Compressed air storage = low efficiency (Score 1)

Using compressed air to store energy is simple and doesn't have the life cycle limits that batteries do. However, *all* gasses when compressed become hotter. A significant portion of the energy put into the compression goes into that heat. If the compressed gas is allowed to cool, it loses a serious portion of its energy which shows up as a reduction in pressure. This is made worse during its expansion in an engine - it cools to below ambient temperature so it doesn't expand as much as its original volume. Only if expanded isothermally will you get its stored energy back. So you lose energy in compression *and* expansion unless each is either isothermal or you store the thermal energy. Making a pressure tank that keeps the heat in is a major thermal trick unless you drive the vehicle right away (in minutes). Otherwise electrical storage is far more efficient. If you want to reduce pollution and energy use, efficiency is of primary importance... Ref: Any Thermodynamics 101 course.