Renewable energy refers to the source of energy that replenishes itself quickly and can be tapped continually without depleting it. It’s generally less environmentally-damaging than nonrenewable energy sources such as fossil fuels.
Examples include wind, solar (photovoltaic), hydroelectric and biomass. The latter refers to burning organic matter such as plants.
1. Wind
Wind energy takes advantage of the kinetic energy in moving air to turn turbines, which generate electricity. This renewable energy source is clean and does not produce greenhouse gases or other climate change-causing pollutants.
It is one of the fastest growing green energy technologies and a key component of a sustainable future. Wind farms are typically built on a large scale, and the turbines are grouped together to provide a more robust energy resource.
Wind power can also be used domestically, at the level of individual homes and small businesses. It is a great choice for consumers who want to reduce their carbon footprint while helping support local economies and communities. Wind plants are nearly completely reversible and can be dismantled and recycled into the materials for new plants at the end of their useful lives.
2. Solar
Solar energy is an endlessly renewable resource that is considered the cleanest of all power sources, as it doesn't emit greenhouse gases when burned. Solar energy also doesn't disturb ecosystems or pollute our air and water supplies.
Solar power is most often harvested using photovoltaic solar panels, which use semiconductor materials arranged edge-to-edge to convert sunlight into electricity. These panels are found on the roofs of homes and businesses around the world, as well as in large-scale solar farms.
Alternatively, concentrating solar-thermal power (CSP) plants utilize mirrors to focus sunlight on a receiver that then heats a fluid to generate electricity. Both processes are rapidly becoming competitive with traditional fossil fuels due to innovation and economies of scale. Solar energy is a reliable source of clean, green energy, but it's not always available – especially during cloudy days and the wintertime.
3. Hydro
Up until 2019, hydropower generated the most electricity from renewables in the United States. Hydropower is based on the Earth’s natural water cycle. Sunlight heats the surface of lakes, rivers and oceans, creating water vapor that eventually falls as rain or snow. This water then flows back into the lakes, rivers and oceans, where the cycle begins again.
While most people think of huge dams that block a river, hydropower plants can be much smaller. Those that use diversions, run-of-river facilities or pumped storage technology operate like giant batteries that can store energy for periods of low demand.
During periods of high demand, these "water batteries" release the water that powers turbines and generate electricity. These plants also provide water for drinking, irrigation and other needs. Hydropower is one of the most reliable, cost-effective and clean sources of renewable energy.
4. Geothermal
Geothermal Energy takes advantage of the Earth’s heat. This heat is used to produce electricity and provides a consistent, reliable source of energy that is not affected by weather or season.
The steam that is drawn from geothermal reservoirs drives turbines to generate electricity. These power plants are most common in the United States, Indonesia, and Iceland. The water that is not used for electricity can be reinjected back into the reservoir. This is called a “closed loop” system.
However, this energy resource also can have its drawbacks. Extraction and reinjection of water can cause land subsidence and may trigger minor earthquakes. In addition, the influx of hot water can disrupt surface and groundwater resources. Geothermal systems can also emit harmful gases including hydrogen sulfide and carbon dioxide. These pollutants can impact vegetation, wildlife, soils and historic buildings/monuments.
5. Biomass
Biomass energy is the process of turning organic matter into electricity using plants and trees. It’s not exactly glamorous but it is a renewable source of the energy and materials we need to survive, including fuel for cars and homes. The sources of biomass include wood from forests or purpose-grown energy crops. It can also be animal waste, like poop (which is used to make methane that can power a turbine).
The fastest-growing biomass energy plants are grasses and other plants with fast growth cycles such as switchgrass, miscanthus, bamboo and sweet sorghum. The use of these types of biomass helps reduce our dependence on fossil fuels and can, in some circumstances, help reduce greenhouse gas emissions. It also diverts material that would otherwise be sent to landfills, which can emit greenhouse gases as they decompose.
6. Ocean
The oceans cover 70% of the world’s surface and contain vast amounts of energy. They absorb the sun’s heat, creating a thermal gradient between warm surface water and cold deep ocean waters. This provides a potential source of power through ocean thermal energy conversion (OTEC).
Waves and tides also generate energy from the gravitational pull of the moon and sun, which creates tidal power. The rise and fall of ocean waves can provide a steady source of electricity, but technology still needs to be developed to capture this energy.
Biomass is another renewable source of energy that can be used to generate electricity through steam turbines. It is derived from organic matter such as dedicated crops, waste wood and trees, agricultural crop residues and marine algae. It can be converted into a fuel such as ethanol.
7. Geothermal
Geothermal energy draws upon the constant flow of heat from deep within Earth to generate electricity. It’s a renewable source that doesn’t produce greenhouse gases and has the lowest environmental footprint of any other green energy.
Humans have been using geothermal energy for thousands of years by accessing the energy from lava flows, hot springs, and fumaroles. Now technology has made it possible to harness that heat for power generation.
To extract geothermal energy, a well is drilled and steam from the underground reservoir is used to rotate an electric generator. The steam can also be condensed into a liquid and returned to the well for reheating. Geothermal power plants can run around the clock and have a low maintenance requirement. They also operate at full capacity non-stop – unlike some renewable technologies that only work intermittently.
8. Biofuels
Biofuels are renewable fuels made from organic material, usually plants. They can be used in place of petroleum diesel or gasoline, reducing greenhouse gas emissions and helping to break the dependence on fossil fuels. Biofuels are also carbon-neutral, meaning that the emissions they produce are offset by gases absorbed during their growth and synthesis.
The most common liquid biofuel is biomass ethanol, which is blended with conventional gasoline at various percentages. Liquid biofuels can also be produced from algae, cyanobacteria, and other microalgae. These are sometimes referred to as second and third-generation biofuels. Other types of biofuels include wood biomass, sustainable aviation fuel and renewable naphtha. These are primarily used for space heating, but may play a larger role in hard-to-electrify sectors like shipping and aviation. The US and Europe have set targets for blending biofuels into their energy mix.
9. Geothermal Biogas
Beaver County, Utah is sunnier than 88% of the nation’s counties, so it’s not a surprise that it produces about 620,000 megawatt hours (mwh) of electricity a year.
In geothermal power plants, hot water or steam is extracted from the earth’s core and used to generate electricity. The steam is then condensed into liquid and sent back down the well for reheating or reinjection.
Biogas is another renewable energy source that provides heat and electricity for off-grid applications such as villages, islands, tourism facilities, industrial, military, and residential use. Diesel, petrol or gasoline engines, turbines, microturbines, and Stirling engines offer feasible options for converting biogas to electricity as prme movers. Hydrogen and transport fuels can also be produced using biogas. Unlike renewable wind and solar power, which can be affected by weather, geothermal energy is stable and dispatchable.
10. Biofuels
Biofuels are fuels made from renewable organic material derived from living organisms and are often used to replace fossil fuels. Popular types include ethanol and biodiesel. They produce fewer types of greenhouse gases when burned than traditional gasoline and diesel. Biofuels are categorized by generation, with first-generation being those made from edible food crops such as corn and sugar cane; second-generation being made from inedible plant materials; and third-generation involving algae and other microorganisms.
Ethanol is produced by fermenting the starches in plants like corn or wheat and is blended with gasoline to power engines. Biodiesel is made from combining ethanol with animal fat, recycled cooking oil or vegetable oils. Other sources of biofuels include wood and straw, waste from municipal solid waste and crop residues such as corn stover, rice straw and sugarcane bagasse.
