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Posted by nicksanspam on September 4, 2004, 12:17 pm

Please tell us more, using numbers. What do they cost, for what size?
How would you glaze them and estimate their solar collection efficiency?

What kind? How much would it cost? What's its efficiency?


Posted by News on September 13, 2004, 10:21 pm

Who cares. The roof is big, so lots of space, and the radiators are cheap
and work well. You can pick them up used for free. If you want highly
efficient solar collectors to collect as much solar gain as possible in a
limited roof area then get thermomax panels and pay the earth.

A cheap indirect internal coiled cylinder is available very cheap, about 80
GBP, well in the UK anyway.  I know of one solar system using cheap panels
radiators and an indirect cylinder  that has been going for about 25 years.
Only one pump has bee replaced in that time.  It requires 1 litre of
inhibitor about every 3 to 4 years which cost about 8 per litre.

Posted by nicksanspam on September 14, 2004, 9:41 am

How well do they work? What do they cost new? How would you glaze them?


Posted by News on September 14, 2004, 2:43 pm


In the UK we don't have a great amount of solar insolation so we can only
pre-heat and occassionally in summer you will have the sun heat all of your
DHW.  ...and that us using an expensive off the shelf system.

e.g., cost of a 600 x 400 is 12 GBP complete with fixing brackets. This is
rated for heating at 1,688 BTU output at 80C temperature through the
radiator. So, I'm sure you can figure how much heat is generated when
painted matt back. They come in all sizes, long and thin, short and square.
You name it they have it.  This is a single panel and has fins on the back.
They can be reversed to have a greater surface area facing the sun if you
want, needing special brackets as the radiator will be reversed. They are
also available a cheaper from specialist dealers than what this web site
offers.  These radiators are not made for solar applications, people just
use them as a cheap way of DIYing a solar panel.  You can make quite large
panels quite cheaply. The larger the more heat you generate. They will not
be the most efficient of solar panels per squ. metre, but being cheap they
can cover more roof space to compensate - quantity rather than quality
(efficiency).  The description of the Red Army is analogous here. People
would say it is large but not efficient and dismiss it.  The Soviets said
"quantity has a quality all of its own", as the highly efficient well
equipped German army found out when it decided to roll.


Fit them in a box and glaze over as you would with any DIY panel. If you
wanted to do the whole roof you could fit them between the rafters, have ply
under the rafters in the attic, preferably hinged so you have access to the
panels (radiators), paint all matt back, insulation under the ply and
glazing over the whole roof. That can't be that expensive to do.  A hell of
a lot of panel.

DIYing a solar panel and using cheap heating panel radiator is common the
UK. It is easy and cheap to do. Not the most efficient, but some commercial
setups will take decades to recoup their capital cost in the UK, these will
when DIYed, and in a short time too.

Posted by nicksanspam on September 25, 2004, 2:17 pm


Ah, numbers... $.37 per square foot.

The heating performance seems irrelevant. No fin + tube spacing compromise,
if they have steel walls and they are completely full of water. They might
absorb 90% of the sun that falls on a single layer of polycarbonate glazing.
In full sun at 800 W/m^2, on a 15 C day, with 20 C water entering and 60 C
leaving, they might gain 0.9x800 = 720 W/m^2 and lose (40-15)5.68 = 142 W/m^2,
for a net gain of 578 and a 72% collection efficiency, but how thick are they?
Thermal mass slows morning wakeup. Steel and water store about 64 Btu/h-F.
Heating an inch from 15 to 40 C takes 225 Btu/ft^2, wasting 2 hours of sun,
or more, after a colder night. What keeps them from freezing? An antifreeze
loop with a heat exchanger?

Energie Solaire's $2/ft^2 2480x860 mm UNglazed stainless steel roof panels
are designed to make a watertight roof, with fewer plumbing connections.
efficiency = 0.959-8.91X-0.47X^2, where X = (Tm-Ta)/I, eg 68% with Ta = 15 C
and Tm = (20+60)/2 = 40 C and I = 800 W/m^2. They only store 2.52L/m^2, ie
20 kJ/m^2 or 1.9 Btu/F-ft^2, so they wake up in 1.9x1.8(40-15)/(0.68x250)
= 0.5 hours.

I stayed in an Irish B&B with a black-painted clawfoot bathtub astride
the roof. It didn't work well, but it looked funky, and it was a cheap
and easy monument to solar heating confusion :-)


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