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April 04, 2008

Comments

Cyril R.

A valid question would be if a serial hybrid bus like this but with a highly efficient diesel genset would be better.

donb

Cyril R. asked:
A valid question would be if a serial hybrid bus like this but with a highly efficient diesel genset would be better.

I think the answer is yes. The data sheet for the C30 microturbine states that its heat rate is 12,700 BTU per kWhr of mechanical output power, or just under 27% efficiency. An optimized diesel engine can do signficantly better.

From a system point of view, a lighter vehicle helps efficiency. The turbine no doubt is lighter than a diesel engine, but the difference that would make in total weight of the bus is not very much.

Cyril R.

Those were my thoughts as well.

The DOE has a program to increase microturbine efficiency to about 40%, which is similar to the efficiency of a modern small diesel generator.

The microturbine is probably a bit more expensive, and it can't be more robust than a diesel.

Emissions could be a plus for the microturbine, but there are cost effective means of dealing with that, especially for a big bus (room for dual filters, nox catalysts/ammonia system etc.)

In the other thread, someone mentioned thee-traction bus gets even better mileage.

Cyril R.

HTML links don't appear to work anymore? That's pretty daft.

Reality Czech

It looks like you left out the href.

Test link.

Looks fine in the preview.

Dave

I wonder if it would be practical to put electric contacts above the bus at some stops. Perhaps the bus could receive a quick charge from these stops when its waiting on passengers.

Benny

The question is, what type of engine is best for a serial hybrid, and does this vary from a bus to a car?

External combustion, microturbines, Wankels, or Fuel Cells? I have read that Wankels could be optimum for some series hybid applications.

padisah

I think the reason why turbine used is, that gas turbines have been tried as (road) vehicel use, but their slow response to higher rpm request made them unuseful in street traffic. With combining in a serial hibrid this factor has been solved, and also the energy buffering can save a lot. Which possibly makes a turbine's less efficeincy still viable.

Despite that, a diesel-electric serial hibrid would be more fuel efficient, but probably with a correspondingly higher production cost.

Kit P

Turbines have very high angular momentum and resist turning. When considering the coefficient of friction of POV tires on wet pavement, this is not a very desirable handling characteristic. This may not be an issue for city buses.

Cyril R.

Despite that, a diesel-electric serial hibrid would be more fuel efficient, but probably with a correspondingly higher production cost.

I don't think microturbines are cheaper than diesels. IIRC microturbines cost more than $ 500 per kW. Diesels are much, much cheaper.

Bob Wallace

"...if a full battery pack of 70Ah batteries is used, the bus has a range of 31 miles on battery only...."

Sounds like there is an opening for all electric buses utilizing exchangeable battery packs.


GreenPlease

Remember that a diesel operates at 40% efficiency in a very narrow range and that 40% is just the thermal efficiency value. Another consideration is the mechanical efficiency of the engine itself (e.g. a lot of losses to friction in the cylindar walls, crank bearings, valve train, etc.). Turbines have a very high ME value which helps to partially compensate for its low(er) TE value.

Diesels have a low specific power compared to turbines. Consider that optimum speed for a diesel is around 2000rpm and it becomes evident that the diesel engine will have to be considerably larger and heavier than a micro-turbine.

The ability to use natural gas is a plus. Cheaper than diesel on a $/BTU basis. Lower emissions to boot.

Turbines have two big things going for them:

1. Noise and vibration. While the exhaust noise of a turbine could be interesting, it inherently vibrates less than a diesel making it more desirable

2. Maintenance/Reliability. These micro-turbines use are bearings and will essentially last forever with practically no maintenance.

GreenPlease

BTW does anyone else wonder why the rear wheels aren't partially covered to enhance aerodynamics?

While I'm at it, how about some turbulators along the side. Just drop the Cd a little on these behemoths and it would make a huge difference.

Joe

I wonder if it's practical to add steam boiler and steam turbine to achieve some extra "cogeneration" power. No doubt that it would add weight and complexity. The complete turbine system efficiency would go to around 50%.

Laurent GUERBY

I couldn't find any data on diesel generator efficiency, any link? I'm wondering about "diesel to generator to electric motor to wheel" vs "diesel ICE" efficiency.

Cyril R.

Remember that a diesel operates at 40% efficiency in a very narrow range and that 40% is just the thermal efficiency value.

It's a genset, which runs at (near) peak efficiency all the time, and the net peak efficiency of a diesel is still better than the net peak efficiency of a microturbine.

Diesels have a low specific power compared to turbines.

Both are lightweight, because little genset capacity is needed (serial drive, only maximum average load required).

The ability to use natural gas is a plus. Cheaper than diesel on a $/BTU basis. Lower emissions to boot.

Sure, if you've got the NG infrastructure for fuelling. Cheaper, yes, cleaner well it's not a big difference if the diesel has a particulate filter and a nox catalyst.

Noise and vibration.

It's a serial hybrid drive, so not really a big deal anyway. Modern diesels are silent.

Maintenance/Reliability

Modern diesels are robust enough for mobile apps. Microturbines are not better for mobile apps.

donb

GreanPlease stated:
The ability to use natural gas is a plus. Cheaper than diesel on a $/BTU basis. Lower emissions to boot.

To which Cyril R. said:
Sure, if you've got the NG infrastructure for fuelling. Cheaper, yes, cleaner well it's not a big difference if the diesel has a particulate filter and a nox catalyst.

If you are going to run natural gas, you can do better than a microturbine with either an optimized lean-burn spark ignition reciprocating engine, or you can mix natural gas with the air going into the diesel engine and use a bit of diesel fuel injection as the combustion initiator. The latter method produces low particulates and minimizes the use diesel fuel. Also, the system allows the engine to return to conventional diesel fuel operation if the natural gas should run out. No need to oversize the natural gas storage system.

Cyril R.

That's a good point, ICE optimized for NG could be very efficient, considering the high octane number of methane.

I think substituting some imported oil for imported natural gas is an improvement. Diversifying will lower the risk of peak oil problems and NG supplies may be a bit less tight than oil supplies.

So building more NG refuelling stations could be a good investment, it's proven technology at a reasonable cost.

Nitpicker

Regenerative BRAKING, not breaking.

Nitpiker, Thanks for the correction, it has been fixed. Myproof reading is rather poor on this sort of thing, where the spell checker is no help. My typing sometimes comes up with the most frequent sequence of letters, even though they are wrong. -- Jim

richy rich

re "Turbines have very high angular momentum and resist turning. When considering the coefficient of friction of POV tires on wet pavement, this is not a very desirable handling characteristic".

Mount them vertically ?

Kim Fenske

What is the North Carolina address and contact information for DesignLine?

Bob Wallace

Google them.

Then click on the "About Us" page, then the "Contact Us" in the upper right of the page.

Buddha

Just a note Capstone has a vehicle engine called IRC 225. It has up to 42% efficiency. The engine operates at 75kWh to conserve fuel but has immediate power of 225kWh available.

Matt

i think you mean 75kw and 225kw....not kwh.
The wrong unti was used in the article as well.
a 480v 42AH battery doesnt store 40kwh.
neither does a 70AH 480v battery store 67kwh

they both can produce 40kw and 70kw respectively.

Matt

Hybrid Car Rental

I wish I could take the bus, I'm way too afraid though.

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Henry Gibson

The total cost of ownership for turbines can be considered compared to diesel engines. Since there is no lubrication and no cooling system required for the microturbines and they have very long life a few more percentage points of efficiency may not be worth using the diesel. Capstone has not yet been able to make the single moving part feature cheap enough to compete with diesels with many moving parts, but somebody in the future will be able to do so. Bus stop charging and ZEBRA batteries could make this vehicle into a first class plug-in-electric hybrid, but so much money is paid for the turbine it ought to be used. ..HG..

Henry Gibson

This would be a very good vehicle to demonstrate EFFPOWER batteries. ..HG..

Cyril R.

Henry Gibson. The turbines are unlikely to be cheaper than diesels. Even HUGE Brayton cycles with lots of economy of plant scale(100,000+ kW) are neither cheaper nor more efficient than a diesel. Brayton cycles have an inherently higher cost structure due to more demanding materials requirements and much more complex engineering in general.

As Donb points out above, a hybrid natural gas/diesel engine is probably the way to go. Mostly natural gas, so it's lower fuel costs, and you still get large emissions reductions.

When it comes to future techs, the SOFC has a strong hand, especially for the serial hybrid market. It should beat microturbines as well as diesels on cost and efficiency.

james

I'd be curious to know what the operation is like once the batteries are fully discharged -- i.e. when you can only use as much power as the 40 hp microturbine (minus electrical generating losses) can produce.

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