Storage of energy. Energy generated by alternative sources is difficult to store. Batteries, stored water, air and heat can all be used to store energy for future use. Fuel cells struggle to overcome high costs. While the size and complexity of energy storage technologies are not critical in static energy production or consumption situations, the energy density of storing devices will be crucial to all transport industries. The development of high energy density batteries and other storage devices must progress in tandem with the growing share that electric motors have in the transport sector.