Increasing the proportion of UK electricity supply from renewable energy sources is a critical part of achieving a sustainable energy framework for the nation. To date, relatively small proportions of wind, solar and biomass based power have been installed in comparison to conventional generation. However, the Government expects the proportion to increase over the next ten years.

As wind and solar power are intermittent forms of generation, when the proportion of these sources increases in the mix, additional measures are needed to maintain a safe and efficient balance between electricity supply and demand on the power system. To date, the adoption of electricity storage technologies as a means of providing power system support has been limited, due to a combination of regulatory, commercial and technical factors.

This position statement examines the role that electricity storage could play in our future networks.

The Institution of Mechanical Engineers urges Government to adopt the following recommendations:

1. Support actions to identify the true system benefit of electricity storage. As a matter of priority the UK’s Department of Energy and Climate Change (DECC) should carry out a detailed analysis to estimate the realistic requirements for electricity storage across the whole UK power system and its corresponding value to the nation.

2. Develop policy frameworks that reward the value of electricity storage in the UK’s power markets. The UK Government’s Electricity Market Reform (EMR), which is examining and revising the commercial and regulatory structure of the nation’s electricity market, should take into account the unique nature of electricity storage and remunerate investors and operators accordingly.

3. Encourage and support UK development of storage technologies for exploitation in world markets. The UK Government should advance the commercial-scale demonstration of electricity storage technologies in the UK, and thereby create technical value that UK companies can exploit in markets worldwide.

Improving the world through engineering

ELECTRICITYSTORAGE

RENEWABLE ENERGY POLICY

Renewable energy provision is a substantial part of current UK energy policy and commitment. The EU Renewable Energy Directive[1] sets the immediate goal of meeting European targets that require the UK to deliver 15% of the nation’s energy from renewable sources by 2020. In addition, the UK’s Climate Change Act 2008[2] places a legally binding longer-term commitment on future Governments to achieve an 80% reduction in the nation’s greenhouse gas (GHG) emissions by 2050 relative to 1990 levels. This target can only be realistically met only through an energy mix consisting of a high proportion of renewable and other low GHG-emitting sources. The Committee on Climate Change (CCC) predicts that a generating system largely free of GHG emissions will be needed by 2030 and that by 2040 most transport and heating will be transitioned to electricity-based technologies [3]. The outcome of such a scenario will potentially be a doubling of UK electricity demand requirements by 2050[4].

Considerable challenges exist in the adaptation of the UK power network to support this anticipated increase in the use of electricity while simultaneously integrating a substantial renewables capacity into the generation mix. The scale of the challenge ahead is evident in the estimates for investment in the system: up to £75 billion for new generation assets[5] and about £35 billion for power transmission and distribution infrastructure[6] over the next ten years. Other changes in the power sector, for example increased loads from digital devices and heat pumps as well as new forms of micro-generation, such as solar power and wind power connected at consumer premises, will also increase the complexity in managing the system. For most of the time the effects of these changes will be small, but there will be periods when they will have serious implications for the network’s operation.

MITIGATION OF VARIABLE GENERATION USING STORAGE

Power systems must be operated so that supply is equal to the real-time demand and system losses at all times. If this balance is not maintained, the system becomes unstable and may quickly fail, with potentially severe economic and technical effects. As the proportion of intermittent renewable energy sources increases substantially on the UK network in the coming decades[3], there will be occasions when nearly all the demand is being met by a combination of wind power and inflexible base-load generating plant (such as nuclear and biomass), resulting in considerable stability challenges. In order to match even small changes in production, the network operator may take a number of steps to manage the system, including:

a) Holding of reserve on existing flexible generation (ie gas turbines)

b) Management of demand (ie voluntary and compulsory disconnections)

c) Use of interconnections to adjoining systems (ie Europe)

d) Electricity storage

Benefits of electricity storage

The key benefit of storage on a power system is to increase the efficient utilisation of assets on the network. In addition to maximising the use of output from renewable-based electricity generation technology, storage enables an increased return on investment (ROI) for base-load plants through enabling longer periods of operation at higher output. Electricity distribution infrastructure, such as transformers, cables and overhead lines can also be installed to meet average loads instead of being oversized to meet peak loads, thereby avoiding unnecessary expenditure on power infrastructure.

A number of other benefits also result for the power system:

• Fast acting and responsive; most storage can be switched to discharge or charge electricity in a time period, often quicker than generating plant can be bought online or switched off.

• Deployable as large or small units; there are a range of technologies, some of which are suitable for small unit sizes and some for larger installations, making it possible to design a project to meet a specific application.

• Deployable where it is most required; storage can be positioned in small units throughout the distribution network, as well as in larger and centralised installations.

• Modularity; many of the technologies can be scaled to suit a required size or configuration and are able to be transported to a new location should the application or circumstances change.

STORAGE TECHNOLOGY

There are a number of electricity storage technologies, each with their own particular characteristics of size, cost and performance[7]. Electricity storage can therefore be considered in terms of its application and it would be incorrect to consider any form of storage as a ‘one-size-fits-all’ device; some applications being more suited to particular technologies, for technical as well as commercial reasons.

Mechanical systems

Pumped hydro electricity storage Energy is stored when electricity is used to pump water from a lower level to a higher level in a reservoir. Electricity can then be generated by water flowing through a turbine as it passes back from the high level to the lower level. This system is typically of large scale and suited to mountainous regions.

Compressed-air energy storage Air can be pressurised using electrically driven compressors, stored, and then the energy recovered when the air is expanded back to atmospheric pressure through a turbo-expander.

ELECTRICITY STORAGE

Storage can be underground, using salt caverns or other structures, or above ground in vessels or pipes (a new variation is to use fabric bags to contain the air underwater). Although large-scale implementation is possible, few large-scale compressed air plants have been built.

Flywheels Electricity can be stored as kinetic energy in a spinning flywheel. A motor is used to increase the rotational speed of the flywheel, and when it slows down the motor operates in reverse as a generator to recover electricity. Large flywheel systems have been used to provide frequency response services to the transmission system.

Thermo-mechanical storage

Cryogenic energy storage Air can be liquefied through cooling using electricity and stored as a cryogen. The liquid air is then expanded upon demand through a turbine and the energy recovered as electricity. The system uses existing technology in a novel configuration and has been recently demonstrated at pilot plant scale by a UK company.

Pumped heat Electrical energy is stored in a thermo-mechanical device when the temperature difference between a hot and cold store is used to drive a reversible heat engine. A UK company is developing this process and is building a demonstrator project.

Latent heat recovery Compressed air can be used for the storage of energy as latent heat and that energy is later recovered by generating electricity from a captive hydroelectric turbine. A UK company is currently demonstrating this system at small scale.

Electro-chemical

Batteries There are many types of battery which can be used for small and large applications of electricity storage on the power network. The lead acid battery is still the most widely used battery type, but recently there have been developments in the use of lithium ion technology. Other battery types include those based on high-temperature systems and flow batteries. The latter store energy in electrolyte tanks outside the battery cell, and have potential for both medium and large-scale applications.

The hydrogen cycle A popular concept for electricity storage is to consider electrolysis of water to produce hydrogen and oxygen. The hydrogen can be used in a fuel cell to generate electricity. Alternatively, the hydrogen can be utilised as a chemical feedstock for manufacturing synthetic liquid fuels, thereby transferring the stored energy to the transportation sector.

POTENTIAL APPLICATIONS OF ELECTRICITY STORAGE

At present there is little storage capability on the UK network, the principal capacity being approximately 2,800 MW of pumped hydro-electricity storage, which can store approximately 27,000 MWh (2.5% of daily average electricity production)[8]. Initial estimates suggest that the reserve requirement will rise from just under 4,000 MW at 2011 levels to 8,000 MW in 2025 if there is penetration of wind of 30%[9].

In addition to considering more pumped storage, other types of electricity storage will be applicable to the UK and there is an urgent need for a thorough, detailed assesment of potential opportunities. Fundamentally, storage should be located where it will be of most value to the network. Table 1 illustrates the relationship between different scales of storage and various operator types. Large-scale storage may be installed at main transformer substations, and medium-scale storage would be appropriate at the distribution transformer substations which supply towns and large industrial sites.

Small Under 1 MW

Medium 1 – 10 MW

Large 10 MW – 100 MW

Very large 100 MW +

Power producers Standalone systems for self-generation and renewables

Energy trading Supply of ancillary services

Network operators Deferral of network reinforcement

Local network management Deferral of network reinforcement

Deferral of system reinforcement Peak shaving

Network constraint management

Consumers of power

Small commercial, domestic users for local load management and tariff reduction

Local load management, smart grid support and external ancillary services

Peak shaving for energy cost reduction and ancillary services

Peak shaving for energy cost reduction

Table 1 Examples of different scale applications of electricity storage and potential users:

Within the urban environment, storage installed at a community level can be used to support the development of distributed energy systems, as well as the smart grid, and defer other expenditure on the network.

COMMERCIAL INFLUENCES AND REGULATORY POLICY

The UK electricity market is dominated by a competitive generation and supply sector and a highly regulated transmission and distribution sector. The opportunities for transmission and distribution companies to own and gain value from storage have been limited by the allowable rates of return on assets, which do not compare favourably against other allowable expenditure. More recently, incentive schemes for novel technologies have allowed network companies to develop demonstration projects to illustrate the value of storage, but the scale of these projects is small and limited to initial demonstrations of applications. Without a change in the regulatory and commercial framework in the power sector, there is unlikely to be a significant expansion of electricity storage.

Numerous past reports on the general value of storage indicate that it is usually necessary to combine income streams in order to achieve a satisfactory rate of return to investors. For example, large users of electricity may install storage so that they can take advantage of off-peak energy prices. As well as reducing their own peak prices, they may choose to use storage to offer frequency response or reserve services to the system operator, or be available to offer network support to the local network operator.

Commercial operators who wish to develop storage, find that rates of return are much lower than for other investments in the energy sector, because of the higher discount rates attached to projects with an uncertain income stream. Technologies such as wind or solar power receive Renewable Obligation Certificates (ROCs) or Feed-In-Tariffs (FIT) which provide a long-term certainty for revenue to incentivise the capital investment. However, because storage is expected to operate in a commercial environment, taking revenue from energy trading, ancillary services and possibly a credit for deferring other capacity, it can be challenging for a developer to consider storage as a commercially acceptable alternative.

This uncertainty creates two significant issues:

a) The development of storage projects is inhibited and the introduction of storage is likely to be delayed further, up until a point when it may be too late to consider storage as a solution for providing flexibility to the operation of the system.

b) Manufacturers, researchers and developers in the UK are not benefiting from early opportunities to demonstrate their technology in the home market and thereby drive costs out through innovation. At best, this means that technology will be exported and the engineering benefit transferred overseas, at worst, the opportunity for the UK to benefit from this significant emerging industry is lost.

VALUE OF STORAGE INDUSTRY IN WORLD MARKETS

Several studies have been carried out that examine and indicate the potential size of the electricity storage market. The Electric Power Research Institute estimates opportunities for up to 50 GW[10] of storage in the USA, at price structure from $4,000/kWh to $300/kWh. Other reports suggest a worldwide market of US $20 – 25 billion annually by 2020[11], which suggests a global opportunity of 13 GW/year for storage[12].

However different countries have different regulations for electricity storage. The USA is embracing storage as an enabler of the smart grid and renewable energy, and changes in legislation are under way at Federal and State levels to ensure storage is considered as part of the future network. Conversely, in Europe directives have been written to exclude network operators from the role of generating and supplying electricity, but the role of storage has been recognised as an integral part of network operation[13/14] and as a major component of future smart networks.

There is an urgent need in the UK for a detailed analysis to be undertaken to estimate the realistic requirements for electricity storage across the power system and realistically determine its corresponding value to the nation.

RECOMMENDATIONS

The large-scale deployment of electricity storage technologies, in common with many other technology-based approaches for mitigating GHG emissions, requires significant initial capital expenditure. The incorporation of storage as a system asset will result in reduced system capital and operating expenditure and maximise the return on investments in electricity generation equipment, whether based on renewable or traditional energy sources, as well as network infrastructure for power transmission and distribution. There is concern that the correct investment decisions should be made now to prevent investment in the wrong technologies that will restrict future options.

The Institution of Mechanical Engineers urges Government to adopt the following recommendations:

• Support actions to identify the true system benefit of electricity storage. As a matter of priority the UK’s Department of Energy and Climate Change (DECC) should carry out a detailed analysis to estimate the realistic requirements for electricity storage across the whole UK power system and its corresponding value to the nation. This should be taken into account by Government when determining the future policy for planning and developing the power system and electricity market. There is a lack of understanding about the flexibility of electricity storage and the wider financial benefits it can deliver, beyond enabling enhanced returns on the deployment of renewable energy-based generation equipment. This is in part related to the rapid development of a number of the newer technologies and needs to be addressed in the near term.

• Develop policy frameworks that reward the value of electricity storage in the UK’s power markets. The UK Government’s Electricity Market Reform (EMR), which is examining and revising the commercial and regulatory structure of the nation’s electricity market, should take into account the unique nature of electricity storage and remunerate investors and operators accordingly. Large-scale installations have to pay charges both as a generator of electricity and as a demand customer when charging up, which increases the costs disproportionately.

Appropriate changes in policies and mechanisms are needed: electricity storage should be considered in a separate market category from generation, transmission, distribution and supply. Market mechanisms will need to be adapted if storage at the distribution level is to receive value both for its role in network reinforcement and for energy and power trading.

• Encourage and support UK development of storage technologies for exploitation in world markets. The UK Government should advance the commercial-scale demonstration of electricity storage technologies in the UK, and thereby create technical value that UK companies can exploit in markets worldwide. This will drive innovation towards lower costs, support the provision of a more flexible and responsive UK power sector, and is an important step if the full benefits of the nation’s renewable energy sources are to be realised.

REFERENCES1 EU, Directive 2009/28/EC of the European Parliament and of the

Council, 23 April 2009.

2 Crown, Climate Change Act 2008, 2008.

3 Committee on Climate Change, The Renewable Energy Review, May 2011.

4 DECC, 2050 Pathways Analysis (HM Government, London, 2010).

5 DECC, Planning our electric future: a White Paper for secure, affordable and low carbon electricity, July 2011 (HM Government, London, 2011).

6 Ofgem, Project Discovery, February 2010.

7 Parliamentary Office of Science & Technology, POSTnote 306, Electricity Storage, April 2008.

8 DECC, Digest of United Kingdom Energy Statistics (DUKES) 2011 Edition, July 2011.

9 National Grid, Operating the Electricity Transmission Networks in 2020, June 2011.

10 EPRI, Electricity Energy Storage Technology Options, EPRI TU 1020676, December 2010.

11 Pike Research, Energy Storage on the Grid, July 2011.

12 Escovale, Market Study for Flow Batteries, 2010.

13 European Commission, Energy Infrastructure Priorities for 2020 and beyond, COM, 2010, 677.

14 European Commission, Proposed regulation for Trans-European Energy Infrastructure, COM, 2011, 658.

Institution of Mechanical Engineers

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PUBLISHED MAY 2012