Electric power points for e-vehicles

Electrically assisted bikes, trikes, etc. that are legal in the UK
kwackers
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Re: Electric power points for e-vehicles

Post by kwackers »

mjr wrote:
kwackers wrote:Typical efficiency of EV's is 4 miles per kwh.
An hours cycling by a moderately fit cyclist is probable around 200w

So if a moderately fit cyclist does 12mph average, does that mean a cyclist does 16 2/3 W/mi and an EV does 250 W/mi?

So an ebike may be 20W/mi due to the extra weight but probably a higher average speed? I suspect an e-recumbent may be the optimal efficiency, with the motor overcoming the notorious disadvantage on hills.

Sounds something like.

You can tweak numbers. Driving carefully I've managed 5.5miles/kwh but it's possible to go as far in the other direction if you drive like a t*t.
Non-linear factors like air resistance can change the numbers significantly hence why an e-recumbent is a winner.
Going slower and avoiding braking (i.e. looking ahead) are the main ways to improve the efficiency of any vehicle.
(Regen helps but in a car it only adds back about 30% of the energy used in accelerating to the braking speed, so braking is still best avoided).
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[XAP]Bob
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Re: Electric power points for e-vehicles

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mjr wrote:
kwackers wrote:Typical efficiency of EV's is 4 miles per kwh.
An hours cycling by a moderately fit cyclist is probable around 200w

So if a moderately fit cyclist does 12mph average, does that mean a cyclist does 16 2/3 W/mi and an EV does 250 W/mi?

So an ebike may be 20W/mi due to the extra weight but probably a higher average speed? I suspect an e-recumbent may be the optimal efficiency, with the motor overcoming the notorious disadvantage on hills.


Let's get the units correct, It really helps.
A watt (W) is a measure of power - energy per second. A watt hour (Wh) is a measure of energy (i.e. work done). Both often have the prefix k for kilo.

An ev does ~4m/kWh so 250Wh/mile
A cyclist at 200W doing 12mph would be doing 12m/200Wh or 17Wh/mile

In other words that cyclist uses 15 times less energy than that car (which makes some intuitive sense, the mass difference is about 15 times 100kg vs 1.5 ton). I would wager 200W will net you more than 12mph though...

BC article wrote:Wind tunnel testing has shown that a 70kg cyclist putting out 200 watts of power would be traveling at 32.4kph if riding upright on the brake hoods.

That suggests 18mph, so 50% more distance per watt hour - 11Wh/mile
In reality I expect the value to be between those two values, but the cycle is more efficient (again, intuitively makes sense, it is a much simpler vehicle).

What that really highlights is how inefficient an ICE is... 1 gallon is about 34kWh - so a car that does 50mpg at 50mph is doing 700Wh/mile (three times more energy than an ev).


Yes, I know I'm ignoring both refinery costs for the petrol (about 10% more energy), and the costs of extraction (5-20% depending on the source*), as well as electricity production costs and transport/conversion losses.
My very hand wavy, based on an absolute minimum of research, excuse is that they are probably a wash, or even give the ICE a slight advantage in the comparison. That advantage will only grow as the proportion of electricity production in the world moves towards more sustainable production.
Anything below ~20mph (fairly old study) is generally particularly inefficient for an ICE as well - the energy required just to keep the engine running starts to become significant (most obvious at 0mph, where they are still using fuel).

Short journeys are also generally a killer for an ICE, but don't really affect an EV much (depending on ambient temperature and battery conditioning on an individual vehicle)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
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Oldjohnw
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Re: Electric power points for e-vehicles

Post by Oldjohnw »

A comment here in parentheses, an aside if you like.

This morning I had to go to Tesco to collect a truck full of medicine. In the car park a guy was sitting in his car, window down, radio on full blast and engine running.

The unreconstructed GOM in me would have asked him to turn his damned radio off. The more venerable gentleman side of my nature politely asked him to switch off his engine.

He ignored me with remarkable indifference.
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Re: Electric power points for e-vehicles

Post by mjr »

Yep, I'm no physicist and had not consumed enough coffee by the previous post. Thanks all.

And I wish the fine for idling was issued more. Is it only police who can issue it? Could Tesco add one to their private parking fines?
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[XAP]Bob
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Re: Electric power points for e-vehicles

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mjr wrote:Yep, I'm no physicist and had not consumed enough coffee by the previous post. Thanks all.

:)
It's very easy to be suckered into the wrong units.

And I wish the fine for idling was issued more. Is it only police who can issue it? Could Tesco add one to their private parking fines?

A million times this... maybe we should all carry around a potato?
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
kwackers
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Re: Electric power points for e-vehicles

Post by kwackers »

[XAP]Bob wrote:A million times this... maybe we should all carry around a potato?

At the bottom of my road a coach comes to pick up kids to take to a school somewhere.
By the time it's due there are maybe 20 cars there of medium to large size all with idling engines.

If there's one place EV's can really score is sitting there with the heater on, not ideal (cars aren't well insulated) but probably a hundred times better than the gross inefficiency of idling engines.
stodd
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Re: Electric power points for e-vehicles

Post by stodd »

mjr wrote:And I wish the fine for idling was issued more. Is it only police who can issue it? Could Tesco add one to their private parking fines?

I suspect even the police can't issue one if the car is on private property (Tesco car park).
It certainly is a common curse.
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[XAP]Bob
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Re: Electric power points for e-vehicles

Post by [XAP]Bob »

stodd wrote:
mjr wrote:And I wish the fine for idling was issued more. Is it only police who can issue it? Could Tesco add one to their private parking fines?

I suspect even the police can't issue one if the car is on private property (Tesco car park).
It certainly is a common curse.

By that metric Tesco can impose whatever fine they want
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
stodd
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Re: Electric power points for e-vehicles

Post by stodd »

[XAP]Bob wrote:
stodd wrote:
mjr wrote:And I wish the fine for idling was issued more. Is it only police who can issue it? Could Tesco add one to their private parking fines?

I suspect even the police can't issue one if the car is on private property (Tesco car park).
It certainly is a common curse.

By that metric Tesco can impose whatever fine they want

Not necessarily. It would certainly be easier to (claim to) impose than to collect.
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Re: Electric power points for e-vehicles

Post by [XAP]Bob »

Yes - I wonder how much success they would have simply charging the fine to the person’s next shop.

But supermarkets value the convenience of their laziest customers over anything else. There is generally no response to misuse of accessible spaces, or parent and child spaces (once your child can get in and strap themselves in you really don’t need one any more).
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
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Re: Electric power points for e-vehicles

Post by sjs »

[XAP]Bob wrote:Yes, I know I'm ignoring both refinery costs for the petrol (about 10% more energy), and the costs of extraction (5-20% depending on the source*), as well as electricity production costs and transport/conversion losses.
My very hand wavy, based on an absolute minimum of research, excuse is that they are probably a wash, or even give the ICE a slight advantage in the comparison. That advantage will only grow as the proportion of electricity production in the world moves towards more sustainable production.


So in round numbers we have about 250Wh/mile for an electric car, something around 15 for person+bike and maybe 700 for a petrol powered car. How to judge relative efficiencies depends on how the electricity was generated; if from gas or (heaven forfend) coal, the thermal energy needed to get that 250Wh of electricity was at least 600Wh, so (still ignoring transmission losses and refinery costs for the petrol) that is the number that should be compared with the 700 for the ICEV. SImilarly the 15Wh for the bike is a lot more than that in terms of the energy input in the form of mars bars or whatever. On that basis, and considering the mass difference, the bike doesn't fare that well.
kwackers
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Re: Electric power points for e-vehicles

Post by kwackers »

sjs wrote:So in round numbers we have about 250Wh/mile for an electric car, something around 15 for person+bike and maybe 700 for a petrol powered car. How to judge relative efficiencies depends on how the electricity was generated; if from gas or (heaven forfend) coal, the thermal energy needed to get that 250Wh of electricity was at least 600Wh, so (still ignoring transmission losses and refinery costs for the petrol) that is the number that should be compared with the 700 for the ICEV. SImilarly the 15Wh for the bike is a lot more than that in terms of the energy input in the form of mars bars or whatever. On that basis, and considering the mass difference, the bike doesn't fare that well.

I'm not sure you can take an absolute worse case for an EV and then compare it to an IC where you've conveniently ignored the extraction to pump costs...
Then of course there's the energy cost of handling pollutants.

The reality is that most EV's charge at night using surplus power which tends to have a higher amount of wind content than the average.
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Re: Electric power points for e-vehicles

Post by mjr »

sjs wrote:On that basis, and considering the mass difference, the bike doesn't fare that well.

Considering what mass difference? The person being transported is the same weight. Is it worth all that extra energy just so the user avoids putting any energy in, goes a bit faster (or not at all in town) and has a longer range, or is it a bit selfish?
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Re: Electric power points for e-vehicles

Post by [XAP]Bob »

sjs wrote:
[XAP]Bob wrote:Yes, I know I'm ignoring both refinery costs for the petrol (about 10% more energy), and the costs of extraction (5-20% depending on the source*), as well as electricity production costs and transport/conversion losses.
My very hand wavy, based on an absolute minimum of research, excuse is that they are probably a wash, or even give the ICE a slight advantage in the comparison. That advantage will only grow as the proportion of electricity production in the world moves towards more sustainable production.


So in round numbers we have about 250Wh/mile for an electric car, something around 15 for person+bike and maybe 700 for a petrol powered car. How to judge relative efficiencies depends on how the electricity was generated; if from gas or (heaven forfend) coal, the thermal energy needed to get that 250Wh of electricity was at least 600Wh, so (still ignoring transmission losses and refinery costs for the petrol) that is the number that should be compared with the 700 for the ICEV. SImilarly the 15Wh for the bike is a lot more than that in terms of the energy input in the form of mars bars or whatever. On that basis, and considering the mass difference, the bike doesn't fare that well.


I've only once had a car that did 50mpg in the real world. I am aware that there are more around nowadays, but I doubt that the ICE fleet is anywhere near that efficient:
38 billion litres of fuel sold "retail" as opposed to "commercial" in 2019, for 21bn miles (lorry+cycle milage) less than the total of 356bn miles = 335bn miles/38 billion litres -> 33 mpg (over 1kWh/m)

Except that the energy used during extraction and refinery are also pulled from the grid, so suffer the same losses - That's why I was ignoring them: a fairly conservative 20% production cost on the 700Wh is 140Wh, over half of the electricity cost for the EV to move - so any "grid penalty" pushes up the cost of both by a reasonably similar amount. Transmission losses on the national grid are measured and reported each month... they are sub 10% (within the margin of error for most mpg or Kwh/m measurements)
Heck, at 33mpg the extraction and refinery are using almost the same energy as an EV :shock: (I honestly didn't expect that to be anywhere near the case until really low efficiency).


The grid penalty for refineries is the overall grid balance, because they tend to run 24/7.
The grid penalty for EVs which predominantly charge at night (off peak) is weighted away from coal and other more "expensive" generation and towards wind/nuclear - that's ignoring the EVs which are charged by local micro solar during the day - yes all of the above have costs to build out, but so do oil wells, refineries, ports etc... I was explicit and honest that I wasn't doing a source to use complete analysis.

The external costs are even more biased against the ICE - since the EV point of pollution is centralised it is much easier to capture (or not generate in the first place, since combustion is better controlled in those cases where combustion is even needed).
Last edited by [XAP]Bob on 5 Feb 2021, 10:05am, edited 2 times in total.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
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Re: Electric power points for e-vehicles

Post by [XAP]Bob »

mjr wrote:
sjs wrote:On that basis, and considering the mass difference, the bike doesn't fare that well.

Considering what mass difference? The person being transported is the same weight. Is it worth all that extra energy just so the user avoids putting any energy in, goes a bit faster (or not at all in town) and has a longer range, or is it a bit selfish?



I was comparing the vehicle mass difference to illustrate that I felt the calculations made intuitive sense, but you are absolutely right that in the vast majority of cases the *cargo* mass is the more useful mass to compare, which is where the humble pedal cycle really pulls ahead.
It's interesting that it's well ahead anyway, even without "just" comparing cargo.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
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