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Calculating watts per square foot

  1. Feb 8, 2009 #1
    I am working on a project to consider how much air conditioning in a metropolitan area raises the temperature outside. I am trying to figure out how to translate the temperature outdoors (say 32 degrees C) into watts per square foot.

    Once I have the watts per square foot of heat already in the air, I plan to estimate the amount of heat dumped by an air-conditioner with a Coefficient of Performance of 3. Then I want to see how many watts per square foot are being dumped into the air by houses. So I will want to calculate the amount of temperature rise as a function of watts per square foot.

    Do I need to use the heat capacity of air?

    *EDIT

    How would I calculate the heat capacity of moist/humid air?
     
    Last edited: Feb 8, 2009
  2. jcsd
  3. Feb 8, 2009 #2

    russ_watters

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    Staff: Mentor

    Watts is a unit of power, whereas temperature is a measure of energy. They can't be mixed in the way you are trying to mix them.

    All you really want is the heat capacity of the outdoor air (specific heat), some assumption about the volume of the air (and it's movement...) and the heat flow rate of the energy in buildings to the outside. That's a lot of assumptions that have a big impact on your answer. But you don't need to know the starting energy of the air in any other units but temperature.

    For the specific heat of air, I would use 1000 j/kg-k (you can google that).

    The big problem you will find with this is that air doesn't stay still. When heated during the day, bubbles of air over cities (and other warm land features) rise and cause convection.
     
  4. Feb 8, 2009 #3
    Thanks for your response.

    Joules per square foot would be fine. But this still doesn't help me calculate the heat capacity of humid air, which is a critical point in the matter.
     
  5. Feb 8, 2009 #4

    russ_watters

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    Staff: Mentor

    Air doesn't contain much moisture, so no need to bother with that. In a weird, but perhaps useful mix of units, it's .322 watt-hours/Cu Ft-F

    No square feet here...
     
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