Batteries have too low energy densities

Electric vehicles are the way to go for a number of reasons. A few of these are the multitude of electric energy sources available (hydrogen cells, PV arrays, outlet power, …), the high efficiency of electric motors, the inherent green character of electric vehicles: no emmission, etc…

An important question arises though: Can one carry as much electric power, as is possible with conventional fuel-powered cars?

The short answer is: no.

The long answer:

When looking at energy densities of batteries and fuels [1], it is clear that at this point even the best batteries have energy densities two orders of magnitude lower than liquid fuels. E.g. newest commercialy available lithium-manganese batteries have an energy density of about 1 MJ/kg or 230 Wh/kg, the batteries used in the newest Belgian solar car had an energy density of about 180 Wh/kg whereas conventional gasoline has an energy density of about 44 MJ/kg or 10.000 Wh/kg.[2] Does this mean there is no future for batteries in electric vehicles?

Not neccesairily. What we need to consider is net mechanical energy delivered to the wheels, and not gross caloric energy stored. This means we need to include contributions from regenerative brakes, take into account  frictional losses in the transmission, the weight of the motor and transmission, the electrical control and power converters, transmission, exhaust, and all associated parts and fluids, which are necessary to convert the stored energy to mechanical work. The total weight is a lot lower for electric vehicles (no transmission, exhaust, lighter motor,…), than it is for classic cars. [3] This gives a whole different figure:

Important on this graph is that Li-Ion has a higher energy density compared to gasoline for lower ranges. An important point on this graph is the crossover range, where both Li-Ion and gasoline have the same energy density. Where this point lies, is dependent on the type of car or truck considered. The more overpowered a vehicle is (like sports cars, trucks, SUVs) the greather crossover range will be. The gasoline curve is more linear because increasing range means increase the size of the fuel tank, which only add slightly to the vehicle mass. Increasing the range of an electric vehicle means increasing the size of the battery pack, which has a pretty poor energy density, as seen above. This makes it clear why electric vehicles are efficient on the lower ranges. This also explains why it’s more difficult to achieve a big range for small, light cars, while maintaining a better energy density than classic cars.

Another conclusion that can be taken, is that for electric vehicles to have a high range, they need to be light, and need better aerodynamics. This can be achieved by using more composite materials in the construction of cars, and pushing aerodynamic studies way further.

And what about hydrogen fuel cells? As can clearly be seen on the topmost figure, hydrogen has an incredibly high energy density per mass. The big downside is that hydrogen has an equaly incredibly low energy density per volume. Hydrogen is a gas, and it can be compressed, but then still, has a very poor volumetric energy density compared to gasoline. Therefore we believe that hydrogen fuel cells are not a one-on-one replacement for gasoline, but could be a valuable addition to electric cars, combined with e.g. batteries and PV cells on the roof.

[1]: http://en.wikipedia.org/wiki/Energy_density

[2]: http://en.wikipedia.org/wiki/Gasoline

[3]: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2W-4VYW6FD-3&_user=1522359&_coverDate=07%2F31%2F2009&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1242215104&_rerunOrigin=google&_acct=C000053496&_version=1&_urlVersion=0&_userid=1522359&md5=b74ef422e5fec69c5421176c01108cb6

Battery energy density is smaller than that of liquid fuels by two orders of magnitude. However, the relevant energy is not gross caloric energy stored, but rather net mechanical energy delivered to the wheels, ηU_f, where η is the “stored energy to mechanical work” conversion efficiency and includes contributions from regenerative brakes as well as frictional losses in the transmission. Additionally, a motor and transmission is necessary to convert the stored energy to mechanical work, so the relevant mass should include the drive train mass, m_d: the motor or engine, electrical control and power converters, transmission, exhaust, and all associated parts and fluids. We introduce an effective energy densityBattery energy density is smaller than that of liquid fuels by two orders of magnitude. However, the relevant energy is not gross caloric energy stored, but rather net mechanical energy delivered to the wheels, ηU_f, where η is the “stored energy to mechanical work” conversion efficiency and includes contributions from regenerative brakes as well as frictional losses in the transmission. Additionally, a motor and transmission is necessary to convert the stored energy to mechanical work, so the relevant mass should include the drive train mass, m_d: the motor or engine, electrical control and power converters, transmission, exhaust, and all associated parts and fluids. We introduce an effective energy density::