Green or Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, wave power, flowing water (hydropower), biological processes such as anaerobic digestion, and geothermal heat flow.
The most common definition is that renewable energy is from an energy resource that is replaced by a natural process at a rate that is equal to or faster than the rate at which that resource is being consumed. Renewable energy is a subset of sustainable energy.
Most renewable forms of energy, other than geothermal and tidal power, ultimately derive from solar energy. Energy from biomass derives from plant material produced by photosynthesis using the power of the sun. Wind energy derives from winds, which are generated by the sun’s uneven heating of the atmosphere. Hydropower depends on rain which again depends on sunlight’s power to evaporate water.
Even fossil fuels derive from solar energy, as fossil fuel originates from plant material. However, while theoretically renewable on a very long time-scale, fossil fuels are exploited at rates that may deplete these resources in the near future, and are therefore not considered renewable.
Renewable energy resources may be used directly, or used to create other more convenient forms of energy. Examples of direct use are solar ovens, geothermal heating, and water- and windmills. Examples of indirect use which require energy harvesting are electricity generation through wind turbines or photovoltaic cells (PV cells), or production of fuels such as biogas from anaerobic digestion or ethanol from biomass.
Renewable energy development is concerned with the use of renewable energy sources by humans. Modern interest in renewable energy development is linked to concerns about exhaustion of fossil fuels and environmental, social and political risks of extensive use of fossil fuels and nuclear energy.
Fossil fuels are nonrenewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. In contrast, renewable energy resources—such as wind and solar energy—are constantly replenished and will never run out.
- Solar – Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy, can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses.
- Wind – The sun’s heat also drives the winds, whose energy is captured with wind turbines. The Earth’s rotation also contributes to the winds, particularly through the Coriolis effect.
- Biomass – Along with the rain and snow, sunlight causes plants to grow. The organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is called biomass energy.
- Geothermal – Not all renewable energy resources come from the sun. Geothermal energy taps the Earth’s internal heat for a variety of uses, including electric power production and the heating and cooling of buildings.
- Hydrogen – Hydrogen can be found in many organic compounds, as well as water. It’s the most abundant element on the Earth. Because energy is always needed to produce hydrogen, it is not an energy source, but a way to store and transport energy, so it is referred to as an energy carrier.
- Yellow floating wave energy point absorber measuring marine hydrokinetics.
- Ocean – The ocean can produce thermal energy from the sun’s heat and mechanical energy from the tides and waves.
- Hydropower – Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower.