Mother of all solar hot water heaters Part 2

[David HK]

Herewith the latest show of my new solar hot water heater. Its designed life span is 50 years or more and it weighs 450lbs including water. All materials are non ferrous.

The water pipes are 42mm copper and I designed for eight, changed a measurement and managed to get nine pipes in the box. The pipes are floating and rest on various pieces of dowel - hence the freedon for movement. Total water capacity is 7.6 gallons or about 34.6 litres.

The insulation is 25mm Kooltherm (polyisocyanurate)sheet which is doing well in the heat.

Temperatures recorded on dry pipes during the installation were 89.9 Celsius, and in the last day or two with pressurised wet pipes 79 Celsius has been observed. I expect higher temperatures.

The unit is fitted with 4 internal sensors which are hooked up through an RS232 interface to my computer. They are:- Inlet pipe, Centre pipe, Outlet pipe, Internal air temperature, and external ambient temperature. The data is dropped into a running Excel file and I am very impressed.

The data has also shown me some short comings in the design especially what happens when cold water is brought into an already hot solar box. As result of this I intend to add more insulation and make a dedicated insulated box for each pipe. I will upload this modification and the results at a later date.

The photographs should be self explanatory, but if you have questions please ask.

Dave HK

[Fiddlehead44]

A very nice unit Dave. I'm impressed. Question or two.

1. What is the purpose of all the sensors?
   
2. It looks like 3/4" copper with 3/8" inlet and outlet. Purpose?
   
3. Is the cover glass?
   
4. What is the overall size?
   
5. What do you use to pressurize it or create the flow?
   
6. What about storage?
   
7. pipe is painted flat black. What kind of paint and how
   
   

   do you prepare the copper for painting?

8. Does it have a pressure relief?
   
9. What about freezing?
   
   

Fiddlehead.

 

[David HK]

1.    What is the purpose of all the sensors?

I have always been curious to know what sort of temperatures could be found in a solar hot water system. This offers me options to investigate heat effects, changes in water pipe temperature, and the opportunity to think about this and contemplate improvements to the design. I can now see two areas for improvement. Thermally isolating each pipe within the solar box to prevent rapid air temperature change (downwards) as cold water runs into the box. Once I have incorporated this design improvement I shall be able to observe the changes.  The aim is also to make good use of technology - this is how designs can be improved and made more efficient.

2.    It looks like 3/4" copper with 3/8" inlet and outlet. Purpose?

The main pipes are 42mm diameter copper with taper reducers down to 15mm copper at the input and outlet. I spent several days working out how much water is held in my wash basins, how much water is used for a shower, how frequently my family take showers, use the wash hand basins, and the sink for washing the dishes. From this data I worked out what quantity of `very hot' solar water would need to be blended with cold to serve the various household uses. From this I was able to determine that what I have built will serve my needs. This keeps costs down and also weight on my car port roof. So far so  good.

3.    Is the cover glass?

Yes the cover comprises double glazed panels with one side of the glass being tempered.

4.    What is the overall size?

Overall size on extremities is 8 feet long by 49 inches wide. (You can metricate if you wish).

5.    What do you use to pressurize it or create the flow?

The system is connected directly to the mains water supply coming from the street into my home. Its usually around 4 - 6 Bar.  

6.    What about storage?

Not sure what you mean here. The system in the box contains 34.62 litres or 7.61 UK gallons. Add on the delivery pipe for a few more litres.

7.    Pipes are painted flat black. What kind of paint and how do you prepare the copper for painting?

The paint is bog standard ordinary paint from the local hardware shop. I figured out that many households around me are painted black and if they can survive direct sunlight and heat in the Hong Kong environment them so can my pipes. I did not make any special preparations prior to painting - just put it straight on. So far so good.

8.    Does it have a pressure relief?

Yes it does. If you look at the photograph with the pressure gauge it's the brass object forming part of that unit.

9.    What about freezing?

[David HK]

Two corrections.

Reference to Hong Kong households being painted black. I meant to write that the wrought iron work and gates of Hong Kong households are mostly black ..... etc

Freezing.

It never freezes in Hong Kong.

Regards,

Dave HK

[johnlm]

Maybe Im missing something, but it looks like the only surface area for collection is the black painted pipe.  The shiny metal (?) behind it helps reflect sunlight back onto the pipe but would be much better served if the copper were thermally connected to the rear metal and the whole works painted black.  As it is you have 20 or 30 (not sure of the box dimensions) square feet of inclosed collector area and only maybe 1 or 2 square feet of absorber plate (the copper pipe).  I doubt you will get much hot water out of it.  But then again maybe Im missing something.

[David HK]

John,

Your quite right with your observation. The omission of black paint (or whatever) is deliberate. I discussed this on this board a month or so ago and decided I would build with the pipes black and leave the background until later.

The reason is that Hong Kong is hellish hot in the summer with 33 Celsius all day and night. I have yet to see what 'real' temperatures will come in the summer and bear in mind we are only in April.

I suspect that the temperatures in the box will be in the 90's come mid summer and expect the pressure relief valve will blow off regularly.

Its fairly easy to change things as I go along. I still have insulation to add to the delivery pipes, but all these things take time.

Regards,

Dave HK

[Ungrounded Lightning Rod]

Painting the innards of the box black will heat the air but then the heat has to transfer via air to the pipe.  Much thermal resistance, so slow power flow.  Thus much overheating of box air and limited output.

Painting the back of the box black and connecting the pipes to it solves the conductivity problem (if the box back is conductive) but creates others:  Differential expansion, increased heat loss due to the collector touching the box rear insulation rather than having an air gap, etc.

What I'd do is get some copper flashing - thin copper sheet, thicker than foil, thinner than heavy copper plates - in strips just a tad narrower than the spacing of the pipes.  Then I'd clean the paint off the (back of the) copper pipe, solder the strips to each pipe run (making a "wing" behind each pipe with a small gap between wings), cut gaps in the wings where the pipe is supported (with clearance around the supports to allow for pipe expansion without the wings hitting the supports) and repaint the pipe/copper wing structures black on both sides.

The copper wings will collect more heat and (if they're not too thin) conduct it efficiently to the pipe.  But (if they're not too thick) they won't appreciably increase the weight of the collector.  With both sides painted black most of the light that passes through the gaps between the wings or around the supports will bounce off the shiny backing and (unless you're pointed right at the sun) be absorbed by the backside of the pipe/wing structure.

With the wings also made of copper the wings and pipe will expand/contract together.  (Solder is soft enough and close enough to the same expansion coeficient that it won't bend the pipes or let them separate from the wings with thermal cycling.)

The whole pipe/wing structure would only contact the supports and the box at the pipe entrance, so contact conductivity will be trivial.

[Ungrounded Lightning Rod]

I note that with an even number of pipe runs your thermal expansion just makes the folded-over far ends of the pipe move around.  With an odd number the inlet and/or outlet have to slide in and out of the box as the pipes change temperature and length.

[dudevato]

Nice work. Is there a pump involved in this project? 'A storage tank? I get the impression you have 7 gallons of HOT water to use and when it's gone you wait for another 7 gallons to heat up.  Is that correct?  I must be missing something.  I'll keep an eye on this story.  I like the solar water heater stories much better than the prodominate wind power threads.  Nice pics too!

[David HK]

ULR,

Yes you are correct about the movement. The inlet is at one end of the box and the outlet at the other. The pipes just rest on the three wooden battens and inserted in these are dowels for the pipes to rest against - they can float or move in any direction they wish.

Another interesting aspect. My television aerial originally sat on a vertical pole and the aerial fins were directly over the spot chosen for the solar box. As usual birds sit on the aerial and crap. Since I did not want the chore of having to keep the glass panels washed and clean I re-configured the aerial pole with a dog leg and have thus avoided the birds problem.

Regarding using up the entire 7 gallons of very HOT water, this has been considered but is unlikely to happen based on the way my family and I live. Each chunk of water removed is of course taken up by cold and I do not expect this to stay cold for long. Perhaps some of the readers have never lived in a hot climate and have little or no experience of high humidity and temperatures so please continue to watch the posts and see how my learning curve grows.

I will keep you all posted about developments over the next few weeks.

[Shadow]

 That looks pretty cool! err hot I guess, I've been collecting some pieces to build something similiar. What would happen if you were to say double the amount of copper pipe you have in your setup, keeping same size box? Have your loops of pipe almost touching each other? Just curious, Thanks for the great post!

[David HK]

Shadow,

I have examined the possiblity of adding smaller diameter copper pipes to run in the gaps between the larger ones shown in the photographs. Yes, it can be done and was something I had in mind during the building stage. It does not provide an awful lot more water, so its left as a thought for the future.

David

HK

[picmacmillan]

good job..looks very similar to some store bought panels i have,...the store bought ones have silver lining facing outwards to the sheet of glass, and foam insulation under the silver lining...they also have dark painted tin sitting right on th pipes themselves..the tin covers the whole interior of the panel...good luck with your project..pickster

[Reno]

Hello Dave

I have a suggestion if you want better performance.

Years ago I built a small model solar heater for my sis's school project.

What I did was corregate the back panel which resulted in a dramatic increase in surface area for the same amount of space. I actually ran the pipe betwwen the corregation and contacting it to improve radiant transfer from the back panel to the pipes. Just remember all the rules of dissimilar metals (good paint coating should take care of this).

Great job and very clean

[Frank06]

I would think that you'd need more surface area.  Adding flashing (painted black) is one good way to go.  You can incorporate storage by using an old electric water heating tank positioned so that it thermosiphons on a constant basis.  That way you'll be able to collect and store heat even if there's no immediate need.  There's sketches all over the internet if you look around; this means you don't need a pump or other controller.

[Shadow]

 I have alot of this heavy black hose, it is about 1 inch outside diameter and 1/2 inch inside, very thick walled. It is used in coil tubing drilling rigs.I'm wondering if it would work for Solar water heating? Or does it have to be metal?Would the thick walls be an advantage?..or disadvantage?

It is very rigid, you can bend a 3 or4 foot piece but not a 1 foot piece.

[dudevato]

Have you thought of a simple 'black tank in the sun'? If it doesn't freeze there it sure would be a simple thing to put together, and if you are not concerned with high water usage a stainless steel beer keg (I'd guess them to be about 10 or 15 gallons in size) sure would weigh less (water weighs about 8.3 lbs per gallon) I'd think it couldn't cost much to have a couple of stainless steel nipples welded on. I'm in southern california and a 40 gallon gas fired water heater, with the sheetmetal and insulation removed will get up to 125*F after a day in the summer sun.  I built an insulated plywood and glass topped box for it and now it reaches like 145*F, The great part about the 'box' is that the water's still at like 90* at sunrise.  Keep in mind also that too hot of water could be a safety problem.  Good Luck

[domwild]

Intersting way of keeping the pipes apart during brazing. The commercial units have a black and flat plate at the back of the pipes, wouldn't your unit have benefitted from a plate??

Another thing: Is this silver solder or what in Oz is called Phoscopper for soldering the bends?

*