Questions about a hydrogen economy

The issue of cost-effectiveness of hydrogen being used in fuel cells is discussed. Per a given equivalent unit of fuel, hydrogen fuel cells in vehicles are about twice as efficient as internal combustion engines. Unlike conventional engines, fuel cells emit only water vapor and heat. However, hydrogen doesn't exist freely in nature, so producing it depends on current energy sources. Sources are either expensive and not widely available, or else they produce undesirable greenhouse gases. Hydrogen may not be the ultimate solution but it can be used for applications such as transportation.