Posted by nicksanspam on June 27, 2004, 2:42 pm
A transparent cover is likely to have a temperature close to the average
"indoor" and outdoor temperatures. On a 100 F day, polycarb might work
until the inside air reaches 2x250 - 100 = 400 F.
Greenhouse polyethylene film has lots of longwave IR transmission
(ie little "greenhouse effect"), so it's less likely to overheat,
altho it can become brittle if it's touching a dark surface, and
it sags a lot when it's warm and it's subject to wind fatigue, esp
if it's installed in flat vs curved sheets.
How can we support it over a dark asphalt roof to make a warm air
or trickle collector? It might be pressurized with a small blower
(as in a greenhouse) that turns on when it's windy outside or very
hot inside, with some web straps over the film to keep it from
Posted by Gary on July 3, 2004, 3:30 pm
I did a not very scientific test with a small sample of left over
corrugated polycarbonate glazing. I supported the PC sample
horizontally a few inches off the floor, and directed the stream of air
from my heat gun onto the top of the PC sample. I measured the
temperature of the top surface with an IR temperature meter (the top of
sample was lightly sprayed with black paint to bring the emissivity into
the 0.9 range (?)). I let things stabilize at each temperature for a
couple of minutes, and then moved the gun closer.
Temperature Sample Condition
top of sample
142F No apparent change (1)
175F No apparent change (1)
200F No apparent change (1)
270F Permanent deformation of mid sample corrugation
and very noticeable loss of stiffness
380F Sample looks like a potato chip
(1) "no apparent change" means that I could not see any deformation, and
no gross loss of strength in flexing the sample with my hands.
The room temperature was 70F.
The temperature on the bottom of the sample was much closer to the top
of sample temperature than to room temp. This was probably because the
heat gun air stream results in much better transfer to the sample than
free convection from the room(?) -- But, this might not be too different
from the real world if there is air flowing through collector, and its
My tentative conclusion is that there is a limit there somewhere above
200F, but less than 270F?
Posted by Toby Anderson on June 30, 2004, 1:01 am
"Brian Graham" Wrote:
(Trouble brewing.. ;-) What I have in mind is the following:
attached to the jet pump) coiled on my roof. Boxed in on the sides,
plexiglass above, but open below. I have a tin roof and want radiant
heating from the tin. (I'll use metal studs and proper tin
screws/washers to ensure a leak-free frame installation.) A food-grade
plastic barrel in the basement to store the heated water, and a small
pump to circulate the water slowly. A 30-60' coil of copper in the
barrel is the "cold water input" for the hot water tank. The roof coil
will be drained for the winter.
Let's say you coil your 3/4" dia. tubing over an 8' x 8' area =
If you could coil the tube to cover the entire 8' x 8' area, then how
long, L, a tube is this?
L ft * 3/4" * 1'/12" = 64sqft
L = 12*64*4/3 = 1024ft --- just use 1 1000ft roll.
Since the tubing covers most of the surface, the sun won't shine on
the tin roof anyway. Since the tubing is black, it's a good color to
aborb the sun as well. You don't need any black or green shadecloth.
The amount of water which this tube holds is about:
Ww = pi*(5/16")^2 * 1 sqft/144sqin * 1000' * 62.4 lb/cuft = 132 lb
Note 1: 132lb * 1gal/8lb = 16.6gal (about 1/3 of a 55gal drum)
If you used a 1" diameter tube, you could hold twice as much ---
The thermal capacitance of the 132lb of water, Cw: Cw = 1Btu/lb/F *
132lb = 132Btu/F
HEre's several very rough assumptions:
1. THe thermal resistance of the 64sqft of glass is: R1 sqft-hr-F/BTU
/ 64sqft = 1/64 BTU/hr-F
THe 'time constant': RC is: 132BTU/F / 64 BTU/hr-F = ~2hr
The temperature increases exponentially using that time constant.
2. The solar radiation falling on the water is 1200 Btu/sqft/day, and
in the winter, most of this occurs in 6 hours.
---i.e. 200 Btu/sqft/hr
200 Btu/sqft/hr * 64sqft = 12800 Btu/hr
3. The daytime temperature is 45F (the average monthly daytime
monthly-averaged temperature in Nevada)
4. The Thermal resistance of 1000' of tubing is: R0.1 R1
sqft-hr-F/BTU / 1000' * 2*pi* (5/16" * 1'/12") = 1/1636 hr-F/Btu
5. ignore heat 'losses' to the attic and the sides of the box
The thermal circuit looks something like:
132Btu/F 1/1636 hr-F/Btu
12800 Btu/hr --->----||---------------wwwww-------------Ts----wwwwww-----45
Hence, in the first hour, assuming the water temp starts at 60F, we
can make this approiximate formula:
Energy from the sun = Energy absorbed by the water + Energy lost from
the sunspace to the outside air
12800 Btu/hr = (Tw-60)F * 132 Btu/F + (Tw-45)/(1/1636 + 1/64)
12800 = 132Tw - 7920 + (Tw-45)*61.6
12800 = 132Tw - 7920 + 61.6Tw - 2772
23490 = 194Tw
Tw = 121F
Now, if you are circulating water to a 50gal hot water tank, then the
situation is a bit different. THe thermal capacitance is now about 3
times more: 3*132= 400 Btu/F
12800 Btu/hr = (Tw-60)F * 400 Btu/F + (Tw-45)/(1/1636 + 1/64)
12800 = 400Tw - 24000 + (Tw-45)*61.6
12800 = 400Tw - 24000 + 61.6Tw - 2772
39572 = 461.6Tw
Tw = 86F
You've gathered: (86-60)F * 400Btu/F = ~10000 Btu's in 1 hour.
The first hour, the water increases by about 26F in temperature.
However, each successive hour, it increases less and less.
I probably made a mistake somewhere in the above calc's....
Posted by nicksanspam on June 30, 2004, 11:28 am
As I recall, this tubing has a 20x3/4" = 15" min bending radius.
It tends to kink badly when coiled into a 2' diameter 55 gallon drum.
Posted by Toby Anderson on June 30, 2004, 8:13 pm
Richard Lambert does it this way:
"BUT, if the coils were mounted in anything but a horizontal surface
would have to be blown out well. I used to have the same outfit that
blew out my underground sprinklers blow out my solar heater too until
they blew UP my $0 multiport pool valve - then I started doing it
myself with a small 3/4 hp compressor. The sprinkler outfit used a
trailer mounted compressor of the type used for jackhammers and it
the system dry (and no it did not unwind my solar coil like a New
eve part favor)." ---see the alt.solar.thermal archives for his
Richard Lambert does it this way:
"I put four 1000' coils of 1" black poly piping
on my (admittedly flat) garage roof. They are
plumbed in parallel to keep the head loss low.
A small Grundfos pump, controlled by a $00
controller that senses the temperature of the
roof as well as the pool water,
circulates water through the system anytime the
roof temperature exceeds the pool water temp.
It also will run the solar pump if the roof
temperature is below 40 to prevent freezing.
Flexible high density polyethylene. I used a 1" 80 psi rated product
under the brand name of "Excel" made by Granse Corporation, 21670
Hamburg Avenue, Airlake Industrial Park, Lakeville, Minnesota 55044,
(612) 469-2191, (800) 328-4509. I paid $10.00 per 1000 foot coil in
1985. ....I laid the pipe on an 85 deg F day in four flat spiral coils
started at about 3 1/2 foot in diameter and went out to about 11 feet
diameter. All four coils fit in an area of about 11 feet by 44 feet."