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Study Finds That Without a Price on Carbon, Regular Hybrids Can Lower Lifecycle CO2 Emissions As Effectively as Plug-in Hybrids, and At Lower Cost

No CO2 price scenario. Final penetration of plug-in hybrids and regular hybrids in 2030 is plotted with corresponding changes in cumulative emissions from 2012 to 2030. The dotted line shows the incremental emission changes of plug-in hybrids compared to regular hybrids. Click to enlarge.

A new working paper from Duke University finds that in the absence of a price signal for CO2, regular hybrids can lower lifecycle carbon dioxide emissions as effectively as plug-in hybrids, and at a considerably lower cost.

In the paper, Eric Williams, co-director of Duke’s Climate Change Policy Partnership (CCPP), compares the two hybrid technologies to see which could lead to lower carbon dioxide emissions, operating costs and overall consumer costs. Williams used six plug-in hybrid penetration scenarios, each of which begins in 2012 and ends in 2030 with a final penetration into vehicle stock ranging from 2% to 56%. He also analyzed four additional scenarios, based on penetrations of 2% and 56%, that have CO2 prices of $20 and $40 per ton. He found that:

With CO2 pricing. Final penetration of plug-in hybrids and regular hybrids in 2030 is plotted with corresponding changes in cumulative emissions from 2012 to 2030. The dotted lines show the incremental emission changes of plug-in hybrids compared to regular hybrids. Click to enlarge.
  • Without a CO2 price signal, plug-in hybrids are essentially no better than regular hybrids at reducing lifecycle CO2 emissions;

  • With a significant CO2 price signal, plug-ins reduce moderately more CO2 emissions than regular hybrids;

  • Plug-in hybrids are significantly more expensive than hybrids at current gas prices; plug-ins become cost-effective at $6 a gallon.

The evening and night-time (off peak) charging of plug-in hybrids makes base-load power more attractive to utilities, Williams reasons. Currently, utilities build only enough base-load power to allow their base-load units to run almost continuously. To meet peak demand, utilities build units with low capital cost and high operating cost, knowing that these units will be needed for only short periods of time and can be turned off when demand drops. The largely night-time plug-in hybrid electricity consumption changes the shape of the demand curve so that utilities can build and run more base-load and fewer peaking units.

Taking a 56% plug-in hybrid penetration as an illustrative example, additional plug-in hybrid electricity consumption is directly responsible for 16.5 GW of new coal capacity by 2030. Renewables also increase by around 2 GW by 2030. Over 17 GW of combustion turbines and nearly 5 GW of oil and gas steam plants are avoided or retired as a result of plug-in hybrids. Overall, capacity needs are lower with plug-in hybrids because the base-load capacity that is built in response runs more frequently and alleviates the need for around 4.5 GW of total capacity. Lower penetrations of plug-in hybrids have similar results, though combined cycle builds tend to be less consistent (builds go up and down) at different plug-in hybrid penetrations. Generally speaking, without a CO2 price present, investment in coal and avoidance of combustion turbines (and to some extent oil and gas steam) is proportionate to plug-in hybrid electricity consumption.

—Williams (2008)

In terms of actual generation, not capacity, Williams concludes that generation increases by around 235 TWh with a 56% penetration of plug-in hybrids whether or not a CO2 price is present. This generation increase is needed to meet the electricity demand of plug-in hybrids. Under this scenario, coal generation increases by 190 TWh by 2030, while generation from natural gas and wood biomass increases by 15 and 18 TWh, respectively, and generation from other sources increases only slightly. Different penetrations of plug-in hybrids follow similar patterns.

If a $40-per-ton CO2 price is present, then a 56% plug-in hybrid penetration results in an additional 132 TWh of coal generation, 75 TWh of nuclear, 14 TWh of wood biomass, and only 5 TWh of natural gas generation by 2030. Both coal and natural gas generation are lower with a $40-per-ton CO2 price than without, and nuclear fills the gap.

It’s not a simple equation. Plug-in hybrids save gasoline but consume electricity. In most of the country, electricity generation relies on fossil fuels, which means that plug-ins would lead to an increase in electricity sector fossil fuel consumption and CO2 emissions. At the same time, plug-ins would reduce direct vehicle emissions. Taking this into account, I wanted to see how net emissions change, regionally and nationally, as a result of plug-ins.

—Eric Williams

The answer to that question, he notes, depends largely on whether there is a price signal for CO2 emissions. If federal or regional climate legislation places a limit on the amount of CO2 allowed, it will create a price signal that will drive the electricity sector to become more efficient and less carbon intensive. In this case, Williams says, plug-in hybrids would typically enjoy lower CO2 emissions nationally and in most regions compared to regular hybrids.

However, in a few carbon-intensive regions where electricity generation relies heavily on coal, plug-in hybrids would have lower net emissions than conventional vehicles, but not lower than regular hybrids. With respect to carbon mitigation, policymakers may want to focus on regular hybrids for certain regions rather than plug-in hybrids, even with a CO2 price signal.

—Eric Williams

In the absence of a price signal for CO2 emissions, Williams’ analysis gives the edge to regular hybrids. Nationally, plug-ins and regular hybrids reduce CO2 emissions by about the same amount without a CO2 price signal, he finds, but regular hybrids can do it more cost-effectively.

CCPP is an interdisciplinary research partnership of Duke’s Nicholas Institute for Environmental Policy Solutions, Nicholas School of the Environment and Center on Global Change. CCPP researches carbon-mitigating technology, infrastructure, institutions and systems to inform lawmakers and business leaders as they lay the foundation of a low-carbon economy.




yeah there has to be technological breakthrough to lower the cost of batteries...Look at NiMH battery. They are widely used now and mass produced yet failed lower the price...


"With respect to carbon mitigation, policymakers may want to focus on regular hybrids for certain regions rather than plug-in hybrids, even with a CO2 price signal."


With respect to carbon mitigation, policymakers may want to focus on increasing the generation of electricity with carbon neutral fuels for certain regions rather than allowing an increase in carbon-based electricity generation, regardless of a CO2 price signal.


All this assumes the NIMBYs will let all these coal power plants be built- http://www.pbs.org/wgbh/pages/frontline/video/flv/generic.html?s=frol02s494q72&continuous=1 -or that any newly built fossil fuel power plants wont have CCS.


Our current electrical energy, being 96% Hydro, 2% Nuke and 2% Wind favors the massive use of PHEVs. We have enough low cost night time electricity for at least one if not two PHEVs per familly.

The current 38% GHG from ground vehicles could be reduced by over 60% with widespread use of PHEVs and clean electricity.

With planned projects, the power mix will be about 75%-85% hydro, 10%-20% Wind and slighly over 1% Nuke by 2020/25. The new extra power will be enough for 2+ PHEVs/BEVs per family. Electricity export could be tripled +.


this is bull! it assumes that burning coal is the only way to meet baseload power needs. If the increased demand at nighttime can be met without increasing the combustion of coal, his argument is bogus.


It also addresses only CO2. The 2nd consideration is energy security, and plug-ins will help a great deal. However, I still believe we could start with plug-ins that get 10-20 miles on a charge and still have a huge impact at reasonable cost rather than looking for 100 mile+ range.


Rich: the generation of and capacity built for electricity in the last 30 years would provide strong support for this "bogus" argument.

I am for HEV over PHEV for the reasons stated: more cost effective CO2 reduction, hence it will have quicker market penetration and a bigger effect. But the analysis is perhaps overly harsh on PHEV: petrol will become more carbon intensive when it is sourced from CTL plants.


Yeah, but who in their right mind would base their energy decisions on the assumption that the future is going to be the same as or similar to "the last 30 years"?

Roger Pham

For energy security, plug-in Natural gas or Biomethane HEV may be a more practical alternative. A flexible fuel vehicle with a single tank for both methane and gasoline will fit the bill. The tank may be of lower pressures of 500-1000 psi to reduce weight and cost, and to allow form-fitting instead of a cylindrical tank, which is less space-efficient. The NG range may not be great, but it can be plugged in the the home NG compressor once every few nights, just like a PHEV. Again, low-pressure NG home compressor to reduce cost. Once NG pressures drop close to atmospheric, then the vehicle will run on gasoline, and can be refilled with gasoline for long trips. The tank will contain a few gallons of gasoline for backing up the low NG content. Biomethane will make it a renewable form of energy.

Biodiesel with Algae oil is another option for energy security, and don't forget H2 vehicles also.

Healthy Breaze

Roger Pham,

You said, "A flexible fuel vehicle with a single tank for both methane and gasoline will fit the bill."

Are you suggesting that you put a mix of gasoline and methane into a pressurized tank? Is it an either/or thing, or would the thing eject gasoline at high speed if breeched? How would you top it off, when you already had half a tank?

Healthy Breaze

Batteries need to get better and cheaper...yep. There's so much activity going on with Lithium chemistries, novel cathodes and annodes, novel membranes, and such, it just seems like the pieces have to come together in the next couple years.

The trick, as always, has been that batteries have to leapfrog a very mature, price-optimized product in petrol. The batteries we know should be possible can be more than adequate for BEV and HEV, and need not be inherently expensive at economies of scale. If we're making long term plans about modes of transportation, we should compare where each technology should be by 2012, since that's when most of the innovations under discussion would ramp up production, anyway.

Roger Pham

@Healthy Breeze,
Yes, unfortunately, the tank will eject gasoline at high speed if breached! For that reason, the tank must be built by high-strength carbon fiber re-enforcement so that it won't rupture in a survivable crash.

Or, alternatively, simply carry a smaller 2-gallon gasoline tank on the side for backup, to avoid putting methane and gasoline together. Once all the methane is consumed, then, gasoline, up to 12-18 gallons of it, can go onto the large main tank. Excellent point you are bringing up!


This is getting complicated. Some outfit I think called the Whole Earth Institute or similar concluded PHEVs were a mile in front. Personally I think cars like this will have more appeal than PHEVs. Of course if NG was diverted to car fuel there would be less available for peak load electrical generation.

Couple more points. What if the 'Car Czar' to be appointed by Obama thinks PHEVs are a winner despite this study? That's where the bailout money will go. Finally who will be able to afford $40k PHEVs with tight credit and a dismal job market that will drive down salaries?


$25K PHEV with 10 mile range would be a good starting point. There is no need to wait for 40 miles, or 100 miles.



I agree with you. Modular (4 KWh?) battery packs could achieve what you mentioned.

Todays electronic battery controls could easily manage 2, 3, 4, 5 or 6 modules.

Buyers would only buy as many modules as they need or can afford.

This common sense approach will come sooner or latter.


I also agree. 10 mile capability will reduce gasoline use by ~ 25% if the average commute is 20 miles. And then the drive system would be in place only requiring added batteries as the price comes down. For folks like me, the 10 mile range may be all I ever need for 90% of travel.


The main utility of PHEVs is not about carbon mitigation. It is about displacing oil use with some other energy source, be it coal or nuclear or solar or wind.

This is yet another one of those stupid studies that asks a stupid question and gets a stupid answer.

Peak Oil will hit us before global warming will. In fact, it already has; oil spiked due to production limits and crashed the world economy. It will keep doing that again and again until we can displace much of its use with something else.


Future is cloudy for ramp-up in coal use--new Secretary of Energy (Chu) is not a fan of coal.


"Peak Oil will hit us before global warming will."

Sweet crude may be running out but if we maintain our dependence on gasoline global warming will continue. We'll only switch to more carbon intense sources [like tarsands, oilshale and CTL] to get our fix.


The results don't seem to be in line with the current trends in new power plant investments. Almost no coal or nuclear. Only things that are being built in quantities are natural gas and wind power plants.

As charging of plug-ins could be controlled to happen during periods of low power prices, plug-ins won't necassarily promote base load any more than they will promote variable power like wind.

Willy Bio

And what about the benefits of Smart Grid? Wait, ignore that since it would render study STUPID.

richard schumacher

Part of the value of studies which use "stupid" assumptions is to point out the stupidities. Within 15 years it will be impossible to build new coal-fired power plants, and probably uneconomical to operate the old ones. Studies such as this one help hasten that day.


This study is correct only if one is considering the reduction of GHGs per unit cost because a HEV would produce more GHGs than a PHEV, except for a rather contrived scenario.

A PHEV in charge-sustaining mode is essentially a HEV, with a bit more weight but with a larger motor to recover energy during braking, i.e., essentially, it gives the same mpg (and produces the same amount of GHG emissions) as a HEV. In charge-depletion mode, a PHEV would produce far less GHGs than a HEV, except perhaps for the pathological case where the PHEV receives all its electrical charge from inefficient coal fired power plants that should be replaced soon anyway.

Coal accounts for about 1/2 of the electricity generated nationwide in the US, with higher proportions in the mid-west, which is ideal wind country.


Well it looks unanimous to me; nobody here is fooled by this 'report.'


The unresolved problems with intermittent power sources (Sun & Wind) is still a major deterrent for power distribution networks.

Europe is studying the possiblity of using a huge multinational DCHV power grid to use more Wind & Sun (and existing Nuke, NG and clean coal plants) to supply base loads and Northern Europe Hydro plants for peak loads and periods with lesser wind and sun power.

The same approach, if used in USA/Canada, could use the huge wind potential to a much higher percentage of total power.

Some 50 000+ very large wind turbines + existing and future Hydro + combined-cycle NG power (with peaking capacity), coupled to a transborder DVHV grid could supply much cleaner reliable power for our future PHEVs and BEVs.

Note: Most NG power plants can be reactivated (cold start) in less than 3 hours. Power ramp up is about 7% per minute.

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