Energy Conversion Efficiency. Before we discuss energy conversion efficiency, let us briefly get familiarized with various forms of energy first.

Transcription

1 Energy and Environment-1: Fossil Fuels and Nuclear Energy Objectives Key words and Concepts Energy Conversion Efficiency Fossil Fuels Nuclear Energy Cold Fusion? Summary Objectives: 1. Gain real understanding about energy conversion efficiency; 2. Capable of explaining major issues associated with fossil fuels both qualitatively and quantitatively; 3. Know the pros and cons of nuclear energy; 4. Possess a basic understanding about radiation safety. Key words and Concepts: Energy Energy Conversion Efficiency Where fossil fuels come from? Production-to-reserve ratio Isotopes Nuclear fission Nuclear fusion Half life Types of radiation Why do we care about energy conversion efficiency? Energy Conversion Efficiency What is energy? Many dictionaries may define energy as the capacity to do work. Here the word work should be understood in its general sense, it may include mechanical, thermal, and radiative functions all together. One key property of energy is given by the first law of thermal dynamics: It cannot be created or destroyed; it is only transformed from one form to another. So in the context of energy resources, we really mean those kinds of energy that can readily be converted to human-usable forms (e.g., mechanical work, heat, or light). Therefore, the process of energy conversion becomes critical, so does the conversion efficiency. Before we discuss energy conversion efficiency, let us briefly get familiarized with various forms of energy first. Forms of Energy: 1. Work (mechanical energy): Energy involved in relative mass movements, e.g., a pendulum in motion. 2. Heat (thermal energy): Energy involved in temperature changes or heat transfers. Heat = [Mass] [Heat Capacity] [Temperature Difference] Heat Capacity = energy used per unit of temperature change per unit of mass 3. Radiation (electromagnetic energy): Energy in the form of light both visible and invisible (e.g., solar energy). All radiation has this relationship:

2 Speed of light = [Wavelength] [Frequency] 4. Gravitational energy: Because of gravitational force, matters at different elevations possess different potential energy. Potential Energy = [Mass] [Acceleration (due to gravity)] [Height] 5. Nuclear energy: energy released from nuclear reactions by converting mass to energy, according to this Einstein equation: Energy = [Mass] [(Speed of light)2] 6. Chemical energy: energy involved in chemical reactions (e.g., fire, photosynthesis). 7. Electric energy: energy involved in electron movement. All the above forms of energy can be measured using various units of energy and related to power and units of power. One of the most commonly used energy units in the U.S. is the BTU, or British thermal unit. One BTU is equivalent to the energy required to raise the temperature of one pound of water by one degree Fahrenheit. Energy Conversion Devices and Their Efficiency An energy conversion device converts one form of energy into another. It is an important element of progress of society. The development of energy conversion devices throught time can be used as a gauge in the history of civilization (From: L.R. Radovic Energy and Fuels in Society. McGraw-Hill): James Watt's steam engine invented in the year 1765 marked the real beginning of the industrial revolution. Similarly, nuclear reactors (or nuclear bombs) got us to the "nuclear age." Both made a significant difference in energy conversion by human beings. An energy conversion device and the definition of energy conversion efficiency: Energy Output = Energy Input (1st Law) Useful Energy Output < Energy Input (2nd Law) In the above definition, the word useful is the key. If a conversion system involves multiple devices and/or several stages, the quantity of "useful" energy can be very different for different devices and/or at different stages. For example, a typical coal-fired power plant will have steam boilers (85% ECE) that drives stream turbines (45% ECE) which are connected to electric generators (95% ECE). The overall system efficiency actually is: 36%. Therefore, the efficiency of the system is always lower than any one of the efficiencies of the individual components of the system. This is governed by the second law of thermodynamics (the entropy law). Here is a table showing typical efficiencies of common conversion devices:

3 Fossil Fuels What are fossil fuels? The fossil fuels used as energy sources today are coal, petroleum and natural gas. Enormous resources of other fossil fuels exist, but they are not yet in large-scale commercial use, e.g., oil shale and tar sands. Where do fossil fuels come from? Apparently fossil fuels are mined out of the Earth's crust. Fossil fuels are derived from the remains of organisms that lived millions of years ago. They have been preserved as geologic deposits in reasonable proximity to the earth's surface (typically <10 kilometers deep). According to our current understanding, these deposits initially resulted from significant imbalance between the rate of net primary production and the decomposition rate in many regions of the world in the late Paleozoic and the Mesozoic period of earth's history, because of a dominant environment that favors abundant growth of living organisms, while preventing their rapid decay to CO2 and H2O. After a certain period of anaerobic decomposition of these deposited organic matter, only insoluble solid material known as kerogen remained. Over the course of geological time (typically, millions of years), the accumulated kerogen was buried more and more deeply in the earth's crust. The more deeply it was buried, the higher the temperature and pressure to which it was exposed. At moderate temperatures inside the earth, the long chains of carbon atoms, characteristic of kerogen derived primarily from planktonic organisms, broke apart into shorter ones, and become a liquid made of mostly compounds between five and twenty carbon atoms. This liquid now is known as petroleum or crude oil. At further higher temperatures inside the earth, the breaking of carbon-to-carbon bonds continued, eventually resulting in the formation of compounds containing only one to four carbon atoms. These molecules are so small that they are gases at ordinary atmospheric conditions. This gaseous mixture is known as natural gas. The type of kerogen produced from the higher plants contained abundant quantities of lignin with very complex molecular structures. As this ring-structured kerogen from higher plants was transformed inside the earth's crust, the original lignin-like materials were gradually changed into a solid material with a complex and somewhat ill-defined molecular structure. This material is known as coal. How much fossil fuels have we burnt? Here is one indication: CO2 emissions through time. Source: CO2 Information Analysis Center, Oak Ridge National Lab. The global amount of CO2 from fossil fuels used since 1750 is roughly half of the amount of CO2 in the atmosphere. Please see this in the global carbon cycle. How fast are we burning these fossil fuels now? Source for the figure above: DOE Energy Information Administration Source for the figure above: DOE Energy Information Administration Source: DOE Energy Information Administration, Energy Perspectives (Quadrillion =1015 ; Energy content for crude oil is roughly 5.8 x 106 Btu/barrel; for coal is roughly 13,000 Btu/lb; and natural gas is roughly 1000 Btu/cubic ft) This is for the US. Source: DOE Energy Information Administration, Energy Perspectives 2007.

4 The bottom line: According to estimates by EIA in 2008, at the global scale the so-called production-toreserve ratio is roughly 143 years for coal, 63 years for natural gas, and 35 years for crude oil. These fossil fuels will not last for long. How is the use of fossil fuels linked to human-enhanced greenhouse effect (global warming) and air pollution? Here is a cumulative picture One report (Meinshausen et al., NATURE, 30 April 2009 issue) predicted that this kind of fossil fuel usage would push the atmospheric CO2 concentration beyond 1000 ppm, and the global average surface temperature would increase from pre-industrial level by 4-8 C by the year Please note that 8 C increase is equivalent to the temperature swing from the peak temperature of an interglacial period (we are in it now) to the lowest level in a glacial period (ice age). Further more, fossil fuel burning also produce air pollutants, e.g., SO2, NOx etc. Nuclear Energy Nuclear energy is also called atomic energy. Nuclear reactions of fission and fusion produce nuclear energy (in the forms of heat and radioactivity) which may be converted to more useful forms such as electricity. In these reactions the atoms lose their integrity. New atoms, either smaller or larger, are formed by the rearrangement of their nuclei. This process is accompanied by a much greater release of energy than that corresponding to the heat of combustion of fossil fuels. However, radioactivity can be potentially harmful, therefore, atomic energy is a very controversial form of energy. How much nuclear energy being used? In the US, nuclear power contributes roughly 8.4% of total energy consumption as of 2007, according to the Energy Information Administration of DOE. In the whole world, nuclear power made up approximately 6% of the total energy supply. Where are these nuclear power plants in the US and in the world? Nuclear power plants in the world: However, the majority of these nuclear power plants were built before the two major nuclear accidents (Three Mile Island in the US, and Chernobyl in the Ukraine). The figure above shows the number of nuclear power plant operating licenses issued per year since 1950's in the US. (Source: Energy Perspective 2007 from Energy Information Administration) In order to understand issues about nuclear energy, let us go through some basic information here. Isotopes: It has long been figured out in physics that an atom of any element consists of a nucleus, which is positively charged, and of electrons, which revolve around the nucleus. Electrons are negatively charged and their mass is much smaller than that of the nucleus. The nucleus consists of protons and neutrons. Protons are positively charged; neutrons are neutral or uncharged. The number of protons in an atom is equal to the number of electrons under normal circumstances. Because neutrons are neutral, their number in the nucleus of an atom is not necessarily equal to the number of protons in the nucleus. It is not uncommon,

5 therefore, to find in nature different forms of an element with the same number of protons, but different numbers of neutrons. Such different forms of an atom, which are chemically identical but can be very different physically, are called isotopes. Isotopes that are energetically unstable and releases their excess energy in the form of radioactive decay (or produce radioactivity) are called radioisotopes. Radioactive decay is a natural process that occurs spontaneously. This process can also be induced in a nuclear reaction or in an atomic bomb. Those isotopes that are energetically stable and do not have radioactivity are called stable isotopes. Here are a few isotopes that are highly relevant to our discussion: There are three kinds of radioactivity: alpha, beta and gamma radiation; and two kinds of nuclear reactions: Fission (a heavy nucleus is fragmented into lighter nuclei) and Fussion (two light nuclei are combined to form a heavier nucleus) Example of Nuclear Fission: Uranium-235 (U-235) is the principal constituent of the fuel rods in many nuclear reactors. U-235 can undergo fission, according to the following nuclear equation: In this fission event, the fissionable uranium nucleus reacts with a neutron, becomes temporarily unstable and is fragmented very soon thereafter into a nucleus of barium (Ba), a nucleus of krypton (Kr), and three neutrons, and give off 57 MJ of energy per gram of U-235 fissioned. The potential nuclear energy stored in one gram of U-235 is equivalent to the chemical energy stored in approximately three million grams of coal. Up to 85% of the energy released in the fission process appears as kinetic energy of the fragments. The rest is in radioactivity. As the high-speed fragments collide with surrounding matter, they induce random motion of the surrounding atoms and molecules; their kinetic energy is thus converted to heat. This heat is used in turn to produce electricity in a nuclear reactor or is allowed to cause an explosion in an atomic bomb. The fate of the neutrons produced in the fission process is the key to understanding the difference between a controlled nuclear reaction, which takes place inside a nuclear reactor, and an uncontrolled nuclear reaction, which leads to the explosion of an atomic bomb. Chain reaction vs. Sustain (or controlled) reaction: U-235 fission is an example of a chain reaction. Each one of the three neutrons produced in the first fission event goes on to collide with other U-235 nuclei. This new collision event will in turn produce three additional neutrons, and more neutrons will cause further more fission events. So this forms a positive feedback--or a chain reaction. Because this exponential increase in fission events is accompanied by heat release, an exponential build-up of heat occurs at the same time. There is no way to dissipate or carry away all this heat, so the temperature of the solid material (within which fission occurs) increases, the material melts and then vaporizes. This in turn causes a pressure build-up which ultimately results in an explosion. This simplified description of an uncontrolled chain reaction represents well the sequence of events in a nuclear bomb or in a major nuclear accident. The China syndrome an expression often used for a hypothetical nuclear accident refers to the meltdown of the Earth caused by such heat release, the sinking of the reactor and the people around it through the molten Earth's crust and their appearance at the other end of the globe, in China! For an atomic bomb it has been estimated that approximately 15 billion BTU of heat are released in just 50 microseconds. How can we control this process? The control of neutron inventory, or the maintenance of neutron balance, is the key. Both the quantity and the quality of neutrons are important for the peaceful and effective utilization of this energy. We first want to maintain a steady number of neutrons. Ideally, only one of the three neutrons produced in each collision will be allowed to carry the chain and continue the fission process until all fissionable material is consumed. Such a process generates a constant quantity of heat. This heat can be dissipated easily and the temperature of the material can be maintained below its melting point. This is a self-sustained chain reaction.

6 A self-sustained chain reaction is often achieved by the use of a material that is capable of absorbing neutrons. A typical example of such a material is the element boron. Its reaction with a neutron is very simple, as shown below: In contrast to U-235 fission, in which a net production of neutrons (three neutrons produced for every neutron consumed) occurs, here neutrons are consumed. If this material is put together with the fissionable uranium-235 in a right mix, it can control the neutron inventory. This is the principle of operation of the control rods in the nuclear reactors used in virtually all nuclear power plants in the world now. Example of Nuclear Fusion: When a deuterium nucleus and a tritium nucleus are combined together forming one helium nucleus plus a neutron at or above the threshold temperature of 45,000,000 C, the amount of energy roughly equal to 330 MJ/g of helium is released as shown below: D + T = 2He4 + n MeV of energy The equation shown above is only one of the possible fusion reactions. Other examples are: D + D = p + T MeV of energy D + 2He3 = p + 2He MeV of energy 3Li6 + n = 2He4 + T MeV of energy Note: D is deuterium (1H2); T is tritium (1H3); He is helium; Li is lithium; p is proton; and n is neutron. The symbol ev represents an energy unit called the electron volt. It is defined as the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt. One electron volt is approximately equal to joules. The advantages of Fusion: One advantage is that the reserves of fusionable isotopes are much larger than those of fissionable isotopes. Another advantage is that the products of fusion reactions are less radioactive then the products of fission reactions. Among the products of the fusion reactions listed above, only tritium and the neutrons are radioactive. The last advantage of fusion lies in its inherent safety. There would be very little fusionable material at any given time in the reactor and the likelihood of a runaway reaction would thus be very small. The challenge of putting nuclear fusion for commercial use: As shown above, nuclear fusion generally has higher energy potential than nuclear fission. However, it also requires more energy to get to the extremely high threshold temperature (>107 C) and other conditions. For nuclear fusion to occur on Earth, other required conditions must be met in addition to the unimaginably high temperature, chiefly, compressing the mixture of the light nuclei to a high density so that the probability of collision among the nuclei can be high enough for fusion to occur; and maintaining the nuclear fusion reaction long enough so that the rate of energy produced is greater than the rate of dissipated energy (as heat and compression). So far, nuclear fusion has been achieved at micro-scales in a couple of highly sophisticated laboratories or used in hydrogen bombs. Because of these challenging

7 requirements, the dreamed commercial use of nuclear fusion technology for electricity generation remains highly elusive at this point of time. Major issues of nuclear power: 1. Pollution of radioactive materials. All fission-based reactors produce some radioactive materials. All radioactive materials can be described by their half-life (Figure), the kind of radiation (alpha, beta, gamma or x-ray, etc), and exposure doses. 2. Nuclear accident. The Three Mile Island Accident--This accident is about the malfunctioning of reactor No.2 at the Metropolitan Edison's Three Mile Island power plant, on March 28, It was the first major accident reported in a commercial nuclear power plant in the U.S. The most authoritative source of information on it is the Report of the President's Commission on the Accident at Three Mile Island (J.G. Kemeny, Chairman), Pergamon Press, New York, October The cause of the TMI accident was a combination of (a) mechanical failures, and (b) human error and lack of adequate training for emergency situations of the kind that developed in those critical hours. The clean-up cost of the radioactive pollution exceeded a billion dollars. For more information about this accident check this out: FACT-sheet. The Chernobyl Accident--The accident at the nuclear reactor in Chernobyl in the Ukraine on April 26, 1986, was much more serious. The cause of the accident was unauthorized experimentation with the reactor (an accidental complete detachment of the control rods). The degree of damage was such that the population from a 30-kilometer radius had to be evacuated; the effects were felt all over Europe. The area around Chernobyl will be radioactive for a long time. For more information about the Chernobyl accident, check this out: 3. Disposal of nuclear wastes. What to do with the old fuel rods? Once the power of a fuel rod dropped to a low level after use, it has to be taken out of the core while still emitting large amounts of radioactivity. And it will continue to be dangerously radioactive for decades and centuries. These old rods have to have a place to stay repositories. The problem is where to find these repositories. 4. Nuclear weapons get to the wrong hands. We all know how dangerous it can be if a nuclear bomb gets to the wrong hands. The atomic bomb dropped on Hiroshima, Japan, on August 6, 1945 was 28 inches in diameter and 10 feet long and it weighed about 4.5 tons. The explosive yield of its uranium fuel was rated at about 13,000 tons of TNT (trinitrotoluene, a conventional explosive). In other words, it released the same energy as 13,000 tons of TNT. The nuclear bomb dropped on Nagasaki a few days later was 5 feet in diameter and 128 inches long. It weighed 5 tons and the explosive yield of its plutonium fuel was 22 kilotons of TNT. More than 200,000 people died immediately as a result of the bombings, and many others contracted cancer afterwards. Today's nuclear weapons are much more sophisticated and even deadlier, having yields of 1 to 2 megatons of TNT. Nuclear Energy to do or not to do? The future of fission-based nuclear reactors primarily depends on the resolution of socio-political issues, in addition to the major technical problems and difficult economic issues. We all understand the reluctance to accept the construction of a nuclear power plant or a waste repository in one's backyard (the not in my backyard syndrome or NIMBY for short). It is probable that the new designs of nuclear power plants will provide a greater degree of safety and will decrease the likelihood of major accidents. It is unlikely, however, that a completely failsafe reactor will ever be designed. After all, accidents like those at Three Mile Island and Chernobyl were considered impossible until they happened. Society will have to weigh the effect of potential nuclear pollution against the problems brought about by increasing use of fossil fuels and other energy sources. In summary, the use of nuclear power remains highly controversial after the two major nuclear accidents happened. However, more nuclear power has to be used if the ever increasing global thirst for more power remains unchanged.

8 Please keep these questions in your mind when you watch the video: 1. If someone asks you what cold fusion is about, what will you tell? 2. What will be your take of the whole story? 3. What would you do if you would be in a similar situation as the two scientists who initially announced the discovery of cold fusion? 4. Do you think there is any hope in cold fusion for the future? Summary: 1. Energy conversion efficiency is a key for our energy utilities. 2. Fossil fuels have been our main sources of energy since the start of the industrial revolution. 3. Fossil fuels are finite, and the current rate of use is not sustainable. 4. Unrestrained use of carbon-based fossil energy is causing global warming through enhanced greenhouse effects. 5. Nuclear energy has been recognized as having huge potentials and associated risks and environmental issues.