Renewable and Sustainable Energy Reviews 13 (2009) 1513–1522 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Overview of current and future energy storage technologies for electric power applications Ioannis Hadjipaschalis, Andreas Poullikkas *, Venizelos Efthimiou Electricity Authority of Cyprus, P.O. Box 24506, 1399 Nicosia, Cyprus ARTICLE INFO ABSTRACT Article history: In today’s world, there is a continuous global need for more energy which, at the same time, has to be Received 3 September 2008 cleaner than the energy produced from the traditional generation technologies. This need has facilitated Accepted 30 September 2008 the increasing penetration of distributed generation (DG) technologies and primarily of renewable energy sources (RES). The extensive use of such energy sources in today’s electricity networks can Keywords: indisputably minimize the threat of global warming and climate change. However, the power output of Power generation these energy sources is not as reliable and as easy to adjust to changing demand cycles as the output from Distributed generation the traditional power sources. This disadvantage can only be effectively overcome by the storing of the Energy storage excess power produced by DG-RES. Therefore, in order for these new sources to become completely Electricity storage reliable as primary sources of energy, energy storage is a crucial factor. In this work, an overview of the current and future energy storage technologies used for electric power applications is carried out. Most of the technologies are in use today while others are still under intensive research and development. A comparison between the various technologies is presented in terms of the most important technological characteristics of each technology. The comparison shows that each storage technology is different in terms of its ideal network application environment and energy storage scale. This means that in order to achieve optimum results, the unique network environment and the specifications of the storage device have to be studied thoroughly, before a decision for the ideal storage technology to be selected is taken. ß 2008 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 1513 2. Flywheel storage technologies . 1514 3. Battery storage technologies . 1515 4. Supercapacitor storage technologies. 1517 5. Hydrogen storage technologies . 1518 6. Pneumatic storage technologies . 1519 6.1. Liquid-piston technology . 1519 6.2. Compressed air energy storage . 1520 7. Pumped storage technology . 1521 8. Conclusions . 1521 Acknowledgements. 1522 References . 1522 1. Introduction In today’s world, the need for more energy seems to be ever- increasing. Both households and industries require large amounts * Corresponding author. Fax: +357 22 201809. of power. At the same time the existing means of energy pro- E-mail address: [email protected] (A. Poullikkas). duction face new problems. International treaties aim to limit the 1364-0321/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.rser.2008.09.028 1514 I. Hadjipaschalis et al. / Renewable and Sustainable Energy Reviews 13 (2009) 1513–1522 levels of pollution, global warming prompts action to reduce the Section 5 the hydrogen storage technologies are presented. The output of carbon dioxide and several countries have decided to pneumatic storage technologies are presented in Section 6 and decommission old nuclear power plants and not build new ones. In the pumped storage technology is discussed in Section 7. The addition, the unprecedented global increase in energy demand has conclusions are summarized in Section 8. meant that the price of conventional energy sources has risen dramatically and that the dependence of national economies on a 2. Flywheel storage technologies continuous and undistorted supply of such sources has become critical. A flywheel is a mass rotating about an axis, which can store Such development brings about the need to replace old energy energy mechanically in the form of kinetic energy. Energy is required production methods with new ones. While several are in develop- to accelerate the flywheel so it is rotating. This is usually achieved by ment, including the promising nuclear fission power, other an electric motor when being used in an electrical system. Once it is production methods are already in commercial use. The penetration rotating, it is in effect a mechanical battery that has a certain amount of renewable energy sources and of other forms of potential of energy that can be stored depending on its rotational velocity and distributed generation sources is increasing worldwide. These types itsmoment of inertia. The faster a flywheel rotatesthe more energy it of energy sources often rely on the weather or climate to work stores. This stored energy can be retrieved by slowing down the effectively, and include such methods as wind power, solar power flywheel via a decelerating torque and returning the kinetic energy and hydroelectricity in its many forms. to the electrical motor, which is used as a generator. These new sources of energy have some indisputable advan- Apart from the rotating flywheel, the other main components of tages over the older methods. At the same time, they present new a flywheel storage system are the rotor bearings and the power challenges. The output of the traditional methods is easy to adjust interface as illustrated in Fig. 1 [5]. The flywheel can be either low according to the power requirements. The new energy sources are speed, with operating speeds up to 6000 rpm, or high-speed with based more directly on harnessing the power of the nature and as operating speeds up to 50,000 rpm [2]. Low speed flywheels are such their peak power outputs may not match the power usually made of steel rotors and conventional bearings. Typical requirements. They may exhibit large fluctuations in power specific energy achieved is around 5 Wh/kg. High-speed flywheels output in monthly or even annual cycles. Similarly, the demand use advanced composite materials for the rotor with ultra-low can vary monthly or annually. Therefore, in order for these new friction bearing assemblies. These light-weight and high-strength sources to become completely reliable as primary sources of composite rotors can achieve specific energy of 100 Wh/kg. Also, energy, energy storage is a crucial factor. Essentially, energy from such flywheels come up to speed in a matter of minutes, rather these sources must be stored when excess is produced and then than the hours needed to recharge a battery. The container for released, when production levels are less than the required high-speed flywheels is evacuated or helium filled to reduce demand. Energy storage technologies form therefore an integral aerodynamic losses and rotor stresses. and indispensable part of a reliable and effective renewable and The power interface includes the motor/generator, a variable- distributed generation unit. speed power electronics converter and a power controller. The There are other reasons why it is necessary to store large motor/generator is usually a high-speed permanent magnet amounts of energy. Depending on how storage is distributed, it machine integrated with the rotor functioning as an integrated may also help the network withstand peaks in demand. Storing synchronous generator. The converter is usually a pulse width energy allows transmission and distribution to operate at full modulated bi-directional converter which can be single-stage (ac capacity, decreasing the demand for newer or upgraded lines and to dc) or double-stage (ac to dc to ac) depending on the application increasing plant efficiencies. Storing energy for shorter periods requirements. Finally, a power controller is required to control may be useful for smoothing out small peaks and sags in voltage. power system variables. There is clearly a need for energy storage, specifically energy The main advantages of flywheel storage systems are the high storage in a larger scale than before. Traditional energy storage charge and discharge rates for many cycles. Indeed, the high cycling methods, such as the electrochemical cell, are not necessarily capability of flywheels is one of their key features and is not applicable to larger-scale systems, and their efficiency may be dependent on the charge or discharge rate. Full-cycle lifetimes range suboptimal. Meanwhile, a number of new and promising methods from 105 up to 107. In fact, the limiting factor in some applications is are in development. Some of these are based on old concepts more likely to be the flywheel lifetime which is quoted as typically applied to modern energy storage, others are completely new 20 years. Also, typical state-of-the-art composite rotors have high ideas. Some are more mature than others, but most can be further specific energies, up to 100 Wh/kg, with high specific power. Their improved. In this work, an overview of the most important energy storage methods available or under development today is carried out. Clearly, the technologies and underlying principles for each storage method can vary to a large extend, thus diversifying significantly the spectrum of available energy storage products. This means that each method can be quite different in terms of its ideal application environment and energy storage scale. More specifically, while one method of storage may be ideal to smooth out annual fluctuations, another may be suitable to satisfy very short peak power requirements. Finally, where applicable, comparisons between the different methods are undertaken, in order to provide a basic understanding of the differences of each technology. In Section 2, the flywheel technologies are presented and, in Section 3, the battery storage technologies are discussed. In Section 4, the supercapacitor storage technologies are illustrated and in Fig. 1. Main components of a flywheel storage system [5]. I. Hadjipaschalis et al. / Renewable and Sustainable Energy Reviews 13 (2009) 1513–1522 1515 energy efficiency is typically around 90% at rated power. The main the chemical energy generated by electrochemical reactions.