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Old 06-08-16, 03:13 AM   #7
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Default efficiency?

Originally Posted by Daox View Post
Can someone help me figure out the calculations for how to get the amount of BTUs you get out of hot air at a specified flow rate? I know that it depends on a lot of different factors like moisture content of the air, but I guess I'm looking for a simplified equation that ball parks things.

For example, lets say I have a solar hot air panel. I know the fan flows 100 CFM and I know that it puts out air that is 20F above ambient. How many BTUs per hour is it going to spit out?
This question created a bunch of other questions. After thinking about it...
The energy out of the system is equal to the solar energy impinging on it.
The efficiency of the system is the ratio of the energy that's actually
in the form you can use (input - losses)/input.
I argue that a cold collector absorbs more solar energy. Colder air
creates less radiated/conducted loss in the piping outside the heated space.
I think you might want more air and less temperature rise.
At some point the cost
of moving the air is more than the solar energy recovered.
Somewhere, there ought to be a peak in the net recovered energy curve.
Would be interesting to measure the delta-t at different air flow rates,
plug that into the BTU formula and plot the result. You'll end up with a
family of curves with solar input and outside temperatures as parameters.

Then add some practical limits.
Like, you may want to set a minimum output temperature for comfort reasons.
Or a maximum air flow for noise reasons.

Depending on the shape of the curves, it might be interesting to use a microcontroller to hunt for the peak in real time.
There's some advantage to measuring the thing you actually want to know.
You could even use a thermostat to divert any excess energy into a storage
tank for use when less solar is available.
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