Energy density by itself is not a sufficient measure of anything. It is only useful in a specific context. For example, one can refer to the energy density of battery technology. And that is an extremely useful comparison because the weight of the battery in an electric car is critical to its success. The General Motors EV1 was abandoned because the battery technology was too heavy.
Context is very important. Comparing the energy density of the battery to the energy density of a fuel is completely useless. And this is an argument that some people use, incorrectly, to defend fuel based vehicles. Gasoline may be 80 times more energy dense than a lead acid battery, but what does that really mean? Of course, gasoline has far greater energy density than a lead acid battery. That’s an absurd comparison if not taken in its full context.
What is the efficiency of the energy conversion process? Internal combustion systems generate a lot of heat which is loss and highly inefficient. Taken with its frictional losses and parasitic loads like the alternator, water pump, etc., input to output efficiency for internal combustion vehicle systems is estimated at 40% or less. The same comparison for an electric car can result in measured efficiency of 90%. So the comparison of system efficiency for internal combustion engine passenger cars and pure electric passenger cars is that the electric system is more than twice as efficient. Lithium batteries are 4 times as energy dense as lead acid and will reduce the required payload of 1800 to 2200 pounds of batteries to a much more reasonably 450 to 600 pounds.
What is the absolute value of the technology? That’s the really important question.
One basis for comparison would be cost per transportation mile. In both cases, the system efficiency is directly affected by the cost of input energy. When gasoline is $2.50 a gallon, a 20 mile per gallon car costs 12.5 cents a mile. When gasoline is $4. a gallon, a 20 mpg car costs 20 cents a mile. Pure electric cars are impacted by the cost per kilowatt hour, but generally are documented as costing about 3 cents a mile.
A more complete comparison would incorporate the maintenance cost in addition to the energy cost per mile. Again the electric vehicle has significant advantages. There are no annual maintenance costs, although some value might be assigned to amortize the cost of the battery pack.
Further, one can include the purchase price of the vehicle. A $20,000 gasoline powered car will cost about $15,000 to operate over 5 years at 12,000 miles per year. An electric car will cost about $1800 to operate over the same period. So if a comparable electric car cost $33,000 the total cost of ownership over 5 years would be the same. Not surprising when you think about it.
It will be interesting to see what comes out of Detroit over the next two years.