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Study: Meaningful GHG Benefit from PHEVs Requires Low-Carbon Electricity

Modeled GHG emissions from conventional vehicles, hybrids, and plug-in hybrids using current US average GHG intensity of electricity. Click to enlarge.

Plug-in hybrids (PHEVs) can displace a large fraction of gasoline use and the greenhouse gas emissions (GHG) associated with its combustion. However, total lifecycle GHG reductions from PHEVs depend heavily on the carbon intensity of the electricity used to grid-charge the PHEV battery pack.

A new study by researchers at Carnegie Mellon University concludes that—given US average GHG intensity of electricity (670 g CO2-eq/kWh)—PHEVs can reduce total lifecycle GHG emissions by 32% compared to conventional vehicles (CVs), but offer only a small reduction compared to conventional charge-sustaining hybrids (HEVs). A paper on their work appears in the 7 April edition of the journal Environmental Science & Technology.

Under a carbon-intensive electricity scenario (950 g CO2-eq/kWh), lifecycle PHEV GHG impacts are 9–18% higher than those of HEVs, Constantine Samaras and Kyle Meisterling conclude. Under a low-carbon scenario (200 g CO2-eq/kWh), however, PHEVs can deliver large lifecycle GHG reductions: 51–63% and 30–47% compared to CVs and HEVs, respectively.

The potential for PHEVs to achieve large-scale GHG emission reductions is highly dependent on the energy sources of electricity production...If large life cycle GHG reductions are desired from PHEVs, a strategy to match charging with low-carbon electricity is necessary.

...For large GHG reductions with plug-in hybrids, public policies that complement PHEV adoption should focus on encouraging charging with low-carbon electricity.

—Samaras and Meisterling

A 20% improvement in liquid fuel economy across the vehicle technologies results in HEVs having 4-13% lower life cycle GHGs than plug-in hybrids. However, if kWh/km (electric energy) requirements for PHEVs improve by 20% while holding liquid fuel economy constant for all vehicles, lifecycle GHGs from PHEVs are 10-13% lower than HEVs.

Lifecycle greenhouse gas emissions for the different vehicles under different scenarios of carbon intensity of electricity;fuel (liquid and electricity) consumption; and E85 use. Click to enlarge.

Their analysis includes GHG emissions associated with vehicle and storage battery production along with energy use and fuel production under three electricity generation scenarios: baseline average, carbon-intensive, and low-carbon. They also consider cellulosic ethanol use.

Vehicles considered in the study are: a conventional internal-combustion (IC) sedan such as the Toyota Corolla (CV); a hybrid electric sedan (HEV), such as the Toyota Prius; and three PHEVs, powered with liquid fuel and electricity from the grid. The PHEVs considered have electric ranges of 30 km (PHEV30), 60 km (PHEV60), and 90 km (PHEV90).

When charging PHEVs with electricity that has a GHG intensity equal to or greater than our current system, our results indicate that PHEVs would considerably reduce gasoline consumption but only marginally reduce life cycle GHGs, when compared to gasoline–electric hybrids or other fuel-efficient engine technologies. With a low-carbon electricity system, however, plug-in hybrids could substantially reduce GHGs as well as oil dependence.

...With the slow rate of capital turnover in the electricity sector, a low-carbon system may require many years to materialize. Considerable reductions in greenhouse gas emissions using plug-in hybrids in the coming decades will likely require decisions within the next ten years to develop a robust low-carbon electricity supply.



Matt Parrish

I think it would also be interesting to see this study with numbers from clean diesel, biodiesel, and diesel-hybrids. I don't know what I would expect to see, but I think it would help give a more complete picture of the options moving forward.


The results of this study are not surprising. PHEVs are first and foremost a strategy for displacing gasoline use, not reducing GHG emissions. However, since PHEVs could spend up to 90% of the day plugged in, this means that intermittent sources of renewable electricity (especially) wind have a great place to send their energy to, namely these vehicles.

Of course, this need not be a concern in any case if we built more nuclear power plants to displace coal.


This is pretty intuitive. If you charge your PHEV using electricity from and old polluting coal fired power plant, you do not reduce GHG emissions as much.

I think most people could have told you this. It is the levels that were not documented and now they are.

Sometimes people mix oil dependence with GHG, with the environment, with conservation with other factors. While they are all related, you can improve one without improving the others much. Sure it would be good to improve them all with one move, but one step at a time.

I see a lot of the idea that if we do not have enough for all cars, then it is not worth much. That if we can not charge in 5 minutes, forget it. Maybe it is a good thing that some people do not make the decisions for all of us.


I'll have a heaping helping of solar and wind energy please, hold the nuclear. It gives me heartburn.

David Anderson

A few PHEVs can charge from base load plants at night with zero additional plant emissions . Base load plants run, and emit, 24/7 anyway even if the electricity generated is not used. It's only when additional capacity, with additional emissions, is brought on line that the boiler emissions should count against the end user. The PHEV.

In addition those PHEVs, can be used to store and return electricity to the grid during peak hours. In effect, recycling electricity that would otherwise be wasted. Thus saving fuel, and emissions, during peak demand hours.

With all due respect to Carnegie Mellon, it's not as linear and straight-forward as they make it sound.

Harvey D

We all know that coal fired Power Generation Plants will have to clean up their act sooner or latter. Otherwise, vehicles electrification will not have as much GHG impact as it should.

Progressive Coal Power Plants replacement with Wind, Solar and even Nuclear could make a worthwhile difference.

Since our electricity is plentiful and about 95% Hydro, 2% Nuclear and 3% Wind (going up to about 6% in 4 to 5 years), Hybrids, PHEVs and BEVs mix could be a very good solution for us. Being amongst the top electicity users (20,000+ KWh/per capita/per year) another 3650 KWh/year/per PHEV would not be an unmanageable increase. Most people could recouperate that much with simple energy conservation measures.

Currently, almost 40% of our total GHG (about 14 tonnes per capita/year) comes from ICE vehicles.


...and what, pray tell, do we plan to do about the enormous volume of batteries we're going to have from this? We hardly recycle the ones we make now! Or is this one of those mysterious industries that'll just happen to pop up, just in time to save the day?

Harvey D


Our GHG was only 12.1 (not 14-tonnes) tonnes/per capita in 2006 (latest stats available).

Harvey D


Think recycling. It will be part of the new clean energy economy.

Healthy Breaze

I second that on battery recycling. There's not enough lithium to make the batteries we want, so we will have to recycle. They will be worth considerably more than a $5 core charge for proper disposal of a Pb acid battery.

Radical Green

This is neat and all, but why don't we start with the basic problem: we're too many people on this planet. As John Feeney points out in his latest interview, more people means more consumption (= ecocide):

Well duh.

Depending on the study, a Parallel PHEV powered by coal ranges from being slightly worse, to slightly better than a conventional hybrid.

Tom Street

I am not surprised since I've previously expressed the opinion that the co2 savings from PHEVs are marginal compared to HEVs.

I would like to see a further analysis,however, that analyzed the effects of larger batteries and electric motors on HEVs. It seems as though less fuel would be required if the battery could be charged to a higher level than currently possible with the Prius, for example. Going down hills, I usually max out the capacity and then don't have as much power to rely on the motor before the ICE kicks in.

Alternatively, perhaps vehicles could be programmed to optimize grid power vs internally generated power depending upon the fuel mix for one's location.


@ Harvey D
Hence the comment, "Or is this one of those mysterious industries that'll just happen to pop up, just in time to save the day?"
We're supposed to be recycling batteries now, yet we don't - we stockpile.

Craig C

Why would CM go to all this trouble to make the obvious point that using electricity from dirty sources isn't green? Who funded the study? Do they disclose funders?


I also did a cost comparison, PHEV against my VW Golf TDI, based on inflated california electricity costs (32cents/kWhr). The PHEV cost per mile was about the same.

The VW Golf TDI has much lower capital costs (bought it used for $10K), but somewhat higher maintenance cost is expected.

So PHEV have been somewhat overhyped for costs and GHG.


the inherent, completely shortsighted assumption here is that PHEVs will necessarily use todays ICE engines as extended range units. that would be braindead.
linear combustion engines, microturbines etc. would all be an option, with way higher efficiencies than current best of the breed Atkinson cycle in Prius.
PHEV opens up the trade space for burning fuels more efficiently in cars, because the power source will be decoupled from torque and RPM requirements.


CO2, 0.0378% of Earth's atmosphere,
water vapor 0.44%
Argon, 0.94% volume

Sequestration of unnecessary water (lakes, rivers, streams) underground lowers global water vapour.


You should be able to run BEvs and PHevs on base load, nuke, wind and gas, all with very low incremental Co2.

You will need "smart charging", but this should be the easiest part of the plan.

Also useful would be to get people into smaller vehicles and efficiency in general.

It will be much easier to build a 1.25 ton PHEv than a 2.0 ton one, even more so for BEVs.

In wealthy countries, you could have 2 cars / family, a BEv for short runs and an ICE for longer ones. All you would need is a tax and insurance code that would encourage people to buy a BEV as a second car (and use it).

Why would CM go to all this trouble to make the obvious point that using electricity from dirty sources isn't green? Who funded the study? Do they disclose funders?
I'd guess that Carnegie Mellon is pushing nuclear.
Healthy Breaze

1. PHEV will get more efficient, just as ICE will.
2. Energy source flexibility (gasoline...or coal, or Hydro, or wind, or solar, or nuclear) wil allow a more robust, stable and optimally green energy profile for PHEV, moreso than straight diesel or HEV.
3. PHEV charging at night will be almost free from a well-to-wheels POV for the first few years, because Coal and nuclear plants produce energy they otherwise can not sell at night, because those plants can't just shut down when demand drops at night.


Too bad you have to have an account or purchase the paper to read it, I was interested in what type of vehicle they used to project the PHEV 30, 60 and 90. I will have to guess it was a Prius.

I wonder how the results would turn out if they used a Camry or Altima instead since you have the exact same vehicle for the CV vs HEV comparison and then you can try to project some numbers for a PHEV based on either of those two cars as well.

Rafael Seidl

For a large state like Germany or California, it would be possible to operate virtual powerplants combining solar, wind, biogas and existing hydro resources to reliably deliver the required amount of electricity 24/7. This is more expensive than nuclear only if you disregard the future cost of dealing with the radioactive waste.

The main problems are the rickety grid in California and the botched electricity privatization there.

@ David Anderson -

base load power stations are called that precisely because they do in fact deliver the amount of electricity that is consumed 24/7. Typically, they run in part load during the night because running some at full power and switching others off causes thermal fatigue and other problems. It's not as if large amounts of electricity are generated merely to be converted to heat in snubber circuits because there is no other consumer for it.

If a large number of PHEVs were to charge during the night, existing base load generating capacity would be better utilized. However, more coal or gas would be consumed in the process. There's no such thing as a free lunch.


Now one might ask
If they are assuming the worst for electricity.

Then are they factoring in Canadian Tar Sands being used to produce 1/4th of the US supply of gasoline?


Now one might ask
If they are assuming the worst for electricity.

Then are they factoring in Canadian Tar Sands being used to produce 1/4th of the US supply of gasoline?

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