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Temperature expectations from solar hot water panel - Page 2

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Posted by nicksanspam on May 22, 2005, 7:57 pm
 

Adding 20% more glazing below the collector, viewed in a fixed font:

                                Tb
                                |
     ---                 1/5.68 | 1.2/5.68
|---|-->|-----------------www-x-*---www--- -8 C
     ---         |  32 C        |
    2307Wh/6h   ---             |
    = 384 W      -              |
                 |              |
                 -              |
     ---                        |
|---|-->|-----------------------
     ---    
    0.2x384
    = 77 W  

With an equivalent circuit to the right of point x:

                                Tb
                                |
     ---                 1/5.68 | 1.2/5.68
|---|-->|-----------------www-x-*---www--- +8.2 C = -8 + 77x1.2/5.68
     ---         |  32 C        
     384 W      ---            
                 -            
                 |          
                 -          
And...

     ---                0.3873
|---|-->|-----------------www--- +8.2 C
     ---         |  32 C
     384 W      ---
                 -
                 |          
                 -          

So the absorber still gains 384 W, but it only loses (32-8.2)/0.3873 = 61.4
watts over 6 hours, for a net gain of 6(384-61.4) = 1935 vs 944 Wh/day. The
box air temp Tb = 8.2 + 61.4x1.2/5.68 = 21.2 C... 20% more glazing doubles
the useful output, with no additional active collector surface :-)


With a mean collector temp Tm = 32 C and Ta = -8 C, X = (32-(-8))/800 = 0.05
and E = 0.959-8.91X-0.047IX^2 = 0.42, ie a 42% collection efficiency in full
sun on an average Dec day in Ottawa. In 2563/6h = 427 W/m^2 sun, X = 0.09364
and E = -0.052, ie they would lose heat.

These unglazed panels are 97.6" long x 33.9" wide, with a 1.93 m^2 active
area. Mounting one in a 4'x8' 2.974 m^2 box with 14.1"x96" of additional R1
polycarbonate glazing from a 4' wide roll might make something like this,
normalized to 1 m^2 of active area:

                                Tb
                                |
     ---               ---      | 1.54/5.68
|---|-->|-------------|-->|--x--*---www--- -8 C
     ---         |     ---      |
    2307Wh/6h   ---  384(1-E)   |
    = 384 W      -              |
                 |              |
                 -              |
     ---                        |
|---|-->|-----------------------
     ---    
    0.54x384
    = 208 W  

With an equivalent circuit like this:

                                Tb
                                |
     ---               ---      |  0.271
|---|-->|-------------|-->|--x--*---www--- +48.4 C = -8 + 208x1.54/5.68
     ---         |     ---      
     384 W      ---  384(1-E)  
                 -            
                 |          
                 -          

If Tb = 48.4 + 384(1-E)0.271 = 152.5-104.1E and X = (32-Tb)/384 and E
= 0.959-8.91X-0.047x384X^2 = 0.959-8.91(32-Tb)/384-0.0001224(32-Tb)^2, then
Tb = 52.67+2.415(32-Tb)+0.01274(32-Tb)^2 = 130.0-2.415Tb+0.01274(32-Tb)^2
= 143.0-3.230Tb+0.01274Tb^2, ie Tb^2-332.1Tb+11223 = 0, and Tb = 38.2 C
and X = (32-38.2)/384 = -0.001611 and E = 1.107, for a 111% efficiency :-)

Nick


Posted by Doug on May 30, 2005, 3:33 pm
 
Hi Nick,
Thanks for responding - I had thought the thread had fizzled.
These panels look interesting and economical.  I looked on their
website but could not find a NA distributor - do you know of one?

Thanks for your calculations.  I wish I had more room to store a larger
quantity of water.

-Doug


Posted by Gary on May 11, 2005, 6:30 pm
 Doug wrote:
Hi Doug,



If you want to supplement radiant floor heat with solar, its best to
use the lowest possible water temperature that will heat the house.
This way you get more out of the solar heated water.  For example, if
you add a 500 gallon storage tank (4000 lbs), and heat it to (say)
150F during the day, then you have (4000lb)(150F-110F)(1 BTU/lb-F) =
160K BTU stored in the tank. If you lower the floor heating temp to
80F (about the lower limit), then you would get (4000)(150-80) = 280K
BTU from the same stored water.  Some radiant floor systems have an
"outside reset" feature that automatically adjusts the circulated
water temp based on the outside temperature.  On cold days it will
circulate hotter water, and on not so cold cays, it will circulate not
so hot water.  This is a really good feature for solar radiant floor
systems.


Someone else is going to have to help with the Ottawa winter weather,
but bear in mind that the heat that a solar collector turns out is the
  product the weight flow through it times the temperature increase
from its input port to its output port (ie it depends on the
temperature and the FLOW).  In general, panels will pick up more heat
when running a relatively high flow rates, and low collector
temperatures.  If the collector temperature at any given time is much
higher than the storage tank temperature at that time, you will have
higher losses and lower efficiency.

There is some information here:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
Scroll down and look for hydronic or radiant floor heating.

And:
http://www.builditsolar.com/References/supplierrefs.htm
Look for the Radiant Heating section

IPEX (listed at the link above) has a downloadable version of "Manual
of Modern Hydroncis" which is good.

Quite a few words to not really answer your question -- eh :-)

Gary








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Posted by News on May 12, 2005, 10:43 pm
 

A radiant floor is 110F?   It usually takes water between 30 and 50C.  Any
hotter and your feet burn.




Posted by daestrom on May 13, 2005, 12:37 am
 

And 110F is 43.3 C, right in the middle of your '...between 30 and 50C'.  So
what's your point??

daestrom



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