Renewable and nonrenewable resources

Nature provides numerous energy resources. Nonrenewable resources were the primary source of energy for the twentieth century. However, with the depletion of nonrenewables, interest in renewable forms of energy has generated increasing research and development of renewables.

Background

Nonrenewable resources cannot be readily replaced after consumption. A renewable resource is one that is continuously available, such as solar energy, or one that can be replaced within several decades, such as wood.

Nonrenewable Energy Sources

Nonrenewable resources may be subdivided into four categories: metals (such as copper and aluminum), industrial minerals (such as lime and soda ash), construction materials (sand and gravel), and energy resources (coal, oil, and uranium). Of the nonfuel substances, metals are most prone to depletion by overproduction, but recycling can prolong their useful lifetime almost indefinitely.

Construction materials, although not readily recyclable, are abundant and ubiquitous in the Earth’s crust, rendering them a virtually unlimited resource. Although less plentiful, the most widely used industrial minerals are unlikely to be depleted in the near future; on the scale of centuries, however, they are an endangered resource if current levels of production are maintained. It is probable that environmental concerns will reduce future production.

The major forms of nonrenewable energy production are fossil fuel combustion (using oil and coal) and nuclear fission (using uranium). In 2022, about 79 percent of total US energy consumption came from fossil fuels, while non-fossil sources (renewables and nuclear) accounted for roughly 21 percent. When looking specifically at electricity generation in 2023, about 60 percent of US electricity came from fossil fuels, 19 percent from nuclear power, and 21 percent from renewable energy sources.

Coal was the first fossil fuel to be used extensively, and it remained the most abundant in the beginning of the twenty-first century. Coal can be burned directly or converted into petroleum or petroleum products, through the expenditure of additional energy. When used as fuel, coal creates many problems. Mines are environmentally destructive, and coal is the most difficult fossil fuel to transport. When coal is burned, vast quantities of sulfur compounds (which form sulfuric acid in the atmosphere) are released, while the carbon in the coal becomes carbon dioxide, a greenhouse gas believed to contribute to global warming. The carbon in coal also has many other valuable (nonpolluting) uses in the chemical industry.

Oil is the world’s major source of energy because it is abundant and relatively inexpensive; however, its high rate of use will result in its depletion during the twenty-first century. When burned as gasoline in cars, it releases carbon dioxide; various dangerous air pollutants, such as carbon monoxide and nitrogen oxides; and uncombusted hydrocarbons (a major cause of photochemical smog). Natural gas, formed when organic materials decompose, is usually found with petroleum reservoirs. Its supply, rate of consumption, and probable future are comparable to those of petroleum. It is widely used because it is relatively inexpensive, clean, and nonpolluting (although it does add carbon to the atmosphere).

Tar sands, principally found in Canada, are a low-grade source of petroleum that is feasible to mine and process only when oil prices are relatively high. Two additional problems limit this source: About as much energy is required to extract usable oil as is created when it is combusted, and the process has raised environmental concerns. Oil shales, abundant in the western United States, appear theoretically to be a major source of future petroleum products. The amount of oil tied up in shale exceeds the remaining total world reserve of oil. To extract oil, however, the shale must be mined and heated by processes requiring large quantities of water in regions where water is scarce. Additionally, the total energy required for extraction exceeds the energy created when the oil is burned.

Nuclear reactors produce energy through controlled fission of uranium 235. No air pollution is produced, the mining operations are relatively small and safe, and the resource being consumed has no other known use. On the other hand, reactor technology is sophisticated and elaborate, and complicated devices are prone to breakdowns. A reactor breakdown can have disastrous consequences if radioactive materials are released into the environment. Of equal or greater concern is how the by-products of nuclear power production—nuclear waste—should be disposed of over the long term.

Renewable Energy Sources

The most abundant renewable energy resource is solar energy, the source of most other renewables as well as the original source of fossil fuels. The supply is enormous and inexhaustible, but most is wasted because it occurs in a dilute form that requires expensive hardware to concentrate. Also, it reaches Earth in its most dilute form during the winter, when it is most needed for heating. In cloudy regions, it is not even available when demand for it is greatest.

Wind energy continues to represent a large and potentially inexhaustible source of power in the United States, but harnessing it requires costly turbines and favorable geographic and meteorological conditions. In 2022, wind accounted for roughly 10.2 percent of US utility-scale electricity generation, while hydropower contributed about 5.7 to 6.2 percent. Renewable sources—wind, hydro, solar, geothermal, and biomass—together supplied around 22 to 23 percent of total US electricity generation that year. Many of the most suitable sites for hydropower development have already been exploited; additional dam projects face growing ecological, regulatory, and aesthetic constraints. At the same time, wind and solar continue to expand rapidly, though their intermittency and spatial limitations mean that they cannot fully replace fossil fuels or traditional baseload power.

Tidal energy utilizes the ebb and flow of tides to create electricity by trapping seawater at the extremes of high and low tide and releasing it through turbines. Although a potentially large energy source, it is economically feasible only where there are naturally high tides—4.5 meters (14.76 feet) or more—and where a narrow inlet encloses a large bay.

Geothermal energy uses the heat from natural hot springs to create steam to power turbines, which are used to create electricity. Because the heat must be close to the surface, there are few known sites from which geothermal electrical energy can be extracted economically. Also, because pipelines must be run over many hectares to collect steam, the power-generating stations tend to be ugly and noisy.

Vegetation-based energy, or biomass, uses plant materials and plant-derived animal products as sources of fuel. This category includes wood, agricultural residues, organic waste, biofuels such as ethanol, and methane produced through anaerobic digestion. Biomass is considered renewable only when harvested and managed sustainably, since trees require time to mature and crops such as corn or sugarcane must be cultivated before conversion to fuel. Although biomass has long served as a traditional energy source, modern systems employ increasingly efficient and regulated techniques, including landfill gas recovery and combined heat-and-power generation. In 2022 biomass accounted for roughly 5 percent of total US primary energy consumption, with contributions from electricity generation and liquid biofuels.

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