• Mar 10th 2011 at 10:56AM
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Honda Accord plug-in hybrid demonstrator – Click above for high-res image gallery

A team at Carnegie Mellon University modeled the net emissions of plug-in hybrid electric vehicles (PHEV) under a slew of different scenarios and discovered that, although the widespread use of PHEVs will likely result in a reduction of CO2 and NOx emissions, the overall environmental impact of PHEV's is highly dependent on several variables. The team concluded that charging strategy, battery pack capacity, driving patterns and the type of liquid-fuel engine used in a PHEV will impact net emissions and, in some cases, could even negate the environmental benefits typically associated with driving a plug-in hybrid.

In its research paper published in Environmental Science & Technology, the Carnegie team estimated the effects of ten percent of the light-duty vehicle fleet becoming PHEVs. The short version is that if coal power plants are used to power PHEVs, we'll get, "lower benefits unless coal units are fitted with CCS [carbon capture and storage] or replaced with lower CO2 generation. NOx is reduced in both RTOs [regional transmission organizations], but SO2 increases."

Whether or not PHEVs will actually contribute to a reduction in net emissions depends upon the average fuel efficiency of the conventional (gasoline- and diesel-engined) vehicle fleet. That is, if CAFE standards soar, then use of PHEVs may increase net emissions of both CO2 and NOx.

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    • 1 Second Ago
      • 4 Years Ago
      Lets take,

      Why spend tons of money converting coal to gas, and or capturing carbon when using the NG that is available now at a reasonable cost......and let's not talk about the cost of fuel cells.

      Having said that I will go study FC/gas turbine combos. sounds interesting.
        • 4 Years Ago
        I'm not suggesting that either coal or NG is preferable - there are no doubt different pros and cons for each - just that they can both be used to create electricity with greater efficiency via a FC and gas turbine than with just a gas turbine alone.

        However, the US has plenty of coal, and to suggest that we won't use it is absurd.

        "A coal gasification power plant, however, typically gets dual duty from the gases it produces. First, the coal gases, cleaned of impurities, are fired in a gas turbine - much like natural gas - to generate one source of electricity. The hot exhaust of the gas turbine, and some of the heat generated in the gasification process, are then used to generate steam for use in a steam turbine-generator. This dual source of electric power, called a "combined cycle," is much more efficient in converting coal's energy into usable electricity. The fuel efficiency of a coal gasification power plant in this type of combined cycle can potentially be boosted to 50 percent or more.

        Future concepts that incorporate a fuel cell or a fuel cell-gas turbine hybrid could achieve efficiencies nearly twice today's typical coal combustion plants. If any of the remaining heat can be channeled into process steam or heat, perhaps for nearby factories or district heating plants, the overall fuel use efficiency of future gasification plants could reach 70 to 80 percent.

        Higher efficiencies translate into more economical electric power and potential savings for ratepayers. A more efficient plant also uses less fuel to generate power, meaning that less carbon dioxide is produced. In fact, coal gasification power processes under development by the Energy Department could cut the formation of carbon dioxide by 40 percent or more, per unit of output, compared to today's conventional coal-burning plant."

      • 4 Years Ago
      • 4 Years Ago
      Do these studies take into account the emissions generated by producing and transporting petrol and diesel? Surely, electricity, which is largely produced by burning coal, is also used in the refining of oil into usable feul.
      • 4 Years Ago
      What other choice is there? Aren't fossil fuels by definition unsustainable? The problem needs to be tackled from all angles at once, we can't say that making one change (moving to EV's) might not yield immediate perfect results simply because the other end of the equation (electricity generation) hasn't been fixed yet.
        • 4 Years Ago
        You are right, BC. It will be a race between technological advance and the proliferation of people who will be able to afford cars in what used to be the third world. The results of research in every direction will yield results in time. As for sustainability, I suspect with natgas, coal, and undiscovered oil deposits there will be enough fossil fuels to last us until we don't need them anymore.
        • 4 Years Ago
        Excellent point. Too many elect to do nothing because one action might result in an undesirable outfall. We cannot expect perfection from a single action. It takes a portfolio of actions to address all issues sustainably.
      • 4 Years Ago
      I'm an old gas turbine guy (entire career @ Garrett Power Systems) and I like SOFC's. We never dabled in this concept...at least when I was there. I love it.

      • 4 Years Ago
      Keep in mind this study used the converted Prius blended plugins. The superior variable blended discharge lowers the Wh/mile electricity consumption, never exceeding more than 200 Wh/mile.

      From the EPA figure, we know Volt consumes 360 Wh/mile so emission from the electricity will be higher. DOE did another study with PHEV40 (series hybrid) that is much closer to the Volt (using 346 Wh/mile). That report said Volt would emit less emission than a non-hybrid BUT Volt would emit more emission than a cordless hybrid.
        • 4 Years Ago
        It could be possible. Don't forget that the Prius only uses the electric only at low speeds and the gas engine cuts in at higher speeds. The sweet spot for EVs is about 20mph. It is these low speeds where we read that EVs including the Tesla Roadster gets about 500 miles range on a single charge. Since the Volt uses electric drive at all speeds, and the EPA rating is the "combined highway/city" cycle it cannot be directly compared.
        • 4 Years Ago
        200 wh/mile for a converted prius in EV mode?? I don't think so. They must have left out charging efficiency or it includes both gas and EV miles combined.
        • 4 Years Ago
        The Volt is actually less efficient than the sticker efficiency of a Tesla Roadster (~30kWh/100 miles).

        And again the results where emissions are higher is from coal heavy assumption, which isn't necessarily the best assumption, given we are below half right now in terms of US average and trends point to less and less coal being used.
      • 4 Years Ago
      Like it or not, the oil companies are already fracking lots of NG out of US fields (some of which are in peoples back yards).ie they are replacing lost production in oil w/ NG. We will slowly see a move to this clean burning fuel. Coal plants will be taken off line and replaced w/ Combined cycle natural gas fueled power plants. These plants, with a gas turbine engine combined w/ a steam cycle bottoming plant have a cycle efficiency approaching 60% PLUS the natural gas as fuel gives an added reduction in CO2 emissions. These plants emit only around 385 g CO2/kwh while a coal plant is anywhere between 800-1050 g/kwh. Natural gas is abundant and cost effective.

      There is a perfect synergy between NGCC plants and EV's.

      AND they solve the problem mentioned in the study.
        • 4 Years Ago
        CCGT's can also make use of solar steam assist and share the costs of the steam cycle, also there is a massive overlap with balancing wind power and using bio, synthetic and unconventional natural gas.

        Also the first and likely IMO only major use of fuel cells will be stationary, high temperature SOFC's running of methane as CHP units, I think it is very unlikely we will see mobile fuel cells outside of shipping but its hard to predict what the technology will do!
        • 4 Years Ago
        @3PeaceSweet Yes! This IS where we will see the first mass use of FCs. We can expect one of the leaders to be Bloom Energy's SOFC CHP unit. Other startups will push this application forward. If we can convert just 20% of residences to CHP connected to local grids - demand to maintain and build new coal plants plummets.

        Not to mention the permanent removal of overhead power lines and attendant vulnerability.

        • 4 Years Ago
        Methane is the primary component of natural gas as well as coal-gas, so I agree that NG and coal-gas fueled SOFCs are going to be part of the initial wave of stationary fuel cell use. They're already sold in the hundreds to hospitals, schools, and businesses that need a reliable source of clean power.
        • 4 Years Ago
        There's a big move to go from burning coal directly to converting into coal-gas to use in either a turbine or a stationary fuel cell. Coal will still be used for a very long time, but with carbon capture/sequestration techniques applied.

        As far as NG (or coal gas), the move there is to a combined FC/turbine combo, which improves the efficiency even higher than a turbine alone.

        "Analysis indicates that with such SOFC/GT hybrids an electrical efficiency of 55% can be achieved at power plant capacities as low as 250 kW, and ~60% as low as 1 MW using small gas turbines. At the 2 to 3 MW capacity level with larger, more sophisticated gas turbines, analysis indicates that electrical efficiencies of up to 70% are possible."


        Keep in mind, that's electrical efficiency. Other studies show that if the waste heat is used, overall efficiency gets into the 80% range.

        The BEVs we will buy in the future will certainly be powered by electricity created via fuel cells.
      • 4 Years Ago
      Who cares? The entire effort is not to reduce these emissions (which they do) but in reality to reduce the cost of transportation in the face of rising oil prices. Anyone who thinks otherwise is being naive.
        • 4 Years Ago
        Well put. The challenge is to lower the use of fossil fuels especially those imported from hostile areas requiring heavy defense efforts. Which means that some of our annual $500B payment for foreign oil will be lower, keeping the money at home - hopefully buying domestic renewables.
      • 4 Years Ago
      Good input Let'stake.
      • 4 Years Ago
      It's the old unintended consequences again. They really don't know what the results will be. What we do know is we will save money on our transportation which could be used for consumer driven economic growth, and use less imported fuel with EVs and various hybrids. They are good for the economy. A good economy results in increased government revenues without adverse economic effects. Increased government revenues could provide more discretionary funds to deal with environmental issues if you can keep the politicians, lobbyists, pork, and other special interests from getting it all.
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