7. Energy, Power and Climate Change. Chapter 7.1 Energy degradation and power generation

Transcription

1 7. Energy, Power and Climate Change Chapter 7.1 Energy degradation and power generation

2 Degradation of energy Energy flows from hot bodies to cold bodies. The difference in temperature between two bodies can make a heat engine work, allowing useful mechanical work to be extracted in the process. Some of the thermal energy transferred from the hot to the cold body can be transformed into mechanical work. Energy flow between two bodies is represented by a Sankey Diagram.

3 Sankey Diagram The width of each arrow in the diagram is proportional to the energy carried by that arrow. hot reservoir 800J 200J Knowing the useful mechanical work done and the energy input we can calculate the machine s efficiency: 600J cold reservoir

4 Degradation of energy As energy flows from the hot to the cold body, they will eventually reach the same temperature and the opportunity to do work will be lost. Heat engines are machines that use the heat transfer to do useful mechanical work. Any practical heat engine works in a cycle as the process must be repeated. thermal energy absorbed gas expands doing mechanical work gas returned to its initial state, so that the cycle can be repeated some thermal energy is released from the engine

5 Degradation of energy The problem with machines that operate in a cycle is that not all of the thermal energy transferred can be transformed into mechanical work. Some energy goes to the cold reservoir. Unless there is a colder reservoir, that energy cannot be used. This energy had become degraded. Energy, while always being conserved, becomes less useful, i.e., it cannot be used to perform mechanical work this is called energy degradation

6 Electricity production Electricity is produced using electric generators by rotating a coil in a magnetic field so that magnetic field lines are cut by the moving coil. According to Faraday s law an emf (voltage) will be created in the coil which can then be delivered to consumers. So, generators convert mechanical energy into electrical energy. fossil fuels nuclear power reactors wind energy hydroelectric turbines wave power solar energy (photovoltaic cells) kinetic energy of rotation generator electrical energy

7 Energy sources Non-renewable sources are finite sources, which are being depleted, and will run out. They include fossil fuels (oil, natural gas and coal), and nuclear fuels (uranium). The energy stored in these sources is, in general, a form of potential energy, which can be released by human action. Renewable sources include solar energy, other forms indirectly dependent on solar energy (wind and wave energy) and tidal energy

8 Energy sources Today, the main energy sources are those that rely on fossil fuels and emit large amounts of carbon dioxide. The world average energy production is give in the table below. Fuel % of total energy produced CO2 emission (g MJ-1) Oil Natural gas Coal Nuclear 7 - Hydroelectric 7 - Others <1 -

9 Energy density Energy density is the energy that can be obtained from a unit mass of the fuel. It is measured in J kg-1. If the energy is obtained by burning fuels, the energy density is simple the heat of combustion. Substance Heat of combustion Coal 30 MJ kg-1 Wood 16 MJ kg-1 Diesel oil 45 MJ kg-1 Gasoline 47 MJ kg-1 Kerosene 46 MJ kg-1 Natural gas 39 MJ m-3

10 Energy density In a nuclear fission reaction, mass is converted directly into energy through Einstein s formula E=mc2. For instance, 1kg of pure uranium-235 releases about 7x104 GJ. Natural uranium produces about 490 GJ/kg and enriched uranium about 2100 GJ/kg. In a hydroelectric power station, considering that the water falls from a height of 100m the kinetic energy gained by 1kg of the water is 103 J. This implies that the energy density of water used as fuel is much less than the energy density of fossil fuels.

11 Fossil fuels Fossil fuels have been created over millions of years. They are produced by the decomposition of buried animals and plant matter under the combined action of the high pressure of the material on top and bacteria. Thermal energy produced when burning these fuels is used to power steam engines. Although these engines are generally efficient (30-40%) they are also responsible for atmospheric pollution and contribute greenhouse gases to the atmosphere.

12 Fossil fuel mining Coal is obtained by mining. This process releases a large number of toxic substances and the coal itself is high in sulphur content and traces of heavy metals. Rain can wash away these substances and cause environmental problems if this acidic water enters underground water reserves. Drilling for oil has also adverse environmental effects, with many accidents leading to leakage of oil both at sea and on land.

13 Fossil fuel mining Advantages Relatively cheap (while they last) High power output (high energy density) Variety of engines and devices use them directly and easily Extensive distribution network is in place Disadvantages Will run out Pollute the environment Contribute to greenhouse effect by releasing greenhouse gases into atmosphere High cost of distribution due to high mass and volume of materials and high cost of storing (needs extensive storage facilities) Pose serious environmental problems due to leakages at various points along the production

14 Nuclear power Nuclear fission is the process in which a heavy nucleus splits into lighter nuclei. When uranium-235 absorbs a neutron, it turns into uranium-236 which will decay into krypton and barium and will release another 3 neutrons Energy In a nuclear fission reaction, mass is converted directly into energy through Einstein s formula E=mc2. Nuclear Power Station

15 Nuclear reactors A nuclear reactor is a machine in which nuclear reactions take place, producing energy. The fuel of a nuclear reactor is typically uranium-235. The isotope of uranium that is most abundant is uranium-238. Natural uranium contains only about 0.7% of uranium-235. The uranium fuel in a reactor is made to contain about 3% of uranium-235 enriched uranium. When uranium-235 captures a neutron, two process can occur:

16 Nuclear reactors The are examples of induced fission. The fission does not proceed by itself neutrons must initiate it. The neutrons produced can used to collide with other uranium-235 nuclei in the reactor, producing more fission, more energy and more neutrons. The reaction is thus self-sustaining and called a chain reaction. For the chain reaction to get going a certain minimum mass of uranium-235 must be present, otherwise neutrons would escape without causing further reactions. This minimum mass is called critical mass.

17 Nuclear reactors Uranium-235 will only capture neutrons if they are not too fast. The neutrons produced in the chain reaction are too fast to be captured and have to be slowed down (they have to go from 1MeV of kinetic energy to less than 1 ev). The slowing down of neutrons is achieved through collisions of the neutrons with atoms of the moderator, a material surrounding the fuel rods (tubes containing U-235). The moderator can be graphite or water, for example. The rate of reaction is determined by the number of neutrons available to be captured by U-235. To few neutrons would result in the reaction stopping Too many neutrons would lead to an uncontrollably large release of energy.

18 Nuclear reactors Thus, the control rods (the material that can absorb excess neutrons whenever necessary) are introduced in the moderator. The control rods can be removed when not needed and reinserted when necessary again. The control rods ensure that the energy from nuclear reactions is released in a slow and controlled way as opposed to the uncontrollable release of energy that would take place in a nuclear weapon.

19 Nuclear reactors

20 Nuclear reactors In a Pressurized Water Reactor (PWR) water is kept under pressure to keep it from boiling, even at 300 C. The pressurized water is pumped through a closed system of pipes called the primary circuit. Heat from the primary circuit warms up water in the secondary circuit. The water in the secondary circuit comes to a boil and its steam turns the turbine. The water in the primary circuit returns to the reactor core after giving up some of its heat.

21 Nuclear reactors Gas Cooled Reactors (GCR) use carbon dioxide as the coolant to carry the heat to the turbine, and graphite as the moderator. Like heavy water, a graphite moderator allows natural uranium (GCR) or slightly enriched uranium (AGR) to be used as fuel.

22 Nuclear reactors The energy released in the reaction is the form of kinetic energy of the produced neutrons (and gamma ray photons). This kinetic energy is converted into thermal energy (in the moderator) as the neutrons are slowed down by collisions with the moderator atoms. A coolant (e.g. water or liquid sodium) passing through the moderator can extract this energy, and use it in a heat exchanger to turn water into steam at high temperature and pressure. The steam can be used to turn the turbines of a power station, finally producing electricity. nuclear energy kinetic energy of particles thermal energy kinetic energy of rotation electrical energy

23 Plutonium production The fast neutrons produced in a fission reaction may be used to bombard U-238 and produce plutonium-239. This isotope of plutonium does not occur naturally. The reactions are: The importance of these reactions is that non-fissionable material (U-238) is being converted to fissionable material (Pu-239) than can be used as the nuclear fuel in other reactors.

24 Problems with nuclear reactors Both fuel and products of the reactions are highly radioactive, with long half-lives. Disposal of nuclear waste is a serious disadvantage of the fission process in commercial energy production. This material is currently buried deep underground in containers that are supposed to avoid leakage to the outside. Another problem is the possibility of accidents due to uncontrolled heating of the moderator. Such heating would increase T and hence the pressure in the cooling pipes, resulting in a explosion. This would lead to the leakage of radioactive material into the environment or, even worse, could lead to the meltdown of the entire core.

25 Problems with nuclear reactors The positive aspect is that nuclear power does not produce larges amounts of greenhouse gases. The nuclei produced in a fission reaction are typically unstable and usually decay by beta decay. This decay produces an additional amount of energy. Even if the nuclear reactor shuts down, production of thermal energy continues because of the beta decay of the product nuclei. The energy produced in this way is enough to melt the entire core of the reactor if the cooling system breaks down. Another worry is that the fissionable material produced can be recovered and be used in a nuclear weapons programme.

26 Uranium mining Like all types of mining uranium mining is dangerous. Uranium produces radon gas, a known strong carcinogen as it is an alpha emitter. Inhalation of this gas as well as of radioactive dust particles is a major hazard in the uranium mining industry. Mine shafts require good ventilation and must be closed to avoid direct contact with the atmosphere. The disposal of waste material from the mining processes is also a problem since the material is radioactive.

27 Nuclear Energy Advantages High power output Large reserves of nuclear fuels Nuclear power stations do not produce greenhouse gases Disadvantages Radioactive waste products difficult to dispose of Major public health hazard should something go wrong Problems associated with uranium mining Possibility of producing materials for nuclear weapons

28 Nuclear fusion A typical energy-producing nuclear fusion reaction is: Deuterium (D) can be extracted from water using electrolysis and tritium (T) can be produced by bombarding lithium with neutrons. The problem with fusion is that, since D and T are both positively charged, the reacting nuclei repel. To get them close enough to each other for the reaction to take place, high temperatures must be reached around 108 K. At this temperature, hydrogen atoms are ionized and so we have a plasma (mixture of positive nuclei and electrons).

29 Nuclear fusion The hot plasma must be confined in such way so that it doesn t come into contact with anything else as this would cause: a reduction in temperature contamination of the plasma with other materials. These two effects would cause the fusion reaction to stop. The plasma is therefore confined magnetically in a tokamak machine (toroidal magnetic chamber). The magnetic field prevents the plasma from touching the container walls.

30 Nuclear fusion Energy must be supplied to the fusion process to reach the high temperatures required. It has not yet been possible to produce more energy out of fusion that has first been put in, for sustained periods of time. For this reason, fusion as a source of commercially produced energy is not yet feasible. There are also technical problems with using the energy produced in fusion to produce electricity.. Compared to nuclear fission, nuclear fusion has the advantage of plentiful fuels, substantial amount of energy produced and much fewer problems with radioactive waste.

31 Solar power The Sun produces energy at a rate of about 3.9x1026 W. This means that, on average, the Earth receives about 1400W per square metre of the surface of the outer atmosphere. Some of this radiation is reflected back into space, some is trapped by the atmosphere s gases and about 1kW m-2 is received on the surface of the Earth. This amount assumes direct sunlight on a clear day and thus is the maximum that can be received at any one time. Averaged over a 24-hour time period, the intensity of sunlight is about 340 W m-2. This high-quality, free and inexhaustible energy can be put to various uses.

32 Active solar devices The sunlight is used directly to heat water or air for heating in a house, for example. The surface is usually flat and covered by glass for protection; the glass should be coated to reduce reflection. A blackened surface below the glass collects sunlight, and water circulating in pipes underneath gets heated.

33 Active solar devices This hot water can then be used for household purposes, such as in bathrooms (the heated water is kept in well-insulated containers). Another possibility is to make the hot water, with the help of a pump, circulate through a house, providing a heating effect.

34 Active solar devices In other schemes, the pipes can be exposed directly to sunlight, in which case they are blackened to increase absorption. The surface underneath the pipes is reflecting so that more radiation enters the pipes. Such a collector works not only with direct sunlight but also with diffuse light like in cloudy days. sunlight warm water out cold water in reflecting surface blackened pipes

35 Active solar devices These simple collectors are cheap and are usually put on the roof of a house. Their disadvantage is that they tend to be bulky and cover too much space. More sophisticated collectors include a concentrator system in which the incoming solar radiation is focused, for example by a parabolic mirror, before it falls on the collecting surface. Such systems can heat water to much higher temperatures (500ºC to 2000ºC) than a simple flat collector.

36 Active solar devices These high temperatures can be used to turn water into steam, which can drive a turbine, producing electricity. Obviously, back-up systems must be available in case of cloudy days.

37 Photovoltaic cells The photovoltaic cell was developed in 1954 at Bell Laboratories for the use in the space programme to power satellites and probes sent to outer space. A photovoltaic cell coverts sunlight into DC current at an efficiency of about 30% Although it was initially very expensive technology, currently the energy cost using photovoltaic cells is slightly higher than that produced by diesel-powered generators. The principle inherent to the working of a photovoltaic cell lies on the physics of semiconductors and must not be mistaken with the photoelectric effect.

38 Photovoltaic cells

39 Photovoltaic cells The price drop of this technology makes it more likely to become more dominant in electricity production around the world. Already, in places far from major power grids, its use is more economical than grid expansion. It can be used to power small remote villages, pump water in agriculture, power warning lights, etc. Their environmental ill effects are practically zero, with the exception on chemical pollution at the place of their manufacture.

40 Photovoltaic cells Advantages Free Inexhaustible Clean Disadvantages Works during the day only Affected by cloudy weather Low power output Requires large areas Initial cost high

41 The solar constant The sun s total power output, also know as luminosity, is P = 3.9 x 1026 W On Earth, we receive only a very small fraction of this total power output. The average distance between the sun and the earth is r=1.5x1011m. The sun s power is distributed uniformly over the surface of an imaginary sphere of radius r=1.5x1011m. The power that is collected by area A is the fraction A r Note that 4πr2 is the surface area of the imaginary sphere.

42 The solar constant The power per unit area received at a distance r from the sun is called the intensity, I, and so This amounts to about 1400 W/m2 and is known as the solar constant. It s the power received by one square metre placed normally to the path of the incoming rays a distance 1.5x1011 m from the sun.

43 The solar constant This amount varies as the sun s output is not constant (variation of ±1.5%). Also, Earth does not keep a constant distance from the sun as the orbit is elliptical (additional variation of ± 0.4%). To find the radiation received on Earth s surface, we must take into account the reflection of the radiation from the atmosphere and the earth s surface itself, latitude, angle of incidence and average between day and night. It is useful to define the total amount of energy received by one square metre of the earth s surface in the course of one day. This is called the daily insolation.

44 The solar constant The reduction of the daily insolation in the winter for high latitudes can be explained by the shorter length of daylight and the oblique incidence of light.

45 The solar constant

46 Hydroelectric power Hydropower, the power derived from moving water masses, is one of the oldest and most established of all renewable energy sources Although dependent of sites, it s capable of producing cheap electricity. Turbines driven by falling water have a long working life without major maintenance costs. It has high initial costs but it s widely used all over the world.

47 Hydroelectric power Hydroelectric power stations are, however, associated with massive changes in the ecology of the area surrounding the plants. To create a reservoir behind a newly constructed dam, a vast area of land must be flooded

48 Hydroelectric power

49 Hydroelectric power The principle behind hydropower is very simple. Consider a mass m of water that falls down a vertical height h. The potential energy of the mass is mgh, and it gets converted into kinetic energy when the mass descends the vertical distance h. The mass is given by ρδv, where ρ is the water s density (1000kg/m3) and ΔV is the volume it occupies. m h

50 Hydroelectric power The rate of change of this potential energy, that is, the power P, is given by the change in potential energy divided by the time taken for that change, so: The quantity Q =ΔV/Δt is known as the volume flow (volume per second) and so:

51 Hydroelectric power Within a time equal to Δt, the mass of water that will flow through the tube is m=ρδv =ρqgh ρ ΔV This is the power available for generating electricity and it s clear that hydropower requires large volume flow rates, Q, and large heights, h.

52 Hydroelectric power A different number of schemes are available for extracting the power of water Water can be stored in a lake, which should be at as high as elevation as possible to allow for energy release when the water is allowed to flow to lower heights. In a pump storage system, the water the water that flows to lower heights is pumped back to its original height using the excess of electricity from somewhere else. This is the only way to store energy on a large scale for use when demand is high. Finally, tidal storage systems take advantage of tides: the flow of water during a tide turns turbines, producing electricity.

53 Hydroelectric power Advantages Free Inexhaustible Clean Disadvantages Very dependent on location Requires drastic changes to environment Initial costs high

54 Wind power Wind power devices have no adverse effects, although there is some evidence that low-frequency sound emitted during the operation of wind turbines affects people s sleeping). However, a very large number of them is not an attractive sight to many people, and there is a noise problem. The blades are susceptible to stresses in high winds, and damage due to metal fatigue frequently occurs. Generally, about 25% of the power of carried by the wind can be converted into electricity.

55 Wind power solar energy kinetic energy (wind) kinetic energy of rotation turbine electrical energy losses in turbine, turbulence Wind speed is the crucial factor for these systems, the power extracted being proportional to the cube of the wind speed. Wind speed of up to about 4m/s are not particularly useful for energy extraction. Serious power production from winds occurs at speeds from 6 to 14 m/s. The dependence of the power on the area of the blades and the cube of the wind speed can be easily understood.

56 Wind power Consider the mass of air that can pass through a tube of cross-sectional area A with velocity v in time Δt. Let ρ be the density of air. Then the mass enclosed in a tube of length vδt is ρavδt. area A spanned by the blades win d This is the mass that will exit the right end of the tube within a vδt time interval equal to Δt. v ρ A ΔV The kinetic energy of this mass of air is thus:

57 Wind power The kinetic energy per unit time is the power, and so dividing by Δt we find: Which shows that the power carried by the wind is proportional to the cube of the wind speed and proportional to the area spanned by the blades. The power extracted by the turbine is: Where Cp is know as the power coefficient. It is simply an efficiency factor that determines how much of the available wind power the wind turbine can extract. Theoretically, it varies between 0.35 and 0.45

58 Wind power In practice, frictional and other losses (mainly turbulence) result in a smaller power increase. The previous calculations also assume that all the wind is actually stopped by the wind turbine, extracting all of the wind s kinetic energy, which in practice is not the case.

59 Wind power Advantages Free Inexhaustible Clean Ideal for remote island locations Disadvantages Works only if there is wind not dependable Low power output Aesthetically unpleasant (and noisy) Best locations far from large cities Maintenance costs high

60 Wave power It has been realised that deep-water, long wavelength sea waves carry a lot of energy. Water waves are very complex and belong to a class of waves called dispersive, i.e., the speed of the wave depends on the wavelength. A water wave of amplitude A carries an amount of power per unit length of its wavefront equal to: Where ρ is the density of water and v stands for the speed of energy transfer in the wave

61 Wave power Many devices have been proposed to extract the power out of waves. The one discussed here is the oscillating water column (OMC). As a crest of the wave approaches the cavity in the device, the column of water in the cavity rises and pushes the air above it upwards. The air then turns a turbine. As a trough of the wave approaches the cavity, the water in the cavity falls and the air drawn turns the turbine again.

62 Wave power Wave patterns are irregular in wave speed, amplitude and direction. This makes it difficult to achieve reasonable efficiency of wave devices over all the variables. For many wave devices, it is difficult to couple the low frequency of the waves (typically 0.1 Hz) to the much higher generator frequencies (50-60 Hz) required for electricity production. The OWC solves this problem by changing the speed of the air by adjusting the diameter of the valves through which the air passes. In this way, very high speeds can be attained, thus coupling the low-frequency water waves with the high-frequency turbine motion.

63 Wave power Advantages Free Inexhaustible Clean Reasonable energy density Disadvantages Works only in areas with large waves Irregular wave patterns make it difficult to achieve reasonable efficiency Difficult to couple low-frequency water waves with high-frequency turbine motion Maintenance and installation costs high Transporting the produced power to consumers involve high costs Devices must be able to withstand hurricane and gale-force storms.