Electricity generated by renewables, such as wind energy and solar power, is based on variable resources. Electricity storage can optimise the energy flows between supply and demand and therefore enable a higher contribution of renewable energy in our electricity mix.
Renewables may also not be fully available at the moment when demand is higher or they may supply an excess when the demand is lower, thus creating an imbalance. Electricity storage can overcome the mismatch between output and demand (the so-called time-shifting) and it can smooth out fluctuations in supply without calling on other back-up capacities. It can also save a supplier from penalties when forecast supply cannot be met (the so-called forecast hedging). The principal electricity storage technologies include hydropower with storage,compressed air energy storage, flow batteries, hydrogen-based energy systems,secondary batteries, flywheels, super capacitors, and superconducting magnetic energy storage.
A wide range of technologies is available to store electricity, including those based on mechanical, chemical and physical principles. Ultimately, the main services that the storage has to provide will dictate the best-adapted technology. For example, there are energy-related applications, where the electricity storage system is designed to discharge for several hours, with a nominal storage capacity of 10 to 500 MW and a time response of 1 to 5 minutes. On the other hand, there are power-related applications, such as maintaining grid frequency, suppressing fluctuations and stabilising voltages, that discharge for between a few seconds to less than an hour and require a response time of a few milliseconds.
The most mature storage technology is hydropower - with either reservoir storage or hydro-pumped storage. The basic principle is to store energy as the potential energy between two reservoirs at different elevations. The wide deployment of hydropower in Europe offers a significant technology base for regulating variable electricity production. The average plant size in the EU-27 is about 270 MW, but can reach 1800 MW, as in France (Isère) and Wales (Dinowig). Most of any increase in pumped hydropower storage in Europe will come from retro-fitting of existing installations, or adding pumped storage to conventional reservoir-based facilities.
Compressed air energy storage systems (CAES) are a hybrid form of storage that is already commercially used for large-scale energy storage. In a CAES system, the compression cycle of a gas turbine is decoupled from its expansion cycle over time. Air is pre-compressed and stored separately in a geological formation, prior to its use in the gas turbine. Despite its reliance on mature technologies, CAES systems are not widespread around the globe.
Ongoing research and development
Flow batteries are at the early commercialisation phase of development. The technology relies on the reversible conversion of electro-chemical potential into electricity and involves storing electrolyte externally in a tank and drawing on it as required.
The concept of hydrogen-based energy storage is currently in a demonstration phase with a focus on wind applications for remote communities. In this technology, hydrogen produced by electrolysis is transformed back into electricity in times of demand by means of a fuel cell or combustion engine/turbine. Future developments aim at increasing efficiency and reducing costs, while exploring the potential for polygeneration (electricity, heat and hydrogen).
Several different types of secondary battery are currently being used for stationary applications, such as lithiumion (Li-ion), sodium sulphur (NaS), nickel cadmium (NiCd), nickel metal hydride (Ni-MeH) and lead acid (Pb-acid) batteries. Each has its own advantages and disadvantages. NaS are of particular interest for energy management applications, with an expected market development of about 1GW worldwide.
Meanwhile, lead-acid batteries are a mature technology, and the most common type in stationary and automotive applications. Worldwide there are several large stationary projects to improve grid performances based on lead-acid batteries. Lithium batteries are of particular interest, as they rely on the properties of lithium metal, the most electropositive and lightest metal. Lithium-ion is the most mature lithium technology.
Flywheel storage systems are still in the demonstration phase for power applications. These systems store energy mechanically in the form of kinetic energy and are generally divided into low speed (up to 5000 rpm) and high speed (50 000 rpm) systems. Although they can reach high efficiency (over 90%), the main drawback in development is the high investment cost. The main potential application is for voltage regulation. A 20MW system is envisaged
for commercialisation in the near future.
Supercapacitors rely on the separation of charge at an electric interface to store energy. Supercapacitors consist of two electrodes of opposite polarity immersed in an electrolytic solution. The use of a liquid electrolyte rather than a dielectric solid material is the major difference with conventional capacitors. Supercapacitors are mainly devoted to very short peak power applications. Nano-carbon materials are currently being investigated.
Superconducting Magnetic Energy Storage (SMES) is a relatively new power storage technology that stores energy in a magnetic field created by a DC current. The response time of SMES is less than a few milliseconds. The main applications foreseen for SMES systems are voltage fluctuations and providing fault-ride through support. To date, only micro-SMES (1 to 10 MW) have been commercialised. One of the major lines of progress of SMES is to develop high-temperature superconductor technology to reduce their cost. At present, the reported energy storage capacity and power accessible domain of SMES lies roughly between less than 1 kWh to 10 kWh and 1 MW to 10 MW respectively, with a long term objective for large scale SMES of about 100 MW, with up to 50 kWh, with efficiencies of 99 % and a lifetime of 40 years based on high-temperature superconductors.
THE INDUSTRY
The European industry has currently a strong market leadership in large-scale energy storage technology. Three market leaders for hydro-pumped storage are based in Europe.
Similarly, although Compressed Air Energy Storage technologies are not widely deployed, one of the two projects currently in operation was built with European technologies. A new facility with a storage capacity of 150 MW is currently in a planning phase in Iowa (USA) and is expected to be in operation by 2011. For fuel cell and hydrogen technologies, the establishment of a Joint Undertaking in 2008 will help develop and strengthen the European Industry.
For intermediate or smaller scale technologies, the European industrial base is weaker. Flywheels and flow battery manufacturers are mostly based outside Europe. For batteries and super-capacitors, although there are world-class European manufacturers, the overall battery market is dominated by Asian manufacturers.
Barriers
The main barriers facing electricity storage can be divided into four categories: market uncertainty; market structure and regulation; current electricity pricing and economics; and performance of storage technologies. Apart from hydropower, most of the other storage technologies still need R&D efforts to reduce costs and improve performance.
Two key priorities are to bring the technologies to a stage of commercial maturity and to accelerate the transition to mass commercialisation.
Needs
With the increasing penetration of variable energy sources, greater attention has to be paid to regulatory aspects. There is also a need to strategically address the transition towards a low-carbon power system. For example, the current hydropower system, with its regional diversity, can be operated in a more flexible way and provide additional storage capacity to the European system as a whole, provided grid connections are in place.
There is also a need for industrial-scale demonstration projects for the near-to-market deployment technologies, such as compressed air energy storage
and lithium-ion batteries. A common assessment framework to evaluate the market potential for electricity storage would enable the industry and public authorities to make decisions on investments in this area.
Installed capacity
In the EU-27 and non-EU Member States of the EEA, about 40 GW of hydropumped storage are in operation, with Italy, Spain, Germany and France having the largest installed base in Europe. Currently, it is estimated that about 75 % of the global potential for hydropower is already developed in Europe.
