IV STORAGE ISSUE
A vehicle should have a stored energy source on-board in order to enable it to move. In the case of ICE vehicles, the stored energy source is the fuel in the form of petrol or diesel. In the case of EVs, the storage device is the battery. Though these are the more common storage mechanisms, there have been vehicles using flywheel storage and compressed air. The onboard stored energy source will infact determine the range of the vehicle before every re-fill or re-charge. The energy density parameter of the fuel or source will impact the size and weight of the vehicle for a given range. Table 1 gives typical values of energy densities for various energy storage devices. The energy density of gasoline is about 12.3 kWh/kg. Most fossil fuel based energy storage systems have energy densities on the order of 10 kWh/kg. On the other hand, the conventional lead-acid batteries have very poor energy densities of about 35 Wh/kg which is about 400 times less than gasoline. Even allowing for the low efficiency of internal combustion engines, this is a huge difference. As a result, energy density has historically been the driving concern and primary challenge for electric vehicles.
Table 1: Energy density comparison
One should note that even though the lead acid batteries have very low energy densities, their manufacturing techniques are highly matured and the key electrochemical components (lead and sulfuric acid) are inexpensive. For EV applications, the deep discharge tubular lead acid batteries are the ones to be used. Nickel-metal hydride (NiMH) batteries have about 50 percent higher energy densitiy than lead-acid batteries and EVs using them can be expected to have a greater range. The lithium-based batteries offer the best long-term hope for commercially practical vehicles. One can also see that hydrogen has a very high energy density. Fuel cells based on hydrogen are being tried for commercially viable storage solutions. It should be noted that hydrogen is also the lightest element and therefore, at standard temperature and pressure, 1 kg will have a very large volume as compared to petrol. This implies that hydrogen must be compressed and used under pressure leading to additional technological complexities and cost. Currently, the least expensive storage solution is lead acid batteries (albiet low grade performance) and the most expensive is hydrogen based solutions. Storage solutions are continuously
evolving and this challenge is yet to be resolved satisfactorily.