Comparing total greenhouse gas emissions between PHEVs

M

Mark Miller

New Member
Some of us care about climate change and want to minimize our own carbon footprint. A year ago, in deciding which new car to buy, I decided to calculate which model would minimize my driving CO2, given my own driving pattern. You may think the largest EV range available in a PHEV would automatically be the best, but there is more to the story, as we will see.

I keep careful records of how many miles I drive each day. 64.5% of days, I drive less than 25 miles, 0.5% of days I drive between 400-499, etc. The overall average is 29.3 miles/day.

Considering the Prius Prime, I allocated the first 25 miles in each range 'bin' to EV, and remaining miles to gasoline. (I did buy a Prime, and my actual EV range is 25 winter, 36 summer).

Adding up all the range bins, this amounts to 16.6 EV miles, 12.7 gasoline miles per day for the Prime. In EV mode, Prime gets 3.95 mi/kWh (EPA est; my experience is 5.0), in hybrid mode a nominal 54 mpg. Dividing, this gives 4.2 kWh/day and 0.23 gal/day. My utility produces 1.24 lb CO2/kWh, and gasoline produces 20 lb CO2/gal. Multiplying, this gives 5.2 lb CO2 from the electricity used, and 4.7 lb CO2 for the gasoline burned, for a total of 9.9 lb CO2/day.

Compare this to the Chevy Volt: 45 miles nominal EV range, 3.23 miles/kWh, 37 mpg. Plugging these in would predict 12.45 lb CO2/day with the Volt, given my driving pattern.

Now, this is not a huge difference (~20% less), but it is significant. And nothing else I could buy within my price range and that I could take on road trips came close to these two in minimizing carbon. So my choice was clear.

Someone who takes more road trips than me would have a comparison even more in favor of the Prime. The most favorable for the Volt would be a person who drives exactly 45 miles/day every day, using the full EV range of the Volt while requiring 20 gasoline miles for the Prime. Even here, the comparison is 15 lb CO2 Prime, 17 lb CO2 Volt. The main thing is, since the Prime is more efficient than Volt both in EV (miles/kWh) and gasoline (mpg), the EV range comparison is less significant.

For folks who live where the grid is cleaner, it would be interesting to see how your numbers compare. Obviously, as the grid gets cleaner, the climate case for a higher EV range gets stronger.
 
Your methodology is good because it is based on your own metrics. Too often we see people trying to advocate for one plug-in over another without realizing individual requirements define the optimum answer. Let me share another approach.

I took the MPG, MPGe, EV range, and gas range to plot for all 2014 models:
i3_pip_020.jpg

Starting with a full charge and tank, around 150 miles, the first generation Prius plug-in begins to outpace the others except for the short range BMW i3-REx.

Here is another chart that reflects our two plug-in hybrids including a 2017 Prime and Volt:
volt_vs_bmw_vs_prime_020.jpg

Again, these are fully charged and full tank. The Y-axis is both MPG and MPGe:
  • <25 miles - Prime is most efficient
  • 25<72 miles - BMW i3-REx is most efficient
  • >120 miles - Prime is most efficient
The Volt has an advantage over the Prime above 25 miles and under 90 miles. But outside of this range, the Prime is supreme.

The BMW i3-REx has an advantage over the other two up to about 120 miles when it loses to the Prime. Its total range is too short to reach Volt range but the second BMW i3-REx tank would soon be the worst of all three.

Bob Wilson
 
This re-inforces just how great the Prius Prime is from an efficiency stand point (regardless of fuel it's using). Too bad Toyota hasn't developed a PHEV or BEV from the ground up. The Prime just feels like an after thought. I'd have one today if it didn't have the battery pack bastardizing the hatch space. The regular Prius is awesome for hatch space and I love my 2010 for that reason. I've always loved Toyota's but would have a hard time buying one today because of how much they've dragged their feet with electric.
 
I could fit all my camping gear, an inflatable kayak and a telescope in my old 2005 Prius. Yes, the Prime has a smaller cargo space, but I measured it carefully, and should be able to do the same. I'll find out later this summer! (Perhaps the telescope will stay home) I would not say an afterthought exactly, but clearly some compromises were made.
 
Bob Wilson, I like your graphs, but I have a problem with putting MPGe and MPG together on the same axis. MPGe is calculated by equating the electrical energy used to a volume of gasoline with equivalent chemical energy content. But this is misleading, because the electricity has already gone through the heat engine conversion losses (at the power plant), while the gasoline has not (yet). Therefore, using MPGe tends to overstate the energy contribution of gasoline vs. electricity, at least if your electricity is coming from a thermal plant, as is typical for most of us. Also, it does not directly lead to a carbon comparison. I'll see if I can reproduce your graphs using CO2/mile on the y-axis instead.
 
Mark Miller said:
I'll see if I can reproduce your graphs using CO2/mile on the y-axis instead.
Click to expand...
Go for it!

The EPA worked hard to make MPGe useful but I've seen some anomalies when looking at the kWh/100 mi and gal/100 mi. I have to think about it but I may be able to do another set of curves using these better metrics.

I was after a quick hack to compare the different cars in a way that shows the Volt 'donut' compared to the Prime.

Bob Wilson
 
Well, I don't have time between my various jobs to find a nice graphing software (better than Excel). Tell me what you are using, Bob? Anyhow here is a simple proof, at least for Prime vs. Volt, and for my utility (Consumers Energy, MI):

+++++++++++++++++++++++++++ 1.24 lb CO2/kWh **** 20 lb CO2/gal

***** EV range * miles/kWh ** mpg *** lb CO2/mile EV **** lb CO2/mile HV
Prime *** 25 *** 3.95 ****** 54 ***** 0.31 ********** 0.37
Volt ***** 53 *** 3.23 ****** 37 ***** 0.38 *********** 0.54

Prime is more carbon-efficient on gas than Volt is in EV. Therefore, Prime will be more carbon-efficient at any range.
 
I switched to OpenOffice because Microsoft office is so expensive. Worse, you pay for upgrades that break features you had been using.

Bob Wilson
 
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