What are hot spots inside the Earth's crust?

$A$ student constructed a box-type solar cooker. He found that it did not work efficiently. Give any four possible mistakes in the construction and operation of the solar cooker.

$(i)$ Write the value of the solar constant of the Sun.
$(ii)$ What type of mirror is used in a box-type solar cooker?
$(iii)$ Why is it difficult to use hydrogen as a source of energy?
$(iv)$ What is the maximum temperature attained in a concave reflector-type solar cooker?
$(v)$ Fossil fuels are classified as a non-renewable source of energy. Explain why.

What do you mean by 'ocean thermal energy'? How can electricity be generated from this energy?

State the reason for calling fossil fuels as a nonrenewable source of energy.

State whether the following statement is true or false:
Solar cells are made of semiconductor materials.