Sources of energy in the Earth System

Energy sources in the Earth system

What comes to mind when someone asks you to name an energy resource? Perhaps you think about the many kinds of fuel, materials that burn or react to produce heat. Alternatively, you may think of windmills and hydroelectric dams, or arrays of solar panels, because they are appearing on the landscape with increasing frequency. Where does the “energy” (the capacity to do work) in these energy resources originally come from? Let’s consider the underlying sources: 

• Energy directly from the Sun: Solar energy, resulting from nuclear fusion reactions in the Sun, bathes the Earth’s surface. It may be converted directly to electricity, or it may be used to heat water.
• Energy directly from gravity: The gravitational attraction of the Moon, and to a lesser extent, the Sun, helps cause ocean tides, the daily up-and-down movement of the sea surface. The flow of water during tidal changes can drive turbines. 
• Energy involving both solar energy and gravity: Solar radiation heats the air, which becomes less dense. In a gravitational field, this warm air rises, while cool, denser air sinks. The resulting air movement, wind, powers sails and windmills. Solar energy also evaporates water, which enters the atmosphere. When the water condenses, it rains on the land, where it accumulates in streams that flow downhill in response to gravity. This moving water can drive waterwheels and turbines.
• Energy via photosynthesis: Algae and green plants absorb some of the solar energy that reaches the Earth’s surface. Their green colour comes from a pigment called chlorophyll. With the aid of chlorophyll, plants produce sugar through a chemical reaction called photosynthesis. Plants use the sugar to manufacture more complex organic chemicals. Burning plant matter releases potential energy stored in the chemical bonds of organic chemicals. During burning, the molecules react with oxygen and break apart to produce carbon dioxide, water, and carbon (soot). People have burned plant material (biomass) to produce energy for centuries. More recently, plant material has been used to produce ethanol, a flammable alcohol. 

While a portion of our energy comes from recently living biomass (wood, sugar cane, etc.) even more comes from the remains of organisms that lived either by carrying out photosynthesis or by eating algae or plants, and were then buried and preserved in sediment after they died. Because the energy stored in these substances was trapped by photosynthesis long ago, and has been preserved in rock over geologic time, we refer to these materials as fossil fuels. 

• Energy from chemical reactions: A number of inorganic chemicals can burn to produce light and energy. A dynamite  explosion is an extreme example of such energy production. Recently, researchers have been studying electrochemical devices, such as hydrogen fuel cells, that produce electricity directly from chemical reactions. 
• Energy from nuclear fission: Atoms of radioactive elements can split into smaller pieces, a process called nuclear fission. During fission, a tiny amount of mass transforms into a large amount of energy, called nuclear energy. This type of energy runs nuclear power plants and nuclear submarines. 
• Energy from Earth’s internal heat: Some of Earth’s internal energy dates from the birth of the planet, while some comes from radioactive decay in minerals. This internal energy heats underground water. The resulting hot water, when transformed to steam, provides geothermal energy, which can drive turbines. 

Sources of energy used by people. The proportion of sources changed through time, but the total quantity used increases. Industrialization in Asia is now driving growth.

During the course of civilization, the sources of energy that people use have changed (figure above). Prior to the industrial age, direct burning of wood and other biomass provided most of humanity’s energy. But by the second half of the 19th century, deforestation had nearly destroyed this resource, and energy needs had increased so dramatically that other fuels came into use. 

Figures credited to Stephen Marshak.

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