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Thursday, December 26, 2013

One Gallon of Gas

Exxon recently released a 'One Gallon of Gas' ad. You can watch it here: http://www.ispot.tv/ad/76i8/exxon-mobil-one-gallon-of-gas

In the ad (spoiler alert) they say that the energy in a gallon of gasoline could run your smartphone for 3000 days. How great would that be? A phone that I didn't have to charge for 8 years!

A gallon of gas would be a lot to carry around, but I would be happy with a smartphone that I only had to recharge refill once a week. According to Exxon's 3000 days for 1 gallon math, a week would be possible with less than half an ounce of gas.

So why aren't we all doing this?

So let's examine this a little more. They didn't mention pollution, environmental impacts, foreign wars, or other issues. These are important, but they only talked about energy, so I'll do the same here.

Since they are talking about running electronic devices with gas, let's see how much energy gasoline has in kilowatt-hours(kWh), since these are the units of energy that run smartphones and other electronic devices. Depending on the summer or winter blend, a gallon of gas has between 33 and 33.6 kWh of energy. Let's be generous and use the higher of the two values.

Ok, it is obvious that Exxon is not truly suggesting that smartphones run on gasoline. Reading the subtext, this is really a jab at alternative fuels and specifically electric vehicles (EV) on the market today.

So let's cut to the chase and compare vehicle fuels. Let's say you have 1 gallon of gas compared to its energy equivalent of a 33.6kWh battery pack.

Depending on the fuel efficiency of the car that one gallon of gas will get you from 18 to 60 miles. The average fuel economy for new cars sold in 2012 (the latest year with current data) was 23.6 miles per gallon on average. So a gallon of gas will, on average, get you less than 24 miles.

Now let's put that one gallon of energy (33.6kWh) in an EV. The EPA rates the 2013 Nissan Leaf at 3.45 miles per kWh (although I consistently get over 4). So, using the EPA numbers, this "one gallon" of energy would get you 115 miles.

There you have it, the electric car will get you more than 4 times farther on a "gallon" of energy.

That concludes cars, let go back and finish the smartphone case.

Exxon does not mention the efficiency of converting gasoline to electricity to run that smart phone. They don't mention that running something like a portable generator would mean that most of the energy in that one gallon would be lost as heat or go out unburned in the exhaust.

Looking at the spec sheet for a Honda portable generator, it is between ~13% and ~18% efficient depending on load. At peak efficiency, the 6kW Honda generator I looked at will produce 6.13kWh per gallon of gas. For completeness: the worst-case was 4.42kWh, and the median was 5.7kWh.

So now let's take another look at Exxon's claim. They said if you could use all the energy in that one gallon of gas, it would run your phone for 3000 days. Well, we cannot extract all of the energy, despite more than 100 years in engine research and advancements.

Using the median value of 5.7kWh of my example generator, that gallon of gas only has enough usable energy to run your smartphone for 500 days. Not bad, but you'd have to drag a gallon of gas and a generator around with you.

Whereas if you were to power your smartphone by charging it in your electric car, other than some minor losses from power converters and self-discharge, you would get nearly all of the energy back and truly be able to power your phone for thousands of days.

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