Solar Hot Water Heater Question

[Industrial Toys]

My salvaged solar collector has tempered frosted shower type glass.

Is there an optimim distance that the glass should be from the collector plate, as far as wavelength or focal distance is concerned?

The housing is about seven inches deep and I want to know how much insulation I should put in the back as more will reduce the cavity between glass and plate.

Thanks

 

[reb]

I doubt that the distance is critical.

BTW, when I attended a class at a local university to build a couple of solar water heater collectors, the instructor said to use fiberglass insulation made for ovens, not the normal stuff used for house insulation. Seems the normal stuff has an additive that will outgas and coat the glass.

 

[Industrial Toys]

I just came back from the store with some foil coated rigid insulation. I wonder if that could present problems. The orignail panel had some of that but just around the perimeter.

Can the glass be cleaned of whatever the off-gassing deposits on it??

 

[kg7il]

I worked for a small company in 1975 that made flat plate collectors The plate was a two ply bonded, but expanded plate.
Long story short, Outgassing is a problem. When you consider the heat, the deposits can be tough to remove.

We actually suspended the flat plate to leave air-insulation on the back side and avoid the problem.

 

[Industrial Toys]

Well, the collector is going to be very close to the glazing just by virtue of my insulation choice. I hope this doesn't hinder the performance.

Anybody know if RTV outgassing is an issue. I am going to seal the foil backed insulation to the aluminium frame with RTV to seal off the foam insulation from my heated cavity.

 

[Raspy]

I don't hink the RTV is the problem as it's good to over 400 degrees. The foil is not a problem either, it's the isocyanerate insulation. Seal all joints to reduce where the insulation is exposed to heat from the absorber. A good high temperature fiberglass, without the fancy pink color or bonding agents, should be good for about 1,000 degrees without outgassing.

Glass spacing to the collector is not a big factor. In theory, a wide spacing may be less efficient because air can circulate better and transfer heat to the glass. The Glass does not focus or magnify the solar energy coming through, in fact the glass reduces the amount of energy getting to the collector, but it helps prevent losses to the environment at higher temperatures so it's a net benefit at higher temperatures. Don't fall for any double glazing hype.

 

[Raspy]

Consider this when deciding on glass and insulation: The box and the glass and the insulation are only there to prevent heat loss to the outside environment. The higher the operating temp of the collector, the more potential losses there are. You want all, or as much as possible, energy to arrive at your storage system, so it's important to keep the losses to a minimum. Insulation and glass do this. But so does operating your collectors at a low temperature.

Bottom line: Collector efficiency is directly related to operating temperature. ALWAYS remember that in your design. A good design mantra is to design the system to COOL the collector, not to make the water hot. Hot means losses to the outside. Cool means you have good circulation and are not leaving any heat behind as the water passes through. It means a small temp rise per pass vs a high temp on the output side. The net result is more BTUs delivered to storage.

 

[Paddy]

Industrial toys,

It can be fun to get one of these old school solar water heaters producing heat. But Have you looked at the heat pipe types that are sealed in vacuum? They are really impressive. Two points that make them so good. First, they have a glass tube in vacuum for insulation and make heat in the dead of winter. You being in Canada is critical to this point. The second neat thing is they don't have water in the panel, just a manifold collector. The way they work is a small diameter copper pipe has a few drops of a liquid that "boils" at a low temp because it's also in vacuum. The hot vapor rises to the top and dumps the heat into the manifold. The vapor then condenses and falls back to the bottom bulb area. Simple heat sensor in the manifold triggers a small circulation pump to charge your storage.
example;
Solar Water Heater | eBay

 

[Raspy]

I'd steer clear of the evacuated tubes. The amount of energy captured per sq ft is less and the reliability of the tubes is questionable. One solar supplier I know won't even stock them anymore while they sell hundreds of flat plate collectors.

Here's an example: Bad experience with evacuated tubes.

 

[Industrial Toys]

I can't believe and am a little discouraged how long all this is taking. I might have forty hours into this thing when done. Thanks for the info as I was having a hard time finding specific info on line.

I did find an article mentioning reduced space to glazing reduces convection currents but then I found out this was for an air tube type of collector and did not know if it applied.

I am surprised that double glazing would not be an improvement, but I have the single tempered shower glass.

As far as freezing protection goes. I built a heavy steel frame with fork pockets that the hot tub sits on and I put the whole hot tub away in the shed for the winter.