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ORNL study finds best current use of natural gas for cars is efficient production of electricity for EVs

24 September 2014

Curran
Top: Components of well-to-wheels pathway. Middle: WTW efficiency for CNGVs. Bottom: WTW efficiency for EVs. Curran et al. Click to enlarge.

A well-to-wheels analysis of the use of natural gas for passenger vehicles by a team of researchers from Oak Ridge National Laboratory (ORNL) has found that, with a high PTW (pump-to-wheels) efficiency and the potential for high electrical generation efficiency with NGCC (natural gas combined cycle) turbines, natural gas currently is best used in an efficient stationary power application for charging EVs.

However, they also noted, high PTW efficiencies and the moderate fuel economies of current compressed natural gas vehicles (CNGVs) make them a viable option as well. If CNG were to be eventually used in hybrids, the advantage of the electric generation/EV option shrinks. Their open access paper is published in the journal Energy.

Because the use of natural gas for transportation requires compressing, liquefying, or conversion, it is important to determine the best use of natural gas as a transportation fuel. Specifically, to minimize GHG emissions and total energy use, is it better to use natural gas in a stationary power application to generate electricity to charge EVs, to compress natural gas for onboard combustion in vehicles, or to reform natural gas into a denser transportation fuel?

—Curran et al.

The study investigated the the WTW energy and emissions from the use of natural gas in CNGVs with a range of CNGV fuel economy and natural gas compressor efficiency. The authors compared these results to a range of fuel economies from an EV that was charged from electricity produced from the US mix and a range of natural gas turbines with varying efficiencies.

The WTW analysis focused solely on the fuel-motive power cycle, disregarding the vehicle cycle—i.e., the associated energy and emissions for the battery, power electronics, and auxiliary systems found only on battery EVs and for the CNG tank and auxiliary systems only found on natural gas vehicles. The analysis did not address the vehicle cycle cradle-to-grave energy use for batteries and CNG tanks. Cost considerations on the total infrastructure or cost of ownership were also outside the scope of this work but are nevertheless important, they team noted.

For modeling both cases of CNG for CNGVs and natural-gas-fired stationary power for EVs, the researchers assumed that both systems are fed from the same North American natural gas pipeline and as such have the same upstream energy use and emissions to the point of the pipeline. This includes the energy and emissions associated with natural gas recovery for North American natural gas, North American shale gas recovery, natural gas processing, as well as transmissions and distribution.

Their analysis also assumed the US mix for sources of electrical generation—in which natural gas is used in a number of different ways. For stationary power for EVs, they varied the fuel mix; for all other calculations including upstream refinery operations, they assumed the US mix. Electricity generation has 8% T&D (transmission and distribution) loss. The share of conventional natural gas and shale gas was assumed to be 77% and 23%, respectively.

For power generation in the US, natural gas is commonly used in both simple-cycle natural gas turbines and combined-cycle natural gas turbines which use waste heat recovery to increase electrical generating efficiency. The efficiency for combined-cycle natural gas turbines ranges from to 36%-50.7%

The ORNL team analyzed two categories of vehicles: current vehicles as well as future technologies that are not currently in the market but are conceptually valid—for example, CNG hybrid electric concepts.

The baseline for comparison is based on a 2012 2.4 L Chevrolet Malibu with a combined fuel economy of 26 mpg (9.0 L/100 km). EV fuel economy is based on a 2012 Nissan LEAF—99 mpg gasoline equivalent (mpgge) (equivalent to 2.4 L/100 km). The CNGV is based on a 2012 Honda Civic natural gas vehicle with a combined EPA label fuel economy of 30.9 mpgge (equivalent to 7.6 L/100 km).

1-s2.0-S0360544214008573-gr10
1-s2.0-S0360544214008573-gr11
Results of WTW analysis for current vehicle technologies. Left: WTW energy use. Right: WTW greenhouse gas emissions. Curran et al. Click to enlarge.

Future technologies. The team compared the WTW results of the analysis of current vehicle WTW technologies to a number of advanced vehicle architectures including both a grid-independent HEV without plug-in capabilities and a PHEV (plug-in HEV) with a 20 mile (PHEV 20) and 40 mile (PHEV 40) all-electric range; a SI ICE, and a CNG engine. For the PHEV cases, both charging from the US mix and charging from a natural gas turbine with a 45% electrical generating efficiency were considered.

Also considered were:

  • Hydrogen fuel cells using hydrogen derived from natural gas and CNG fuel cell vehicles, where the CH4 to H2 conversion takes place onboard;

  • Methanol from natural gas in an SI ICE;

  • A CIDI (compression ignition direct ignition) vehicle running on ULSD (ultra-low sulfur diesel) fuel;

  • E85 flex-fuel vehicles using conventional corn-based ethanol and cellulosic ethanol.

They did not consider other gas-to-liquids pathways—e.g., FT diesel.

They also assumed that as future regulations on RPS (renewable portfolio standards) are enacted, the GHG emissions factor associated with the US mix will change. They assessed scenarios for 25% (RPS-25) and 50% (RPS-50) renewable portfolio standards for EV use along with the current US mix, natural gas, and coal.

Curran
Estimated WTW GHG emissions for future vehicle technologies. The researchers commented that the figures show that even factoring in the very high TTW fuel economy of the electric vehicle, the upstream efficiencies from generating electricity can significantly degrade the WTP efficiency and therefore the total GHG emissions and energy use.

The high-efficiency CNG hybrid case illustrates the importance to fuel economy of ICE engines of keeping the WTW energy use and emissions low, regardless of WTP efficiencies.

The RPS cases illustrate the effectiveness of renewable power generation on the EV.

Significant WTW GHG reductions would be expected for both CNG and EV scenarios that used bio-methane or landfill gas. Curran et al. Click to enlarge.

[The results] can be generalized to say that the most effective use of natural gas in transportation ultimately depends on the efficiency of the combustion prime mover, whether on vehicle or in a stationary power plant. The difference in WTW energy use and emissions between CNGVs and EVs depends on the method of producing electricity from natural gas. The results presented here for the high-efficiency CNG hybrid case also illustrate the potential benefits of increasing the engine efficiency for CNGVs, which could be realized by optimizing engine operation around the high octane of CNG.

… The efficiency of both the prime mover and the fuel pathway processes is critical for keeping WTW energy use and GHG emissions low for the both the EV and CNGV scenarios. In each case there are multiple processes to convert natural gas to motive power, all of which have losses. With an EV, the primary energy use is in converting fuel into electricity for grid charging, while for a CNGV, the primary energy use is in converting fuel into vehicle motion.

—Curran et al.

Resources

  • Scott J. Curran, Robert M. Wagner, Ronald L. Graves, Martin Keller, Johney B. Green Jr. (2014) “Well-to-wheel analysis of direct and indirect use of natural gas in passenger vehicles,” Energy, Volume 75, Pages 194-203 doi: 10.1016/j.energy.2014.07.035

September 24, 2014 in Electric (Battery), Hybrids, Lifecycle analysis, Methanol, Natural Gas, Power Generation | Permalink | Comments (37) | TrackBack (0)

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Comments

This confirms what many posters have been saying for many months/years. Electrified vehicles (buses, trucks, locomotives, light trucks and cars) make more efficient use of energy and should be used as widely and as quickly as posible.

Fossil and bio-fuels should be restricted for commercial-military airplanes and selectively for power plants with appropriate clean-up Equipment.

Yep! The move here in California is replacing the NatGas plants with renewables and storage batteries. The oil and coal plants are gone.

How's your state doing? Still burning high mercury coal and building up mountains of ash?

It depends how you make the electricity, many of the new plants are combined cycle, but out of a thousand large power plants only a small percentage are combined cycle. There is a natural gas steam plant near us that produces 2 gW, but is less than 35% efficient.
Only the very latest designs that are combined cycle can claim close to 60%. NG to H2 is more than 80% efficient, PEMFCs are 50%, so you end up with transport that rivals EV.

Here is the average efficiency of natural gas plants in the US in 2012:
8039 Btu thermal for 3412 Btu/kwh works out at 42.4% efficiency.

Yep, replace those all with the latest combined cycle turbines, and maybe rebuild the grid so it doesn't lose another 6-8% of that transmitting it, and it would be pretty efficient.

Easy, really. :-O

BTW so that power can be ramped up and down swiftly, for instance to allow for renewables to vary from passing clouds, gusts of wind and so on, then gas turbines are often used as spinning reserve, ie they are running at low levels without producing any power.
Under those circumstances you can have as efficient turbines as you like, the overall efficiency will still suffer.

The article seems to gloss the issues over somewhat.

SJC> NG to H2 is more than 80% efficient...

Until you have to compress it to 10,000 PSI

@evi:
They are aiming towards 95% compression efficiency for 2015:
http://www.hydrogen.energy.gov/pdfs/review12/pd048_lipp_2012_o.pdf

Every other target for hydrogen and fuel cells has been met or exceeded, so there seems no reason why this one should not be too.

In addition, the PEM fuel cells used in cars, as opposed to stationary applications, are up to 60% efficient:
http://en.wikipedia.org/wiki/Fuel_cell#Efficiency_of_leading_fuel_cell_types

So: 0.8*0.95*0.6 = 0.456 total efficiency.

In practical terms that should mean that hydrogen from natural gas through a fuel cell car is somewhere north of 40% efficient, which is pretty good compared to the real electricity grid.

There is not much in it for efficiency between battery electric cars and fuel cell ones, despite some of the tendentious figures floated around claiming vast disparities.

With compression 70% x 50% is 35%. 40% power plant minus 5% for transmission, 10% for conversion, 10% for battery round trip, you end up with 32-35% for EV.

I find it ironic that researchers from Oak Ridge did not include the combination of nuclear-generated electricity charging EVs in their comparison of greenhouse gas emission scenarios.

Elephant in the room, that...

Breeder reactors would use the 700,000 tons of depleted uranium in the U.S. enough to power the country for hundreds of years. You could charge a WHOLE lot of EVs with that.

The end user cost tells the true story of efficiency of H2 vs EV.

$5-$6 gge for H2
vs
$1 gge or less for EV

When H2 can meet or beat EV cost of operation and CO2 emissions, I'll be all for it.

(Except by then my solar installation will be paid for, so personally, I probably will stick with charging at home)

You should include another 50 cents per GGE for taxes to fix the roads and another dollar per GGE for the battery replacements.

'The end user cost tells the true story of efficiency of H2 vs EV.'

You'll have to explain why it tells us about efficiency.

It certainly tells us about the cost of the fuel, although it does not tell us about total running costs.

But efficiency, why?
I think the figures given above tell the story of the efficiency pretty well, and your notion is entirely extraneous.

Or simply:

Efficiency figures tell you about efficiency.
Cost figures tell you about cost.

It sounds to me as though you did not much fancy what the efficiency figures were saying, and so changed the subject!

BTW VW's notion of a PHEV FCEV would do pretty well over the year on both counts.

It takes about two therms of natural gas at about $1 total to make one kilogram of hydrogen. Add compression costs of 10% and 50 cents per GGE for highway taxes then you get $3-4 hydrogen. When you add all the costs for EVs you get $3 per GGE, so costs are comparable. However the FCV can refuel in minutes and range is extensive.

If the cheapest was always the best, we would be driving coal powered steam vehicles and sill be using locomotives and steam boats.

The switch was to oïl because it is more convenient not because it was cheaper. The same applies to house heating.

Another switch was to NG because it is as convenient and as cheap as oïl. Salemanship and abundance did this one, not because it is 25% cleaner..

The next switch will be to electricity because it is really the most convenient, cheapest and cleanest energy source. We have to find ways to store it in larger quantities at a lower cost. It will come by 2020 or shortly thereafter.

The H2 venue may become a way to store energy for Extended periods but it won't be essential for light vehicles. However, it may become a solution for larger trucks, ships etc.

SJC, where can I buy this $3-4 gge hydrogen? Retail prices in Southern California, one of the only regions in the US where you can buy H2, quote prices up to $12 per kg.

Does the time to refuel include the travel time to and from the H2 fueling station, which is not conveniently located right around the corner?

H2 is expensive because it is energy intensive to get in a form suitable for a transportation fuel. Toyota quotes $50 for 250 mile range. EVs cost 1/4 of that for fuel.

Hyundai quotes the cost of the Tucson FCV at ~144,000, although it's available on lease only. Not cheap either.

At 80% efficiency, it would take less than 1.5 therms to make a kilogram of hydrogen. Today's quoted price per therm is 40 cents. So it takes less than 60 cents for the feed stock. Those are the real numbers, I would imagine that gasoline was quite expensive at first 100 years ago.

@eci:

H2 is not currently expensive because is it is energy intensive to get into a form suitable for transport, but currently due to underdeveloped infrastructure.

Here is the DOE in June 2013:
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/hptt_roadmap_june2013.pdf

The goal (pg 9) is produced, delivered and dispensed costs of $2-4 gge, as SJC said

For natural gas reforming all the targets are met, with only detail work to do-

'However, to fully commercialize small-scale hydrogen production by natural gas reforming, additional development will be needed in areas that DOE has not addressed, including system integration, optimization, and technology
validation'

(pg 12, table 1.2, distributed natural gas reforming)
Getting the costs of renewably produced hydrogen etc down to competitive levels will take more work, and there are taxes to add, but since a small SUV gets 68mpgge (Toyota)there is plenty of margin to allow for that and still have a petrol competitive price, or better.

Gasoline cost $0.17/gal in USA in 1914 dollars = $4.04 in 2014 dollars.

1914 - GASOLINE UP AGAIN; PRICE IS 22 CENTS

The Standard Oil Company of New York put up the price of gasoline another cent yesterday, making the wholesale price 22 cents a gallon.

http://query.nytimes.com/gst/abstract.html?res=9E00E4DC1531E733A05755C0A9679C946796D6CF

That makes it over $6 per gallon wholesale in adjusted dollars. Wholesale price today is under $3 per gallon. SO, hydrogen may sell for $8 per kilogram today, but could come down to $4 by the end of the decade.

DM > The goal (pg 9) is produced, delivered and dispensed costs of $2-4 gge

Ok, we can agree on the goal. But I don't drive my car on goals, I drive my car on realities.

DM> For natural gas reforming all the targets are met, with only detail work to do...

Detail work like carbon sequestration? Or were you going to just dump it in the atmosphere, like the current crop of H2 dispensing stations, which use methane reforming do?

Details, details...

eci:
You don't seem to have actually read the link I researched to answer your question, and seem only interested in offering refutation without serious consideration.

We were not talking about carbon sequestration, which does not happen for the grid electricity used to power BEVs either at the moment.

You asserted that the reason for high current costs of hydrogen is that it is energy intensive.

I showed conclusively that that is not the case, and that for the initial roll out it is the inevitable process of fully optimising the equipment, which is always the case for any roll out of new technology.

Without even acknowledging that point, you seek to switch the subject.

Many of us have learnt that 'and another thing' arguments are rarely productive.

Whatever carbon is 'just dumped in the atmosphere' initially is at any rate much less than petrol cars do, just as electric cars rely at the moment on carbon being 'just dumped in the atmosphere' from the grid, and in pretty comparable quantities per mile driven to fuel cell cars too.

To return to the point at issue from your diversion though, it is quite clear that hydrogen can be produced, dispensed and delivered at $2-4gge as stated plus tax.

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