End Uses of Energy

There are four different end-use sectors responsible for energy consumption: transportation, residential, commercial and industrial. In addition, the electric power sector consumes a lot of primary energy that is subsequently used in the other four sectors.

The transportation sector depends on petroleum as the source of more than 90% of its primary energy for moving cars and trucks. The petroleum is mostly consumed in the form of gasoline, diesel, and jet fuel. Petroleum-based fuels are effective for transportation because they have high energy density and are therefore convenient to store on board the vehicle. For an interesting anecdote about the U.S. Secretary of Energy Stephen Chu discussing the excellent technical performance of petroleum-based fuels, see Chapter 3 of Russell Gold’s book The Boom (New York: Simon and Schuster, 2014).

The relationship between fuels (Source, left side) and end uses (Sector, right side). The percentage of each source and sector is labeled on the connector lines.

In contrast to the transportation sector’s heavy reliance on a single primary energy source, the electric power sector depends on a more diverse array of fuels, drawing from coal, nuclear, natural gas, and renewable sources. Coal has historically been the most important fuel for the power sector, but as natural gas and renewable prices leveled or dropped in the first two decades of the 21st century, its relative contribution for power generation has dropped. Because of the expense, petroleum is used for very little power generation in countries where other options are available. In the United States and western Europe, petroleum is used primarily as an emergency or backup fuel for the power sector, with the typical exception of distant islands or remote locations.

Hawaii is the only U.S. state that depends heavily on petroleum for electrical generation. Primarily because it lacks local sources of traditional thermal fuels, Hawaii uses petroleum, which is easy to ship and store. 

Image Credits: U.S. Energy Information Administration, Monthly Energy Review /.

Renewable Energy

As a reminder, three original sources—the earth, the moon, and the sun—ultimately provide all the energy we consume. The earth provides nuclear fuels and geothermal resources. The moon induces tidal energy. The sun provides the rest, including the renewables such as wind, solar, hydro, and bioenergy. The sun also provides the fossil fuels, which are forms of bioenergy converted under millennia of geological compression and heat into oil, gas, and coal.

Of the forms of renewable energy, hydroelectric energy ranks as one of the most important sources historically and remains so today. Hydroelectric turbines harvest energy from water flowing downhill to generate electricity. Sailboats employ wind as a form of mechanical energy, and modern turbines convert wind into electrical power. Solar energy can be used for direct heating or for electrical power. The earth provides geothermal energy for passive heating, passive cooling, and power generation. Bioenergy includes crops, trees, and plants, as well as organic waste materials, such as cow dung, old tires, municipal solid waste, agricultural waste, and landfill gas. Producers usually combust bioenergy sources in their original form to generate power, heat, or motion, though sometimes the combustion occurs after conversion to a liquid fuel, like ethanol. Energy from the ocean includes the forms that can be harnessed from waves, tides, ocean thermal energy conversion (OTEC), and salinity gradients where freshwater and saltwater mix, such as at the mouth of a river.

The major sources of renewable energy include hydroelectric generation, wind power, solar power, geothermal energy, biogas and biofuels, and ocean energy.

Renewable vs. Sustainable

Renewable energy sources replenish continually or annually. Wind, solar, and hydro can never deplete, as meteorological, astronomical, and geological forces are not anticipated to end on any timeline that is relevant to human planning purposes. However, renewable energy and sustainable energy are not synonymous. Demand and mismanagement can deplete renewable energy sources faster than they replenish. Overuse can deplete bioenergy resources faster than they grow back, as demonstrated by deforestation. Rate of consumption also affects geological resources. For example, geothermal resources sometimes play out. Hydroelectric dams can silt up over time, which means they might not be sustainable even though they produce renewable energy. Interestingly, fossil fuels can replenish because of ongoing growth and sedimentation of organic matter, but renewal occurs so slowly that human society treats fossil fuel reserves as a fixed asset that will only deplete with time.

Land Use

Renewable energy is typically very land intensive. Hydroelectric dams and their reservoirs can require many square miles of land. Wind farms can consist of hundreds of turbines each requiring dedicated space. Solar photovoltaic harvesting stations and concentrated solar power are often constructed in large arrays. Bioenergy requires biomass grown fairly traditionally in vast fields. However, many facilities and sites of generating renewable power are compatible for dual use. Wind farms can also be used for agriculture, ranching, or other land intensive activities. Rooftop solar panels can be used in urban areas with buildings that serve a different purpose. Hydroelectric reservoirs can also be used for flood control, storage, irrigation, and recreation. Balancing tradeoffs between land intensity and zero-emissions operation is one of the key challenges of renewable energy.

Reservoirs and Transportation

Water projects are closely tied with political systems and power structures. In the nineteenth century, Abraham Lincoln ran on a platform of enhanced water infrastructure—better canals for navigation andcommerce.1 Water serves an important role for transportation. While railroads are more energy-efficient at moving goods than trucks, cars, or planes, many people do not realize that waterborne commerce is even more energy efficient. In the United States, 18,000 kilometers (km) (11,000 miles) of inland waterways move 725,000 tonne-km (500,000 ton-miles) of freight annually, which is nearly a third of the 2.5 million tonne-km (1.7 million ton-miles) of freight moved by 227,000 km (141,000 miles) of railroads.2 In particular, the lower Mississippi River and its famous barges move a lot of cargo.

Decades after Lincoln, the federal government built and named the largest dam in the world for President Herbert Hoover. Before the Hoover Dam, the very first hydroelectric dams had not been very large in size. They more closely resembled medieval overshot water wheels that were used for mechanical power to grind grain, cut wood, or polish glass. For these older wheels, the flowing water of the river rotated a massive wooden wheel that was connected by wooden gears and axles to a workhouse on the bank of the river. Reservoirs maxed out around 3 meters (10 feet) high, and these structures were not considered to be that impactful to the river’s natural flow.

The ecological damage of dams and their massive reservoirs is hard to assess. While dams make electricity relatively cleanly, they distort the landscape with a permanent mark. Fish and other animals cannot freely travel the rivers once dams are put in place. Famously, salmon fight currents to go upstream many hundreds of miles, jumping over obstacles along the way to spawn. While they can impressively jump over small rapids a few feet tall, they cannot jump over dams. Consequently, some dams have installed “fish ladders” that are a cascading series of waterfall steps that allow salmon to bypass the dam. Even if they successfully navigate the fish ladder, their navigational systems, which benefit from a distinct current in the water, can get confused by the slack water on the other side of the dam. Fish also get caught going downstream. The whirling blades of the hydroelectric turbines have been known to harm the fish while they pass through. More recently, with the advent of “fish-friendly’’ turbines, fish can pass through with less likelihood of being injured.

Former University of Texas at Austin student Lily Xu explains the ecological impacts of dams and the benefits of the fish-friendly turbine.

Silting is another major problem for dams. Sometimes silt is needed downstream because its minerals are useful for agriculture. Dams stop the flow of silt, causing it to accumulate behind the concrete walls. Eventually, the silt will fill up the reservoir, causing the dam to lose its function. This phenomenon happens for natural dams and lakes. In Yosemite Valley, receding glaciers left a natural dam of rocks andrubble that caused water to pool up, forming Mirror Lake. Over millennia, silt has been filling the natural lake, so it is now more of a pond than a lake and is only filled in the spring when the water levels are high. Eventually it will disappear altogether.

Mirror Lake reflects the surrounding mountains in Yosemite Valley. Because of silting, the lake is gradually becoming more shallow. 

Dams can negatively impact river temperatures in substantial ways. Usually temperatures are relatively uniform for a free-flowing river. After a reservoir is built, the temperature can vary significantly from the water surface (relatively warm) to the bottom of the water column (relatively cold). Water flowing through the dam from the bottom of the reservoir can lower the temperature of the river downstream. Native river species are often not adapted to the cooler water. In some cold environments with surface-release dams, the opposite can happen: dams release water downstream from the relatively warmer surface, making downstream temperatures slightly higher than they would have been otherwise. Both situations can affect fish reproduction. Consequently, native river species must often migrate upstream of the dam to reach normal conditions or move downstream until temperatures stabilize.

Chinese leaders sought the Three Gorges Dam for decades and finally saw construction in the 2000s. While it has helped reduce the risks of flood-related disasters and improved the navigability of the Yangtze River, its creation flooded entire valleys, displacing people and erasing towns.4 Geologists worry about the earthquakes and underwater landslides caused as the soft, soaked soils around the reservoir settle to accommodate the new load. In the first decade of its operation, the reservoir, which is as long as Great Britain, triggered more than 500 earthquakes with a magnitude greater than 2.0 on the Richter scale and more than 400 landslides. If the Three Gorges Dam were to collapse, it would put approximately 15 million or more lives at risk downstream. In the event of a collapse, the biggest man-made wall of water ever conceived would move quickly down the canyons, making it difficult for people to escape.

The 1889 Johnstown Flood, triggered by a collapsed dam, killed more than 2,200 people in Pennsylvania. The Saint Francis Dam collapse in Santa Clarita, California, in 1928 killed 600 people and still reverberates through Southern California water politics. The collapse of the Grand Teton Dam along the Teton River in eastern Idaho in 1976 was caught on film, and it shows the powerful force of the water. Thankfully, its collapse was in the middle of the day, so only 11 people were killed. The wall of water wiped out two small towns and so if the flood had occurred in the middle of the night, it is likely that thousands of people would have perished.

Filmmaker Jared Dann tells the story of the Teton Dam disaster and its aftermath in this short film. (8:00)

The Reynolds Transport Theorem

Keeping track of water is done through a scientific accounting methodology known as the Reynolds transport theorem, which is used to track the flows, fluxes, and storage of water in a physical system.

\( \frac{{dB}}{{dt}} = \iiint\limits_{cv} {\beta \rho d\forall } + \iint\limits_{cs} {\beta \rho v} \cdot dA \)

A simpler way to express this bookkeeping is as follows:

Wfinal = Winitial + Wadditions – Wwithdrawals

That is, total water in a system (Wfinal) is equal to the initial water (Winitial), plus added water (Wadditions), minus withdrawn water (Wwithdrawals). The paradigm resembles that of financial bookkeeping: the amount of money we have in the bank tomorrow is the sum of what we had yesterday, plus today’s deposits, minus withdrawals. A surplus depends on whether we withdraw more than we deposit. If society withdraws water from an aquifer faster than water returns from rainfall, then the aquifer will go dry.

The connection between water bookkeeping and financial bookkeeping is a way of life in some societies. For example, the unit of currency in Botswana is the pula, which literally translates as rain, (brought to the attention of the author by Dr. Ashlynn Stillwell). Terms, such as petrodollars and black gold, suggest that energy and money are also synonymous in some contexts.

Turning On the Lights: Conversions and Efficiencies

Flipping a light switch begins a sequence of energy conversions before you ever see the light. Tap the switch to begin the journey of energy from the fuel to your light bulb. Tap on the left or right of the image to continue.

Every time energy is converted, some of it is lost. The efficiency (η) of the entire process is the product of the efficiency of each conversion. In this example, only 0.6% to 1.1% of the energy contained in the fuel becomes useful light, while the rest is lost as heat.

Large power plant boiler (makes steam)E1 = 90 – 98%c→t
Large steam turbine (makes motion)E2 = 40 – 45%t→m
Large electricity generator (makes electricity)E3 = 98 – 99%m→e
Utility electric grid (distributes electricity)E4 = 94 – 95%e→e
Traditional incandescent lightbulb (makes light)E5 = 1.8 – 2.6%e→r
Total: ETotal = E1 × E2 × E3 × E4 × E5ETotal = 0.6 – 1.1%c→r

The Kaya Identity

Japanese energy economist Yoichi Kaya developed the Kaya identity as a function relating factors that determine the level of human impact on climate, in the form of emissions of the greenhouse gas, carbon dioxide.1Kaya, Yoichi; Yokoburi, Keiichi (1997). Environment, energy, and economy : strategies for sustainability. Tokyo [u.a.]: United Nations Univ. Press.

This identity states that total emissions can be expressed as the product of four inputs.

\(F = P \times \frac{G}{P} \times \frac{E}{G} \times \frac{F}{E}\)

F = global CO2 emissions from human sources

P = global population

G = global gross domestic product

E = global energy consumption

Policy experts and scientists focus on the ratios which make up this identity, which are crucial to understanding the relationship between emissions, population, gross domestic product (GDP), and energy consumption. 

G/P = affluence (GDP per person)

E/G = energy intensity (Energy per $ of economic activity)

F/E = cleanliness of energy (emissions per BTU of energy consumed)

This equation is both simple and tricky, as it can be reduced to only two terms. However, as the data requested are generally available, this identity straightforwardly relates how many of us there are, how rich we are, how much energy we use, and how the energy we use impacts the world around us.

Energy Transitions

Over time, energy use changes significantly. Bioenergy, such as wood, remained the dominant fuel from antiquity through the late 1800s. Starting around the Second Industrial Revolution in the mid-1800s, coal and petroleum production increased. Domestic and industrial users began widely adopting coal around 1850. In parallel, oil production started in earnest in 1859 in Titusville, Pennsylvania.

By 1885, coal surpassed wood as the primary fuel in the United States. Coal’s superior characteristics as a fuel and deforestation throughout the Upper Midwest and American Northeast contributed to the transition.

By most measures coal performs better than wood as a fuel. It has twice the energy density of wood at approximately 23 gigajoule per metric tonne (20 million British thermal units (Btu) per ton) of coal versus approximately 12 gigajoule per metric tonne (10 million Btu per ton) of wood, generates less smoke and ash, releases less carbon per unit of energy, and produces higher temperatures when burning. Metal making requires high temperatures, and coal’s qualities established a long relationship between coal and steel, for which the coal served as a source of both heat and carbon.

Although considered environmentally problematic today due to mining, emissions, and disposal concerns, coal helped save North American forests by slowing rates of deforestation. Similarly, the development of the petroleum industry helped eliminate commercial whaling. Today’s energy solutions often become tomorrow’s problems.

Petroleum use grew at about the same time as coal use, but much more slowly. Initially, petroleum consumption included only kerosene for illumination and polyolefins for lubrication. Similar to coal, petroleum became popular because the conventional fuel source—animal fats for lubricants and whale oil for lighting—became scarcer and more expensive.

While animal-fat lubricants would clog up industrial machinery, oil-based lubricants had less friction and better consistency over a wider range of operating temperatures and pressures. Similarly, kerosene burned longer and more brightly with less smoke and odor than whale oil.

Reproduction of a retouched photograph showing Edwin L. Drake, to the right, and the Drake Well in the background, in Titusville, Pennsylvania, site of the first commercial oil well, drilled in 1859.

Until 1950, petroleum use paled in comparison to coal and wood use in the United States. The advent of the internal combustion engine in automobiles, which transformed society near the start of the 1900s, provided an additional application for petroleum and led to its dominance today. 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.

These historical transitions from wood to coal to oil reveal three very important aspects: transitions take a long time; transitions have a distinct trend towards decarbonization; and there has been an unmistakable pattern of growth in energy consumption ever since the Second Industrial Revolution.

Image Credits: cosma/Shutterstock.comLibrary of Congress/public domain.

Carrying Capacity 

As absolute energy and water use increases due to population growth and economic growth, it is worth considering what the ultimate carrying capacity of the Earth is. That determination depends on factors such as how many people inhabit the Earth and the number of resources they use for their existence. It turns out that those values are not consistent from place to place and that they change over time.

Four macro trends that have been underway for decades and are likely to continue include urbanization, industrialization, electrification, and mobilization, all of which increased overall demand for energy from 1800 to the present day and are expected to continue that trajectory into the future. Economic growth typically implies higher per capita energy consumption as people gain affluence, and that outcome has been evident for many decades since the early 1800s. However, in many medium-economic developed countries (MEDC), per capita consumption leveled off or decreased slightly since the energy crises of the 1970s. Since those years, the industrial mix in the United States shifted from very energy-intensive industries such as manufacturing and chemical production to less energy-intensive, more service-oriented industries such as banking and research.

Overall, the world consumes more than 550 Exajoule (EJ) of energy. U.S. consumers use the same types of energy as global citizens but on a different scale. Roughly speaking, the average global citizen consumes about 75 EJ each year. The average U.K. resident consumes twice as much as the average global citizen. The average U.S. resident consumes twice as much as the average U.K. resident. On average, U.S. residents consume four times as much energy per person as the typical resident of China or India.

If everyone on Earth consumed energy at British rates, global energy demands would double. If the world’s population consumed at U.S. rates, energy demand would quadruple. Population growth compounds the pressure. By 2050, between nine and eleven billion people will inhabit Earth. With an increasing population and increasing resource demand as more people consume more energy per person, it is possible to imagine a moment at which Earth’s atmosphere, oceans, and resources reach their capacity to provide resources or to take society’s waste products, much of which is from the energy system.

Image Credits: Pradit.Ph/Shutterstock.com; ESB Professional/Shutterstock.com; xuanhuongho/Shutterstock.com; Nuk2013/Shutterstock.com.

Fossil Fuels

Conventional fossil fuels include coal, natural gas, and liquid petroleum accumulated in Earth’s crust. The geologic time scale over which fossil fuels form—hundreds of millions of years—lies outside human time horizons, so these resources are considered for practical purposes to be finite and exhaustible. Synthetic forms of petroleum and natural gas can also be fabricated. For example, synthetic gasoline or synthetic diesel can be made from coal or natural gas through processes known as coal-to-liquids and gas-to-liquids. In addition, the anaerobic decomposition of organic matter, such as food waste or manure, from agricultural operations produces renewable natural gas (RNG), also called biogas.

Fossil fuels include coal, natural gas, and liquid petroleum.

Unconventional fossil fuels include nonliquid forms of petroleum, such as oil shale, shale oil, oil sands, tar sands, and heavy oils. Unconventional forms of natural gas include shale gas and coalbed methane. Oil shale is a form of solid kerogen rock found in Utah and Colorado that releases energy when burned. Shale oil, also known as tight oil, is the liquid produced from impermeable shales.

Fossil fuels gained a significant market share in the 1860s with the Second Industrial Revolution. Despite the diversity of fuel options, fossil fuels—coal, petroleum, and natural gas—remain the dominant primary energy sources today and still provide approximately 85% of the world’s energy.

How Coal Was Formed

Millions of years ago, dead plant matter fell into swampy water, and over time, a thick layer of dead plants lay decaying at the bottom of the swamps. Over time, the surface and the climate of the Earth changed, and more water and dirt washed in, halting the decay process and forming peat.

The weight of the top layers of water and dirt packed down the lower layers of plant matter. Under heat and pressure, this plant matter underwent chemical and physical changes, pushing out oxygen and leaving rich hydrocarbon deposits. What once had been plants gradually turned into coal.

Coal can be found deep underground (as shown in this graphic), or it can be found near the surface.

How Petroleum Was Formed

Tiny sea plants and animals died and were buried on the ocean floor. Over time, they were covered by layers of sediment and rock.

Over millions of years, the remains were buried deeper and deeper. The enormous pressure turned them into oil and gas.

Today, we drill down through the layers of sedimentary rock to reach the rock formations that contain the oil and gas deposits.

In addition to their role as fuels in combustion, coal, petroleum, and natural gas serve as feedstocks for manufacturing materials. This role is similar to how wood can serve as fuel in the form of firewood or as a building material in the form of lumber and beams. Plastic, pesticides, pharmaceuticals, cosmetics, paints, dyes, and cleaners are formed from petroleum. Fertilizers, ink, glue, and paint are formed from natural gas. Steel and iron production and the cement-making process incorporate heat and carbon from coal. Even some solid wastes from coal combustion, including bottom ash found at the bottom of coal boilers and fly ash that rises through the smokestack, can be used to make drywall for buildings and aggregate for roads.

Fossil fuels are in nearly everything we use in our modern life, from plastics and pigments to common building materials. Tap items to learn more.