This little test measures the collector internal temperatures and glazing temperatures for our $2K system collector during a stagnation event on a clear, warm, sunny day in September.
This time of year is probably worst for stagnation for this vertically oriented collector. The ambient air temperatures are still high, the weather tends to be quite sunny, and the sun is getting lower in the sky and shines much more directly on the collector. This is coupled with the hot water demand on the low side (no space heating), and the tank reaching its high limit temperature so that the controller shuts off the pump and stagnates the collector.
I recorded collector temperatures over the peak of the stagnation and also took some thermal camera pictures of the outside of the glazing and surrounding areas on the house.
The chart below shows the internal collector temperature with the collector stagnated (no flow).
Date: Sept 10, 2012
Weather: Clear except for a little forest fire haze, little wind, ambient temperature 80F
The collector is vertically mounted on a near south facing wall. The collector glazing is twinwall polycarbonate. The ground in front of the collector is light colored concrete and some grass, cars, ...
The collector is stagnated because the storage tank reached its high limit temperature of 165 F, which caused the controller to stop the pump. This happens fairly often this time or year because hot water demand is low (no space heating), weather is usually sunny, and the sun is getting lower in the sky this time of year so it shines more directly on the collector.
so, the collector reached a maximum of 238F, and then started down. At about 3:00 pm, I took a long shower which got the tank temperature low enough that the pump kicked back on and brought the collector temperature quickly down into the 170 F's. The collector temperature normally runs a few degrees over the tank temperature when the pump is running.
So, the effect of stagnation is to increase the collector temperature from about 170 F to about 240F -- a 70F increase.
I don't consider this as any kind of problem in that all the materials are good for prolonged exposure to to temperatures well over 240F. The only component that is getting even close to its limit is the glazing, and it runs at a considerably lower temperature (see below).
On my vertical single glazed collector on the shop, I have not seen stagnation temperatures over 185 F, and I believe that the double glazing is responsible for most of the difference.
I took some pictures of the outside of the collector with the thermal camera to get the temperatures on the exterior of the glazing. The polycarbonate glazing is opaque to the IR camera, so the temperatures recorded are the outside surface of the glazing.
The picture above is of the glazing of the collector at about the same time that the inside of collector temperature was about 235F. So, as you would expect, the outside surface of the glazing runs much cooler that the inside of the collector. Don't know what the inside surface of the glazing is running, but I would guess substantially lower than the 235F. This does give the glazing, which is good for at least 270F, some more breathing room over the maximum temperatures inside the collector box.
The picture above is taken backing out a ways so that the siding around the collector and the roof temperatures are visible. The siding next to the collector at 128F is nearly the same temperature as the collector glazing. The roof is considerable hotter at up to 170F. The roof tilted at 45 degrees. (All three of these materials have emissivity's close to 0.95, so the temperature on the image can be directly compared).