What is the minimum wind velocity required for obtaining useful energy from a windmill?

The energy liberated by burning $1 \, g$ of wood is approximately: (in $, kJ/g$)

Which is the ultimate source of energy?

What is the basic cause for winds to blow? Name a part of India where wind energy is commercially harnessed. Compare wind power and power of water flow in respect of generating mechanical and electrical energies. What is the hindrance in developing them?

What is the use of a glass sheet in solar heating devices?

Why is $CNG$ considered an environmentally friendly fuel?