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Primary energy is any energy that exists naturally and has not been modified into another form by human beings. Examples of primary energy resources include both renewable resources, such as wind and solar energy, and non-renewable resources, such as fossil fuels. When primary energy resources are processed in a power plant or other facility, they are transformed into “carriers,” or secondary energy sources. These include fuel and electrical energy. Primary energy is a pertinent topic for statistical analysis of energy usage because it determines the supply of energy available for human activities.
As of 2011, the most widely used primary energy sources worldwide were fossil fuels such as oil, coal, and natural gas. Fossil fuels produce a large amount of energy proportional to their unit weight, making them currently the most efficient choice for energy production. Problems with fossil fuels include their environmental impact — the burning of these materials releases a large amount of carbon dioxide into the atmosphere, which cannot readily be absorbed — and the fact that they are non-renewable. Substances like coal and oil are produced over millions of years from the decomposition of biological organisms. Human society draws on fossil fuels at a much greater rate than they can be renewed, making eventual shortages inevitable.
Alternative energy sources have been increasingly explored as options as the supply of fossil fuels declines. Nuclear reactors generate power from uranium, a primary energy resource found naturally in the earth. Although nuclear plants can generate large amounts of power from small amounts of uranium, nuclear power still only accounts for a small percentage of worldwide energy generation due to environmental and safety hazards as well as political factors. Uranium also technically constitutes a non-renewable resource, as it is not replenished in the earth at a high enough rate to counteract human usage.
Solar, wind, and hydroelectric power are examples of technologies that draw on renewable primary energy sources — energy sources that are replenished constantly by natural processes. Biomass is another example. Energy from the sun is sequestered in plants through photosynthesis, the biochemical process by which plants convert sunlight to energy. When combusted, plants release this energy. Biofuels, solid or liquid fuels refined from plants, are secondary energy products that make use of this principle.
As primary energy sources such as fossil fuels become more difficult to harvest due to decreasing supply, a ratio known as energy return on investment (EROI) comes into play. EROI is the ratio of the amount of energy gained from a primary energy source to the amount of energy expended to obtain it. If it requires more energy to obtain a resource than can be gained from it, then it is no longer economically viable to pursue that resource, since energy will be lost in the process. Although EROI has been criticized for oversimplifying the complex process of harvesting energy, it remains a factor for industries and governments in determining how to economize energy usage and invest in various primary energy sources.