How to calculate meter to sqmeter?

How do I calculate thermal energy coming out of a solar heater?

• 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.dT. The problem is I can't exactly calculate the mass of air going in. I could find mass by using the density formula m=rho.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). So if I assume that the flow rate is equal to volume then my equations should be something like this: Et = m.c.dT (general equation) = (rho.v).c.dT (subbed in density multiplied by volume for mass) = (rho(A.v)).c.dT (subbed in flow rate equation for volume , which I think is wrong but right) = (rho(pr.r^2)(v)).c.dT (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. here are some values that I have obtained. (info that may be useful in the calculations): 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" dT = 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 rho of air = 1.165 kg/m³ at 30 degrees C 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..

• Answer:

  Your approach is sound. However, the air coming out will probably not have a uniform velocity; it will be fastest in the scenter and slower at the edge. When you have a problem that seems to require measuring something that is going to be a real beast to measure, see if there is some way to recast the problem to eliminate the need for that measurement. One solution would be to take the exiting air and run it through a heater of known wattage. Measure temperature before solar panel, between panel and heater, and after heater. Since heat will be in proportion to DeltaT you will be able to find the wattage of your solar heater.