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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.




This study just imply the business as usual scenario, discarding completely the role of the PHEVs in displacing fuel use in transportation, which is totally strategic considering the limits in oil production. After all, the alternatives to hydrocarbons are all based on production of electricity.


I think RS is on to something. If California can show the way to renewable energy and PHEVs then it would be a proven case. Arnold talks about the hydrogen highway, but what we need is action among state, county and cities with the utilities to prove that you can run your cars on clean energy. California has led the way on other issues, why not this one?
It would be good for the environment and good for the economy.


Well Caliornia already has one of the Greenest grids in the US.

It's primarily all Natural Gas, or better.


I grew up in an area that has a lazy river running through it. On the river was a dam with a rush. For those who do not know what a rush is: a ditch dug around the dam so water can rush around the dam and be used to turn a turbine to produce electricity. For some unknown reason a move is afoot to destroy these dams. The government is paying big bucks to have the dams torn down. This seems odd to me since we are in need of ways to produce more electricity.

Kit P

I have been asking the EV advocates for 20 years the same question for20 years. Where is the electricity going to come from?

I get an answer that demonstrates that they are clueless, something like this,

“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.”

Then how they explain that large coal plant just down the road, they pretend it does not exist or that it is not running.

Then there is this,

“Well Caliornia already has one of the Greenest grids in the US.
It's primarily all Natural Gas, or better.”

Then why are they building LNG terminals to import natural gas to California?

The electricity for US PHEV is going to come from imported LNG which is the worse choice I can think of.

This is a sad fact. It is a direct result of anti nuke, anti coal, and anti drill policies. For those who want to pretend that some banana republic has higher environmental standards, than the US, go ahead and call me names. For those want to pretend that countries with no regard for human rights, give a crap about AGW, please feel offended when when I suggest you are clueless.


Kit P,

Since you obviously are not clueless, what is your solution?

Harvey D


You should know that it is easy to generate clean electricity in very large quantity without using coal. We DO NOT USE coal power plants but we have more then enough Hydro & Wind generated electricity (right now) for at least 2 PHEV per residence. Please do not use your own local situation to pre-judge and proclaim that it is impossible to produce the clean electricity required for PHEVs (even if coal producers and ICE vehicle makers keep on saying so).

Denmark and Germany could do it with wind power. About another 100 countries could do likewise.

California and many other (US) States and countries could do it with Sun Power.

Others like France (+ all other countries) could do it with Nulcear.

Producing the clean electricity required for PHEVs is NOT a major challenge and is NOT even a good argument to support on-going use of ICE vehicles and coal fired power plants.



Some dams are being torn down because it is more important for people to have water than to have the electricity.

Go take a look at the Coloroda river from start to finish (in Mexico) today compared to 50 years ago. Essentially, we have turned it into a veritable stream by the time it hits Mexico when it used to be a life giving river.


Where the electricity now comes from does not matter. What matters is that it is getting greener every year. Eventually, in 100+ years , it will all be renewable.


It is interesting to speculate on what might happen if they found a BIG deposit of coal at the surface or a huge easy to get oil field. Would we back slide to the old ways once again? I do not think so. We have seen the problems with burning fossil fuels and that they are not sustainable. Maybe people see them as a starting point, but we do not want to continue to be dependent on them totally.


At 200 g CO2 per kWh (their low carbon scenario) and 5 miles per kWh assuming a Prius-size PHEV, that equates to 40 g CO2 per mile vs current Prius emissions of 166 g CO2 per mile.

PHEV lowers CO2 by 75% under low carbon regime, not 30-47% vs HEV as they claim.

But the biggest advantage of the PHEV is the capacity to buffer renewable energy, making it an enabler of yet more CO2 reductions overall. HEVs can't do that.


Clett, the 40 g CO2 you mention would be for the electricity only. We also included gasoline combustion and fuel cycle impacts (a PHEV would also use gasoline), battery production, and vehicle production. The use of PHEVs as a distributed resource to buffer intermittent renewables and provide ancillary services or other power back to the grid is a potential large benefit, and this is being investigated by several research groups.


FYI- California and other desert southwest states have 10 solarplants in the advanced planning stage that on sunny days will provide the equivalent of 3 nuclear power plants if built: They can be built quickly and with less cost and energy than any nuclear power plant, producing clean power while a nuclear plant is still under construction.

BTW- the DOE states that there is enough electricl capacity for 180 million plug in hybrids *right now* :


Hi CSamaras...Thankyou for that clarification to Clett's email. I was curious too.

Since my Northwest generates E. by hydro & increasing wind power, I'm curious how pure EVs might do compared to your various machines listed. I would assume any further extensions of your study to include pure EVs would include electric motors' greater efficiency to ICE & recycling potentials for various batteries? Also, would potential downsizing of EVs from ICE counterparts be offset by the weight of batteries needed to get a marginally viable range?

I would assume Americans would not be satisfied with an average EV the weight of a Prius & '5 miles per Kilowatt' would not be the consumption of the average EV.

Craig C

Response by DS: I'd guess that Carnegie Mellon is pushing nuclear.

Wow, it looks like Carnegie Mellon is beating up on all kinds of green solutions, they appear the NYT article,, cited above too:

“The one thing that’s eventually going to raise its head is desert biodiversity, and the land area itself,” said Terrence J. Collins, an environmental expert and professor at Carnegie Mellon University.

Building the plants in deserts poses another obvious problem: deserts are not exactly teeming with power lines. “Whatever you do, you’ve got to have the wiring,” Mr. Collins said.

Kit P

“what is your solution?”

Well Anne, it depends on what the problem is. Twenty years ago EVs were being touted as a solution to Los Angles air quality issues. At the time, the source of electricity was likely to be from a local single cycle gas fired power plant without pollution controls. In this case, EVs would have the unintended consequence of making air quality worse.

Various forms of EVs have been touted as a solution for AGW, oil depletion, and energy security. In other words, a solution in search of a problem. LCA has been developed to help us evaluate the environmental benefits. Based on this LCA (and others), EVs will not be a good solution until we can build nukes and renewable energy generations fast enough to reduce the demand for LNG.


EVs will happen when they get the right battery at the right price and enough people want to buy them. It has little to do with renewable energy. EVs can be charged at night from base load. There are 3 major huge nuclear plants around Southern California, the utilities have already said that they can handle charging 100s of thousands of vehicles at night. Southern California Edison already has 20% renewable energy.

BTW- Sempra Energy is putting a very large LNG terminal in Ensenada Mexico about 50 miles south of the boarder. It will provide natural gas for California and provide NG for combined cycle plants south of the boarder supplying electricity to Mexico and California. If we can get Mexico city to go EV, it might clean their air up too, they could use it. I do not think EVs are waiting for nuclear and renewable to solve the LNG problem, they are ready to go when they are ready to go.

Kit P

Wow, two years to build a 64 MWe solar thermal plant that does not run very well and 4 years to build a 1200 MWe nuke plant with a 95% capacity factor. So how did the journalist at the NYT come up with, “On sunny afternoons, those 10 plants would produce as much electricity as three nuclear reactors, but they can be built in as little as two years, compared with a decade or longer for a nuclear plant.”

No math skills.


Kit P,

You re-iterate you opinion that EV's are not a real solution (yet). But re-read your post, there is not a single idea in it on how to significantly reduce CO2 emissions from cars.

I'm an EV advocate too, and I'll be honest to say that I have no clear defined vision for the future of electricity generation. But the main advantage of electricity is that there are so many options: wind, solar, hydro, nuclear, geothermal, even coal (with CCS), etc. That's why I think EV's are smart. Wherever the technology leads us, we're not dependent on the public to change their habits/perceptions.


Wow, two years to build a 64 MWe solar thermal plant that does not run very well and 4 years to build a 1200 MWe nuke plant with a 95% capacity factor. So how did the journalist at the NYT come up with, “On sunny afternoons, those 10 plants would produce as much electricity as three nuclear reactors ..."

The 64 MWe solar Nevada one plant is already built and it didn't take 2 years. The 2 year quote was for the building of 10 plants already in the advanced planning stage. The 10 years for nuclear may very well include the planning and licensing. The 4 years for 1200 MWe is the best case scenario given by the builders. Very unlikely to be that good. The AP1000 reactors ordered by the chinese have operational start times 5-8 years from the time construction starts for instance. The EPR is 1-2 years behind schedule. The pressure vessels needed for nuclear reactors is produced in one japanese plant, that is behind orders. Even if the capacity factor was in the 95% range, it would require the retirement of assests probably not near the end of their lifecycles. Lastly, where did you get the idea that the solar thermal plant doesn't operate well?

Interesting that you interpreted the article in the fashion that you did, KitP, but it was an interpretation.

stas peterson


Pinnacle West, my local utility, and the majority owner and operator of the 3 operating reactor park at Palo Verde, is also the builder and operator of the world's largest Solar Plant, here in metropolitan Phoenix. They have been planning this solar plant described in this article for at least 5 years, and have been building it for at least the past two years, and expect it to be finished in under another 2 years or so.

It is expected to produce 120MW peak, of electricity output, rating from a 280 MW (thermal) plant. It is a large demonstration plant with an as delivered cost of over 0.15 dollars per KWH estimated. (Some estimates place this price at $0.25 per KWH)

This facility could never be justified, and would never have built, except for publicity protection it buys for Pinnacle to address the environmental community It also provides such protection to the Utility commission which would never have allowed such expensive electricity generation into the rate base, otherwise.

Pinnacle West is building it, along with the half dozen or so small demonstration solar plants, they have also built, already. Most (or all) of which are rusting in the desert, and are never used, except they make fine backdrops for photos in its brochures.

This solar electricity is by far the most expensive electricity from the mix of nuclear, gas, coal and solar plants that Pinnacle West and its operating subsidiary APS, do or will operate. In comparison, the Palo Verde Nukes have a delivered electric cost of between $0.0175 and 0.030 or less per KWH, or roughly one tenth to fifth the cost per KWH. It is difficult to discern from blended costs, and what is included in the accounting.

Once again the paper of record assigned an uninformed individual to writing this article. Quoting the thermal output of a solar-electric plant is equivalent to quoting the raw thermal output of a natural gas, coal, or nuclear electricity generating plant. It doesn't include the roughly 30-40% efficiency in conversion to electricity. A 1000 Megawatt (electricity) Nuke is actually a 3300 Megawatt(thermal) plant.

In that case, there is no way ten such solar plants, equal the output of even one Nuclear plant, except for the few peak hours per day that they operate, versus the 24x7 hours/day that a Nuke operates.

Ditto for Natural gas and coal facilities. This reveals the lack of knowledge on the part of the author. Ignoring other considerations, unless it is a conscious effort to be un-informative.

It is also perhaps the source of the "2 years to build" statement. The fact is that this is a custom plant of ganged together smaller installations, sized up, to this thermal rating.

A utility management, like any other business, when it needs some equipment, goes to commercial vendors to buy such. If it needs a truck, it doesn't open an engineering facility and a construction plant and assembly line, to make one. It simply buys a truck from a vehicle maker.

If it needs paper, it does not become a paper maker. It buys some paper, instead. Ditto for pencils, and any other items.

The same applies when purchasing some electric generating equipment. It goes to a electric generating equipment vendor, such as Combustion Engineering, or General Electric Company and requests a proposal.

There are ZERO makers of such solar plants in the world. There are no plant builders it could go to buy a solar plant of this size.

So Pinnacle West is doing the equivalent of becoming a truck builder to build this solar plant, because it needs a truck.Certainly it might be done in two years. Sure.

BTW, Pinnacle West is considering eventually ordering one or more new Nukes for it Palo Verde nuclear park facility. PV was originally sized for 5-10 such plants. These are not included in the 34 plants that are included in the pipeline, nationally.

California's electricity generation is indeed clean. It has no coal plants any longer; half its nuclear generation has been shut in, too. It also must import NG, LNG or electricity. Arizona, Nevada, Oregon, Washington, export electricity to California in massive amounts.

Rush Limbaugh

Mega dittos Stan. All funding for wind, solar, wave, or geothermal energy should be immediately cut, and any plants already built should be immediately shut down. They pose a grave national security risk.


Kit, we could always build electric plants to run on the displaced oil. At a minimum, our energy security would improve greatly as would our air quality. Both BEVs and Plug-ins are much cleaner than the typical ICE. The only car that compares is an HEV.

Stan, it is funny how you are willing to take all of the accounting liberties (omitted costs and subsidies) in the delivered cost of nuclear at face value but contest the estimated 15 cent per KWH of a solar demonstration plant.



the article had pictures of the factories where this stuff is being manufactured. Saying it doesn't exist when it does is your problem. For a solar thermal plant, it uses standard thermal components that have to be built. I'm sure the turbines for a nuclear power generator are mass produced and can be bought off the shelf...not.

You are fast in interpreting what the writer has done. But that is mere speculation and you know it. You haven't checked the sources and you haven't looked it up. But there you go, bolding going where no reasonable man would. On a rant.

Kit P

“how to significantly reduce CO2 emissions from cars.”

Stop driving them. Plant a tree and sit in the shade while tending your compost pile. Drink tap water but if you must consume packaged beverages recycle the beverage containers.

It is interesting how people have a vision of the future that requires engineers to ignore the laws of nature while ignoring the simple solutions.

All solar system do not operate very well. It is the physics of the energy source. So aym if you would like to provide some data to the contrary I am interested. The start of construction and commercial date of Nevada Solar one are know facts. The amount of electricity produced is a known fact. There is a reason the solar industry does not talk about performance.

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