Chapter 3

Energy Storage

Whilst the growth in variable generation over the last decade has increased the opportunity for energy storage in some roles, the wider capability of storage to improve the reliability and resilience of the whole system will be increasingly required at multiple time-scales and in different locations.

Storing energy is not an objective in itself, but it allows enhanced system operation, by providing a number of specific services, that are explored below. High-level ‘purposes’ of storing energy may be described as:

Geopolitical securityManage disruptions to imported energy.
Economic securityFuel bought in advance hedges against price swings.
System resilienceProvide security of energy supply against system shocks.
Meeting demandTo meet peak demand for energy services.
Supply-side efficiencyAllow producers to run efficiently, and reduce curtailment.
Coupling energy servicesEnable the transformation of energy to another service.
Efficient network utilisationMaximising use of infrastructure.
System stabilityManaging short-term supply/demand variations.

The relative importance of each purpose has shift as the energy system has evolved – with new fuel supplies and generation capacities, changes in demand, increasing interconnections for electricity and gas, and policy drivers such as decarbonisation or reducing consumer costs.

3.1 Conventional fossil fuel and thermal energy storage

The UK’s central stocks of stored energy have been reducing since 2005 (Table 1). Over this time, there has been a reduction in fossil fuel used for electricity generation, efficiency improvements and changes in the supply of natural gas (with declining domestic production, increased imports of LNG and additional pipelines between the UK and mainland Europe). The changes in stock levels of natural gas over a year are shown in Figure 9.

Storage of heat at a household level has been common in the UK, but is also reducing. Many British homes have traditionally had hot water tanks, and a significant proportion have used electrical storage heaters for space heating. However, the rise of combination boilers to provide instantaneous hot water has reduced the embedded thermal storage, with 40% of (English) households having a hot water tank in 2017, down from 62% in 2007 (MHCLG, 2019). In France, with most electricity generated by nuclear power stations, domestic hot water tanks also act as distributed thermal energy storage (TES), reducing peak demand by 5 GW (IEA, 2014).

Stored energy (GWh)

Crude oil and oil products170,000135,000-35,000
Natural Gas55,00031,000-24,000
Pumped storage30300

Table 1: Stored energy in Great Britain (source: Wilson, 2020).

Figure 9. Stock levels of medium range storage for natural gas, July 2019 – July 2020 (National Grid, 2020b).

3.2 Energy storage services

A review of available reports (National Grid, 2017b; ERP, 2011; Brandon et al, 2018; Energy UK, 2017) and input from the expert workshop helped us identify the key services that energy storage could provide under decarbonisation scenarios, summarised in Table 2. Given the increasing electrification of energy demand, many of these services are in the electricity sector where instantaneous balancing is necessary. However, with heat being a key energy service, balancing thermal energy supply and demand is equally important to consider.

It is important to note that ‘flexibility’ and other services described in Table 2 can also be provided by a number of options including Demand-Side Response (DSR), interconnection and generation (BEIS & Ofgem, 2017; Carbon Trust, 2016).

Whilst the growth in variable generation over the last decade has increased the opportunity for energy storage in some roles, the wider capability of storage to improve the reliability and resilience of the whole system will be increasingly required at multiple time-scales and in different locations.

Type Service Opportunity for storage Drivers System-level scale / duration
Power Ancillary services Balancing electrical supply and demand on a near real-time basis, including voltage support, frequency response . Non-synchronous generation leading to a drop in voltage and additional voltage support, with reduced system inertia from thermal generation . GW
sub-sec to mins
Power Reserve power Provides power at short notice when demand is greater than forecast. Deployment of variable renewable supply technologies, and reduction of dispatchable generation. GW
mins to hours
Energy Intra-day electrical peak shifting Reducing generation during daily peak demand periods, using energy stored from off-peak demand periods. This is likely to include behind-the-meter users. Peak generation from renewables does not coincide with peak demand; changes in demand from EV charging and electrical heating; reduced generation from fossil-fuel generation. In the case of behind-the-meter users price arbitrage will be the main driver, this will be determined by price differentials in electricity tariffs to reflect changes peak generation and peak demand. GWh
<24 hours.
Energy Inter-day electrical demand levelling Storing energy during days of higher supply to be discharged during days of lower supply; Weather-related supply variability from renewable generation. GWh – TWh
>24 hours
Energy Seasonal electrical peak shifting Capturing excess electrical energy generated in the summer, discharging in winter to lower the additional power capacity required. Electrification of heat generation leading to increasing winter power demand, with excess generation from renewables during low demand in the summer. >TWh
weeks to months
Energy Uninterruptible power supply Provides electricity to customers in the case of an outage. Back-up generation is standard for many industries, demand may increase if there is a perception of reduced supply reliability. kW to MW
seconds to hours.
Thermal Intra-day thermal load shifting Reducing thermal generation (usually from electric sources) during daily peak demand periods, using energy stored from off-peak demand periods. This is likely to be at a domestic scale as well as larger thermal stores for district heat networks. Peak generation from renewables does not coincide with peak demand; electrification of heat (Gas and liquid fuels are relatively easy to store whereas electricity is not). GWh – TWh
<12 hours
Thermal Seasonal thermal peak shifting Capturing energy generation (as thermal or converted from electricity) in the summer, discharging in winter to lower capacity of heat generation required. Reducing use of natural gas for heating. >TWh
weeks to months
Infra Black Start Recovering from a shutdown of an electricity network; with generators started individually and gradually. Fewer large generation plants with black start capability and a more decentralised system make it likely that a greater number of smaller plants with black start capacity will be required . MW – 100s MW
Infra Electricity network upgrade deferral Storing electricity generated during times of low demand to avoid network congestion, and reduce the need for infrastructure upgrades. Increased variable and decentralised renewable generation in areas with good resource but low demand is increasing network congestion in these locations. MW+

3.3 Electrical energy storage

The main provision of electricity system flexibility in the UK has come from 57 GW of non-nuclear thermal generation capacity, fuelled by natural gas or coal (BEIS, 2020d). Pumped-hydro storage (PHS) was built by the nationalised energy sector from the 1960s – 1980s to store excess generation from nuclear power overnight when demand was low, and to provide fast response for grid stability (Wilson, 2010). An example of how generation and storage act to meet demand is given in Figure 10.

Installed electrical energy storage generation capacity in the UK for 2019 was 3,465 MW, with storage potential of 39.3 GWh, and supplying 1.8 TWh (BEIS, 2020e; National Grid, 2020; BEIS, 2020f). The generation capacity comprises 2,828 MW of pumped hydro storage (PHS), 632 MW battery, 5 MW liquid air (BEIS, 2020e). Although non-PHS is 20% of electrical energy storage capacity, the generally short battery discharge times of 1 – 2 hours mean that the stored energy is likely to be 2 – 3% of the total.

Figure 10. Output from energy storage and flexible generation to meet daily electrical demand in GB. Data from

Non-PHS storage has grown since 2016 to almost 600 MW (Figure 11), with over fifty units operational, all under 50 MW capacity (Figure 12(a) and Table 3) . Planned storage, where applications have been submitted, are awaiting construction or under construction, is almost 10 GW from over 250 proposals, comprising 2,900 MW of new PHS and 6,800 MW of battery projects (Figure 12(b)). For both operational and planned battery facilities, about 75% are stand-alone network-connected, with the rest split evenly between co-location with fossil fuel or renewable generation. (BEIS 2020e)

Figure 11. Growth in operational battery capacity in the UK (BEIS 2020e).

(a) Operational.

(b) Planned.

Figure 12. Number of battery facilities according to capacity.

OperatorSite NameTechnology TypeCapacity (MW)
First Hydro CompanyDinorwigPHSStand-alone1,728.0
Scottish PowerCruachanPHSStand-alone440.0
EFDA JETEFDA JJET Fusion FlywheelFlywheelsStand-alone400.0
First Hydro CompanyFfestiniogPHSStand-alone360.0
SSE RenewablesFoyersPHSStand-alone300.0
Statera EnergyCreyke BeckBatteryStand-alone49.9
Statera Energy/ InfraRed CapitalPelhamBatteryStand-alone49.0
EDF Energy RenewablesWest BurtonBatterywith fossil fuel plant49.0
Gresham House Energy StorageBloxwich BatteryBatteryStand-alone41.0
Low CarbonHuggin's HallBatteryStand-alone40.0
ForesightPort of TyneBatteryStand-alone35.0
GridserveBoscar Grange FarmBatterywith RE27.0
Enel (formerly Element Power)Tynemouth Energy Storage System (TESS)BatteryStand-alone25.0
VattenfallPen y Cymoedd Energy StorageBatterywith RE22.0
Renewable Energy Systems (RES)BroxburnBatteryStand-alone20.0
Anesco (formerly owned by Energy Reservoirs)Land to the west of Clay Hill Pit / Larport FarmBatteryStand-alone20.0
Hazel Capital/ Noriker PowerNoriker Power StaunchBatterywith fossil fuel plant20.0
Zenobe (formerly Energy Reservoir 15)Claredown FarmBatteryStand-alone20.0
ØrstedLand at Carnegie RoadBatteryStand-alone20.0
AnescoLascar WorksBatteryStand-alone20.0
Aura PowerLockleaze Energy StorageBatteryStand-alone15.0
AES Kilroot Power (AES KPL)AES Kilroot Station Battery Storage ArrayBatterywith fossil fuel plant10.0
E.ON UKBlackburn MeadowsBatterywith RE10.0
Anesco/ Green Hedge Energy UKHill Farm - Energy BarnBatteryStand-alone10.0
Anesco/ Green Hedge Energy UKKings Barn Farm - Energy BarnBatteryStand-alone10.0
EelpowerLeverton Farm - Energy BarnBatteryStand-alone10.0
Belectric UK / ForesightNevendonBatteryStand-alone10.0
Low Carbon Storage Investment Company/ VLCCleatorBatteryStand-alone10.0
Kiwi PowerCleveland PotashBatteryStand-alone6.0
Grid Battery Storage (GBSL)Dorking Battery Energy Storage SystemBatteryStand-alone6.0
AnescoClayhillBatterywith RE6.0
Viridor/Highview Power StoragePilsworth Landfill SiteLAESStand-alone5.0

Table 3 Energy storage in the UK >5 MW generation capacity (BEIS 2020e).

3.4 Energy Storage In Scenarios

Integrating energy storage into energy system models is challenging. There is limited availability of data, and there are large uncertainties in forecast technoeconomic parameters, with ESC’s report ‘Non-battery electrical storage’ noting the ESME model’s ‘relatively coarse representation of timeslices (National Grid, 2020c; ESC, 2020b). A report by the Energy Research Partnership states that ‘current storage models alone are not robust enough or meet the economies of scale needed for future energy demands and generation scenarios’ and calls for Working Group to be convened, ‘focussed on analytical and modelling frameworks that include thermal, mobility and power services, to assess the potential contribution of energy storage and its technical characteristics.’ (ERP, 2020)

Nevertheless, storage has been represented in scenarios of the UK energy system, with the outputs in terms of electrical energy storage capacity and generation shown in Figure 13.

The NG FES scenarios includes:

  • Various types of battery technologies;
  • Pumped hydro storage;
  • Compressed air electricity storage; and
  • Liquid air electricity storage.

ESC’s scenarios include

  • Pumped hydro storage;
  • Electrochemical and flow batteries
  • Pumped heat electricity storage
  • Compressed air energy storage

(a) Energy storage capacity.

(b) Energy storage generation.

Figure 13. Energy storage in scenarios

A separate exercise by ESC ran the ESME model with varying storage capex parameters, giving a ‘best’ and ‘worst’ case. The best case had 35 GWh of electrical energy storage, with 27 GWh existing PHS and 8 GWh of a generic long-duration technology; the worst case had 1.4 GWh of mechanical/thermal storage and less than 1 GWh of battery storage, on top of the same amount of PHS (ESC, 2020b).

The Carbon Trust and Imperial College modelled flexibility options in the electricity sector under twelve scenarios each making different assumptions regarding level of electricity demand, the cost of energy storage, the cost of DSR and timings of interconnector deployment (Carbon Trust 2016). The study found that deployment of these flexibility options could save the UK £17 – 40 bn across the electricity system by 2050, with savings by 2030 equating to £1.4 -2.4 bn/year. Table 4 shows the minimum and maximum capacity (GW) found for each flexibility option across the scenarios. Although gas-fired power plants have a high capacity it should be noted that they have low capacity factors, in the region of 5% by 2050, and are increasingly used to provide reserve capacity for use in the event of ‘low probability peaks’ such as particularly harsh winters. The optimal deployment of storage across the scenarios is shown in Figure 14 with the range in 2050 being between 5 and 28 GW, but nine of the scenarios are in the range 10 – 20 GW.

Figure 14. Range of optimal deployment of energy storage to 2050 across twelve core scenarios considered by (Carbon Trust, 2016)

2030 2050
Min Max Min Max
Demand-Side Response 3.3 15 6.7 35
Interconnectors 6.2 14.7 16.1 18
Energy Storage 2.8 7.6 4.8 27.6
Gas (unabated) power plants 38.4 58.3 41.5 90.7

Table 4. Range of installed capacity (GW) of flexibility options, modelled across different scenarios (Carbon Trust, 2016).