We have no knowledge of what energy is. . . . However, there are formulas for calculating some numerical quantity. . . . It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
—Richard Feynman, The Feynman Lectures on Physics, 1963
ENERGY IS THE FUNDAMENTAL driver of most of the processes we care about. It helps drive photosynthesis to make plants grow, it drives our cellular functions, and its modern forms are what make today’s societies different than those of antiquity. Energy even has its own language, with nuanced differences in the meaning of different vocabulary terms that can be confusing.
Energy is the ability to do work, where work is defined as exerting force over a distance. Lifting a rock and pushing a wheelbarrow are examples of work. While energy can be evaluated, predicted, and controlled, it remains an abstract concept that is hard to define, touch, or describe. Despite the fact that energy surrounds us—and is embedded within us—in many ways, it is hard to say what energy is. Even the Nobel laureate physicist Richard Feynman, who worked deeply with quantum mechanics and electrodynamics, and who is credited with introducing core concepts of nanotechnology, pointed out that we don’t really know what energy is.
Power is closely related to energy, but is different. While energy is a quantity, power is a rate. It is the rate at which energy is produced, moved, or consumed. That means power is a measure of energy per unit of time, or work per unit of time. Power measures how quickly something is being done or how quickly energy is being consumed.
As a scientist, engineer, and thermodynamicist, I find it is easier to describe the consequences and transformations of energy than it is to describe the fundamental nature of energy itself. And this ambiguity is just one of energy’s many challenges. The definition I use in the classroom while teaching thermodynamics is that energy is something we use to predict and explain how things happen. I can teach you what energy does. I can explain how to harness it, convert it, manipulate it, and clean up after it. But we cannot touch it or see it. Yet the evidence of its existence is all around us, omnipresent—and maybe omniscient, too, if you include the information contained within energy—like a mysterious force.
We owe the modern definition of energy to the nineteenth-century physicist and inventor Sir Benjamin Thompson. He challenged established physical theory and spurred a revolution in thermodynamics, which is the underlying science of energy. Just the word “thermodynamics” itself is telling. Thermo means heat (like a thermos that keeps your soup warm, thermal underwear that keeps the heat close to your body, or a thermometer that measures the heat), which is a proxy for energy. And dynamics means change (or changes), which is the opposite of static or steady. So, the field of thermodynamics is the study of changes in heat or changes in energy. Thermodynamicists study those changes, to understand how energy works and how to harness it better.1
Energy’s relevance is based on transformations from fuels such as coal, oil, wood, or gas to motion that allow us to perform useful functions such as mechanical work or moving a car or crushing rock. Or we can change the chemical energy in the fuels to heat for thermal activities such as warming and cooking. While providing useful services, these transformations, which are governed by the laws of thermodynamics, also cause pollution and waste.
As a professor who gives many lectures for the general public and teaches in many disciplines across campus for experience levels ranging from freshmen through Ph.D. students, I am often in position to give introductory lectures on energy to nonengineers. Most nonengineers groan when I warn them that I am about to give a lecture on thermodynamics. I usually reply that thermodynamics is like Shakespeare for engineers. And, since nonengineers make us engineers take classes on Shakespeare whether we want them or not, it seems only fair to return the favor.
For that matter, engineers often groan, too. Thermodynamics, which is the science at the heart of energy, is usually offered during the sophomore year of college and is considered one of the last weed-out classes for most major engineering programs at top-tier universities. The class is notoriously hard. Interestingly enough, despite the difficulty of the subject, it is not unusual for engineering students many years removed from the class to fondly recall thermodynamics as the first course when they actually felt like they understood how the world works.
The topic is important because the laws of thermodynamics provide the key principles by which energy is harnessed or wasted. That is, there are physical laws that describe changes in energy. The typical joke is that when hearing about the laws of thermodynamics for the first time, and hearing that the laws have some undesired outcomes for humanity, a savvy politician suggested that legislation could be passed to change the laws and a high-dollar lawyer started looking for loopholes in the law. Unfortunately (or fortunately?), such a legislative option is not available and no loopholes exist, as these physical laws are immutable, as dictated by nature.
Compared with the teachings of philosophy or the emergence of the world’s major religions, the knowledge of these laws came about only relatively recently. The science of thermodynamics developed in the 1800s from the desire to improve the performance of early steam engines. These engines were increasingly valuable in Britain just after the Industrial Revolution as they provided mechanical power at factories and mills.
As if to illustrate that the nexus of energy and water has been important since the dawn of the Industrial Revolution, one of the most important early applications of steam engines was to pump water out of coal mines. As the coal near the surface was extracted first, miners eventually had to dig deeper to find productive coal seams. As mines went deeper into the ground, water would pool at the bottom of the pit and get in the way of the miners. Pumping the water out of the mines took tremendous effort, so steam engines were used to do the heavy lifting. With a seemingly circular loop of the energy-water-energy-water-energy nexus, energy in the form of coal was used to drive a steam engine (steam is a form of water) that was used to provide mechanical energy to pump water so more coal could be mined. Notably, some coal mines today still need to be dewatered.
Most of the early work inventing steam engines was accomplished with the slow march of progress from trial and error. Early tinkerers needed to understand the theory of the engines to make them work better, so they needed new science. The names of some of those early scientists—Thompson, Carnot, Watt, and Joule—are still recognizable today and some are immortalized as units of power and energy.
As prominent science historian Bruce Hunt noted, it is not clear who helped whom the most: while we often think that innovation follows a path from new scientific discovery in the lab leading to better engineering design in the field leading to a better product available for consumers, the history of thermodynamics might have gone the other way.2 Rather than going from fundamental science to applied science to prototype to product, the direction was reversed. That is, the tinkerers inventing and improving engines out at the mines and in the factories might have done more to advance the science of thermodynamics than the thermodynamicists might have done to improve the engines.
There’s the old adage among engineers that asks, “We know it works in theory, but does it work in practice?” For the early study of the science of energy, it might have gone the other way around: “We know it works in practice, but does it work in theory?” The engineers were able to figure out how to convert heat into motion before the scientific theory existed to explain what was going on. But today, we know that what’s happening with energy is governed by three laws of thermodynamics.
Of these three laws, the first two are most important in the context of our energy future. The First Law of Thermodynamics is one of the fundamental governing laws of the physical universe, and includes three separate but related concepts:
The most important implication of the First Law of Thermodynamics is that energy is conserved. That is, we can never get more energy out of a system than what is put into it. If a system produces more energy than is put into it, then that system violates the First Law of Thermodynamics.
A quick search of the Internet will reveal all sorts of innovative designs for machines that their inventors claim will provide perpetual motion. That is, these machines, once started, never need fuel or additional inputs to propagate their motion. According to the First Law of Thermodynamics—and practical judgment, too—such machines are impossible. So save your money. That is, one cannot get energy for free—it has to be paid for in some way.
The First Law of Thermodynamics simply states that all energy is conserved. No matter what we do, in a closed system, energy will always be conserved; it is neither created nor destroyed. It can only be changed from one form to another or transferred from one body to another. The total amount of energy remains constant. Another way to consider this concept is that the best one can ever do with a closed system is break even: there will never be more energy than what it started with. Energy cannot be earned for free; it has to come from somewhere else.
While the First Law of Thermodynamics says that energy must be conserved, it also says that energy can exist in different forms. The different forms of energy are sometimes in raw form found in nature, such as the chemical energy stored in the bonds of molecules of fuels such as petroleum, coal, wood, or natural gas. Sometimes it exists in a more useful form such as directed radiant energy—think of the energy in a laser beam or spot lighting from incandescent bulbs—or mechanical energy of a moving object. The typical forms of energy include chemical (c), atomic (a), electrical (e), mechanical (m), radiant (r), and thermal (t).
While energy is conserved, and energy has many forms, what makes the First Law of Thermodynamics particularly useful is that we can convert energy from one form that is convenient for storage—for example, bound up as chemical energy in a cord of firewood—into another form that is useful for our comfort—for example, as thermal energy emanating from a fireplace that warms our house. In fact, the intentional and thoughtful transformation of energy from one form to another is what enables many of the great aspects of modern society: physical mobility, climate control, refrigeration, and so forth. Manipulating these forms of energy is one of the behaviors that distinguish humans from other species.3 While many animals uses muscle power to break open shells or to crack nuts, no other species controls fire or manipulates chemical energy in such an intentional way.
While this ability to convert energy has no end of practical uses, it is still bound by the requirement that energy is conserved. In practical terms, whenever a fuel is transformed—for example by burning a cord of wood in a fireplace—the magnitude of energy contained in the heat, light, and waste that is produced by the burning will be the same as the original chemical energy contained within the unburned wood.
To illustrate the concept, consider the process of lighting an incandescent bulb using coal-fired electricity. While flipping a light switch seems so simple, that action triggers a sequence of four energy conversions through five different forms of energy. This process begins with chemical energy (c) in the form of coal, and ends with radiant energy (r) in the form of light from the bulb.
To begin, the chemical energy in coal (c) is converted to heat or thermal energy (t) when the coal is burned. A boiler uses the heat to convert liquid water into steam that drives a turbine, similar to the way steam shooting out of a tea kettle would spin a pinwheel if you were so inclined to put one in front of the spout, giving mechanical energy (m) from its rotation. The turbine spins electricity generators that rotate magnets giving electrical energy (e), which is then distributed to the home or building where the bulb will be used. The incandescent lightbulb converts the electricity to radiant energy (r) in the form of light. The overall process goes from chemical to radiant energy (c → r) through a series of individual sequential conversions (c → t → m → e → r).
But something happens along the way: there are losses and inefficiencies and waste heat. What causes those losses? The Second Law of Thermodynamics: it prevents our processes from breaking even.
The Second Law of Thermodynamics dictates that entropy (or disorder) increases for a closed system. For example, imagine a dorm room on the first day of college. The room starts off neatly ordered. Flash forward a few weeks and the room is highly disordered, with books, clothes, and snacks scattered about. That familiar anecdote is an example of disorder increasing with time. Left to their own devices, dorm rooms and their inhabitants naturally go from clean to messy. Scientifically, this requirement describes the direction of different processes. Consequently, some people refer to entropy as the arrow of time.
For another example, imagine a beautiful maple tree in the autumn. When the weather changes, the leaves go from an ordered state (neatly attached to the tree), to a disordered state (scattered about the ground). Our intuition tells us that it is easy to imagine leaves falling from a tree down to the ground, but hard to imagine the leaves jumping up from the ground and reattaching themselves to the tree. This intuition is actually a reflection of the Second Law of Thermodynamics, which determines for us the natural direction of the leaves’ motion: from tree to ground, and not the other way around.
The changes in entropy as described by the Second Law of Thermodynamics also reveal that heat flows from a higher to a lower temperature, a phenomenon that we experience frequently. Let’s say a cup of hot herbal tea is brewed and the phone rings. That the tea cools during the phone conversation seems to be an obvious outcome. The heat from the tea is flowing to the room; as that happens, the tea cools off and the room gets just a bit hotter. Now imagine setting out a cup of room temperature tea and then walking away for a phone conversation. The idea that the cup would spontaneously heat itself up, becoming hot tea while you are away, seems preposterous. And in fact, it is preposterous.
The leaves falling to the ground and the hot tea cooling off are all different expressions of the Second Law of Thermodynamics at work. This law determines the direction of those processes. And, regrettably, it also requires that those processes will have losses that show up as waste streams. While the First Law of Thermodynamics indicates that the best you will ever be able to do with any particular energy system is break even, the Second Law of Thermodynamics says you will not even be able to do that. Those losses are the cause of the environmental impacts of energy, and those losses often impact the water system through thermal or chemical pollution.
The Second Law of Thermodynamics manifests itself in the form of inefficiencies or losses during energy conversion. Because of entropy, which increases for a closed system, inefficiencies are introduced, causing losses. These losses show up as waste heat, lost fuel, or suboptimal operation of systems. Inefficiencies are simultaneously a vexing problem and an enticing opportunity for the global energy system. In the United States, we consume about 100 quads (or quadrillion Btus) of energy each year. More than half of that energy—about 55 quads—is rejected as waste heat into the atmosphere from our smokestacks and tailpipes or into our waterways from cooling our power plants.4 That means we have the confounding situation where we waste more energy than we actually use. If we could only find a way to successfully harness our waste streams, then we will have made significant progress toward solving our energy problems. Those waste streams include food waste, municipal solid waste, agricultural waste (such as manure), wastewater, and the waste products that come out of smokestacks in the flue gases, such as waste carbon dioxide and waste heat. The latter two are relatively abundant, and distributed wherever people and combustion take place, which happens to be a convenient location because it’s next to where people often need energy.
No conversion from one form of energy to another occurs with 100 percent efficiency, so there are always losses for every conversion.5 One way to think about efficiency is by measuring what you get out of a conversion compared with what you put into it. Highly efficient systems will convert more than 90 percent of the incoming energy into useful output energy, though in a different form. Some anthropogenic processes, for example electricity generators and boilers, are very efficient, while others such as steam turbines and incandescent lightbulbs are not. Surprisingly, many processes found in nature, while resilient and robust, are not very efficient. For example, photosynthesis typically has efficiency lower than 1 percent for converting energy in photons from the sun into chemical energy stored in a plant’s biomaterial.
Unfortunately, the Second Law of Thermodynamics means that electricity production typically wastes about two thirds of the energy content of the original fuel. That is, for every one hundred units of fuel energy that enters a power plant at the start of the process, only thirty-three units of useful electricity leave the plant, and sixty-seven units of waste heat are generated. These effects are exacerbated by the losses from transmission and distribution, followed by the lightbulb itself. Including all the losses end-to-end, usually less than 1 percent of the original energy content of the coal is used to illuminate a room. For every one hundred units of fuel energy in the coal at the start, less than one unit of useful light energy is generated. That outcome is a travesty, and is one of the reasons why the Nobel Prize for Physics in 2014 was awarded to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura, inventors of an efficient blue light-emitting diode (LED). The invention opened up the pathway for white LED lights that are twenty times more efficient than incandescent lightbulbs.
Automobiles are also inefficient. The next time you get into your car, realize that only a small fraction of the energy contained in the original crude oil is being used to move you to your next location, with the rest being released into the atmosphere as heat and other waste products, including pollutants.
A related concept is that entropy can be reversed, but only by investing energy. That is we can reattach the fallen leaf to the tree, but to do so requires energy to pick the leaf up, climb the tree, and then affix it in place. We can clean our dorm rooms, but only if we invest effort. We can clean up our pollution, but it takes work to do so.
Another aspect of entropy and the Second Law of Thermodynamics is the conclusion that nonrenewable energy reserves are bound to become depleted. As we pull oil, coal, and gas out of the ground, the amount that remains is smaller than before. And if we keep up that process, eventually we will run out. Or a more likely scenario is that the prices to extract the fuels will increase to the point where they are uneconomical and other options will be more attractive. This point is in important contrast with water. The volume of water in the world is for all practical purposes fixed: it does not deplete and it does not grow, though it might move in place and time to be less available or dirtier. Thus, in this important way, energy and water are different.
All in all, the Second Law of Thermodynamics is a powerful and important edict. It says that our energy systems are and always will be wasteful. It says that we cannot avoid environmental impact from our energy conversions. And it says our conventional energy sources will eventually run out. If we heed its rules, then we would probably seek to design a system that is less wasteful, has smaller environmental impact, and uses fuels that replenish. Many of those options have water implications, too.
One of the most useful outcomes of the First Law of Thermodynamics is that energy can embody many different forms. To work our way through these forms, there are different classifications in the modern parlance we use for our convenience. One of the key distinctions is between primary and secondary energy.
The primary energy supply is made up of the original unconverted fuels in natural form, such as petroleum, natural gas, coal, biomass, flowing water, wind, solar, and uranium. Secondary energy is the converted fuels, including such forms as electricity, hydrogen, and stored energy. The difference is that you can “mine” for primary energy, but not for secondary energy. We can mine for coal and uranium and drill for oil and natural gas, but you cannot mine for electricity. Electricity, as a secondary form of energy, must be produced from something else. That is too bad, especially given how convenient, quiet, and clean electricity is to use with our appliances.
There are only a few different original sources for these primary fuels: the earth, moon, and sun. The earth provides radioactive materials for nuclear energy and geothermal resources that can be used for heating and cooling. However, much of the geothermal energy is generated from the heat of decaying radioactive materials, so that source could also be deemed nuclear. The moon is a source of energy, as its gravitational pull provides tidal forces that can be harnessed for mechanical or electrical power. Other than nuclear, geothermal, and tidal energy, all other energy forms originate from the sun.
Solar energy comes to us in a direct form, as incoming electromagnetic radiation—or light—that can be converted into electricity through photovoltaic (PV) panels or heat that can be turned into steam using mirrors that focus the solar beams. That steam can be used to spin turbines for making electricity or to drive a propulsion system for an old-fashioned Stanley Steamer automobile. Plus, there are many other forms of energy that are indirectly created by solar energy. For example, global wind patterns are caused by solar energy. Cycles of solar heating and cooling of the continents and oceans create the temperature differences that drive wind. That makes wind an indirect form of solar energy. Subsequently, waves are driven by wind, making wave energy a twice-removed form of solar energy. The sun also is a primary driver of the global hydrologic cycle, with evaporation and subsequent rain driven by sunshine. In fact, much of the sunshine that comes to earth is used to evaporate water from the oceans, doing the heavy lifting of raising water into the atmosphere for us.6 So the energy forms embedded in flowing water—such as power derived from hydroelectric dams, natural thermal differences in the ocean between the relatively warm surface and cooler depths (ocean thermal energy conversion or OTEC), river currents, and salinity gradients where fresh and saltwater meet—all originate with the sun.
Notably, the sun provides the key input—sunshine—for photosynthesis, which allows plants to grow as feed, food, fuel, feedstock, and fiber. These items can be broadly categorized as bioenergy, and they represent solar energy that has been stored over time as chemical energy in the bonds of the plant’s materials. Crops typically represent solar energy that has been stored for months, while old-growth forests represent solar energy that has been stored for decades or centuries. Solar energy has been the primary source of energy for most of mankind’s historical endeavors, including farming, raising livestock, building houses with wood, making cloth from plants directly with cotton grown by the sun or indirectly with wool from sheep that were fed plants grown by the sun, and ensuring our survival by eating. It is only recently—since the 1860s—that mankind has engaged in large-scale use of an older, fossilized form of solar energy that was trapped in old plants and marine life that has been compressed and heated for hundreds of millions of years to form coal, oil, and natural gas.
These new fuels that gained significance in the 1860s are the fossil fuels. Although fossil fuels had been used for thousands of years—even the Bible refers to ointments, which might have been from natural oil seeps in the Middle East—that use was at a very small scale. It was only in the 1860s that their use grew to be a nontrivial fraction of the overall energy supply. Ironically, fossil fuels are simply a collection of old bioenergy that had stored solar energy over many millennia. This solar energy stored in biomass was then subjected to geological pressures and timescales that converted forests and swamps into coal and algae into petroleum and natural gas. Thus, even fossil fuels are a form of solar energy, though with a much slower rate of replacement, beyond the duration of human life, than the conventional notion of solar energy. Adding complexity, savvy nuclear scientists are quick to gloat that the sun is basically a massive nuclear power plant. Solar energy—and therefore all fossil fuels—are really derivative, indirect forms of nuclear energy.
Our familiarity with energy comes from the ways we buy and use it. We buy cords of firewood, gallons of gasoline, barrels of petroleum, tons of coal, cubic feet of natural gas, and kilowatt-hours of electricity. We use natural gas or wood to heat our homes and businesses. We use petroleum products to operate our machines in factories and our cars and trucks. And, we use many fuels to make electricity. Electricity is particularly valuable, because once you have flowing electrons you can use them to make heat and to move machinery.
In addition to using these resources as a fuel that is burned, their important role as feedstocks for manufacturing materials is often overlooked. Petroleum is used to make plastic, pesticides, pharmaceuticals, cosmetics, paints, dyes, and cleaners. Natural gas is used to make fertilizer, ink, glue, and paint, among other products. Wood is used to make paper, fenceposts, and other building materials. Coal is used as a source of heat and carbon for steel and iron production. Coal is also used in the cement-making process. And, the solid wastes from coal combustion, including bottom ash, which comes out the bottom of coal boilers, and fly ash, which flies through the smokestack, are used to make drywall for buildings and aggregate for roads.
Because there are so many forms of energy with different end-use applications and implications for society, they are organized into different categories. Labels such as fossil fuels, alternative energy, renewable energy, sustainable energy, green (or clean) energy, and unconventional energy color our conversations. Unfortunately, these different classifications are sloppy at best, and intentionally misleading at worst.
The (conventional) fossil fuels include coal, natural gas, and liquid petroleum. The key consideration regarding fossil fuels is that they were formed tens to hundreds of millions of years ago—in the fossil era—and are considered to be finite and nonreplenishing. According to the Second Law of Thermodynamics, if we keep using them, we will run out of them someday. However, just to make things confusing there are renewable forms of natural gas, sometimes called biogas or renewable natural gas (RNG), that can be produced from decomposing organic matter such as food waste or manure from agricultural operations. And, there are people who manufacture synthetic fuels (synfuels) such as synthetic gasoline and synthetic diesel, which are fuels with similar characteristics to the petroleum products, but made from coal or with some fraction of biogenic sources.
Unconventional fossil fuels include nonliquid forms of petroleum such as oil shale, shale oils, oil sands, tar sands, and heavy oils. Unconventional forms of natural gas include shale gas and coalbed methane. The word choices are interesting, as it is not obvious at first blush what the difference is between oil shale and shale oil. Oil shale is the kerogen that is essentially a form of rock found in Utah and Colorado. Shale oil, also known as tight oil, is the liquid produced from impermeable shales. In the former case, the oil is the shale. In the latter case, the oil is held by the shale and has to be cracked open by hydraulic fracturing or some other technique.
And, these words are wrapped up in different agendas. A college schoolmate of mine was a senior drilling engineer producing oil for one of the major oil companies in Alberta. In one of my communications I discussed the growing production of the tar sands in Canada, and he wrote back a terse email, excoriating me for referring to them as “tar sands” instead of “oil sands.” To paraphrase his reply, tar sands conjure up images of strip mining operations with significant environmental impact, whereas oil sands are relatively cleaner and an abundant, secure resource. The words matter.
For a variety of reasons—their carbon emissions, the fact that they are depleting, and because some forms are imported—fossil fuels have become unpopular with large swaths of the population and with policymakers. And, they seem particularly unpopular in certain circles of the current generation of college students. In fact, it is not unusual for me to field inquiries from eager students who want to join my research group to do research on alternative energy. I usually begin by asking them “alternative to what?” In typical usage, alternative energy, renewable energy, and clean energy are often used interchangeably even though they mean different things. Usually what these prospective students broadly mean is “clean energy.” But distinguishing between “alternative” and “clean” is important. Petroleum was an alternative to whale oil, and coal was an alternative to wood. Early in the atomic age, nuclear energy was considered an alternative to fossil fuels. However, today in the modern parlance, nuclear is typically one of the options for which an alternative is sought. And today, natural gas is an alternative to coal in the power sector, though both are fossil fuels. Also, the context becomes important depending on the end-use. For example, electricity is widely considered an alternative transportation fuel for cars, despite electricity being powered almost completely by coal, natural gas, and nuclear. And, natural gas as a fuel for vehicles is also considered an alternative to petroleum, even though natural gas is often produced alongside petroleum.
Renewable energy typically includes any form of energy that is renewed continually or annually. This category includes energy forms that are not depletable, such as solar, wind, water, and tidal. It also includes forms that renew quickly, but are depletable, such as bioenergy. “Renewable” refers to the characteristic of the fuel’s availability and the rate at which it renews itself. Fossil fuels are not considered renewable because they replenish themselves over tens of millions of years, which is a timescale too slow to be practical from a human perspective. By contrast, no matter how much solar energy we use on a Tuesday, just as much solar radiation will hit the earth on the following Wednesday, so the solar radiation “renews” itself daily. Even though renewable energy comes back regularly, it is not inherently sustainable.
The dividing line between renewable energy and sustainable energy is not always clear. Roughly, these two are used synonymously, though they have distinctly different interpretations. In particular, it is possible to use renewable energy in unsustainable ways. It is possible to cut down trees faster than they grow back. Just because trees are renewable does not mean we use them sustainably. Forests typically take many decades or centuries to grow, but it only takes years or a few decades to cut them down, as was witnessed in the upper Midwest and northeastern United States in the 1800s.7 So, while renewable energy is a classification dependent on whether a fuel is replenished quickly by nature, sustainable energy is a comment about the rate of consumption. To illustrate, solar energy is inherently sustainable: it is rapidly renewable—it comes back every day—and no matter how much we use, there is still sufficient solar energy. By contrast, finite forms of fossil fuels are inherently unsustainable: even if they are used at a slow pace, the replenishment rates are even slower and so the amount of geological resource in place only declines. Keep in mind that even though the resource will decline, our ability to extract it might increase with time, so the actual production rates might increase in the foreseeable future. Other renewable forms of energy also are not necessarily sustainable. While geothermal energy is renewable as it is driven by internal heating of the earth, some geothermal sites get played out. And hydroelectric dams can silt up after a century or so, limiting the sustainability of their usefulness despite the renewability of the water flows that power them. Dredging to remove silt extends the lifetime of dams, but that costs a lot of money and effort. Overall, it is important to remember that just because an energy form is renewable does not mean it is necessarily sustainable, clean, or green.
Green (or clean) energy are those forms of energy that have small environmental impacts. This idea is particularly contentious to define as it mixes technical characteristics of energy sources such as their emissions and behavioral choices about how we produce or use those energy sources. Clean energy once referred to whether it was clean at its point of use, and was primarily a reference to air quality. However, the vision has expanded to include the entire life cycle, such as extraction and production of the fuel itself (which implies a renewable or nondepletable resource base), processing (which implies a low energy intensity for upgrading and distributing), and at its use (which implies a low carbon intensity and toxicity). But all energy choices have an environmental impact: even clean options like wind energy have effects on land use, and solar energy has impacts from pollution at the mines that produce silicon used in manufacturing photovoltaic panels.
The world consumes a lot of energy from many sources for a variety of applications. Despite the diversity of options, fossil fuels—coal, petroleum, and natural gas—are the dominant historical primary energy sources and still provide approximately 85 percent of the world’s energy today.8 The rest of our energy comes from nuclear and renewables such as wind, solar, geothermal, bioenergy, and hydro. We put that energy to work, using those fuels in different ways for the different end-use sectors.
The transportation sector is highly dependent on petroleum, with more than 95 percent of the energy for moving our cars and trucks coming from gasoline and diesel. The reason for this reliance on petroleum-based fuels is that they are particularly good transportation fuels, with high energy density that makes them convenient to store on board the vehicle.9 We essentially have a monopoly of fuels for transportation: we can choose our retail vendor—Exxon versus Chevron, for example—but we do not have much choice about the fuel. When people declare that we have an “oil problem,” they are essentially saying we have a transportation problem.
In contrast to the transportation sector’s dominance by a single fuel (petroleum), the electric power sector draws on a much more diverse array of fuels. While still heavily dependent on coal for more than a third of its energy, the power sector also draws heavily from nuclear, natural gas, and renewable sources. And, as coal prices increased steadily through the first decade of the twenty-first century, and as natural gas and renewable prices leveled or dropped, that distribution has diversified further in the United States. Because it is so expensive, very little petroleum is used for power generation in countries where other options are available, serving mostly as a backup source with diesel generators or for peaking units during the hottest or coldest hours of the year when demand is highest. In contrast with the other forty-nine states, the power sector in Hawaii is heavily dependent on imported petroleum because there are no local sources of other traditional fuels and oil is the easiest to ship and store.
Annual energy consumption by fuel, globally, in trillion British thermal units, or Btu. [BP Statistical Review of Energy, 2014]
This point about the power sector’s lack of dependence on oil is very important. Many observers in the United States are fond of saying we need more wind power as a way to reduce oil imports. Environmentalists and conservationists are fond of saying we should turn off our lights to reduce oil consumption. But these statements do not make any sense: we use very little oil in the power sector, so, outside of Hawaii, saving electricity does not save oil. We should still turn off our lights, but the way to reduce our oil consumption is through changes in our transportation sector or by replacing our dirty old fuel oil heaters in our homes with something better.
In addition to the different fuels and end-uses, the scale of global annual energy consumption is impressive. Overall, the world consumes more than 525 quadrillion Btu of energy.10 One Btu is about the same energy as in a kitchen match, so that means the world’s 7 billion people consume 525 million billion kitchen matches each year. Even more notably, despite having less than 5 percent of the world’s population, the United States is responsible for nearly one-fifth (100 quads) of that global energy consumption. U.S. consumers use the same types of energy as global citizens: we just consume a whole lot more of it. Roughly speaking, the average global citizen consumes about 75 million Btu of energy each year. The average U.K. resident consumes twice as much as the average global citizen, and the average U.S. resident consumes twice as much as the average U.K. resident. So U.S. residents consume four times as much energy per person on average as the typical resident of China or India.
But, there are only 60 million British, and 315 million Americans. That leaves about 7 billion others who consume energy like global average citizens. If they all tomorrow started consuming energy like the British, then we would need to double annual energy production and consumption. If they consume like the Americans, the energy system would have to quadruple. And, the world’s population is growing. By 2050 there might be 9 to 11 billion people. Factor in the additional people who also want to consume energy like Americans, and then it’s hard to imagine that the earth’s atmosphere or oceans could take it. It’s also not clear that the global energy industry could meet the demand. Is it even possible to extract, refine, move, consume, and clean up that much energy at that rate?
This conundrum is essentially the grand challenge of the twenty-first century: how do we bring the value of that energy consumption—the clean water, indoor lighting, comfortable quality of life—to every global citizen in a way that does not leave behind a wake of environmental destruction?
While the modern-day energy policy debates hinge on a snapshot in time of current energy consumption and production, one of the most important lessons to learn is that the energy system is continually changing. Many of the key factors for energy vary with time, and there are many timescales for these changes. For example, we use decades or centuries to describe energy transitions from one dominant fuel to another, such as the transitions from wood to coal and coal to oil. We use years to decades to contemplate macro demographic trends such as population growth or for the construction of major energy facilities. We use months to years to evaluate seasonal shifts in energy production and consumption due to changes in the weather or other prevailing conditions that affect renewable energy sources’ production or the demand for air-conditioning. We use hours to describe shifts in demand for electricity from baseload (at night) to peak load (in the afternoon on a hot day), and we use minutes or seconds to balance the power grid.
It turns out that preventing the failure of the power system requires mastering the second-by-second variations of electricity demand along with the multidecade process of grid planning. This mismatch in timescales is rife with problems and seemingly invites disaster at every step as some solutions for rapid grid-balancing such as sophisticated algorithms do not work for the political-economic-cultural process of building large-scale long-lived capital assets like power plants or oil export facilities that are used to produce or distribute conventional forms of energy. A capital lock-in effect of bad decisions ensues: if a company builds a $5 billion power plant that is expected to operate for forty years, they generally want to use it for the entire lifespan of the design so that they can get their money back. That means it is important for us to understand the broader trends before committing ourselves to expensive, big-ticket items that we might come to regret.
Over time, energy use has changed significantly. The dominant fuel from antiquity through the late 1800s was wood and other forms of bioenergy, which were used for heating, cooking, and as a feedstock for materials (such as lumber and paper). Starting at about the time of the Industrial Revolution in the mid-1800s, coal and petroleum production increased. Coal had already been used minimally in the early 1800s in a few discrete applications, but then was adopted more widely as a fuel for domestic and industrial use starting around 1850. In parallel, oil production started in earnest in 1859 in Titusville, Pennsylvania.
By 1885, coal had surpassed wood as a primary fuel in the United States. There were several reasons for this transition, including coal’s superior characteristics as a fuel and deforestation throughout the upper Midwest and the northeastern United States. Many of today’s beautiful forests in New England, Wisconsin, and upper Michigan are second-growth forests. By the late 1800s, logging had been so extensive that many of these forests had been almost completely wiped out, leaving an indelible environmental footprint and making wood more expensive and harder to come by.11 Because of wood’s increasing scarcity and price, alternative fuels were particularly appealing, which made coal—an affordable alternative to wood—an attractive option.
Annual energy consumption (in quadrillion Btu) in the United States from 1800 through 2013, showing transitions from one dominant fuel to another. [U.S. Department of Energy, Energy Information Administration]
In addition, by most measures coal was simply a better fuel than wood. It has twice the energy density of wood at approximately 20 million Btu per ton of coal versus 10 million Btu per ton of wood, generates less smoke and ash, is less carbon intensive per unit of energy, and produces higher temperatures when burning. That last point is particularly valuable, as it made coal a better fuel for metalmaking applications. Prior to the use of coal, wood and char had been used for metalworking. However, because wood’s combustion temperatures were lower, workers had to put more energy into the metal by hammering against an anvil, which explains the stereotypical image of the strong, sweating blacksmith hammering heated metal.
The unexpected irony here is that in the 1800s coal essentially saved the forests. Because a better and more abundant fuel—coal—came along, the rampant deforestation slowed down dramatically. This concept is difficult to digest in the modern context, for which coal is considered a threat to the forests through mountaintop removal mining, which wipes out entire mountains, valleys, streams, and forests, and through acid rain (composed of sulfuric and nitric acids), formed when coal combustion emits pollution that mixes with water vapor in the air.
Petroleum use grew at about the same time, but much more slowly. Initially, petroleum consumption was almost entirely in the form of kerosene as an illuminant and polyolefins as lubricants. Similar to the trajectory that coal faced, petroleum became popular because the conventional fuel sources—animal fats for lubricants and whale oil for lighting—were becoming scarcer and more expensive. While many people argue today whether peak oil is worthy of concern, in the mid-1800s, peak whale was a very real phenomenon: whale oil production peaked around 1850.12 Literature from the time even featured whaling, revealing its importance to society. Herman Melville’s Moby-Dick, published in 1851, was practically a technical manual for whaling.
These new fossil fuels had good performance. While the lubricants made from animal fats would clog up the new machinery that was becoming prevalent during the Industrial Revolution, oil-based lubricants had less friction and better consistency over a wider range of operating temperatures and pressures. Those improved the coal-fired machines’ operation and helped spawn more growth in factories and automation. At the same time, kerosene was a much better illuminant than whale oil. It burned brighter, burned longer, generated less smoke, and did not have the pungent odor that was produced by burning whale oil. So, once again, depleting renewable resources (animal fats or whale oil) were displaced by the new fossil fuel alternative, which was more abundant, cheaper, and had better performance characteristics.
Just as coal saved the forests, petroleum saved the whales from extinction. Today, petroleum is considered by many to be a threat to whales and other marine life because of risks that are induced by noise and spills from coastal pipelines, offshore production, and ocean-borne tankers. These two examples are illustrative of another broader philosophy about energy: today’s energy solutions often become tomorrow’s energy problems.
Ultimately, petroleum use did not surpass coal use in the United States until approximately 1950. While petroleum was growing in popularity, the use of oil for lubricants and kerosene for lighting was small overall compared with coal and wood for heating and materials. However, with the advent of the internal combustion engine for automobiles, which transformed society near the start of the 1900s, petroleum found additional applications. After that, its use grew rapidly for several decades. For transportation, petroleum’s high energy density and liquid form (which made for easy handling and portability) yielded much better performance in terms of power and endurance than biomass, which was the conventional fuel for other forms of transportation, namely trains, which used wood for boilers, and horses, which were fueled by feed. John D. Rockefeller, who had already become the world’s wealthiest man from selling kerosene, became even richer once he could also sell gasoline.13
These historical transitions from wood to coal to oil reveal three important aspects: transitions take a long time; transitions have a distinct trend toward decarbonization; and there has been an unmistakable pattern of growth in energy consumption since the Industrial Revolution.
While the ability for a society to shift its fuel mix over time is evident, it is also clear that these transitions do not happen quickly. While biomass was the dominant energy source for millennia, its modern use as a source of energy for activities beyond sustenance and shelter picked up in earnest in the late 1700s to early 1800s with wood as its preferred form. Wood’s decline as the dominant fuel source in the United States spanned centuries, until it was surpassed by coal in 1885. Subsequently, the rise and fall of coal as the most popular fuel source spanned sixty-five years, from 1885 to 1950. After that, petroleum has reigned supreme for more than sixty-five years, and continues today as the nation’s most popular source of energy, though I expect natural gas to overtake petroleum in the United States by 2025.14 If society’s goal is to wean ourselves from fossil fuels entirely, then history’s lesson is that it will take a while and so we better get started. Because these trends take so long, one role of government might be to set policies in place that accelerate the transition toward lower-carbon, sustainable fuels.
In addition to the long duration of these transitions, they also show a distinct trend of decarbonization. Wood emits more carbon per unit of energy content than coal. Similarly, coal emits more carbon than petroleum. Natural gas, whose primary molecular constituent is methane, CH4, is the least carbon intensive of the four. As we switched from wood to coal to petroleum and then to natural gas, we have been preferentially selecting fuels with lower carbon intensity. Despite the decreasing amount of carbon emitted per unit of energy, the total carbon emissions in the United States increased throughout the twentieth century up until about 2008 because total energy consumption grew by so much. But our fuel choice became cleaner along the way.
This trend, which spans centuries, makes very clear that decarbonization is not a modern concept invented by Al Gore or environmental warriors. Rather, this pattern indicates that decarbonization is what societies do as they get richer. Civilizations naturally desire to “clean up their act” over time. In that context, modern efforts to combat global change through policy efforts that seek decarbonization would not be a departure from business-as-usual, as many critics allege, but rather would be a direct continuation of a trend that has been in place for more than one hundred years. Though to be clear, the prior shifts were not targeting carbon directly—they were targeting cost, performance, or cleanliness—and the decarbonization was a useful by-product. The difference for the ongoing transition is that carbon reductions are explicitly targeted because of climate change concerns.
Alongside our changing fuel mix was growth in total energy consumption since the early 1800s. This trend is the consequence of population growth—more people, each of whom consumes energy—and economic growth—more rich people, who consume more energy than poor people. In addition, society was undergoing several shifts, including urbanization, industrialization, electrification, and mobilization, each of which brought along with it additional energy requirements.
Economic growth typically implies higher per capita energy consumption as people gain affluence, and that trend is evident for many decades since the early 1800s. However, with many industrialized countries, per capita consumption leveled off or even dropped slightly since the energy crises of the 1970s, as the industrial mix shifted from highly energy-intensive industries such as manufacturing and chemical production to less energy-intensive service-oriented industries such as banking and research not to mention wide-ranging investments in energy efficiency as a way to reduce energy costs. In the early 2010s, as energy prices remained high and new appliance efficiency and fuel economy standards kicked in, our per capita energy use dropped further.
Increasing energy consumption brought with it many benefits, such as prosperity, better health, and improved quality of life. But that energy consumption also brought along several downsides. As energy consumption grew, so did energy imports, environmental degradation, emissions, and global climate change. For people who are energy poor, increases in modern energy consumption usually lead to a better life. Electric lights help poor students in remote villages do their homework at night, and cleanburning stoves spare their users from the risks of inhaling too much smoke and soot. But, for those who are already energy rich, increases in energy consumption actually might worsen our quality of life as the accumulating effects of the environmental impacts from energy consumption start to undermine the benefits. Solving this balance of good and bad is the main challenge moving into the future.