As the term implieas, they are sources that can be renewed and never run out! Non-renewable sources include fossil fuels, e.g. oil, coal, gas, and their deposits are limited and can be exhausted. Renewable energy sources include solar, wind, biomass, hydro, geothermal and ocean power.
Solar Energy
With this term we refer to the energy provided by the sun through radiation. The sun is the most powerful source of energy and this energy is free, as long as we exploit it! Technologies are categorized as either active or passive. Active technologies convert solar energy into a form we can use directly either electrical or thermal. Such are photovoltaic cells that convert sunlight directly into electrical energy, like the picture over right, the solar collectors for domestic hot water heating or even solar space heating and cooling, the solar concentrators that use mirrors to focus solar irradiation and generate intense heat, turning water to steam and generating electricity using certain machines and even solar ovens. Passive technologies seek to place buildings in a favorable orientation towards the sun or use special materials and architectural designs to exploit solar energy.
Wind Energy
The energy of wind has been known and exploited for millenniums, either in windmills or in sailing. Nowadays, we can exploit wind energy using special blades to capture wind and machines to transform it to electrical energy. Wind turbines are installed both onshore and offshore in places where wind speed is generally high and constant.
Biomass
Biomass has been used since man invented fire and used to burn wood to heat or cook! It refers to the energy embodied in plants and organic material. Plant biomass comes from the sun through the photosynthesis process, when they capture solar energy and on the other hand animals eat plants! Biomass includes a wide variety of materials including wood, energy crops (grown specifically for energy applications), agricultural and forest residues, food waste and organic components from municipal and industrial waste. A variety of conversion technologies exist to convert biomass energy into other usable form. Such technologies either converts the energy in forms that can be used directly (heat or electricity), or may convert it to another form, such as liquid biofuel or combustible biogas.
Hydropower
Water also, is a renewable energy source since it is recharged through the cycle of evaporation and precipitation. Its power was known since ancient years and was exploited through dams, water mills and irrigation systems. The energy of the falling or moving water can be harnessed by various technologies. Water wheels can transform it directly into mechanical energy (i.e. water mills), turbines and electrical generators can transform it into electricity.
Geothermal Energy
Geothermal energy refers to the energy stored and created inside the earth in the form of thermal energy. At times this energy is released to the surface through volcanoes and geysers, or is available constantly e.g. through hot springs. Geothermal energy can be harnessed to produce electricity or for heating and cooling purposes. It can be acquired from deep underground reservoirs through drilling, or from other geothermal reservoirs closer to the surface. Geothermal energy can be used in residential applications also, e.g. small geothermal heat pumps.
Ocean or Marine Energy
This is a renewable energy that is underexploited so far. It refers to the mechanical energy carried by ocean waves and tides or to the thermal energy of the ocean coming from the sun. Covering almost 70% of earth surface, oceans may prove to be the renewable energy of the future, however harnessing ocean energy to produce electricity is not cost-effective currently.
