Solar water heater

[David HK]

I am mid way through making my solar heater box.

The frame is stainless steel and I am lining it with polyisocyanurate sheets. These are provided with an aluminium cover on one face and I intend to place this face on the sunny side up.

Is there any merit in painting this aluminium face covering black, or, shall I just leave it 'as is'?

The copper pipes will of course be painted black.

I hope to put up photos of the construction on the web site in the next week or so.

Early advice and opinion would be very welcome.

Dave HK

[picmacmillan]

someone gave me a couple store bought solar water heaters and there is some aluminum sheeting covering the pipes and it is in fact covered with dark paint..in a nutshell the solar heater had thick foam insulation with a silver coating on it all around the inside of the frame.., then the pipes went over that, then the dark painted aluminum sheets were placed right on the pipes themselves...in fact they were shaped to wrap around the pipe slightly to have more surface area directly on the pipes..these sheets covered the whole inside area of the heater...this was then covered with a sheet of glass..hope this helps..good luck on your project...pickster

[zap]

Hi Dave, 2 cents coming your way.

I think we need a little more info on what you're planning. If you can provide a picture or drawing of what you're expecting for the final look of the collector that would help.

Are the copper pipes going to be attached to the polyiso? And if so, how? From what you've described so far I'd say you will definitely want to paint the polyiso but without a lot of pipe your efficiency will suffer. Not painting the polyiso will allow most of your sunlight to be reflected back out of your collector without being changed to heat.

GaryGary will probably jump in on this and provide his expertise but till he does try to visit his site if you never have and take in some of the mass of info there. http://www.builditsolar.com/

Ok Dave I've looked over some of your old posts and if you're still planning something like this



I'd suggest moving your pipes up and have them sit on the polyiso like this



or maybe even raised up off the polyiso to get them up to where the hottest air will be. I know you're trying to keep the cost down but maybe Gary can give you some more pointers to up the efficiency without breaking your budget. Have you looked at his site? Good luck and keep us informed of your progress.

[georgeodjungle]

all black works best.

it will get above 350f.

but your insulation "ployisoc" is only good for about 200f.

it will start to smell at like 180 and the turn to goo after that.

i'd pick another insulation or go with all silver with black pipe.

[GaryGary]

Hi David,

Can you explain in a bit more detail how the absorber in your water heater is constructed?  Sounds like its made of multiple copper pipes? Of what diameter?  Are you pumping water through the pipes when the sun is out, or is this more like a batch heater where the pipes hold a lot of water which is heated by the sun?

For batch heaters (ie a tank in an insulated box), you can use either the black paint or the reflective approach, but they work in different ways:

• For the black painted interior, the sun is absorbed by the black box walls, which get hot.  This heats the air in the box, which then transfers heat to the batch tank by conduction/convection.  There is also some radation transfer from the hot black surface to the batch tank.  
  
• For the reflector interior, sun is reflected off  the interior walls, and onto the batch tank (which is always black) and absorbed by the tank.  In this configuration, the reflective walls of the box should be angled to reflect toward the tank.
  
  

The Bainbridge book on batch water heaters (which I am hoping to have available for download from my site in a few days) says that in one test where both of these methods (black and reflective) were tried on the same batch water heater, that there was very little difference in the performance.

I agree with the comment above that the Polyiso insulation painted black and directly exposed to the sun inside a glazed box may get to hot.  The Polyiso is good, and is routinely used behind absorbers in solar collectors, but I'm not sure its good enough for the situation you describe.  I guess you could always do a little test with a piece of Polyiso painted black in a small insulated and glazed box -- ie a shoebox size experiment.

Gary

[David HK]

Thanks for the response folks.

Below is a photo of my solar box. Its 8 feet by 4 feet internally and you can see the polyisocyanurate insulation panels below the copper pipes. The wooden battens will carry the pipes which in turn will 'just touch' the aluminium foil on the panels.

There are more pipes to add plus holes for water inlet and outlet, and temperature sensors. I will up load more photographs as the project proceeds.

The copper pipes are 42mm diameter and the box top will be covered with 4 sheets of clear tempered glass.

For the moment I just need views on whether or not I should paint the insulation panels.

In Hong Kong our day and night ambient temperature in the summer is around 33 degrees Celcius. With the sun shining directly on these pipes they are going to get really hot.

Dave HK

[GaryGary]

Hi Dave,

!! Nice workmanship !!

I still don't understand exactly how the collector is plumbed into your homes hot water system?  Is the collector a "batch" style collector, in which the hot water storage is the collector itself, and the cold water only makes one pass through the collector, or is there a pump that pumps water through the collector to a hot water storage tank?

This makes a difference, in that if its a batch collector, you  have a very high ratio of glass area to water storage area -- about 32 sqft of glass for 8.5 gallons of water in the collector, or about 3.5 sqft of glass per gallon of water.  The usual would more like half a sqft of glass per gallon of water.  So, if its a batch collector, then with the large amount of glazing per gallon of water, I think it hardly matters whether you paint the insulation or not -- the water is going to get quite hot either way.  This is especially true with your high ambient temperatures.

If its a collector that you are pumping water through all the time, and you want to maximize the heat transfer to the copper pipes, I'm not sure which scheme (paint or no paint) would work best.  With no paint, you will lose some heat because some incoming sun will get reflected back out the glazing.  With paint, the box air temperature will be high, and you will lose heat out the glazing by convection/conduction.

The usual way to get the heat from sun that does not direcly strike the pipes into the pipes would be to have fins that are thermally bonded (eg soldered) to the pipes, and that extend half way to the next pipe.  This is a good arrangement in that the heat from the sun that strikes the fins gets conducted directly into the pipes.  You could still do this on yours by attaching aluminum or copper strips to the pipes above the insulation.  The better the bond you could provide from the fins to the pipe the better it would work.  These fins would also protect your insulation from direct sun exposure.  One way to attach such fins to copper pipe is to use a collection of boards to make a groove that is just a touch wider than the pipe and half as deep.  Then place the light aluminium or copper fin material over the groove, and using a piece of the copper pipe and a mallet, pound the pipe into the groove.  This forms the fin material into a half circle that fits the copper pipe.  Then put some silicone between the copper pipe and fin, and clamp them togehter until the silicone sets.  This is not as good a solder, but (I'm told) works pretty well.  Its similar to this scheme: http://www.builditsolar.com/Experimental/CPVCCollector/jig.htm

Another possibility that might work is to use a trough type reflector behind each tube.  The best type of reflector would be whats called a non-imaging reflector -- these will reflect heat onto the pipe for a wide range of sun anlges so that you don't have to track the collector on the sun as would be required for a parbolic reflector.  This sounds like more work than it is (I think :-)) the reflector contours would not be critical, and you could make a would tool to shape the troughs.  It would be an interesting experiment.  If you are interested in this, I can probably help you determine a good shape for the trough relfector -- let me know.

I am assuming that the 4 pieces of glazing go side by side, not one on top the other?

Gary

[windyknight]

Dave

Have made an identical unit here in uk where temps not so high (unfortunately!)

I Bought an aerosol tin of heat resistant stove paint and sprayed the lot.  I was concerned that aluminium on copper may increase corrosion (dim and distant past chemistry lessons about dissimilar metals reacting)- also felt that ANY reflective surface cant be as good at absorbing heat as a matt one

Hope this helps

k

[GaryGary]

Hi Again,

I was curious about the temperature that the surface of your polyiso insulation would get to if painted black.  I have a little test collector made from Polyiso painted black, so I put a surface mount thermocouple on the back wall of the collector, with the absorber out (so sun shines right on the black painted polyiso back surface).  Here are the results:

   Back surface (poliso painted black)     211F  (as measured with surf mt TC)

   Air in collector                        147F

   Outside ambient                          32F

I took some readings on the back wall as best I could with an IR thermometer, and they were close to the 211F.

I blocked off the entry and exit vents as best I could, but there was some leakage, so it probably would have been a little hotter if the vents were well sealed.  It was full sun, but not quite direct incidence, so direct incidence would have also made the temp hotter.  

I did not see any damage to the insulation for the half hour it was exposed to this condition, and did not notice any odor other than the usual hot smell.

Still, your collector with 33C (90F) ambient will run considerably hotter than this -- it might get surface temps in the high 200's.  If it was me, I would not trust the polyiso to withstand this for a long time.  I would protect it with something, or leave it reflective.  

Here is the test collector I used:

http://www.builditsolar.com/Experimental/FlowOrganizer/FlowOrg.htm

The thermocouple was mounted on the back wall, and the absorber screen was rolled up so that the back wall and thermocouple were directly exposed to the sun.  I taped the thermocouple on with black duct tape, and sprayed black paint over it.

K's comment on copper and alum corrosion is good, but if you don't get moisture inside the collector, you should not get corrosion.  Some commercial collectors (like SunRay) are made with alum fins pressed on to copper pipes.

Hope this helps.

Gary

[David HK]

Thank you for all the very useful observations.

The note about dissimilar metals is useful.

Gary Gary, you are quite right the water will be a single pass through - a simple batch heater in effect. There is no water recirculation function.

Having read all the comments I am going to take the obvious solution which is to paint the copper pipes black and leave the insulation in its original form and un- painted. This will then allow me to monitor its progress through the year and observe its reaction to high heat conditions. The data from this should be useful to many people.

If at some later date there is a need to paint the insulation black all I need to do is buy a pot of paint and get on with the job.

The week end is about to occur and apart from some final welding the frame is complete. This should allow me to take some photographs and up load this weekend. Keep posted.

Dave HK

[Clifford]

You don't want reflectors inside of the "box" (exposed to the sun).  There may be minor benefits of a reflective surface hidden from the sun, but I am not convinced of the benefits of hidden/obsucred reflective surfaces.

Your goal is to absorb as much sun as possible, thus everything exposed to the sun should be a FLAT BLACK.

You should have some kind of a heat conductive material (painted black) taking up as much space as possible, and bonded to the pipes (air is a relatively poor heat conductor).

Both Aluminum and Copper are excellent heat conductors.  Steel and Brass have somewhat lower heat conductive ratings.

Interesting question...  pipes on top of or below the heat conductor material.

Not sure about corrosion (copper/AL)

(an aside) I believe that steel and copper is fine (you can get copper plated steel).

Steel and Zinc also works well together.

But, mix steel, zinc, and copper, and you get horrible corrosion.

[GaryGary]

Hi Clifford,

Batch water heaters are often built with reflectors inside the box that reflect heat onto the batch tank.  This seems to work about as well as painting everything black.  The Bainbridge book on batch heaters ( http://www.builditsolar.com/Projects/WaterHeating/ISPWH/ispwh.htm ) talks about a test that was run with the same batch heater, version one painted black all over and version two had reflectors that reflect onto a black tank -- they had the same performance.

I think that external reflectors are the best deal of all, because they increase the effective collection area without increasing the size of the glazing, and since the losses depend pretty much on the glazing area alone, its almost a "free lunch".

I agree that it would be more efficient to attach black metal fins to the copper pipes, and let them transfer in the heat, but I don't think David wants to do that at this point, and he has a very large ratio of glazing to water volume for his batch tank, so I would guess he will get good heating without the fins.

Gary