This is a book about energy. Between 1960 and 1970 we in the United States consumed as much electrical energy as in all time before 1960. It is quite likely that between 1970 and 1980 we again will consume more electrical energy than in all time before 1970. We cannot do this without impact on ourselves, our environment, and the world. Drastic changes in the trends of energy utilization must somehow be made in the years not far ahead. These matters are already becoming the subject of intense public debate. To act responsibly, and intelligently, one must develop an understanding of what energy is, how we use it, and its impact upon society; only then can one participate rationally in the decisions that must be made.
Rational thinking about a subject such as energy has two important aspects. First, there are quantitative matters for which numbers can be developed by computation based on scientific principles. One might think that there is not much room for argument here; but there is, for to make such a calculation one must always approximate the real world by a simplified model, and then use data (often incomplete) gathered from measurements (often inaccurate). A well-trained engineer or scientist will be able to identify the pitfalls of a particular quantitative argument, and will produce a number of analyses covering the range of probable realities. Then there is room for considerable debate as to which end of the range is most realistic, and the debate can get very heated when one end spells doom and the other paradise. The second aspect of rational thinking is value judgment. There is usually a great deal of room for argument here, even between persons purportedly espousing the same values, but especially between persons who fail to articulate their values clearly. The engineer can calculate the size of a bridge that could safely span the Grand Canyon, without much room for debate on the quantitative issues. But the question whether or not to put the bridge there in the first place is a value judgment that relates to why the bridge is there and to its impact upon other aspects of the setting. In the situation where a new power plant is proposed for a particular location, an environmentalist might claim that the ecology of the river will be upset. A biologist might be able to provide some numbers relating to the ecological impact (probably not very reliable because of poor or nonexistent data), and a heated debate over the quantitative issues could ensue. Various value judgments might be espoused: "That power plant will foul up our river." "The plant is essential to the economic survival of our community." Ultimately the decision should be made by considering the quantitative facts against a background of important values; the better understood the quantitative statements, the clearer the decision.
In this book we shall emphasize the quantitative aspects necessary for rational thinking in decisions about energy. The basic science of energy is simple, relates well to personal experience, and does not involve very complicated mathematics. If you do not already know about the simple scientific matters that we will discuss here, you may be amazed to find how much you will be able to do with some very simple tools for quantitative analysis. We expect, for example, that before long you will be able to make a fair estimate of the temperature is of the river used by that new power plant that is proposed for your community, or to assess quantitatively the impact of a national emphasis on generation of electric power from solar energy. You may find that your ability to analyze these situations quantitatively is very satisfying, in which case you will experience some of the emotions of an engineer. You will probably find that your ability to communicate with your engineering friends will improve dramatically as you develop your ability to think and talk about matters of mutual interest. Very soon you will be able to read articles in such journals as Scientific American, Mechanical Engineering, Power, Technology Review, and the Society of Automotive Engineers Journal (are these in your library?). From these journals you will be able to extract quantitative information that you can understand and can use in your arguments for or against a particular issue. Please always bear in mind that our objective is to give you some appreciation for what is involved when an engineer does a quantitative analysis, not to make you a skilled engineer! If you already have this quantitative bent, and have studied math, science, or engineering, look for new ways to think about the scientific matters discussed here; you may find new understanding, and perhaps some simple ways to explain your life to your nontechnical friends.
"Science" is the systematic structure of ideas that are used to describe nature. The task of creating things that take advantage of scientific principles to do certain jobs is called "engineering." Thus, the engineer must understand and use science, and in addition have some inclination for the nuts-and-bolts aspect of making real things work properly, cheaply, safely, reliably, and responsibly. The technique of doing all this is called "technology." The technology of road building is relatively simple, and does not involve much science or mathematics. The technology of satellite communi-cations is very sophisticated, and involves very newly developed scientific ideas (some discovered by engineers and some by scientists) and high-powered mathematics. Some engineers are attracted by the simplicity and effectiveness with which they can apply low technology, and others are attracted by the complexity of high technology. The technology of energy spans the spectrum from the relatively low (windmills on the farm) to high (magnetohydrodynamic power generation); so there is room for all kinds. Our objectives are to give you some background in energy science, some experience with energy engineering, and some feeling for energy technology.
To help augment your feeling for hardware and your understanding of energy technology, we suggest that you make arrangements to visit a variety of places nearby, preferably as you are studying something about that particular technology. Be sure to visit a large central power station. If you are on or near a university, visit their mechanical engineering laboratory, where you will probably find examples of engines, heat exchangers, turbines, compressors, boilers, etc. Or perhaps you can find a factory with a central heating and cooling plant, or a large air-conditioning system, where you can see things like cooling towers, pumps, etc. During each visit get a quantitative feel for the size of the facility, say in terms of the amount of energy or powered handled, try to learn the capital-equipment cost, and then calculate the cost per unit energy or power. Ask yourself what a larger unit would be like, what the impact of the facility is upon its community, and how such systems directly affect your life. You can do a lot on your own to gain more appreciation of energy technology; this book should be only the start.
Throughout the book we will raise questions that do not have unique answers, but for which a combination of quantitative analysis and value judgments provides the best basis for rational decisions. Try out your tools for quantitative analysis on these questions; try to identify the pitfalls in your analyses, and estimate the possible ranges of the quantitative results. And try to articulate the values that you feel are important to the questions and your own particular position on these values. Are your values shared by your society? By other cultures? How do the quantitative facts of life alter or influence your values and those of society? How could (or should) the values of your society be changed?
In this book we will concentrate on the technical aspects of energy. You must be the author of your values, but we will try to help you understand, clarify, and extend them. We hope that this experience will help you develop a style of critical thinking that will serve you in many other aspects of your life.
What is energy? A good portion of this book will be devoted to the job of developing an understanding of energy. As with all scientific concepts one develops good understanding only over a long period of time, with frequent exposures from many angles. Think back; when did you first hear about energy? Perhaps in the early elementary grades, if not before. As a young child, you may have read breakfast cereal-boxes, where the contribution of the cereal to your minimum daily demands for energy was proudly announced. You may have been scolded for wasting energy by leaving on lights unnecessarily; through such experiences you became aware of the fact that energy costs money. You no doubt saw commercials describing how a particular high-energy gasoline makes cars perform better, and you perhaps visited a large power station near a dam on a summer's vacation. If you grew up on a farm, you soon learned the notion that machinery could do a lot more work than you yourself could do by hand. If you studied biology, chemistry, or physics in school, you were again exposed to the concept of energy. The totality of your exposure to various aspects and kinds of energy puts you in a very good position really to understand the concept of energy. Imagine how frustrated you would feel if you began a study of energy without this personal background! Our first job is to tidy up your understanding of the concept of energy; you have made this easy for us simply by living to your present age in a world in which energy plays such an important role.
There are three important aspects of the concept of energy; probably you already know all these, but you may not have heard them articulated. They are:
- All matter and all things have energy.
- The energy of the whole is the sum of the energies of the parts.
- Energy is conserved.
The first recognizes that energy is a property of matter; the molecules in a certain chunk of matter, the electromagnetic waves in a certain field of radiation, or the cells in a certain living organism have energy. The second aspect tells us that the amount of energy in a complex system is the sum of the energies in its various parts. This may seem trivial, but matter has other properties (for example, temperature) which do not behave in this manner. The third states that the energy of a system that does not interact in any way with anything else is constant, in other words, that the universe always contains the same amount of energy (though perhaps the form of the energy might change).* Why are these three statements true? You might think that they are experimental observations, but in fact they are really scientific axioms. We can't really tell anybody what energy is without invoking these three ideas; they are part of the basic concept of energy. They are as fundamental as energy itself.
- * We shall use the term "conservation" of energy only to reflect this idea, which should not be confused with the popular idea that "energy resources" must be conserved for the benefit of later generations.
Often we want to analyze systems that interact strongly with their surroundings, and to do this we use energy bookkeeping.As any good bookkeeper knows, we first must define the system that we are dealing with and the energy flows to and from it very carefully. (If you don't distinguish carefully between deposits and withdrawals, and make them for the proper bank account, you are likely to end up bouncing checks!) Engineers usually express their choice by drawing dotted lines around the part of the world that they wish to consider as "the system." Figure 1.1 shows a schematic of a large central power station in the process of burning a fossil fuel for the purpose of producing electric power. Not all the fuel energy can be converted into electrical energy, and some rejected energy leaves through the boiler stacks and some more through the cooling-tower exhaust. One possible choice for the "system" (sometimes called "the control volume") that would be analyzed in the study of the energetics of this power station is shown by dotted lines in Fig. 1.1. Figure 1.2 (omitted) is a photograph of a real power station that uses ocean water for cooling rather than cooling towers. We have drawn a dotted line on this picture to show the equivalent system boundary. The three ideas discussed above tell us the following:
- There is some energy in this system stored in the mass within; there is also a flow of energy into the system with the fuel, air, and water and out of the system with the stack and cooling-tower exhausts. And of course there is an energy outflow associated with the electric current.
- Since energy is conserved, the energy that flows into the system during one hour must account for the energy that flows out during that period plus any increase in the energy stored within the system. This is the statement of correct energy bookkeeping.
- At any instant in time, the energy within the system is the sum of the energies of all the parts (when the plant is operating steadily, this energy would be the same at all instants in time, and this can lead to important simplifications in the energy analysis).
The second gives us the basis for writing down a simple equation describing the conservation of energy for this system, called the "energy balance,"
Here E stands for energy; E(in) represents the amount of energy that enters the system in the time period under consideration, E(out) the amount that leaves the system in this period, and δE(stored) the change in the amount of energy stored within the system. In the language of mathematics the symbol δ is used to denote "final minus initial" (which of course is the "change"), and so
To do our bookkeeping properly, the three terms in Eq. (1.1) must of course be evaluated for the same time interval and in consistent measuring units. And we must include all significant inflows and outflows of energy in the evaluation of E(in) and E(out). Thus, for Fig. 1.1, we might write
thereby neglecting other energy flows that we decide are not significant for this system (such as the energy input from the sun).
What could we do with all this? If we can develop numbers for each of the terms symbolized above, we could first examine the system to see if indeed we had an "energy balance." An imbalance would not indicate that the conservation of energy notion is invalid but would instead indicate either that our numbers or arithmetic were in error or that we had neglected some significant terms. If the plant is operating steadily, the amount of energy stored within the control volume should be the same at all times. This "steady state" condition would allow us to set
in which case the total energy input must be precisely balanced by the total energy output. Since the objective of the power plant is to convert the energy in the fuel to electrical energy, we could quantify the ability of the power plant to do this by defining an "overall plant efficiency," which normally is denoted by the greek symbol "eta" but here we'll use "h",
In a typical power plant perhaps 30% of the fuel energy would be converted to electrical energy (h = 0.3). We could then look at E(stack) and E(tower) to see where the rest of the energy is going. If we are clever, perhaps we could figure out some way to make use of this "waste energy." Or perhaps the values of the times would dictate that it is "best" simply to waste this energy.
The notion of an energy balance on a well-defined system, as illustrated above, is very important. It is the primary tool by which one derives quantitative information needed for the design, analysis, and decision making involved in energy systems. It is not very difficult; it is simply a matter of good bookkeeping (evaluating the various terms in the energy balance can be more tricky). You will learn how to do some simple energy problems yourself, and this will help you understand the harder ones.
Let's begin to acquaint ourselves with numerical values of energy. There are several important measures of energy, or energy units, in common use today. These include
- The calorie, which is roughly the amount of energy that must be added to 1 gram of water to increase its temperature by 1°C.*
- The Btu (British thermal unit), which is roughly the amount of energy that must be added to 1 pound of water to increase its temperature 1°F.**
- The joule, which is precisely the amount of energy that is used by a one-watt light bulb in one second.
- * °C stands for "degrees Celsius." You may know of it as "degrees centigrade," which is an older way of naming the same temperature interval.
- ** °F stands for "degrees Fahrenheit," as you probably know. An interval of 1 deg F corresponds to an interval of 5/9 °C.
One Btu corresponds to 252 calories; equivalences of these and other energy units are given in the Appendix, Table A.3.
You will want to develop some personal "feel" for the magnitudes of energy. One "calibration" point is that your daily intake of food energy is around 2 to 4 million (i.e. 2 to 4 x 106) calories,*** depending upon your particular size, shape, and activity. In terms of Btu, this is of the order of 8,000 to 16,000 Btu.
- ** 1,000 calories = 1 kcalorie (kilocalorie). Food values are usually stated in kcalories, often denoted simply as Calories (note the capital C that is often left out inadvertently by dieticians!).
Check the specification plate on your water heater at home; it probably says something around 10,000 Btu, which indicates the amount of energy that the heater is able to put onto the water in one hour. Then, you can calculate how many pounds of water your heater can warm up 100°F in this period.
Another helpful reference is the "toaster" calibration. A typical toaster requires about 1,000 joules of electrical energy per second of operation. It takes perhaps 20 seconds to make tasty toast, or about 20,000 joules. A joule is about 1/4 calorie (see Table A.3). So the amount of electrical energy required to cook two pieces of toast for your breakfast is about 5,000 calories, or about 0.2% of your body's energy requirement for the day.
If you are not familiar with the various energy units, do simple calculations like these as you study the subject, and relate the energy requirements or output of various devices to the "you" calibration and the "toaster" calibration; before long you will have a very good feel for the magnitude of energy numbers.
An important aspect of systems that use or produce energy is the rate at which this happens. A battery that could deliver 20 x 108 joules would be ideal for an electric urban automobile; but the battery would be useless unless this energy could be delivered in a few hours (nobody wants to take three days to go to the supermarket). The "power" of the device is the characteristic that describes the rate at which it supplies or uses energy; the power is the energy divided by the time required for the energy transfer. Denoting the power by P, the energy by E, and the time interval by t, the power is
For example, your calorific intake of 3,000 kcal in 1 day corresponds to an average of P = 3,000 kcal/24 hr = 125 kcal/hr, which corresponds to a steady diet of one cookie per hour. If you eat a 2,000 kcal dinner in 1 hr, your power input is 2,000 kcal/hr, 16 times the power input of the cookie nibbler.
The power used by a toaster is about 1,000 joules/sec. The unit combination joules/sec (joules per second; per means "divided by") is termed "watts" after a famous contributor to energy technology; 1,000 watts is termed a kilowatt ("kilo" = 1,000), abbreviated by Kw or kw. So, your toaster power is about 1 kw. Power units are often given in kw, which is why the "toaster calibration" is so useful. Another unit of power that is in common use is the horsepower, abbreviated hp or HP, after a famous horse. This is roughly the amount of power that a horse can deliver; it is also equivalent to about 3/4 kw. In other words, a horse working hard could deliver enough power to cook your breakfast toast. (Are you more or less powerful than a horse?)
Since electrical power is usually measured in kw, electrical energy is usually given in kwhr ("kilowatt hours"; power times time equals energy). Check the electric meter outside your residence; read the amount of energy (in kwhr) used to date, and then read it again later and compute the average electrical power that was used over this time period. Most appliances, lights, etc., have power requirements in kw or watts stamped on them; check these to see if you can account for the energy that the electric company says you used. By the way, they probably charge you about 2.5 cents per kwhr (check your electric bill). You might compute the cost of making toast, or of leaving a light burning all night, to further increase your personal feel for energy and power.
The horsepower unit is used for motors, pumps, and other devices that produce or use energy. Get a feel for horsepower; remember that a kilowatt is about 4/3 of a horsepower. Your kitchen refrigerator probably has about a 1-hp motor; your heart uses about 0.01 hp; your automobile engine may be rated at 300 hp but probably seldom actually produces more than 100 hp, even under drag-strip conditions. Large jet engines produce about 25,000 hp. The largest modern steam turbines in central power stations produce about a million horsepower.
Figure 1.3 gives some typical energy and power numbers for some interesting systems. Note that the range of interest is enormous, and hence a logarithmic scale has been used in each case. You will recall that multiplication is equivalent to adding logarithms; this means that a fixed percentage change is represented by a fixed interval on the logarithmic scale, irrespective of where on the scale the change is taken. Logarithmic scales such as those used here are very helpful in studying phenomena involving rapid growth, such as the national consumption of energy; if they are new to you, buy some "log-log" paper and some "semilog" paper at your local bookstore, and play with these until you feel comfortable plotting or reading such graphs. It takes only courage, not mathematical wizardry!
At this point it is important that you work seriously with the ideas described above to get a reasonable feeling for energy and power magnitudes. You may not be quite sure just what energy is, but we'll clarify this in the next chapter. The remainder of this chapter provides an overview of the patterns of energy and power utilization in our contemporary society. A reasonable feeling for energy and power magnitudes will help make this material meaningful to you. You may also want to read it again after you have learned more about energy technology.
All material (except for some code, external links and Optional Readings) © Jeffery A. Schneider, 2003