What is ocean thermal energy?

Draw a labelled diagram of a floating gas holder type biogas plant and describe its working. Mention any two advantages of using animal dung for making biogas over using it as a fuel in the form of dried dung cakes.

Mention three advantages of converting domestic sewage to sewage gas.

Write true or false for the following statement:
Glass cover in a solar heating device allows the short wavelength of ultraviolet radiations to enter the solar cooker.

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)

Ocean thermal energy is due to