Open system collectors?

[Skindancer]

I have a question that i am hoping one of you can answer or maybe point me in the direction of an answer.

Every thing I have seen for solar collectors has been about closed collectors, either evacuated tube, flat plate, serpentine or some other system where your heated medium flowed through an enclosing pipe or structure; has anyone ever done any studies on an open system? basicly a water tight box with a set of energy collecting baffles and a dribble tube at the top that trickled liquid over the baffles. A drain at bottom of the box would allow the liquid to flow back to a collection tank to be recirculated.

I can see a few problems with such a system but I can also see several benifits. Getting your flow rates correct could be challenging and condensation on the glazing medium could be a problem and you would have to do your insulating on the outside of the box but on the plus side you could increase your surface area inside the collector considerably and overheating would not be a problem since if you shut the water down everything would drain back to your tank and there are no pipes to overheat.

Am I chasing a March Hare? Has this been tried and given up as a lost cause? Is the Hoki Poki really what it's all about?
Please enlighten me.

[LucMan]

Solar thermal systems are closed because household water systems are pressurized.
An open system would allow the water to evaporate leaving behind clogging minerals, and other nasty things such as bacteria. Ever heard of Legionairs disease?
An open system may be ok for camping but I will take my shower pressurized please.
If you want to see what an open system would be like after a few days or weeks take a look at a local industrial cooling tower, lots of maintainance and chemicalsrequired.to keep them functional.
Look into a sealed drain back system, it does everything that you are talking about without any draw backs.

[Skindancer]

Clarifying details

Solar thermal systems are closed because household water systems are pressurized.
An open system would allow the water to evaporate leaving behind clogging minerals, and other nasty things such as bacteria. Ever heard of Legionairs disease?
An open system may be ok for camping but I will take my shower pressurized please.
If you want to see what an open system would be like after a few days or weeks take a look at a local industrial cooling tower, lots of maintainance and chemicalsrequired.to keep them functional.
Look into a sealed drain back system, it does everything that you are talking about without any draw backs.

I may not have been clear in my explanation. This would not be for a direct water heating system but an indirect one where a heat exchanger would be used to heat the potable water. Also the accumulater system would not be open to atmosphere, the collectors would be sealed tight to presvent any evaporation but there would not be any tubing inside the collectors the glycol/water/whatever would just flow in a very thin sheet over/around a surface rather like a waterfall then when it reaches the bottom of the collector drain back to a receiver tank to be pumped out to the heat exchangers and then back to the top of the collectors.

I am going to build a small scale prototype this weekend and see if it is feasable and if it looks promising I will let you know.

[J.P.M.]

I may not have been clear in my explanation. This would not be for a direct water heating system but an indirect one where a heat exchanger would be used to heat the potable water. Also the accumulater system would not be open to atmosphere, the collectors would be sealed tight to presvent any evaporation but there would not be any tubing inside the collectors the glycol/water/whatever would just flow in a very thin sheet over/around a surface rather like a waterfall then when it reaches the bottom of the collector drain back to a receiver tank to be pumped out to the heat exchangers and then back to the top of the collectors.

I am going to build a small scale prototype this weekend and see if it is feasable and if it looks promising I will let you know.

Check builditsolar.com. Systems of this type have been around for a long time.
First I heard of it was from a guy named Thomason who built some homes in the late '50's incorporating south facing black painted aluminum roofing panels with distribution pipe at the top and glazing over the aluminum. I believe the water flowed to a gutter, collected and pumped back to the top. No reason a HX can't be inserted in that loop prior to returning to the top. Don't know long term results, but you put something black in the sun and run water over it, the water will get warmer.

The devil's in the details, one being evap. when the water/glycol is exposed to the space between the absorber and the glazing. Another is ensuring cross contamination potable H2O<-->glycol doesn't occur. One way to help ensure that is a double wall HX, which reduces HX efficiency a fair amount. A 2d method as more insurance involves ensuring that the potable loop is ALWAYS at a higher pressure than the glycol loop.