[TRNSYS-users] Type553 Unglazed Collector Theory

[Nicholas LaHam]

Hey Jeff,

I changed the time step and tolerances and still see the same trend. Here is
what I changed them to. The simulation took about 5 times longer.

Time Step: 0.01 [Hour]
Tolerance Integration: 0.000001 [-]
Tolerance Convergence: 0.000001 [-]

Also so you know, I attached the TRNSYS schematic of what I ran. A very
simple system for testing as you can see. I set a constant flowrate of 5 gpm
on the load side of the HX that never changed, and I varied the source side
from 3 gpm to 12 gpm over the course of 6 trials. The HX is also 100%
effective for testing purposes, but the same trend occurs with
any effectiveness under 1.

I attached a pdf of the results if you want to take a quick look. The
numbers from the default tolerance test and tighter tolerance test are
virtually the same, and show that same trend: the performance of the
collector and the heat transfer in the HX increase from 3 to 5 gpm, and
decrease from 5 gpm and up.

This does not happen when do not use a HX.

Is this what the trend should actually be or is TRNSYS (or me) doing
something weird?

Thanks for any help Jeff. I appreciate it.

On Thu, May 19, 2011 at 8:57 PM, Nicholas LaHam <ndlaham at gmail.com> wrote:

> I'll try that tomorrow Jeff.
>
> I am using a constant effectiveness and I am leaving pipes out of the
> simulation for now to figure out what is going on.
>
> Should this be the trend?
>
> Thanks,
> Nick
>
> On May 19, 2011, at 2:11 PM, Jeff Thornton wrote:
>
> Tighten your tolerances way up and shorten your timestep dramatically and
> see if the trend changes.  I'm also guessing that you're using a constant
> effectiveness HX?  Are you using pipes for capacitance or just a collector
> and HX?
>
> Jeff
>
> Jeff Thornton
> President - TESS, LLC
>
> 22 North Carroll Street - Suite 370
> Madison WI 53703 USA
>
> Phone: 608-274-2577
> Fax: 608-278-1475
> E-mail: thornton at tess-inc.com
> Web: www.tess-inc.com
>
>
>
> *----- Original Message -----*
> *From:* "Nicholas LaHam" <ndlaham at gmail.com>
> *Sent:* Tue, May 17, 2011 10:55
> *Subject:* [TRNSYS-users] Type553 Unglazed Collector Theory
>
> 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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