Mention the minimum temperature difference required between surface water and the water at depths of up to $2\, km$ in an ocean thermal energy plant. (in $^{\circ} C$)

$A$ student constructed a box-type solar cooker. He found that it is not working efficiently. What could this be due to? Give any four possible mistakes in the construction and operation of the solar cooker. What maximum temperature can ordinarily be reached inside a solar cooker?

The waste materials collected from a market complex are scrap paper,thermocol,vegetable waste,tin cans,and glass bottles. Which of these can be used for producing biogas? Mention three advantages of converting these into biogas rather than burning them.

Name the largest component of biogas.

Minimum velocity of wind required for a functional windmill is about:

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