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ORC Heat Recovery Hybrid Electric Vehicle

  1. Dec 22, 2014 #1
    I have been thinking of a concept for a class 7-8 semi-tractor trailer that is hybrid electric. The basic idea is a fully electric drive system that propels the truck with electric motors only. A lithium-ion battery pack stores energy for the drive system. A diesel generator provides electricity to charge the batteries and motors. An ORC Scroll Expander (from Air Squared) is powered by the waste heat from the engine coolant and exhaust; the electricity from the scroll expander also provides charge to the batteries and motors. This combined cycle power-plant coupled to an electric drive train makes the perfect, most efficient hybrid electric vehicle available.

    The coolant system for the diesel engine is modified to flow through the engine block and a heat exchanger in the exhaust manifold. This hot coolant then passes through another heat exchanger that has the refrigerant being pumped through for the scroll expander. In this setup most of the wasted thermal energy has been converted into electric energy.

    Air squared offers 1, 5, and 10Kw scroll expanders with two options; oil lubricated or oil-free. The oil version offers an 80% efficiency, while the oil-free version runs at 70%. I'm in communication with them trying to figure out some of the details; like what type of oil does it uses and how often does it need to be changed. My guess is that the oil version will offer the highest pay-back on investment; so I would like to use the 10Kw oil version in my setup.

    I've been trying to figure out the total thermal efficiency of the diesel engine and scroll expander. So if the diesel gets 40% efficiency that means there's another 60% lost as heat. If the oil scroll expander can capture 80% of that heat then the total efficiency would be: 0.4 * 0.8= 0.32 * 100= 32%+ 40%= 72% Is that calculation correct? Here's another: diesel 50%, scroll 80% 0.5 * 0.8= 0.4 * 100= 40% + 50%= 90%

    Then, to get the MPG you would have to factor in the efficiency of the electric drive system. I will do that calculation tomorrow since it's getting late, but there's a lot more variables involved since it depends on the batteries, motors, motor controller; and a bunch of other things.

    The vehicle could have a longer battery pack if it has access to regular charging;or a smaller pack if it's constantly over the road. Check out Air Squared's website, I think what they have is a breakthrough in waste heat recovery energy production.

    There's a lot of different setups you could do to improve a vehicles efficiancy, extended-range electric vehicle, strong hybrid electric vehicle, mild hybrid electric, or non hybrid vehicle.

    Thanks for you inputs!

    http://airsquared.com/
     
  2. jcsd
  3. Dec 22, 2014 #2

    mfb

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    You cannot recover "most of the wasted thermal energy", you are always limited by the Carnot efficiency. The numbers on their website might be relative to this maximal efficiency. You cannot use 80% of the waste heat for non-thermal applications.

    An additional notation issue: 0.32 * 100= 32%+ 40% is not correct.
     
  4. Dec 22, 2014 #3
    Can you please explain this better? If a gallon of diesel contains 139,000 BTU's of energy and you have a diesel engine that has a working efficiency of 40%, wouldn't that leave the remaining 60% or 83,400 BTU's as thermal energy? A Kilowatt hour of electricity contains 3412 BTU, so if we could recover 100% of that 83,400 BTU and turn it into electricity we would get 24.443 Kw/H. But the scroll expander recovers 80% of the available thermal energy so we would get 19.555 Kw/H of electricity.

    I know the efficiencies of the diesel engine and scroll expander could be lower in working conditions, but isn't the general math theory that I'm doing correct? What am I missing here?

    If people are wondering why you would even bother with a 40% efficient diesel engine when you have an 80% efficient scroll expander; the answer is power output, space, and weight. It is possible to fit the 10Kw scroll expander on a truck with this system; but the truck needs about 80Kw of electricity continuously provided to the batteries and motor; a small diesel engine can easily provide 70Kw of power. Another important point is that internal combustion engines are the best way to generate a lot of heat quickly while being able to control emissions. Also, you can get a higher efficiency than by just using the scroll expander alone. Even if your ICE engine was only 10% efficient, your total efficiency would be 82%, 2% higher than without the ICE. Unless if my calculations are totally wrong.

    One more point, combined cycle Diesel engine/ Steam turbine powerplants are becoming standard; they have real world efficiencies of 60% or higher; which is higher than either can achieve alone.

    Ok, so I did some calculations based off the thermal efficiencies in BTU and I got a different answer. Here's my math:

    For the diesel engine and one gallon of diesel
    139,000 * 0.4 = 55,600
    For the 60% wasted thermal energy
    139,000 * 0.6 = 83,400
    For the scroll expander that captures 80% of that wasted thermal energy:
    83,400 * 0.8 = 66,720
    The total sum BTU of the diesel engine and scroll expander:
    55,600 + 66,720 = 122,320
    The total sum combined efficiency of the diesel engine and scroll expander:
    122,320 / 139,000 = 0.88 * 100 = 88%

    Which answer is correct? Or am I wrong on both of them?
     
    Last edited: Dec 22, 2014
  5. Dec 23, 2014 #4

    mfb

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    Right.
    It is impossible to recover 100% or even 80%. The optimum depends on the temperature ratio (hot engine to cooling fluid), but it will be significantly lower.
    The diesel engine has the best possible conditions to convert the thermal energy to work, and it still just gets 40%. Anything outside the engine will have a smaller temperature difference and less ideal conditions, so even 30% conversion of the remaining heat would be very good.
    You cannot compare those numbers. I don't know what exactly the 80% mean, but they certainly mean something different from the 40% of the diesel engine.

    Right, but they use expensive equipment to squeeze every percent out of it. Worth the effort for a power plant that produces a gigawatt of electricity, but unreasonable for something the size of a car.
     
  6. Dec 27, 2014 #5

    billy_joule

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    Your calculations are not correct. To get realistic results you need to accurately model a Rankine cycle with your fluids properties. Including all losses (pump inefficiency, pressure drops, heat exchanger inefficiencies etc etc)

    Overall ORC efficiency at the temperatures you could expect are typically in the ~5% range.

    I am currently designing a pilot ORC unit at my student internship. We hope for ~10% efficiency between sinks of 40 and 200deg C, so 500kW of waste heat will become 50kW shaft power. Total material costs will be ~$100k which I'm told is cheap by ORC standards.

    There are many papers around on the performance of functioning ORC units running off waste heat from diesel engine. You'll probably need to do a thermodynamics course to understand much of it then start here:
    http://scholar.google.co.nz/scholar...a=X&ei=zzWfVLWLCKX5mAWq84DIBw&ved=0CBkQgQMwAA
     
  7. Feb 23, 2015 #6
    Thank you for your posts, I really appreciate the discussion!

    Wow, I knew the calculations I was doing were theoretical calculations, but I didn't think they would be that far off! I guess it makes sense to me now that the total sum efficiency cannot be higher than either of the individual efficiencies. If it is possible to turn only 5% of the wasted thermal energy into shaft power I can see why nobody else is already doing this. I was hoping for maybe 40-50%. I sent an email to Air Squared to see if they could clear this up; I also sent them a link to this thread. I will post their reply here when I get it.
     
  8. Feb 23, 2015 #7

    y2j

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    Are you used any software for theoretical calculations,?
    I am use the EES and there is a problem in calculate the exhaust temperature outlet from the evaporator become below the dew point of the exhaust gases
    of the engine
     
  9. Feb 25, 2015 #8

    y2j

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    Hi
    if anyone simulated the heat from the exhaust of the vehicle by an electric heaters inside the evaporator of 'ORC' and covered with working fluid , then what is the control strategy that applied to sure a continuous operation of expander by vapor generated in the evaporator ,since the pressure raise not by pump but by setting a specified valve at the desired pressure
     
  10. Feb 25, 2015 #9

    billy_joule

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    It's a little hard to interpret your questions..
    The high side pressure is set by the pump, you don't want to throttle the flow with a valve, you want the pressure drop over the expander, not a valve. Controlling the pump with a VFD is the usual method.
     
  11. Feb 25, 2015 #10

    billy_joule

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    And if the fluid is condensing in your expander then you need to alter the low side conditions to prevent that. In the same way you'll need to ensure there is no cavitation at the pump.
     
  12. Feb 26, 2015 #11

    y2j

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    thanks
    sorry
    I am not talking English language professionally
    my idea similar to that one in a paper available online http://www.sciencedirect.com/science/article/pii/S0360544200000633?np=y
    and another paper on this location http://papers.sae.org/2011-01-1339/
    in this papers the authors used electric heaters in the evaporator as a simulation of heat recovery
    I understand from this papers that to allow only vapor inlet the expander must keep the working fluid at specified level and only vapor out of from the evaporator
    by a valve (e.g. solenoidal valve set at specified pressure )
     
    Last edited: Feb 26, 2015
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