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In this module we begin our study of nonrenewable energy sources by looking at patterns of energy use throughout the world and in the United States. We will see how evaluating energy efficiency can help us determine the best application for different energy sources. Finally, because electricity accounts for such a large percentage of our overall energy use, we will examine the ways in which electricity is generated.

Learning Objectives

After reading this module, you should be able to

Fossil fuels are fuels derived from biological material that became fossilized millions of years ago. Fuels from this source provide most of the energy used in both developed and developing countries. The vast majority of the fossil fuels we use—coal, oil, and natural gas—come from deposits of organic matter that were formed 50 million to 350 million years ago. As we saw in Chapter 3 (see FIGURE 7.2 on page 83), when organisms die, decomposers break down most of the dead biomass aerobically, and it quickly reenters the food web. However, in an anaerobic environment—for example in places such as swamps, river deltas, and the ocean floor—a large amount of detritus may build up quickly. Under these conditions, decomposers cannot break down all of the detritus. As this material is buried under succeeding layers of sediment and exposed to heat and pressure, the organic compounds within it are chemically transformed into high-energy solid, liquid, and gaseous components that are easily combusted. Because fossil fuel cannot be replenished once it is used up, it is known as a nonrenewable energy resource. Nuclear fuel, derived from radioactive materials that give off energy, is another major source of nonrenewable energy on which we depend. The supplies of these energy types are finite.

Every country in the world uses energy at different rates and relies on different energy resources. Factors that determine the rate at which energy is used include the resources that are available and affordable. In the past few decades, people have also begun to consider environmental impacts in some energy-use decisions. In this section we will look at patterns of energy use worldwide and in the United States.

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To talk about quantities of energy used, it is helpful to use specific measures. Recall from Chapter 2 that the basic unit of energy is the joule (J). One gigajoule (GJ) is 1 billion (1 × 10⁹) joules, or about as much energy as is contained in 30 L (8 gallons) of gasoline. One exajoule (EJ) is 1 billion (1 × 10⁹) gigajoules. In some figures, we also present the quad, a unit of energy used only by the U.S. government to report energy consumption. The quad is 1 quadrillion, or 1 × 10¹⁵, British thermal units, or Btu. One quad is equal to 1.055 EJ.

Worldwide Patterns of Energy Use

As FIGURE 34.1 shows, in 2011 total world energy consumption was approximately 550 EJ per year. This number amounts to roughly 75 GJ per person per year. Oil, coal, and natural gas were the three largest energy sources. Peat, a precursor to coal, is sometimes combined with coal for reporting purposes in certain countries, mostly in the developing world.

Energy use is not evenly distributed throughout the world. FIGURE 34.2 shows that energy consumption in the United States was 325 GJ per person per year, almost 5 times greater than the world average. In fact, although only 20 percent of the world’s population lives in developed countries, those people use 70 percent of the world’s energy each year. Note that of the countries shown in FIGURE 34.2, China has the greatest total energy consumption, whereas Canada has the greatest per capita energy consumption. At 0.12 EJ per year, Tanzania has the lowest annual energy consumption of the countries shown; annual per capita energy consumption in Tanzania is less than 3 GJ per person per year.

There are a variety of reasons for the patterns we see in FIGURE 34.2. In developed countries and in urban areas of some developing countries, individuals are likely to use fossil fuels such as coal, oil, and natural gas—either directly or indirectly through the use of electricity that is generated by burning those fuels. However, people living in rural areas of developing countries primarily still use such fuels as wood, charcoal, or animal waste. These differences lead us to distinguish between commercial and subsistence energy sources. Commercial energy sources are those that are bought and sold, such as coal, oil, and natural gas, although sometimes wood, charcoal, and animal waste are also sold commercially. Subsistence energy sources are those gathered by individuals for their own immediate needs and include straw, sticks, and animal dung. There is much greater use of subsistence energy sources in the developing world, especially in rural areas.

Changes in energy demand generally reflect the level of industrialization in a country or region. As energy demand increases, societies change the types of fuels they use. Today, we see the same patterns of changing energy use in developing countries that have been observed historically in the United States. For example, as more people own automobiles, demand for gasoline and diesel fuel increases. As industries develop and factories are built, demand for electricity and nuclear fuel increases. Although worldwide energy use varies considerably, the United States is a particularly large energy consumer.

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Patterns of Energy Use in the United States

FIGURE 34.3 shows the history of energy use in the United States. Wood was the predominant energy source until about 1875, when coal came into wider use. Starting in the early 1900s, oil and natural gas joined coal as the primary sources of energy. By 1950, electricity generated by nuclear energy became part of the mix, and hydroelectricity became more prominent. The 1970s saw a decline of oil and a resurgence of coal. These changes were the result of political, economic, and environmental factors that will continue to shape energy use into the future. Today, the three resources that supply the majority of the energy used in the United States—in order of importance—are oil, natural gas, and coal. The recent increase in the consumption of natural gas evident in FIGURE 34.3 is a result of its increase in availability and decrease in price due to hydraulic fracturing (fracking).

Energy use in the United States today is the result of the inputs and outputs of an enormous system. The boundaries of the system are political and technological as well as physical. For example, oil inputs enter the U.S. energy system from both domestic production and imports from other countries. Hydroelectric energy comes from water that flows within the physical boundaries of the country, as well as from neighboring Canada, but it is not an energy input until we move it into a technological system, such as a hydroelectric dam. One major output from the system is work—the end use of the energy, such as in transportation, residential, commercial, and industrial. The other major output is waste: heat, CO₂, and other pollutants that are released as energy is converted and entropy increases.

As you can see in FIGURE 34.4, U.S. energy consumption in 2012 was approximately 100 EJ (1.0 × 10¹⁸ J) per year. The energy mix of U.S. consumption is 82 percent fossil fuel, 8 percent nuclear fuel, and 10 percent renewable energy resources. The United States produces 85 percent of the energy it needs, while the remainder—roughly 15 percent—comes from other countries, primarily in the form of petroleum imports. The percentage of energy imports has been decreasing as domestic production of natural gas and oil increases; these trends are expected to continue. Industry uses the most energy, followed by the transportation sector.

Energy use varies regionally and seasonally. In the midwestern and southeastern states, coal is the primary fuel burned for electricity generation. The western and northeastern states generate electricity using a mix of nuclear fuels, natural gas, and hydroelectric dams. Highly populated areas tend to use less coal, which creates more air pollution than any other fuel. Regional use of fuel varies according to the climate and the season. Northern areas consume more oil and natural gas during the winter months to meet the demand for heating while southern areas consume more electricity in the summer months to meet the demand for air conditioning.

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Expanded domestic production of natural gas and oil has led to a rapidly changing energy portfolio in the United States and elsewhere. However, the type of energy used for a particular application is a function of many factors, including its characteristics. Factors to be considered include the ease with which the fuel can be transported and the amount of energy a given mass of the fuel contains.

The best form of energy to use depends on the particular purpose for which it is needed. For example, for transportation, we usually prefer gasoline or diesel fuel—liquid energy sources that are relatively compact, meaning that they have a high energy-to-mass ratio. Imagine running your car on coal or firewood: To travel the same distance as a conventional car on one tank of gasoline, you would have to carry around a much larger volume of material. Gasoline, the current fuel of choice for personal transportation, gets you a lot farther on a much smaller volume.

Energy-to-mass ratio is not the only consideration for fuel choice, however. A wood or coal fire starts relatively slowly and if used in an automobile, would not allow it to accelerate quickly. Gasoline and diesel are also ideal fuels for vehicles because they can provide energy quickly and can be shut off quickly. Unfortunately, compared with other energy sources, such as natural gas or hydroelectricity, gasoline produces large amounts of air pollution per joule of energy released. In addition, unlike coal or wood, gasoline requires a good deal of refining—chemical processing—to produce. So there are many factors to consider when determining the most suitable energy source for a particular application.

Quantifying Energy Efficiency

Although all conventional nonrenewable energy sources have environmental impacts, an understanding of energy efficiency can help us make better energy use decisions. Energy efficiency refers to the efficiency of the process we use to obtain the fuel and the efficiency of the process that converts it into the work that is needed.

In Chapter 2 we discussed energy efficiency as well as energy quality, a measure of the ease with which stored energy can be converted into useful work. The second law of thermodynamics dictates that when energy is transformed, its ability to do work diminishes because some energy is lost during each conversion. In addition to these losses, there is an expenditure of energy involved in obtaining almost every fuel that we use.

FIGURE 34.5 outlines the process of energy use from extraction of a resource to electricity generation and disposal of waste products from the power plant. The red arrows indicate that there are many opportunities for energy loss, each of which reduces energy efficiency. You may recall that the efficiency of converting coal into electricity is approximately 35 percent (see FIGURE 5.6, on page 48). In other words, about two-thirds of the energy that enters a coal-burning electricity generation plant ends up as waste heat or other undesired outputs. If we included the energy used to extract the coal as well as the energy used to build the coal extraction machinery, to construct the power plant, and to remove and dispose of the waste material from the power plant, the efficiency of the process would be even lower. These other energy inputs will be discussed in greater detail in Chapter 16.

Every energy source, from coal to oil to wind, requires an expenditure of energy to obtain. The most direct way to account for the energy required to produce a fuel, or energy source, is by calculating the energy return on energy investment (EROEI), the amount of energy we get out of an energy source for every unit of energy expended on its production. EROEI is calculated as follows:

For example, in order to obtain 100 J of coal from a surface coal mine, 5 J of energy is expended. Therefore,

As you might expect, a larger value for EROEI suggests a more efficient and more desirable process. “Science Applied 6: Should Corn Become Fuel?” following Chapter 13 calculates the EROEI for ethanol, a fuel made from corn.

Finding the Right Energy Source for the Job

When deciding between two energy sources for a given job, it is essential to consider the overall system efficiency. Sometimes the trade offs are not immediately apparent. The home hot water heater is an excellent illustration of this principle.

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Electric hot water heaters are often described as being highly efficient. Even though it is very difficult to convert an energy supply entirely to its intended purpose, converting electricity into hot water in a water heater comes very close. That’s because heat, the waste product that usually makes an energy conversion system less efficient, is actually the intended product of the conversion.

If the conversion from electricity to heat occurs inside the tank of water, which is usually the case with electric hot water heaters, very little energy is lost, and we can say the efficiency is 99 percent. By contrast, a typical natural gas water heater, which transfers energy to water with a flame below the tank and vents waste heat and by-products of combustion to the outside, has an efficiency of about 80 percent. The overall efficiency is actually lower, though, because we need to include the energy expended to extract, process, and deliver natural gas to the home. However, if a coal-fired power plant is the source of the electricity that fuels the electric water heater, we have to consider that conversion of fossil fuel into electricity is only about 35 percent efficient. This means that even though an electric water heater has a higher direct efficiency than a natural gas water heater, the overall efficiency of the electric water heating system is lower—35 percent for the electric water heater compared with something close to 80 percent for the gas water heater. There may be many situations in which it is a better choice to heat water with electricity rather than with natural gas, but from an environmental perspective, it is important to look at the overall system efficiency when considering the pros and cons of an energy choice.

Efficiency and Transportation

Because nearly 30 percent of energy use in the United States is for transportation, energy efficiency in transportation is particularly important. The movement of people and goods occurs primarily by means of vehicles that are fueled by petroleum products, such as gasoline and diesel fuel, and by electricity. These vehicles contribute to air pollution and greenhouse gas emissions. However, some modes of transportation are more efficient than others.

As you might expect, public transportation—train or bus travel—is much more efficient than traveling by car, especially when there is only one person in the car. And public ground transportation is usually more efficient than air travel. TABLE 34.1 shows the efficiencies of different modes of transportation. Note that the energy values report only energy consumed, in megajoules (MJ, 10⁶ J), per passenger-kilometer traveled and do not include the embodied energy used to build the different vehicles. Trains and motorcycles are the most energy-efficient modes of transportation shown. If a car contained four passengers, we would divide the value in TABLE 34.1 for a lone driver by four, and thus the car would then be the most efficient means of transportation. However, cars traveling with four riders are relatively rare in the United States; single-occupant vehicles are the most common means of transportation. “Do the Math: Efficiency of Travel” shows you how to calculate the efficiencies of different modes of transportation.

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Transportation efficiency calculations do not take into account convenience, comfort, or style. Many people in the developed world are quite particular about how they get from place to place and want the independence of a personal vehicle. They also tend to have strong feelings about what type of personal vehicle is most desirable. In the United States, light trucks—a category that usually includes sport-utility vehicles (SUVs), minivans, and pickup trucks—account for roughly one-half of total automobile sales, while hybrid electric vehicles account for roughly 3 percent of total sales.

Light trucks are comparatively heavy vehicles and so generally have mileage ratings of less than 8.5 km per liter, or 20 miles per gallon (mpg). Because they are exempt from certain vehicle emission standards, they emit more of certain air pollutants per liter of fuel combusted than passenger cars. Smaller cars with standard internal combustion engines can travel up to 19 km per liter (45 mpg) on the highway. Hybrid passenger cars, which use a gasoline engine, electric motors, and special braking systems, obtain closer to 21 km per liter (50 mpg). Electric cars and plug-in hybrid electric cars, which are becoming more common in the United States, obtain even better fuel efficiency.

Despite the availability of fuel-efficient vehicles, many people drive vehicles that yield relatively low fuel efficiencies. As FIGURE 34.6 shows, the overall fuel efficiency of the U.S. personal vehicle fleet declined from 1985 through 2005 as people chose light trucks and SUVs over cars. Only in the last 8 years have vehicle choice changed and vehicle efficiency slowly increased. Recently, legislation was passed to increase the average fuel efficiency of new cars and light trucks sold each year so as to deliver a combined fleet average of 15 km per liter (35 mpg) by 2016.

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Energy efficiency is an important consideration when making fuel and technology choices, but it is not the only factor we must consider. Determining the best fuel for the job is not always easy, and it involves trade offs among convenience, ease of use, safety, cost, and pollution.

Because electricity can be generated from many different sources, including fossil fuels, wind, water, and the Sun, it is a form of energy in its own category. Coal, oil, and natural gas are primary sources of energy. Electricity is a secondary source of energy, meaning that we obtain it from the conversion of a primary source. As a secondary source, electricity is an energy carrier—something that can move and deliver energy in a convenient, usable form to end users.

Approximately 40 percent of the energy consumed in the United States is used to generate electricity. But because of conversion losses during the electricity generation process, of that 40 percent, only 13 percent of this energy is available for end uses. In this section we will look at some of the basic concepts and issues related to generating electricity from fossil fuels. Chapter 13 discusses electricity generation from renewable energy sources.

The Process of Electricity Generation

Electricity is produced by conversion of primary sources of energy such as coal, natural gas, or wind. Electricity is clean at the point of use; no pollutants are emitted in your home when you use a light bulb or computer. When electricity is produced by combustion of fossil fuels, however, pollutants are released at the location of its production. And, as we have seen, the transfer of energy from a fuel to electricity is only about 35 percent efficient. Therefore, although electricity is highly convenient, from the standpoint of efficiency of the overall system and the total amount of pollution released, it is more desirable to transfer heat directly to a home with wood or oil combustion, for example, than via electricity generated from the same materials. The energy source that entails the fewest conversions from its original form to the end use is likely to be the most efficient.

Many types of fossil fuels, as well as nuclear fuels, can be used to generate electricity. Regardless of which fuel is used, all thermal power plants work in the same basic way—they convert the potential energy of a fuel into electricity.

FIGURE 34.7 illustrates the major features of a typical coal-burning power plant. Fuel—in this case, coal—is delivered to a boiler, where it is burned. The burning fuel transfers energy to water, which becomes steam. The kinetic energy contained within the steam is transferred to the blades of a turbine, a device with blades that can be turned by water, steam, or wind. As the energy in the steam turns the turbine, the shaft in the center of the turbine turns the generator, which generates electricity. The electricity that is generated is then transported along a network of interconnected transmission lines known as the electrical grid, which connects power plants together and links them with end users of electricity. Once the electricity is on the grid, it is distributed to homes, businesses, factories, and other consumers of electricity, where it may be converted into heat energy for cooking, kinetic energy in motors, or radiant energy in lights or used to operate electronic and electrical devices. After the steam passes through the turbine, it is condensed back into water. Sometimes the water is cooled in a cooling tower or discharged into a nearby body of water. As we saw in Chapter 9, once-through use of water for thermal electricity generation is responsible for about half the water consumption in the United States.

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Efficiency of Electricity Generation

Whereas a typical coal-burning power plant has an efficiency of about 35 percent, newer coal-burning power plants may have slightly higher efficiencies. Power plants using other fossil fuels can be even more efficient. An improvement in gas combustion technology has led to the combined cycle natural gas–fired power plant, which uses both exhaust gases and steam turbines to generate electricity. Natural gas is combusted, and the combustion products turn a gas turbine. In addition, the waste heat from this process boils water, which turns a conventional steam turbine. For this reason, a combined cycle plant can achieve efficiencies of up to 60 percent.

Capacity

A typical power plant in the United States might have a capacity—its maximum electrical output—of 500 megawatts (MW).This means that when the plant is operating, it generates 500 MW of electricity. If the plant operated for one day, it would generate 500 MW × 24 hours = 12,000 megawatt-hours (MWh). Most home electricity is measured in kilowatt-hours (kWh). So the typical power plant we’ve just described would generate 12,000,000 kWh in a day. If it operated for 365 days per year, it would generate 365 times that daily amount.

Most power plants, however, do not operate every day of the year. They must be shut down for periods of time to allow for maintenance, refueling, or repairs. Therefore, it is useful to measure the amount of time a plant actually operates in a year. This number—the fraction of time a plant is operating in a year—is known as its capacity factor. Most thermal power plants have capacity factors of 0.9 or greater. As we will see in Chapter 13, power plants using some forms of renewable energy, such as wind, may have a capacity factor of only about 0.25. “Do the Math: Calculating Energy Supply” shows you how to calculate the amount of energy a power plant can supply.

When it is time to start up a power plant, both nuclear and coal-fired plants may take a number of hours, or even a full day, to come up to full generating capacity. Because of the time it takes for them to become operational, electric companies tend to keep nuclear and coal-fired plants running at all times. As demand for electricity changes during the day or week, plants that are more easily powered up, such as those that use natural gas, oil, water, or wood, are used.

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Cogeneration

The use of a fuel to generate electricity and produce heat is known as cogeneration, also called combined heat and power. Cogeneration is a method employed by certain users of steam for obtaining greater efficiencies. If steam used for industrial purposes or to heat buildings is diverted to turn a turbine first, the user will achieve greater overall efficiency than by generating heat and electricity separately. Cogeneration efficiencies can be as high as 90 percent, whereas steam heating alone might be 75 percent efficient, and electricity generation alone might be 35 percent efficient.

There are over 17,000 power plants in the United States. In 2012, they generated approximately 3.7 billion MWh. FIGURE 34.8 shows the fuels that were used to generate this electricity. As we can see, coal-fired power plants are the backbone of electricity generation in the United States, responsible for 40 percent of all electricity produced. Natural gas, at 28 percent, and nuclear energy, at 20 percent, account for most of the remainder of the generating capacity. Together, these three sources, plus a very small amount of oil, account for 89 percent of electricity generation in the United States. Water and other renewable energy resources such as wind and solar energy account for the remainder of the electricity generated in the country. As we discussed earlier, expanded domestic production of natural gas has increased the fraction of electricity generated from natural gas while the fraction of electricity generated from coal has decreased. In 2005, coal accounted for 50 percent of electricity generation.