Renewable Energy Sources
Sun light, wind, gravity and natural processes are providing the base for renewable energies. By definition, renewable energy is all energy transformed of renewable or ultimately available sources.
The means of transformation are considered to be at least greenhouse gas neutral.
Technologies
The most commonly know technologies to transfer energy from natural or renewable sources into electricity or usable heat or alternative fuel are:

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Photovoltaic (PV)
The direct transformation of light into electricity is called Photovoltaic (PV). Photovoltaic technology is the most scalable renewable energy technology existing. Systems with 100 Wp for simple home applications, as well as utility scale solar power solutions with 100 MWp are installed in all regions of the world. Over the last few years, PV had been the fastest growing source of renewable energy and has already reached peak energy parity in certain areas of the world.
Advantage:        scalability, low maintenance and operation cost
Disadvantage:   non-continuous, low conversion efficiency
Residential Solar Thermal
Heating water, under floor heating and air conditioning are well known applications of solar thermal systems. Specifically for residential use, millions of residential solar thermal systems are installed world wide on houses, schools, small and medium size businesses as well as hospitals and other communal facilities. The sun is captured by flat panels or heat pipe systems to warm-up water or an intermediate transfer fluid, which will than transport the heat to air conditioning or heating systems.
Advantage:        simple technique, scalable
Disadvantage:   none
Wind Energy Converter (WEC)
Onshore wind is a mature technology that can provide low cost energy production in many parts of the world. Wind power is the best case example of successful government subsidies supporting a technology until it has become competitive with traditional energy forms. In Germany many wind park developers are beginning to opt out of the feed-in tariff whilst in the US many wind farms would be economical without the production tax credit.
Advantage:        gird-parity, mature, reliable
Disadvantage:   environmental impact, unpredictable generation
Concentrated Solar Power (CSP)
Solar thermal power plants absorb and capture heat generated from solar radiation and convert this into electricity. Concentrating solar power focuses large areas of sunlight into a narrow beam utilizing lenses or mirrors combined with sun tracking systems. This captured heat energy is fed into a steam cycle. As with conventional power plants, the generated steam is used in a turbine to produce electricity.
Advantage:        almost continuous generation of electricity
Disadvantage:   high maintenance cost, not scalable
Geothermal Energy
Geothermal power plants are capable of producing continuous power and are unique in the space in that they are base load power plants. Geothermal is considered a clean source of energy and in many locations it is a very cost competitive. While the market is relatively small in terms of percentage to world energy mix, with 10,000 MW of installed capacity it is one of the most established renewable energy sources.
Advantage:        constant generation of electricity
Disadvantage:   high initial cost, low conversion efficiency
Hydropower
Hydro power is the world’s most used renewable energy resource, about 20% of total installed capacity worldwide. Hydro power is comprised of three main technologies, dams, run-of-river and pumped storage. While dams are required to maintain certain flows, they as well as pumped storage facilities can regulate output to meet demand. This compares to run of river projects which do not have storage capacity.
Advantage:        on demand generation of electricity
Disadvantage:   huge environmental impact, high investments
Ocean Energy
Tidal and Wave Power are still in its infancies at the moment although research on the technology started around 40 years ago. Nevertheless it constitutes a promising technology to meet the world’s rising demand in renewable energies. Only some years ago, government backings could be secured which increased research process significantly. Currently, worldwide significant research projects are going on, resulting in very different and competing shapes and modes of operations.
Advantage:        easy scalable, predictable generation
Disadvantage:   non-mature technology, high maintenance cost
Biomass Energy
Biomass, a renewable energy source, uses biological material, such as wood, waste, (hydrogen) gas, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. The most conventional way in which biomass is used, however, still relies on direct incineration. Forest residues, for example (such as dead trees, branches and tree stumps), yard clippings, wood chips and garbage are often used for this. Biomass may also include biodegradable wastes that can be burnt as fuel.
Advantage:        continuous generation of electricity
Disadvantage:   partly competition food vs. energy, maintenance intense