Why are solar heating devices painted black from inside?

The power generated in a windmill:

Why is charcoal considered to be a better fuel than wood? What are the disadvantages of converting wood into charcoal?

Ranvir lives in a village and uses wood as a fuel. He studied in his school that charcoal is a better fuel than wood. He decided to use charcoal instead of wood.
$(a)$ How can Ranvir obtain charcoal from wood?
$(b)$ Why is charcoal considered a better fuel than wood? (Give two reasons)

Justify the statement 'Hydrogen is a cleaner and better fuel than $CNG$'.

Why should copper tubes be given a higher number of coils in a solar water heater?