[Greenbuilding] solar heat collection

[Clarke Olsen]

All heat exchangers have a problem; transfer can never be total. Shouldn't running through the
greatest differential extract the most? I can appreciate the concept of obtaining a high temp. in the
upper strata, but storage considerations suggest the efficiency of more uniform temperatures.

Clarke Olsen
clarkeolsendesign
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at taconic.net




On Jul 5, 2012, at 7:44 AM, Corwyn wrote:

> On 7/4/2012 8:39 PM, Nick Pyner wrote:
> 
>> >You will get better stratification if the incoming water from the
>> >collector, is heating the warmest water at the top of the tank, it will
>> >also be a more efficient heat exchanger.
>> 
>> I submit that takes a lot of explaining. Heat exchangers fundamentally
>> rely on the temperature difference, the "heat gradient". The greater the
>> difference, the steeper the gradient. The incoming water from the
>> collector will not find that at the top of the tank.
> 
> So I will try to explain.  Basically what you are trying to achieve is the same thing as a cross-flow heat exchanger.  Let's assume the incoming water is 180°, and the tank is 150° at the top and 120° at the bottom.  Instead of considering the whole tank as a single thing, divide it into two boxes.  Now imagine running the incoming water through two exchanges, through one or both boxes in any order.  Which arrangement gets you the most heat?  Assume 100% efficiency and equal amounts of water in the tube and in the boxes to make the calculations obvious.
> 
> If you run the water through just the cooler box, both end up at 150° ( and obviously, the warmer box stay 150°).  If you run through the warm box and then the cool box, the top box (and the water when it is done) end up at 165°, then the cooler box and the water end up at 142.5°.  So you have extracted 7.5 more degrees of warmth from the incoming water, so more efficient; plus you have a max temperature of 165° rather than 150° so more stratification (higher usable energy).
> 
> Thank You Kindly,
> 
> Corwyn
> 
> -- 
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.greenfret.com/
> topher at greenfret.com
> (207) 882-7652
> 
> 
> 
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