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.

Global Energy Flows and Reservoirs

The earth and its atmosphere have flows and reservoirs of energy sources that are converted into useful energy services such as heating and motion. The resources, shown in the bubbles, represents the different types of energy that have been accumulated (shown in ZJ, or zetajoules) over millenia on the Earth’s surface, its crust, or the oceans. The lines and bars represent the fluxes (shown in TW, or terawatts), andthe arrows show the portion humans use. Each resource has a different abundance, availability andaccessibility. For example, deuterium is very abundant, but is difficult to harness through nuclear reactions to generate useful energy. The most important flux of energy is the incoming solar radiation, which is about 10,000 times greater than the combined global rate of human energy consumption. However, much of that solar radiation is reflected back to space or consumed through the evaporation of water from oceans. That evaporated water is held in the atmosphere, which can be harnessed with hydroelectric power plants as it falls back to earth by gravity. Another significant portion of solar radiation drives photosynthesis, which converts solar energy into chemical energy stored in plants. Those plants can be used for bioenergy or are converted through geological processes into fossil fuels such as coal, petroleum and natural gas. 

This figure is based on the one prepared by Wes Hermann and A.J. Simon, Global Climate and Energy Project, Stanford University, 2007. The underlying details are from Weston A. Hermann, “Quantifying global exergy resources,” Energy 31 (2006) 1685—1702

Image Credits: Es sarawuth/

Primary Resources and Secondary Energy

Earth is endowed with a significant amount of primary energy forms and water resources. Some of those primary energy resources, such as fossil fuels and minerals (e.g., uranium), deplete over time as they are extracted. Some of them, such as renewable energy resources, replenish over time. In contrast, the volume of water in the world neither decreases nor increases. The amount of water in the atmosphere, earth and oceans is essentially fixed. However, the water availability and purity differ greatly over time and place because of usage patterns and the hydrologic cycle that moves water around the planet. Thus, in this important way, energy and water differ.

Primary energy resources are those found in nature. Secondary energy resources are those forms that must be produced by conversion of primary resources. There are only a few different original sources for primary fuels: Earth, the Moon, and the Sun. Earth provides radioactive materials used to harness nuclear energy and geothermal resources that can be used for heating and& cooling. The moon’s 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

Electromagnetic radiation in the form of light from the sun can become electricity directly through conversion within photovoltaic (PV) panels. Also, mirrors that focus solar beams concentrate the sun’s energy to heat water, molten salt, or other fluids for other conversions. However, solar energy indirectly creates many other forms of energy. For example, cycles of solar heating and cooling of the continents andoceans create the temperature differences that drive wind. Wind drives waves, making wave energy a twice-removed form of solar energy. Also, the sun powers the global hydrologic cycle by evaporating water from the oceans, which raises water into the atmosphere. Hence, energy embedded in flowing water, such as power derived from hydroelectric dams or river currents are also indirect forms of solar energy. Natural thermal differences in the ocean between the warm surface and cooler depths (Ocean Thermal EnergyConversion or OTEC), and differences in salinity also originate with the sun.

Photosynthesis relies on sunshine, which allows for plants to grow as feed, food, fuel, feedstock, and fiber. These forms of bioenergy represent solar energy stored over time as chemical energy in the bonds of plant materials. Crops represent solar energy stored for months, while old-growth forests represent solar energy stored for decades or centuries. Fossil fuels formed from old biomaterials thereby represent solar energy stored for hundreds of millions of years.

Primary Resource or Secondary Energy?

It’s not always obvious if a fuel is a primary resource or secondary energy. Sometimes, the same thing may be primary or secondary, depending on its source. For instance, the heat from a geothermal vent could be considered a primary resource, but the heat made from burning fossil fuels is secondary energy.

Primary energy resources are those found in nature. They include the fossil fuels (petroleum, natural gas, and coal), uranium, blowing wind and flowing water, biomass, and the radiant energy of the sun.

Secondary energy comes from transforming primary energy, and includes things like gasoline and liquid fuels, refined biofuels, electricity, hydrogen, and heat.

Entropy: The Second Law of Thermodynamics

The second law of thermodynamics describes changes to entropy (or disorder) for a system. The law arises from the empirical observations of increasing disorder and the conclusion that processes have a direction. For example, leaves move from an ordered state (neatly attached to the tree) to a disordered state (scattered about the ground). Many have seen leaves falling, but none have witnessed fallen leaves reattaching themselves to the tree.

A burning kitchen match releases about 1 Btu of energy. 

Similarly, heat flows from a higher to a lower temperature. For example, a cup of hot herbal tea cools when left alone in a house at room temperature as heat from the tea flows to the room around it. The tea cools while the room warms very slightly.

The second law of thermodynamics manifests as inefficiencies, losses, and waste streams during energy conversion, such 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. The United States consumes about 105 Exajoules (EJ)  (about 100 quads or quadrillion British thermal units (Btu)) of energy each year. More than half of that energy—about 62 EJ (59 quads)—enters the atmosphere as waste heat from smokestacks and tailpipes or the hydrosphere as warmed water from power plants. Other waste streams include food waste, municipal solid waste, agricultural waste (manure), wastewater, and flue gases. Successfully harnessing wasted energy would be a valuable step for solving global energy problems. 

We can measure efficiency by comparing the energy output of a system to energy inputs. Highly efficient systems convert more than 90% of the incoming energy into useful output energy, though in a different form. Some anthropogenic processes, such as electricity generators and boilers, are very efficient, while others such as steam turbines and incandescent lightbulbs are not. Many resilient and robust natural processes are not very efficient. For example, photosynthesis converts on average less than 1% of energy in photons from the sun into chemical energy stored in a plant’s biomaterial.

Types of Energy

Energy—the ability to do work—can be divided into six different forms.

Mechanical Energy (m)

Mechanical energy is the energy associated with motion. The amount of energy depends on the mass and velocity of the relevant system or component. Objects that have greater mass or motion at a higher speed have higher kinetic (or mechanical energy). It also includes the potential for motion, such as with gravitational potential energy (for example from elevated water) or elastic potential energy (for example from a coiled spring).

Thermal (Heat) Energy (t)

Thermal energy (or heat) is a measure of kinetic energy at the molecular level. Heat and temperature are proxy measurements for the motion of molecules. One way we feel as heat is when energytransfers to our bodies from molecules colliding against us. Molecules move faster in hot weather, colliding against us with higher velocity, which feels warm. In many systems, thermal energy is held in working fluids such as steam, refrigerants, molten salts, or specialty oils. In power plants and heating districts, thermal energy is typically carried in steam. In cooling districts or refrigeration systems, thermal energy is carried in chilled water or refrigerants, such as R-142a or ammonia.

Electrical Energy (e)

Electrical energy (or electricity) is ubiquitously familiar to the developed world as a common form of secondary energy in the built environment. This form of energy is carried in currents and driven by voltages across loads (or resistances).

Radiant Energy (r)

Radiant energy includes those forms of energy that travel in waves. Examples include electromagnetic radiation, such as light and magnetic forces. Thus, light waves carry energy, which is a simple explanation for how sunlight can cause burns. Acoustic waves also carry energy. Microphones detect this energy to make an audio recording. Conversely, very powerful weapons destroy buildings using intense acoustic waves.

Chemical (Fuels) Energy (c)

Chemical energy is the energy stored in the chemical bonds of molecules. During combustion, fuels burn and the chemical bonds break, releasing heat. The energy content of fuels depends on the composition, masses, and other properties of the molecules. Chemical energy is responsible for more than 85% of the energy consumption around the world in the form of fossil fuels (coal, petroleum, and natural gas) and bioenergy (wood, alcohols, straw, and cow dung).

Atomic (Nuclear) Energy (a)

Atomic energy is the energy stored in the nucleus of atoms. During reactions, atomic bonds break, releasing heat. Einstein’s famous equation relates mass and the energy contained within: E = mc2. For this equation, E is energy, m is the mass lost during a conversion, and c is the speed of light. In other words, because the speed of light is so large (light moves at a rate of 30 billion centimeters per second), small changes in mass yield tremendous energy. During atomic reactions, minuscule amounts of mass disappear, releasing significant amounts of energy.

The First Law of Thermodynamics

The first law of thermodynamics is one of the fundamental governing laws of the physical universe and comprises three separate but related concepts:

Conservation of energy grounds all processes in the natural world. A system can never produce more energy than it consumes.

The total amount of energy in a closed system remains constant. It can only be changed from one form to another or transferred from one body to another. Another way to consider this concept is that the best outcome in a closed system is to break even; there will never be more energy at the end than at the beginning.

The second implication of the law states that energy exists in different forms. The different forms of energy manifest in nature, such as the chemical energy stored in the bonds of molecules of fuels such as petroleum, coal, wood, or natural gas. Sometimes energy exists in a more tangible form, such as directed radiant energy—a laser beam or lighting from incandescent bulbs—or the 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).

The third implication of the first law provides the most use because energy can be converted between its different forms. For example, forms convenient for storage, such as the chemical energy in a cord of firewood, become useful thermal energy emanating from a fireplace that warms a house. Intentional and thoughtful transformation of energy from one form to another enables many aspects of developed society: physical mobility, climate control, and refrigeration. And as noted in an earlier section, this intentional transformation distinguishes humans from other species.

Image Credits: Oleksiy Mark/

The Steam Powered Revolution

The relationship between energy and water has been important since the dawn of the Industrial Revolution. One of the most important early applications of steam engines was pumping water from coal mines. As miners removed coal near the surface first, mines extended deeper into productive coal seams. In deep mines, water would pool at the bottom of the pit, interfering with the labor of the miners. Pumping water from mines requires tremendous effort, so mining companies used steam engines for the heavy lifting. In this scenario, energy in the form of coal fueled a steam engine using water vapor to provide mechanical energy to pump liquid water so operations could mine more coal.

Statue of James Watt, engineer and inventor, 1736–1819.

Energy is Wealth

One can measure energy across a variety of different scales. For example, power is the rate of energy production or the rate of energy consumption, which is measured in watts (W). Overall U.S. power consumption totals around 3.34 × 1012 W, or 3.34 terawatts (TW), compared to 17.5 TW globally. About 10,000 times more energy shines on Earth every day than is needed to power all of humanity.

As earlier established, more affluent countries tend to use more energy. A photographic map of the world at night reveals this relationship clearly. Lights shine in the night where there is a robust infrastructure for electricity, and the map of wealth lines up closely with the map of electricity. The Nile River Valley stands out in the darkness of the surrounding arid landscape, as the Eastern Seaboard of the United States appears bright between the Atlantic Ocean and rural Appalachia. North Korea, which is poor and authoritarian, is dark in contrast with South Korea, which is wealthy and free. Because energy is so cross-cutting, energy options that simultaneously encourage a robust economy and protect the environment without undermining national security are preferred.

The Earth at night. Everywhere there are lights, there is wealth.

The solutions to our problems are both technical and cultural, and they are key to a sustainable future. Each energy technology and option has benefits and risks. Thus, as we evaluate options for fuels and technologies for the future, we must keep tradeoffs and technology trends in mind.

Image Credits: SPF/; NASA Earth Observatory/NOAA NGDC.

Math of Pumping Water

The energy that is needed to raise water by overcoming potential energy is essentially the reverse of how hydroelectric dams extract the potential energy of falling water:

where ρ is the density of the water (1,000 kilograms per cubic meter (kg/m3)), V is the volume of water raised, g is the acceleration due to Earth’s gravity (9.8 meters per second squared (m/s2)), and h is the net or cumulative change in height (in m). Raising a 10-liter (2.75-gallon) bucket of water through a distance of about 100 meters (330 feet) requires approximately 10,000 joules (10 British thermal units (Btu)) of energy. Pumping the water up only 10 m (33 feet), for example by raising the water from a river to the top of a nearby riverbank, requires 1,000 joules (1 Btu) of energy.

For continuous flows, it is more useful to look at the power required for pumping, which is the energy required over time:

For this equation, the term P is the change in potential energy per unit time (or the pumping power needed), ηp is the efficiency of the pump (approximately 80% or 0.8), and Q is the volumetric flow rate (m3/s). The other terms are the same as the prior equation. Pumping 100 liters per second—enough water for 1,900 average Americans—out of an aquifer 100 meters below the ground requires a pump with about 100 kW of pumping power. Keep in mind that a typical house needs 1 to 3 kW of power on average to run the whole place, so a pump that size consumes the same power as approximately 30 to 100 homes.

Pumps must also overcome friction within pipelines. The water industry terms the losses from friction “head losses,” which is analogous to the additional “head” or elevation that must be overcome for pumping. These losses can be described for turbulent flow by the Darcy-Weisbach equation:

where hf is the head loss due to friction (in meters, m), f is the unitless friction factor specific to physical parameters of the pipe, ΔL is the length of pipe through which the water is piped (m), v is the velocity of the fluid on average (m/s), and D is the diameter of the inside of the pipe. The gravitational constant g is the same as before.

Image Credits: benjamas154/