Capstone Microturbine . . .

Discussion in 'Hybrid' started by auriel, Aug 13, 2011.

  1. auriel
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    auriel Junior Member

  2. Yellowjacket
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    Yellowjacket Senior Member

    Auriel,

    The reason that the Capstone unit is a poor choice for something like a smaller vehicle is actually one of their main selling points. The Capstone engine has air bearings that don't require oil or maintenance. Great if you are sitting on a skid in Iowa. However, the use of air bearings is a problem because air bearings have a very low spring rate. This means that the clearances in the engine have to be greater to accomodate the motion of the rotor when the vehicle hits a bump. Not a big deal for a bus that has a soft floaty ride, or a houseboat on a river, but it would be a big deal for smaller vehicles and a distaster in something as stiffly sprung as a racing car. The large clearances that you would need in the turbomachinery for that kind of an application would result in poor efficiency.

    Also air bearings have issues with air contamination. Again that isn't a problem if you are sitting on a skid in Iowa. If you have a lot of salt in the air, the heat evolved in the air bearing could percipitate the salt out and that could cause things to come to a screeching halt. Not an issue on a houseboat inland, but not something that is good if you are at sea.

    Large simple cycle turbines aren't quite as efficient as similar size diesels, but the an order of magnitude lighter. Small simple cycle turbines aren't very efficient, but the addition of a heat recovery system (as is done with the Capstone unit) bring the efficiency to about the same as a diesel. The heat exchanger is expensive, and one of the primary reasons the cost of the Capstone unit is as high as it is. While there aren't any other small turbines in the market Capstone has to compete with diesels, so they can't just charge what they want. If they could make it and sell it for less they would, because the would sell a heck of a lot more than than they are now. If you look at what's in the Capstone unit, the cost isn't going to come down very much unless they went to very high production rates, and I don't see that happening.

    One thing that none of the puff pieces mention is that the Capstone unit uses a permanant magnet alternator. The problem with these things is that if it is spinning, it is making voltage and power. This is potentially more dangerous than an induction machine where you can cut the power by shutting off the field. If you were to have an electrical fault it could slag down in a few seconds, where an induction machine could be shut down safely. Again, not a big deal if you are sitting on a slab in Iowa, but a whole nother' thing if you are in a boat.
     
  3. auriel
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    auriel Junior Member

  4. auriel
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    auriel Junior Member

  5. brian eiland
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    brian eiland Senior Member

    Is an air bearing creating more heat than a contact (friction) bearing?
     
  6. DCockey
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    DCockey Senior Member

    Efficiency of the Capstone generator sets is shown in their catalog as 25% to 33%. I don't know how that compares to high efficiency Diesel powered generator sets but I doubt it's significantly higher.

    There are reasons, such as very short start-up time, why gas turbines orders of magnitude larger than the Capstone units are used for power generation. Likewise there are reasons, such as efficiency, why large merchant vessels use extremely large Diesel engines. The best powerplant for a particular application depends on the application.

    Interesting that Capstone touts the efficiency of a hybrid bus with one of their turbines compared to a bus with a conventional powertrain, but doesn't say anything about comparison with a Diesel/CNG powered hybrid bus.
     
  7. Yellowjacket
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    Yellowjacket Senior Member

    Air bearings use a thin film of air to support the shaft. This very thin film, between the rotating shaft and the bearing surface creates heat by shearing the air and the air leaves the bearing at a higher temperature than it goes in at. If you have any any salt in the air, the heating process can perciptate out the salt.

    The alternative is rolling element bearings (conventional roller and ball bearings) that require a lube system. There are advantages and disadvantages to each. There are no friction bearings in a gas turbine.

    Auriel,

    The air bearing, as noted above has a very low spring rate. If you apply a shock load the shaft responds as a supported spring mass sysetm. Low spring rates mean more travel. More travel means larger clearances. Larger clearances result in poor efficiency of the turbine and the alternator. I can tell you for a fact that if you put one in a car and shock it to 5 g's you won't be able to maintain the same efficiency that you would if you run it on a stationary skid. Just because you can run one as a stunt doesn't mean it is viable. Are there any plans for production?

    Turbines like this are very clean.... once they are running. Startup emissions, when compared to a gasoline engine are far far worse. Basically you blow a hurricane's worth of air inside a small burner, add a stream of fuel and a big spark and (sooner or later) it lights off and sends a big wad of unburned fuel and flames out the back of the engine. I don't believe that you could ever get a gas turbine running only on liquid fuel past the EPA requirements for an automobile. The cold start test would kill it. Once the emissions tester stopped laughing you would be out of there. Maybe if you used a gaseous preburner and did some type of soft lightoff it could be done, nobody does that now. One of the things that killed the automotive gas turbine programs that GM and Ford had was cold start emissions.


    Yea, that's an inland ship, not an ocean going ship. Big difference in the environment isn't there. An inland ship is about as challenging as mounting the unit on a concrete pad in Iowa. I think this is probably a good application for a Capstone unit, but that doesn't mean yours is.

    As an example, there is the issue of startup and shutdown. These systems use a recouperater to capture waste heat from the exhaust and put it back into the cycle just before the burner. This heat exchanger on the Capstone unit is actually the life limiting component on the engine in many installations. The recouperator wears out because of thermal stress caused by startup and shutdown. It eventually cracks and leaks and the engine performance falls off. There was a winery that was running Capstone units and got more than 10,000 hours out of them. The catch was that the units were run continously. Starting them up and shutting them down causes the heat exchanger to crack sooner. Daily use, where you start them in the morning and shut them down at night greatly reduces the life of the unit. In an automobile where there could be 3 or 4 colds starts in a day, it would be shot in a few months. You might get a year or two out of one if you started it once daily.


    Just don't believe everything in every press release that you see. Capstone doesn't talk at all about life of their units in anything buy continuous use. Before you buy one you need to look at all of the things that will impact the life in your particular application. Number of start/stop cycles is one of the many aspects that you need to look at. If it doesn't fit your use profile it could get really expensive, compared to a more conventional diesel engine.

    You seem to be fixated on using one of these, and good luck if you do. Just be prepared to spend a lot for the first cost, and if you don't run it continuously, be prepared to spend a lot to maintain it. Rather than just surfing the web you would do well to talk to a Capstone distributor or their customer applications engineering department.
     
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  8. brian eiland
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    brian eiland Senior Member

    more Air Bearing questions

    I can appreciate your informed reply on much of this discussion, but I still have questions about the bearings.

    Are these not a turbine bearings?
    http://www.obbco.com/babbitt-bearings/turbine-bearings.html?gclid=CLGGztLz0aoCFYiC5QodRkU50Q

    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003251_1969003251.pdf

    http://www.globalspec.com/reference/73495/203279/chapter-16-gas-turbine-bearings-and-seals
    "However, all industrial gas turbines use journal bearings. Journal bearings can be split or full-round. Large-size bearings used for heavy machinery normally have heavy lining. Precision insert-type bearings used commonly in internal combustion engines have a thin lining."

    In fact I don't see a lot of information related to 'air bearing" use in gas turbines? I did find this reference that indicates the air bearing tech in turbines is still in formative stages?
    http://www.grc.nasa.gov/WWW/RT/RT2001/2000/2200bream.html

    Contamination?
    "Foil air bearings can handle severe environmental conditions such as the ingestion of sand and dust. A reversed pitot design at the cooling flow inlet prevents large particles from entering the bearing's flow path, and smaller particles are continually flushed out of the bearing by the cooling flow. This ability to withstand contamination eliminates the need for filters in the airflow."
    http://www.memagazine.org/backissues/membersonly/july98/features/foil/foil.html

    One of the reasons this subject interest me is there was lots of discussions about such bearings for use in Flywheel Energy Storage systems. They desired extremely low friction bearings for the flywheel itself, but also needed to try and maintain the vacuum chamber for the flywheel to operate in....double duty.
     
  9. brian eiland
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    brian eiland Senior Member

    KER, Kenetic Energy Recovery

    The rule changes we're most interested in are those concerning the introduction of the Kinetic Energy Recovery System (KERS) that will eventually make every future Formula One race car a hybrid. KERS is not mandatory in 2009 but will be in 2010 and as a result some teams who have no chance of challenging for the world championship have opted not to use KERS immediately. To remain competitive in 2009, the usual race winning teams will all be running KERS this weekend and for the full season.

    The FIA rules governing KERS are fairly simple but very restrictive. From this season teams are allowed to use KERS to draw 60 Kw of energy from the rear axle on the car, which can be stored up to a total of 400kJ (111 watt hour) of energy per lap, to be reused in the form of a 'boost' button. In effect the system uses regeneration to collect and store energy during braking which allows the drivers to use 60 Kw (82 hp) for 6.6 seconds per lap. The teams are free to choose between either mechanical or electric hybrid systems. Of the ten teams in Formula One, all bar one have chosen the electric hybrid system with only Williams pioneering a flywheel mechanical system.

    http://www.gizmag.com/formula-one-kers/11324/
     
  10. brian eiland
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    brian eiland Senior Member

    Ricardo is, "Building on its experience in the research and development of advanced energy management concepts—including the engineering of kinetic energy recovery systems (KERS) for motorsport—Ricardo has devised Kinergy, a high-speed, hermetically-sealed flywheel energy storage system concept with an innovative and patented magnetic gearing and coupling mechanism"

    http://www.greencarcongress.com/2009/11/kinergy-20091124.html
     
  11. DCockey
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    DCockey Senior Member

    Kinetic Energy Recovery Systems have very limited applications in marine propulsion.
     
  12. auriel
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    auriel Junior Member

    ?
    Seems like Kinetic Energy Recovery Systems would adapt very well to marine applications.
     
  13. DCockey
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    DCockey Senior Member

    How would it be used? How it provide a gain greater than the losses due to it's weight?
     
  14. Yellowjacket
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    Yellowjacket Senior Member

    Brian,

    NASA has an oil free turbomachinery initative that they have poured a lot of money into. They have also sponsored research into magnetic bearings. The biggest problem for both of these concepts is that they provide a relatively soft spring rate mounting system and that isn't good for turbomachinery (or alternator) efficiency.

    Air bearings have been produced in numbers for air cycle machines in aircraft. That is an application where maintenance cost is more important than high efficiency. Giri mentioned one of my engines in his article that you quoted, he has been one of the few guys to make air bearings that work. Odly enough, I haven't seen that application in any other publications, it's kind of a stealth thing. (Funny side story here, when NASA came down to try to get us to buy into the air bearing initiative and put one in an advanced engine, you should have seen the look on their faces when we camly told them "We already have an engine running with air bearings in it." They were totally shocked, we just went out and did it and they weren't even aware of it.) Anyway I'm real familar with what works and doesn't work with air bearings and small turbine engines.

    If particles evolve within the bearing, if they are larger than the film thickness, that's a problem. Or, if the deposits evolve within the bearing and build up, that is another potential problem. One of the keys to air bearings is the dry film lubricant on the stainless steel leaves. If the film goes away the bearing locks up when it starts or shuts down. Air bearings in a high salt environment haven't, to my knowledge been widely used, so there are unanswered questions as to how they will handle it. Giri wants to sell air bearings, so he paints them as the greatest thing since sliced bread, nice guy and a smart guy, but he is in a sales mode in that article. Designing an engine to use them is not an easy thing to do. Been there, done that, and what we did worked, but it wasn't easy or simple and there was a performance penalty when we were done.

    Journal bearings use an oil film, and air bearings are simply a journal bearing that uses air as a working fluid, as opposed to a thicker fluid like oil. The air film is very thin and since the friction would tear up the bearing in a few revolutions at 90,000+ rpm you have to go to great lengths to prevent the shaft from bottoming out in the bearing. What has evolved is a set of wound thin sheets that wrap around the bearing (as shown in the article you quoted). These sheets are less stiff than the air film. So if you put a high load on the bearing, the sheet deforms out of the way and the air film doesn't bottom out. I'm simplyfing it a bit here, but that's part of the idea. As the bearing deforms the area with higher pressure increases and the load capacity increases, so there are other things going on. Suffice it to say that this is a relatively soft bearing system in terms of radial spring rate. If you can live with a low rate then it could work for your application.

    Journal bearings aren't generally used in small gas turbines. The reason for this is that the tip clearance of the engine has to be that much larger than it would otherwise be if you had the shaft constrained in a rolling element bearing. Typical tip clearances in a small turbine are .007 inches, and to use a journal bearing with .005 inches of clearance would result in almost doubling the clearance in the engine. Big industrial turbines use them, but the amount of clearance as a percentage of the tip clearance isn't as big an issue in that case. Moreover, the maintenance of the oil filled journal bearing is nil and the load capacity is huge (which you need for big heavy industrial turbine rotors), so what's not to love.

    Automotive turbochargers use oil filled journal bearings at very high speeds, but the most efficient and expensive turbochargers use rolling element brearings to help them reduce the tip clearance and improve efficiency.

    It is all a tradeoff. In some applications air bearings are a good thing. In others they don't make sense. Where you have to hold the rotor in place and not have it move around much they aren't so good.
     
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  15. Stumble
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    Stumble Senior Member

    Auriel,

    Kinetic recovery requires some recoverable energy to work. In boats without a braking system there is no place to recover the energy from. I guess it would be possible to add a turbine that would only drop in the water while slowing down, but for most boat users stopping quickly is rarely a problem, and the gains from regenerative braking just aren't there.
     
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