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8
INTRODUCTION
I
n Chapter 2, energy was defined as the capacity to do work. The various forms of energy can be classified as:• Mechanical (kinetic and potential) • Heat • Radiant (electromagnetic) • Chemical (potential) • Sound • Electrical/magnetic • Nuclear
Mechanical energy includes both kinetic and potential
energy. Friction is mechanical energy as it is caused by kinetic energy and potential energy of a body as a force is applied through a distance.
Heat energy is the energy a body possesses because of
its internal energy due to the motion of the particles it contains.
Radiant energy is the source of all life on the Earth and
is the greatest potential energy resource available for the future. When radiant energy in the form of light is absorbed by plants, it is converted into stored chemical energy. This energy is available as food or biomass. Biomass is used to produce fuels. It is electromagnetic in nature and is possessed by all components of the electromagnetic
spectrum (γ–rays, X–rays, UV radiation, visible light, IR radiation, microwaves and radio waves). Infra-red radiation falling on a body is converted into thermal energy. Solar energy is a form of radiant energy.
Chemical energy is the energy locked up in fuels and
other chemicals. The energy obtained by the combustion of fuels represents a major source of energy in current use. All food that we eat is a store of chemical energy. It can be considered to be latent potential energy that a body possesses.
Sound energy is produced by longitudinal waves that
have an organised and periodic pattern that causes the vibration of the particles in the same direction as the transfer of energy.
Electrical energy is the energy carried by moving
charges and these moving charges produce a magnetic field. Through the process of electromagnetic induction, electricity has become the greatest form of energy used by man in everyday life.
Nuclear energy is the potential binding energy released
during a nuclear reaction when mass is converted to thermal and perhaps light energy. Nuclear fission reactors are starting to gain a renewed acceptance and nuclear fusion has potential for the future when the technology becomes available.
8.1 Energy degradation & power generation
8.2 World energy sources
8.3 Fossil fuel power production
8.4 Non-fossil fuel power production
8.5 Greenhouse effect
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8.1.1 State that thermal energy may be completely converted to work in a single process, but that continuous conversion of this energy into work requires a cyclical process and the transfer of some energy from the system.
8.1.2 Explain what is meant by degraded energy.
8.1.3 Construct and analyse energy flow diagrams (Sankey diagrams) and identify where the energy is degraded.
8.1.4 Outline the principal mechanisms involved in the production of electrical power.
© IBO 2007
8.1.1 THERMAL
ENERGYIN
A
CYCLICALPROCESS
Suppose a piston was placed on a heat reservoir, such as the hot plate of a stove. Thermal energy is supplied by the heat reservoir, and work is done by the gas inside the piston as it expands. But this is not an engine as it only operates in one direction. The gas cannot expand indefinitely because as the volume of the piston increases the pressure decreases (Boyle’s Law) as the force exerted on the walls of the piston by the gas molecules per unit area is continually decreasing. Some point will be reached when the expanding gas will not be able to move the piston. For this simple engine to function, the piston must eventually be compressed to restore the system to its original position ready to do work.
A thermodynamic engine is a device that transforms thermal energy to mechanical energy (work) as in an engine, or mechanical energy to thermal energy such as in refrigeration and air-conditioning systems. Cars, steam trains, jets and rockets have engines that transform fuel energy (chemical energy) into the kinetic energy of their motion. In all presently manufactured engines, the conversion is accompanied by the emission of exhaust gases that waste some of the thermal energy. Consequently, these engines are not very efficient as only part of the thermal energy is converted to mechanical energy. An engine has two crucial features:
• It must work in cycles to be useful.
• The cyclic engine must have more than one heat reservoir.
A thermodynamic cycle is a process in which the system is returned to the same state from which it started. That is, the initial and final states are the same in the cyclic process.
Figure 801 shows a series of schematic diagrams for the cycle of an internal combustion engine as used in most automobiles.
• With the exhaust valve closed, a mixture of petrol vapour and air is drawn into the combustion chamber through the inlet valve as the piston moves down during the intake stroke. • Both valves are closed and the piston moves
up to squeeze the mixture of petrol vapour and air to about ⅛th its original volume during the compression stroke.
• With both valves closed, the mixture is ignited by a spark from the spark plug.
• The mixture burns rapidly and the hot gases then expand against the piston in the power stroke. • The exhaust valve is opened as the piston moves
upwards during the exhaust stroke, and the cycle begins again. intake valve closed exhaust valve closed compression stroke crankshaft Gas vapor and mixture intake valve open exhaust valve closed intake stroke crankshaft piston intake valve closed exhaust valve closed ignition intake valve closed exhaust valve closed power stroke intake valve closed exhaust valve open exhaust spent fuel gases
Figure 801 The Internal Combustion Engine
For a cycle to do net work, thermal contact with the original heat reservoir must be broken, and temperatures other than that of the original heat reservoir must play a part in the process. In the above example, if the piston is returned
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to its original position while in contact with the hot plate, then all the work that the gas did in the expansion will have to be used in the compression. On a p –V diagram, one would draw an isotherm for the expansion and an isotherm for the compression lying on top of the expansion isotherm but in the opposite direction. Therefore, the area enclosed by the cycle would be zero. However, if the gas is compressed at a lower temperature the internal pressure of the system will be lower than during the expansion. Less work will be needed for the compression than was produced in the expansion, and there will be net work available for transformation to mechanical energy.
Motor cars usually have four or six pistons but five and eight cylinders are also common. The pistons are connected by a crankshaft to a flywheel which keeps the engine turning over during the power stroke. Automobiles are about 25% efficient.
Figure 802 shows the p-V graph for the Otto cycle designed by Nikolaus Otto in 1976. The Otto cycle is similar to the present day car engine.
Figure 802 The Otto cycle
The fuel-air mixture enters the piston at point A. The compression AB is carried out rapidly with no heat exchange making it an adiabatic compression. The ignition and combustion of the gases introduces a heat input QH that raises the temperature at constant volume from B to C. The power stroke is an adiabatic expansion from C to D. Thermal energy QL leaves the system during the exhaust stroke, and cooling occurs at constant volume from D to A. The net work is represented by the enclosed area ABCD.
Diesel engines use diesel instead of petrol, and there is no spark from a spark plug to cause ignition. They do not have a carburettor which is used to produce a spray of the fuel-air mixture. Rather, the air is sucked in and the diesel is introduced through a valve when the piston is at the top of the compression stroke. During the adiabatic
compression, the air is squeezed to one-sixteenth of its volume. This makes the air so hot that the fuel ignites of its own accord and explodes as soon as it enters through the valve. Diesel engines with 40% efficiency are amongst the most efficient engines used today. Figure 803 demonstrates the diesel cycle patented by Rudolf Diesel in 1892.
A B C D maximum temperature minimum temperature QH QL V1 V2 p V adiabatic expansion adiabatic compression constant volume (V1) constant pressure
Figure 803 The Diesel cycle
Jet engines burn fuel continuously. They suck air in the front of the engine and this air is compressed by the compressor fans. The air becomes so hot that it burns in the continuous fuel supply. Exhaust gases are blown out the back of the engine propelling the engine forward. These gases also turn a turbine that supplies electricity to the jet, and keeps the compressor fans turning.
8.1.2 ENERGY
DEGRADATION
When energy is transferred from one form to other forms, the energy before the transformation is equal to the energy after (Law of conservation of energy). However, some of the energy after the transformation may be in a less useful form. We say that the energy has been ‘degraded’. For example, in a simple battery operated flashlight, an energy input of 100 units of chemical potential energy will give a 10 unit output of light energy and the light energy is enhanced by placing a curved mirror behind the lamp to concentrate the light into a beam. The other 90 units of output is used in heating up the filament of the light bulb and in heating the battery and the surroundings. These 90 units of energy output have become degraded. The thermal energy that is transferred to the surroundings, the filament and the battery is no longer available to perform useful work.
The Second Law of Thermodynamics in one form states that engines are theoretically inefficient users of energy. The efficiency of an energy conversion process is a ratio of the useful energy output to the total energy input usually expressed as a percentage. In practice, the efficiency is even lower than this theoretical value. Figure 804 gives
A B C D maximum temperature minimum temperature QH QL V1 V2 p V adiabatic expansion adiabatic compression constant volume (V1) constant volume (V2)
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examples of the efficiency attainable by some devices in their energy conversion process.
Chemical energy and electrical energy are considered to be high-grade energy because they can be converted to other forms of energy. However, there is a gradual degradation of the high-grade energy to low-grade energy in the operation of machines as the entropy (amount of disorder in a system) increases. It has become increasingly more important that man explores the renewable energy sources so that the energy demands of the future can be met with new high-grade energy.
8.1.3 ENERGY
TRANSFERDIAGRAMS
One useful way of showing the energy degradation is by using energy transfer diagrams. For a certain flashlight, the energy transfer can be represented as shown in the
Sankey diagram in Figure 805.
100 J chemical energy 5 J light energy 95 J thermal energy
Figure 805 Sankey diagram for a torch
In a Sankey diagram, the thickness of each arrow gives an indication of the scale of each energy transformation. The total energy before the energy transfer is equal to the total energy after the transfer otherwise the Law of conservation of energy would be violated. The problem is that once the thermal energy is transferred to the surroundings, it cannot be used to do useful work. Scientists are becoming more aware of this waste and there are many innovations being made in building designs to use some of this energy for heating purposes.
The efficiency of this simple flashlight is 5%. The efficiency of any system can be determined by using the relationship:
useful energy output
total energy input × 100% Efficiency =
8.1.4 POWERSTATIONS
Power stations rely on thermal energy, gravitational
potential energy or wind power to supply the kinetic energy to rotate a turbine. The turbine contains blades that are made to rotate by the force of water, gas, steam or wind. As the turbine rotates, it turns the shaft of a generator. The electrical energy can be produced by rotating coils in a magnetic field as previously discussed in the topic on electromagnetism.
Typically, fossil fuel power stations have a higher efficiency than nuclear power stations because current technology permits a higher temperature of 650 K versus the 570 K of a nuclear power plant.
The principle mechanisms involved in the production of electrical power with fossil fuels can be demonstrated by looking at the energy conversions in a coal-fired power station using a Sankey energy transfer diagram as shown in Figure 806. 100 units of energy stored in the fuel 9 units wasted in the furnace 47 units wasted to the cooling towers 3 units wasted by friction in the turbine and the
generator 40 units of useful energy
Figure 806 Sankey diagram for a coal-fired power station
Another useful energy transfer diagram is shown in Figure 807. The rectangles contain the different forms of energy, the circles show the conversion process, and the
Mechanical Electric Radiant Chemical Thermal
Mechanical 99% (electric generator) 100% (brakes)
Gravitational potential
85% (water turbine) 90% (hydro- electricity)
Electric 93% (electric motor) 40% (gas laser) 72% (wet cell battery) 100% (heating coil) Radiant 55% (wind power) 27% (solar cell) 0.6% (photosynthesis) 100% (solar furnace)
Chemical 45% (animal muscle) 10% (dry cell battery) 15% (chemical laser) 88% (steam furnace)
Thermal 52% (steam turbine) 7% (thermocouple) 5% (fluorescent tube)