What percentage of solar energy can be converted into electricity by a modern solar cell?

Define the term 'fuel'. What are primary and secondary fuels? To which class of fuel do the following belong: Coke,Wood,Petroleum,$LPG$?

$(a)$ Classify the two fuels, $CNG$ and hydrogen, as renewable and non-renewable.
$(b)$ Justify the statement, "Hydrogen is a cleaner and better fuel than $CNG$."

$(a)$ $A$ hydropower plant converts one form of energy into another. Name these two forms of energy.
$(b)$ State two advantages of generating hydroelectricity compared to the generation of electricity in a thermal power plant.

Explain how geothermal energy is harnessed to produce electricity.

Why are solar heating devices painted black from inside?