Energy storage can refer to a broad family of technologies with different characteristics that affect the charging and discharging rates, and the scale and form of energy that can be stored. Energy storage types are commonly classified according to the processes involved: mechanical (e.g. pumped-hydro, flywheels), thermo-mechanical (e.g. pumped thermal, liquid air, compressed air energy storage), electrical (e.g. capacitors), electrochemical (e.g. lithium-ion batteries), thermal energy storage (e.g. hot water tanks, molten salt), and chemical (e.g. fossil fuels, hydrogen).
Often energy is converted from one form to another in the storage process, such as from electrical to mechanical, to operate a pump, and to gravitational potential energy, as water is raised up a hill for PHS. Most storage types under consideration for system-level applications take electricity from the nationally connected infrastructure, and supply it either back to the network (‘network-connected’), or for a specific consumer (‘behind-the-meter’ storage), as electricity. Generation-Integrated Energy Storage (‘GIES’) systems store energy at some point along the transformation between the primary energy form and electricity (Garvey et al, 2015). Electrical and thermal energy may also be stored on the demand-side using locally produced energy, e.g. from solar panels to generate electricity or heat. Thermal energy storage (TES), described more below, may use electricity, excess process heat or renewable sources (e.g. solar or geothermal) to charge, and will normally discharge energy as heat at temperatures according to end-use, though it may be converted to electricity.
Globally, there is 176 GW of installed non-primary energy storage power capacity, 96% of which is PHS, with 1.9% as TES, and the remainder split equally between batteries and other mechanical technologies (IRENA, 2017).
4.1 Energy storage technology development
Although a limited range of energy storage technologies have been deployed commercially, many other options are in development. This first edition of the Roadmap assesses twelve electrical energy storage technologies and thermal energy, as summarised below in boxes 1 and 2, with comprehensive descriptions of the technologies can be found in the references, though several reports review the technologies (e.g. Brandon et al, 2018; ESC, 2020; IRENA, 2017). In Figure 15, the technologies are placed on a chart to show the maturity and discharge duration. Other emerging technologies will be included in future editions. Key considerations were current and projected future (2030) costs and performance, storage services they can provide, research activity and gaps. The services these technologies could provide are shown in Table 5.
4.2 Thermal energy storage in heat networks
Large-scale TES could be also used alongside district heat networks to shift thermal demand to times of greater supply. UKERC estimate that for every 1,000 consumers on a district heat network, up to 24 MWh of thermal storage would be required to allow for 3-hours of intra-day load shifting (Eames et al, 2014). With a recent study from the ETI suggesting that up to 12 million households may be connected to a district heat network by 2050 (ETI, 2018) this results in a total storage demand of 691 GWh by 2050.
There are currently only a small number (tens) of district heat networks with TES in the UK using hot water tanks for storage. One such network is the Pimlico District Heating Undertaking in London which provides heating and hot water to over 3,000 homes, 50 commercial properties and 3 schools using a series of boilers and CHP engines alongside 18 MWh of water tank storage (Eames et al, 2014). Outside of the UK there are examples of other TES technologies being used alongside district heat networks such as the Drake Solar Landing Community (DLSC) in Alberta https://www.dlsc.ca/index.htm). The DSLC heating network provides 90% of space-heating and 60% of domestic hot water demands for 52 households and uses borehole thermal storage to provide seasonal peak shifting.
Significant efforts are in progress for heat networks considering their potential towards achieving net zero targets, including Government’s multi-million awards for district heat networks (BEIS, 2020g), reports on thermal storage technologies and associated policy and barriers (ESC, 2020c).
Box 1: Electrical Energy Storage Technology Summaries
Pumped Hydro Storage (PHS)
PHS stores energy at times of low demand by pumping water from a lower reservoir, often a lake, to an upper reservoir. At times of high demand water is released from the upper reservoir to the lower reservoir, through a turbine (located in the machine hall) to generate electricity.
Compressed Air Energy Storage (CAES)
Compressed Air Energy Storage (CAES) stores energy by using excess electricity to compress air at high pressure which when required the compressed air is heated using natural gas, to drive turbines to generate electricity (Brandon, 2017). An electric motor is used to compress the air which is then stored in a storage reservoir, usually an underground cavern (for example a salt cavern). To avoid using natural gas a new CAES system known as Adiabatic CAES (A-CAES) has been developed which avoids using natural gas (Huang, 2017). Air is adiabatically compressed and pumped into the storage reservoir and the heat generated through the compression process is also stored. When the system is discharging the compressed air is heated by the stored thermal energy.
Flywheel systems provide fast responding, short term energy storage (Brandon, 2017). They consist of one or more flywheels which store electrical energy in the form of rotational kinetic energy by using a motor to spin a large-mass rotating mechanical device. When discharging the flywheel drives the motor which now acts as a generator. Flywheels have relatively high power and energy densities; lower than batteries but higher than ultracapacitors. Power and energy is decoupled and can be optimised independently by selection of the motor-generator and main rotor respectively.
Liquid Air Energy Storage (LAES)
Liquid-Air Energy Storage (LAES) typically uses air or nitrogen as its storage medium and stores energy by using electricity to cool the storage medium, in this case air or nitrogen, until it liquefies at around 78 K (Morgan, 2015; EASE, 2017). The liquid air is then stored in an insulated tank until power is required. At this point the liquid air is pressurised and then superheated to ambient temperatures, where as a pressurised gas it can then drive turbines to generate power. Waste heat from industrial processes or the recycling of otherwise wasted cold energy from the liquid air can be utilised to improve round-trip efficiency (Sciacovelli, 2017).
Lithium-ion Batteries (LiB)
A Lithium-ion battery (LiB) consists of an anode (negative electrode, often graphite), a cathode (positive electrode, often a layered oxide or a polyanion type cathode) and an electrolyte (a Lithium salt) supporting the lithium-ion transfer between electrodes during charge and discharge processes (Brandon, 2017)During the charging process the lithium ions move from the positive to the negative electrode through the electrolyte in between. Electrons flow through an external circuit in the same direction. To produce electricity, the reaction is reversed, so that lithium ions move from the negative to the positive electrode.
Sodium-ion Batteries (NaB)
Sodium-ion Batteries (NaBs) operate in a similar way to LiBs and so have similar technological characteristics (Brandon, 2017). NaBs were first introduced in the 1980s but have not yet been fully commercialised for grid-scale applications (Kubota, 2015). The major advantage of NaB’s is the cost and abundance of the raw materials compared to those for LiB’s. However the gravimetric and volumetric energy densities of NaBs can be lower than LiBs, and operating voltages are typically around 0.3V lower than for equivalent LiB systems (Liu, 2016).
Metal-air batteries consist of a metal anode and an external cathode of ambient air (Brandon, 2017). There are a number of variations of metal-air batteries depending on the metal used including lithium-air, zinc-air, magnesium-air, and aluminium air. Due to their higher energy density and lower energy cost metal-air batteries could in the future replace Lithium-ion batteries (Institution of Mechanical Engineers, 2014; Rahman, 2013), however they are currently not a viable option due to their limited technological performance and the difficulty of the electrical recharging process (Larcher, 2015).
Lead Acid Batteries (PbA)
PbA’s are the oldest form of rechargeable battery and in their basic form consist of a lead anode, a lead dioxide Cathode and a sulphuric acid solution electrolyte (May, 2018; Brandon, 2017). PbA’s were developed in 1859, and currently dominate the global market for small scale battery applications such as automotive applications including starting, lighting and ignition.
Redox Flow Batteries (RFB)
RFB’s discharge electricity by releasing electrons from the electrolyte on the anode side of the power cell through an oxidation reaction, these electrons flow to the cathode side of the power cell where they are accepted through a reduction reaction by the electrolyte on the cathode side, generating an electrical current. RFB’s then store energy by using electricity to reverse the oxidation and reduction reactions and therefore the flow of electrons. The electrolyte used in RFB’s is usually a liquid but can take solid or gaseous forms. The electrolytes are stored in tanks separate from the power cell so that the power density and energy density are decoupled.
A supercapacitor consists of a porous separator and two electrodes which store electrical charge by electrostatic adsorption of electrolyte ions in an electric double layer (EDL) (Brandon, 2017). Supercapacitors can bridge the power and energy gap between dielectric capacitors and electrochemical batteries. However, to be applied for grid applications they still require a cost reduction and an improvement in energy density, while maintaining the high power density and long cycle life.
Superconducting Magnetic Energy Storage (SMES)
Superconducting magnetic energy storage (SMES) stores electrical energy in the form of a magnetic field which is a created by passing a current through a superconducting coil . The superconducting coil has been cooled to cryogenic temperatures to ensure there is no resistance to the current. This allows the current to continuously flow thus storing energy in the form a magnetic field. The stored magnetic energy can be converted back to electricity by discharging the coil (Taylor, 2012).
Box 2: Thermal Energy Storage Technology Summaries
Thermal Energy Storage (TES)
Thermal energy storage (TES) refers to a collection of energy storage technologies that store energy in the forms of heat, cold or their combination (Eames, 2014; Brandon et al, 2018; Radcliffe, 2015; IRENA, 2020). Thermal energy storage can produce both thermal energy and electrical energy. This section includes thermal energy storage technologies that produce heat or cold while thermal energy storage technologies that produce electricity are included in the electricity storage section.
There are a number of different TES technologies which can be used for a variety of applications, however they can be categorised into three broad sections by the material they use to store the heat or cold energy. These categories are: 1) sensible thermal energy storage materials; 2) phase change (or latent heat storage) materials; and 3) thermo-chemical (TC) storage materials.
Sensible Thermal Energy Storage (STES) systems are the most widely accepted form of TES and store energy by raising or reducing the temperature of the material whilst maintaining its current state. Therefore the maximum thermal energy storage capacity is equal to the specific heat capacity and temperature change of the material being used. To avoid the loss of the thermal energy, container and insulation systems are required.
Phase-change material (PCM) storage, also known as latent heat storage can store more energy than STES by heating materials that can melt or boil at a specific ‘transition’ temperature. There are three types of PCM; solid-solid PCM, solid-liquid PCM, and liquid-gas PCM. The crystal structure of solid-solid PCM’s changes as energy is applied so can be used for thermal energy storage (Li, 2007), however only a limited number of these materials provide sufficient latent heat that they are useful as a thermal energy storage material, and even then solid-solid PCM storage systems are generally of relatively low latent heat. However they do have several advantages such as such as simpler design for material storage requirement, small erosion, no leakage, no additional storage container for encapsulation and longer lifespans (Sari, 2011; Wang, 2000).
Thermal energy can trigger a chemical reaction that will store or generate heat. If the chemical reaction caused by thermal energy is completely reversible, the material can be used for thermochemical material energy storage (TCES). Commonly used thermochemical materials are metal chlorides, metal hydrides, and metal oxides (Xu, 2014). There are some technological advantages of TCES including high energy densities, small heat loss, long storage duration, and heat-pumping capability which is useful for inter-seasonal thermal storage (Radcliffe, 2015). However TCES systems are generally more complex compared with other storage systems and subsequently have a higher capital cost. Furthermore TCES systems are still at a research stage in terms of technological development.
|TES Principle||TES Technology||Technology Summary|
|Sensible heat||Underground TES||Thermal energy is stored underground in the summer season for use during winter, storage mediums include water (aquifers), earth and bedrock.|
|Pit Storage||A pit is excavated and then insulated before being filled with the storage medium usually water or a gravel-water mix. Used for seasonal thermal storage.|
|Solid Materials||Solid thermal storage materials include rocks, metal, concrete, bricks and sand .|
|Liquid Materials||Liquid thermal storage materials include water (e.g. hot water tanks), and oil-based fluids .|
|PCM||Salt hydrates||Covers a temperature range between -50oc and 120oc, salt hydrates which can be used for thermal energy storage include Calcium chloride hexahydrate (CaCl2.6H2O), Magnesium nitrate hexahydrate (Mg(NO3)2.6H2O) and Magnesium chloride hexahydrate ( MgCl2.6H2O) (Kenisarin, 2016).|
|Molten Salt||Often used in conjunction with concentrated solar plants (40% of CSP’s have a molten salt storage system). Salts used include Sodium Chloride (NaCl) and Nitrate salts .|
|Ice||A proven technology; ice storage can use cheap electricity during periods of low demand to make ice. Ice storage is used to replace air conditioners to reduce room temperature during peak hours .|
|Thermochemical||Thermochemical||Currently the least developed form of thermal energy storage with more research around reactor design required, although they potentially have high energy densities; possible thermochemical materials include Calcium hydroxide (Ca(OH)2) and Ferrous carbonate (FeCO3) (Eames, 2014).|
A summary of TES technologies
|Ancillary services||Reserve||Intra-day peak shifting||Inter-day levelling||Seasonal electrical peak shifting||Seasonal thermal peak shifting||Black Start||Network Upgrade Deferral||UPS|
|Electrical Energy Storage|
|Compressed Air Energy Storage (CAES) (Adiabatic)||In development||✓||✓||✓||✓||✓|
|Compressed Air Energy Storage (CAES) (Diabatic)||Commercial||✓||✓||✓||✓||✓|
|Liquid Air Energy Storage (LAES)||Demonstrated||✓||✓||✓||✓||✓|
|Metal-air Batteries||In development||✓||✓||✓|
|Pumped Hydro Storage (PHS)||Commercial||✓||✓||✓||✓||✓|
|Superconducting Magnetic Energy Storage (SMES)||Commercial||✓||✓|
|Thermal Energy Storage|
|Solid Sensible TES||Commercial||✓||✓||✓||✓||✓|
|Liquid Sensible TES||Commercial||✓||✓||✓||✓||✓|
|Thermochemical Energy Storage||In Development||✓||✓||✓|
4.3 Technology development priorities
Research gaps for individual technologies were initially identified by reviewing available literature, these gaps were then considered by technology experts both individually and at the expert workshop. Through this process a series of recurring needs were found:
- Improved manufacturing technologies and larger manufacturing base; these can lead to lower costs and are clearly beneficial for most technologies but were specifically identified as being required for CAES (Brandon, 2017), Flywheels (EASE EERA, 2017), PbA (EASE EERA, 2017), LiB (JRC, 2013), NaB (EASE EERA, 2017), supercapacitors (Brandon, 2017) and TES (Brandon, 2017).
- Improved electrode materials (batteries); all battery technologies (including RFB’s) assessed in this report and supercapacitors were found to have a need for improved electrodes. The exact improvements required were technology specific but in all cases this would result in an improved performance, normally in terms of energy and power density leading to a reduction in costs (European Commission, 2011; EASE EERA, 2017; McKeon et al, 2014; Zhou et al, 2014).
- Improved safety; energy storage technologies are generally safe and stable but it is acknowledged that for a number of technologies there are developments which would further improve safety. In particular minimum standards for flywheel rotor housing (Amiryar, 2017) , and alternative material and better thermal management for many (LiB, LiS, Metal-air) electrochemical batteries (European Commission, 2011; Larcher, 2015).
- Better recycling processes and reduction in environmental impact; many batteries contain materials which can be toxic if they enter the wider environment so improved recycling processes would reduce environmental impact (Lv, 2018; US Environmental Protection Agency, 2016). There is limited availability of lithium and cobalt so improved recycling processes would also help to keep resource cost low as well as reducing the environmental impact of mining (Zeng, 2014). Other technologies which can have significant environmental impacts are PHS due to its large surface footprint and need for two reservoirs (Institution of Mechanical Engineers, 2014), and large-scale SMES from the magnetic field (AEA, 2010). In both cases additional siting studies to identify the most suitable sites would be beneficial.
- Reduction in cost and/or additional revenue streams required to allow energy storage technologies to compete; energy storage technologies are currently competitive in some markets, for example LiB’s in the National Grid’s enhanced frequency response tender. However, in many cases energy storage options are not yet competitive therefore there is a need to reduce costs and/or identify additional revenue streams (Castagneto Gissey, 2018). For batteries and some thermal energy storage technologies a better understanding of their degradation mechanisms could lead to improved performance and therefore an improvement in their competitiveness (Brandon, 2017).
In summary significant levels of investment and innovation are still required to meet the recurring needs described above to improve the performance and cost characteristics of energy storage technologies.