Renewable energy

DEFINITION: Energy derived from natural, unlimited, and replenishable sources

The burning of fossil fuels such as coal, natural gas, and petroleum releases emissions that contain greenhouse gases and cause air pollution and acid rain, whereas most forms of renewable energy are nonpolluting. In addition, because the earth has a finite supply of fossil fuels, the development of renewable energy sources is important to the long-term future of humankind.

The environmental movement and the oil crises of the 1970s led to interest in the development of energy sources that would offer alternatives to the use of fossil fuels. Fossil fuels are limited resources, and the burning of fossil fuels to generate energy creates emissions of carbon dioxide, toxic chemicals, and air pollutants that harm the environment and human health. Because renewable, or clean, energy systems use natural, local sources that are inexhaustible and such systems have fewer negative impacts on human life and the environment, governments have provided increasing support for the development of renewable energy technologies. By the early 2020s, renewable energy made up 13 percent of total energy consumption in the US, according to data from the US Energy Information Administration.

Biomass

The oldest renewable energy source is biomass, which is organic animal and plant material and waste. Biomass resources include grass crops, trees, and agricultural, municipal, and forestry wastes. Since the discovery of fire, humans have burned biomass to release its chemical energy as heat. For example, wood has been burned to cook food and to provide heat. Biomass energy has also been used to make steam and electricity. Biomass oils can be chemically converted into liquid fuels or biodiesel, a transportation fuel. Ethanol, another transportation fuel, comes from fermented corn or sugarcane. Crops such as willow trees and switchgrass are also cultivated for biomass energy generation.

Biomass energy has many environmental benefits when compared with fossil-fuel energy. It contributes little to air pollution, as it releases 90 percent less carbon dioxide than do fossil fuels. Energy crops, such as prairie grasses, require fewer pesticides and fertilizers than do high-yield food crops such as wheat, soybeans, and corn, so they cause less water pollution. Energy crops also add nutrients to the soil. About 5 percent of the energy used in the United States came from biomass energy in the early 2020s. This represented about 60 percent of the total renewable energy consumed in the US.

Solar Energy

One of the most promising and popular kinds of renewable energy is solar energy, which uses radiant energy produced by the sun. Solar energy was used as early as the seventh century BCE, when a magnifying glass was used to concentrate sunlight to light fires. In 1767, Swiss scientist Horace-Bénédict de Saussure invented the first solar collector, a device for storing the sun’s radiation and converting it into a usable form, such as by heating water to create steam. In 1891, American inventor Clarence Kemp patented the first commercial solar water heater.

Sunlight can be converted directly into electricity at the atomic level by photovoltaic (PV) cells, also called solar cells. The photovoltaic phenomenon was first noted in the eighteenth century and became more practical with the use of silicon for the cells in the twentieth century. The cells are joined together in panels, often connected in an array. They can be placed on rooftops and connected to a grid. In the twenty-first century, solar cells are used worldwide in home and commercial electrical systems, satellites, and various consumer products.

Solar energy has numerous environmental benefits. Photovoltaics produce electricity without gaseous or liquid fuel combustion or hazardous waste by-products. Decentralized PV systems can be used to provide electricity for rural populations, saving more expensive conventional energy for industrial, commercial, and urban needs. By providing electricity for remote and rural areas, solar energy also reduces the use of disposable lead-acid cell batteries, which can contaminate water and soil if they are not disposed of properly. The use of solar energy in rural areas also reduces air pollution by decreasing the use of diesel generators and kerosene lamps.

Since solar energy depends on sunlight, the efficiency and performance of solar energy systems are affected by weather conditions and location. Nevertheless, the amount of inexhaustible solar energy that could be generated around the globe exceeds the amount needed to meet the world’s energy requirements. It has been estimated that if PV systems were installed in only 4 percent of the world’s deserts, they could supply enough electricity for the entire world. As solar technologies improve and costs decrease, solar energy has the potential to be the leading alternative energy source of the future. In 2023, the amount of electricity generated from solar in the US had increased to about 238 billion KWh, accounting for 11 percent of the total renewable energy used in the country. Globally, China and the US were the largest producers of solar energy, accounting for approximately one-half of the world's total solar electricity generation.

Wind Energy

One of the fastest-growing types of renewable energy, wind energy has been used by humans for centuries. Windmills appear in Persian drawings from 500 CE, and they are known to have been used throughout the Middle East and China. The English and the French built windmills during the twelfth century, and windmills were indispensable for pumping underground water in the western and Great Plains regions of the United States during the nineteenth and early twentieth centuries. These windmills converted wind into mechanical power.

The modern windmills used to convert wind energy into electricity are called wind turbines or wind generators. In 1890 Poul la Cour, a Danish inventor, built the first wind turbine to generate electricity. Another Dane, Johannes Juul, built the world’s first alternating current (AC) wind turbine in 1957. In the twenty-first century, large wind plants are connected to local electric utility transmission networks to relieve congestion in existing systems and to increase reliability for consumers. Wind energy is also used on a smaller scale by homeowners in what is known as distributed energy; home-based wind turbines, with batteries as backup, can lower electricity bills by up to 90 percent.

The use of wind energy has long-term environmental benefits. Unlike nuclear and fossil-fuel electricity generation plants, wind generation of electricity does not consume fuel, cause acid rain and greenhouses gases, or require waste cleanups. For example, it was estimated that Cape Wind, America’s first offshore wind farm proposal in Nantucket Sound, would reduce greenhouse gas emissions by 734,000 tons annually; however, that project never came to fruition after it received backlash from Indigenous groups and wealthy homeowners who complained about their views being disrupted.

In 2010, the annual wind energy generating capacity of the United States was more than 35,000 megawatts, enough electricity to power 9.7 million homes. This amount of electricity generated by fossil-fuel-burning plants would have released some 62 million tons of carbon dioxide; avoiding the release of so much carbon dioxide is the equivalent of keeping 10.5 million cars off the roads. By the early 2020s, wind electricity made up over 10 percent of the total electricity generation in the US, increasing to 434 billion kWh in 2022. Wind energy technology has advanced to the point that wind power is affordable and can compete successfully with fossil fuels and other conventional energy generation.

The main disadvantage of wind energy is that it is intermittent because wind velocities are inconsistent even in areas of strong winds. In addition, some environmentalists have objected to the establishment of wind farms because of their potential to harm wildlife and the aesthetic damage they do to natural landscapes.

Hydropower

Long before electricity was harnessed, in about 4000 BCE, ancient civilizations used hydropower, or energy from moving or flowing water, in the waterwheel, the first device employed by humans to produce mechanical energy as a substitute for animal and human labor. Running water in a stream or river moves the wooden paddles mounted around a waterwheel, and the resulting rotation in the shaft drives machinery. The earliest waterwheels were used to grind grain, and the technology went on to be used worldwide for that purpose, as well as to supply drinking water, irrigate crops, drive pumps, and power sawmills and textile mills.

In the nineteenth century, the water turbine replaced the waterwheel in mills, but then the steam engine replaced the turbine in mills. The hydraulic turbine reemerged, however, to power electric generators in the world’s first hydroelectric power stations during the 1880s. By the early twentieth century, 40 percent of the US electricity supply was hydroelectric power. Modern large hydropower plants are attached to dams or reservoirs that store the water for turning the turbines and are connected to electrical grids or substations that transmit the electricity to consumers.

Hydropower was the leading renewable energy source for generating electricity in the US, but its numbers decreased during the second half of the twentieth century, as electricity generation from other renewable sources grew. It has both negative and beneficial effects on the environment. Building dams and reservoirs changes the environment and can harm native habitats and their fish, animal, and plant life. In addition, reservoirs sometimes emit methane, a greenhouse gas. Water is a natural and inexpensive energy source, however. No fuel combustion takes place in the generation of hydropower, so the process does not pollute the air, and energy storage is clean. Hydroelectric power accounted for about 17 percent of electricity used worldwide and 6.2 percent of electricity in the United States in the early 2020s. Hydroelectric energy capacity grew by approximately 3.3 percent per year throughout the twenty-first century, although widespread drought in 2015 slowed growth in this area.

Geothermal Energy

Geothermal energy comes from heat produced deep inside the earth. Deep wells and pumps bring underground hot water and steam to the earth’s surface to heat buildings and generate electricity. Some geothermal energy sources come to the surface naturally, including hot springs, geysers, and volcanoes. The ancient Chinese, Native Americans, and Romans used hot mineral-rich springs for bathing, heating, and cooking. Food dehydration became the major industrial use of this form of energy. In 1904, the first electricity from geothermal energy was generated in Larderello, Italy.

Although not as popular a renewable energy source as wind or solar energy, geothermal energy has significant advantages and benefits for the environment. Because the earth’s heat and temperatures are basically constant, geothermal energy is reliable and inexhaustible; it is also not affected by changes in climate or weather. It is very cost-efficient as well; heat pumps can be operated at relatively low cost. The steam and water used in geothermal systems are recycled back into the earth.

Geothermal plants are environmentally friendly. Because they do not burn fuel to generate electricity, they release little or no carbon dioxide and other harmful compounds. Geothermal plants produce no noise pollution and have minimal visual impacts on the surrounding environment because they do not occupy large surface areas.

The US Environmental Protection Agency and the Department of Energy support the use of geothermal heat pumps. The American Recovery and Reinvestment Act of 2009 (ARRA) provided for grants and tax incentives worth $400 million to the industry, which added 144 geothermal energy plants in fourteen states at the beginning of 2010. Indeed, ARRA measures benefited the development of all renewable energy sources; it included a Treasury Department grant program for renewable energy developers, increased funding for research and development, and a three-year extension of the production tax credit for many renewable energy facilities. By the end of 2014, the United States had an installed geothermal energy capacity of about 3.5 GW. A decade later, geothermal power plants were found in seven states in the US, all of which were located in the western half of the country. Furthermore, energy from geothermal plants was produced in two dozen countries around the world, generating 92 billion kWh of electricity from geothermal energy.

As the threats from climate change continued to grow in the early twenty-first century, increased emphasis was placed on renewable energy. According to the International Energy Agency (IEA), about 3,700 GW of new capacity in renewable sources was planned during the five-year period from 2023 to 2028, to be supported by over 130 countries. The IEA has further predicted that by 2025, renewable energy will surpass coal to become the largest source of electricity generation. To reach such milestones, the World Bank, the World Solar Decade, and the World Solar Summit designated $2 billion for projects focused on renewable energy resources and the environment. Likewise, many federal, state, and local governments offered incentives to citizens to invest in and use renewable energy sources.

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