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The growth of renewable energy technologies, mainly wind and solar, demands the development of practical and economically viable energy storage technologies in order to balance the availability of supply with demand. This book explores the current state-of-the-art of energy storage and examines the likely environmental impacts of the main categories based on the types of energy involved.

The first chapter, written by Robert Lynch of the University of Limerick, Ireland, and his collaborators, provides an overview of energy sources and electricity supply grids, including fossil fuels, nuclear, renewables, biomass and geothermal sources. Relevant issues such as variable demand, smart grids, distributed generation, ramp times and the growth of electric vehicles are explored in relation to such issues as load levelling and stability of electricity grids. By far the largest providers of mechanical energy storage are pumped hydroelectric storage (PHS) and compressed air energy storage (CAES); these methods are examined in some detail in the second chapter by David Evans of the British Geological Survey (BGS) and his colleagues. Both storage technologies are well developed and offer the potential for better integration and penetration of renewable electricity sources and thus the reduction of greenhouse gas (GHG) emissions to the atmosphere. The chapter reviews the issues relating to the general operational parameters and also the legislative and environmental aspects of the two storage types, mainly in the context of the UK but also with reference to worldwide developments.

The third chapter, written by Noel Buckley of the University of Limerick and his collaborators, provides a wide-ranging overview of electrochemical storage systems with a focus on the three important types: rechargeable batteries, fuel cells and flow batteries. The relative strengths and weaknesses of these, from lead–acid batteries, through lithium and lithium-ion batteries, sodium–sulfur and nickel–metal hydride types, to the several kinds of fuel cell and flow battery are considered from the points of view of both the technologies and their applications and also their potential environmental impacts. This is followed in the fourth chapter, written by Fernando Rhen, also of the University of Limerick, and his collaborators, by a review of electrical storage technologies involving devices such as supercapacitors and supercapatteries, flywheels, superconducting magnets and synchronous condensers. These devices, having fast response times, can serve to correct short-duration fluctuations of electricity supply and demand that can cause instability in electrical grids and utility systems.

In the fifth chapter, written by Alexander Cowan and his colleagues at the University of Liverpool, photochemical energy storage methods are reviewed. This chapter highlights energy storage strategies that utilise solar energy to drive the formation of chemicals, fuels and feedstocks. The production of solar fuels that can be stored and transported is an attractive way to address the intermittency of terrestrial solar and provide sustainable access to the fundamental feedstocks on which society has come to rely. The solar energy-driven reactions considered here are the splitting of water to produce hydrogen and oxygen, and the coupled oxidation of water and reduction of CO2 to produce a variety of higher-value carbon products and oxygen. The chapter provides an introductory overview to both direct (photochemical) and indirect solar (photovoltaic-enabled electrolysis) routes to these fuels. The sixth chapter, the final chapter on storage technologies, deals with thermal (sensible heat and latent heat) and thermochemical energy storage and is written by Yukitaka Kato and Takahiro Nomura of the Tokyo Institute of Technology and Hokkaido University, respectively. This introduces the concepts of phase-change materials and chemical heat pumps and discusses encapsulation, composite materials, heat exchangers, application to concentrated solar power plants and the various types of chemical heat pump currently under development.

Smart energy systems are reviewed in the seventh chapter, by Rasmus Lund and his colleagues at Aalborg University, Denmark. A smart energy system is a combination of the currently isolated energy sectors, such as electricity, heating and transport, and it includes three smart energy grid infrastructures, namely the electric, thermal and gas grids. These grids connect the energy resources with the demands, energy production, energy storage and interconnection points. From the case studies examined in detail, it is concluded that hydroelectric storage, batteries in electric vehicles, thermal storage in district heating systems and storage of renewable electrofuels are important and provide a cost-efficient flexibility to the overall energy system, although large-scale batteries on the grid level and stationary batteries in buildings are not feasible from an energy system perspective. In the eighth chapter, Heidi Hottenroth of Pforzheim University, Germany, and her colleagues review the application of life-cycle assessment (LCA) for determining environmental impact in the context of stationary energy storage systems. The LCA technique is applied to three different case studies involving pumped hydroelectric storage, lithium-ion batteries and combined heat and power plants in order to determine which energy storage system is the best option in a specific setting. Finally, business opportunities and the regulatory framework are examined in the ninth chapter, by Reinhard Madlener and Jan Martin Specht of Aachen University, Germany. These involve assessment of the economic viability and cost competitiveness of the different storage methods and the various flexibility options competing with each other to balance supply and demand. Market and regulatory conditions and also underlying uncertainties are considered in relation to business models and return on investment.

We are pleased to have engaged this international group of experts to produce wide-ranging overviews of the important area of energy storage. We are confident that this volume will provide a valuable resource for decision makers, scientists and engineers, and equally for practitioners and students involved with the globally ongoing sustainable energy transition.

Ronald E. Hester

Roy M. Harrison

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