They are: “all those energies derived from a natural process, which are renewed on a human time scale”. Among these energy sources are: hydropower, solar, wind, geothermal, oceanic and biomass, which exist due to the energy of solar radiation, the gravitational attraction of the moon, sun and heat of the Earth.
About solar photovoltaics
Solar photovoltaic (PV) systems directly convert solar energy into electricity.
The basic building block of a PV system is the PV cell, which is a semiconductor device that converts solar energy into direct-current electricity. PV cells are interconnected to form a PV module, typically up to 50 to 200 Watts. The PV modules, combined with a set of additional application-dependent system components (e.g. inverters, batteries, electrical components, and mounting systems), form a PV system. PV systems are highly modular; i.e. modules can be linked together to provide power ranging from a few watts to hundreds of megawatts.
Biomass is any organic, i.e. decomposable, matter derived from plants or animals available on a renewable basis. Biomass includes wood and agricultural crops, herbaceous and woody energy crops, municipal organic wastes as well as manure.
Bioenergy is energy derived from the conversion of biomass where biomass may be used directly as fuel, or processed into liquids and gases.
Traditional biomass use refers to the use of wood, charcoal, agricultural resides and animal dung for cooking and heating in the residential sector. It tends to have very low conversion efficiency (10% to 20%) and often unsustainable supply.
Wind energy is kinetic energy of wind exploited for electricity generation in wind turbines.
Wind energy, like other power technologies based on renewable resources, is widely available throughout the world and can contribute to reduced energy import dependence. As it entails no fuel price risk or constraints, it also improves security of supply. Wind power will enhance energy diversity (unless it is already the dominant source) and hedges against price volatility of fossil fuels, thus stabilising costs of electricity generation in the long term. Wind power entails no direct greenhouse gas (GHG) emissions and does not emit other pollutants (such as oxides of sulphur and nitrogen); additionally, it consumes no water. As local air pollution and extensive use of fresh water for cooling of thermal power plants are becoming serious concerns in hot or dry regions, these benefits of wind become increasingly important.
Hydropower derives energy from turbines being spun by fresh flowing water. This can be from rivers or from man-made installations, where water flows from a high-level reservoir down through a tunnel and away from a dam.
Geothermal energy can provide low-carbon base-load power, heat (and cooling) from high-temperature hydrothermal resources, deep aquifer systems with low and medium temperatures, and hot rock resources.
About ocean power
Five different ocean energy technologies under development aim to extract energy from the oceans.
Tidal power: the potential energy associated with tides can be harnessed by building a barrage or other forms of construction across an estuary.
Tidal (marine) currents: the kinetic energy associated with tidal (marine) currents can be harnessed using modular systems.
Wave power: the kinetic and potential energy associated with ocean waves can be harnessed by a range of technologies under development.
Temperature gradients: the temperature gradient between the sea surface and deep water can be harnessed using different ocean thermal energy conversion (OTEC) processes.
Salinity gradients: at the mouth of rivers, where freshwater mixes with saltwater, energy associated with the salinity gradient can be harnessed using the pressure-retarded reverse osmosis process and associated conversion technologies.