TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

"Solid State" Thermoelectric AC

Discussion in 'Technical' started by caps04, Jan 22, 2015.

  1. caps04

    caps04 Member

    Joined:
    Aug 14, 2014
    Messages:
    237
    Location:
    NJ USA
  2. Saghost

    Saghost Active Member

    Joined:
    Oct 9, 2013
    Messages:
    2,965
    Location:
    Delaware
    Got it pretty much right - but the difference in efficiency is rather large, and I believe that Peltier stacks are also heavier (though likely more compact) for a given cooling power, and I suspect they are more expensive.

    AFAIK there's no reason one couldn't design an automotive HVAC system using Peltiers if one were so inclined.

    There aren't many locations where they seem to make functional and economic sense right now, though that may change as the technology improves.
    Walter
     
  3. Danal

    Danal electricmotorglider.com

    Joined:
    Dec 11, 2014
    Messages:
    431
    Location:
    Fairview, TX, United States
    The most common way to express efficiency of cooling is the "Coefficient of Performance". If COP were 100%, then every unit of energy expended (like a watt) would result in the same amount of heat removal. A true 100% cannot be achieved in the physical world in which we live... still, the closer the better.

    • Compressor/refrigerant based systems have demonstrated COPs as high as 60%. A more typical real world value of an economically viable implementation is 45%.
    • Peltier (solid state) have demonstrated 5%. A more typical real world deployed number is 2%.

    So, yeah, there is a reason. I don't know the exact capacity of a Tesla A/C; many sedan sized vehicles are in the 24000 to 36000 BTU per hour area, max capacity. Of course, they don't run all the time; the peak is used to quickly cool a hot car and after that, they cycle, and they cycle a lot. Let's assume (pulling this out of thin air) they run 20% of the time. Assuming the same for a Tesla:

    • 7 to 10kW * .2 (for cycling) / .45 (for efficiency) means a compressor based A/C will take about 3 to 4 kW from the pack spread over one hour, which makes it very easy to say 3 to 4 kWh. That's about 4 to 5 % of the total pack (i.e. reduction in range, per hour).
    • If Tesla invested in a very efficient compressor based system, like 55% efficient, that could be as low as 2 or 3% of the pack.
    • 7 to 10kW * .2 (for cycling) / .02 (for efficiency) means a solid state based A/C will take about 70 to 100 kW from the pack spread over one hour. That's the entire pack.


    Those numbers are based on a lot of assumptions and averages. Reality in a Tesla may be different... but the relative efficiency will be somewhere in that ballpark, namely "a few percent" vs. "pretty much everything".
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

    Joined:
    Mar 8, 2012
    Messages:
    12,752
    Location:
    Texas
    The Tesla uses a variable speed scroll compressor rather than a piston type so it doesn't really cycle, it just goes slower or faster. This uses far less energy than the cycling of a pulley driven system. While it can use 3-4 kW, it almost never does so (at least in my car).
     
  5. Larry93428

    Larry93428 Member

    Joined:
    Feb 26, 2014
    Messages:
    371
    Location:
    Cambria California, United States
  6. TEG

    TEG TMC Moderator

    Joined:
    Aug 20, 2006
    Messages:
    17,249
    Location:
    Silicon Valley
    I think you would need a heck of a lot of Peltier devices (space & $$$) to try to match that...

    How many BTUs does a Peltier cooling chip generate? | Answerbag
     
  7. Danal

    Danal electricmotorglider.com

    Joined:
    Dec 11, 2014
    Messages:
    431
    Location:
    Fairview, TX, United States
    Cool! (Pun Intended).

    So the gap is likely to be even larger than my assumption-laden estimates above.
     
  8. NickInLab

    NickInLab Member

    Joined:
    Jan 9, 2014
    Messages:
    11
    Location:
    many places in the western US
    #8 NickInLab, Jan 28, 2015
    Last edited: Jan 28, 2015
    So I think you're thinking of the related (but different) concept of thermal efficiency as it relates to heat engines, primarily because practical heat pumps (i.e., air conditioners) usually have COP well over 100% [1]. Thermal efficiency, of course, can never exceed 100%. COP measures the amount of heat removed from a cold reservoir per amount of work used to run the pump. The sum of those two quantities is pumped into the hot reservoir.

    This differs from measurement of thermal efficiency, which is measuring work performed per amount of heat entering from the hot reservoir. The difference of these quantities is exhausted as waste into the cold reservoir.

    A heat engine run backwards is the same as a heat pump. However, people conventionally measure the "waste heat" (cold) side of the machine when talking about heat pumps and the "combustion" (hot) side of the machine when talking about heat engines. People also flip the fraction (work on top for engine vs. bottom for pump) depending on the context. Hence the confusion. In both cases, the heat flux on the hot side of the engine/pump is the sum of the work in/out and heat flux on the cold side.

    [1] Trane, the first hit for "SEER air conditioner" on Google, advertises units with a SEER rating of 21, which corresponds to a COP of around 6 (600%). I suspect that's at the high end of "economically practical heat pumps for stationary applications."


     
  9. Danal

    Danal electricmotorglider.com

    Joined:
    Dec 11, 2014
    Messages:
    431
    Location:
    Fairview, TX, United States
    Good point. Bad terminology on my part... yet the math still holds. Thermal efficiency of compressor based systems vs. Peltier on the same scale is so far apart that it does represent the difference between a few percent of the traction pack for cooling vs. all (or almost all) of it.
     

Share This Page