Name a non-conventional renewable source of energy which can be harnessed for $24$ hours throughout the year.

Ram visited Manikaran Sahib in Himachal Pradesh where he saw an experimental geothermal plant. When he saw a hot spring,he wanted to know more about it. His father explained to him about geothermal energy and hot springs. Now answer the following questions:
$(a)$ What did Ram's father explain to him about hot springs?
$(b)$ Why is geothermal energy not generated on a large scale?
$(c)$ What is the main advantage of using geothermal energy?

Define a nuclear fusion reaction. Describe the conditions for the occurrence of a nuclear fusion reaction.

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

In which forms is solar energy stored in the oceans? Mention any two forms that could be harnessed to obtain energy in a usable form.

What is a windmill? With the help of a diagram,describe in brief the construction and working of a windmill. Give uses of wind energy.