Posted by SJC on January 13, 2007, 8:32 pm
No by finned tubes I mean:
Use the term flate plate collector, whatever you want.
It is a solar thermal collector like Sunearth collectors:
Fluid (water) is circulated through the tubes and sun shines on the black
surface and heats the surface and the tube and the water. Come on guys,
you know the drill! :)
What I am saying is if the air in the box (the Sunearth box!) gets to 100F
and you have more than 60 square feet of surface area and the outside
temperature is 50F you are going to lose a lot of heat. Cut your loss and
run that box air through and air to air exchanger to the house during the
I am not talking about air heaters. I am talking about fluid solar thermal
collector also being air heaters.
Posted by Morris Dovey on January 13, 2007, 8:56 pm
| No by finned tubes I mean:
| Use the term flate plate collector, whatever you want.
| It is a solar thermal collector like Sunearth collectors:
| Fluid (water) is circulated through the tubes and sun shines on the
| surface and heats the surface and the tube and the water. Come on
| you know the drill! :)
| What I am saying is if the air in the box (the Sunearth box!) gets
| to 100F
| and you have more than 60 square feet of surface area and the
| temperature is 50F you are going to lose a lot of heat. Cut your
| loss and
| run that box air through and air to air exchanger to the house
| during the
| I am not talking about air heaters. I am talking about fluid solar
| collector also being air heaters.
Thanks for the links! The tubing I used looks like
<http://www.mcelroy.com/fintube/images/ifin.gif> . I think I need to
update my old design and build a couple of full-sized panels for show
and sell. :-)
DeSoto, Iowa USA
Posted by nicksanspam on January 13, 2007, 10:33 pm
Hot air can help more. In full sun, a box with 1 ft^2 of R1 glazing with
90% solar transmission would gain 0.9x250 = 225 Btu/h. If it contains 1 ft
of 3" fin-tube with a 5 Btu/h-F air-water conductance, the fin tube might
gain 225x3/12 = 56 Btu/h from direct sun.
Meanwhilst, on a 30 F day, we might have something like this, viewed
in a fixed font:
--- | R1 R1 |
|--|-->|---*---www---30 F ---www---*----
--- | | I---> |
225-56 | | |
w | w
w 1/5 - | 199 F w 1/5
w - | w
| - --- |
| 70 F - | 70 F
| | |
--- - ---
I = (199-70)/1.2 = 107.5 Btu/h, so Ta = 70+107.5x0.2 = 91.5 F, and
the fin tube gains another 107.5 Btu from the hot air.
Posted by DJ on January 14, 2007, 2:20 am
That is true, in part. But the heated fins warm the air inside the box,
they're in direct contact; conversely, they are also *cooled* by the
air inside the box if the air is cooler than the fin is. It is in a
homeowner's best interest to keep that box as hot as possible to
maximize transfer. That's why the boxes are so well insulated
That's also the theory behind why vacuum tubes work as well as they do:
they can capture solar radiation on their fin surface, and can't
radiate it back to the environment because of the vacuum surrounding
the fin; heat transfer is impossible.
The downside of vacuum tube systems in that respect is that they are
incapable of melting snow and ice cover off their tubes in the winter,
whereas a flat plate system will shed enough heat usually into the box
(and thus the glass) to melt the ice and snow off them.
Posted by nicksanspam on January 14, 2007, 5:31 am
Sneaky, huh? :-)
But that can be less costly and more efficent than 2 collectors, if
we want to collect solar heat at 2 different temperatures, eg room heat
and DHW. Fin tubes need their own inner air heater glazing to do this
kind of thermal cogeneration, but collector panels do not.
In full sun (250 Btu/ft^2-h), 1 ft^2 of polycarbonate R1 glazing with 90%
solar transmission will transmit 0.9x250 = 225 Btu/h, regardless of what's
inside. If the outdoor temp is 30 F and the sunspace is 70 F, with lots of
airflow, the glazing will lose (70-30)1ft^2/R1 = 40 Btu/h to the outdoors,
so the room gets 225-40 = 185 Btu/h of heat.
Put 0.5 ft^2 of 140 F collector panel (eg a homemade Big Fin with 1/2"
copper pipe pounded into a circular groove in alumimum flashing, with
silicone caulk) with 1.5 Btu/h-ft^2 of airfilm conductance and good back
insulation in the sunspace, and it will collect 112.5 Btu/h of sun and
lose (140-70)1.5x0.5 = 52.5 Btu/h to the 70 F sunspace air, for a net
water gain of 60 Btu/h, so the room only gets 225-40-60 = 125 Btu/h of
heat. We can bring the room heat back up to 185 by using 1.32 vs 1 ft^2
Or keep the room heater at 1 ft^2 and build another outdoor box with
another 1 ft^2 of glazing to collect 60 Btu/h at 140 F with 0.5 ft^2
of collector gaining 112.5 Btu/h and losing 52.5 Btu/h to 70 F box air,
with no heat gain for the room from the outdoor box.
The combined box with 1.32 ft^2 of glazing is cheaper and more efficient
for the same outputs, no? And it's more flexible. If we need room heat
but no DHW, we can turn off the collector pump.
Right. They might destroy themselves :-) Bummer.