What Are Renewable Energy Resources?

Renewable energy resources are naturally occurring energy sources that replenish on timescales comparable to human use, making them effectively inexhaustible for practical planning purposes. They include solar radiation, wind, flowing water, geothermal heat, and biomass, and they stand in contrast to fossil fuels, which accumulate over geological timescales and are depleted by consumption. The U.S. Energy Information Administration classifies renewable energy by five primary types and tracks their share of national electricity generation and end-use consumption. Harnessing these resources requires engineering disciplines spanning photovoltaics, power electronics, fluid mechanics, and electrical grid design, reflecting the breadth of technology needed to convert diverse natural energy flows into usable electricity and heat.

The share of renewable energy in global electricity generation has grown substantially since 2000, driven by cost reductions in solar photovoltaic and wind technologies, regulatory mandates, and decarbonization commitments. Integration of large quantities of variable renewable generation into power grids requires corresponding advances in energy storage, transmission infrastructure, and grid management software.

Solar and Wind Energy

Solar photovoltaic (PV) systems convert solar radiation directly into electricity through the photoelectric effect in semiconductor cells, typically silicon. Utility-scale PV installations now represent the lowest cost source of new electricity generation in many regions. Concentrated solar power (CSP) uses mirrors or lenses to focus sunlight onto a heat transfer fluid that drives a conventional turbine cycle, and it can incorporate thermal storage to provide dispatchable output after sunset. Wind turbines convert the kinetic energy of moving air into mechanical rotation through aerodynamic lift on the rotor blades, then into electricity through a generator. Both onshore and offshore wind installations have grown rapidly, with offshore arrays benefiting from stronger and more consistent wind resources at the expense of higher installation and maintenance costs. The International Energy Agency's renewable energy tracking system reports on global capacity additions and generation trends for solar and wind.

Hydropower and Geothermal Energy

Hydropower generates electricity by directing the flow of water through turbines, accounting for the largest share of renewable electricity generation worldwide. Conventional hydropower relies on dams that create reservoirs to regulate water flow and store energy, while run-of-river plants exploit natural stream gradient without large-scale water storage. Pumped-storage hydropower, in which water is pumped from a lower to an upper reservoir using surplus electricity, represents the dominant form of large-scale grid energy storage globally. Geothermal energy extracts heat from the Earth's interior, either directly from shallow ground for building heating via heat pumps or from deep hydrothermal reservoirs to generate electricity in steam-cycle power plants. Enhanced geothermal systems (EGS) extend geothermal power to regions lacking natural hydrothermal resources by hydraulically fracturing hot dry rock formations to create permeable heat exchangers.

Biomass and Ocean Energy

Biomass energy uses organic materials, including dedicated energy crops, agricultural residues, forestry waste, and municipal solid waste, as fuel for combustion, gasification, or biological conversion to liquid biofuels. Combustion biomass plants can be co-fired with coal to reduce net carbon emissions from existing thermal generation facilities. Biogas, produced by anaerobic digestion of organic waste, provides a renewable substitute for natural gas in heating and power generation. Ocean energy technologies, including tidal stream turbines, wave energy converters, and ocean thermal energy conversion (OTEC) systems, are at earlier stages of commercial deployment but represent large resource bases in coastal regions. The breadth of these technologies and the grid integration challenges they present are examined in USGS remote sensing of renewable resource assessment applications, among other survey resources.

Applications

Renewable energy resources have applications across a wide range of sectors, including:

• Utility-scale electricity generation through solar, wind, and hydropower installations
• Distributed residential and commercial power through rooftop photovoltaics
• Grid energy storage using pumped hydro and combined renewable-plus-storage systems
• Building and district heating through geothermal heat pumps and biomass boilers
• Transportation fuel through cellulosic ethanol, biodiesel, and renewable hydrogen
• Remote and off-grid power systems in locations without access to transmission infrastructure