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 .

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.

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).





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 CH 4 to H 2 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.



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.

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