nuclear power (left) and energy share (right). Bottom: Electricity storage power capacity relative to peak power demand (left) and energy capacity relative to annual electricity demand (right) as a function of wind, solar and nuclear power (left) and energy share (right).

A more comprehensive approach to assessing the system value potential of electricity storage in integrating low-carbon power is to analyse the overall flexibility capacity requirements, regardless of which technology provides them, as a function of wind, solar and nuclear power penetration (Figure 3, left panel). It shows that up to 40% penetration, less than 20% flexibility capacity relative to peak demand is required. This increases to a range of 40-100% above 80% penetration.

There seems to be a difference in the flexibility capacity requirements modelled in industry or government compared to academic studies (Figure 3, right panel). Academic studies identify less than 75% flexibility capacity required beyond 80% penetration compared to above 75% in the other studies. 

The linear regression of both data sets reveals two potential approaches for planning low-carbon power systems. The more conservative suggests that flexibility capacity is only needed once wind, solar and nuclear make up 30% of the generation portfolio and will then increase by 1.7% relative to peak demand with each additional 1% of low-carbon capacity. The less conservative approach suggests that no flexibility capacity below 17% wind, solar and nuclear power penetration and an increase of 0.8% of peak demand for each additional 1% low-carbon capacity in the power mix.


Figure 3 - Flexibility capacity requirements relative to peak demand for a future low-carbon GB power system. Left: Results from individual studies. Right: Differentiation of studies along commissioning institution. Trendline formulae in right panels displayed in respective colour code. Negative term in bracket denotes trendline intersection with x-axis.

These two approaches can be useful in planning low-carbon power systems to assess flexibility capacity requirements that could be fulfilled with electricity storage. This idea is implemented in Figure 4 as a “thought experiment” on the amount of flexibility capacity required globally if the power generation mix changes in line with projections made in the IPCC 1.5°C report to keep global average temperature increase below 2°C.


Figure 4 - “Thought experiment” on global flexibility capacity requirements. Global installed capacity based on IPCC 1.5°C report. Conservative and optimistic approaches reflect flexibility capacity requirements as identified in Figure 3. Red numbers indicate additional flexibility capacity required on top of projected capacities for hydro and oil-based generation and 2015 capacity levels for electricity storage, interconnection and demand-side response. In 2050, global annual electricity demand is modelled at 48,000 TWh, noncoincidental peak demand at 10,000 GW, total capacity at 15,700 GW with 2,000 GW hydro and oil-based generation. For comparison, 2015 values are 20,500 TWh (annual demand), 4,200 GW (peak), 5,500 GW (total capacity), 1,400 GW (hydro, oil). The result from a study by Jacobson et al. for a 100% wind, water and solar power based energy system for 139 countries is also displayed for comparison (peak: 11,800 GW; Hydro and ‘peaking/storage’ capacity: 7,060 GW)

Project in progress. First results have been published here: Dr Iain Staffell, Dr Malte Jansen, Professor Richard Green, Dr Rob Gross, Professor Tim Green and Dr Oliver Schmidt (2019). Drax Electric Insights Quarterly – Q3 2019. Drax Group. (Publication)