DIY solar pre-heater

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Re: DIY solar pre-heater

Postby andarm » Tue Jul 06, 2010 9:32 am

The above two pics were something I did a while back using Corflute. Worked fine using 20mm pressure pipe and fittings sawn lengthways with a circular saw.

The following is a link to a multi wall Laserlite type product.
http://www.danpalon.com.au/c4.php?star=Mjg%3D
Hope this helps
Andrew
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Re: DIY solar pre-heater

Postby Tracker » Tue Jul 06, 2010 5:53 pm

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great minds think alike..
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I started with the same idea but with a different product.
I wanted to make a patio roof that also heated water.

So - There is also the Polycarbonate equivalent of corflute (just slightly bigger), and I think that it comes in different colours from clear to even possibly black. It is used for structural awnings, window awnings etc.

What I was going to do was notch the upstream and downstream ends of a sheet, by one or two "tubes" and to a depth of 10mm and every (say) 150mm.
You would then use PolyCarbonate tube, and route corresponding slots such that the broad sheet fingers fit into the slots, and when glued (Sikaflex) , locks it all together.
ie.. you would have 150mm sheet inserts and 150mm tube slots, and where the cut-outs are -there would (naturally) be no water flow..

All - the dead-same as you are thinking, but with the similar materials (thermal coefficients) , and where the slotting and the unused tubes/gaps, means that the whole assemble is far more rigid, because you don't have a "Split Tube".. a big No-No because it can't take ANY pressure..

Can you picture my thoughts?

The product(s) used to be available from Cadillac Plastics at Silverwater (when I last visited them)
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CADILLAC PLASTICS AUSTRALIA. Sales Inquiries. Phone: 03 9775 0407 Fax: 03 9775 1546. Email: info@plasticwebsite.com.au


Just noticed this in a previous post - THIS is the same stuff..
http://www.laserlite.com.au/residential/laserlite_multiwall.asp
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Re: DIY solar pre-heater

Postby 120ThingsIn20Years » Wed Jul 07, 2010 8:19 pm

[quote="andarm"][/quote]
Sweet!
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Re: DIY solar pre-heater

Postby 120ThingsIn20Years » Wed Jul 07, 2010 8:30 pm

Tracker
yeah I think I understand.

My construction was about laying a sheet of glass on the ground and sticking everything to that. Then when everything was tested and leak free Id glue some foam to the back and some brackets on the corners.

the structural rigidity was to come from the glass.

But the test of the poly pipe worked much better than I had expected as far as heat exchange goes. There was only 5 (I think) vertical hoses going through my collector, and the air temp was almost identical to the water temp in my collection tank (small collection tank)

I think the heat exchange is self regulating in that the hotter it gets in the collector-> the faster the water moves -> the greater the heat exchange -> the cooler it gets in the collector etc.My original plan to use corflute or similar was because I expected the weak link would be the heat exchange so I was trying to maximise surface area, but as it happens, the great pumpkin did pretty well on the poly pipe heat exchange specs after all.
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solar pre-heater

Postby Tracker » Thu Jul 08, 2010 11:51 am

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Something to consider with a Solar-Pre-Heater, is that there is actually an issue with it..
You only Pre-Heat when the sun is shining best, which is in the middle of the day, which is when you use the least hot water.
I would think that there are only two way of benefiting from the concept.. Either have a separate tank so that the pre-heated water is waiting for it's ultimate re-use, or you try to store the heat in Gordon's Phase-Change wax, and then have another heat exchanger within, to warm the mains-pressure water when it's needed for night-time usage.

My idea with the concept of the patio roof was that it would provide more subdued light (UV Free) whilst trapping the energy from the sun's heat.. Win-Win.

I might actually re-look at the project with a view to using the Phase-Change wax as an intermediate heat storage..

Gordon - where do you buy the wax??
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Re: DIY solar pre-heater

Postby Gordon-Loomberah » Thu Jul 08, 2010 12:12 pm

After a bit of research, I bought mine from here: http://www.naturalcandlesupply.com.au/

In many cases palm oil production is causing the destruction of rainforests, this company buys from a member of the Roundtable on Sustainable Palm Oil. "This organization was founded by the World Wide Fund for Nature (WWF) and heads of government and industry involved in palm cultivation. The RSPO was founded to develop and define best practices for sustainable palm cultivation, and protection of the environment."

I think it's worthwhile buying from them if you dont want your purchase of wax to come at the expense of rainforest and animals.
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Re: DIY solar pre-heater

Postby Tracker » Thu Jul 08, 2010 1:26 pm

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Thanks for that Head's-Up.. on the Palm Oil/Wax
I was thinking that it was something even more sophisticated than just plain wax.
Next job - to find a sturdy smallish Recycled PET tank that can be adapted. I wonder if they do that?
or - one of those BLUE 44Gallon HDPE drums, which would be easily insulated
At least we don't have to contend with MAINS-PRESSURE..
From your experience in Eng-Thermo.. Do we place the Input heat exchanger at the bottom and the output one at the top, or try to distribute BOTH throughout the mass of the medium.
With the Top/Bottom thingo - I assume you always have the hottest at the top,
but - in the transition phase from liquid to solid, there would still be residual heat (at the bottom) that could be sucked from the bottom cooling wax
but - is the thermal conductivity of wax, such that once you have a layer of cold wax, you have lost contact with the heat in the adjacent wax... anyway...?... :cry:

I think that I have convinced myself that a TOP and BOTTOM system would be best, due to the very poor conductivity of solid wax.... :(

To be functional, this has to be as cheap as possible, or it's not an "Environmentally Friendly Proposal"..
At least on the Hot-Water side - you have mains pressure to work for you, and you would then need a circulating pump/differential thermometer to gather the heat, whether it's from HDPE tube or the Lexan-Multiwall sheeting..

What temp. does your wax melt at???
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Re: DIY solar pre-heater

Postby Gordon-Loomberah » Thu Jul 08, 2010 7:44 pm

Tracker wrote:Next job - to find a sturdy smallish Recycled PET tank that can be adapted. I wonder if they do that?
or - one of those BLUE 44Gallon HDPE drums, which would be easily insulated


does HDPE retain its strength at up to 100C?

I think that I have convinced myself that a TOP and BOTTOM system would be best, due to the very poor conductivity of solid wax.... :(


Glad you have done that, because thats what is needed. In my experiments with melting ~10kg in a large pot, it melts outwards from the heat source and basically pools on top of solid wax in the bottom. It cools from the outside in- leaving a liquid reservoir in the middle which eventually solidifies after about a day. Its clear the heat source and drain need to be in close proximity, and distributed throughout the wax mass. It is a poor conductor, so you dont want any great thickness of it away from the water pipes/radiators or whatever.

I'm looking at either:
1/ alternating small car radiators on the low pressure hydronics circuit with high pressure radiators or coils of soft copper pipe for the ~250kPa house hot water
or
2/ nested paired coils of soft copper pipe, or Pex-Al-Pex which should be easy enough to form up, but wont offer quite as much surface contact area as a radiator.

I've got to do some calculations of amounts needed and costs before deciding which way to go.


What temp. does your wax melt at???


pretty close to 56C
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Re: DIY solar pre-heater

Postby Tracker » Thu Jul 08, 2010 10:18 pm

Gordon-Loomberah wrote:Glad you have done that, because that's what is needed. In my experiments with melting ~10kg in a large pot, it melts outwards from the heat source and basically pools on top of solid wax in the bottom. It cools from the outside in- leaving a liquid reservoir in the middle which eventually solidifies after about a day. Its clear the heat source and drain need to be in close proximity, and distributed throughout the wax mass. It is a poor conductor, so you dont want any great thickness of it away from the water pipes/radiators or whatever.


Gordo - don't confuse me.. I do that well enuf by myself... .. . :|
You started saying that my self-convinced use of coils at the top and bottom were best, but then described a mechanism where the heat in and heat out devices are distributed through the medium, because the solid wax is a rotten thermal conductor.

Again my thought process would be that I heat from a bottom coil, and after a long time - the column would be melted.. As more heat is added, the hottest 'oil' will rise to the top.
At a time in the day, the wax can take no more heat from the pre-heater, so it sits there with the hottest oil at the top, and perhaps that at the base would start to solidify if there was not enuf heat to complete the job.
When you start to use hot water and the cold flows through the transfer pipes in the hot oil to pre-heat the cold water headed for your water heater, the oil in contact with the cold pipe will cool and hopefully sink to the bottom of the column.
The cooling oil will solidify at the base (below 56c) more rapidly, and eventually when the wax reaches 55c, it will be largely solid, immobile and hence incapable of transferring any more of it's heat to the transfer pipe.
So the last lot of heat below 55c will be largely lost to the system.
ie.. surely it will react like any other hot liquid, and as is the case with a Hot-Water-Tank, I could have COLD water at the bottom and scalding water at the top...

So you are thinking in terms of having the entire mass of the wax, in intimate contact with transfer devices like radiators.
I would think that that would be especially good for getting all the heat from all the wax below the 55c mark, but would it make any difference if the actual heating device (charging) was just the simple coil in the bottom, such that convection distributes the heat whilst charging, but the dispersed/intimate contact of the radiators does the job during heat draining process.

1/ alternating small car radiators on the low pressure hydronics circuit with high pressure radiators or coils of soft copper pipe for the ~250kPa house hot water

I hate the sound of finding 25 good car radiators?? :(
Almost all modern radiators have plastic header tanks etc. YUCK !
or
2/ nested paired coils of soft copper pipe, or Pex-Al-Pex which should be easy enough to form up, but wont offer quite as much surface contact area as a radiator.


Copper Refrigeration tubing is easily obtained in quantities (but is still copper and hence not real cheap).
Take two rolls, and roll them out, with the ends in a vice.
The far ends go in a big drill chuck - now pull like a madman and twist them tight in a figure 8, and then roll the whole lot back into a broad helix..

Re -the temperature capabilities of HDPE drums -- I would assume (silly me) that it would be like black polly pipe in the sun, being used as an alternative collector????
Same material - same pressure, different location????? . . . . . :mrgreen:
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Re: DIY solar pre-heater

Postby Gordon-Loomberah » Thu Jul 08, 2010 11:22 pm

Tracker wrote:Gordo - don't confuse me.. I do that well enuf by myself... .. . :|


err yea, I quoted the wrong bit! Just above you suggested a distributed arrangement, which is the bit I was going to quote ... not enough sleep, going to bed at 6am and getting up at 11am is never a good idea... even though I do it rather a lot here at work.

So top and bottom is good, as long as they extend well into the mass of wax ;)

When you start to use hot water and the cold flows through the transfer pipes in the hot oil to pre-heat the cold water headed for your water heater, the oil in contact with the cold pipe will cool and hopefully sink to the bottom of the column.


I think when cold water flows through the pipe, initally the pipe would gain a coating of solid wax within the surrounding liquid, further along where the water has warmed somewhat, some sinking of cooler wax would probably start to happen...if we were talking about a straight pipe through the wax, but with coils and or finned pipes/radiators etc, it would be a bit more complicated. Given enough surface area for the heat transfer to heat the water sufficiently and small initial accumulation of solid wax would melt after the water flow stopped.
In my case the wax will generally always have heat going into it from the wet back during periods when the hot water is required, so there shouldnt be too much accumulations of solids within the system.

The cooling oil will solidify at the base (below 56c) more rapidly, and eventually when the wax reaches 55c, it will be largely solid, immobile and hence incapable of transferring any more of it's heat to the transfer pipe.
So the last lot of heat below 55c will be largely lost to the system.
ie.. surely it will react like any other hot liquid, and as is the case with a Hot-Water-Tank, I could have COLD water at the bottom and scalding water at the top...


I think that effect would be minimised with distributed heating throughout the wax.

So you are thinking in terms of having the entire mass of the wax, in intimate contact with transfer devices like radiators.


yes, thats the plan

I would think that that would be especially good for getting all the heat from all the wax below the 55c mark, but would it make any difference if the actual heating device (charging) was just the simple coil in the bottom, such that convection distributes the heat whilst charging, but the dispersed/intimate contact of the radiators does the job during heat draining process.


It would need more than just the coil in the bottom, since there is no convection to distribute the heat once the wax solidifies, and the conductiviity is fairly low.

I hate the sound of finding 25 good car radiators?? :(
Almost all modern radiators have plastic header tanks etc. YUCK !


yes, older ones are all metal, but in any case, they are designed to withstand over 100C.

I'm thinking I would only need 3 or 4 of them, car wrecking yards are full of them. I have an internal car heater radiator, which I picked up for $35, its a bit smaller than I would like, but most car radiators are a bit large. I'm sure there is something available in between. Maybe I can find some used AC radiators for scrap value. I think they would be good for a bit of pressure too.
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