PEH:The 21st Century Energy Mix
Petroleum Engineering Handbook
Larry W. Lake, Editor-in-Chief
Volume I – General Engineering
John R. Fanchi, Editor
Copyright 2007, Society of Petroleum Engineers
Chapter 18 – The 21st Century Energy Mix
Energy demand is expected to grow during the next century as more countries seek a better quality of life for their citizens. Increasing trends in population and consumption, price volatility, supply instability, and environmental concerns are changing the energy mix and energy strategies in the 21st century. The energy mix is the set of energy sources that are used to meet energy demand. Energy demand will be met by a global energy mix that is transitioning from a mix dominated by fossil fuels to a more balanced energy portfolio.
The emerging energy mix will rely on clean energy. Clean energy refers to energy that has little or no detrimental impact on the environment. The goal is sustainable development—the integration of social and environmental concerns into development activities that optimize economic profitability and value creation as the world undergoes the transition from nonrenewable fossil fuels to renewable fuels and a sustainable, secure energy infrastructure.
This chapter presents an overview of energy sources available for use in the 21st century. Following the overview, a prediction of the contribution of hydrocarbon oil and gas to the 21st century energy mix is presented. This prediction is only one possible scenario selected from the literature for analysis. The scenario assumes a gradual transition from the dominance of fossil fuels in the current energy mix to a more balanced energy portfolio. The projected energy portfolio for the 21st century is presented, and the implications for society and the emergence of an energy industry are discussed.
The literature contains several sources that present a description of the energy sources that are available or are expected to be available during the 21st century. Today’s energy options include fossil fuels, nuclear energy, solar energy, renewable fuels, and alternative sources.
Fossil fuels are the dominant energy source in the modern global economy, but environmental concerns are prompting changes to an energy supply that is clean. Natural gas is a source of relatively clean energy. Oil and gas fields are considered conventional sources of natural gas. Two unconventional sources of natural gas are coalbed methane and gas hydrates.
Coalbeds are an abundant source of methane. Miners historically have known the presence of methane gas in coal as a safety hazard, but it is now being viewed as a source of natural gas. Coalbed methane exists as a sorbed monomolecular layer on the internal surface of the coal. Its composition is predominantly methane but can also include other constituents, such as ethane, carbon dioxide, nitrogen, and hydrogen. The gas, which is bound in the micropore structure of the coalbed, can diffuse into the natural fracture network when a pressure gradient exists between the matrix and the fracture network. The fracture network in coalbeds consists of microfractures called cleats. Gas subsequently flows through the microfractures to the production well. (See the chapter on gas properties and correlations in this section of the Handbook for more information about methane.)
Gas hydrates are chemical complexes that form when one type of molecule completely encloses another type of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water molecules form a cage-like structure in which mobile molecules of gas are absorbed or bound. Although gas hydrates occur through¬out the world, difficulties in cost-effective production have hampered development of the resource. Gas hydrates generally are considered troublesome for oil- and gasfield operations because they can reduce the flow capacity of wells and pipelines, but their potential commercial value as a clean energy resource is changing the industry perception. Gas hydrates have the potential to be a significant gas resource because of the relatively large volume of gas contained in the gas-hydrate complex. (See the chapters on phase behavior of water systems and water hydration in this section of the Handbook for more information about gas hydrates.)
Currently, nuclear fission provides nuclear energy. Nuclear fission is the process in which a large, unstable nucleus decays into two smaller fragments. Fission depends on a finite supply of fissionable material. Nuclear fusion is the combination, or fusing, of two small nuclei into a single larger nucleus. Many scientists expect nuclear energy to be provided by nuclear fusion sometime during the 21st century. The sun supplies energy through fusion reactions. Attempts to harness and commercialize fusion energy have been unsuccessful so far because of the technical difficulties involved in igniting, containing, and controlling a fusion reaction. Fusion energy is expected to contribute significantly to the energy mix by the end of the 21st century, even though a prototype commercial-scale nuclear reactor is not expected to exist until 2015 at the earliest. Both fission and fusion reactions release large amounts of energy, including significant volumes of waste heat that must be dissipated and controlled. The decay products of the fission process can be highly radioactive for a long period of time, while the byproducts of fusion are relatively safe. One of the fission products, the plutonium isotope with atomic number 239, is of special concern because it is a radioactive material with a half-life of 24,000 years, and it can be used for creating nuclear weapons. Half-life is the time it takes half of the nuclei in the radioactive sample to decay. Fusion byproducts, in contrast, include the stable, naturally occurring isotope of helium with atomic number 4 and short-lived neutrons.
Solar energy is available in three forms: passive, active, and electric. Active and passive solar energy generally are used for space conditioning, such as heating and cooling. Active solar energy technologies are typically mechanical devices, such as solar hot-water heaters, that collect and distribute solar energy. Passive solar heating integrates building design with environmental factors enabling the capture of solar energy. A simple example is south-facing windows in a house. Solar electric devices, such as photovoltaic cells, convert sunlight into electricity. Groups of photovoltaic cells can provide electricity in quantities ranging from a few milliwatts to several megawatts and can power devices ranging from calculators to power plants. To get an idea of the scale of these values, a large color TV requires approximately 1 kilowatt of power, while a power plant for a modern city requires approximately 3 gigawatts.
Renewable fuels range from hydroelectric and wind to synfuels and biomass. The kinetic energy of wind and flowing water are indirect forms of solar energy and are considered renewable. Wind turbines harness wind energy, and hydroelectric energy is generated by the flow of water through a turbine. Both convert the mechanical energy of a rotating blade into electrical energy in a generator.
The oceans are another solar-powered source of energy. Waves and tides can be used to drive electric generators. Temperature gradients in the ocean exist between warm surface water and cooler water below the surface. If the temperature gradient is large enough, it can be used to generate power with ocean thermal-energy-conversion power plants. Similarly, temperature gradients and steam generated by geothermal sources can drive electric generators as a source of energy.
Biomass refers to wood and other plant or animal matter that can be burned directly or can be converted into fuel. Historically, wood has been a source of fuel. Technologies now exist to convert plants, garbage, and animal dung into natural gas. Methanol, or wood alcohol, is a volatile fuel that has been used in racecars for years. Ethanol, which can be produced from sugarcane, can be blended with gasoline to form a blended fuel (gasohol) and used in conventional automobile engines or used as the sole fuel source for modified engines. Synthetic fuels (synfuels) are fossil fuel substitutes created by chemical reactions with such basic resources as coal or biomass. Synfuels are used as substitutes for convention¬al fossil fuels such as natural gas and oil.
There are several ways to convert biomass into synfuels. Oils produced by plants such as rapeseed (canola), sunflowers, and soybeans can be extracted and refined into a synthetic diesel fuel that can be burned in diesel engines. Thermal pyrolysis and a series of catalytic reactions can convert the hydrocarbons in wood and municipal wastes into a synthetic gasoline.
One difficulty with the exploitation of biomass fuels is the potential impact on the fertility of the region. For example, excessive use of dung and crop residues for fuel, instead of fertilizer, can deprive the soil of nutrients needed for future crops.
Synthetic liquid hydrocarbon fuels can be produced from natural gas by a gas-to-liquids (GTL) conversion process that consists of the following three major steps:
- Natural gas is partially oxidized with air to produce synthetic gas (syngas).
- The synthetic gas is reacted in a Fischer-Tropsch (F-T) reactor to polymerize it into liquid hydrocarbons of various carbon-chain lengths.
- The heavy fraction of the F-T products is separated and cracked back to transportation fuels in a hydrocracking reactor.
The F-T process produces a hydrocarbon mixture with a range of molecular weight components by reacting hydrogen and carbon monoxide in the presence of a catalyst. The primary product of the GTL process is a low-sulfur, low-aromatic, high-cetane diesel fuel.
Alternative sources of energy include hydrogen fuel cells and cogeneration. Hydrogen can be used as fuel for a modified internal combustion engine or in a fuel cell. Fuel cells are electrochemical devices that directly convert hydrogen or hydrogen-rich fuels into electricity through a chemical process. Fuel cells do not need recharging or replacing and can produce electricity as long as they are supplied with fuel. Hydrogen and oxygen are fuel-cell fuels that can be produced by the electrolysis of water. Ausubel has suggested that the potential of nuclear energy will be realized when nuclear energy can be used as a source of electricity and high-temperature heat for splitting water into its constituent parts.
The environmental acceptability of hydrogen fuel cells depends on how the hydrogen is produced. If a renewable energy source such as solar energy is used to generate electricity for electrolysis, vehicles powered by hydrogen fuel cells would be relatively clean. Hydrogen combustion emits water vapor, but it also emits NOx compounds. Nitrogen dioxide (x=2) contributes to photochemical smog and can increase the severity of respiratory illnesses. The shipping and storage of hydrogen are important unresolved issues that hinder the widespread acceptance and implementation of hydrogen fuel-cell technology.
Cogeneration is the simultaneous production of two or more sources of energy. The most common example of cogeneration is the simultaneous generation of electricity and useful heat. In this case, a fuel like natural gas can be burned in a boiler to produce steam. The steam drives an electric generator and is recaptured for heating or manufacturing. Cogeneration is most effective when the cogeneration facility is near the site where the excess heat can be used.
Several energy forecasts have appeared in the recent literature. Although the assumptions, methods, and results presented in each of these predictions are debatable, they all show an energy infrastructure in transition. The trend in the 20th century has been a move away from fuels with many carbon atoms to fuels with few or no carbon atoms. This decarbonization process is discussed by Ford and Ausubel. Ausubel defines decarbonization as "the progressive reduction in the amount of carbon used to produce a given amount of energy." Two energy forecasts are discussed here. One is a projection of world energy consumption through 2100, published by Schollnberger. Schollnberger’s forecast of world oil production is compared with a projection that uses a different method.
Energy forecasts rely on projections of historical trends. Figs. 18.1 and 18.2 are based on data presented by Schollnberger and show the dominance of fossil fuels in the energy mix at the end of the 20th century. Fig. 18.1 shows historical energy consumption in quads, a unit of energy that is often used in global energy discussions because it is comparable in magnitude to global energy values. One quad equals one quadrillion BTU, or 1015 BTU. In SI units, one quad is approximately 1018 J. Beginning at the bottom of Fig. 18.1, we see that firewood, coal, oil, natural gas, water, and nuclear energy were the major contributors to energy in the latter half of the 20th century.
Fig. 18.2 illustrates the dominance of fossil fuels in the energy mix at the end of the 20th century as a percent of total energy consumed. For example, oil accounted for approximately 22% of world energy consumed in 1940 and approximately 45% of world energy consumed in 2000. Schollnberger’s estimate for 2000 was a projection from historical data that was complete through the end of 1996. According to the U.S. Energy Information Agency (EIA), actual oil consumption was approximately 40% of world energy consumed in 2000. This shows that Schollnberger’s forecast overestimated oil consumption over a relatively short forecast of 4 years; however, Schollnberger’s focus was on long-term trends, not short-term forecasting. In addition, Schollnberger was more interested in the combined forecast of oil and gas consumption or demand because gas can be substituted for oil in many instances. If we combine oil and gas, Schollnberger forecast oil and gas consumption to be approximately 53% of world energy consumed in 2000, while EIA statistics show oil and gas consumption to be approximately 62% of world energy consumed. From this perspective, Schollnberger underestimated oil and gas consumption in 2000.
Schollnberger’s forecasts were designed to cover the entire 21st century and predict the contribution of a variety of energy sources to the 21st century energy portfolio. Schollnberger considered three forecast scenarios:
A. "Another century of oil and gas" corresponding to continued high hydrocarbon demand.
B. "The end of the internal combustion engine" corresponding to a low hydrocarbon demand scenario.
C. "Energy mix" corresponding to a scenario with intermediate demand for hydrocarbons and an increasing demand for alternative energy sources. Schollnberger views Scenario C as the most likely scenario. It is consistent with Smil’s observation that, historically, the transition from one energy source to another has taken several generations. Leaders of the international energy industry have expressed a similar view that the energy mix is undergoing a shift from liquid fossil fuels to other fuel sources.
There are circumstances in which Scenarios A and B could be more likely than Scenario C. For example, Scenario B would be more likely if environmental issues led to political restrictions on the use of hydrocarbons and an increased reliance on conservation. Scenario B would also be more likely if the development of a commercially competitive fuel cell for powering vehicles reduced the demand for hydrocarbons as a transportation fuel source. Failure to develop alternative technologies would make Scenario A, which assumes that enough hydrocarbons will be supplied to meet demand, more likely.
Scenario C shows that natural gas will gain importance as the economy shifts from a reliance on hydrocarbon liquid to a reliance on hydrocarbon gas. Eventually, renewable energy sources such as biomass and solar will displace oil and gas (see Fig. 18.3).
Society’s demand for petroleum fuels should continue at or above current levels for a number of years, but the trend seems clear (see Fig. 18.4). The global energy portfolio is undergoing a transition from an energy portfolio dominated by fossil fuels to an energy portfolio that includes a range of fuel types. Schollnberger’s Scenario C presents one possible energy portfolio, and Fig. 18.4 illustrates the historical and projected energy consumption trends.
Schollnberger’s forecast is based on demand. An alternative approach is to base the forecast on supply. Beginning with Hubbert, several authors have noted that annual U.S. and world oil production approximately follows a bell-shaped (Gaussian) curve. Analyses of historical data typically predict that world oil production will peak in the first decade of the 21st century, and that cumulative world oil production will range from 1.8 to 2.1 trillion barrels. Note that Laherrère used proprietary reserves data for non-U.S. fields from a consulting firm’s database to prepare his forecasts.
Forecasts based on Gaussian fits to historical data can be checked readily with publicly available data. Fig. 18.5 shows a Gaussian curve fit of world oil production data from the U.S. EIA database. The fit is designed to match the most recent part of the production curve most accurately. This gives a match that is similar to Deffeyes. The peak oil-production rate in Fig. 18.5 occurs in 2010, and cumulative oil production by year 2100 is a little less than 2.1 trillion barrels.
If a Gaussian fit of historical data is accepted as a reasonable method for projecting oil production, future oil-production rates can be estimated as a percentage of the oil-production rate in the year 2000. Fig. 18.6 shows this estimate. According to this approach, world oil production will decline by the middle of the 21st century to 50% of the 2000 world oil-production rate. This forecast can be compared with Schollnberger’s Scenario C.
According to Scenario C, fossil fuel consumption will increase relative to its use today until about the middle of the 21st century, when it will begin to decline (see Fig. 18.7). By the end of the 21st century, fossil fuel consumption will be approximately 70% of what it is today. Gas consumption will be considerably larger, while oil consumption will decline to approximately one-half. This illustrates the range of uncertainty in existing forecasts. A more immediate test of forecast validity is the peak of world oil production.
Forecasts of the world oil-production peak tend to shift as more historical data is accumulated. Laherrère points out that curve fits of historical data should be applied to activity that is "unaffected by political or significant economic interference, to areas having a large number of fields, and to areas of unfettered activity." Furthermore, curve-fit forecasts work best when the inflection point (or peak) has passed.
Changing Industry Policy
One of the most pressing environmental concerns facing the world today is global climate change. A purported cause of adverse global climate change is the greenhouse effect. Increasing levels of carbon dioxide and other greenhouse gases such as methane in the atmosphere absorb infrared radiation rather than letting it escape into space. The resulting atmospheric heating is attributed to excessive emissions of carbon dioxide into the atmosphere. Although the evidence for global climate change and its implications for society are in dispute, the petroleum industry is considering methods for mitigating the emission of greenhouse gases.
One way to reduce the emission of carbon dioxide into the atmosphere is to collect and store carbon dioxide in oil and gas reservoirs as part of a process known as CO2 sequestration. The goal of CO2 sequestration, as stated by the U.S. Dept. of Energy, is to provide economically competitive and environmentally safe options to offset all projected growth in baseline emissions of greenhouse gases. Carbon sequestration programs are designed to reduce the climatic greenhouse effect by collecting and storing carbon dioxide in nonatmospheric sites. Sequestration of greenhouse gases is part of the more general problem of sustainable development.
The goal of sustainable development is to integrate social and environmental concerns into a development plan that optimizes economic profitability and value creation. One industry response to environmental and social concerns in the context of sustainable development is the "triple bottom line." The three components of sustainable development and the three goals of the triple bottom line are economic prosperity, social equity, and environmental protection. The focus of the triple bottom line is the creation of long-term shareholder value by recognizing that corporations are dependent on licenses provided by society to do business. In this regard, major energy companies are undertaking the task of developing methods for determining the extent of greenhouse gas emissions that result from their activities. This positive step indicates the recognition by energy companies that environmental issues are legitimate resource management concerns.
One of the key elements of the triple bottom-line policy is the development and implementation of strategies that will enable the energy industry to meet future global energy needs and environmental objectives. Studies show that energy consumption correlates positively to quality of life, with quality of life measured by such factors as infant mortality, literacy, life expectancy, and university attendance. Cassedy and Grossman provide a good discussion of two diametrically opposed ethical positions that apply to the global distribution of energy. Their discussion is outlined in the following section to illustrate some of the issues that may affect the business decisions of global energy industry organizations.
Ethical Issues in Energy Distribution
According to Cassedy and Grossman, future energy distribution will be affected by the distribution of energy between nations with a large per-capita energy base and those without. Traditional ethics would favor a policy of helping those nations without energy resources, but opinions differ on how to proceed. Two of the more important ethical positions being considered are "lifeboat ethics" and "spaceship ethics."
Proponents of lifeboat ethics oppose the transfer of wealth by charitable means. In this view, the more developed industrial nations are considered rich boats and the less developed, overcrowded nations are poor boats. The rich boats should not give the poor boats energy because their help would discourage the poor boats from making difficult choices about issues such as population control and investment in infrastructure. Lifeboat ethics is a "tough love" position.
Proponents of spaceship ethics argue that everyone is a passenger on spaceship Earth. In this view, some passengers travel in first class while others are in steerage. A more equitable distribution of energy is needed because it is morally just and it will prevent revolts and social turmoil. Thus, the wealthy should transfer part of their resources to the poor for both moral and practical reasons. If the size of the resource is increased, the need for sacrifice will be lessened.
Implications for Engineers
In an attempt to respond to market realities, some oil and gas companies and electric power companies are beginning to transform themselves into energy companies. The trend is expected to continue, and these companies will be pioneers in the emerging energy industry.
Improvements in technology and an increasing reliance on information require a high level of technical expertise to acquire resources on behalf of society. To meet the technical demands, Walesh predicts that engineers will need periods of dedicated learning or retraining between periods of full-time employment throughout their careers. Engineers in energy companies will need to understand and appreciate the role of alternative energy components in the energy mix. They will need to be able to identify and solve problems in the acquisition and environmentally acceptable use of several energy components. This will give engineers additional flexibility and help them thrive in an energy industry that is evolving from an industry dominated by fossil fuels to an industry working with many energy sources.
Fossil fuel (e.g., coal, oil, and gas) was the fuel of choice during the last half of the 20th century. The 21st century will see a gradual transition from the dominance of fossil fuels in the current energy mix to a more balanced energy portfolio. The goal is to integrate social and environmental concerns into a development plan that optimizes economic profitability and value creation as the world undergoes the transition from the use of nonrenewable fossil fuels to the use of renewable fuels and the creation of a sustainable, secure energy infrastructure.
I would like to thank Ken Larner, Don Williamson, and Wolfgang Schollnberger for their comments.
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SI Metric Conversion Factors
|Btu||×||1.055 056||E + 00||=||kJ|
|kW-hr||×||3.6*||E + 06||=||J|
|quad||×||1.055 056||E + 12||=||J|
Conversion factor is exact.