Deciding if I should build this solar heater

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rogerdw Guru Joined: 22/10/2019 Location: Australia Posts: 955	Posted: 05:23am 11 Apr 2021         Davo99 said   I had a different Idea of how these worked from the last Discussion. I thought the end of the things got hot at a point and the heat was removed ( generally with water) from there. Didn't realise they were hollow. In my head it looks a bit tricky to get the air in and out of what I imagine is a fairly small area but if you have it figured out, that's no longer a problem. You are quite right in that they do normally harvest the heat from the tube with a "heat pipe"  ...  but my 'cheap' pipes don't have them fitted so my experiments revolved around just the bare tubes. The heat I was obtaining still seemed very worthwhile even without them, so I was trying to avoid the expense and complication of buying or making them.   Quote   This is a REALLY interesting concept and One I'll follow with great interest. If you could post some pics of the tubes particularly the ends that would be great as still not entirely sure I have them right in my head. I take it they are like a big test Tube? That's a good description, they do look like giant long skinny test tubes  ...  though they have a second tube inside the outer one  ...  with a vacuum inbetween  ...  like a good old thermos flask. The inner one has a black coating that absorbs the suns energy  ...  which cannot escape due to the vacuum between the tubes. The overall OD is ~60mm and ID is 44mm.   Quote   Something I can add some real world hands on experience with and have been playing with it the last few days and previously, Fans. Those Bilge fans will be absoloutley useless for this job.  I think your Idea of using the C channel in the shed is bloody brilliant and I can see how one would work there, just, but these things have no pressure  and there is no way they are going to force air though narrow orifices.  Thats been what I have been looking at, using them to supply air for burners and  even though 1.5" pipe or necked down in any way, they fail miserably. Thanks for that Dave, I can never work out the pros and cons of all the different types. The fan I used pushing air down the C-Section does the job ok though it's not super speed or volume  ...  just a nice continuous waft of warm air coming out from under the bench. It may have been better having a fan at the bottom pulling air down  ...  but I wanted the noise further away!     Quote   With a ducting the diameter of the outlet of the fan or larger with no restrictions, yep, no problem. Put any restriction in them  and the airflow becomes non existent. The FAR better alternative I tested  Just yesterday is a Cabin Blower / AC fan.  The one I had is out of subaru's from about '03 to '14, they are all the same in all the models pretty much. The housing is designed by a Madman but  with a bit of cutting, shaping some 90mm storm water pipe with a heat gun and some Duct tape, I got a very workable outlet. The thing supplys huge amounts of air and at good pressure too. They are a scroll type blower not an enclosed fan. I tested a Bilge blower at under 2A. The cabin Blower was 17A which gives a good relevance to their outputs. Not comparison in other words. Sounds like a good find Dave, it's obviously horses for courses  ...  though 17A is a bit of a jump up from 2A. Depends how often it has to run in your burner I suppose.   Quote   I have fans on my solar inverters direct connected to some old panels and a voltage reducer.  With a bit of resistance built in, you may be able to " Tune" the system to give a fairly consistent heat output.  The brighter the sun the harder the fan blows and the greater the available heat. Yes, that's a great idea if I can find a blower (or sucker) fan that will work on DC happily. Move the air easiest when the sun is strongest.   Quote   My only other suggestion is if you are going to use PVC pipe for the Ducting, use Pressure not DWV. That thin drainage stuff is useless crap which I personally hate with a passion. Have a job I'm about to go and do now for a repair where I touched some with a shovel in the garden and demolished it. This is going to be one of those blue word aplenty simple but incredibly annoying jobs I can just see as I took out and elbow and spilt both sides of the pipe.   Might have helped if they sank it more than about 150mm down but typical of this crappy cheap built place. The previous owner whom built it was a shocking tight arse and it shows in every way. When you can't dig a hole to a decent depth..... I'm not sure DWV would stand up to much heat either. I formed it for the blower the other day with great ease with a heat gun and my bare fingers and it was like rubber and wasn't even hot.  The thicker pressure stuff would be much more resilient I think or better still if you can use proper 4" ducting, go to an AC supply and get some of that. I had sort of taken PVC and plastics off the list and was contemplating a timber or composite structure for the manifolds and housing. Not really sure yet but I could certainly see plastic turning into a floppy mess.   Quote   As to the setup, would seem to me as these tubes are round, with the correct spacing you should be able to get near 180o  sun coverage as it moves in its arc if the array is properly aligned.  Wouldn't matter too much if the sun hit them side or straight on unlike a panel as long as they were spaced so one didn't shadow the other too much. Keep us informed how you go with this.. It's a very interesting concept that's for sure! Yes, that's one of the touted benefits in the sales pages for the tubes and means they are no where near as fussy as eg pv panels. I'll keep you up to date  ...  just gotta keep some momentum.