Figure 6: Efficiency – energy density product
Figure 6 gives an interesting conclusion on the feasibility of EVs and ICE vehicles. The figure shows one line indicating the energy density of storage components. On the EV side the energy density of batteries hovers around 35-250 Wh/kg, and on the other side (ICE), the energy density of the fossil fuels are around 12000 Wh/kg. Another line called the energy efficiency line is at around 70% on the EV side and around 15% on the ICE side. These two aspects contradict each other leading to the possibility of an optimal mix. Today, it would probably make sense to have a HEV which demands 40% electric storage and 60% fossil fuel storage on-board. This probably would be a good mix to start off with. Eventually, as technology of electric storage systems improve, the dependence on petrol/diesel fuels can gradually be reduced.
Apart from the issue of energy density, there is another serious challenge related to refueling or recharging that is not yet satisfactorily resolved. In conventional ICE vehicles, at the end of range, the infrastructure of refueling stations and pumps are available wherein the ICE vehicles will stop to refuel. On an average the refuelling time will be around 5 minutes per car. This may vary to some extent with city traffic and highway traffic conditions. However, in the case of EVs, infrastructure is not yet established for recharging the batteries or swapping the batteries. The existing ICE refuel stations with some modifications can double as battery recharge or swap stations in order to cater to EVs too. The batteries require to be charged in a controlled manner. The recharge times would be in the order of 4 to 5 hours. There are fast charge solutions that have also been proposed. However, in a country like India, the electric grid is a supply-deficit grid. This means that electric load demand is more than supply. Hypothetically, if to this supply-deficit electric grid, the massive transport loads are also included by way of battery recharge for milliions of vehicles throughout the country, the national power grid can become unstable. Daytime fast charging may not be appropriate solutions here.
If the batteries are pre-charged under controlled conditions and lean electric demand conditions, then swaping the discharged batteries for pre-charged ones can be a possible long term solution. The refuel stations can offer such battery swap services. Battery swap approach itself will throw up several challenges. Detecting the best time to pre-charge the batteries in a controlled manner is one area of work that needs to be resolved. Monitoring the state of health of the batteries continuously and updating the info within a RF tag embedded on the battery will provide useful information and tariff control while performing the swap. This is an interesting area to research on.