Quote:
Originally Posted by NeilBlanchard
Why is it so hard to understand how it is that EV's are so much more efficient? My electric lawn mower uses the equivalent of just 0.0012 gallons of gasoline in an HOUR -- this is less than a teaspoon full. A gasoline powered lawn more would burn through ~0.09375 gallons (1.5 cups) in an hour -- about 78X more.
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Well I think one of us must be doing the math wrong. My Black and Decker electric lawnmower uses 12 amps at 120 volts AC: 12 X 120 = 1440 watts (about 2 hp). 1440 watts/1000 = 1.44 kilowatts. 1.44 kilowatts x 1 hour = 1.44 kilowatt hours. A gallon of gasoline contains 33,705 watt-hours of energy. 33,705/1000 = 33.71 kilowatt hours. 1.44/ 33.71 = 0.043 gallons of gasoline equivalent used.
I don't know of any gasoline lawnmower that would only use .09375 gallons in an hour. I wish I did. I couldn't find any fuel consumption figure for lawnmowers in particular, but I did find this for compressors that use similar engines.
SS.FM -- Fuel Usage Table If we take the Honda 2.8hp engine as an example, the fuel consumption is 0.24 gallons per hour (and it has 40% more power than my electric lawnmower).
So 0.24 gph (gas) / 0.044 gph (equivalent electric) =
5.6 times as efficient, not
78 times. But, of course, this ignores conversion and transmission losses as you cannot just pour electricity into your lawn mower.
Let's look at it from a cost standpoint. Around here regular gas is about $2.67/gallon and I pay $0.10/kW-hr for electricity. So the gas lawnmower would cost $2.67 X 0.24 = $0.64 to run for an hour. The electric lawnmower would cost 1.44 X $0.10 = $0.144 to run for an hour. 0.64 / 0.144 = 4.44 times as much to run the gas mower, or 0.64 - .144 = $0.50 an hour more. Sounds really attractive right? Yes, until you have the hassle of an extension cord and the "range" limitation of the length of that cord. Lots of people are willing to pay the 50 cents/hour more not to have to drag a cord around. Now if only we could come up with an EV that had a 100-mile long cord reel. Hmm . . .
Anyway, how does my math look? Did I miss anything?
Quote:
Originally Posted by NeilBlanchard
And, gasoline has far more "embedded" energy in it than does electricity -- far more. Nissan says that it takes about 7.5kWh of electricity to do part of the refining of petroleum into gasoline (and it takes some natural gas, too!). So the embedded energy of that electricity IS ADDED TO THE GASOLINE. So, the gasoline has all of the exploration, drilling, transportation, refinement -- which includes enough electricity to drive 30-60 miles -- and you still have to transport it and store it, etc., and then you get to burn it in your car.
So, without ANY of the other embedded energy -- just use that 7.5kWh of electricity directly in a car -- and you have saved ALL of that other energy.
!
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Well, this sounds good on the surface. But how do we get that "embedded" electricity into our electric car? If our transmission system has 50% losses (just a PFA number, I have no idea what they really are), then the 7.5kWh of electricity at the plug has 7.5kWh of "embedded" electricity, too.
A gallon of gas has 33,705kWh of energy. So the "embedded" part of the electricity would be 7.5 / 33.7 = 22.2% of the total energy available. The transmission losses would have to be less than 22% for electricity to be a net winner on that basis. Maybe they are, I don't know, but the higher the losses the less weight the "embedded" argument holds.
If we could generate the electricity right on site and put it straight into our electric cars it would be great. But, unfortunately that is not the case for most people.
I still think that the well-to-wheels evaluation is the only fair way to look at the situation, as it takes into account all of these variables.
Quote:
Originally Posted by NeilBlanchard
A gallon of gasoline contains a whole lot of energy -- about SIXTY THREE sticks of dynamite. And yet, we use less than 1% of that to actually move the driver...and about 80% or more of it is waste heat. How horribly inefficient is that?
To wait any longer to start being much more efficient -- by driving EV's, would be a crime. We are immorally using up this amazing resource AS FAST AS WE CAN, and we are completely altering our world, that we depend on. We cannot wait a moment longer. Let's not let the perfect be the enemy of the good!
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I agree with your last sentence, but the problem for most people is that the cost/benefit ratio isn't there yet. The practical EVs available today (or in the near future), like the Tesla, Nissan Leaf and Chevy Volt either cost a whole lot more than comparable gas cars, have limitations the gas cars don't, or both. Perhaps with the goverment incentives and eventual economies of scale we will get there someday.