In solar collectors, stagnation refers to the situation in which the sun is shining on the collector and no fluid is flowing through the collector to remove heat. Since there is no fluid removing heat, the collector heats up until the heat losses out the glazing and sides and back of the collector balance the solar radiation coming into the collector. This results in high temperatures. How high depends on the collector construction, the glazing (single or double and transmittance, insulation levels, and absorber construction (absorbtance and emittance), and on the collector tilt angle.
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. The tactic I am using to keep stagnation temperatures within the capabilities of PEX is to tilt the panel at a steep angle (around 70 degrees). This results in much hjgher incidence angles in the summer, and therefore less radiation absorbed and lower stagnation temperatures.
Based on the data I've seen on PEX, I would expect that it can take stagnation temperatures around 220F -- 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.
My tentative conclusion is that the combination of high tilt angle, single glazing, and non-selective surface makes for acceptable stagnation temperatures. I do plan to take the collector apart later and cut one of the PEX tubes open to see if any damage is visible in either the PEX or the silicone.
When the collector has water flowing through the PEX tubes (as would be normal for collection periods), the collector temperatures are, of course, much cooler -- usually pretty close to the storage temperature.
<pic of collector in stagnation with sensor placement>
<other solutions> venting etc.
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