We are going to have a big energy problem in the future.Today,the world uses 320 billion kilowatt-hours of energy a day.By 2100,we will use three times as much energy.How will we got the energy? Today,we get a lot of energy from fossil fuels:coal,oil,and natural gas.But fossil fuels are dirty,and they will not last forver.In the long term,we will have to find alternatives.We will need renewable energy