Kwh required to replace existing Diesel engine in SUV

This is a very difficult problem.

You need only look at attempts by the United States Environmental protection agency to do just what you are trying to do. The best the EPA could come up with was the MPGe as a way to equate fuel energy to electrical energy. http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent

You are trying to do the same.

Lets looks at this problem by breaking it down piece by piece and see where we end up…

A diesel engine converts the chemical energy in diesel fuel to mechanical energy.

An electric motor takes the chemical energy in batteries and converts it to mechanical energy.

This is one of those apples to oranges problems. We need to look at apples or we need to look at oranges. We must not compare apples to oranges. What would the bananas think?

You mention kWh is what you are interested in. For this problem we will equate everything to kWh.

We will change our apples and oranges all to be kWh.

I am going to ignore everything else (carbon emissions, the mining of minerals mind, environmental impact, etc).

If I were to try to include those items in the calculation it would get very complicated.

I am also going to assume you want to keep the performance of your SUV exactly the same as what you have now.

No compromise in performance.

For the purposes of coming up with some sort of usable and meaningful calculation I am also going to assume you are not going to change anything else. No changes to aerodynamics, You want to drive exactly the same as you did before. No changes zip. Nada. No change other the source of energy your vehicle uses.

Here, I am just interested in energy. The energy it takes to do a job. The energy here makes your SUV move. And it will come from one of two sources either Diesel fuel of the electrical energy stored inside a battery.

Here we go…

I am going to warm up with some math and hopefully something pops out along the way that you can use.

1) Energy in Diesel Fuel

Wikipedia states, Diesel fuel has an energy density of 35.86 MJ/Litre with a specific density of 0.832 kg/Litre (http://en.wikipedia.org/wiki/Diesel_fuel)

We could also say that 1 Kg of diesel fuel has: 35.86 MJ/Litre * 1Litre/0.832 Kg = 43.1 MJ/Kg

Using the conversion factor: 1 watt-hour =3600 Joules

We can calculate that 1 kg of diesel fuel has 11.9kWh of energy contained within its molecules (43.1MJ/kg *1 watt-hour/3600J = 11972.2 watt-hours / kg or 11.9 kWh/kg)

2) Lead Acid Batteries

What is the energy density (MJ/kg) of a lead acid battery? (http://en.wikipedia.org/wiki/Lead_acid_battery)

The specific energy of an average lead acid battery is said to be 30-40Wh/Kg (for calculation sake lets say the average is 35)

How does this compare to Diesel fuel?

We calculated above that Diesel fuel contains 11.9 kWh/kg of energy

So... remembering to keep the units the same.

11.9 / 0.035 = 342

This means the energy content of 1 kg of diesel fuel is 342 times larger than the energy content of 1 kg lead acid battery.

To have access to the same amount of energy as you have in the diesel fuel contained in your fuel tank you would need 342 times per kilogram the number of lead lead acid batteries.

A 70 litre diesel tank of fuel contains 693 kWh or energy (70 Litres * 0.832 Kg / 1L * 11.9 kWh/kg = 693 kWh)

You would need 19800 kg of lead acid batteries (693 kWh * 1 kg / 0.035kWh = 19800 kg) to make up the same energy as contained in your 70 litre diesel fuel tank

Whatever lead acid based power train is installed it would need to be 342 times more efficient than the diesel power train in order to have the same performance with the same base machine.

This calculation shows the enormous advantage diesel fuel has over lead acid batteries when it comes to energy density.

This calculation also shows the enormous task before electric car designers trying to bring electrically powered vehicles to market.

Just swapping out your diesel engine for electric motors is a waste of money because the electric system will never perform as well as the diesel engine does.

If its not 342 times more efficient then your kWh/kg usage is actually worse and not better.

This is an example of why its troublesome to convert existing internal combustion powered vehicles into electrically powered ones. There is so little energy in batteries that it cannot compensate for how inefficiently modern day automobiles are designed to use energy.

Why are todays cars so poorly designed in their use of energy? There is so much energy in petroleum fuel that it compensates for bad design.

If you do want to do something for the environment, just drive your SUV when you really, really need to and walk or bike the other times.

If you just drive your vehicle 1 less day a week and walk or bike on those days you will have increased efficiency of using the energy available by 14% (1 / 7 = 0.14)

If you have a long commute and you want to have access to a vehicle then buy one that is more efficient than your SUV is today.

If someone tells you they can save you money by converting your existing vehicle to electric they are lying to you. Its absolute nonsense.

Save your SUV for those times when its power is needed. Use a more energy efficient vehicle for all those times a more modest automobile will do.

If you need access to your SUV all the time then get smarter in how you drive it. checkout http://en.wikipedia.org/wiki/Hypermiling

My name is John Vukelic and my email address is jvukelic [at] urbee.net

If I made a mistake in my calculations then I am very sorry and embarrassed.

If I got it right and saved you some cash. Then I want my Guiness.

Regards,
John Vukelic

how much energy required is a fuction of how far and fast you go, how many stops/starts and how many hills.

I normal use a scaling factor of 3kwh/L for diesel,

so you would need roughly 335wh/km at the battery for your SUV, give or take 25%.

so for 100km range you need a usable capacity of 33.5kwh,
with 200Ah TS cells that means you should be aiming for 65cells.

Matt

First off - I agree with Matt's estimate - it "seems" right to me. I've never seen the 3kWh/L for diesel rule of thumb before - but the wh/km seems right.

Secondly - another way of approaching the problem would be to look at EV Album and take a sample of similar size vehicles and their efficiencies.

Thirdly - and just for fun I decided to see if I could come up with a comparable figure to Matt's by starting with the info on that wikipedia page...

(a) 1 kWh is equivalent to 3,412 BTU
(b) 1 US Gallon of diesel contains 129,488 BTU
(c) 1 US Gallon = 3.785 liters
(d) Your SUV travels 100km on 11.5 liters of fuel
(e) Diesel SUV complete system efficiency is approximately 25%
(f) Electric SUV complete system efficiency is approximately 80%

So...

(i) 1 liter of diesel contains 3,4207 BTU or about 10 kWh
(ii) Your SUV consumes about 115 kWh worth of diesel in 100 km @ 25% efficiency
(iii) That's 1.15 / 4 = 0.29 kWh per km @ 100% efficiency
(iv) Which is 0.36 kWh per km @ 80% efficiency

And 360 wh/km is close enough to 335 wh/km.

Now, come on, tell me that wasn't fun! :-)

(Oh, and Matt's 65 cells comes from this equation: num_cells x 3.2V x 200Ah x 80% DoD = 33.5kWh)