Primary energy sources are those found in nature that can be used to either directly power end uses, such as cars, or to create secondary energy sources, such as electricity. The main primary energy sources are as follows:

1. Fossil fuels - coal, oil, natural gas. Acid rain, smog, and smoke befouled the air of industrialized countries and large cities for more than a century. This visibly dirty air was the result of burning the carbon-based fossil fuels: oil for transportation fuel and coal for energy production. Much of this pollution has been remedied in the US, by legislation that requires energy companies to clean or sequester the waste products of energy production. However, the most important current problem with the use of fossil fuels is the over-production of carbon dioxide, the invisible "greenhouse gas" most closely associated with global warming.

The importance of fossil fuels, petroleum, coal and natural gas, in modern civilization presents the world with a dilemma.

On one hand, we have come to rely on large, consistent, instantly available supplies of fossil fuels for almost all our cars, trucks, trains, boats and airplanes. Many homes use oil or natural gas for heating, and we use fossil-based chemicals to produce many of the products of our daily life, from our home furnishings to the clothes on our backs. Most of our electricity is generated by the burning of coal or natural gas. In addition, economic growth in countries around the world creates increasing demand for more and more of the energy so readily available from the plentiful supply of fossil fuels available on our planet.

At the same time, we have an increasing understanding that extremely harmful global climate change could result from our protracted, massive use of such fuels. Additionally, we know that all fossil fuels are finite as resources. They will eventually have to be replaced by other sources of energy, possibly at considerably higher cost than we are paying today. Due to the deeply destructive, profoundly negative impact fossil fuels have on our global environment, there is a growing movement to switch our energy sources to fuels that do not emit greenhouse gases.

Given our current predicament, how do we successfully transition to cleaner, sustainable fuels?

2. Nuclear fission. Of all our current power generators, nuclear reactors produce the greatest amount of energy per unit of fuel weight. They do this by splitting the atoms of specially processed uranium, releasing large amounts of energy. Under the right conditions the splitting process can become a self-sustaining chain reaction, allowing a continuous flow of energy and power. The process is efficient, low in cost and emits no air pollution or carbon dioxide.

However, nuclear reactors are expensive to build, and the fission process results in highly radioactive waste products that must be isolated from human exposure in safe and secure storage conditions for many hundreds of years. Nuclear produces a substantial portion of our power, but no new plants have been built in over 20 years due to public fears of environmental and security dangers. It is possible that improved technology and increasing energy needs may result in the construction of more nuclear power plants. However, transportation and long-term storage of used fuel rods have yet to be fully addressed.

3. Renewables - hydroelectric, biomass, wind, geothermal and solar. In 2017 these power sources accounted for only about 13% of our energy supply. However, they are the most hoped-for solution to our growing need for clean energy.

Hydroelectric is the leading source of renewable power. A hydroelectric station is usually built by damming a river and erecting turbines, which are run by falling water. The energy produced is cheap, clean and constant. However, dams are costly to build, may cause flooding and can disrupt ecological plant and animal systems.

Biomass is now used mostly for the transportation fuels ethanol and biodiesel. Biomass fuels, (i.e., fast-growing "energy" crops, agricultural wastes and methane from landfills) may be burned, fermented or "digested" by bacteria to provide an inexpensive, relatively clean power source.

Wind has increased substantially as a source of renewable power since 2007. It works by using the wind to turn tall propeller-like turbines, whose spinning hubs connect to generators which convert mechanical to electrical energy. Wind is relatively cheap, very clean, and a wind farm takes only about 2 years to build. It's drawbacks are that it is intermittent as a power source, can kill birds, and the turbines can be annoyingly noisy.

Geothermal power stations tap into reservoirs of deep underground water heated by surrounding hotbeds of rock and volcanic magma. Steam from that water is what ultimately powers geothermal generators to produce renewable energy. Small geothermal systems can heat and cool buildings efficiently, though at considerable installation cost.

Solar power currently comes in two forms: concentrated solar power (CSP) and photovoltaic (PV).

Concentrated solar power (CSP) is a method of capturing solar energy which concentrates the sun's heat on an absorbent surface. The heated surface then transfers the heat to water. The resulting steam drives a turbine linked to an electric generator. CSP plants employ large arrays of mirrors, that tilt with the movement of the sun to focus solar rays on an absorbing surface. That surface, which can be in a variety of forms, contains a fluid that transfers the heat to the power plant's boilers. For small applications like heating and cooling buildings or heating water, flat heat-absorbent panels pass heat to tubes of fluid which transfer it to an insulated water tank. One of the world's largest solar plants, built in the 1980's in the Mohave Desert of California, uses the CSP method.

Photovoltaic (PV) cells, when exposed to the radiant energy of the sun, produce an electrical voltage. At a photovoltaic power station, multiple panels of silicon-based PV cells collect electromagnetic energy directly from the sun and turn it into direct current (DC) electricity. That direct current is usually converted to alternating current (AC) and immediately transmitted to end users.

Secondary energy sources are sources that have been created using primary energy sources. The principal secondary energy source is electricity, which is usually produced using either coal, natural gas, solar, wind, hydroelectric or nuclear power. Another secondary energy source is hydrogen, which can be efficiently burned to provide energy in numerous circumstances.