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Author: Subject: Electric cars - just how efficient are they really?
Ivan

posted on 6/6/09 at 10:30 AM Reply With Quote
Electric cars - just how efficient are they really?

I have seen a lot of blurb about electric cars but they never answer the basic question about overall efficiency - so did the following calc for myself using info from Google - any thoughts:

Assume petrol consumption rate = 10 litres/ 100 km

Therefore you use 320 MJ per 100 km or 320/3.6 = 89 kW per 100 km

But Petrol engine is only 30% efficient therefore you actually need 89 X 0.3 = 26.7 kW to move car 100 km

For electric car you get around 70% efficiency therefore need 26.7/0.7 = 38.1 kW from battery.

Now battery conversion rate is 23% therefore you need 38.1/0.23 = 165.8 kW from mains to get equivalent of 10 litres of petrol.


Factors used – Petrol Supplies 32 MJoule/ litre
1 kW/hr – 3.6 MJ/hr

To take it further:

Electric grid efficiency is 92.5%

So you need 165.8/.925 = 179.2 kW generated at source

And coal fired power station is 35% efficient so need 179.2/0.35 = 512.1 kW worth of coal to move your 10 litre/100 km car 100 kilometers

Overall efficiency of electric car is 26.7/512.1 = 5.2 % and its ultimate fuel source is the most polluting there is.

[Edited on 6/6/09 by Ivan]

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speedyxjs

posted on 6/6/09 at 10:34 AM Reply With Quote
I dont thing electric cars are the way forward. I think hydrogen will make improvements because they are much more efficient





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Dangle_kt

posted on 6/6/09 at 10:34 AM Reply With Quote
classic case of a "solution" just moving the problem.
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UncleFista

posted on 6/6/09 at 10:35 AM Reply With Quote
I read recently, that the best, most expensive and newest in battery technology gives a fully charged electric car the power equivalent of 1.1 US gallons of petrol before needing charging again.

It obviously won't do the distance a petrol powered car would, as it's pulling a gert 'uge battery pack

I reckon the technology needs to "mature" a bit first





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blakep82

posted on 6/6/09 at 10:37 AM Reply With Quote
conservation of enegry and all that, the more processes you put between fuel and driving, the more energy is lost/wasted as heat and noise i guess

the most efficient way would be to develop a engine that runs on crude oil

edit: oh yeah, or hydrogen

[Edited on 6/6/09 by blakep82]





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b16mts

posted on 6/6/09 at 10:39 AM Reply With Quote
Sounds perfectly reasonable, would be interesting to see the energy use to get the petrol from the ground to the pump aswell.

Tbh, the only way I'd have an electric car, and feel "green" about it, would be to charge it from a private solar or wind powered generator, which is perfectly feasible

Interesting calcs though!

The future is veg oil anyway, the only simple renewable fuel! And I use it everyday!





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Snuggs

posted on 6/6/09 at 10:39 AM Reply With Quote
quote:
Originally posted by Ivan
I have seen a lot of blurb about electric cars but they never answer the basic question about overall efficiency - so did the following calc for myself using info from Google - any thoughts:

Assume petrol consumption rate = 10 litres/ 100 km

Therefore you use 320 MJ per 100 km or 320/3.6 = 89 kW per 100 km

But Petrol engine is only 30% efficient therefore you actually need 89 X 0.3 = 26.7 kW to move car 100 km

For electric car you get around 70% efficiency therefore need 26.7/0.7 = 38.1 kW from battery.

Now battery conversion rate is 23% therefore you need 38.1/0.23 = 165.8 kW from mains to get equivalent of 10 litres of petrol.


Factors used – Petrol Supplies 32 MJoule/ litre
1 kW/hr – 3.6 MJ/hr

To take it further:

Electric grid efficiency is 92.5%

So you need 165.8/.925 = 179.2 kW generated at source

And coal fired power station is 35% efficient so need 179.2/0.35 = 512.1 kW worth of coal to move your 10 litre/100 km car 100 kilometers

Overall efficiency of electric car is 26.7/512.1 = 5.2 % and its ultimate fuel source is the most polluting there is.

[Edited on 6/6/09 by Ivan]





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b16mts

posted on 6/6/09 at 10:41 AM Reply With Quote
quote:
Originally posted by blakep82[/I]

the most efficient way would be to develop a engine that runs on crude oil


Surely the pinto is nearly there?





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MautoK

posted on 6/6/09 at 10:55 AM Reply With Quote
From a cursory scan, your units don't stack up.

Joules are energy or work.
Watts are power or rate of work
Watts are Joules/second
kW/hour is meaningless.
John.

But I know what you mean and entirely agree that electric cars are the most misguided concept.


[Edited on 6/6/09 by MautoK]





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Ninehigh

posted on 6/6/09 at 11:24 AM Reply With Quote
We could always harass Honda to bring their hydrogen cars over here, their website says there's a solar/wind home filling kit available too






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clockwork

posted on 6/6/09 at 11:40 AM Reply With Quote
Not sure about your figures at all, also you ignore the energy required to find the oil, drill for it (not find it, go drill somewhere else), pump it, refine it, deliver it... plus it is a finite product

I think Hydrogen makes much less sense than battery or petrol. The figures are even worse. It takes a lot of energy to split hydrogen, then compress it, all to give the abilty to refill a tank to go further.

Vegetable oil is an inefficient use of agricultural land (but I agree is of benefit if used as a by product).

Another idea not mentioned is compressed air, but compressing air is horrendously inefficient.

I think though that there are bigger problems than the petrol we put in our cars.

Personally I favour electric cars, but I don't think we are there yet with regard to battery technology. Until we are (and we will be) efficient use of petrol is our best bet.

2c

As an aside, I do not think that the truth is ever considered an option in the green debate, and I always consider a lot of commentary with suspicion. Years ago I read an article in New Scientist that examined dust to dust environmental vs £ benefit of recycling. Recycling paper was PROVEN to have been far more energy inefficient than recovering energy through
burning it as a fuel. It also did not have the environmental pollution associated with the extra bleaching required the second time through. A discussion of the findings with the green party at the time produced a comment very much along the lines of "We need to get people recycling, paper is a good clean substance to start with. Burning paper is not something that we would like to see encouraged".
I could see their point, recycling one thing encourages you to recycle more, but personally I fealt their whole mandate ought to have been what was best for the planet now... not in 10 years time.





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smart51

posted on 6/6/09 at 12:01 PM Reply With Quote
I've never seen so many assumptions used as "facts" upon which to base an argument. That and the use of kW as a unit of energy lead me to believe that the OP is just 8ollocks. Maybe electric cars are a total waste of time. You wouldn't know from the OP.

For the record, I don't believe that a typical petrol car is anything like as good as 30% efficient, not from Jules in the fuel to miles on the road. Similarly, you can make electric cars way better than 70% A good electric motor can top 95% and a good controller in the same region. Copper losses will be small but you can also factor in regenerative braking Wheel motors mean you don't need any kind of gearbox, diff or prop shafts to lose power.

Another obvious mistake in the original post was to take account of electricity distribution losses but not the diesel used by fuel bowsers or power used by filling stations

You then have to compare oil wells / super tankers / refineries against coil / oil / gas / Uranium mining and power generation. Well to wheel figures they're called. Look them up rather than guessing.

I reckon an electric car to be more efficient than an internal combustion car but I also believe the best solution NOW is an electric car fueled by a small generator, say 30 BHP or so, optimised to run at 1 speed, generating 1 power at 1 throttle opening. Then your engine will exceed 30%.

For the future, who knows. I'll tell you this though, as soon as a good way of making / storing hydrogen is invented, the petrol engine will die. I attended a lecture on the development of the fuel cell powered car. There's still a way to go but the smart money will go into developing electric cars now so that when hydrogen fuel cells are available, they'll slot right in.






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blakep82

posted on 6/6/09 at 12:02 PM Reply With Quote
hmmm, clockwork....

clockwork cars i wonder how that would work? how much it would take to wind up a car would it be possible for peopl eto wind them up? lol





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bram boekestein

posted on 6/6/09 at 12:03 PM Reply With Quote
quote:
Originally posted by Ivan
I have seen a lot of blurb about electric cars but they never answer the basic question about overall efficiency - so did the following calc for myself using info from Google - any thoughts:

Assume petrol consumption rate = 10 litres/ 100 km

Therefore you use 320 MJ per 100 km or 320/3.6 = 89 kW per 100 km

But Petrol engine is only 30% efficient therefore you actually need 89 X 0.3 = 26.7 kW to move car 100 km

For electric car you get around 70% efficiency therefore need 26.7/0.7 = 38.1 kW from battery.

Now battery conversion rate is 23% therefore you need 38.1/0.23 = 165.8 kW from mains to get equivalent of 10 litres of petrol.


Factors used – Petrol Supplies 32 MJoule/ litre
1 kW/hr – 3.6 MJ/hr

To take it further:

Electric grid efficiency is 92.5%

So you need 165.8/.925 = 179.2 kW generated at source

And coal fired power station is 35% efficient so need 179.2/0.35 = 512.1 kW worth of coal to move your 10 litre/100 km car 100 kilometers

Overall efficiency of electric car is 26.7/512.1 = 5.2 % and its ultimate fuel source is the most polluting there is.

[Edited on 6/6/09 by Ivan]


The petrol engine uses 10L/100km so it uses 89kW, although it only delivers 89x0.3=26.7kw but it still uses 89kW so efficientcy doesn't matter.

It's 89kW vs. 38.1kW, the battery conversion rate can't be included, grid efficiency and coal power station likewise. Because the petrol calculation doesn't include used energy from the fuel truck, used energy for making petrol and winning petrol etc. etc.

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nstrug

posted on 6/6/09 at 12:07 PM Reply With Quote
I'm afraid I'm not convinced by your figures.

Specific energy of petrol = 44x10^6 J/kg
density of petrol = 720kg/m^3

So, 10 litres/100km = 7.2 kg/100km
equivalent to 3.17x10^8 J/100km

A petrol engine is only 30% efficient, so an ideal car with a 100% efficient engine with the same characteristics would only use:

3.17x10^8x0.3 = 9.5x10^7J/100km

An electric motor is about 90% efficient and a lead-acid battery about 80%, so taking the same car and powering it with an electric drivetrain (all other things being equal) uses:

9.5x10^7/(0.9x0.8) = 1.32x10^8 J/100km

So if we take efficiency at the point of distribution (petrol station or wall socket) then we see that the electic car uses only 41% as much energy as an equivalent petrol-driven car.

Now obviously we should figure in transmission and generation losses too, but these figures are extremely hard to get hold of in the case of the petrol driven car (perhaps why you ignored them?). We need to calculate the efficiency of the transmission, generation and primary fuel source mining in the case of electicity, and in the case of petrol we need to calculate distribution, refining and crude extraction costs.

I understand that the electric car still wins out after these calculations, and clearly if you factor in a clean primary source (nuclear or renewables) is far preferable.

In any case, hydrogen produced using nuclear power is a far more efficient fuel than either grid electricity or petrol, and will be the standard automotive fuel of the future I believe, unless there is a huge breakthrough in battery energy density.

Interestingly, there are several projects on going to produce synthetic petrol and diesel using water and CO2 feedstock.

Perhaps in 50 years time, we will see modern cars running on hydrogen fuel cells, and old classics running on synthetic (and carbon neutral!) petrol produced using almost unlimited electricity from fusion power.

nick

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tendoshingan

posted on 6/6/09 at 12:18 PM Reply With Quote
Sorry not convinced by the efficiency of anything said so far.
There is only one environmentally friendly and efficient car that i know of....
Fred Flinstones
Old tinkle toes could beat anything when he put his mind to it.

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dinosaurjuice

posted on 6/6/09 at 12:32 PM Reply With Quote
Electric cars are simply 'toys' in my view until we have a sutainable power network.
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Fred W B

posted on 6/6/09 at 01:11 PM Reply With Quote
Dennis goes into some real world detail on

THIS PAGE

and HERE

If you haven't see the dpcars site before, best you go get a fresh cuppa/beer before you click.

Cheers

Fred W B

[Edited on 6/6/09 by Fred W B]





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MautoK

posted on 6/6/09 at 01:12 PM Reply With Quote
Another aspect of electric cars is the horrendous amount of heavy metals that go into their construction.

And while I'm here, when will people realise that there's no such thing as 'renewable energy'. All our energy comes from the sun - whether immediate, as direct radiation, or delayed, as captured in the specific energy content of oil or gas, or for example from the gravitational effects of sun + moon giving tides. I'm all in favour of using the energy encapsulated in tidal flow, but tapping that will have an effect on the moon's orbit (eventually.........)

The fatuous use of the wind as an energy source is not like a magic bucket where you scoop out a ladleful of energy and it mysteriously remains full - it gets refilled (i.e. more winds blow) only as a result of more energy from the sun.

All energy ends up as low-grade heat.

It's very simple!





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smart51

posted on 6/6/09 at 02:08 PM Reply With Quote
quote:
Originally posted by MautoK
when will people realise that there's no such thing as 'renewable energy'. All our energy comes from the sun


Perhaps because most of us can't wait for coal to "renew"






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MikeRJ

posted on 6/6/09 at 11:12 PM Reply With Quote
quote:
Originally posted by Ivan
Now battery conversion rate is 23%


What is the battery conversion rate? Not heard this term before and Google doesn't seem to know either.

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cloudy

posted on 6/6/09 at 11:50 PM Reply With Quote
quote:
Originally posted by MautoK

And while I'm here, when will people realise that there's no such thing as 'renewable energy'. All our energy comes from the sun


Nuclear / Zero Point Energy?





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Liam

posted on 7/6/09 at 12:19 AM Reply With Quote
Shoddy units aside, I'm really not sure where you get 23% battery 'conversion rate' from. If you mean efficiency, it's much higher. In reality, power station fuel to car wheels, an electric car is about 25% efficient, which slightly beats the crude oil to wheels figure of a conventional car.

But the efficiency of an electric car is rather less important in the long run. Even now we can generate electricity from clean plentiful/renewable/free sources, and will do so much more in the future (I do wish we'd hurry up with nuclear fusion - we're well behind the predictions of Sim City 2000). Burning oil is always gonna be burning oil - until there's none left that is.

Liam

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Badger_McLetcher

posted on 7/6/09 at 12:35 AM Reply With Quote
quote:
Originally posted by cloudy
Nuclear / Zero Point Energy?


All heavy metals (such as fissionables) were created in a star (via fusion).
Zero point and fusion yes. But good luck with both of those. In my opinion we need to stop putting money into wind farms etc that create tiny amounts of energy when the fusion research programs are underfunded but may be the saviour of humanity (I suscribe the the Fallout perception- one day, too many people, too few resources, boom.) Just my opinion.





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gazza285

posted on 7/6/09 at 01:33 AM Reply With Quote
You are all looking at the wrong sort of efficiency, when electric cars become more profitable that petrol/diesel is when they will become viable.




quote:
Originally posted by Badger_McLetcher
one day, too many people, too few resources, boom



What we need is a good, old fashioned epidemic, kill off a few billion or so. How about swine flu? As long as I'm immune!





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