What is renewable energy?

Renewable energy is the energy that is collected from renewable resources, which are naturally replenished on a human timescale, such as sunlight, wind, rain, tides, waves, and geothermal heat (1). This is the most obvious and popular understanding of the renewability understanding.  The concept of renewability was invented to highlight the difference between renewable energy sources and fossil fuels. Please note that fossil fuels once burned will be lost forever and burning leads to carbon dioxide emissions, not to mention methane emissions associated with extraction.
The graph below compares the energy content of various renewable energy sources.  The energy potential of hydropower (nascent most popular renewable energy source) is a point of reference, please note that energy generation potential of the hydropower plants is relatively small when compared to other renewables – for example, amount of solar energy available on the Earth is almost 3 000 times bigger than the hydro-energy available.
Availability of the different renewable technologies, https://www.nrel.gov/gis/re-potential.html 
The abundance of the energy source does not mean that the energy would be effectively utilized. For example, geologists were well aware of the existence of gas in shale rocks, but the extraction was technically impossible. Due to technological development, previously unreachable energy sources (shale gas) appeared to be one of the cheapest fuels, revolutionalizing energy markets and setting new rules for international politics.
There are factors determining the real potential of the energy source:
– availability – tells if the energy source physically present at the place, for example sometimes utilization of the biomass would be unfeasible due to the very high transportation costs,
– physical features like enthalpy or caloric value – for example, low-temperature geothermal water cannot be utilized for electricity production (but still can be used for heat generation)
– other physical properties – like for example moist content or corrosiveness of the underground water
– the cost of technology – some technologies are just too expensive to be used, even if they are available on the market. For example, PV technologies have been present of the market for years, at very high costs.
the case of PV
– financing – sometimes financing makes some technologies more feasible than others – for example, generous EU feed-in tariffs available at the beginning of the century made the PV technology feasible despite high costs.
The availability of the energy source has to be transformed into useful energy available in the market
Growing penetration of renewable energy sources is already very visible – share of renewable energy sources rises continuously from the beginning of the 20th century. International Energy Agency forecasts, that due to the development of the technology in thirties of 21st century PV sector will generate more electricity than the coal sector.  The PV sector is considered as the most promising way of clean energy production.
Global energy generation by renewable energy sources + nuclear power plants. IEA 2019, Global Energy Outlook.
(1) Ellabban, Omar; Abu-Rub, Haitham; Blaabjerg, Frede (2014). “Renewable energy resources: Current status, future prospects and their enabling technology”. Renewable and Sustainable Energy Reviews. 39: 748–764 [749]. doi:10.1016/j.rser.2014.07.113 (from Wikipedia 2020 https://en.wikipedia.org/wiki/Renewable_energy#cite_note-3)

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