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Thermal Losses
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Author:  azizsye1 [ Wed Nov 21, 2012 5:55 pm ]
Post subject:  Thermal Losses


I found the following information on the PVSYST Help section:

The thermal behaviour is characterised by a thermal loss factor designed here by U-value (formerly called K-value), which can be split into a constant component Uc and a factor proportional to the wind velocity Uv :

U = Uc + Uv · v (U in [W/m²·k], v = wind velocity in [m/s]).

These factors depend on the mounting mode of the modules (sheds, roofing, facade, etc...).

I want to determine how the Uv Factor is effected with a higher tilt angle. When going from 5 to 10 Degrees in a set-up on a flat rooftop, I would expect that the Uv value would increase. Currently I have Uc set to a default of 20 and Uv at 0.

Please advise.


Author:  cnrikio [ Sat Dec 29, 2012 11:41 am ]
Post subject:  Re: Thermal Losses

The expression of U-Value is used to revise the effection of wind. And as you may know, higher wind speed may result in a stronger thermal transferring. But I think it is hard to say that the wind speed would be obviously higher when tilting 10 degrees, though the air flow would be more turbulent. So, my point is, the Uv would be different, but may not that big.

Author:  André Mermoud [ Sun Jan 06, 2013 6:22 pm ]
Post subject:  Re: Thermal Losses

These u-values are really not well established.
I have never found publications, results of systematic measurements about them in any conditions.
There is certainly a tilt dependence, but nobody knows how to quantify it...

The only way is to measure it on-site. This is a heavy project: it cannot be done without careful long-term measurements of Modules and Ambient temperatures, as well as Irradiance and if possible wind velocity.

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