Chapter 1
Introduction

Billions of years ago, during the Big Bang nucleosynthesis, chemical energy was stored in the chemical elements. We now store electrochemical energy in our modern-day batteries. Over the estimated 14 billion years life of our universe, energy storage has been and will continue to be an essential part of all things in existence.

In the last few decades, we have become dramatically more dependent on reliable and long-lasting energy storage. Several reasons have contributed to this increased reliance, including the widespread use of modern portable or wearable devices, significant growth of human population, and the rising demands of the 21st century lifestyle. Meanwhile, there are other driving factors at play. Our conventional sources of energy like non-renewable fossil fuels continue to decline, and the environmental, economic and political concerns surrounding the generation and use of energy are growing, leading to a wide range of energy challenges and concerns.

To resolve and manage the unique energy challenges of the 21st century, much effort has been directed toward identifying the most efficient, effective and durable energy storage solutions that can adequately meet our multifaceted life requirements. In this book, we will address a wide range of energy storage methods with emphasis on their basic mechanisms. In this chapter, we will first discuss the "energy problem" and the motivation underpinning the development of efficient approaches to store energy. We'll then classify, compare and discuss various methods of energy storage. In the final section, an overview of this book will be provided.

1.1 The Energy Problem

All the discussions and dire announcements in technical literature during recent years have certainly made everyone aware of the "energy problem." There is not much doubt that we are confronted with a real problem of domestic and international importance. The critical issues concerning the availability of energy sources and their efficient use are rapidly becoming vitally important. Increasing population, in conjunction with the greater-than-ever energy and materials demands that people are making in order to increase their comfort, travel, and other lifestyle choices is indeed causing greater stress. All of these require not only an increased availability of energy but also more effective ways of utilizing what is available.

1.1.1 Increasing Population and Energy Consumption

Human population is growing at a rapid rate. The world population is projected to reach about 9 billion by 2040, as depicted in Figure 1.1. Consequently, energy consumption has risen significantly due to the increasing human dependency on various technologies and the power and energy they require.

Figure 1.1 World population increase.

According to the 2016 International Energy Outlook (IEO) by U.S. Energy Information Administration (EIA), the total world energy consumption is projected to surpass 900 quadrillion Btu by 2040. British thermal unit (Btu) is defined as approximately 1,055 Joules. The electrical power and industrial sectors are projected to each increase over 300 quadrillion Btu by 2040. The measured and projected total delivered energy consumption per sector is shown in Figure 1.2. Clearly, we must seek sustainable and global energy solutions to combat the significantly large energy consumption demands of the 21st century.

Figure 1.2 World energy consumption by sector type.

The main efforts of research and development have been directed toward the development of new alternatives or finding more primary sources of energy. For the present, and until the discovery of a new class of phenomena, we have a fairly good idea of what can be accomplished. We know what alternative sources are possible - alternative presumably to petroleum products. Yet none of them are nearly as attractive for portable or motive power unless we significantly lower our criteria.

It would appear that sources of energy are plentiful on planet Earth. However, they are often locally unavailable, too bulky, too unpredictable (solar and wind), and/or too dangerous to be portable. An effective method for storing energy would greatly reduce the problem and would provide low-cost energy for everyone. It seems that not nearly as much attention or support has been directed toward the problem of storage as that which has been directed toward generation. Perhaps this difference is due to the absence of many promising approaches to accomplishing the latter. This book presents a different approach and aims to stimulate additional efforts toward the search and development of better storage.

1.1.2 The Greenhouse Effect

The Earth is believed to be approximately 5 billion years old. For a relatively long time, life could not be sustained outside the ocean. This is mainly due to the absence of a properly formed atmosphere that could protect life from lethal radiations. Several billions of years later (about 600 million years ago) the Earth's atmosphere, capable of protecting life, was formed.

For millions of years, the atmosphere and the surface of Earth have effectively reflected the solar radiation to Space, and the consequent infrared (IR) radiation has passed through the atmosphere back to Space as depicted in Figure 1.3. However, it is believed that more recently, some of the IR radiation has been absorbed by the greenhouse gases below the Earth's atmosphere and the absorbed IR radiation is re-emitted back to Earth. This phenomenon is commonly referred to as the "Greenhouse Effect". It can be mainly attributed to the use of fossil fuels and the human industrial evolution. The greenhouse effect presents yet another major reason for the global community to actively move toward environmentally friendly and renewable energy harvesting, conversion and storage.

Figure 1.3 (1) Solar rays radiated toward Earth, (2) some reflected by Earth's atmosphere, (3) some reflected by Earth's surface, (4) infrared radiation passes through the atmosphere and out to Space, and (5) some of the infrared radiation is absorbed and re-emitted back to Earth by the greenhouse gases.

1.1.3 Energy Portability

Energy portability is another major challenge. We cannot carry windmills around - they are huge and dangerous. A waterfall, due to topographical considerations, is not available everywhere, and its size is immense for the intermittent power and energy produced. Solar cells can be designed to be portable but their usage is confined to the available sunny days and limited daytime hours. There are really not too many attractive choices for portable energy harvesting and storage.

Batteries and supercapacitors are the least obtrusive and the most predictable limited secondary sources, but they are not practical as large-scale primary or secondary sources. Windmills and photovoltaic cells are almost useless without either storage or the assistance of an electric utility power grid, which operates on nuclear power or coal fuel. It would appear that the energy source trap has merely changed shape.

Ideally, high energy and power density "batteries" of some sort that are charged by nuclear or fossil fuel would be a good solution for smoothing the irregularities in the distribution and availability for the planet's population. The term "batteries" used here only refers to some mechanism for practical storage. So far, the most promising is probably an electrochemical method. Compressed air, metal springs, flywheels, etc., all have very serious drawbacks. Most generating facilities are not portable, nor would most people wish to live with them in their midst.

1.2 The Purposes of Energy Storage

Storing energy in its many forms in nature is a vital part of all processes as well as life itself on Earth. As we explore these processes and their importance to us, we can gradually make observations that lead to some revealing conclusions.

One of our purposes is to examine the general area of energy storage and to identify the key mechanisms that have significant roles in nature and civilization. Then we will develop a description and a reasonably detailed understanding of why we need to store energy and how the various mechanisms we employ work to satisfy these needs. Most of this book is devoted to electrochemical processes and, in particular, full flow electrolyte cells that are frequently referred to as redox batteries.

In a very general sense, there are only three purposes for the storage of energy: to make an energy supply portable from essentially non-portable sources, to store from an ongoing source for use at a later time, and to change the ratio of power-to-energy, as accomplished by flywheels, capacitors, etc.

All applications of energy storage can be put into one or more of these categories. Certainly, if we wish to power a portable power tool or an electric automobile, a hydroelectric plant is hardly practical. However, if we use the energy produced by the hydroelectric station and store part of what is not immediately needed in an electric battery, it becomes a pragmatic concern for the electric vehicle. Nuclear energy sources are hardly portable on a small scale. But in similar fashion, as for non-portable hydroelectric stations, storing portions of the generated energy in some sort of device such as a battery could become useful in mobile electric vehicles.

In the second instance, we might have the need to...