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

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

Comments

SJC

The CO2 ends up in the air at natural gas fired power plants, your furnace, hot water heater and car, at least you don't have the smog causing NOX from internal combustion. The power plant making the electricity for the EV using natural gas in a turbine sure creates a lot.

Davemart

Note that the DOE assess that target of $2-4gge, which you seek to dismiss as wishful thinking, as already met for natural gas.

Davemart

12.3% efficient solar to hydrogen!

http://phys.org/news/2014-09-h...

'These high efficiency values are based on a characteristic of perovskite cells:
their ability to generate an open circuit voltage greater than 1 V
(silicon cells stop at 0.7 V, for comparison).

"A voltage of 1.7 V or more is required for water electrolysis to
occur and to obtain exploitable gases," explained Jingshan Luo. To get
these numbers, three or more silicon cells are needed, whereas just two
perovskite cells are enough. As a result, there is more efficiency with
respect to the surface of the light absorbers required. "This is the
first time we have been able to get hydrogen through electrolysis with
only two cells!" Luo adds.'

Shame it uses lead, but tin may be possible which is much more environmentally benign.

electric-car-insider.com

Promising technology, but a long way from market. The authors acknowledge that the cycle life is very short.

I'd love to see them solve it.

electric-car-insider.com

DaveMart, I actually enjoy reading the links you post, so thank you for posting them.

I don't actually disagree with many of your points: $4gge from natural gas is possible now, but unfortunately, is not the retail price reality. In fact, from the report linked to, the authors are careful to point out:

The hydrogen threshold cost is a DOE threshold cost and not a Partnership goal or target.

Who are the partners? Listed in the front of the report they are:

Chevron
ExxonMobil
Phillips 66
Shell Oil Products US

So that price is a government target, not a manufacturer goal.

The timing of that accomplishment is also an issue. We're not talking about a current acheivement, but a goal that is six years distant:

"DOE’s goal is to reduce the cost of hydrogen production to ≤$2.00 per gge1 ($2.00 to $4.00 delivered and dispensed2) by 2020."

And the report also makes clear, and you acknowledge in your post, that :

"With the exception of natural gas reforming, all hydrogen production technologies discussed in this roadmap require significant advancements and additional development prior to commercial use."

By pointing out that natural gas reforming does not actually accomplish the zero emissions part of the objective of this whole ZEV exercise, I'm not changing the subject, just pointing out that the only cost-effective path for hydrogen fuel is built on a slight of hand. That is certainly relevant to the argument.

EVs, when using grid mix, are certainly not carbon free. But the capability is there, and it is there in the near term and at very reasonable cost, and individuals can make that choice by purchasing green energy (where they are allowed that option) or installing solar

That's not theoretical or an aspirational goal, that's a current market reality.

I believe my point of $1gge today with EV, or $4gge for H2 in 2020, or possibly later, stands.

SJC

So neither includes road taxes, but EV does not include battery replacement. If they can get the solar hydrogen efficiency up and do it inexpensively, there is an alternative.

Even though CO2 will probably be released from the methane to hydrogen reformer, there is no smog producing NOX like there is with a turbine making electricity for the EV.

electric-car-insider.com

If climate change wasn't one of the worlds most urgent and potentially catastrophic problems, we might be able to ignore that CO2 "detail". But we can't.

If you don't mind warmer weather that threatens crops (I live in California, the farmers are about to take pitchforks to the Capitol), maybe ocean acidification will get your attention.

Battery replacement is a canard. The batteries in EVs are designed to last the life of the car. The Tesla Model S 85, for example, has a 8 year unlimited mile warranty. Most other EVs have an 8 year 100,000 mile warranty.

If hydrogen vehicles ever become competitive with EVs on price, performance, utility and infrastructure cost and availability, I'll be a big supporter. But when Toyota's senior VP for technology says that hydrogen won't be cost competitive until 2030 - I believe him.

T2

A well-to-wheels analysis of the use of natural gas for passenger vehicles should involve the BIG picture which here is not the case. Their model seems to align with the old school method of distributed electrical power.

To most of us it should be irrelevant how efficient megascale generating plants can be made if the residences - outside of which these passenger vehicles are parked - happen to have their space heating provided by open flame natural gas furnaces and as we all know these forced air systems have a thermodynamic efficiency of ZERO per cent. A serious W to W study should at least consider home generation of electricity. You know, CHP.

Engineer-Poet
A serious W to W study should at least consider home generation of electricity. You know, CHP.

T2, some of us considered this years ago, at length.  My own analysis touched on the use of CHP as a buffer for unreliable sources such as wind.

All contributions count, the megascale as well as the micro-scale.  They add to the totals (plural).  What matters is which contributions can get the GHG contributions heading toward 350 ppm fast enough to make a difference.

T2

@ E-P,
Thank you for providing us the link, very useful. I have to say that it is probably the best aggregation of data to the subject that I have seen so far. Thanks again.

Davemart

I've been checking back on the actual study linked, and some points are strange indeed.

They show the figures for using natural gas with 25% and 50% renewables, but fail to show the same calculations for hydrogen fuel cell cars.

Why? The gap is actually quite narrow, like for like, but looking at the charts sure does not give that impression.
This is even more important, as hydrogen for transport is mandates to be 33% from renewables in California, so that will reduce them comfortable under the levels shown for most of the EV's from various sources in the chart, with only EV's charged from 50% renewables much ahead.

Even more seriously, in the link they show the assumed mpgge for fuel cells using hydrogen as identical to the 38.5 mpgge they give for their so far as I know never on the road on board reformed CNG fuel cell car.

This is way lower than the mpgge of any fuel cell car that I am familiar with, where over 60 mpgge is much more typical.

What is going on?
This sure looks like not only a loaded presentation, but loaded data.

Davemart

Checking further they give the well to pump efficiency of the CNG FC at 85%, and the WTP of the H2FC at 55%, but then put them on identical mpgge.

They have clearly shown reforming losses twice for the H2FC, as reforming takes energy so of course an H2FC gets more mpgge once it is in hydrogen form.

Using the mpggee on the Hyundai com website the much bigger Tucson FCEV is rated at 50mpgge combined, so the ~175GGE gm/km shown comes down to around 135g/km and turns their statement that using natural gas to produce electricity is the most efficient way around to equality with H2fuel cells, which are also way better than using electricity at the US grid mix to power an EV.

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