Want to better understand Tesla e-mpg

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Grinkle

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Summary
What is Tesla equivalent miles per gallon comparing at the macro level?
From here -


I see:

246629


1 gallon of gasoline burned in an internal combustion auto engine will yield 33.7 kWh of usable work - is that what this means?

I am trying to get my head around whether e-vehicle use takes more fossil fuel or less fossil fuel than gasoline / Diesel engine use. Because electricity is generated from fossil fuel, I want to immediately conclude that the 2nd law of thermodynamics says that I don't need to think very hard on it, electricity must use more fossil fuel than burning raw fossil fuel directly - but a conversation with a friend has me uncertain.

If it is the case that a large generator can be very efficient at turning fossil fuel into electricity, and there is not much loss in the distribution, then perhaps the efficiency of the electric vehicle engines can be better enough relative to gasoline engines that in the net it takes less fossil fuel to generate and distribute and utilize an electric vehicle than a gasoline vehicle.

Is this known either to be the case or to not be case, or is this conclusion very judgement driven and hard to quantify?

I must also consider the below, which surprised me so I am glad I looked it up. Only 63% of electricity in the US is generated from fossil fuel. I mistkenly thought it was closer to 90%.


So I think that means if I am looking for the break-even point on fossil fuel consumption, I must give e-vehicles an approximate 30% margin in the above net calculation thinking if I am not talking about a hypothetical situation where the only root source of usable work is fossil fuel.
 

phyzguy

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The energy content of gasoline is about 45 MJ/kg, which works out to about 33 kWh/gal. But internal combustion engines are only about 25% efficient at conveying the fuel energy into mechanical energy. Power plants are better, but still under 40% efficient. I think the main gain is what you said, 1/3 of our electrical energy comes from non fossil fuels, and this number is climbing.
 
Summary: What is Tesla equivalent miles per gallon comparing at the macro level?

I am trying to get my head around whether e-vehicle use takes more fossil fuel or less fossil fuel than gasoline / Diesel engine use. Because electricity is generated from fossil fuel, I want to immediately conclude that the 2nd law of thermodynamics says that I don't need to think very hard on it, electricity must use more fossil fuel than burning raw fossil fuel directly - but a conversation with a friend has me uncertain.

If it is the case that a large generator can be very efficient at turning fossil fuel into electricity, and there is not much loss in the distribution, then perhaps the efficiency of the electric vehicle engines can be better enough relative to gasoline engines that in the net it takes less fossil fuel to generate and distribute and utilize an electric vehicle than a gasoline vehicle.

Is this known either to be the case or to not be case, or is this conclusion very judgement driven and hard to quantify?
I am closely associated with small personal electric vehicle designers, and I have created some electric vehicle efficiency calculators personally, but I have also used this calculator (which I did not write) which as far as I know is based on equations relevant to the permanent magnet motor found in the tesla 3:

http://www.bavaria-direct.co.za/constants/

Entering some parameters which can be achieved in some of the small do it yourself personal vehicles I am familiar with, we get around 90.3% electrical to mechanical conversion efficiency:

246634
 

Grinkle

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internal combustion engines are only about 25% efficient
Power plants are better, but still under 40% efficient.
we get around 90.3% electrical to mechanical conversion efficiency
If I am understanding correctly -

1G of gasoline will produce 0.4 x 33.7 kWh = 13.48kWh
and the electric motor will net 0.9 x 13.48 kWh = 12.42 kWh used work in moving a vehicle

1G of gasoline will net 0.25 x 33.7 kWh = 8.425 kWh used work in moving a vehicle

So there is roughly a 47% advantage to using the gas (sic) to generate electricity as opposed to burning it directly in an internal combustion engine.

This isn't really apples to apples, because one burns coal or natural gas or something besides gasoline to generate electricity, but at least it shows that in general, its potentially more efficient to use fossil fuels to generate electricity en-masse and then use that electricity to power vehicles.

@metastable I don't think your calculator accounts for transmission loss from the initial generator to the destination battery charger, does it? If not, do you have any swag for that one?
 

anorlunda

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Power transmission losses are around 2%. Plus an additional 10-15% loss in local power distribution.

The best fossil power plants are combined cycle gas turbine and steam. Those plants can get 60-65% thermal efficiency. Because of that, they are popular among the most recently built fossil power plants.

Computing accurate numbers for the actual trade-off is very difficult, but the conclusion that EVs recharged from the grid cause less CO2 is correct.

But there's another problem. The power grid may have problems expanding fast enough to meet the demand for electric vehicles. The figure below is from


As you can see, to totally shift the transportation sector onto the electric sector means nearly doubling the electric capacity. But some transport such as ships and planes do not switch to EV, so let's say in round numbers 50% more electric power.

That is more than just power plant capacity, but also the transmission lines, distribution, and perhaps even upgrading the electric service entrance to every house and building. That can be done, but not overnight and not at zero cost.

246642
 

berkeman

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As you can see, to totally shift the transportation sector onto the electric sector means nearly doubling the electric capacity.
Oh wow, I did not know that.
 

anorlunda

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As you can see, to totally shift the transportation sector onto the electric sector means nearly doubling the electric capacity. But some transport such as ships and planes do not switch to EV, so let's say in round numbers 50% more electric power.
I forgot another factor that would tend to mitigate. It is the topic of this thread. EVs use less energy than their fossil powered predecessors. Therefore, shifting all transportation to electric, does not shift all of today's transport energy consumption. The 50% increase in electric rather than 100% is closer to correct.

Still, it is a very big deal. I see many households with 4-5 pickup trucks parked in the driveway at night. Charging 5 of them at 7 kw each means 35 KW. Not many residences have that capacity today, nor does the line on the poles running down the street.
 

Grinkle

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Not many residences have that capacity today, nor does the line on the poles running down the street.
If charging can scale to large banks of batteries and battery transport and swapping can be made as convenient as gasoline distribution is today, might that be a more efficient / realistic path than fat-piping all electrical networks?
 

anorlunda

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If charging can scale to large banks of batteries and battery transport and swapping can be made as convenient as gasoline distribution is today, might that be a more efficient / realistic path than fat-piping all electrical networks?
Yes, that may help. But the details might be difficult. A typical gas station might have room on their shelves for 50 car battery packages. How many vehicles per day can it serve with liquid fuels? I think you would need one station per block in residential areas. It might be easier for people to park their cars at the block charging station every night, then walk home.

The other key factor would be for all the EV makers to standardize on the size/shape of batteries and make them easy to swap. By analogy, AA and AAA size batteries dominate portable markets, but custom batteries for each make/model dominate cell phones, and many cell phones don't have swappable batteries at all.

But we are getting kind of off topic for this thread. Better to start a new thread to explore that theme.
 
The main problem with Electric vehicles are batteries, basically cost, chemicals and number of cycle counts of batteries. Until recently, the main rechargeable battery available was Lead Acid and the technology is surprisingly 150 years old with poor Energy Density and Cycle count. Typically you get 1200 cycles with 50% DoD (Depth of Discharge) and 600 cycles with 80% DoD for Lead Acid.

Lithium based rechargeable batteries are relatively recent invention (around 1980s) , gained momentum with laptops and mobiles. They have good Energy/Power Density and Cycle Count and with more and more research, it is increasing. As the technology improved, it has percolated into Battery based EVs also.

Looking from another side, Electric Transpiration is nearly a century old, both Electric Locomotives
and Trolleybuses are a very mature technology, as they use live electric power.
 

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