Clean Energy » Climate Change

What is Renewable Energy?

Forms of Renewable EnergyRenewable Energy comes from sources that do not deplete over years such as sun, wind, oceans and plants.

Renewable energy technologies convert those source of primary energy such as wind and solar to energy forms that we can readily use, including heat and electricity. There are numerous ways to convert primary energy forms into consumable forms of energy. Electricity is perhaps the most precious form of energy, as it is most versatile in usage. However, electricity cannot be stored and has to be consumed instantly. Due to the intermittent nature of many renewable sources, the issue of storing electricity is of particular importance. Surplus energy can be used to pump water up to damed mountain lakes or to re-charge batteries. Hydrogen, on the other hand, is an energy carrier, a fuel that is man-made.

Renewables in comparison

We are showcasing a number of renewable energy technologies here on the website. One question that is often asked is: Which one is the best? As always, it depends on circumstances such as geographic location, availability of space, capital costs, operational costs, and environmental concerns. Hence, one renewable energy technologies don't necessarily compete with each other purely based on price. Here is a comparison table for rough comparisons.

  Solar Photovoltaic Solar Thermal Biofuels Wind Energy Wave Energy Tidal Range Tidal Stream Hydro Geothermal
Source Sun Wind Gravitational Forces Rivers Inner Heat
Conversion Process Photovoltaic Collector Photo Synthesis Turbine Wave device Turbine / Dam Turbine Turbine  
Where? Anywhere Anywhere Anywhere Best off-shore or near coast Oceans Few locations worldwide Near shore, few locations most rivers Few locations. E.g. Iceland
Installed Base 20GW     200GW n/a   10MW    
Capital Cost [$/kW]

3,000 (PV) 5,000 (CPV)

Levelised cost of energy [$/kWh] 0.28 (CPV)                
Capacity Factor 25%     25 - 40% 25%   35 - 50%    
Power depends on ~ annual radiation     wind speed ~ v³ wave height (H²) and wave period Height (squared) Flow speed (cubic) Height (squared) Temperature
Land use: Annual energy (kWh/m²) ~128   ~30 ~200       n/a  
Power density of the source (W/m²) 1,000     300 (at 9m/s wind speed)     62,500 (at 5m/s speed)    


Three Sources of Renewable Energy

Sources of Renewable Energy

The various forms of renewable energy can be traced back to three primary sources: Solar radiation, heat from within the earth and gravitational forces between the earth, the sun, the moon and other stars.

Solar Energy

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30% of the solar radiation that could hit the earth, is reflected back into space by the earth's atmosphere.

Most of the renewable energy potential stems from solar, though some forms are indirect solar forms:
  • Hydro power: Around 1/3 of the terrestrial solar energy is consumed by the hydrological cycle of evaporation and precipitation, feeding rivers, which can drive turbines.
  • Wind & wave energy: Temperature differences on the earth's surface cause winds, and ultimately waves.
  • Bio-energy: Plants convert solar radiation into carbhydrates (photosynthesis), which can be used as "bio" - fuels.

The solar radiation that is not consumed by the indirect uses, is available for conversion in solar thermal, ocean thermal or photovoltaic devices.

Renewable energy sources that do not depend on solar radiation are:
  • Tidal Energy comes from gravitational forces between the earth, sun and moon.
  • Geothermal Energy is heat from within the earth.


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