Solar Energy Storage

Bent Sorensen

• Publisher: Academic Press
• Year: 2015
• ISBN:
  9780124095496
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

While solar is the fastest-growing energy source in the world, key concerns around solar power's inherent variability threaten to de-rail that scale-up. Currently, integration of intermittent solar resources into the grid creates added complication to load management, leading some utilities to reject it altogether, while other operators may penalize the producers via rate increases or force solar developers to include storage devices on-site to smooth out power delivery at the point of production. However these efforts at mitigation unfold, it is increasingly clear to parties on all sides that energy storage will be pivotally important in the drive to boost the integration of variable renewable sources into power infrastructures across the globe. Thoughtfully implemented storage technologies can reduce peak demand, improve day-to-day reliability, provide emergency power in case of interrupted generation, reduce consumer and utility costs by easing load balance challenges, decrease emissions, and increase the amount of distributed and renewable energy that makes it into the grid. While energy storage has long been an area of concern for scientists and engineers, there has been no comprehensive single text covering the storage methods available to solar power producers, which leaves a lamentable gap in the literature core to this important field. Solar Energy Storage aims to become the authoritative work on the topic, incorporating contributions from an internationally recognized group of top authors from both industry and academia, focused on providing information from underlying scientific fundamentals to practical applications, and emphasizing the latest technological developments driving this discipline forward.

• Expert contributing authors explain current and emergent storage technologies for solar, thermal, and photovoltaic applications
• Sheds light on the economic status of solar storage facilities, including case studies of the particular challenges that solar energy systems present to remote locations
• Includes information on: chemical storage mechanisms, mechanical storage tactics, pumped hydro, thermal storage, and storage strategies for systems of all sizes—from centralized utilities to distributed generation

Chapter 1

Introduction and Overview

Bent Sørensen    Department of Environmental, Social and Spatial Change, Roskilde University, Roskilde, Denmark

Abstract

The contents of the book is introduced by way of a short overview of the reasons for considering energy storage in connection with either thermal or electric solar energy systems.

Keywords

solar thermal energy

solar electric energy

storage technologies

For some five decades, the cost of solar photovoltaic systems has declined from space industry levels toward affordability for individual consumers, helped by the occasional government subsidy programs in effect in one country or another for a limited time. The current decade is seeing solar panel prices being increasingly accepted by customers without subsidies; consequently, a rapid augmentation of installed capacity has taken place (IEA, 2014). A similar but less dramatic development has happened for solar thermal systems, having become common first as hot water providers, and now also for space heating in the intermediate latitude regions that need winter space heating but are still close enough to the equator to have a decent number of sunshine hours in winter. These developments make it timely to think of the obstacles for solar energy techniques to eventually become a major component in our energy systems. Among these problems, intermittency seems to be the most important. Either solar energy should be combined with other energy sources capable of complementing supply during dark hours, or devices for storing and regenerating energy must be part of the solar energy system. The best solution is probably a combination of including energy sources with a different delivery profile, having energy storage available, and taking advantage of energy transmission and trade possibilities, as well as any load management options that would be acceptable to the energy users.

This book focuses on energy stores suitable for integration into solar energy systems for delivering electric or thermal power to the end users. The combination of all the methods hinted at above is the subject of another recent book on energy intermittency, being the first to chart the global intermittency problem and to look at all the options for a sustainable energy system based on several renewable energy sources, storage, trade, and demand management, including the possible synergies offered by combining the solutions for dealing with intermittency (Sørensen, 2014).

In looking specifically at energy storage, two distinct areas of application for solar energy can be discerned. One is that of storage systems aimed at handling the obvious day-to-night storage requirement, caused by human activities not being restricted to hours of high solar radiation input. The second area is the seasonal storage that will store energy generated in summer or adjacent periods for use in winter at mid or high latitudes. Electricity demand varies over the year, with differences between different types of societies, but it is still fairly evenly distributed over the year. This is at least true compared with the situation for solar space heating, where the demand is obviously inversely correlated with solar supply, as soon as one departs substantially from the equator. Between these two extremes are the intermittency problems caused by passage of weather systems, with clouds and other atmospheric obstacles to receiving solar radiation at the Earth's surface. If solar intermittency is to be handled by energy stores, the storage systems must comprise units operating on diurnal, weekly, and seasonal time scales.

Several of the energy stores currently developed and ready for use are directed at shorter periods of storage. The shortest storage periods, related to fluctuations of solar energy production over seconds or minutes, are usually unimportant for heating purposes; for electric power production, any grid-connected system with some kilometer spacing between solar panels will smooth out such small-scale fluctuations. For isolated systems, short-term fluctuations may be handled by adding capacitor stores or flywheels, but even in this case the importance is declining, because the present generation of electricity-using equipment is either not very sensitive to modest variations in voltage or frequency (if AC), or it has built-in short-term storage, usually in the form of supercapacitors.

Moving on to storage levels capable of handling the variations in solar energy production during the day, or the nighttime nonproduction, the relevant storage periods would be 12-18 h, considering that the reduced solar radiation away from noontime may not correlate with the variations in demand. Stores of interest for this length of drawing time (in most cases also with a prolonged store-filling time) include various types of batteries, compressed air underground stores, pumped hydro, as well as fuel containers, which in a renewable energy scenario would have to contain fuels that can be created from biomass or electricity (such as certain biofuels and hydrogen). A range of such stores will be described and evaluated in the following chapters.

Finally, for storage lasting more than a week (as needed during episodes of heavy cloud cover or extended rainy periods) and seasonal storage (needed at latitudes with a substantial difference between summer and winter levels of incoming solar radiation, and for heat also because of temperature-related variations in space-heating demands), there are fewer storage options available. Economic considerations rule out storage in containers as compressed air and probably also in batteries, due to the cost being proportional to the amount of energy stored. Pumped hydro will remain an option as long as the upper reservoir is large enough, and so will fuel storage if containers can be replaced by inexpensive underground cavity options. Such cavities can be found in abandoned oil or gas wells, or they can be formed in certain aquifers and geological salt deposits (Figure 1.1). Both aquifer bends offering sections suitable for displacing water by a gas and slowly washed out cavities in salt domes are in substantial use today for natural gas storage, such as the strategic stores common in some European countries, holding 4-6 months of gas demand as a buffer against disruptions caused by major rupture of undersea gas supply pipelines or by political delivery blockades (Russian gas).

Figure 1.1 Types of underground gas stores (for hydrogen, natural gas, or air at various levels of compression): (a) storage in salt domes, optionally using canisters if cavity walls are too porous or do not accept liners; (b) drilled rock cavities or abandoned wells, possibly with a compensating surface reservoir; and (c) storage in an upward bend of an aquifer running between nonporous layers (e.g., of clay). From Sørensen (2010), used with permission from Elsevier.

In connection with solar power and other sources of power generated by variable renewable energy flows, the relevant gas to store would be hydrogen. This is independent of the fate of the current efforts to develop hydrogen fuel cells for the automotive sector, because the solar-produced hydrogen stored could be used to regenerate electricity by conventional gas turbines, a low-cost technology available today. By the way, one should remember that fuel cells and electrolyzers are the same thing: all fuel cells can be used in reversed mode to generate hydrogen from electricity. Currently, common high-efficiency electrolyzers are alkaline fuel cells, but also the proton-exchange membrane fuel cells relevant for the transportation sector or the solid-oxide fuel cells aimed at electric power sector applications may be operated in reverse mode as electrolyzers (electricity to hydrogen) (Sørensen, 2011).

The underground hydrogen stores seeming to offer the most viable solution for longer time storage of solar electricity are often missing from the scientific literature on stores for renewable energy systems, but have in recent years taken the foreground in national and regional planning studies both in the United States (Lord et al., 2011) and in the European Union (Simón et al., 2014). Here, they are dealt with in the chapter on environmental issues.

As regards thermal energy storage, the most common technology is heat capacity storage using water, either in connection with individual building solar installations or in a central location for district heating based on solar collector arrays. Alternatives using gravel or other materials have been tested, as have phase change materials, but without pointing to any clear winners. In many locations, the best option for covering winter space loads in the absence of sufficient solar energy collection is to use renewable electricity or electricity from stores through heat pumps (Sørensen, 2014). A selection of such systems is discussed in the chapters below.

References

IEA, 2014. Trends 2014 in photovoltaic applications. PVPS report T1-25:2014. International Energy Agency, Paris.

Lord, A., Kobos, P., Klise, G., Borns, D., 2011. A life cycle cost analysis framework for geological storage of hydrogen: a user's tool. Report SAND2011-6221. Sandia National Laboratories, Albuquerque/Livermore.

Simón, J., et al., 2014. Assessment of the potential, the actors and relevant business cases for large scale and long term storage of renewable electricity by hydrogen underground storage in Europe. Final report D 6.3 from the European Commission funded project “HyUnder”, Zaragosa.

Sørensen B. Renewable Energy—Its Physics, Engineering, Environmental Impacts, Economics & Planning. In: fourth ed. Burlington: Academic Press/Elsevier; 2010 (5th edition currently in preparation).

Sørensen B. Hydrogen and Fuel Cells. second ed. Burlington: Academic Press/Elsevier; 2011.

Sørensen B. Energy Intermittency. Baton Rouge: CRC Press, Taylor & Francis; 2014.

Part I

Solar Energy Storage Options

Chapter 2

Solar Electrical Energy Storage

Yulong Ding¹; Yongliang Li¹; Chuanping Liu²; Ze Sun^{3 1} School of Chemical Engineering & Birmingham Centre of Energy Storage, University of Birmingham, Birmingham, UK
² Department of Thermal Engineering, University of Science and Technology Beijing, Beijing, China
³ National Engineering Research Centre for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai, China

Abstract

Solar power is expected to play an important role in the future electricity supply chain. However, many challenges remain to be overco...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Preface
• Chapter 1: Introduction and Overview
• Part I: Solar Energy Storage Options
• Part II: Economic Assessment of Solar Storage
• Part III: Environmental and Social Impacts
• Part IV: Case Studies
• Index