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NREL Publishes Final Version of Study Comparing Series-Hybrid and CNG Transit Buses
10 November 2006
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| The fuel-economy of the hybrids surpassed that of the diesel and CNG buses. Click to enlarge. |
The US Department of Energy’s National Renewable Energy Laboratory (NREL) has published the final version of a study comparing 10 new compressed natural gas (CNG) and next-generation diesel series-hybrid electric bus propulsion systems operated at New York City Transit (NYCT) against a baseline of conventional diesel buses.
The report is part of a series of evaluations from NREL that track and evaluate new propulsion systems in transit buses and trucks. DOE/NREL evaluated the original 10 prototype diesel-hybrid buses from Orion and BAE Systems (model Orion VI buses) operated at New York City Transit (NYCT) .
That evaluation was reported in July 2002 and provided results from the prototype buses from 1998 through 2001. This report focuses on 10 new compressed natural gas (CNG) buses and hybrid propulsion systems.
This report describes the evaluation results for new Orion VII low floor buses at NYCT with CNG propulsion (equipped with Detroit Diesel Corporation Series 50G CNG engine) and new hybrid propulsion (equipped with BAE Systems’ HybriDrive propulsion system). These final results represent a 12-month evaluation of these two groups of buses (October 2004 through September 2005).
The buses evaluated are the same age, the same bus platform, have been operated on similar duty-cycles, and experienced similar maintenance practices.
Among the study’s findings:
With a fuel economy of 1.70 miles per diesel-equivalent gallon, the CNG study group has a 25%-28% lower fuel economy than the diesel buses. The low average speed of the NYCT operation is the key to this significantly lower fuel economy—spark ignited (SI) natural gas engines typically have a lower thermal efficiency at low speed and load than compression ignition (CI) diesel engines. Consequently, lower natural gas fuel economy is expected in this type of operation.
The hybrid study fleet—with average fuel economy of 3.19 mpg) achieved 37% higher average fuel economy that the diesel buses, and exhibited 88% higher fuel economy than the CNG study fleet.
The hybrid bus average fuel economy had a much larger decrease/fluctuation in fuel economy in the summer months (June through September) than any of the other three study bus groups. According to BAE Systems, much of this decrease is due to an increase in energy consumption for air conditioning.
Resources:
New York City Transit (NYCT) Hybrid (125 Order) and CNG Transit Buses (NREL/NREL/TP-540-40125)
November 10, 2006 in Diesel, Fleets, Hybrids, Natural Gas | Permalink | Comments (38) | TrackBack (0)
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It sounds like you might want a series hybrid CNG diesel bus that loads the engine properly for maximum mileage and gets the economy of a hybrid. These are good studies that show the magnitudes of performance with various configurations.
Posted by: SJC | Nov 10, 2006 10:05:03 AM
Notice the effects of warm/hot weather on the Orion VI. The need for cooling takes a toll on fuel mileage as the diesel engine runs to keep the bus cool. Perhaps a electric compressor, running off of stored regen braking energy, could be used to run the AC during summertime. Larger, better battery packs would be needed during hot months.
Posted by: allen_Z | Nov 10, 2006 11:07:11 AM
With all the high grade waste heat, they could use abosorption cooling and use not extra power at all. It is a bus, with a lot more room than a truck or car for the cooling unit.
Posted by: SJC | Nov 10, 2006 11:14:43 AM
SJC,
That may be feasible w/a modular pack that can be removed during cool/cold months. It might be a single unit, or 2 part, with one onboard and the other in on trailer.
Posted by: allen_Z | Nov 10, 2006 12:02:21 PM
wow, so the thing that i've been saying all along, this whole time, has finally been proven by months of testing – a CNG bus is simply not a "clean air" vehicle. i still cannot understand for the life of me why people want to insist on buying CNG buses instead of going for the diesel-hybrid route.
Posted by: lensovet | Nov 10, 2006 12:07:19 PM
I see nothing in the article about clean air, just the mileage figures are shown. I would imagine CNG is much cleaner than diesels.
As far as absorption cooling from waste heat, a 10 ton cooling unit is smaller than the average refrigerator. I can think of several places under the floor of a bus for such a unit.
Posted by: SJC | Nov 10, 2006 12:33:03 PM
CNG is considered "clean air" because it releases far less PM and other local pollutants, relative to conventional diesel buses, even given its lower fuel economy. Cleaner diesel technology has narrowed the gap, but CNG still has the edge there.
Hybrid diesels that combine low emissions and high economy are nearly as clean as CNG in terms of local pollutants per mile driven, but cost a whole lot more to buy on the front end. They can make that up in fuel and repair cost savings (hybrid buses wear out their brake pads much more slowly than conventional buses), but only if they are substituted into appropriate routes -- stop and go city center routes, not high speed, hilly or more suburban ones.
Still, considering previous studies which examined the fuel economy performance of hybrid buses, this study is encouraging. Previous studies would often put hybrids into service on routes which did not take full advantage of their capabilities; this study seems to openly acknowledge the importance of route selection, and confirms the hybrid advantage on well chosen routes.
Posted by: NBK-Boston | Nov 10, 2006 2:18:38 PM
right, i didn't phrase that quite properly. NBK-Boston said my thoughts exactly, thanks man.
Posted by: lensovet | Nov 10, 2006 2:43:42 PM
SJC -
an absorption chiller would be a great way to deliver airco in city buses. Since diesels are turbocharged, it could also be used to drive a chiller between the intercooler and the intake manifold to further cut the temperature of the fresh charge. With an air/water intercooler, the waste heat from the first stage can actually help drive the second. Charge chilling should boost rated power and/or reduce engine-out NOx levels.
The fly in the ointment is that traditional absorption chiller designs (other than small hotel/RV refrigerators) rely on the gravity-induced flow of a finely dispersed mist to achieve adequate re-absorption by the weak ammonia solution. Experiments on ships have shown that the chiller power of such designs is highly susceptible to vibration, making them unsuitable for mobile applications. The porous membrane absorber stacks under development in Germany are much more compact and impervious to vibrations.
http://www.itt.uni-stuttgart.de/~schaal/index.en.html
Keep in mind, though, that absorption chillers are quite expensive. Once mechanically robust solutions are available, they should initially be considered for buses and refrigeration trucks with life expectancies of 1-1.5 million kilometers to permit amortization of the up-front premium.
Posted by: Rafael Seidl | Nov 10, 2006 3:33:21 PM
Yazaki makes an absorption unit that works with lithium bromide and not ammonia. It runs on hot water temperatures down to 200F and if you have higher temperatures you get higher efficiencies. Yazaki makes components for the auto industry as well, so perhaps they could design a unit for busses.
http://www.yazakienergy.com/waterfired.htm
Posted by: SJC | Nov 10, 2006 3:43:51 PM
Actually, numerous studies have shown that CNG is typically much worse in NMHC emissions, including the associated air toxics. Even when equipped with catalysts, CNG NMHC emissions are somewhat higher than clean diesel NMHC emissions (which are usually near or below detection limits) according to a study by CARB.
NMHC is the worse possible pollutant for urban air quality.
Posted by: Carl | Nov 10, 2006 4:32:58 PM
Here is a link to BAE Hybridrive
Posted by: amx | Nov 10, 2006 5:47:40 PM
SJC,
The Orion VII hybrid is a low floor bus. It may fit on the roof, but that could change the center of gravity. One may be able to fit a unit on the rear of the vehicle, or mounteed on a trailer fluid&electric w/couplings. The Orion Bus site has the Thermo King AC at 100,000+ BTU unit.
___The batteries could use updating. I went over the Resources link and the PDF states that they are lead acid units. Li-ion could be make large improvments to fuel mileage in stop and go driving. It would also:
a) drop the mass of electrical energy storage.
b) increase energy storage capacity.
c) lend to plug-in bus experimentation and implementation.
On the flipside, Li-ion units are expensive vs lead acid.
_
Rafael,
That is why they are found mostly on buildings. Many office towers/large buildings away from the waterfront in Manhattan use absorption chillers (+ heating during winter) powered via ~350 F steam from co-generation electric+steam plants.
Posted by: allen_Z | Nov 10, 2006 6:25:12 PM
Oh, I don't care where the heck they put it, just put it!
Posted by: SJC | Nov 10, 2006 8:55:46 PM
NMHC is the worst possible pollutant for urban air quality
What's NMHC?
Posted by: George | Nov 10, 2006 9:11:09 PM
George,
The internet is the data keeper to most of your questions.
Posted by: allen_Z | Nov 10, 2006 10:14:35 PM
NMHC refers to "non-methane hydrocarbons," and Carl's phrasing should therefore read "NHMCs *are* the worst possible..."
But as to the substance of that statement, NMHCs are a range of compounds, and the statement that they are "the worst possible" form of local air pollution is a vague one at best. By what standard? Which NHMCs are particularly nasty, and under what circumstances? I am better acquainted with the dangers associated with PM and NOx emissions, so I would appreciate a link to the CARB study mentioned earlier in order to get more background information on these questions.
Posted by: NBK-Boston | Nov 10, 2006 11:01:29 PM
There are couple of defined emissions like HC (hydrocarbons), VOC (volatile organic carbons) and NMHC (non-methane hydro-carbons). Methane emissions, quite common to NG engines and somehow to propane engines are not really troublesome, because methane is not smog-forming emission, and naturally occurring methane emissions are way higher then produced from IC engines.
That’s why emission legislation usually targets VOC or NMHC. In case of gasoline or diesel engines it practically means all HC emissions.
Posted by: Andrey | Nov 11, 2006 3:56:21 AM
Methane is a GHG. If we have fuel cells with reformers to turn SNG into H2, we would have no tailpipe emissions other than CO2 (neutral) and H2O.
Posted by: SJC | Nov 11, 2006 6:37:05 AM
Interesting comments, as usual. A few additional points:
BAE has chosen to stick with lead acids based upon a cost model factoring in replacement frequency and cost. They model the costs to the customer as less, with the current battery technology landscape, than if NiMH were used. That said, they are testing NiMH, and will make the transition eventually.
The decision to go CNG or diesel hybrid is really specific to each fleet. There are infrastructure and facility conversion costs associated with CNG, as well as price volatility of fuel. Hybrids of course cost a bit more per unit right now. It is expensive to install fueling and convert old buildings in places like NYC, which is why they are moving more in the direction of hybrids. Other fleets may have a different situation, and CNG can make more sense.
It is important to note that the comparison of CNG to diesel hybrid to diesel is not a perfect one. NYCT has not purchased diesel buses in some time, so diesel baseline is comprised of older buses, with consequently higher maintenance costs. Diesels are not factored into the maintenance cost comparison. The CNG and diesel hybrids are of similar vintage, thus the maintenance cost comparison is made in addition to fuel economy. As noted above, the NYC duty cycle is severe, and not the best showcase for SI engine technology. Also, the CNG buses are powered by the DDC S50G, which is no longer produced, and was generally considered an inferior powerplant.
We take pains to match duty cycles when making comparisons. The question of series versus parallel, and what control strategy is employed, are very specific to to duty cycle and application. One size does not fit all.
BAE/Orion have a refined version of the hybrids we tested, which we are in the process of evaluating without a baseline. We also hope to emissions test one on our chassis dyno. Stay tuned.
Posted by: Robb | Nov 11, 2006 7:53:08 AM
It seems to be an ideal vehicle for 140+ KWh Altairnano new quick charge-discharge batteries.
In city traffic situation, these batteries would allow much more breaking energy recuperation to operate the cabin heating/cooling system.
Some 40+ square meters of high efficiency (25% to 30% = about 1.1 x .25 x 40 x 6 = 66 KWh/day) solar panels mounted on the large bus roof would also help to keep the batteries charged when most needed = during the 6 to 8 hrs/day of bright sunshine.
An up-to-date very light weight, compact, very high efficiency 23-SEER, 8 to 10 Ton heat pump, (preferably 2 units x 4 to 5 Ton) equipped with variable ECM motors would to the HVAC job year round in most cities. Why re-event the wheel when it is there already.
This type of city bus could be built now and would need a much smaller diesel genset.
Dozens of USA/Canadian firms could do it with adequate R & D support.
Posted by: Harvey D. | Nov 11, 2006 8:10:11 AM
SJC-
To his credit, Andrey noted that methane is "not smog-forming," and therefore not a problem in the air quality arena. Its status as a greenhouse gas is well known.
Posted by: NBK-Boston | Nov 11, 2006 9:49:21 AM
I was not responding to anyone, just pointing out that methane is a GHG. Keep methane out of the air, simple as that.
Posted by: SJC | Nov 11, 2006 12:44:31 PM
NBK-Boston - you're correct, NMHCs are a class of compounds (hydrocarbons) so it should have been plural. Should QC my posts more closely.
Here's the link to the CARB study to which I referred:
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2003/session5/deer_2003_ayala.pdf
See also:
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2003/session5/deer_2003_lowell.pdf
My response was more to convey that CNG engines are not necessarily uniformly "cleaner" than low-emitting diesel engines based on most studies. My comment about NMHCs being the most problematic of the regulated emissions was in regard to urban ozone (smog) production.
Posted by: Carl | Nov 11, 2006 1:24:08 PM
Carl-
The two presentations you posted are certainly eye-opening when it comes to comparing CNG to clean-diesel in the 2002 time frame. I wonder if newer CNG buses have oxydation catalysts (these presentations seem to imply that the older ones did not), and if so, whether that puts them decisively over 2002-era clean diesels as far as local air quality issues go. All the same, the newest diesel technologies can make the real target a moving one.
The cost issues identified in the second presentation in particular confirmed what I had long suspected, which is that a conversion to a CNG fleet is quite expensive. Taking into account infrastructure costs, my gut always leaned towards the diesel-hybrid approach, as against a conventional CNG drivetrain. Though a good series-hybrid layout might be able to take maximum advantage of CNG, and be worth the cost.
Thanks for posting a response.
Posted by: NBK-Boston | Nov 11, 2006 2:13:52 PM
Harvey D. -
your suggestion of attaching solar panels to the roof of a bus is intriguing, I had not come across it before. Unfortunately, panels that get 25-30% efficiency are still very expensive indeed. Also, city centers tend to feature high-rise buildings that cast shadows on the streets below.
Therefore, in the real world, I suspect you'd be lucky to get a third of the solar energy you estimate. On the plus side, the panels may have a longer life expectancy than the rest of the bus - if so, it would reduce the TCO. I suspect it would still be cheaper to invest in ultracaps for efficient recuperative braking to achieve similar fuel savings.
Posted by: Rafael Seidl | Nov 11, 2006 3:47:39 PM
400 square feet of 25% efficient solar cells might produce 10Kw of electricity maximum, under fully optimal conditions. Since the bus takes about 10 times that much energy, I don't think it would help much and would cost a whole lot.
Posted by: SJC | Nov 11, 2006 4:29:47 PM
Carl, NBK:
CNG buses with oxidation catalyst is antiquated technology. With tree-way catalytic converter NG bus engine is cleaner then cigarette lighter.
Posted by: Andrey | Nov 11, 2006 5:44:18 PM
SJC:
The high efficiency solar panels were not intended as the main power source but as a complemtary (optional) free power source for part of the on board HVAC. Solar panels may be too expensive now but the price per KWh should go down by about 10+% per year as did large LCD panels. If so, the price would be acceptable-affordable by 2012/2015.
Not all city street have 300/400-ft buildings. The average city bus is exposed to sunlight most of the time. Their large flat roof is an ideal place for solar panels.
The idea is to reduce dependance on ICE genset as much as possible by making vehicles as electric as possible.
Posted by: Harvey D. | Nov 11, 2006 7:30:19 PM
Rafael & SJC:
Please check the MIT Technology Review (10 Nov. 2006) for 'Cheap Superefficient Solar Power Modules' using sunlight concentrators + very high efficiency wide spectrum tri-junction solar cells.
When mass produced those power modules could convert sun energy into useful electricity at a relatively low cost.
These power modules made replace the current steel bus roof-top.
Posted by: Harvey D. | Nov 11, 2006 8:13:05 PM
Harvey D. -
do you really think sunlight concentrators are practical in a mobile application? Tracking mirrors would have to deal not just with the gross motion of the bus but also the elastokinematics of the suspension, plus vibrations. In addition, they would generate substantial aerodynamic drag and reaction forces on the roof structure at elevated speeds. The alternative, heavy fresnel lenses, is even less practical on top of a bus.
If Sharp's technology really is cheap enough, it should be used in stationary solar electricity farms to address energy security and climate change, cp.
http://www.greencarcongress.com/2006/10/solar_systems_t.html
For a public transport concept based on all-electric propulsion with occasional grid connectivity (at selected bus stops), please see
http://www.greencarcongress.com/2006/11/fraunhofer_test.html
Posted by: Rafael Seidl | Nov 12, 2006 4:27:36 AM
Rafael:
These concentrators have no moving parts, are very rugged, relatively thin, light and could last up to 30 years. They can be made tile or strip shape of various size and could be fixed or glued to a flat roof tops.
The idea is to reduce the size of the very high efficiency (up to 50%) wide band expensive energy converter components in favor of low cost mainly plastic concentrators to reduce the total cost.
Posted by: Harvey D. | Nov 12, 2006 7:43:50 AM
These panels have 10x concentration with no tracking, but you still have the 1/10 power generation to consumption fact to deal with.
http://www.siliconsolar.com/shop/catalog/Concentrator-Solar-Panels-SSPV-10-p-16339.html
Posted by: SJC | Nov 12, 2006 2:43:47 PM
Andrey-
Could you point me to a curret study or spec sheet detailing the emissions performance of a CNG bus fitted with the latest catalysts? I would like to compare it to the 2002 data presented in the studies that Carl linked mentioned.
Posted by: NBK-Boston | Nov 12, 2006 3:12:44 PM
A bus-type diesel engine with gears, generator and rectifier gets roughly 10 kWh out of every gallon of diesel. That means that the cost of electricity is three times as high as wholesale electricity for consumers. Thus, PVs mounted on buses should be three times as economical as home installations, no matter what efficiency is employed. Furthermore, the electric control unit needed in home installations is already paid for. Last, but not least, I will claim that installation of PVs in mass/series production (of cars/buses) is cheaper than installation in homes with all their variability.
But in the end, it is a matter of economics - including marketing. Will diesel savings pay for the solar panels or not? And will "solar driving", however small percentage, entice more customers?
Posted by: Thomas Pedersen | Nov 12, 2006 3:35:24 PM
I would agree that the payback would be better with busses than with homes. You could produce enough energy to power the A/C, or use absorption cooling and save fuel with PV electric for the motors. PV payback for the home might be 40 years, PV payback for the bus might be 20 years. Absorption cooling payback might be 10 years. Since city busses can cost $500k each, you have to ask if they want to spend another $100k with a 20 year payback when the bus might last 10 years.
Posted by: SJC | Nov 13, 2006 8:18:46 AM
Federal regulations aim for a transit bus lifespan of 12 years. Payback for any extra technologies should therefore take place within that time frame, accounting for the time-value of money. This assumes that the bus will be scrapped at the end of that period and not live on with a secondhand operator. If some of these suggested technological units can be expected to outlive the bus, they might be salvageable -- they can be stripped from the junked bus, refurbished, and installed in a new bus. The residual salvage value can therefore be subtracted from the extra up-front cost.
City buses tend to cost $350,000 or so for conventional models. A $500,000 figure is more accurate for hybrid city buses.
Posted by: NBK-Boston | Nov 13, 2006 9:08:44 PM
If a bus were 40 feet long and ten feet wide its top surface would have 400 square feet. That is less than 40 hp at full sunlight and only 4hp with a standard 10% solar cell conversion factor, at $10 a watt fabricated and installed the solar collector would cost $30,000. The average output over day and night would be one horsepower or less. Solar energy is too weak and too expensive and inefficient to convert for transportation units.
Super light weight street cars on tracks drawn by horses fed with grass from Central Park is a better option. Corn from the midwest could also be fed to them instead of making ethanol. Oh wait, every even slightly advanced nation has tried using biomass energy with the result of the nearly total destruction of Virgin forests and grasslands.
Low pollution power plants that feed power to plug in electric hybrids is the answer to all stop and go bus service. Recharge facilities can be built at every stop.
APUs, probably small diesel piston engines(OPOC), would be used after ten miles without a recharge stop. Very high speed, super-charging and exhaust-energy-turbines would allow high power output from low weight units, but only simple units should be installed in buses on routes where they would seldom be used.
Because of the maintenance costs, any battery other than ZEBRA batteries are too expensive for HEV use in busses. L-Ion batteries of all types are too expensive to monitor and cool. If large numbers of ZEBRA batteries were built on an efficient assembly line they could be one fifth the price or less.
With careful design of components, a hydraulic hybrid bus would be the least expensive bus to build and operate. The UPS hydraulic hybrid points the way, add a large flywheel, as in PARRY PEOPLE MOVERS, and you have a lot of regenerative power. Put in a small Zebra battery for lights and air conditioning from the engine generator...HG...
Posted by: Henry Gibson | Apr 24, 2008 10:16:59 AM
