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-   -   Let's be fair on the high-mileage EVs (https://ecomodder.com/forum/showthread.php/lets-fair-high-mileage-evs-8099.html)

Ernie Rogers 04-25-2009 10:37 PM

Let's be fair on the high-mileage EVs
 
I see that the four highest-mileage cars listed on the left margin of the home page are all electric cars, with the exception of the Mustang HP2g (see more on that elsewhere). The data provided in the EV mileage logs indicates the assumed energy content of gasoline. Here are the numbers I found:

Citicar................36.6 kWh per gallon
Electro Metro......36.5?
1992 Metro.........33.6
Fiero EV.............36.6

Except for the third car, one wonders if these people are intent on violating the first law of thermodynamics. I think the most widely-accepted energy content (low heat value) for gasoline for use in an internal combustion engine is 116,090 BTU per gallon. This was adopted by Wang at Argonne Nat'l Laboratory (e.g., the GREET model) and (I think) is the value used for calculations for the Auto X Prize. From my CRC handbook, the conversion factor from kWh to mean BTUs is 3409.5. Dividing these numbers gives a HEAT energy content of gasoline of--

34.05 kWh (heat) per gallon.

I acknowledge that the higher value of approx. 36.6 kWh (heat) per gallon is obtained if you use 125,000 BTU per gallon, but I believe that would be incorrect for referencing to real gasoline in an internal combustion engine that exhausts products as gases.

Let's be kind and assume that the calculations are based on a strict 100% conversion of gasoline heat of combustion to electricity. However, this represents an implicit violation of the second law of thermodynamics, which states that heat cannot be converted entirely to work.

If the EV car owners would like to be entirely fair in representing the efficiency of their cars, then I suggest they agree to abide by both the first law of thermodynamics and the second law. We could be generous and let them use say the theoretical (Carnot) efficiency of a gasoline engine, or possibly a high (power plant+transmission) efficiency for an advanced power system, at about 50%.

In such a case, I would suggest that they use a kWh-gallon equivalence of either 17 or 18 kWh per gallon.

Making this adjustment will allow a much more fair comparison between EVs and highly-efficient IC cars.

Ernie Rogers

Big Dave 04-25-2009 11:54 PM

Wowsers! Somebody familiar with Carnot efficiency.

Ryland 04-26-2009 02:55 AM

When I get my self better organized and a kill-a-watt meter on the charger for my comuta-car, I plan to keep track of power going in to the charger, it sounds like your formula is assuming that people are tracking the watt hours as it comes out of the batteries, not what it takes to charge the vehicle and that charging is what people tend to keep track of, energy in and miles driven.

Ernie Rogers 04-26-2009 11:56 AM

Quote:

Originally Posted by Ryland (Post 100479)
When I get my self better organized and a kill-a-watt meter on the charger for my comuta-car, I plan to keep track of power going in to the charger, it sounds like your formula is assuming that people are tracking the watt hours as it comes out of the batteries, not what it takes to charge the vehicle and that charging is what people tend to keep track of, energy in and miles driven.

Hello, Ryland,

I'm curious about your photo--that's not one of your vehicles, is it?

About the expression, "energy in and miles driven," I would be quite happy to work in those terms, but few people do. In this kind arrangement, you would report----

kWh (at the wall socket) per mile for the electric car

BTU (at the pump) per mile for the fuel burner.

The common term for this kind of vehicle efficiency is "pump-to-wheels." The problem with it is there is no satisfactory way to relate the two types of vehicles because the electricity is fundamentally different than the fuel.

Let's say you burn a gallon of gasoline and get 116,000 BTU of heat.

If you run 34 kWh of electricity through a resistor, you also get 116,000 BTU of heat. Electricity is "work." According to thermodynamics, you are allowed to convert work to heat with 100% conversion, like through a resistor.

But, the second law of thermodynamics says that you are not allowed to convert heat to work at 100%--it's impossible. The best you are allowed to do is the theoretical maximum, generally called the "Carnot limit." The machine that converts heat to work is what we call a heat engine. Fuel-burning cars use engines that always convert less than 50% of the heat of the gasoline to work. Work can move a car, but heat can not.

So, is it fair for the EV driver to assert that his car would get 120 miles per gallon running on gasoline? Not if he assumes the gasoline can produce 34 kWh of electricity--that's impossible. But, it would be fair to say that he might be able to get half of that, 60 mpg (or 17 kWh of electricity per gallon)because it is at least theoretically possible that at some future time a heat engine may reach 50% efficiency.

Ernie Rogers

Matt Herring 04-26-2009 01:04 PM

Is Ernie Rogers the famous Ernie, "owner of the re gen braking system Citi car", that bennelson posted the vid from you tube?

dcb 04-26-2009 01:05 PM

I see were you are coming from Ernie, but I don't think this will lead to complete "fairness" in the comparisons.

I think if you were to consider where and how we get petroleum from one place on the globe to another so it can be put in your car vs getting coal/wind/water/sun/bagasse/??? to a power generation station and maintaining transmission then the table would be slanted very much towards electric vehicles.

I don't have the research to say that with certainty, but if price per unit of energy is any indication then that pretty much tells the story.

Ernie Rogers 04-26-2009 02:04 PM

Wow, two hypermiling Ernies
 
Quote:

Originally Posted by Matt Herring (Post 100519)
Is Ernie Rogers the famous Ernie, "owner of the re gen braking system Citi car", that bennelson posted the vid from you tube?

Nope, I am not that Ernie. I'm this one--

Ern's Blog

Ernie Rogers

Ryland 04-26-2009 03:07 PM

yes, that light blue vehicle is one two electric cars I own, it's a 1975 Seabring Vanguard, Citi-car, like what TomEV drives, the electric car that I drive every day is a later version, of that car, a 1981 Comuta-car, I don't have any good numbers on the energy usage of it because my batteries are 2nd hand and in rough shape so I've been leaving them on the charger more then I would normally to keep them topped off and to equalize and desulphate the plates, I'm also using the original on board charger from 1981 that does not turn off, it just tapers the charge slowly as the batteries get closer to full, this makes it hard to get exact numbers on energy usage, but if I had to make an educated guess based off Kill-a-watt meter readings at the outlet.
Ernie I would like to see, based off your math, what kind of mileage you think my electric car gets, with my current lead acid batteries I would say I can go one mile on 250-300 watt hours, or about 3-4 miles per KWH as it comes out of the outlet, before the charger.
So what does that come out to in MPG? and at what point am I losing that 50%?

Concrete 04-26-2009 09:35 PM

Cost is a pretty good dipstick
 
Quote:

Originally Posted by dcb (Post 100520)
I don't have the research to say that with certainty, but if price per unit of energy is any indication then that pretty much tells the story.

I like this reasoning - cost tells us a lot about a system

Some data
losses are 7.2% in the US for electrical transmission
Electric power transmission - Wikipedia, the free encyclopedia

hard to compare to pumping and boating but ...
5-10% of oil cost is transportation
International Oil Transportation
I know this is not apples for apples - but transportation cost are practically identical

and if percent usage in vehicles is any indication
then that pretty much tells us a completely different story about cost

The costs are in another piece of the puzzle
part of it is in the subject Ernie is talking about - unit accounting, if you will

roflwaffle 04-26-2009 10:11 PM

The energy efficiency of distribution is not strictly related to the financial cost of distribution. If we know what those relationships are, then we can make a reasonable comparison. If not, all we're doing is shooting in the dark.


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