A renewable energy source can be defined as an energy source that is continually replenished, is available over the long term at a reasonable cost that can be used with minimum environmental impacts, produces minimum secondary wastes, and is sustainable based on current and future economic and social needs. This definition of renewable energy resources includes many forms such as wind energy, solar energy, biofuels, geothermal energy, and ocean wave energy.
It is natural to believe that human civilization is not prepared to make sacrifices in the quality of life and inhibit energy consumption - driven growth due to the decline in finite fossil-fuel-based energy resources. Therefore, humans who have already come this far are smart enough to realize that renewable energy is the alternative to finite fossil energy sources. In addition to this, there are many encouraging points for the development and use of renewable energy sources like diversity in energy supply options, both for developed and developing nations.
Except in the case of geothermal energy, the sun is the primary source of all renewable energy, and currently the total energy generating capacity of all energy conversion systems built by mankind amounts to about 14 TW (terawatt). In comparison to this, the solar input is extremely large, and the continuous solar input is equivalent to 90000 TW, of which about 1000 TW could in principle be captured for energy conversion to forms we can use . Of course, there are significant losses due to poor conversion efficiencies and land use constraints that need to be taken into account, but even so, there should be sufficient raw energy from the sun to meet our needs many times over. The challenge is development of efficient green technologies. Energy scenarios are widely used to describe possible paths ahead and the sustainable growth scenario produced by Shell International in 1995 has been very influential. It suggested that, by around 2060, renewables sources could meet about half of the world's total energy needs. Subsequent studies have suggested that in principle, by 2100, renewables could perhaps meet over 80% of global energy needs, assuming that they were seen as a priority for environmental reasons. Inevitably, long-term projections like this are very speculative. In 2012, modern renewables supplied around 8.2% of the world's energy, which included about 3.3% provided by hydropower electricity. The contribution is expanding rapidly, stimulated by some quite demanding targets. For example, the European Union aims to have 12.5% of its electricity produced from renewable sources by 2020, with some member countries aiming for even higher targets. Denmark aims for 29%, Finland 21.7%, Portugal 21.5% and Austria 21.1%, and these figures exclude the contribution from large hydropower plants .