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Introduction

Energy is essential to our lives. Our bodies need energy to function and to perform physical activities. And the technological age in which we live needs a reliable energy supply for heating, lighting, communication, transport, food production, manufacturing goods, and so on. Because of their importance, issues such as the supply and cost of energy and the environmental impact make frequent appearances on the daily news. In this introductory chapter we consider energy consumption and the energy resources available to us. We consider the general characteristics of energy sources and the transformation of energy from one form to another to suit the end use. We also consider the role of energy storage.

1.1 Energy consumption

We consume energy in maintaining our vital bodily functions, such as the operation of the heart and lungs, the maintenance of body temperature, brain function and digestion of the food we eat. Roughly speaking, in maintaining these functions we consume energy at the rate of ∼100 J/s; a power of ∼100 W. We also expend energy when we do physical work. Suppose, for example, that we climb stairs and rise at the rate of 0.5 m/s in vertical height. If our mass is 75 kg, our rate of doing work is 75 kg × 9.8 m/s2 × 0.5 m/s = 368 W. The amount of physical activity that a person does depends on their lifestyle. Suppose, however, that, averaged over the course of a 24-hour period, we consume energy at the average rate of 125 W in maintaining our metabolic rate and performing physical work. This amounts to ∼10 MJ of energy per day. This energy comes from the chemical energy stored in the food that we eat; a tin of baked beans, for comparison, contains ∼1.5 MJ of energy. We also need energy to heat and light our houses, to run washing machines and refrigerators, to travel to work, to use computers, to fly to a foreign country on holiday, and so on. Furthermore, energy is needed to produce the food we eat, to manufacture and transport the goods we buy, etc. Overall, the total energy consumption per person per day in the UK is ∼450 MJ. When we consider energy consumption, it is perhaps more meaningful to use the kilowatt-hour (kWh) unit of energy. This is the energy consumed by a 1 kW electric fire in 1 hour and the conversion factor is 1 kWh = 3.6 MJ. So 450 MJ/day = 125 kWh/day, which is the amount of power consumed by five 1 kW electric fires running day and night. This figure of 125 kWh per person per day is typical for a European country. In the USA, the energy consumption per person is about twice as high, while in underdeveloped countries it is considerably lower. Averaged over all countries, energy consumption is ∼60 kWh per person per day and this amounts to a total global energy consumption of ∼5 × 1020 J/year.

Global consumption of energy continues to increase because of advances in technology, growth in world population and economic growth, factors that are interrelated. Figure 1.1 illustrates the dramatic increase in annual global consumption of energy that occurred between 1820 and 2010. As an example of a technological advance, James Watt patented his steam engine in 1769 and this enabled the Earth's deposits of fossil fuels such as coal to be unlocked. This signalled a sharp increase in energy consumption, and once industrialisation occurred, the rate of consumption increased dramatically; over the course of the 20th century, global use of energy increased more than 10-fold. The world's population has also increased dramatically over the last few hundred years, rising from 1 billion in 1800 to 7.4 billion in 2016. Indeed the curves for global energy consumption and global population follow each other quite closely. Presently, global population is increasing at a rate of just over 1% per year. The rate of economic growth is different for different countries. However, averaged over all countries, economic growth also increases at about 1% per year. Taking the various factors into account, it is predicted that the growth in global energy consumption over the next 30 years will be ∼2% per year.

Figure 1.1 Illustration of the dramatic rise in annual global energy consumption that occurred between 1820 and 2010.

A complementary aspect of energy consumption is the efficiency with which energy is used. No source of energy is cheap or occurs without some form of environmental disruption, and it is important that energy is used as efficiently as possible. One particular advance can be seen in the use of electric light bulbs. It is estimated that lighting consumes about 20% of the world's electricity. Traditional incandescent light bulbs with a wire filament are only about 5% efficient, while new types of lighting are much more efficient. LED lighting, for example is about 20% efficient.

1.2 Energy sources

The main sources of energy available to us are:

• fossil fuels
• solar energy
• biofuels
• wind energy
• nuclear energy
• waves and tidal energy
• hydroelectric energy
• geothermal energy.

Most of the energy available to us comes directly or indirectly from the Sun. The Sun gets its energy from nuclear fusion reactions that heat its core to a temperature of ∼107 K. Energy is transported to the Sun's surface and maintains the surface at a temperature of ∼6000 K. The hot surface acts as a blackbody radiator emitting electromagnetic radiation and it is this radiation or sunlight that delivers solar energy to the Earth. The total solar power that falls on the Earth is enormous, ∼1.7 × 1017 W, which is about 25 MW for every person in the world.

Sunlight provides us with energy in various ways. Photosynthesis is the process by which plants and other organisms use sunlight to transform water, carbon dioxide, and minerals into oxygen and organic compounds. Fossil fuels that we burn, including oil, coal and natural gas, were formed over millions of years by the action of heat and pressure on the fossils of dead plants. Bioenergy comes from biofuels that are produced directly or indirectly from organic matter, including plant material and animal waste; an example is rapeseed oil, which produces oil for fuel. Wood also fits into this category and, indeed, burning wood is by far the oldest source of energy used by humankind. Hydroelectric power, wind power and wave power can also be traced back to the Sun. Solar energy heats water on the Earth's surface, causing it to evaporate. The water vapour condenses into clouds and falls as precipitation. This fills the reservoirs of hydroelectric plants, and the potential energy of the stored water provides a supply of energy. The Sun's warming of the Earth's surface produces winds that circulate the globe and which can be used to drive wind turbines. The winds also produce ocean waves whose kinetic energy can be harvested. More directly, solar energy can be captured by solar water heaters or alternatively by photovoltaic devices, which convert sunlight into electrical energy directly. The Sun even plays a role in the formation of the tides, which result from the motions of the Moon, Sun and Earth. The rising and falling tides contain potential and kinetic energies that can be harvested.

We also get energy from human-induced nuclear reactions. So far, nuclear power has exploited fission reactions of heavy, radioactive elements such as uranium. However, as we will see, nuclear fusion of light elements such as deuterium and tritium has great potential as an energy source of the future. Finally, the Earth itself is a source of energy called geothermal energy. This is stored as thermal energy beneath the Earth's surface. It results from the processes involved in the formation of the Earth and from the decay of radioactive elements within its crust and appears, for example, as hot water springs in various regions of the world.

The annual consumption of energy with respect to energy source varies from country to country and from year to year. However, to get an impression of energy consumption by energy source, Figure 1.2 shows the data for the USA in 2014. We see that 81% of energy consumption came from fossil fuels, while nuclear energy and renewable sources provided the remainder.

Figure 1.2 Annual energy consumption for the USA in 2014, by energy source – 81% of energy consumption came from fossil fuels, while nuclear energy and renewable sources provided the remainder.

The energy sources listed above are called primary energy sources. Electricity, on the other hand, is described as a secondary energy source, as it derives from the conversion of energy from a primary source. Electricity has significant advantages as an energy carrier. It can be conveniently transported and distributed via a national grid, and for many energy needs it is easier to use than the primary energy source itself. The other important secondary energy source is hydrogen gas, which can be burnt or used in fuel cells.

1.3 Renewable and non-renewable energy sources

Energy sources can be classified as either renewable or non-renewable. We define a renewable source as one in which the energy comes from a natural and persistent flow of energy that occurs in the environment. Hydroelectric energy, solar energy, wind energy, wave energy, tidal energy and geothermal energy are...