G'day Tim, All
At 04:53 PM 14/06/2011, Tim wrote:
>You might find the following article of interest:
>http://www.sciencedirect.com/science/article/pii/S0301421509001323
I might find it of interest, but I'm not going to pay for it on the off-chance.
>It posits the question: "Batteries: Higher energy density than gasoline?"
>
>In part due to these assumptions: "The energy density of batteries
>is two orders of magnitude below that of liquid fuels. However, this
>information alone cannot be used to compare batteries to liquid
>fuels for automobile energy storage media. Because electric motors
>have a higher energy conversion efficiency and lower mass than
>combustion engines, they can provide a higher deliverable mechanical
>energy density than internal combustion for most transportation applications."
What battery chemistry are they assessing the energy density of? I
suspect that with figures of two orders of magnitude the batteries
are not lead-acid, but more likely a Lithium chemistry. So does the
energy density (power for mass) of a Lithium cell compares reasonably
favorably with the mass of a liquid hydrocarbon fuel when you look at
overall system efficiency?
I think that their figures are unlikely to be right. Take the Tesla
roadster as an example of a vehicle with an advanced lithium battery
pack. Driven very conservatively it is possible to get over 500km out
of its' 450kg battery pack. A similar vehicle using liquid
hydrocarbon fuel in an internal combustion engine would do that
distance under the same circumstances for about 60kg of fuel, so when
you compare mass for range you need 450kg of batteries to do the same
trip as 60kg of liquid hydrocarbon fuel.
Comparing the volume of the Tesla battery (I was unable to find a
stated volume, but going by a photograph) it looks like the battery
volume is similarly about 7.5:1 over a liquid fuel tank.
The Tesla battery contains about 53kWh of electricity, equivalent to
20, 220-amp-hour 12V lead-acid batteries (when discharged over 20
hours), and weigh in at about 560kg plus all the interconnects and
battery mounting guff. (The big difference in Lithium is when pulling
power at the rate that fully discharges a battery in an hour or two,
a lead-acid battery will only give about half its' nominal stored
energy, where the Lithium will give it all).
So regardless of considering lithium or lead, you still fall a fair
way short of stored energy capacity in comparison to a liquid fuel.
The consideration then has to become, how much of the available space
and carrying capacity of my vessel am I going to give up for fuel? If
you are only using 10% of the fuel range in any considered time
period, then battery/electric is totally viable. If your boat is a
floating fuel tank (can't practically carry any more fuel) and you're
using a significant proportion of the fuel in each considered period
then electric power needs to wait for another two battery breakthroughs.
Hope this helps
Regards
[Technik] James
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