[TRNSYS-users] Type553 Unglazed Collector Theory

[Nicholas LaHam]

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
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