Renewability

Introduction

An energy resource is considered renewable if natural processes are constantly replenishing it. Wind, sunshine, geothermal heat, wave power, and wood are typical renewables. However, fossil fuels can also be renewable over many millions of years, and a small minority of geologists (especially in Eastern Europe) believe that geological processes are continually generating new petroleum, though not fast enough to refill oil fields as they are drawn down. Standard renewables, on the other hand, cannot be renewed perpetually because the sun itself will not last forever. For these and other reasons, some scientists prefer to define renewability in terms of the ongoing human relationship to a given energy resource.

Historical Background and Scientific Foundations

Scientists who view renewability in terms of the human relationship to an energy resource define the renewability of a resource in terms of the rate at which humans exploit it compared to the rate at which nature renews it. On this view, there are many resources that are renewable under some conditions of use, but not all. If the resource is regenerated at a higher rate than it is used, it is considered renewable, but if it is exploited at a higher rate than it is regenerated, it is considered non-renewable.

The renewability of an energy resource can change, therefore, depending on human activity. For example, wood used as fuel is currently a non-renewable resource

wherever people are logging forests more quickly than new timber is growing. However, if growth were to increase or cutting decrease, fuel wood would eventually become renewable again.

Renewability is a key parameter for evaluating energy technologies according to their environmental impact. The more renewable an energy resource is (that is, the faster it can be regenerated at the level it is being used), the less of an environmental impact it has. At present, coal is a non-renewable resource as it takes many millions of years for it to form. It is unlikely that the renewability of this resource is going to change due to coal's high level of exploitation by humans and its extremely slow rate of regeneration.

Wind electricity, however, has high renewability, as there is essentially no time lag between its use and regeneration. On these grounds alone, one would expect coal mining to have a greater environmental impact than wind generation. Exploitation of forests at rates low enough to permit renewal would likewise be expected to have less environmental impact than exploitation above renewal rates.

Impacts and Issues

Coal and wind are extremes of low renewability and high renewability. It is of most interest to examine what energy technologies have intermediate renewabilities that can be realistically increased by policy choices. Renewability might be increased by more efficient use of a resource (e.g., wood), allowing lower harvesting rates. It might also, in a technical sense, be increased by discovering how to turn a non-resource renewable material into a useful resource. Renewable energy sources presently supply about 22% of total world primary energy use, while finite, or depletable energy sources make up about 78%.

In 1999, scientists Fabio Manzini and Manuel Martinez examined the environmental impact of several energy technologies. They suggested that several presently available energies that now have low renewability may become more renewable in the future. For example, bagasse (the fibrous material remaining after raw sugar and juice is extracted from the sugarcane plant) is not currently considered a renewable energy resource—not because the world is exhausting its holdings of bagasse, but because there is no affordable way to extract energy from it.

WORDS TO KNOW

BIOFUEL: A fuel derived directly by human effort from living things, such as plants or bacteria. A biofuel can be burned or oxidized in a fuel cell to release useful energy.

DEPLETABLE ENERGY SOURCE: Energy source that diminishes as it is used: same as a non-renewable energy source (to “deplete” means to use up). All fossil fuels are depletable energy sources.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GEOTHERMAL ENERGY: Energy obtained from Earth's internal heat, which is maintained by the breakdown of radioactive elements. Geothermal means, literally, Earth-heat. Geo-thermal energy may be used either directly as heat (e.g., to heat buildings or industrial processes) or to generate electricity.

RENEWABLE ENERGY: Energy obtained from sources that are renewed at once, or fairly rapidly, by natural or managed processes that can be expected to continue indefinitely. Wind, sun, wood, crops, and waves can all be sources of renewable energy.

However, the cellulose (a structural carbohydrate of the cell walls of plants) in bagasse is now being tested for production of commercial quantities of cellulosic ethanol. Cellulosic ethanol is a type of biofuel. According to the U.S. Department of Energy, corn-based ethanol provides 26% more energy than is required for its production, while cellulosic ethanol provides 80% more energy. Therefore, as the development of biorefineries for the production of cellulosic ethanol increases, the renewability of certain biomass energy resources, such as bagasse, may increase.

See Also Biofuel Impacts; Ethanol; Renewable Energy.

BIBLIOGRAPHY

Periodicals

Manzini, Fabio, and Manuel Martinez. “Choosing an Energy Future: The Environmental Impact of End-Use Technologies.” Energy Policy 27 (1999): 401–414.

Stefansson, Valgardur. “The Renewability of Geothermal Energy.” Proceedings World Geothermal Congress (2000): 883–888.

Web Sites

“Cellulosic Ethanol: Benefits and Challenges.” U.S. Department of Energy Office of Science, June 4, 2007. <http://genomicsgtl.energy.gov/biofuels/benefits.shtml> (accessed December 3, 2007).

Michele Chapman