P7 Energy Resources - Cambridge IGCSE Combined and Co-ordinated Sciences PDF

Okay, here is the converted markdown format of the book pages you sent to me. # P7 Energy resources This chapter covers: * the different energy resources we use * the advantages and disadvantages of different energy resources * how we rely on the Sun for most of our energy resources. ## P7...

Okay, here is the converted markdown format of the book pages you sent to me. # P7 Energy resources This chapter covers: * the different energy resources we use * the advantages and disadvantages of different energy resources * how we rely on the Sun for most of our energy resources. ## P7.01 The energy we use Here on Earth, we rely on the Sun for most of the energy we use. The Sun is a fairly average star, 150 million kilometers away. The heat and light we receive from it have taken about eight minutes to travel through empty space to get here. Plants absorb this energy in the process of photosynthesis, and animals are kept warm by it. The Earth is at a convenient distance from the Sun for living organisms. The Sun's rays are strong enough, but not too strong. The Earth's average temperature is about $15^\circ C$, which is suitable for life. If we were closer to the Sun, we might be intolerably hot like Venus, where the average surface temperature is over $400^\circ C$. Further out, things are colder. Saturn is roughly ten times as far from the Sun, so the Sun in the sky looks one-tenth of the diameter that we see it, and its radiation has only one-hundredth of the intensity. Saturn's surface temperature is about $-180^\circ C$. Most of the energy we use comes from the Sun, but only a very little is used directly from the Sun. On a cold but sunny morning, you might sit in the sunshine to warm your body. Your house might be designed to collect warmth from the Sun's rays, perhaps by having larger windows on the sunny side. However, most of the energy we use comes only indirectly from the Sun. It must be converted into a more useful form, such as electricity (Image P7.01). Figure P7.01 is a chart showing the different fuels that contribute to the world's energy supplies. This chart reflects patterns of energy consumption in the early years of the 21st century. Many people today live in industrialized countries and consume large amounts of energy, particularly from fossil fuels (coal, oil and gas). People living in less-developed countries consume far less energy - mostly they use biomass fuels, particularly wood. A thousand years ago, the chart would have looked very different. *** Image P7.01 showing a high voltage power line with the sun as its center. Image P7.01 We use energy from the Sun in many different ways - for example, for producing electricity. **World energy use, by fuel (2006)** | Fuel Type | Percentage | | ------------------ | ---------- | | Nuclear | 6% | | Hydro | 3% | | Other renewables | 1% | | Biomass | 14% | | Oil | 33% | | Gas | 20% | | Coal | 23% | Figure P7.01 World energy use, by fuel. This chart shows the contributions made by different fuels to energy consumption by people in 2006, across the world. Three-quarters of all energy is from fossil fuels. Fossil fuel consumption was much less important then. Most people relied on burning wood to supply their energy requirements. We will now look at these groups of fuels in turn. ### Energy direct from the Sun In hot, sunny countries, solar panels are used to collect thermal (heat) and light energy from the Sun. The Sun's rays fall on a large solar panel, on the roof of a house, for example. This absorbs the energy of the rays, and water inside the panel heats up. This provides hot water for washing. It can also be pumped round the house, through radiators, to provide a cheap form of central heating. We can also make electricity directly from sunlight (Image P7.02). The Sun's rays shine on a large array of solar cells (also known as a photocells). The energy of the rays is absorbed, and electricity is produced. As this technology becomes cheaper, it is finding more and more uses. It is useful in remote locations - for example, for running a refrigerator that stores medicines in central Africa, or for powering roadside emergency phones in desert regions such as the Australian outback. Solar cells have also been used extensively for powering spacecraft. Ideally, a solar cell is connected to a rechargeable battery, which stores the energy collected, so that it can be available during the hours of darkness. ### Wind and wave power Wind and waves are also caused by the effects of the Sun. The Sun heats some parts of the atmosphere more than others. Heated air expands and starts to move around - this is a convection current (see Chapter P11). This is the origin of winds. Most of the energy of winds is given up to the sea as waves are formed by friction between wind and water. There are many technologies for extracting energy from the wind. Windmills for grinding and pumping are traditional, and modern wind turbines can generate electricity (see Image P7.03). *** Image P7.02 This array of solar cells provides electricity for a water pump in a Kenyan village. *** Image P7.03 showing large, three-blade wind turbines as part of a wind farm at Xinjiang in China. Image P7.03 These giant turbines are part of a wind farm at Xinjiang in China. They produce as much electricity as a medium-sized coal-fired power station. Wave technology is trickier. The up-and-down motion of waves must be used to spin a turbine, which then turns a generator. This is tricky to achieve, and rough seas are a hazardous place to work. On calm days, the system produces no power. **QUESTIONS** * P7.01 Explain why wind and wave power could not be relied on to provide a country's entire electricity supply. * P7.02 A solar cell (photocell) produces electricity when the sun shines. What energy conversion is going on here? * P7.03 When a wave travels across the sea, the water moves up and down. What two forms of energy does a wave have? ### Biomass fuels For many people in the world, wood is the most important fuel. It warms their homes and provides the heat necessary for cooking their food. Wood is made by trees and shrubs. It stores energy that the plant has captured from sunlight in the process of photosynthesis. When we burn wood, we are releasing energy that came from the Sun in the recent past, perhaps ten or a hundred years ago. Wood is just one example of a biomass fuel. Others include animal dung and biogas, generated by rotting vegetable matter. These can be very important fuels in societies where most people live by farming. As you can see from Figure P7.01, biomass fuels account for about one-seventh of all energy consumption in the world. This figure can only be a rough estimate, because no one keeps track of all the wood consumed as fuel. However, we can say that this segment of the chart represents the energy consumption of about three-quarters of the world's population. The remaining one-quarter (who live in developed, industrial nations) consume roughly six times as much. ### Fossil fuels Oil, coal and gas are all examples of fossil fuels. These are usually hydrocarbons (compounds of hydrogen and carbon). When they are burnt, they combine with oxygen from the air. In this process, the carbon becomes carbon dioxide. The hydrogen becomes 'hydrogen oxide', which we usually call water. Energy is released. We can write this as an equation: hydrocarbon + oxygen carbon dioxide + water + energy Hence, we can think of a fossil fuel as a store of energy. Fossil fuels store energy as chemical energy. Where has this energy come from? Fossil fuels are the remains of organisms (plants and animals) that lived in the past. Many of the Earth's coal reserves, for example, formed from trees that lived in the Carboniferous era, between 286 and 360 million years ago. ('Carboniferous' means 'coal-producing'.) These trees captured sunlight by photosynthesis, they grew and eventually they died. Their trunks fell into the swampy ground, but they did not rot completely, because there was insufficient oxygen. As material built up on top of these ancient trees, the pressure on them increased. Eventually, millions of years of compression turned them into underground reserves of coal. Today, when we burn coal, the light that we see and the warmth that we feel have their origins in the sunlight trapped by trees hundreds of millions of years ago. **QUESTIONS** * P7.04 a Name three fossil fuels. b Name three non-fossil fuels. * P7.05 What energy conversion is happening when charcoal is used as the fuel for a barbecue? Oil and gas are usually found together. They are formed in a similar way to coal, but from the remains of tiny shrimp-like creatures called microplankton that lived in the oceans. The oilfields of the Persian Gulf, North Africa and the Gulf of Mexico, which contain half of the world's known oil reserves, all formed in the Cretaceous era, 75 to 120 million years ago. ### Nuclear fuels Nuclear power was developed in the second half of the 20th century. It is a very demanding technology, which requires very strict controls, because of the serious damage that can be caused by an accident. The fuel for a nuclear power station (Image P7.04) is usually uranium, sometimes plutonium. These are radioactive materials. Inside a nuclear reactor, their radioactive decay is speeded up so that the energy they store is released much more quickly. This is the process of nuclear fission. Uranium is a very concentrated store of energy in the form of nuclear energy. A typical nuclear power station will receive about one truckload of new fuel each week. Coal is less concentrated. A similar coal-fired power station is likely to need a whole trainload of coal every hour. A wind farm capable of generating electricity at the same rate would cover a large area of ground - perhaps 20 square kilometers. In some countries that have few other resources for generating electricity, nuclear power provides a lot of energy. *** Image P7.04: A person wearing protective clothing while checking equipment outside of a nuclear power plant. Image P7.04 This nuclear power station generates electricity. Its fuel is uranium. As the fuel is used up, highly radioactive waste products are produced. These have to be dealt with very carefully to avoid harm to the surroundings. Here, checks are being carried out to ensure that the level of radioactive materials near the power station is safe. In France, for example, nuclear power stations generate three-quarters of the country's electricity. Excess production is exported to neighboring countries, including Spain, Switzerland and the UK. Nuclear fuel is a relatively cheap, concentrated energy resource. However, nuclear power has proved to be expensive because of the initial cost of building the power stations, and the costs of disposing of the radioactive spent fuel and decommissioning the stations at the end of their working lives. ### Water power One of the smallest contributions to the chart in Figure P7.01 is water or hydroelectric power. For centuries, people have used the kinetic energy of moving water to turn water-wheels, which then drive machinery of all sorts - for example, to grind corn and other crops, pump water and weave textiles. Today, water power's biggest contribution is in the form of hydroelectricity (see Image P7.05). Water stored behind a dam is released to turn turbines, which make generators spin. This is a very safe, clean and reliable way of producing electricity, but it is not without its problems. A new reservoir floods land that might otherwise have been used for hunting or farming. People may be made homeless, and wildlife habitats destroyed. ### Geothermal energy The interior of the Earth is hot. This would be a useful source of energy - if we could get at it! People do make use of this geothermal energy where hot rocks are found at a shallow depth below the Earth's surface. (These rocks are hot because of the presence of radioactive substances *** Image P7.05 of the Itaipu Dam on the Paraná River in South America Image P7.05 The giant Itaipu Dam on the Paraná River in South America generates electricity for Brazil and Paraguay. inside the Earth.) To make use of this energy, water is pumped down into the rocks, where it boils. High-pressure steam returns to the surface, where it can be used to generate electricity. Suitable hot underground rocks are usually found in places where there are active volcanoes. Iceland, for example, has several geothermal power stations. These also supply hot water to heat nearby homes and buildings. ### Renewables and non-renewables Figure P7.01 showed that most of the energy supplies we use are fossil fuels - coal, oil and gas. There are limited reserves of these, so that, if we continue to use them, they will one day run out. They are described as non-renewables. Once used, they are gone for ever. Other sources of energy, such as wind, solar and biomass, are described as renewables. This is because, when we use them, they will soon be replaced. The wind will blow again, the sun will shine again - and, after harvesting a biomass crop, we can grow another. Ideally, we should develop an 'energy economy' based on renewables. Then we would not have to worry about supplies that will run out. We would also avoid the problems of global warming and climate change. ### Comparing energy sources We use fossil fuels a lot because they represent concentrated sources of energy. A modern gas-fired power station might occupy the space of a football ground and supply a town of 100000 people. To replace it with a wind farm might require 50 or more wind turbines spread over an area of several square kilometers – the wind is a dilute source of energy. This illustrates some of the ideas that we use when comparing different energy sources. If you look back through this chapter, you will find many comments about different energy sources. Each has its advantages and disadvantages. We need to think about the following factors: * Cost. We should separate initial costs from running costs. A solar cell is expensive to buy but there are no costs for fuels - sunlight is free! * Reliability. Is the energy supply constantly available? The wind is variable, so wind power is unreliable. Wars and trade disputes can interrupt fuel supplies. * Scale. As discussed above, a fossil-fuel power station can be compact and still supply a large population. It would take several square meters of solar cells to supply a small household. * Environmental impact. The use of fossil fuels leads to climate change. A hydroelectricity dam may flood useful farmland. Every energy source has some effect on the environment. **QUESTION** * P7.07 Explain whether the following energy sources are renewable or non-renewable: * a uranium-fuelled nuclear power * b wave power. ## P7.02 Energy from the Sun Most of the energy we use can be traced back to the Sun. We have seen the following in the previous section: * Fossil fuels are stores of energy that came from the Sun millions of years ago. * Radiation (light and heat) from the Sun can be absorbed by solar panels to provide hot water. Sunlight can also be absorbed by arrays of solar cells (photocells) to generate electricity. In some countries, you may see these on the roofs of houses. * The wind is caused when air is heated by the Sun. Warm air rises; cool air flows in to replace it. This moving air can be used to generate electricity using wind turbines. * Most hydroelectric power comes ultimately from the Sun. The Sun's rays cause water to evaporate from the oceans and land surface. This water vapor in the atmosphere eventually forms clouds at high altitudes. Rain falls on high ground, and can then be trapped behind a dam. This is the familiar water cycle. Without energy from the Sun, there would be no water cycle and much less hydroelectric power. However, we make use of a small amount of energy that does not come from sunlight. Here are three examples: * A small amount of hydroelectric power does not depend on the Sun's energy. Instead, it is generated from the tides. The Moon and the Sun both contribute to the oceans' tides (the Moon's effect is greater than the Sun's). Their gravitational pull causes the level of the ocean's surface to rise and fall every twelve-and-a-bit hours. At high tide, water can be trapped behind a dam. Later, at lower tides, it can be released to drive turbines and generators. Because this depends on gravity, and not the Sun's heat and light, we can rely on tidal power even at night and when the Sun is hidden by the clouds. * Nuclear power makes use of nuclear fuels - mostly uranium - mined from underground. Uranium is a slightly radioactive element, which has been in the ground ever since the Earth formed, together with the rest of the solar system, 4.5 billion years ago. So uranium did not get its energy from the Sun. * Geothermal energy also depends on the presence of radioactive substances inside the Earth. These have been there since the Earth formed; they have been continuously releasing their store of energy ever since. ### The source of the Sun's energy The Sun releases vast amounts of energy, but it is not burning fuel in the same way as we have seen for fossil fuels. The Sun consists largely of hydrogen, but there is no oxygen to burn this gas. Instead, energy is released in the Sun by the process of nuclear fusion. In nuclear fusion, two energetic hydrogen atoms collide and fuse (join up) to form an atom of helium. Nuclear fusion requires very high temperatures and pressures. The temperature inside the Sun is close to 15 million degrees. The pressure is also very high, so that hydrogen atoms are forced very close together, allowing them to fuse. Scientists and engineers would like to be able to make fusion happen in a similar way here on Earth. Experimental reactors have been built, but it is very tricky to create the necessary conditions for fusion to happen in a controlled way. Perhaps, one day, fusion will prove a safe, clean way of producing a reliable electricity supply. **QUESTION** * P7.08 Name three energy resources for which the original energy source is not radiation from the Sun. **Summary** You should know: * about renewable and non-renewable energy resources * that most of our energy comes from the Sun * that nuclear fusion is the source of energy in the Sun. ## End-of-chapter questions 1. Copy and complete the sentences that follow, filling in the gaps with words from the list below: wind Sun electricity resource non-renewable fossil fuels renewable a. An energy ........... is anything from which we can obtain energy. b. Most of the energy we use comes originally from the............ c. A ........... resource is naturally replaced after we have used it. d. Coal and other ........... are examples of ........... energy resources. e. Energy carried by the ........... can be used to turn turbines to generate ........... 2. Explain how the following energy resources rely on energy from the Sun: a. biomass fuel, such as wood b. electricity from a hydroelectric power station. 3. In a hydroelectric power station, water is stored behind a dam. It flows down past a turbine, so that the turbine spins. This causes a generator to turn and produce electricity. a. What form of energy is stored by the water when it is behind the dam? b. What form of energy does the spinning turbine have? c. Write down the two energy transformations that occur in a hydroelectric power station. 4. Fission and fusion are two nuclear processes that release energy. a. i Which is used in a nuclear power station? ii What is the fuel used for this? b. i Which is the Sun's energy source? ii What element is the fuel? iii What element is produced? 5. Electricity supplied by solar cells is expensive. This is because, although sunlight is free, the cells themselves are expensive to produce. a. Explain why solar cells are a suitable choice for powering a spacecraft but are less likely to be used for providing domestic electricity to consumers in a city such as London, Dubai or Hong Kong. b. Suggest one other situation in which solar cells would be a good choice, and justify your suggestion. c. Why are solar cells often used in conjunction with a battery? 6. Here is a list of energy resources available to the world. Some of these are renewable and some are non-renewable. | | Renewable | Non-renewable | | -------------------- | --------- | ------------- | | coal | | | | hydroelectricity | | | | nuclear energy | | | | oil | | | | solar energy | | | | tidal energy | | | | wind energy | | | 7. a. Here is a list of some **energy resources** that might be used to generate electricity: oil hydroelectricity nuclear fission wind waves List any of these that rely on a fuel being consumed. b. Here is a list of devices that convert **energy** from one form to another: battery electric motor gas lamp gas fire generator loudspeaker microphone Which of these is designed to convert: i chemical energy into light **energy** ii electrical **energy** into mechanical **energy** iii sound **energy** into electrical **energy**