Physics Topics are also essential for space exploration, allowing scientists to study phenomena such as gravitational waves and cosmic rays.
Types of Natural Sources of Energy
We use heat energy obtained by burning fuels like wood, coal, kerosene or LPG for cooking our food. The energy of fuels like petrol and diesel is used to run cars, buses, trucks and trains. Diesel is also used to provide energy to run pump-sets for irrigation in agriculture.
Coal, fuel oil and natural gas are used to provide energy in factories and run thermal power plants. Electrical energy (or electricity) is used for lighting bulbs and tubes, and to run radio, television, household appliances, electric trains and factory machines.
In fact, all our everyday activities use energy in one form or the other. And if there is no energy, all our activities will come to a stop. For example, if there is no energy, we won’t be able to cook food. If there is no energy, we won’t be able to run cars, buses, trucks, trains and aeroplanes. And if there is no energy, we won’t be able to run machines in factories, power plants or pump-sets in agriculture. So, energy is essential for our survival in this world. We will now discuss the various sources which can provide us energy.
A source of energy is one which can provide adequate amount of energy in a convenient form over a long period of time. All the sources of energy can be divided into two main categories : Non-renewable sources of energy and Renewable sources of energy. These are described below.
1. Non-Renewable Sources of Energy
Those sources of energy which have accumulated in nature over a very, very long time and cannot be quickly replaced when exhausted are called non-renewable sources of energy. For example, coal is a non-renewable source of energy because coal has accumulated in the earth over a very, very long time, and if all the coal gets exhausted, it cannot be produced quickly in nature (see Figure).
The non-renewable sources of energy are : Fossil fuels (Coal, Petroleum and Natural gas), and Nuclear fuels (such as Uranium). Non-renewable sources of energy are dug out from the earth.
The non-renewable sources of energy are also called conventional sources of energy. The non-renewable sources of energy like fossil fuels (coal, petroleum and natural gas) are present in a limited amount in the earth. Once exhausted, they will not be available to us again.
The nuclear fuels (like uranium) have been put in the category of non-renewable sources of energy because the nuclear materials which can be conveniently extracted from the earth are limited and hence they will get exhausted one day. Please note that since the non-renewable sources of energy can get exhausted one day, they are also known as exhaustible sources of energy. Another point to be noted is that though nuclear fuels are non-renewable source of energy dug out from the earth, they are not conventional source of energy.
2. Renewable Sources of Energy
Those sources of energy which are being produced continuously in nature and are inexhaustible, are called renewable sources of energy. For example, wood is a renewable source of energy because if trees are cut from the forests for obtaining wood, then more trees will grow in the forest in due course of time. So, the loss of wood by cutting trees is made good by nature (see Figure 3).
The renewable sources of energy are : Hydroenergy (Energy from flowing water); Wind energy; Solar energy; Energy from sea (Tidal energy, Sea-wave energy and Ocean thermal energy); Geothermal energy; Biomass energy (Energy from bio¬fuels such as Wood, Biogas and Alcohol); and Hydrogen.
The renewable sources of energy are also called non-conventional sources of energy. These sources of energy can be used again and again, endlessly. They will never get exhausted. Since renewable sources of energy will never get exhausted, so they are also known as inexhaustible sources of energy. A yet another name for renewable sources of energy is the alternative sources of energy.
According to the above classification, wood (obtained by the cutting of trees) is a renewable source of energy. Now, a newly planted sapling usually takes more than 15 years to grow and mature into a tree. So, replenishment of cut down trees takes a very long time.
Another disadvantage is that the cutting down of trees on a large scale (for obtaining wood) causes depletion of the forests leading to an imbalance Figure. Wood is a renewable source in nature. Due to these reasons, using wood as a source of energy is not °f energy, a wise decision (even though it is a renewable source of energy).
What is a Good Source of Energy
Whenever work has to be done, energy is needed. This energy is supplied by a ‘source of energy’. Different sources of energy are used depending on the ‘type of work’ to be done. Some sources of energy may be good whereas others may not be so good. We can find out whether a given source of energy is good or not by making use of the following ‘characteristics’ or ‘qualities’ of a good source of energy. A good source of energy is one :
- which would do a large amount of work per unit mass (or per unit volume),
- which is cheap and easily available,
- which is easy to store and transport,
- which is safe to handle and use, and
- which does not cause environmental pollution.
The most common sources of energy available to us are the fuels. These are discussed below.
The materials which are burnt to produce heat energy are known as fuels. Examples of fuels are : Wood, Coal, Cooking gas (LPG), Kerosene, Diesel and Petrol (see Figure). In our day to day life, the most important sources of energy are fuels. Fuels are, in fact, the concentrated store-house of energy. This energy is released in the form of heat when the fuels are burnt.
Since a variety of fuels are available to us, we should have some means of deciding which fuel is the best for our purpose. Before we describe some of the characteristics (or properties) of fuels which will help us choose the best possible fuel for a particular use, we should know the meaning of two terms calorific value of a fuel’ and ‘ignition temperature’ of a fuel. These are discussed below.
All the fuels produce heat energy on burning. Different fuels produce different amounts of heat on burning. Some fuels produce more heat whereas others produce less heat. The usefulness of a fuel is measured in terms of its calorific value. Fligher the calorific value, better the fuel will be. The amount of heat produced by burning a unit mass of the fuel completely is known as its calorific value.
The unit of mass usually taken for measuring the calorific value of a fuel is “gram”. So, we can also say that “The amount of heat produced by burning 1 gram of a fuel completely is called its calorific value”.
For example, when one gram of a carbon fuel (like charcoal) is burned completely, it produces about 33000 joules of heat, so the calorific value of charcoal is 33000 joules per gram or 33000 J/g. Since joule is a very small unit of heat energy, so the calorific value is usually expressed as kilojoules per gram (kJ/g).
Thus, the calorific value of charcoal becomes 33 kilojoules per gram which is written in short form as 33 kJ/g. Thus, the common unit of measuring calorific value is kilojoules per gram (kJ/g). Please note that the ‘calorific value’ of a fuel represents the ‘heat value’ of the fuel. The calorific values of some common fuels are given below.
Calorific Values (or Heat Values) of Some Common Fuels
From the above table we find that the calorific value of kerosene oil is 48 kj/g. Now, by saying that the calorific value of kerosene oil is 48 kilojoules per gram, we mean that if 1 gram of kerosene oil is burnt completely, then it will produce 48 kilojoules of heat energy.
Actually, the term “calorific value” comes from the fact that the earlier unit of measurement of heat was calorie. These days, although we mostly use joule as the unit of heat but the term calorific value continues to be used as such. If we look up the table of calorific values, we will find that hydrogen gas has the highest calorific value of 150 kilojoules per gram.
Thus, because of its high calorific value, hydrogen is an extremely good fuel. Most of the common fuels are the compounds of hydrogen and carbon called ‘hydrocarbons’. Since hydrogen has the highest calorific value, therefore, a fuel containing higher percentage of hydrogen will have a higher calorific value than another fuel which has a lower percentage of hydrogen in it, For example, LPG has a higher percentage of hydrogen than coal, so LPG has a higher calorific value than coal. We will now describe the ignition temperature of a fuel.
We know that a fuel has to be burnt to obtain heat energy. Now, before a fuel can catch fire and burn, it must be heated to a certain minimum temperature. The minimum temperature to which a fuel must be heated so that it may catch fire and start burning, is known as its ignition temperature.
When we apply a burning matchstick (or a lighter spark) to the burner of a gas stove, we actually supply a little heat to cooking gas coming out of gas burner so that it gets heated to its ignition temperature and start burning. No fuel can burn unless it is heated to its ignition temperature.
Choice of a Good Fuel
The fuel which we choose for our domestic or industrial use should have a high calorific value. This means that the fuel chosen should be such that it gives us more heat per unit mass. The fuel should burn without giving any smoke or harmful gases. In other words, the combustion products produced by the burning of fuel should not be poisonous, and they should not pollute the environment.
Carbon dioxide and water vapour are the harmless products produced by the burning of fuels. But some fuels produce poisonous gases like carbon monoxide (CO) and sulphur dioxide (SO2), etc., which pollute the air around us. The fuel should have a proper ignition temperature which should be well above the normal room temperature.
The fuel which we choose should be cheap and easily available. Another point which we have to keep in mind is that the fuel should be easy to handle, safe to transport and convenient to store. The fuel should have low percentage of non-combustible materials. This means that the residue left behind after the burning of a fuel should be as low as possible.
For example, coal is a fuel which leaves behind a lot of ash after burning. The removal and disposal of ash poses a big problem in factories and power plants where large quantities of coal are burnt everyday. The fuel should burn smoothly and at a steady rate.
This is because if a fuel burns too fast, then most of the heat produced by it will be wasted. And if the fuel burns too slowly, then the heat produced may not serve our purpose. The fuels should have a low rate of evaporation at room temperature. We will now write down the characteristics of an ideal fuel (or good fuel) point-wise.
Characteristics of an Ideal Fuel (or Good Fuel)
While choosing the most appropriate fuel for our domestic use or for use in industry, we should keep in mind the following characteristics of an ideal fuel or good fuel.
1. It should have a high calorific value. In other words, an ideal fuel (or good fuel) is that which gives us more heat per unit mass.
2. It should burn without giving out any smoke or harmful gases. That is, an ideal fuel (or good fuel) is that which does not pollute air on burning by giving out smoke or poisonous gases.
3. It should have a proper ignition temperature, so that it can be burned easily. The ignition temperature of an ideal fuel (or good fuel) should neither be too low nor too high. Because if the ignition temperature of the fuel is very low, then the fuel will catch fire too easily and hence it will be very unsafe to use it. On the other hand, if the ignition temperature is too high, then it will be very difficult to light the fuel.
4. It should be cheap and easily available. That is, an ideal fuel (or good fuel) is that which is not expensive and which is available in plenty everywhere.
5. It should be easy to handle, safe to transport, and convenient to store. That is, an ideal fuel (or good fuel) is that which does not create any safety risks during handling, during its transportation from one place to another or during its storage.
6. It should not leave much ash behind after burning. That is, an ideal fuel (or good fuel) should have low percentage of non-volatile materials which do not burn, so that it may burn completely without leaving much ash.
7. It should burn smoothly. That is, an ideal fuel (or good fuel) should have a moderate rate of combustion, and burn at a steady rate. In other words, the fuel should not burn either too fast or too slow.
From the above discussion we conclude that the various factors which should be kept in mind while choosing a proper fuel for homes and factories are : high calorific value; absence of smoke and harmful gases (or polluting gases) on burning ; proper ignition temperature ; low cost and easy availability ; ease of handling, transportation and storage ; low non-volatile content; and a moderate rate of combustion. We will now take some examples to decide which of the two given fuels is a better fuel or a more ideal fuel.
Suppose we have two fuels A and B. The calorific value of fuel A is 55 kJ/g and its ignition temperature is 20°C whereas the calorific value of fuel B is 50 kJ/g and its ignition temperature is 80°C. Now, we find that fuel A has a higher calorific value of 55 kJ/g than that of fuel B. So, at first sight it would appear that fuel A is a better fuel than fuel B. But it is not so.
This is because fuel A has a very low ignition temperature of 20°C due to which it can catch fire easily and hence it is unsafe to use. On the other hand, though the calorific value of fuel B is little less (than that of fuel A), but it has the right ignition temperature of 80°C which is neither very low nor very high. So, here fuel B will be a better fuel on the basis of its appropriate ignition temperature.
Again suppose that, on burning, fuel A produces gases like CO2, H2O, CO, SO2, SO3, etc., whereas fuel B produces only CO2 and H2O. Out of these gases carbon monoxide (CO), sulphur dioxide (SO2) and sulphur trioxide (SO3) are poisonous gases or harmful gases. Now, since fuel A produces poisonous gases like CO, SO2 and SO3, on burning, therefore, fuel A is not a good fuel.
On the other hand, burning of fuel B produces only harmless products like carbon dioxide (CO2) and water vapour (H2O), so fuel B is a better fuel. Similarly, if a fuel burns with an explosion, it will not be a good fuel and if it leaves behind a lot of ash on burning, even then it cannot be a good fuel” Let us solve one problem now.
The calorific value and ignition temperature of fuel A are 55 kJ/g and 20°C respectively. These values for another fuel B are 50 kJ/g and 80°C respectively. Which of the two will be more ideal fuel if, on burning, the fuel A produces CO2, SO2 and SO3 as by-products while the fuel B produces CO2 and H2O ? Give two reasons in support of your answer.
Fuel B will be a more ideal fuel because of the following reasons :
- Fuel B has a moderate ignition temperature of 80°C which is neither very high nor very low. On the other hand, the ignition temperature of fuel A is very low (20°C) and hence it is unsafe to use.
- Fuel B does not produce any harmful gases on burning. On the other hand, fuel A produces poisonous gases like SO2 and SO3 on burning.