# DIY Solar Thermal Panel - Page 2

Posted by Zombie Wolf on July 2, 2006, 3:55 pm

Not really.. my 4 collectors are designed to heat 240 GALLONS of water to
195 deg F in two hours. if you divide by 4, that is 60 gallons per
collector, and the temperature is much, much higher. 40 lbs of water aint
much. That would be about 5 gallons.  And the water these collectors are
heating is 50 degrees to begin with, which means it has to bring the
temperature up by 145 degrees, not 40 degrees. The heat is collected into
three 80-gallon tanks, and there is quite a bit of heat energy left over,
since this thing has to function in the winter. No, I dont have "5 dollars"
invested. I have about 2000 in this system, which I will recover in less
than one year, since the collectors save me about 200 dollars a month on my
power bill.... The collectors will be profitable in 11 months.

Posted by Jeff on July 2, 2006, 5:26 pm

Zombie Wolf wrote:

Let's see

60 gallons = ~ 500 lbs
500lbs * 145 r,645 BTU's for 2 hours

That's 1135 BTU per square foot per hour. You must be a *lot* closer to
the sun than I am!!!

Originally I thought the guy with the refrigerator collector was
fantasizing, but now I think you are too!

Jeff

The heat is collected into

Posted by Zombie Wolf on July 2, 2006, 10:17 pm

I'm not any closer to the sun. 12 kilowatts of heating power is more than is
required for this job . your average water heater draws about 1500 to 1800
watts. If I heated the water with electricity, only about 7.2 kilowatts
would be required by a conventional water heater... there is nothing
"fantastic" about these figures. As a matter of fact, there is excess
capacity.

Posted by Jeff on July 3, 2006, 12:05 am
Zombie Wolf wrote:

Sure you are.

1135 BTU per square foot per hour is 3.58 kilowatt hours per square
meter. The sun isn't that bright. All I'm doing is pointing out that
your performance calculations are off. Of course I agree that you have
enough to heat all the water you could want!

The refrigerator box calculations are possible, but probably off
also. If they were accurate that thrown together collector would be
working pretty well, which really was my original point (that they were
either no doubt wrong, or something else was going on)!

I wouldn't have said anything except it was obviously off and I had
some spare time to run some basic calculations. No offence intended!!!

Cheers,
Jeff

12 kilowatts of heating power is more than is

Posted by daestrom on July 2, 2006, 3:48 pm

One interesting thing is the use of reflective foil on the back inside.  The
author explains this is to keep the backing from getting hot.  Okay, a cool
backside means it doesn't lose a lot of heat out the uninsulated back.  But
a reflective surface means a lot of light energy is just being reflected
back out the glass.  Only the reflected light that happens to land on the
back side of the tubing/wires turns into heat.

So the reflective foil means you get collection on the entire tube surface,
not just one side.  But still, seams like a lot of the solar energy is just
'bouncing' back out the glass from the reflective foil.  Might be
interesting to calculate the total surface area of the tubing, and add to it
the total surface area of all the 'wires' that are soldered to the tubing.

As far as size, yes it is very small.  But then he was working with scrap
parts, so you take the size of parts you can get.

As to the total tubing length, its always a compromise.  The collector
*plate* is what intercepts the solar energy.  But you have to get the energy
out of the plate before it is lost, otherwise you have nothing.  So you want
tubing to gather up the energy and transfer it to the water with a minimal
temperature difference between collector plate and water.  And large
distances between tubes means the plate area half-way between must be the
hottest part of the plate.  Too hot, and more lost energy.

The author mentions that since both the inlet and outlet are at the same
end, he couldn't get a thermosyphon.  I disagree.  If he turned it around so
the inlet/outlet were at the top, and put a 'cold' storage tank above the
unit, I think it would naturally thermosyphon.  The amount of heat picked up
by the straight run down to the coil bottom would be less than the heat
picked up by the rest of the coil, and it would work.

daestrom

•
• Subject
• Author
• Date
 Re: DIY Solar Thermal Panel Mary Fisher 06-25-2006
 Re: DIY Solar Thermal Panel Zombie Wolf 07-02-2006
 Re: DIY Solar Thermal Panel Zombie Wolf 07-02-2006
 Re: DIY Solar Thermal Panel Zombie Wolf 07-02-2006
 Re: DIY Solar Thermal Panel Zombie Wolf 07-02-2006
 Re: DIY Solar Thermal Panel Zombie Wolf 07-02-2006
 Re: DIY Solar Thermal Panel Mary Fisher 07-03-2006
 Re: DIY Solar Thermal Panel nicksanspam 07-03-2006