[TRNSYS-users] Type553 Unglazed Collector Theory

[Nicholas LaHam <ndlaham@gmail.com>]

Hey all,

I have a question about the Type553 Unglazed collector and how it's performance interacts when changing flowrates in two different scenarios. I was wondering if someone could help explain the trend in a practical way for me so I can explain it to my colleague.

First off, when just running a constant flowrate and constant incoming temperature as the inputs to the collector, I can see that increasing that flowrate increases the useful energy out of the collector. This makes sense and is intuitive.

Now when I use a loop instead for the same collector, (say with a pump operating at 1 GPM just as a starting point), and a perfect heat exchanger with the same incoming water temp now on the load side of the HX, I see a different trend I'm having trouble explaining to my colleague.

What is happening is that the useful energy increases like the first scenario, but only up to the flowrate that is on the load side of the HX. ex. So if I have 5 GPM on my load side, I see an increase in performance up to 5 gpm on my collector side, and then the performance begins to drop after increasing it any further.

Another way to say it is that whenever my flowrate of the collector array is greater then my load I have weakened performance. It seems that a 1:1 flow ratio on each side of the HX is the optimal method (assuming same specific heats).

Can anyway explain in a conceptual manner why there is this decrease only when using the HX? I was thinking that because the load side of the HX is the limiting flow (the one that isn't changing) then because the collector flow is higher, that not all the heat from the collector can be transfered, leaving the outlet of the HX on the collector side hotter than if the flows were balanced. Thus in turn lowering the efficiency and available energy to be collected the next time it passes through the collector.

I can actually see the source side temp coming out of the HX hotter than the incoming water when the collector flow is higher, but my colleague still thinks the energy should not drop. He believes that when increasing the flow up to a very large number over many trials, the useful energy will always rise up until it levels off asymptotically, whereas in my case, it is doing more of a bell curve and dropping off after it becomes higher than the load flowrate.

Thanks for any help.

Nick LaHam