What is renewable energy?

Unlike fossil fuels, which are finite, renewable energy sources regenerate. There are five commonly used renewable energy sources:
• Biomass—includes:
? Wood and wood waste
? Municipal solid waste
? Landfill gas and biogas
? Ethanol
? Biodiesel
• Hydropower
• Geothermal
• Wind
• Solar
Biomass—renewable energy from plants and animals

Biomass is organic material that comes from plants and animals, and it is a renewable source of energy.
Biomass contains stored energy from the sun. Plants absorb the sun’s energy in a process called photosynthesis. When biomass is burned, the chemical energy in biomass is released as heat. Biomass can be burned directly or converted to liquid biofuels or biogas that can be burned as fuels. Examples of biomass and their uses for energy:
• wood and wood processing wastes—burned to heat buildings, to produce process heat in industry, and to generate electricity
• agricultural crops and waste materials—burned as a fuel or converted to liquid biofuels
• food, yard, and wood waste in garbage—burned to generate electricity in power plants or converted to biogas in landfills
• animal manure and human sewage—converted to biogas, which can be burned as a fuel
Converting biomass to other forms of energy
Burning is only one way to release the energy in biomass. Biomass can be converted to other useable forms of energy such as methane gas or transportation fuels such as ethanol and biodiesel.
Methane gas is a component of landfill gas or biogas that forms when garbage, agricultural waste, and human waste decompose in landfills or in special containers called digesters.
Crops such as corn and sugar cane are fermented to produce fuel ethanol for use in vehicles. Biodiesel, another transportation fuel, is produced from vegetable oils and animal fats.
How much biomass is used for fuel?
Biomass fuels provided about 5% of the primary energy used in the United States in 2016. Of that 5%, about 48% was from biofuels (mainly ethanol), 41% was from wood and wood-derived biomass, and about 11% was from the biomass in municipal waste. Researchers are trying to develop ways to use more biomass for fuel.
Biomass—Wood and wood waste
People have used wood for cooking, for heat, and for light for thousands of years. Wood was the main source of energy for the world until the mid-1800s. Wood continues to be an important fuel in many countries, especially for cooking and heating in developing countries.
Hybrid poplar wood chips being unloaded in Crookston, Minnesota

Source: National Renewable Energy Laboratory, U.S. Department of Energy (public domain)
In 2016, about 2% of total U.S. annual energy consumption was from wood and wood waste (bark, sawdust, wood chips, wood scrap, and paper mill residues).
Using wood and wood waste:
Industry, electric power producers, and commercial businesses use most of the wood and wood waste fuel consumed in the United States. The wood and paper products industry uses wood waste to produce steam and electricity, which saves money because it reduces the amount of other fuels and electricity that must be purchased. Some coal-burning power plants burn wood chips to reduce sulfur dioxide emissions.
About 19% of total U.S. wood energy consumption in 2016 was by the residential sector, and wood energy accounted for about 2% of total residential energy consumption.
Wood is used in homes throughout the United States for heating as cord wood in fireplaces and wood-burning appliances and as pellets in pellet stoves. In 2015, about 11.5 million U.S. households used wood as an energy source, mainly for space heating, and 2.3 million of those households used wood as the main heating fuel.
Energy from municipal solid waste

Municipal solid waste (MSW), often called garbage, is used to produce energy at waste-to-energy plants and at landfills in the United States. MSW contains
• biomass, or biogenic (plant or animal products), materials such as paper, cardboard, food waste, grass clippings, leaves, wood, leather products
• nonbiomass combustible materials such as plastics and other synthetic materials made from petroleum
• noncombustible materials such as glass and metals

In 2014, about 258 million tons of MSW were generated in the United States, of which
• 53% was landfilled
• 35% was recycled and composted
• 13% was burned for energy
Waste-to-energy plants make steam and electricity
MSW is usually burned at special waste-to-energy plants that use the heat from the fire to make steam for generating electricity or to heat buildings. In 2015, 71 waste-to-energy power plants and four other power plants burned MSW in the United States. These plants burned about 29 million tons of MSW in 2015 and generated nearly 14 billion kilowatthours of electricity. The biomass materials in the MSW that were burned in these power plants accounted for about 64% of the weight of the MSW and contributed about 51% of the energy. The remainder of the MSW was nonbiomass combustible material, mainly plastics. Many large landfills also generate electricity by using the methane gas that is produced from decomposing biomass in landfills.
Waste-to-energy is a waste management option
Producing electricity is only one reason to burn MSW. Burning waste also reduces the amount of material that would probably be buried in landfills. Burning MSW reduces the volume of waste by about 87%.
Collecting and using biogas from landfills
Landfills for municipal solid waste can be a source of energy. Anaerobic bacteria—bacteria that can live without the presence of free oxygen—living in landfills decompose organic waste to produce a gas called biogas. Biogas contains methane. Methane is the same energy-rich gas found in natural gas, which is used for heating, cooking, and producing electricity.
Landfill biogas can be dangerous to people and the environment because methane is flammable, and it is a strong greenhouse gas. In the United States, regulations under the Clean Air Act require landfills of a certain size to install and operate a landfill gas collection and control system.

Source: Adapted from National Energy Education Project (public domain)

Some landfills control the methane gas emissions simply by burning or flaring methane gas. Methane gas can also be used as an energy source. Many landfills collect biogas, treat it, and then sell the methane. Some landfills use the methane gas to generate electricity.
Using biogas from animal waste
Some farmers produce biogas in large tanks called digesters where they put manure and used bedding material from their barns. Some farmers cover their manure ponds (also called lagoons) to capture biogas. Biogas digesters and manure ponds contain the same anaerobic bacteria found in landfills. The methane in the biogas can be used for heating and for generating electricity on the farm.
What is ethanol?
Ethanol is an alcohol fuel made from the sugars found in grains such as corn, sorghum, and barley.
United States Department of Agriculture (USDA) research geneticists study switchgrass as a source of ethanol.

Other sources of sugars to produce ethanol include:
• Sugar cane
• Sugar beets
• Potato skins
• Rice
• Yard clippings
• Tree bark
• Switchgrass

Most of the fuel ethanol used in the United States is distilled from corn. Scientists are working on ways to make ethanol from all parts of plants and trees rather than just grain. Farmers are experimenting with fast-growing woody crops such as small poplar and willow trees and switchgrass to see if they can be used to produce ethanol.
Ethanol is blended with gasoline
A biodiesel and standard gasoline pump

Nearly all of the gasoline now sold in the United States is about 10% ethanol by volume. Any gasoline-powered engine in the United States can use E10 (gasoline with 10% ethanol), but only specific types of vehicles can use mixtures with fuel containing more than 10% ethanol. A flexible-fuel vehicle can use gasoline with ethanol content greater than 10%. The U.S. Environmental Protection Agency ruled in October 2010 that cars and light trucks of model year 2007 and newer can use E15 (gasoline with 15% ethanol). E85, a fuel that contains 51%–83% ethanol, depending on location and season, is mainly sold in the Midwest and can only be used in a flexible-fuel vehicle.
What is biodiesel?
Biodiesel is a fuel made from vegetable oils, fats, or greases—such as recycled restaurant grease. Biodiesel fuel can be used in diesel engines without changing the engine. Pure biodiesel is non-toxic, biodegradable, and produces lower levels of most air pollutants than petroleum-based diesel fuel. Biodiesel is usually sold as a blend of biodiesel and petroleum-based diesel fuel. A common blend of diesel fuel is B20, which is 20% biodiesel.
Energy from moving water

Source: Adapted from National Energy Education Development Project (public domain)

Source: Tennessee Valley Authority (public domain)

Hydropower generates electricity
Hydropower is the largest renewable energy source for electricity generation in the United States. In 2016, hydropower accounted for about 6.5% of total U.S. utility-scale electricity generation and 44% of total utility-scale electricity generation from all renewable energy.
Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source.
Hydropower relies on the water cycle
Understanding the water cycle is important to understanding hydropower. The water cycle has three steps:
• Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate.
• Water vapor condenses into clouds and falls as precipitation (rain, snow, etc.).
• Precipitation collects in streams and rivers, which empty into oceans and lakes, where it evaporates and begins the cycle again.

The amount of precipitation that drains into rivers and streams in a geographic area determines the amount of water available for producing hydropower. Seasonal variations in precipitation and long-term changes in precipitation patterns, such as droughts, have a big impact on hydropower production.
Hydroelectric power is produced from moving water
The volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water. Swiftly flowing water in a big river, like the Columbia River that forms the border between Oregon and Washington, carries a great deal of energy in its flow. Water descending rapidly from a high point, like Niagara Falls in New York, also has substantial energy in its flow.
At both Niagara Falls and the Columbia River, water flows through a pipe, or penstock, then pushes against and turns blades in a turbine to spin a generator to produce electricity. In a run-of-the-river system, the force of the current applies pressure on a turbine. In a storage system, water accumulates in reservoirs created by dams and is released as needed to generate electricity.
History of hydropower
Hydropower is one of the oldest sources of energy for producing mechanical and electrical energy. Hydropower was used thousands of years ago to turn paddle wheels to help grind grain. Before steam power and then electricity were available in the United States, grain and lumber mills were powered directly with hydropower. The first industrial use of hydropower to generate electricity in the United States occurred in 1880, when 16 brush-arc lamps were powered using a water turbine at the Wolverine Chair Factory in Grand Rapids, Michigan.
The first U.S. hydroelectric power plant opened on the Fox River near Appleton, Wisconsin, on September 30, 1882.
What is geothermal energy?
The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is heat within the earth. People can use this heat as steam or as hot water to heat buildings or to generate electricity.
Geothermal energy is a renewable energy source because heat is continuously produced inside the earth.
Geothermal energy comes from deep inside the earth

Source: Adapted from a National Energy Education Development Project graphic (public domain)
The slow decay of radioactive particles in the earth’s core, a process that happens in all rocks, produces geothermal energy. The earth’s core is hotter than the sun’s surface.
The earth has a number of different layers:
• The inner core is solid iron and is surrounded by an outer core of hot molten rock called magma.
• The mantle surrounds the core and is about 1,800 miles thick. The mantle is made up of magma and rock.
• The crust is the outermost layer of the earth. The crust forms the continents and ocean floors. The crust can be 3 to 5 miles thick under the oceans and 15 to 35 miles thick on the continents.

The earth’s crust is broken into pieces called tectonic plates. Magma comes close to the earth’s surface near the edges of these plates, which is where many volcanoes occur. The lava that erupts from volcanoes is partly magma. Rocks and water absorb heat from magma deep underground. The rocks and water found deeper underground have the highest temperatures.
People around the world use geothermal energy to heat their homes and to produce electricity by drilling deep wells and pumping the hot underground water or steam to the surface. People can also use the stable temperatures near the surface of the earth to heat and cool buildings.
Energy from moving air
How uneven heating of water and land causes wind

Source: Adapted from National Energy Education Development Project (public domain)
Wind is caused by uneven heating of the earth’s surface by the sun. Because the earth’s surface is made up of different types of land and water, it absorbs the sun’s heat at different rates. One example of this uneven heating is the daily wind cycle.
The daily wind cycle
During the day, air above the land heats up faster than air over water. Warm air over land expands and rises, and heavier, cooler air rushes in to take its place, creating wind. At night, the winds are reversed because air cools more rapidly over land than it does over water.
In the same way, the atmospheric winds that circle the earth are created because the land near the earth’s equator is hotter than the land near the North Pole and the South Pole.
Wind energy for electricity generation
Today, wind energy is mainly used to generate electricity. Water pumping windmills were once used throughout the United States and some still operate on farms and ranches, mainly to supply water for livestock
Energy from the sun
The sun has produced energy for billions of years and is the ultimate source for all of the energy sources and fuels that we use today. People have used the sun’s rays (solar radiation) for thousands of years for warmth and to dry meat, fruit, and grains. Over time, people developed devices (technologies) to collect solar energy for heat and to convert it into electricity.
Radiant energy from the sun has powered life on earth for many millions of years.

Source: NASA
Collecting and using solar thermal (heat) energy
An example of an early solar energy collection device is the solar oven (a box for collecting and absorbing sunlight). In the 1830s, British astronomer John Herschel used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into useful heat energy for a variety of purposes.
We use solar thermal energy systems to heat
• water for use in homes, buildings, or swimming pools
• the inside of homes, greenhouses, and other buildings
• fluids to high temperatures in solar thermal power plants
Solar photovoltaic systems convert sunlight into electricity
Solar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes.
Solar energy has benefits and some limitations
The two main benefits of using solar energy:
• Solar energy systems do not produce air pollutants or carbon dioxide.
• Solar energy systems on buildings have minimal effects on the environment.

The main limitations of solar energy:
• The amount of sunlight that arrives at the earth’s surface is not constant. The amount of sunlight varies depending on location, time of day, season of the year, and weather conditions.
• The amount of sunlight reaching a square foot of the earth’s surface is relatively small, so a large surface area is necessary to absorb or collect a useful amount of energy.

Environmental impacts
Renewable energy entails a number of other potential environmental impacts. On the negative side, renewable energy can make large tracts of land unusable for competing uses, disrupt marine life, bird life and flora/fauna, and produce visual and noise pollution. Generally though, these potential environmental impacts are site-specific and there are a number of ways to minimise the effects, which are usually small and reversible. There are environmental benefits from renewables other than reduction of greenhouse gas and other air emissions. For example, hydroelectric schemes can improve water supplies and facilitate reclamation of degraded land and habitat.
The use of bioenergy can have many environmental benefits if the resource is produced and used in a sustainable way. If the land from which bioenergy is produced is replanted, bioenergy is used sustainably and the carbon released will be recycled into the next generation of growing plants. The extent to which bioenergy can displace net emissions of CO2, will depend on the efficiency with which it can be produced and used. Bioenergy plants have lower emissions of SO2 than do coal and oil plants, but they may produce more particulate matter.
These emissions are controllable but they increase generating costs. The environmental and social effects of large-scale hydropower are site specific and are the subject of much controversy. Large-scale projects may disturb local ecosystems, reduce biological diversity or modify water quality. They may also cause socio-economic damage by displacing local populations. A number of projects in developing countries have been stalled or scaled down for these reasons; obtaining loans from international lending institutions and banks for major projects has become more difficult. Although these ill effects can be managed and mitigated to some degree, they may affect the future of hydropower in general.
Geothermal plant:
Geothermal plants may release gaseous emissions into the atmosphere during their operation. These gases are mainly carbon dioxide and hydrogen sulphide with traces of ammonia, hydrogen, nitrogen, methane, radon, and the volatile species of boron, arsenic and mercury.
This could slow the future development of geothermal resources. Emissions can be managed through strict regulations and by control methods used by the geothermal industry to meet these regulatory requirements.
Hydrogen sulphide abatement systems reduce environmental damage but are costly to install. Wind-power generation has very low emissions on a life cycle basis, but has a number of environmental effects that may limit its potential.
The most important effects on the environment
Visual Effects
Wind turbines must be in exposed areas and are therefore highly visible. They are considered unsightly by some people, and concerns have increased with the larger size of new generation turbines.
Wind turbines produce aerodynamic noise, from air passing over the blades and mechanical noise from the moving parts of the turbine, especially the gearbox. Better designs have reduced noise, and research continues. Wind farms developed far from highly populated areas are, by definition, less offensive.
Electromagnetic Interference
Wind turbines may scatter electromagnetic signals causing interference to communication systems. Appropriate siting (avoiding military zones or airports) can minimise this impact.
Bird Safety
Birds get killed when they collide with the rotating blades of a turbine. Migratory species are at higher risk than resident species. Siting the turbines away from migratory routes reduces the impact.