The plot below shows the results of a stagnation test run on the prototype PEX collector for several days toward the end of August.
The collector was constructed as shown here, and was facing due south with a tilt of 70 degrees.
As mentioned in the design overview, PEX tubing will not take the kind of temperatures that occur in a collector that is tilted to face directly into the sun during the middle of summer. I have tilted the collector at about 70 degrees to reduce the summer heat gain and increase the winter heat gain. This, along with a somewhat higher than usual collector area should result in improved year round performance.
I have also avoided double glazing and a selective coating -- both of these can increase the collector efficiency a bit, but also raise the inside temperatures. This is not unusual -- nearly all commercial collectors are single glazed, and many do not use a selective coating on the absorber.
Based on the data I've seen on PEX, I would expect that it can take stagnation temperatures around 230F -- perhaps a bit more. Since the drain back system is not pressurized, the PEX will not be under pressure when stagnated (or any other time) -- this makes life a bit easier.
I exposed the prototype PEX collector to stagnation for a couple weeks in August and shorter periods at other times during the summer. The plots below show the temperatures measured in the collector. The collector tilt angle for these tests was 70 degrees -- lower tilt angles with more direct solar incidence would result in higher temperatures.
The green trace shows ambient temperature.
The red trace is the collector air temperature near the top of the collector.
The black trace shows the temperature registered by a K thermocouple placed between the PEX pipe and and aluminum fin.
Ambient temperatures ranged from about 70F up to about 100F.
Collector air temperatures range up to just over 200F on hot days.
The fin/PEX thermocouple gets to just about 250F on hot days.
- The collector would not normally be subjected to long periods of stagnation like this. In normal operation, the water being pumped through the collector would reduce the collector air and PEX temperatures down into the 140F range even on hot days. While the collector must be able to withstand some exposure to stagnation temps, it will not be the norm.
- This seems to me to about as high a temperature as you want to subject the PEX to for long life, so I would be reluctant to make any design or tilt changes that would increase stagnation temps.
- This is probably the worst time of year for stagnation around here -- the sun is not as high in the sky as it is in mid summer, but the ambient temps are still high -- both of these factors raise stagnation temperatures.
- I am inclined to tentatively conclude that the combination of high tilt angle, single glazing, and low internal pressures of the drainback system provide enough protection from occasional bouts of stagnation to allow a long life for the collector.
At some point, I will cut away the fin in an area of the collector near the top, and cut open the PEX to see if it shows any signs of distress.
If the stagnation temperatures seem to high for comfort to you, there are some other alternatives that might work.
1 - Covering all or part of the collector with shade cloth during the summer and early fall would cut the temperatures down, and would probably still result in enough hot water. A roll up shade cloth shield could be included in the collector design. It would not have to cover all of the collector -- just enough to lower the stagnation temperatures a few degrees.
Covering with shade cloth is recommended by some commercial collector manufacturers for extended periods of stagnation.
2 - Some form of venting system that would allow a little air to flow through the collector when temperatures rise above some level would work. One scheme for doing this is described here...
Gary August 31, 2008