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Constructing collectors for a solar hot water system

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Posted by mark.fox on September 27, 2007, 6:26 pm
 
I've been thinking about building a solar hot water system for our
cabin, but hadn't committed. A couple of weekends ago, I picked up a
couple of 33 1/4" x 75 1/8" plates of glass for $0 each. So now I'm
committed. Ideally, the system would be totally passive, but I have
photo-voltaic supplied electricity and a DC water pump at my disposal.

Many are my questions, but I'll limit it two for this post: Does
foregoing the flat plate in a flat plate collector seriously impede
its performance? Assuming the flat plate was done-away with, would
mirroring the rear of the panel (collection pipes painted flat black)
make sense?


Thanks,

Mark


Posted by Cosmopolite on September 28, 2007, 1:00 am
 
mark.fox@gmail.com wrote:

    The effectiveness of a solar thermal collection system depends on
several things.

   1  The surface area of the collector/s, the larger, the better,
therefore, the plate.

   2  The effectiveness of tranferring the heat, collected by the plate,
to the pipes.

   3  The effectiveness of tranferring the heat from the pipes to the fluid.

   4  The ability to conserve the heat collected. ie. the insulated box
and the glazing, as well as selective coatings to abate re-radiation

   Hope this helps.

Posted by Jeff on September 28, 2007, 2:00 am
 mark.fox@gmail.com wrote:

   Good deal.

  So now I'm

A few places to look. With all things solar:

http://builditsolar.com

has a wealth of information.

   You can find an excellent flat plate software utility here:

<URL: http://sel.me.wisc.edu/codepro/new_codepro.html  />

and for thermosyphon collectors you'll need to have the tank above the
collector.

Jeff




Posted by mark.fox on September 28, 2007, 1:17 pm
 
Came across that one early on. It's an excellent place to start.


Thanks. I'll check it out. It certainly looks useful.


Yup. I keep forgetting that. I will likely build a simple solar hot
water heating system to keep my livestock's water from freezing. That
will be a thermosiphon system. The one for the cabin will have to be
somewhat active as the panels will hang off the southern edge of the
roof and the tank will be on the ground floor. (We're far enough north
that the collectors have to be positioned steeply enough that hanging
them from a roof-edge works well. It also makes them easy to adjust
over the course of the year.)

Mark


Posted by Jeff on September 29, 2007, 3:36 am
 mark.fox@gmail.com wrote:

Why don't you consider a Thomason type trickle collector. The main
disadvantages are that you need glass glazing (polycarbonate will
deteriorate) and they are not as hot (I believe) as a flat plate. It
could be made out of your glass sheets over some corrugated metal (with
some insulation behind) painted with black etching primer. You would
need a small pump though. Except for the pump, it sounds perfect for
what you probably have on hand and for the lower temp needs of your
livestock.


  That

Collector efficiency is determined by the amount of sun / water temp -
ambient temp. That is called instananeous collector efficiency.  You can
get solar solar and weather data from NREL. If you are in very harsh
conditions then you'll want to consider double glazing, extra insulation
and selective coatings. Those things will boost efficiency in poor light
and low temperatures. (other things will too)

  On another note, you may consider generating some methane from your
livestock waste.

   If you want to go with a conventional flat plate collector I've
collected much of the wisdom of Mike in NZ on building these. I don't
think he would mind if I reposted it (let me know Mike!). I see Daestrom
has posted some good general guidelines also... You should be thinking
about 4" (or maybe less) spacing between tubes for about .02" thick
aluminum.

   I've got a half dozen collectors I started on some time ago and ran
out of time before winter (and friends) set in. I'll be finishing them
soon... I'm not an expert like the others but have been listening for a
while!

   Jeff

  enough that hanging


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