Tuesday, June 15, 2010

Electric Cars and Energy Density


An Under-Powered Electric Car

Electric Cars
The Truth about Milage
and Batteries

Petrol gives approximately 9 kWh per litre. Diesel can deliver 10 kWh/L. Now compare this to the 0.25 kWh/l produced by current types of lithium-ion batteries, and you will see one heck of a difference.

Put it another way, if your gas tank was say the size of a large suitcase, a lithium-ion battery, to go the same distance, would have to be some 40 suitcases in size!!

That’s why, despite having lots and lots of space filled up with batteries, in the engine space, under the floor etc. etc., most electric cars are lucky to be able to go for 100 miles on a charge. A diesel car, on the other hand, can go for 500 - 600 miles on a fill.

The latest claim to fame for packing the most energy into a litre sized battery is by Ecolocap, who in their latest set of figures recently published, state that they have achieved the amazing energy density of 1.17 kWh per litre. That’s about 5 times greater than the very best current types of battery can pack in, all assuming that their claim can be substantiated.

If Ecolocap come up with the goods, it will make the electric car a much more a viable prospect for anyone other than the city driver.


1 comment:

McAlfa said...

Hi Tony, I just wanted to comment on your figures quoted above as I believe that the scales is slightly off-kilter!
Firstly you're comparing petrol and diesel, in litres, to a battery also in litres. In most situations the energy density (in MegaJoules/m3) of a battery is more critical than the 'volume' that the battery takes up, so, in my humble opinion you should consider reapplying the maths based on this.

For example, petrol with an energy density of 34.2 MJ/L means that a standard petrol tank of 60l will have the following attributes:
Energy in joules for 60L: 2052 MJ
Weight for 60L: 45 Kg (at 0.75 Kg/L).

Similarly for diesel:
Energy Density: 37.3 ML/J
Energy in joules for 60L: 2238 MJ
Weight for 60L: 51 Kg (at 0.85 Kg/L)

Now, knowing the comparable weight of an average fuel-tank we can then do the maths on the battery to provide a fair comparison. Some of the energy densities of batteries are discussed on the Wikipeda page I've linked below, but I'd need more information to calculate the equivalent results as above.

Secondly, let's not neglect to mention that that efficiency of an internal combustion engine is significantly less than that of an electrically driven system, even when energy recovery systems are used. I don't believe that combustion engines achieve better than 40% but an electrical motor may achieve 80% or more.
So, for the reduced energy density of the battery, you have a 100% gain from the engine in efficiency alone.


Good luck and keep up the great blogging.