The range of sources of renewable energy requires a leap forward when it comes to innovation in energy storage and other enabling technologies that will help achieve the energy transition, including by balancing supply of and demand for power. These enabling technologies include electrolysis for the production of hydrogen, which can be used for chemical energy storage or as an alternative fuel itself.
Electrochemical, heat and mechanical energy storage technologies are available in many forms, and future scenarios may incorporate any of the diverse technologies explored below, each favouring particular end-use and economic environments.
The 1.5°C Pathway report issued by the International Renewable Energy Agency (IRENA) predicts that hydrogen and derivatives will need to account for 12% of final energy use by 2050. Green hydrogen from water electrolysis using renewable energy is expected to be both a key strategic energy source and storage medium.
Several countries have already included targets for electrolyser capacity in national hydrogen strategies. Future electrolysers would ideally be built out of raw materials that are cheap and abundant, and use renewable electricity. According to a recent joint study published by the EPO and IRENA on innovation trends for electrolysers in hydrogen production, investment costs for electrolyser plants can be reduced by 40% in the short term and by 80% in the long term through improved design and construction, economies of scale, replacing scarce materials with abundant metals, and enhanced operational efficiency and flexibility. This would help increase global electrolyser capacity from today's 0.5 GW to the 5 000 GW by 2050 proposed in IRENA's 1.5°C Pathway report.
Five major groups of sub-technologies are important for reducing the cost of electrolysis:
Operating electrolysis cells at a higher temperature and pressure increases the efficiency of electrolysis without compromising durability, while also reducing costs. The electrolysers must be designed to withstand these operating conditions.
A key element in optimising cell efficiency is the contact surface between the membrane and the electrode, reducing electrical resistance and heat generation:
The scarcity of materials, and of noble metals in particular, is a major barrier to reducing the costs of electrolysers and scaling up production. Alternatives are needed that make use of non-noble materials, for example. Developments include noble metals (including oxides), non-noble metals, alloys and ceramics, and organic, diamond and non-diamond materials.
The following search statements include electrodes that incorporate catalyst materials based on:
Electrodes, bipolar plates and porous transport layers can contribute significantly to the stack cost. Improvements in these components, including their manufacture, can help to reduce capital costs and increase scalability. While bipolar elements containing stacks remain the most important development area, the optimal combination of these with the membrane electrode assemblies is key:
Photoelectrolysis using photovoltaic cells as the energy source can integrate electricity and hydrogen production in a single element, potentially leading to higher efficiency. Although currently at a low technology maturity and process optimisation stage, promising developments for the future include:
The electrolysis of water alone produces hydrogen that is difficult to store. An emerging area of development is co-electrolysis, producing hydrogen in a more storable compound. Examples include nitrogen-containing compounds such as ammonia, presented below, and others such as syngas (CO/H2) and organic fuel production (methane, alcohols).
 IRENA (2020), Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5⁰C Climate Goal, International Renewable Energy Agency, Abu Dhabi.
The electricity we use is traditionally generated just moments before it is consumed. It is a challenge for today's energy industry to plan for the current variable and uncontrollable energy demands. With more complex future energy needs, e.g. from electric mobility, and with renewable energy from weather-dependent sources such as wind, sun and waves, balancing supply and demand has become a key challenge.
Better energy storage technologies enable the integration of larger quantities of renewable energy into the energy system, helping to replace fossil fuels in a variety of applications. A wide range of energy storage technologies are currently at various stages of development. Key technology categories include mechanical storage, such as pumped hydropower storage and compressed gas; thermal storage using water, solids or steam accumulators; and electrochemical solutions including batteries, which can be complemented by technologies such as supercapacitors.
Electrochemical inventions (e.g. batteries) account for 88% of all patenting activity in the field of electricity storage, far outweighing electrical (9%), thermal (5%) and mechanical (3%) solutions. Growth in the markets for electric vehicles and stationary electricity storage make electrochemical solutions even more important for the future.
According to the Sustainable Development Scenario outlined by the International Energy Agency (IEA) in 2021, close to 10 000 gigawatt-hours of batteries and other forms of energy storage will be required annually across the energy system by 2040, compared with around 200 GWh available today.
Patenting activity has been increasing for a range of
inventions including lead acid, redox flow and nickel-based batteries. However,
innovation in the field has been spearheaded by lithium-ion (Li-ion) batteries,
with 45%, compared with just 7.3% for other chemistries, and the remainder for manufacturing
and engineering aspects.
The choice of cathode material is key. While traditionally lithium cobalt oxide was used, concerns related to cost and supply have now shifted the focus towards nickel- and/or manganese-containing materials (together with lithium iron phosphate or LFP). The following cathode technologies have been chosen as key to this trend:
Supercapacitors can complement Li-ion batteries by addressing specific needs. Supercapacitors can be charged and discharged within seconds. However, they cannot store electricity in quantities as large as batteries can. Their ability to provide bursts of power makes them valuable in combination with other higher-capacity battery types, for example in electric cars.
Much of today's major innovation in solid-state electrolytes is geared towards finding alternatives to the liquid or polymer gel electrolytes used in current Li-ion batteries, which pose a flammability risk. In addition to providing improved safety, solid-state electrolytes can offer a high level of specific energy and a high degree of stability and durability.
For some applications, redox flow batteries can provide a safer, more durable and more scalable alternative to Li-ion batteries. Redox flow batteries use porous electrodes, in which an ion-exchange membrane separates the active materials in the form of positive and negative liquid solutions containing redox-active species.
Redox flow batteries can have different chemistries, with vanadium being the most commonly used redox-active cation. Their scalability makes them particularly interesting for residential and large-scale stationary applications.
Electrical energy can be converted into various forms of mechanical energy, such as gravitational potential energy and kinetic energy, and can also be used to compress a gas, such as air. Some of these forms of energy are suitable for large-scale, long-duration energy storage (LDES). Mechanical energy systems tend to have large environmental footprints and often require a favourable geological setting to be viable in the first place.
Pumped-storage hydroelectricity (PSH) is a type of gravitational energy storage; water is pumped from a lower elevation reservoir to a higher elevation. When electricity is needed, e.g. for load balancing, the stored water is released through turbines to produce electric power. This represents some 90% of existing electric grid storage today. Existing PSH could also play a larger role in balancing supply and demand in other renewable energy generation technologies.
Liquid air energy storage (LAES) is a type of cryogenic energy storage: it involves storing air in liquid form at a very low temperature but near-ambient pressure. To generate electricity, the liquid air is heated to a gas, which is then used to drive a turbine.
Compressed air energy storage (CAES) systems store pressurised air underground in cavities or above ground in tanks. Some CAES systems also store the heat that is generated when the air is compressed. CAES has been widely discussed as a potential grid-scale energy storage option, but faces significant hurdles for deployment at scale, including cost.
Pumped thermal electricity storage (PTES) uses a heat pump to turn electricity into heat. The heat is then stored in a medium, such as water, gravel or sand, inside a thermally insulated tank. Using a heat engine, the heat is then turned back into electricity when needed.
Dry gravity energy storage (GES) systems use an electric motor to lift a mass, such as a very heavy rock mass, so that it acquires potential energy. This energy is then released by lowering the mass and using the motor as a generator of electricity.
Flywheel energy storage (FES) systems accelerate a rotor (flywheel) to a high speed using a motor, such that rotational kinetic energy is stored in the system. When energy is required, the rotational speed of the flywheel is reduced as electricity is generated.
Thermal energy storage (TES) means storing energy as heat. The energy stored in this way can be used on a short-term, daily basis, or on a longer-term basis, including seasonal storage, such that summer heat is stored in a certain medium and then re-used to heat buildings in winter. Materials can also be stored at a lower temperature for air-conditioning purposes.
Alternatively, energy can be absorbed or released during the phase transition of water or another medium, such as a salt or polymer, for the purposes of latent heat storage or release.
Thermochemical heat storage relies on a reversible exothermic/endothermic chemical reaction involving thermochemical materials (TCM), such as potassium oxide, calcium hydroxide or nitrosyl chloride. Depending on the reactants, this technique can yield an even higher storage capacity than latent heat storage. Exothermic/endothermic adsorption of e.g. water vapour by zeolites may provide a practical method of heat storage with a potentially unlimited lifetime.
Water has a relatively high heat capacity and is the obvious choice for liquid heat storage. However, molten salt may be stored at up to 1 400°C for the purposes of energy storage (molten salt energy storage, MSES). It can then be used on demand e.g. for the generation of superheated steam, which can in turn drive turbine generators that produce electricity.
Hot rocks, stone and concrete, potentially in the form of granular packed beds, can provide a low-cost but high-volume means of energy storage that can withstand high temperatures. Their use may be enhanced by means of e.g. heat pumps to store and extract the heat. Metals including alloy combinations can also offer a means of storage that is conducive to rapid heat transfer.
Phase change materials include salts, polymers, gels and alloys. Their different melting points enable tailored use for sensible heat storage according to demand. Ice-based technologies are also used for cooling purposes. These are often used in combination with heat pump exchange systems.
Steam accumulators use a combination of water and steam and can be applied on an industrial scale. The steam that is used to drive steam turbines can also be used for storage purposes if it is allowed to partially condense, creating a certain latent heat capacity. Steam power plants may also be combined with local heat storage using other technologies.
Solar thermal power plants may incorporate heat storage facilities to overcome the variability of solar (and wind) energy. This can be achieved by generating and accumulating steam, or even through thermal energy storage using molten salt (molten salt energy storage, MSES). Electricity can then be generated accordingly, on demand.