Sources of Energy

Energy is the ability or capacity to do work. Everything that happens around us — a moving car, a glowing bulb, a growing plant, our own bodies working — needs energy. Energy exists in many different forms, and one of the most important facts in science is that energy can change from one form to another. Understanding the forms of energy and how they transform is the key to understanding all the sources of energy we use.

There are several important forms of energy. Kinetic energy is the energy of a moving object (a moving ball, flowing water, blowing wind). Potential energy is the stored energy an object has because of its position or condition (water stored high in a dam, a stretched spring). Heat (thermal) energy is the energy that makes things hot. Light energy lets us see. Sound energy is carried by sound. Chemical energy is stored in fuels and food and released in chemical changes. Electrical energy is carried by electric current. Nuclear energy is stored in the nuclei of atoms. Each form can do useful work.

A central principle is that energy can be transformed (converted) from one form into another. For example, in an electric bulb, electrical energy changes into light and heat energy; in a fan, electrical energy changes into kinetic energy; in our bodies, the chemical energy of food changes into kinetic energy and heat. Devices and natural processes are constantly converting energy from one form to another to do useful work.

These transformations follow the Law of Conservation of Energy, which states that energy can neither be created nor destroyed; it can only be changed (transformed) from one form to another. This means the total amount of energy always stays the same — when energy seems to "disappear", it has really just changed into another form. For instance, when a moving ball stops, its kinetic energy has not vanished but has changed into heat and sound. Knowing that energy is conserved, and that it merely changes form, helps us understand how all energy sources work: they are really ways of converting energy from a natural store into the useful forms we need.

To run our homes, vehicles, and industries, we need large amounts of energy, which we obtain from various sources of energy. These sources are broadly divided into two groups: conventional (traditional) sources, which have been used for a long time and are the main sources today, and non-conventional (alternative) sources. In this topic we look at the conventional sources, which are mostly non-renewable — that is, they exist in limited amounts and will eventually run out.

The most important conventional sources are the fossil fuels: coal, petroleum (oil), and natural gas. Fossil fuels were formed over millions of years from the buried remains of ancient plants and animals, under heat and pressure deep in the Earth. Coal is a solid fuel burned in power stations and industries; petroleum is a liquid fuel from which petrol, diesel, and kerosene are made for vehicles; and natural gas is a gaseous fuel used for cooking and power. Because they store a lot of chemical energy, fossil fuels are burned to release heat, which is used to do work or generate electricity.

Thermal power is a major use of fossil fuels: in a thermal power plant, coal (or gas/oil) is burned to heat water into steam, the steam spins a turbine, and the turbine drives a generator that produces electricity. Here the chemical energy of the fuel is converted, through heat and motion, into electrical energy. A large part of the world's electricity is generated in this way. Another conventional source is hydropower (hydroelectricity), in which the energy of falling or flowing water (stored behind a dam) is used to spin turbines and generate electricity; hydropower uses the potential and kinetic energy of water.

Conventional sources, especially fossil fuels, have serious drawbacks. They are non-renewable and are being used up far faster than they form, so they will eventually be exhausted. Burning fossil fuels also causes pollution — releasing smoke and harmful gases that pollute the air and contribute to environmental problems. (Hydropower is cleaner and renewable, but building large dams can affect land and people.) These drawbacks of conventional sources are the main reason we are turning more and more to non-conventional, renewable sources of energy, which we study next.

Because fossil fuels are limited and polluting, the world is turning to non-conventional (alternative) sources of energy. Most of these are renewable, meaning they are naturally replenished and will not run out — the source keeps being renewed, unlike fossil fuels. Renewable sources are also generally cleaner, causing far less pollution. Using these sources helps meet our energy needs in a way that can continue far into the future, which is why they are increasingly important.

The most important renewable source is solar energy — energy from the Sun. Sunlight is freely available, will last for billions of years, and causes no pollution. Solar energy can be used to heat water (in solar water heaters), to cook food (in solar cookers), and, using solar cells (panels), to produce electricity directly from sunlight. The Sun is in fact the original source of most of our energy, including the energy in fossil fuels and in wind and water.

Several other renewable sources harness natural processes. Wind energy uses the energy of moving air to turn the blades of windmills (wind turbines), generating electricity. Tidal energy uses the rise and fall of the tides in the sea to turn turbines. Geothermal energy uses the heat from inside the Earth (hot rocks and underground water), which can heat buildings or generate electricity. Biomass energy comes from plant and animal matter — such as wood, crop waste, and dung — which can be burned or processed for energy; biogas, a fuel gas made from such waste, is one important example.

These non-conventional sources have great advantages: they are renewable (will not run out) and largely clean and non-polluting, making them well suited for a sustainable future. They do have some limitations — for example, solar and wind energy depend on the weather and time of day, and setting up the equipment can be costly at first. But because they are renewable and clean, solar, wind, tidal, geothermal, and biomass energy are being developed worldwide to reduce our dependence on fossil fuels. In the next topics we look more closely at two practical examples — solar energy devices and the biogas plant.

Two renewable sources are especially important in everyday life: solar energy and biogas. Solar energy is the energy that comes from the Sun. It is the most abundant source of energy available to us — the Sun pours out vast amounts of energy, it is freely available, it will last for billions of years, and it causes no pollution. Because of these qualities, many devices have been developed to capture and use solar energy directly.

Several solar devices put the Sun's energy to use. A solar cooker uses sunlight to cook food: it has a black box (black absorbs heat well), a glass cover to trap the heat, and often a mirror to reflect more sunlight in, raising the temperature inside enough to cook food slowly without any fuel. A solar water heater uses the Sun's heat to warm water for use in homes. Solar panels (solar cells) convert sunlight directly into electricity; they are used to power lights, pumps, calculators, satellites, and even homes. These devices let us use clean solar energy in place of fuels and grid electricity.

Another very useful source is biogas, a fuel gas produced from biomass — animal dung, plant waste, and other organic matter. Biogas is made in a biogas plant. In the plant, a mixture of dung and water, called slurry, is put into a sealed tank (digester), where, in the absence of air, microorganisms break down the organic matter and produce biogas (which is mainly methane), a clean-burning fuel. The biogas is piped out and used for cooking and lighting, and the leftover slurry makes good manure for fields.

Biogas has clear advantages over burning wood (firewood). Biogas burns with a clean flame and does not produce smoke, so it does not pollute the air or harm health the way wood smoke does. It is made from waste that would otherwise be thrown away, so it puts waste to good use, and it leaves behind useful manure. Using biogas also saves trees, since less firewood is needed, helping protect forests. So solar devices and biogas plants are practical, renewable, and clean ways of meeting everyday energy needs for heat, light, cooking, and electricity.

A very powerful source of energy is nuclear energy — the energy stored in the nucleus (centre) of atoms. This energy is released during certain nuclear reactions, in which the nuclei of atoms change. A tiny amount of nuclear fuel can release an enormous amount of energy, far more than the same amount of fossil fuel, which is why nuclear energy is so important. There are two main types of nuclear reactions that release energy.

Nuclear fission is the splitting of a large, heavy nucleus into smaller nuclei, which releases a great deal of energy. This is the reaction used in nuclear power plants to generate electricity. Nuclear fusion is the joining (combining) of two small, light nuclei into a larger one, which also releases a huge amount of energy. Fusion is the reaction that powers the Sun and other stars. At this level, we understand fission as "splitting" and fusion as "joining" of nuclei, both releasing energy.

In a nuclear power plant, the energy released by nuclear fission is used to heat water into steam, which spins a turbine and drives a generator to produce electricity — much like a thermal power plant, but using nuclear fuel instead of burning fossil fuel. Nuclear energy can generate large amounts of electricity from a small amount of fuel and does not produce the smoke that fossil fuels do. However, it must be handled with great care: it produces dangerous radioactive waste, and accidents can be very serious — as the disasters at Chernobyl and Fukushima showed, when nuclear plants released harmful radiation.

Finally, the world faces an energy crisis — a growing shortage of energy. Our use of energy keeps rising as populations and industries grow, but the fossil fuels we mainly rely on are being depleted (used up) and cause pollution. This makes it urgent to conserve energy (use it carefully and avoid waste) and to develop sustainable, renewable sources like solar, wind, and biogas. By using energy wisely, switching to renewable sources, and developing nuclear energy safely, we can meet our needs without exhausting our resources or harming the planet. This completes our study of the sources of energy and the importance of using them responsibly.