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Homework Help: (Please help!) Calculating the thermal energy coming out of a homemade solar heater.

  1. May 24, 2012 #1
    This is for a physics finals project. So it's not really a homework question. But I didn't know where else to post it and I need immediate help.
    I have made a passive solar heater out of pop cans and I need to find out how much energy (thermal) is produced. I know the equation for thermal energy is Q=m.c.ΔT. The problem is I can't exactly calculate the mass of air going in. I could find mass by using the density formula m=ρ.v, but then that leaves finding volume. But then, you also can't exactly calculate how much volume air can hold. So I was advised to calculate the flow rate of air using this equation Q = A.v where v is velocity. But I wasn't sure how the flow rate of air be equal to volume and come out as thermal energy (in joules).

    Here's some details about my box frame.
    my box is 2'x4' using 2"x4" wood (3.5"x1.5" in real life). so the volume of my box is 21"x45"x3.5" = 3307.5 cubic meter. I have 2 holes cut with a diameter of 4.5" so the radius would be 2.25"

    now what I don't know is how to obtain mass.
    Can someone please explain to me what I'm doing wrong and guide me to determine the thermal energy coming out of this solar heater..

    1. The problem statement, all variables and given/known data
    What is the equation to calculate thermal energy coming out of the solar heater ?

    ΔT = 0.5-1 degree C every 5 minutes ( I collected temperatures for 2 hours every 5 minutes and they ranged from 34-42 degrees C)
    c = 1.006 kJ/kgC
    ρ of air = 1.165 kg/m³ at 30 degrees C
    volume of box = 3307.5 cubic meter
    r = 2.25"

    2. Relevant equations
    Et = m.c.ΔT (general equation)
    Q = A.v where v is velocity and Q is flow rate
    m=ρ.v (density equation, solving for mass)

    3. The attempt at a solution

    If I assume that the flow rate is equal to volume then my equations should be something like this:

    Et = m.c.ΔT (general equation)
    = (ρ.v).c.ΔT (subbed in density multiplied by volume for mass)
    = (ρ(A.v)).c.ΔT (subbed in flow rate equation for volume , which I think is wrong but right)
    = (ρ(pr.r^2)(v)).c.ΔT (worked everything out and this is what I get)

    See where I'm going with this? I don't fully understand it. And I'm not sure how to calculate velocity either without using some sort of instrument that just shoots out a value.

    Unit check:
    kg/m^3 . m^2 . m/s . KJ/Kg K . K
    I have done a unit check and after manipulating common variables I get KJ .s
    Is this correct?

    I don't need to get a specific value in the end. I just need to check if my variables are correct and if it's the right equations I'm using and if my final answer will come out as joules.
    If there's anything I'm missing. please let me know.
  2. jcsd
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