Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermal energy conversion

  1. Oct 6, 2010 #1

    CIm

    User Avatar

    Hello! I was wondering if someone could take a look at this idea and tell me if there is something wrong with it:

    In attempting to shut down every part of a hypothetical spacecraft except some sort of highly insulated crew pod, the total amount of heat produced over a period of time is equal to the total calories consumed minus any useful mechanical work performed. If a person consumes an average of 2400 kilocalories per day, the average heat produced is 100 kilocalories per hour or 116 watts.

    Ignoring all other assumptions, if you were to convert that heat buildup to electrical storage, you get 20 crew x 1 day, 55.68 kilowatts. A kilogram of Li-ion stores 128 Wh[1], so you need 435 kilograms to store the heat energy of the crew for the day. Multiply that by 120 days and you get a little over sixty tons of Li-ion batteries required for a four month journey, easy to fit in on a ten thousand ton ship. With lithium thionyl chloride, used in extremely hazardous or critical applications such as space flight and deep sea diving, you get a little over 10 tons needed.

    Now you probably won't get all of the thermal energy, but using new technological advances [2] and say dunking the lot in water to slowly absorb the rest for four months, could you conceivably keep a vessel of this sort at or as near to ambient as makes no odds?

    [1]http://www.allaboutbatteries.com/Battery-Energy.html" [Broken]
    [2]http://www.sciencedaily.com/releases/2010/09/100930154610.htm"
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 6, 2010 #2
    Uhm seems like you are ignoring the second principle of thermodynamics. To convert thermal energy to work you need to reservoirs at different temperatures. A lower ratio between the lesser temperature and the upper temperaturemakes the efficiency of you thermal engine higher. Assuming you have an internal temperature equal to 300 Kelvin, what do you plan the lower temperature resorvoir to be?
     
  4. Oct 6, 2010 #3

    CIm

    User Avatar

    Since the idea is to keep the ship, except the small crew pod, which is inside the ship, at ambient space temperature, it would be around three kelvin. The apparatus to convert the thermal energy to electrical energy and the storage medium would be at the ambient ship temperature, and inside the ship as well.
     
  5. Oct 6, 2010 #4

    CIm

    User Avatar

    So it should be technically doable?
     
  6. Oct 7, 2010 #5

    CIm

    User Avatar

    Maybe I'm reading this wrong, but the Carnot limit on thermal efficiency is

    2c38c2bf9b428765f846f836e7a81786.png

    So the maximum possible efficiency would by 99% in this case, so you'd only need to absorb around 60 kilowatts of heat for the entire journey? Note that you could vent it afterwards, its just being stored long enough to get there.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook