Direct solar energy, which provides 10,000 times the world's energy requirements, is the largest renewable energy source. Wind, which provides 100 times the world's energy requirements, is next. The average power per square meter from wind turbines is comparable to that from direct solar energy, because the Earth's wind power is concentrated within a thin atmospheric shell.

Hi Alberto,

Here are the ones that I would "vote for" - in falling order of "apparent promise":

Cheers !

-Sanjay

In some places whereas sun light is available for more than 80% of year days.

Wind energy in mountainous areas.

Biogas from waste 

Hydropower whereas Dams  and river currents.

There are many sources of energy that are renewable and considered to be environmentally friendly and Emits low carbon. in my opinion Here are the main sources of renewable energy:

Solar Energy

Wind Energy

Hydroelectricity

Hydrogen and Biomass Energy 

Ocean Energy

Geothermal Energy

And after all Nuclear Power

by the way i forgot about Radiant Energy (TESLA Free Energy ) regard to

(We can identify emerging energy technologies those still far away from the maturity, with a Technology Readiness Level of 1-3, researched at the laboratory level or not successfully tested in pilot plants yet)

Renewable Energy Technologies:

1.      Hydroelectricity

2.      Solar Energy

3.      Wind Energy

4.      Bio-Fuels

5.      Ocean Energy

Fossil fuels and mining and mineral fuels:

6.      Natural Gas

7.      Nuclear Power

8.      Geothermal Energy

9.      Aluminum-Rocket Propellant Fuel

10.  Hydrogen Energy-Fuel Cell Technology

https://shapeenergy.eu/wp-content/uploads/2017/07/SHAPE-ENERGY-Annotated-Bibliography_COMPETITIVE_SECURE_LOW-CARBON-ENERGY-SUPPLY.pdf