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 security||Manage disruptions to imported energy.|
|Economic security||Fuel bought in advance hedges against price swings.|
|System resilience||Provide security of energy supply against system shocks.|
|Meeting demand||To meet peak demand for energy services.|
|Supply-side efficiency||Allow producers to run efficiently, and reduce curtailment.|
|Coupling energy services||Enable the transformation of energy to another service.|
|Efficient network utilisation||Maximising use of infrastructure.|
|System stability||Managing 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 products||170,000||135,000||-35,000|
Table 1: Stored energy in Great Britain (source: Wilson, 2020).
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
|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
|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
|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.
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 12. Number of battery facilities according to capacity.
|Operator||Site Name||Technology Type||Capacity (MW)|
|First Hydro Company||Dinorwig||PHS||Stand-alone||1,728.0|
|EFDA JET||EFDA JJET Fusion Flywheel||Flywheels||Stand-alone||400.0|
|First Hydro Company||Ffestiniog||PHS||Stand-alone||360.0|
|Statera Energy||Creyke Beck||Battery||Stand-alone||49.9|
|Statera Energy/ InfraRed Capital||Pelham||Battery||Stand-alone||49.0|
|EDF Energy Renewables||West Burton||Battery||with fossil fuel plant||49.0|
|Gresham House Energy Storage||Bloxwich Battery||Battery||Stand-alone||41.0|
|Low Carbon||Huggin's Hall||Battery||Stand-alone||40.0|
|Foresight||Port of Tyne||Battery||Stand-alone||35.0|
|Gridserve||Boscar Grange Farm||Battery||with RE||27.0|
|Enel (formerly Element Power)||Tynemouth Energy Storage System (TESS)||Battery||Stand-alone||25.0|
|Vattenfall||Pen y Cymoedd Energy Storage||Battery||with RE||22.0|
|Renewable Energy Systems (RES)||Broxburn||Battery||Stand-alone||20.0|
|Anesco (formerly owned by Energy Reservoirs)||Land to the west of Clay Hill Pit / Larport Farm||Battery||Stand-alone||20.0|
|Hazel Capital/ Noriker Power||Noriker Power Staunch||Battery||with fossil fuel plant||20.0|
|Zenobe (formerly Energy Reservoir 15)||Claredown Farm||Battery||Stand-alone||20.0|
|Ørsted||Land at Carnegie Road||Battery||Stand-alone||20.0|
|Aura Power||Lockleaze Energy Storage||Battery||Stand-alone||15.0|
|AES Kilroot Power (AES KPL)||AES Kilroot Station Battery Storage Array||Battery||with fossil fuel plant||10.0|
|E.ON UK||Blackburn Meadows||Battery||with RE||10.0|
|Anesco/ Green Hedge Energy UK||Hill Farm - Energy Barn||Battery||Stand-alone||10.0|
|Anesco/ Green Hedge Energy UK||Kings Barn Farm - Energy Barn||Battery||Stand-alone||10.0|
|Eelpower||Leverton Farm - Energy Barn||Battery||Stand-alone||10.0|
|Belectric UK / Foresight||Nevendon||Battery||Stand-alone||10.0|
|Low Carbon Storage Investment Company/ VLC||Cleator||Battery||Stand-alone||10.0|
|Kiwi Power||Cleveland Potash||Battery||Stand-alone||6.0|
|Grid Battery Storage (GBSL)||Dorking Battery Energy Storage System||Battery||Stand-alone||6.0|
|Viridor/Highview Power Storage||Pilsworth Landfill Site||LAES||Stand-alone||5.0|
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
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.
|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).