Energy is usually defined as the ability to do work. This is an anthropocentric and utilitarian perspective of energy; however, it is a useful definition for engineering where the aim of machines is to convert energy to work. As a more general description, we would say that energy is a fundamental entity whose availability and flow are required for all phenomena, natural or artificial.
An understanding of how energy is generated and measured is central to our decisions concerning the use and conservation of energy. Large-scale production of energy evolved over centuries but grew radically in the last 400 years and especially since the Industrial Revolution. A century of development and commercialization of electric power technology has ensured an easy supply, and continuous measurement.
Energy is derived in usable forms from numerous sources, such as flowing water, fossil fuels (e.g., coal and natural gas), uranium, and the sun. Electricity is a widely used form of energy. Any of these sources can be used to generate electricity. Liquid fuels such as gasoline and diesel derived from fossil fuels are a widely used source of energy. These fuels form the basis of our easy transportation. A complete understanding of the complexities of the energy systems within the natural environment requires knowledge of some basic physics and chemistry. This is discussed later in this unit in the sections under "Science Notes."
In Figure 1, each of the arrows shows transformation or transmission of energy -- that is, the energy changes form or is moved from one place to another. Plants and humans are the agents shown that store and/or transform the energy.
This natural energy system is part of a larger system that includes nutrients from the soil as input, other energy for cooking as input, etc. Figure 1 is drawn to show the parts of transformation of this initial solar energy up to its final dissipation and one storage system (fossil fuels). A complete concept map would show all the other factors. The numerous energy systems in nature include the food chain, the climate and ocean systems, and the cycles of various materials such as water, carbon, and nitrogen.
Most of the energy systems currently in use, both natural and man-made, originate in the Earth-Sun relationship. The fossil fuels we use today are stores of solar energy. Photosynthesis is an example of solar radiant energy transformed into stores of chemical energy that plants and animals (including humans) use to maintain themselves. The conversion of solar radiant energy through photosynthesis is a fundamental natural energy system. The food chain is an example of a natural, solar-based energy system that has sustained human life on Earth. Often we take for granted that energy will always be available for us to use. We fail to recognize the complexities of the energy systems that drive these environmental phenomena and sustain life on Earth. We are intricate parts of the system as end users, completing the dissipation of energy to forms that are so spread out that it is impossible to use that energy again.
(coal, oil, gas) result from a transformation of plant and animal material
over millions of years. The solar energy originally stored in the plant
or animal is eventually converted into energy stored in carbon and hydrogen
bonds of the fossil fuel. The fuels that took millions of years to make
are being used at an enormously rapid rate. Figure 2 is a representation
of the use of fossil fuels over time, including an estimation of how long
they might last.
Fossil fuels and fuels like uranium are "spent" once they are used to obtain energy. These are called non-renewable sources of energy. Although new plants can be planted that eventually turn to coal, the process takes millions of years and that is why coal and other fossil fuels are considered non-renewable. Solar and wind energy arrive or circulate air on the Earth everyday. These sources are called renewable.
Wood and trees used as fuel are called renewable, because they can be replanted. However, when we use them so that the rate of use far exceeds the rate of replenishment (trees take time to grow), referring to these sources as "renewable" can be a misnomer!
in each human activity has grown exponentially since the early days of
human civilization. For example, technological capabilities enable us
to travel more and process more food. Figure 3 shows the amount of energy
(in calories) we spend for each calorie of food we get. It shows that
technologies have mechanized and made large production systems of cultivation
and fishing. These systems involve large expenditures of energy, as seen
in Figure 3. The figure shows that for wet rice production in Asian countries,
it takes between 0.02 and 0.1 calories of energy to produce 1 calorie
worth of rice as food. Large-scale food production consumes enormous amounts
of energy. For example, it takes over 2 calories of energy input to produce
1 calorie worth of eggs in large-scale farms, and it takes 10-15 calories
of input for every calorie worth of beef produced in the U.S.. Note how
the intensity of energy consumption for U.S. food production has grown
almost ten-fold in the 20th century! Add to this the fact that for every
calorie of energy our body gets, we have to take in over 5 calories worth
Table 1 shows that as we become more industrialized, each human consumes more calories daily as well. This, along with population increase, has resulted in an enormous increase in the daily calories consumed by humans.
More information on human impacts on the food chain and on ecological economies can be found in the Ecological System.
History of the Energy System
the Beginning: Pre-Industrialization
Pre-Industrial society depended primarily on muscle power and biomass for their energy needs. Biomass consisted primarily of wood or peat and its energy delivery had a low efficiency. Amory Lovins, an expert on energy, states, "Most of the energy generated by wood or peat went up in the chimneys rather than into the room or cooking pot of pre-industrial societies."
Animal power in the form of horse mills, wind power in the form of windmills, and water power with the use of a water wheel were major energy sources harnessed until the 19th century; especially for "industrial uses." Wood and charcoal were the main fuels for cooking, heating, and other domestic uses, but coal and oil were available as well. "In the Middle East crude oils have been known for millennia from natural seepage and pools, but they were used only rarely as fuels, and more frequently as protective coatings."1 Coal has its origin in "the lithification of peats produced by accumulations of dead plant matter in wetlands. Difference in original vegetation and, more importantly, in magnitudes of durations of transforming temperatures and pressures, have produced a large variety of coals."2 As early as the 13th century, coal pits were mined and coal energy was used specifically for the forcing and smelting of metals. In the 1600's, England experienced an energy crisis due to a shortage of wood and began using coal as a substitute fuel source for domestic purposes. Even in the 1700's, wood was the major fuel source in colonial America.
Steam power was developed in the 1600's in conjunction with coal mining to help pump water out of the mines. It had been known since ancient times that heat could be used to produce steam, which could then do mechanical work. However, it was only in the late eighteenth century that commercially successful steam engines were invented. The first commercially successful steam engine was invented by Thomas Savery (1650-1715), an English military engineer. In 1712, this engine was refined by Thomas Newcomen (1663-1729), another Englishman. The Newcomen engine was widely used in Britain and Europe throughout the eighteenth century, but had very low energy efficiency.
A greatly improved steam engine was designed and built in 1763 by James Watt who was asked to repair a Newcomen engine. Watt built and then sold or rented his engines to mining companies, charging them for the "power" in the rate of work the engine produced. Today, the unit for power is called a Watt.
The sun was also studied as an energy source in the 18th century. In 1767, the first solar thermal collector was developed by the Swiss scientist Horace de Saussure. Solar thermal power was used in the American west as an energy source for cooking until oil and natural gas became a more reliable way to generate energy. For simple cooking solar energy was absorbed by black cast iron pots. Solar thermal collectors were also used in the form of hot boxes to cook food.
In 1839, Alexandre Becquerel discovered that an electric current could be generated when certain elements were exposed to light. The scientific explanation of this phenomenon by Albert Einstein, called photoelectricity (light-induced electricity), came much later in 1905. Photoelectricity is the basis of the photovoltaic cells, now used to convert light into electricity. Despite the century and a half since it discovery, photovoltaic means of generating electricity have not been developed with enough vigor for it to become a major source of electricity. This is because the material technology for photovoltaic panels developed slowly. As coal and other fossil fuels were easier to use, and available in plenty, not much effort has gone into photovoltaic research.
Until the early 1800's our understanding of the science of energy was not well developed. The theory at that time was the caloric theory, which said that heat is a substance called "caloric" that flowed from hotter to colder bodies. In the 1840's the English physicist James Prescott Joule did a long series of experiments that showed that heat is a form of energy. Joule found the relationship between a unit of mechanical energy and a unit of heat. This helped Joule finalize what chemists and natural philosophers had come to believe--that the total energy in the universe is constant, although energy is continuously changing forms.
The study and invention of the heat engine and steam power established and confirmed the Laws of Thermodynamics. From 1840-1880, Joule, Lord Kelvin, and James Clark Maxwell in England; Sadi Carnot and Rudolf Clausius in France; and Ludwig Boltzmann in Austria formulated a theory of heat engines, laying the foundations of Thermodynamics, literally the science of "motion from heat." (Thermo=heat and dynamics=motion).
In 1820, the advances in mechanical and materials engineering made the railroad the most efficient and fastest means of transportation. Coal and wood were used as the primary fuel source for the steam engine. The locomotive also changed society's perception of travel and transportation.
Wind energy was developed on a large scale in the United States as an energy source for farms and railroad stations, using tall windmills to pump water from underground wells. There were specific design developments that made these windmills more efficient, although they still generated relatively little power. The height of these windmills helped to ensure they caught the wind and a tailfin generally kept the fan facing the wind.
Another result of the Industrial Revolution was an energy distribution infrastructure built into cities that promoted domestic convenience. As early as 1816, natural gas was piped into cities for domestic uses such as cooking, home illumination, and street lighting. The steam engine was used to pump water into homes and sewage away from homes. The city was undergirded with networks that usually began with water pipes and gas lines and gradually expanded to include sewers, electrical conduits, and telephone lines.3
In 1859, when petroleum was drilled in Titusville, Pennsylvania, an apparently plentiful energy source began to replace coal. Oil was distilled into kerosene (referred to as coal oil) and used as a lamp oil. It replaced dwindling supplies of whale oil used for lamps. There were many reasons oil became a more desirable fuel source than coal: it was easy to obtain and transport; it emitted less particulate pollution than coal; it replaced scarce whale oil as a fuel for lamps; and coal had become an unreliable fuel source because of the labor issues surrounding the mining of coal. Miners were striking for safer work environments and more money, which affected the amount of coal available to the consumer.
But the most significant use of crude oil was as the liquid fuel for the internal combustion engine, designed in 1861 by Nikolaus August Otto. The internal combustion engine became one of the most influential inventions of the Industrial Revolution. Although this engine is low in efficiency, it could produce enough work to move a large metal vehicle far distances. The fuel of the internal combustion engine was also easier to use than, for example, shoveling coal into a furnace to power a locomotive. This was the beginning of the use of liquid fuel to advance transportation.
In 1879, Thomas Edison invented the incandescent light bulb -- a major step in the human use of storable energy leading eventually to large-scale electrification. Electricity is similar to a liquid fuel in that it can be transported easily (although not efficiently) from one place to another. One of Edison's goals was to make electricity affordable for all homes. Edison began with the distribution of electricity through a direct current (DC). This meant that electrons would flow one way through a wire to bring electricity to a home; however, a good portion of the energy was lost as the electrons moved through the wire. This loss of energy using direct current to move electricity meant that power plants had to be built close to the homes the plant serviced and was eventually considered impractical.
Nikola Tesla, an inventor employed by Edison, discovered that electrons would alternate or travel back and forth on a wire and travel longer distances with less energy loss. This was called alternating current (AC) and had an advantage because AC could be more easily generated. Edison had so much money invested in his DC power plants that he discredited Tesla's alternate current as dangerous -- thus beginning a "war of the currents." Tesla eventually joined forces with George Westinghouse and began developing power plants using alternating current (AC).
In the late 19th and early 20th centuries the steam turbine, using coal as a fuel, was developed as a cheap power source that generated electricity. In 1882, the first functional steam turbine was designed by Charles Parsons, an English engineer. He used the high pressure of steam to hit the blades of a rotor. The principle of the turbine was a major step toward today's production of electricity.
In 1893, Westinghouse demonstrated a "universal system" of generation and distribution at a Chicago exposition. The universal system meant that power or energy could be used in a variety of ways at many different voltages. Westinghouse, using Tesla's invention of the transformer and the electric motor, as well as steam turbines, transformed Niagara Falls into one of the first hydroelectric plants in the world
In 1910, Henry Ford opened the 60-acre Highland Park automotive plant with a moving assembly line. This was the beginning of an eventually enormous use for fossil fuels. Fossil fuels were used not only to propel the automobiles that were made at the plant, but also to generate electric power for the automotive plant.
Energy technologies developed rapidly during the 20th century. Although the current version for solar thermal collectors was designed in 1908, they were not developed well enough for mass distribution. In the 1920's, 30's, and 40's, there was large-scale construction and development of hydroelectric plants/dams to support increasing population in the Southwest.
In 1938, Otto Hahn and Fritz Strassman demonstrated nuclear fission and within four years (1942), Oak Ridge, Tennessee, was chosen as the site for the first functional nuclear reactor plant, and for the preparation of uranium and plutonium used to the create the atomic bomb at Los Alamos. The first nuclear chain reactor was demonstrated at the University of Chicago in December 1942. In July 1945, the testing of the first atomic bomb at Alamogordo, New Mexico, demonstrated the technology used to release nuclear energy on a large scale. In 1957, the first commercial nuclear power plant opened in Shippingport, Pennsylvania.
The first large scale use of photovoltaic (PV) solar energy in conjunction with satellite technology developed in the 1950's. The United States Vanguard I was the first PV-powered satellite.
By the early part of the 20th century, crude oil and its products had become an indispensable part of the industrial economy. James Young had patented a process in England in 1850 to distill oil from coal and shale. Oil refining is not just about gasoline. The distilled chemicals from crude oil have many purposes -- for example, petroleum is used for plastics manufacturing. Young's process of fractal distillation forms the basis of the world's oil refining industry.
Figure 4 shows the oil reserves that we know for sure as of 1987.
a large amount of oil occurs in many parts of the world, the largest
stores are located in the regions governed by the Arab countries. The
Oil and Petroleum Economic Cartel (OPEC) is the economic coalition of
these countries that control the flow of that oil. In the 1970's, OPEC
placed an embargo on their oil sales. This "energy crisis"
brought energy scarcity to the consciousness of all nations -- and especially
the U.S., with its higher dependence on imported oil. This crisis began
to generate interest in the exploration of renewable energy sources
for large-scale generation of electricity and other energy needs.
Human Energy Needs
Our energy consumption has led us to develop new energy sources and technologies. In a century, liquid fuels and electricity have improved our standard of living and provided us with more mobility than people in any other era. This section reviews our human energy needs, how we currently meet them, and what the future may have to offer.
is essential for all we do as individuals and as societies. Energy production,
use, and distribution also cause some of the most pressing environmental
problems. Figure 6 shows the overall picture of human energy needs,
the ways in which we meet our energy needs, and the impacts.
Although industrialized countries use the most energy at present, newly industrialized countries are increasing their rate of use. Figure 7 graphs the projected energy needs of industrialized countries, developing countries, and Eastern Europe and the former Soviet Union. Many environmental and economic issues arise from the escalating energy use all over the world. Understanding the science and technology driving the energy system enables us to better understand our relationship to the environment.
Energy Use and Sources - Data
The factors that affect energy use most are the level of industrialization, the climate of the region, and the population. These factors are not independent of each other. All of these factors influence energy choices, production, distribution, and usage. The average use of energy per person in different countries varies widely. Table 3 highlights world energy consumption for electricity generation by region and fuel source. It is evident in these tables that industrialized countries use more energy.
Information describing our energy use often can be evasive. Figure 8 shows the main heating fuels the U.S. uses for residential home heating. Electricity use for heating is on the rise; however, electricity is not a direct fuel source like the others in this figure. It is a form of energy. Different sources, such as coal and uranium, are used to generate electricity. Therefore, if electricity use is on the rise, the use of coal and uranium are also on the rise.
We are all affected by and pay for the life cycle of electricity. As end users of electricity, understanding the process of electricity generation gives a clearer picture of what you are paying for and helps in your decision making as a consumer.
as an Energy Sector
use for transportation is the least efficient use of fossil fuels. The
automobile loses far more energy than it uses. As shown later (in the
Energy Transformation section), for every 20 gallons put into a car
we only get about 2 gallons worth of actual work. The rest is dissipated
as heat. The other inherent inefficiency is in the fact that to move
one person, we have to spend energy moving over a ton of extra material
(the automobile itself).
Use of crude oil is escalating as developing countries emulate industrialized mobility. Figure 8 graphs the use of crude oil for transportation needs. This increase in oil consumption specifically for transportation not only impacts the environment, it will deplete the limited oil reserves in the earth's core. Increased consumption of oil for transportation could also affect a variety of industries (i.e. plastics) which in turn affects the economy.
A review of the science supporting our current energy systems can be found in the section "Science Notes."
The definition of energy as the ability to do work came from the 19th century as steam engines and other work-producing machines were developed. The first engines converted heat (thermal energy) into motion (dynamics). The science of heat engines developed by Lord Kelvin in England and Joule and Clausius in France founded the science of thermodynamics. They showed that two rules always held when energy was used to produce motion or work. These are called the two Laws of Thermodynamics. It was noticed that when engines performed work, heat was always produced in addition to the work, and that this represented wasted energy.
Two Laws of Thermodynamics
Conservation of energy was an idea proposed by numerous people. Julius Robert Mayer, a German physician, proposed in 1844 that energy was conserved by observing physical processes involving heat and respiration, but with no quantitative measurements. The first formal statement was in a paper by a young physicist, Hermann von Helmholtz, in 1847. (Helmholtz is known as one of the greatest physicists of the 19th century). Joule's experiments from 1843 and his paper in 1849 publicizing his most precise experiments stimulated gradual acceptance of the principle. However, it met with a lot of skepticism early on because it seemed speculative. Even as late as 1858, William Barton Rogers, the founder of Massachusetts Institute of Technology (MIT), wrote to his friend that the principle of conservation of energy was "mysticism"! This is an example of both the slow pace of science and of the fact that great discoveries are often a leap of speculation, although based on observations.
The First Law of Thermodynamics states that energy cannot be created or destroyed. Simply stated: energy can change forms; mass and energy may even change into each other (this happens only in special reactions within nuclei of atoms) but the total energy in the universe remains constant.
law says that you can convert energy into work, and into the simultaneous
production of heat. If the total energy is E to start with, and all
of it is spent doing an amount of work (W), with a production of heat
(Q), the First Law states:
More correctly, we are talking of the change of energy, work, and heat in a system. Using the symbol "" to denote the changes, the first law is written as:
All the descriptions above are equivalent.
As energy changes forms, the energy becomes more and more spread out and inaccessible to us. For example, the energy that is stored in a compact form within a gallon of gasoline in a car tank becomes transformed into work in moving the car, and dissipated into the energy of the surrounding air and road. As the molecules are heated they spread over a much larger area. This energy, now spread out all over, is not destroyed but has become dissipated and therefore unavailable for us to do more work with it. In this process, matter (such as the heated molecules in the air surrounding the car) has also become more disordered. These two facts combined are known as the Second Law of Thermodynamics. This law states that the unavailable energy in the universe increases, or equivalently, that the disorder (also called entropy) in the universe increases (as energy is used).
The second law then simply states that within each process of producing work, we are increasing the unavailable energy and the disorder in the universe. This means that even though the total energy in the universe is constant, we are decreasing the "quality" of the energy. We are decreasing the amount of available energy -- the energy that we can use to produce work.
We cannot harness all the energy coming out of a process of energy transformation. We define efficiency as:
Based on the First Law of Thermodynamics, we neither create nor destroy energy. Whenever we say that we are producing energy, what we really mean is that we are transforming energy from one form to another that is more usable. Energy that is the result of work usually manifests as change in position or as the motion of an object. Energy that is stored is called potential energy -- energy that has the 'potential' to do work. A second form of energy -- energy of motion -- is called kinetic (meaning "moving") energy. Both kinetic and potential energy can be transformed into work.
Later in this unit we present the physical basis of energy and work. Without going into the details that will come then, let us discuss two transformations of energy: a waterfall and a pendulum. Every gallon of water in the fall has potential energy at the top of the waterfall by virtue of its position. At the bottom of the waterfall, this gallon of water travels faster and has gained kinetic energy at the expense of potential energy. As a pendulum swings back and forth, the energy changes from potential energy at the top position to kinetic energy at the lowest position and potential energy again as it goes to the top. We will explain the deeper meanings of potential and kinetic energy later in the unit.
We begin by reviewing some fundamentals of physics and chemistry relevant to understanding the basic principles of energy transformation. In particular, we focus on concepts related to the fundamental aspects of energy: Matter, Force, and Energy and the Fundamental Forces of Nature. Then we describe the physics and chemistry of Measuring Energy, Work, and Power. Chemical reactions and energy release are then described to understand more clearly how the chemical combustion of fossil fuels produces the carbon dioxide and other products that cause environmental problems.
Science Notes: Measuring Matter, Force, and Energy
Matter, force, and energy are common terms that have scientific meanings close to the common use of the words. Matter is the stuff of which everything is composed, and force is something that is capable of changing the motion of an object. Energy is a property that tells us how much work we can get out of the object that possesses that energy. In keeping with the fact that science is based on measurements and observations, all of these entities are described in terms that we can observe and measure. Therefore, we first discuss these entities by examining and measuring their effects.
Matter and force are the two fundamental entities of which the universe is composed. All that exists can be classified in these terms. All environmental phenomena occur because of the interactions between matter and transformations of matter in space and time. As the arrangements between forces and masses change, the change is manifested in terms of energy. Table 4 gives the abbreviations for the physical qualities and their definition and units.
Matter is discussed in terms of the quantity of matter. This is called the mass. The unit we will use to measure mass is the kilogram (gram, milligram) in the MKS system that is globally adopted as the scientific system of units. For most engineering applications, and for common trade in the United States, the British system is used, in which the unit of mass is the pound.
Force is a more complicated concept. We experience or observe force through its action on matter (as a push or pull). When a force acts on matter, it changes the way that quantity of matter is moving (Newton's first law). So we measure force by its capacity to produce acceleration in a mass. Acceleration is a measure of change of motion, measured by how much speed (m/sec) changes in a second. So the units for acceleration are meter per second per second, or, m/sec2. (Newton's second law: F = ma). Thus the unit to measure force is a composite of the measure of mass, and of acceleration. The unit is called a newton (kg.m/sec2), written as NT or N. So a one-newton force can change the acceleration of a 1 kg mass by 1 m/sec2.
When a force (F) is applied to an object and moved through a distance (d, measured in meters, m) in the direction of the force, we define the work (W) done as: Work = Force * Distance moved in the direction of the force, or symbolically,
= F · d
When F is measured in newtons (NT, or N) and distance in meters, the resulting quantity of work is expressed in Joules. So, a force capable of producing an acceleration of 2.3 m/sec2 by acting on a mass of 3 kilograms is a 6.9 NT force. The weight of an object of mass m on Earth is the force due to Earth's gravitational pull on that object. The gravitational acceleration of the Earth is about 9.8 m/sec2. (This means that the gravitational force exerted by the Earth on a 1 kg object is 9.8 nt.). Thus the weight of a 5-kilogram object on Earth is 5 kg * 9.8 m/sec2, or 49.0 nt.
A 2.5 NT force moving something through a distance of 4 m (meters) in its direction does a work of 2.5 nt * 4 m, or 10 Joules. When a 5 kg object falls a distance of 4 m, the work is done by the Earth's gravitational force. As the gravitational force on 5kg is 49 NT, the work done by the Earth on the 5 kg object in pulling it down by 4 m is 49 nt * 5 kg = 245 nt-m = 245 Joules.
Energy: Work, Energy, Heat, and Power
Due to historical reasons, different measures of energy were developed and used in physics and chemistry. In the early times, physics dealt mainly with motion -- of bodies such as planets and stars, as well as smaller masses. Thus forces and motion were the focal points of early physics. Physics measured energy by means of the force required to change the state of motion. The units of physics dominated the emerging fields of engines as well, where forces were used to produce motion. How much energy could be produced every second was the question in designing engines. The amount of energy per unit of time is defined as power. Thus power has the units of Joules (energy) per second, also known as a watt. One watt is one Joule per second. We also have the Horsepower, which is the unit in the British system. It is understandable that with the horse as one of the important "animal engines," the early engines were compared to the power of a horse to move things.
Chemistry started as the study of changes in the nature of substances. Heat was a common method used to change substances. Temperature, or the feel of heat, was used to measure the amount of heat in a substance. Thus heat energy was measured by chemists in terms of the energy required to change the temperature of a common substance - water. Thus the unit of energy (heat) most used by chemists was the calorie, defined as the amount of heat required to change the temperature of one gram of water by one degree Celsius. Of course, there are also two different measures of temperature depending on whether you follow the British or the Metric system. We will confine ourselves to the Metric system, and hence to degrees celsius (or centigrade). Count Rumford (Benjamin Thompson) and James Joule, scientists in the eighteenth century, were among the earliest to show that mechanical energy and heat could be changed into each other, primarily by noting that when mechanical work is done, the friction produces heat.
Joule in fact determined that 4.18 Joules of mechanical work is equivalent to 1 calorie of heat. In practice, a calorie is a very small amount of heat, and so a kilocalorie, also written as Calorie (or kcal) is used. A kilocalorie is therefore 4,180 Joules. The units of energy, heat, and power are summarized in Table 5. Because of the different origins of the ways of measuring energy, and the numerous manifestations of energy, there are several units for measuring energy. Units also vary depending on the practices in different fields, and on the type of energy being measured. This can be confusing at times. The tables below summarize most of the units and contexts.
Work and energy transform into one another. They are measured in the same units. So a "Joule" is also a unit for measuring energy. As described in more detail later, all energy is either stored (potential energy) or is in the process of causing motion of an object (kinetic energy). Thus we can represent the energy work relationship in a continuous state of mutual transformation in a system, including the energy that is "lost" (has become unavailable).
Depending on the context, the different units above are used in physics or mechanical engineering:
For physics, mechanics, and engines, the work is typically stated in terms of moving something in a unit of time, e.g., energy per second, with power (watts) in terms of joules per second. For chemistry, the work is changing the nature of substances, therefore the units are in terms of the amount of heat needed to change the temperature of water (calorie).
Science Notes: Energy Accounting and Balance
Once we understand the various transformations of energy that are possible, an energy balance can be used to track energy through a system, and is a very useful tool for determining resource use and environmental impacts. The idea is to use the First and Second laws to determine how much energy is needed at each point in the system and in what form that energy is. The accounting system keeps track of energy in, energy out, and non-useful energy versus work done, and transformations within the system. An energy balance diagram is used. Non-useful work is what is often responsible for environmental problems.
Science Notes: Fundamental Forces of Nature
All forces in nature may be classified into four types. The gravitational force holds together the universe at large, plus the atmosphere, water, and us to the planet Earth. The electromagnetic force governs atomic level phenomena, binding electrons to atoms, and atoms to one another to form molecules and compounds. The strong nuclear force holds the nucleus together. The fourth force, the weak nuclear force, is responsible for certain types of nuclear reactions and has little bearing on energy sources today.
7 shows the four forces, the property on which each acts, and examples
of each force. Gravitation and electromagnetism are the two forces
with which we will be primarily concerned, as these are the two forces
that operate at the macroscopic level of environmental systems. These
also currently form the basis of our most prevalent sources for energy
technologies. The strong nuclear force is the strongest of the forces.
Nuclear fusion reactions on the surface of the sun are the result
of the nuclear strong force.
Every force that we experience belongs to one of these four categories, even when the connection is hard to see. The force of friction, for example, is an electromagnetic force. The force of the explosion of a chemical explosive is also electromagnetic in origin. The energy of an atomic bomb (more correctly, a nuclear bomb) is released as a result of the action of strong nuclear forces.
Energy changes occur when matter changes position or matter changes state in the presence of these forces. What we call energy "production" is really energy transformation -- that is, energy is converted from a potential form to a form available to us for use.
Each type of force acts on a specific property of an object. The specific property refers to the aspect of an object that is necessary for that object to be influenced by the force -- or "to feel" the force. For example, gravitation acts on the mass of an object. Strictly speaking, as an object fall towards the Earth, the Earth is falling towards the object. Because of the mass difference, the total effect of the Earth on the object is much more than that of the object on the Earth. However, the gravitational force on each kg of Earth is the same as on each kg in the object.
In the case of the electromagnetic force, the object must have an electric charge. But if that object has no net electric charge, then an outside source of electromagnetic force cannot exert a force on that object. The space in which a force is felt is called the "field" of the force. Thus all objects on Earth are in Earth's gravitational field, the planets and moons are in the sun's gravitational field, as well as in their mutual gravitational field.
and Kinetic Energy
Potential energy, therefore, is the energy associated with different positions in the force field. The water at the top of a waterfall has higher gravitational potential energy than at the bottom because of the different positions in the gravitational field. Consider two points (A and B) in the Earth's gravitational field (g) where B is h meters higher than A. Then a mass (m) has a potential energy mgh higher than its potential energy at A. At a point 2h above A, the mass has a potential energy of 2mgh. So height is a measure of the potential energy.
Thus, an analogy with water and gravitational potential energy gives us a way to represent energy levels showing the potential energy state of a system in terms of horizontal lines. Thus we could say that the 100 m point above the lowest level in a waterfall has 980 Joules gravitational potential energy per kg of water above the lowest point.
m * g * h = E (Energy)
1 kg * 9.8 m/sec2 * 56 m = 549 J
These formulas also demonstrate that potential energy is a representation of the position of a system in a field of force. The 1 kg of water in our example has higher potential energy when it is further away from the center of force (center of the Earth). At point A, the water is more "bound" (to the Earth) than at point C. We will use this idea later to draw the analogous levels to represent chemical potential energy.
Science Notes: Energy and Chemical Stability
systems left to themselves move towards states of lower potential
energy. For example, water flows down a hill or a ball rolls down
a hill, if free to do so. States of lower potential energy are more
stable. As a rule, the lower the potential energy of a system, the
more stable it is. As a result, left to themselves, systems attempt
to reach the configuration with the lowest energy possible under a
given set of constraints. To change the state of a system from lower
to higher potential energy, one must therefore supply energy to the
As more stable states have lower potential energy, we can get energy for use by moving a system to a lower potential energy. This is the basis for all energy transformation technologies.
These lower energy configurations of elements happen when the elements get an electronic configuration similar to the nearest inert gas. Electronic configurations of inert (noble) gases are the most stable in a given period (horizontal segment) of the Periodic Table. In fact, that is why these specific elements do not "need" to react with anything and are, therefore, chemically inert (or "noble"). These elements have no "need" to combine because their electron shells are completely filled with electrons. The noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Chemical activities of the elements are simplest to predict when they are close to the inert gases in the periodic table. Look at the first few periods of the Periodic Table in Figure 13. The activity can be described as a result of elements "wanting" to complete their shell. Although this is an anthropomorphic description, it is a helpful analogy.
Lithium and sodium would tend to "lose" one electron to become more stable (more like the closest inert element). They can do this, for example, by combining with elements that can gain stability by adding one electron to their shell -- elements such as F and Cl. These pairs therefore favor ionic bonds, in which the electron is actually transferred.
When similar atoms such as H or O or N come together they can gain stability by sharing electrons in a covalent bond. Shared electrons spend time with either of the H atoms in H2 for equal amounts of time, for instance. In a case of a covalent bond such as those in H2O, however, the electrons spend more time on the average in the neighborhood of the oxygen.
The water molecule, which has an angle of 105° between the two H-O bonds, is therefore a polar molecule, being more negative at the oxygen end of the molecule, because the negatively charged electrons spend more time near the oxygen atom.
Nitrogen, hydrogen and oxygen atoms go to more stable configurations by forming the diatomic gases, N2, H2, and O2 respectively rather than remain in the atomic form: N, H, and O. When hydrogen atoms are produced in any reaction, pairs of these hydrogen atoms form covalent bonds with each other so that each has the helium (nearest inert gas) configuration at least a fraction of the time. Hydrogen has one electron and needs a total of two to be like He. So two hydrogen atoms share a pair of electrons, each belonging to one of the original atoms, thus forming H2. Schematically, it could be written as HxxH (x representing an electron). This schema is represented by H-H where the single line represents a bond consisting of two shared electrons.
Again, the same can be said for oxygen and nitrogen. Oxygen has an atomic number of 8, and has four electrons in the outermost shell. It needs two more to be like neon (nearest inert gas).
Following this logic, we can figure out the most frequent bond configurations for carbon, nitrogen, oxygen, phosphorus, hydrogen, with 4, 3, 2, 3 and 1 bonds respectively, as schematically in Figure 14.
Energy transformations using chemical sources consist of changing the mutual configurations of these compounds accompanied by the release of energy, which we can then use for something. Chemical bonds "contain" energy that may be released when the bonds are made.
Let's look at the example of nitrogen (atomic number = 7). Nitrogen, being 3 electrons short of its nearest inert gas (neon, atomic no. = 10), tries to bond so as to share 3 of it's electrons with other atoms bonding with it, in order to get a complete shell of 10 whenever possible. Thus it may share electrons with another N atom (forming N2), or with hydrogen (forming NH3).
Similarly, carbon can bond with four hydrogens to form CH4 (a gas called methane), or with two oxygen atoms to form CO2 as depicted above. Note that we always have four bonds coming out of carbon, one out of hydrogen, and one out of oxygen. Look at their position in the periodic table to see why this is so. Atoms in the middle of the Periodic Table and their bonding become more complicated, and we will not need to deal with them here.
Carbon is the basis for all our life forms. It is a very versatile atom because of its capability to form four bonds. Depending on the amount of hydrogen available to bond, and the temperature and pressure conditions, carbon can form a plethora of compounds with hydrogen alone. One such family is the hydrocarbons, important in our context because they are the basis of fossil fuels.
Note how some of these compounds have double and triple bonds between carbons. This happens when carbon and hydrogen combine under circumstances in which there is not enough hydrogen to satisfy all four bonds of each carbon. For example, if there is plenty of hydrogen to combine with carbon, we get CH4 or C2H6 (Ethane), with all single bonds. With less hydrogen we get C2H4 or C2H2 (less hydrogen for the same number of C atoms). C2H4 has a double bond between the carbons, and C2H2 has a triple bond. Compounds with double and triple bonds are called unsaturated, while single bond compounds like C2H6 are said to be saturated. Unsaturated compounds are more reactive than saturated compounds because not all the C atoms are bonded to four other atoms. Hydrocarbons are not the only compounds that can be unsaturated. Carbon monoxide is a good example of an unsaturated compound, "looking" for another oxygen atom to form CO2, a more saturated compound. When carbon (in coal or wood, for example) burns in an environment with insufficient oxygen, it forms CO which is deadly when breathed in. This is the reason to ensure plenty of access to fresh air when we have a fireplace or running car engine.
Note that many representations are two-dimensional, and that in actuality, the electrons forming the bonds are distributed in three dimensions. In a compound like CH4, the carbon is in the middle of a tetrahedron with the 4 H atoms at the vectors.
Similarly there can also be C3H8 (propane), C4H10 (butane), C5H12 (pentane) and so on. When a formula is written as CH4 just showing the proportion of atoms, it is called an empirical formula. When the bonds are shown as in Figure 15, it is called a structural formula. A single empirical formula may represent different compounds because the structures may be different for the same number of atoms combining.
Try drawing propane, butane, and pentane. Note that there are always four bonds coming from carbon. The linear structures are called aliphatic hydrocarbons. In addition to the linear hydrocarbon molecules, hydrocarbons may also be formed into ring structures. The ring structure possesses the property that enables us to smell these compounds! So they are called aromatic hydrocarbons. The simplest aromatic hydrocarbon is C6H6, benzene. The structure of benzene was long a puzzle in chemistry, with chemists wondering what the structural formula for C6H6 could be. They knew the empirical formula was C6H6. It is said that the great organic chemist Kekulé, who had been wondering about this, dreamed one night of a snake swallowing its tail and was inspired to draw the ring structure! Note the alternating single and double bonds, a clever way of ensuring four bonds from each carbon shell.
The versatility of carbon in forming bonds, ring structures and various configurations is the basis of life on our planet. The chemistry of carbon compounds is therefore called organic chemistry. More complicated carbons compounds are described in the Ecological System and Materials System. For now, let us look at some additional aromatic and aliphatic compounds, and note some aspects that are relevant to energy storage and release.
Aliphatic hydrocarbons are the basis of fossil fuels. All saturated hydrocarbons react with oxygen at high temperatures to form carbon dioxide and water, and give off energy. This oxidation reaction is the basis of the internal combustion engine. Gasoline normally contains hydrocarbons from C6 to C18, a mixture of over 100 compounds! An example reaction of the combustion of a hydrocarbon is:
C7H16 + 11O2 7CO2 + 8H2O + energy
"Burning" (or a combustion reaction) consists of combining with oxygen at high temperatures. The combustion reaction of acetylene (C2H2) with oxygen gives off such a large amount of energy that it is used as a welder's torch.
Ring compounds do not play as large a role in energy production but often occur as byproducts or waste products. These polyaromatic hydrocarbons (PAH's) pose a serious pollution problem.
Ring compounds, based on the benzene ring, are so common in biochemistry that we just draw to represent C6H6. Adding one more carbon and two hydrogens to the benzene ring gives us C7H8 which is methyl benzene or toluene (at right).
Ring compounds can get very complicated. Several organic compounds playing an important role in our physiology are shown in the Ecological System.
Energy Release and Bond Energies
Table 8 gives the energies for bonds we will commonly encounter. The table defines these energies in units of kcal/mole. A mole is an abbreviation for "gram-molecular weight" of a substance.
Let us see what the energy values in Table 8 mean. The bond energy of H-H is 104 kcal/mole. This means that when hydrogen atoms combine to form molecular hydrogen H2, represented by the reaction H + H H2, for every mole (2g) of H2 formed, 104 kcal of energy are released. Conversely it takes 104 kcal to break apart a mole (6 x 1023) of hydrogen molecules. From this we can draw a simple chemical energy level diagram for the above reactions, analogous to Figure 12 of gravitational potential energy.
One mole of an H2 (2g) molecule has 104 kcal total potential energy less than 2 g of H atoms. This is why when H atoms are formed in a reaction, and these atoms are the only atoms available, they combine to form H2 (roll down the potential energy "hill" towards a more stable state). In forming the H-H bond, 104 kcal of energy are released for every 2 g (6.02 x 1023 molecules) of hydrogen gas (H2) formed. Similarly oxygen, and nitrogen exist as O2 and N2 rather than in the atomic form as O and N. So whenever we say hydrogen, oxygen, or nitrogen gas, we mean H2, O2, N2. For H, O, N we specifically say atomic hydrogen, atomic oxygen, and atomic nitrogen.
Science Notes: Chemical Formations
of Ammonia (NH3) as Example
The reaction is N2 + 3H2 2NH3
This equation says that one mole of N2 requires three moles of H2 for a complete reaction, and this would then yield two moles of NH3. Note that we can also say that one molecule of N2 reacts with three molecules of H2 to yield two molecules of ammonia (NH3). But the table gives energies in units of kcal/mole, which is why it is easier to work with moles. The energy involved in the reaction involving just one or two molecules of ammonia is too small.
following table contains the variety of ways in which you can write
the reaction that forms ammonia. All the descriptions below are equivalent.
The atomic masses are rounded off values from the Periodic Table.
What are the energies of the reactions? In order to get the molecular structure of ammonia, we have to break one NN bond and three H - H bonds. This requires that we supply energy to break the bonds. From Table 8, we get the bond energies:
To form each molecule of NH3, we break one bond of N2 and three of H2, and then three N - H bonds form to make NH3. To calculate energy released (or absorbed) in the reaction, we have to calculate the energy needed to break the bonds of N2 and H2, and the energy released when the atoms rearrange to form NH3.
More energy is released than absorbed in the formation of ammonia, so this reaction is exothermic. We can also say that 10.5 kcal are released when 17g of ammonia are formed. The energy level diagram here is more complex than that for the H + H = H2 reaction because of the steps involved.
For this reaction, we had to put in some energy to "activate" the reaction, which was the energy required to break the N2 and H2 bonds. N2 and H2 brought together with no addition of energy would not spontaneously react. This is analogous to our striking a match to start the burning of coal. The energy required to start the reaction is called activation energy. Then, left to themselves, the N and H form bonds to release 568 calories.
What is a corresponding example with the gravitational force?
of Water (H2O) as Example
Write equation and balance:
2H2 + O2 2H2O
H-H + O=O 2
Bonds: break old (spend energy), recombine to form new bonds (release energy).
Break 2 H-H, break O=O and in recombination, 4 H-O bonds are made and release energy. Using the bond energies of H-H 104 kcal/mole, O-O 119 kcal/mole, and H-O 111kcal/mole from Table 8, we can see how much energy is released when bonds are broken and formed.
Energy released is greater than energy required to break bonds so there is a net energy release. The reaction is exothermic. The amount of energy released is 444 kcal minus 327 kcal which equals 117 kcal per mole of oxygen burnt or:
The energy released per unit mass may also be calculated in Joules. When we talk about released energy as the output of a power plant, Joules are the customary units; therefore it is often necessary to convert the energy from kcal to Joules.
Kilocalories are units of heat. 4,190 kilojoules make 1 kilocalorie. So, multiply kilocalories by 4,190 to convert to kilojoules. This is necessary to convert heat (or chemical energy) units to work (or mechanical energy) units. Recall that these two systems of units evolved separately. Heat units were used by chemists and chemical engineers and mechanical units by physicists and mechanical engineers.
325 x 4,190 = 1,361,750 kJ of energy is released per kg water formed. This reaction can theoretically do 1,361,750 kJ of work. We usually get less useful work because of Second Law of Thermodynamics--some energy is lost as heat.
Science Notes: Chemistry of Fossil Fuels
and Energy Release
+ O2 CO2
We can show that these reactions release energy. The basic reaction of the burning of C is the basis of our largest energy source -- fossil fuels of various types, including coal, natural gas, and oil. Recall the energy in these bonds came originally form the solar energy captured by plants and then "processed" for millions of year (transformed over millions of years) under the pressure in the Earth.
Most of the carbon in coal is bound so that there is only one C-C bond for every C atom. Thus, for calculating the energy release of C + O2 CO2 in the case of coal, one assumes only the breaking of one C-C bond.
Following the previous example,
from Coal Combustion
Insufficient oxygen supply during combustion -- as for example, burning coal (or wood) in a closed environment such as a room without adequate ventilation or a fireplace without a proper chimney -- produces carbon monoxide. As the oxygen is depleted, the reaction C + CO2 2CO becomes possible. Carbon monoxide is a colorless, odorless, and very poisonous gas. When breathing in CO, the CO takes the place of O2 in the hemoglobin molecules in the blood supply in the lungs, causing asphyxiation.
Energy Use, Efficiency, and the Future
Garrett Hardin who originated the idea of the Tragedy of the Commons, summarizes the two laws of Thermodynamics in terms of human significance as:
can't win, you are sure to lose;
Whether we can get more "out of the game" is the central question of energy production. The examples above show what a small fraction of the input potential energy is actually used as output work. Designing for energy efficiency means in order to get a higher ratio of output, the input needs to become a central concern of intelligence design and practice.
Most of the energy planning is done by looking at the supply side. We examine how we can increase the supply of the resource in question, rather than by asking how the demand side (all our uses of energy) can be managed. Energy availability and use are good indicators of the standard of living in our technological world. In the U.S. the "average consumption per capita" is 55 barrels of oil. In the poorer countries, the consumption is 6 barrels per year. Figure 20 shows the projections of world energy supplies from 1970-2020. The increased coal supply is based on mining coal that is harder (and hence more costly) to extract.
Demand-side management instead of or in addition to supply side management would mean a focus on increasing efficiency of use and considerations of how to reduce the demand. The CAFE (Corporate Average Fuel Economy) standards legislated in the U.S. in 1980's required a certain level of fuel efficiency of U.S. automobiles. By demanding that the corporations figure out an overall fuel rating for all their fleets, the decisions on design and distribution of big and small cars in the total fleet were left to the industry, as long as the total corporate fuel economy goals were met.
There are also some efforts to recapture some of the "waste energy" from the processes of energy generation. Co-generation described in Figure 21 is an example of industries working together to see how exchanges of energy and materials could minimize waste.
Example of "Waste Power" Use
Four companies (a power plant, a refinery, a gypsum facility for producing wall board, and a pharmaceutical plant) effect the exchange shown in Figure 22. The "waste" products from the power station (including heat in the form of warm water) are used to warm the greenhouse and other facilities. Such a co-generation system provides an industrial ecosystem with a much higher efficiency for overall energy use than if any of the organizations had organized independently for their material and energy needs. The ten years of planning required shows that such processes take time to explore the possibilities, develop the relationships, then plan and execute.
Energy Sources, Technologies, and Impacts
Historical, geographical, and political contexts have led to the adoption of different fuels and related technologies to produce energy. As described in the history section, we have progressed from using above ground, easily accessible sources of energy, such as wood and direct solar energy, to fuels such as coal and oil that require large infrastructures and energy to mine and process before extracting energy from them. Table 9 at the end of this section outlines different energy sources and the information relevant to the environmental impact of these sources.
As described before, over 95% of the world's energy requirement is currently met by fossil fuels -- coal, oil, and natural gas. In various technologies, they release energy by the process of combustion. Major byproducts are carbon dioxide and various residuals such as fly ash. The environmental problems relating to fossil fuel use are described in detail in the Atmospheric System. Environmental pollution, especially air, global climate change, and resource depletion are the greatest drawbacks of heavy fossil fuel use. Another problem (particularly for the U.S.) is dependence on foreign resources. Development of oil in the Atlantic has been a response to the U.S. need for fuel independence. Alaskan oil exploration involves destruction of pristine land and unique natural habitats.
Coal burning, while a simple technology, has numerous side effects in addition to carbon dioxide emission noted above. Coal occurs in combination with sulfur in many places. High sulfur coal, when burned, produces sulfur dioxide, which is the source of acid rain. Countries like the U.S. regulate the amount of sulfur that can be in coal used for power production; therefore high sulfur coal must be cleaned before it can be burned. Coal burning also produces large amounts of particulates in the form of fly or bottom ash, which must be disposed of or recycled.
Currently, coal power plants with the encouragement of government and to the liking of environmental groups, are developing ways to burn coal and reduce the large amount of coal byproducts. Coal-gasification is one such technology. Coal is pulverized, mixed with water and then combined with gases such as nitrogen and oxygen. The gaseous mixture is then heated and a synthetic gas is produced as the particulates or ash falls to the bottom of the burner. As the gas cools, sulfur particulates are separated and used to make sulfuric acid. This is an example of getting more than one product from a fuel source. The gas is then used to produce steam that spins turbines and generates electricity.
The energy in the bonds within the nucleus may be released through nuclear fission or nuclear fusion reactions. The technology used to produce nuclear power is based on nuclear fission. The fuel for fission reactions are heavy nuclei, particularly uranium, thorium, and plutonium (a material that no longer occurs in nature, but of which the U.S. and states of the former Soviet Union have enormous supplies because of the bomb programs). The U.S. led the world in nuclear power production, producing about 728 billion kilowatt-hours. France ranked second, with 375 billion kilowatt-hours, and Japan was third, at about 309 billion kilowatt-hours produced.8 In 1999, the nuclear share of total electricity generation for France was 75%, for Japan was 33%, and for the U.S. was 20%.9 Nuclear energy is often considered the desirable alternative to coal, because it does not release carbon dioxide. The materials involved in nuclear power are, however, heavily radioactive varying from uranium, the starting material, to the various byproducts during all phases of the energy production cycle, and the last byproduct, generally termed "radioactive waste." These byproducts are "radioactive," that is, they emit particles of radiation and high-energy electromagnetic radiation, such as gamma rays. This quality makes the materials dangerous. People and other parts of nature exposed to this radiation can suffer serious long-term damage. Many of the radioactive materials are also very long-lived, continuing to emit radiation for hundreds of years or more. Nuclear fission power technologies have been designed with numerous safeguards and extreme caution as to be "safe." However, if the global economy were to depend predominantly on nuclear power, radioactive material transport over air, land, and water could pose a very large exposure risk. Radioactive waste disposal is also a challenging problem, as it has to be kept isolated for thousands to millions of years!
Nuclear fusion is the reaction responsible for the production of energy in the sun. Hydrogen is the main fuel for energy source. In fact, the sun is a huge nuclear fusion reactor. But the extreme high temperature and pressure needed for fusion to take place has been a formidable obstacle to designing fusion systems on any usable scale. Nuclear fusion would not produce many radioactive wastes like fission but will produce radioactive tritium (an isotope of hydrogen) for which we would have to design safeguards.
Hydropower, wind, direct solar radiation, and geothermal power are all renewable resources. Of these, hydropower is the best developed. Starting with water wheels that converted the kinetic energy of running water into various kinds of motion, to vast projects like the Hoover Dam, the technology for conversion of hydropower to electricity has long been explored. While it produces no byproducts, hydroelectric power requires waterfalls or dams with a large volume of flow. In the case of dams built for hydropower, the devastation of land and distinctive ecological niches can be large. The conflict between development of cheap hydroelectricity, preservation of habitat, and economic interests are encapsulated in the example of the endangerment of salmon in the Pacific Northwest, due to extensive dam building on the Columbia River. Hydropower can also come from hot water springs, where the kinetic energy of water comes from the heat at the Earth's core.
Sun and Energy
On average, one square meter on the side of the Earth facing the sun receives 1400 W (Joules per second). In a 24-hour period the total amount of energy reaching the upper atmosphere is 14.4 million calories. One-third of it is reflected back into space by cloud cover, and the rest, traveling through the atmosphere, powers the wind and water cycle, and drives the Earth's climate. The total sunshine entering our atmosphere every year is equivalent to 500,000 billion barrels of oil or 800,000 billion metric tons of coal! On a bright sunny day in the northern latitudes, when the sun is at the highest point, about 1000 Watts/ m2 reaches the ground. On cloudy days, it can be as low as 200 Watts/m2.
The main problem with solar energy for high levels of use comes from the fact that it is so diffuse and spread out, and has to be collected over large areas. In effect, this is what the foliage of plants does, storing some of the energy through photosynthesis. It is important to note that only a small fraction of the solar spectrum -- in the violet and some in the red region -- is used for photosynthesis. Sunlight has a large quantity of energy in the green and yellow regions, most of which is reflected by the leaves. However, it is the capture of energy through photosynthesis, combined with elements like carbon, oxygen, and hydrogen from the Earth and its atmosphere, which results in biomass immediately. This biomass eventually results in a favorite fuel of today (coal) after millions of years of "processing" by the Earth. Oil and natural gas are similarly produced from organisms buried for millions of years under rock foundations. Ancient humans used the gentle, spread out solar energy and biomass for drying, cooking, and heating. Today's needs demand much larger quantities concentrated in space and time. This tendency promotes rapid depletion of the solar "capital" invested into coal formation over millions of years.
One of the less thoughtful energy uses in our convenience-dominated society is the use of "high-quality" and concentrated energy even when "low-quality" spread-out energy would suffice. A good example is our use of fossil fuel energy driven clothes dryers, even in the summer when clothes could just dry on a clothesline from direct solar energy. In the ideal use of energy, we would distinguish between the needs requiring high or low "quality" energy. Using energy at the appropriate level and from renewable resources is what is referred to by Amory Lovins as a "soft energy path." One soft technology philosophy argues that we adapt our life styles to suit the energy available to us.
Figure 23 is derived from the proceedings of UNERG, the United Nations Conference on New and Renewable Sources of Energy held in Nairobi, Kenya, in 1981. The figure shows the different grades of energy derived from direct solar energy. Passive collectors are static and collect heat energy that falls on them. Active collection involves mechanisms for storing and/or following the direction of the sun's rays to receive the maximum energy. Many ancient civilizations, including the Egyptians, Pueblo and Anastasi Indians, and Greeks, built their houses to take maximum advantage of the sun's apparent movement in the sky. However, as more technological energy systems developed, building with the sun's position in mind became a lost art, especially in industrialized countries.
Photon collection refers to collecting the energy through natural or artificial chemicals -- in the form of biomass (firewood, animal dung, and waste materials) or in water or chemicals such as certain salts, which hold the heat.
To convert solar radiation into electricity, we use photovoltaic cells. Photovoltaic cells are based on the phenomenon of photoelectricity -- that light can release electrons from certain materials. Using these materials, light can be converted directly into electricity. This phenomenon was discovered in the late 1800's by George May in Ireland. Rudolf Hertz in Germany produced the first photoelectric cell soon thereafter using the element selenium and Albert Einstein explained the physics of photoelectricity in 1905. However, the first practical solar cells were only developed in the 1950's by the Bell Telephone Company. The early version of the cell cost thousands of dollars per watt of electricity yielded. The first large scale testing occurred in space on the NASA satellite Vanguard in 1958.
The conversion efficiency (amount of electrical energy output per input of light energy) of the largest photoelectric cells is still below 20%. This means we have to collect sunlight over a large area for any useful application. For example, if the conversion efficiency is 15%, we need a 6.5 square feet of photovoltaic material to power a 100-watt light bulb.
Solar technologies have made the largest inroads in space applications. Collection of solar energy by satellites for Earth's applications have been long considered. The basic idea is that if the collection were in a geosynchronous orbit around the Earth (35,890 km or about 22,500 miles above the equator), we could capture the energy before so much of it was absorbed by clouds and the atmosphere. Various technical difficulties have essentially halted this SPS (Solar Power Satellite) project.
If we followed a philosophy of ecologically friendly design, the best use of solar would be in the passive or active collection, rather than the conversion to electricity. However, as a society, we have chosen to use electricity as the form of energy for almost all applications and a large source of our environmental problems -- pollution, resource depletion, habitat loss -- lies in this societal choice.
Wind is derived from solar energy moving large masses of air. The two basic phenomena that are responsible for wind patterns are a large global circulation and local effects. Cool polar air is drawn towards the tropics to replace lighter, warmer air that rises and moves towards the poles. This creates areas of high and low pressure and circulation patterns are set up differently in the northern and southern hemispheres because of the Earth's rotation. This sets up the global patterns, such as the trade winds. Locally, the circulation depends upon whether the air is over land or water. Air over oceans and large bodies of water is cooler than that over land, and cooler air is drawn toward the land as the warmed air rises. Together these two patterns produce movements of enormous complexity.
The wind represents kinetic energy of air arising from the thermal energy of sunlight. A large part of this energy is lost in various functional forces, and only a small portion can be captured by windmills. High tech windmill designs have been developed by various aircraft companies, because of their expertise in wind dynamics.
In summary, the sun it still our most valuable source, powering most of our energy sources. Our survival may depend upon a wise and judicious use of the numerous, versatile sources the sun provides.
Cycle of Electricity Generation
Electrical energy is the combined potential and kinetic energy of electrons in materials. Materials that have electrons that are mobile, rather than being confined to orbits around the atomic nucleus, can conduct electricity. Metals are prime examples of conductors. A discovery by Michael Faraday of England in 1831 is the cornerstone of our large-scale electricity generation. Faraday discovered that when a conductor moves in a magnetic field, an electric current is produced in the conductor. This Faraday's Law, and the fact that we can make large magnets is the basis of an electric generator. If we can use an energy source to move a conducting coil of wire that is placed in a magnetic field, the current produced in the wire can then be transported to deliver electric energy. Figures 24.A-D demonstrate the sequence of electric power production and distribution from four sources: running water, coal, nuclear fission, and wind.
Figure 24.A-D: Electricity Generation Methods
A power plant consists essentially of a huge generator -- a gigantic coil of wire capable of rotating in the space between the poles of an enormous horseshoe magnet. The motion of the coil is caused by a shaft connecting it to a turbine, whose rotation spins the coil. In all power plants then, the energy obtained from the sources has to cause the turbines to turn, which then rotates the coil. This energy is delivered directly to the turbines by the falling water in a hydroelectric power plant, and by the wind in a wind farm. In the case of coal and nuclear fission, the primary energy is used to transform water into steam or high-pressure water, which then drives the turbines. Figures 24.A-D illustrate the steps prior to this, and show the similarity of the final steps of electricity distribution for all sources.
The current from the coil is then carried to the final place of use through transmission and distribution lines. Some of the energy is lost along the wires. To minimize this loss, the electricity is transmitted at very high voltages (thousands of volts) along transmission lines and the voltage "stepped down" near the location of use using transformers. Electricity is then carried over smaller distribution lines to homes and businesses for use. The huge steel towers typical of transmission lines are a familiar sight, as are the distribution lines -- the smaller wires near buildings attached to "telephone poles." Transformers are the ceramic structures on local distribution line poles.
of Energy Production and Use
Fossil fuels cause some of the largest impacts. In order for a typical 500-Megawatt plant to produce about 158 Terawatt-hours (tera = 1012) of electricity per year, it takes 1.5 million tons of coal and 0.15 million tons of limestone. It produces emissions to the air of 1 million tons of carbon as carbon dioxide, plus 10000 tons of ash and 193000 tons of scrubber sludge -- both of which contain large quantities of sulfur. (check numbers) Global climate change, resulting from atmosphere increases in CO2, is described in detail in the Atmospheric System. Even seemingly slight temperature changes can cause changes in weather patterns, climate, melting of polar ice caps, and sea-level rise.
Gasoline combustion releases pollutants that, under certain conditions, give rise to photochemical smog and high levels of atmospheric ozone. Impacts from oil drilling include destruction of ecosystems. The many impacts of extensive fossil fuel use is discussed in detail in the Atmospheric System.
From the mining and processing of the fuels to the production stage, nuclear power requires the handling of radioactive material. The potential for accidental release of these materials and exposure to people, and the problem of long-term disposal of radioactive wastes, are the main environmental concerns of nuclear power.
Hydroelectric power causes disturbances in ecosystems from dams and large land use. A striking example of the loss of biodiversity is the rapidly declining populations in the remaining species of salmon in the Pacific Northwest. (See exercise: Salmon Management in the Pacific Northwest.)The Three Gorges Dam project currently underway in China requires the displacement of one million people, in addition to the devastation of land and ecosystems. But the People's Republic of China has made rapid industrialization a national priority, and this requires an enormous development of power production systems.
Alternative energy sources, such a wind and solar energy, also have large land use implications.
Table 9: Energy Sources and related information.
 Smil, Vaclav. Energies: An Illustrated Guide to the Biosphere and Civilization. The MIT Press: Cambridge, MA, 1999.
 Nye, David E. Consuming Power: A Social History of American Energies. The MIT Press: Cambridge, MA, 1999.
 Nye, p. 94..
There is an anecdote that Joule did this experiment for the first time by noting the temperature difference between the bottom and top of a beautiful waterfall in Switzerland while he was on his honeymoon there!
 Hardin, Garret. Filters Against Folly: How to Survive Despite Economists, Ecologists, and the Merely Eloquent. Viking Press, 1985. (p. 173)
 Energy Information Administration, International Energy Annual 1999, DOE/EIA-0219(99) (Washington, DC, January 2001.)
 Energy Information Administration, International Energy Outlook 2001, DOE/EIA-0484(2001) (Washington, DC, March 2001.)