What is Renewable Energy?

Forms of Renewable Energy Renewable 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, renewable energy technologies do necessarily compete with each other purely based on price. Here is a 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 [$/kW]	< 800 (PV)			1,000				huge range
Levelised cost of energy [$/kWh]	0.06 and less			0.07
Capacity Factor	< 25%			25 - 40%	25%		35 - 50%
Power depends on	~ annual radiation		crops	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.

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.