[TRNSYS-users] Solar thermal collectors / absorber temperature

[Hemmatabady, Hoofar]

Dear Jeff,

Thank you for your reply. I need it for 2nd law efficiency calculation.

So it can be considered as outlet temperature when the collector is idle and approximately as average temperature when the collector is working.

But sometime there is the problem that when there is no flow, the inlet and outlet temperatures show unacceptable numbers (e.g., 300 °C which cannot be the mean plate temperature!) and they get back to the reasonable values when there is flow again (the attached image). What could be the problem?


Best regards,

Hoofar

________________________________
Von: TRNSYS-users <trnsys-users-bounces at lists.onebuilding.org> im Auftrag von Jeff Thornton via TRNSYS-users <trnsys-users at lists.onebuilding.org>
Gesendet: Donnerstag, 14. Januar 2021 21:23:04
An: TRNSYS users mailing list at OneBuilding.org
Cc: Jeff Thornton
Betreff: Re: [TRNSYS-users] Solar thermal collectors / absorber temperature


Hoofar,

The mean plate temperature is calculated by several models - but none that I know of that take the "simple" parameters like Type 1. The reason for it is simple - the models like Type 1 aren't doing a full energy balance on the absorber plate.  So calculating a mean plate temperature (factoring in both the changes in the flow direction and across the tubes) isn't all that easy - or maybe even possible.  If the collector has a linear efficiency equation, you can estimate the mean plate temperature from:



Tplate,mean = Tfluid,in + Qu/Area/FrUl * (1-Fr)



This is courtesy of Duffie and Beckman.  But in simple collector models you only know FrUl - and you don't know Fr separately.



As for the case with no thermal mass and no flow, the calculated outlet temperature of these simple models is also equal to the mean fluid temperature - and with no heat transfer into the fluid - is also then the mean plate temperature.



May I ask why you need the mean plate temperature?  I do a lot of solar collector modeling and have never needed it before...



If it's critical, you could estimate the mean fluid temperature (for a collector with a small temperature difference) as the average fluid temperature (in/2 + out/2) and then add a couple of degrees for the resistance between the plate and the fluid.  Typically the mean plate temperature is slightly higher than the mean fluid temperature - for cases where the collector is receiving radiation and the inlet temperature is close to the ambient temperature.



Jeff



---
Jeff Thornton
President - TESS LLC
3 N. Pinckney Street, Suite 202, Madison WI USA 53703
Office: (608) 274-2577  Fax: (608) 278-1475
www.tess-inc.com
E-Mail: thornton at tess-inc.com


On 01/14/2021 10:48 am, Hemmatabady, Hoofar via TRNSYS-users wrote:

Dear TRNSYS users,



For a thermodynamic assessment, I need mean absorber plate temperature of flat plat solar collectors. The temperature is needed even if the collector is idle (e.i, there is no flow rate). Here are my questions:



1) Is there a TRNSYS type, which have plate temperature as an output with simplified required parameters (like those of type 1)?



2) Can TRNSYS components be used for guessing their physical behavior? (e.g., using the average inlet and outlet temperatures of the solar collectors as mean plate temperature even if there is no flow)



Best regards,

Hoofar


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