THERMAL PARAMETER LOSSES PVSYST

[Michalis Angeli]

Can you explain to me which is the different between these types of 

free-standing (open-rack) systems,

fully insulated backside

(semi-integration, air duct below the collectors)

domes, a manufacturer has measured the U-value on several installations  (height about 40 to 70 cm above the ground). Actuall i would like to know how to consider the factor Uc and Uv for an installation on a tiled roof. Thank you

 

[Lazare Fesnien]

Dear Michalis Angeli,

In the absence of reliable measured data, PVsyst proposes default values without wind dependency   (i.e. assuming an average wind velocity):

- For free-standing (open-rack) systems, i.e. with air circulation all around the collectors), according to our measurements on several installations:  

Uc =  29  W/m²·k,            Uv = 0   W/m²·k / m/s

- Therefore for fully insulated backside  (no heat exchange at the backside, only one side contribution to the convecting heat exchange), the U value should be divided by 2:  

Uc =  15  W/m²·k,            Uv = 0   W/m²·k / m/s

- For intermediary cases (semi-integration, air duct below the collectors), the value should be taken between these 2 limits, but preferably lower than 22  W/m²·k as the air heat removing is often not very efficient. The default value proposed by PVsyst for any new project is  

Uc =  20  W/m²·k,            Uv = 0   W/m²·k / m/s

 We have chosen this value as general default, because we consider that it is more representative of usual rooftop systems, managed by "less professional" people who will not necessarily modify the PVsyst default. For big systems, we suppose that trained engineers will indeed adjust this parameter (for example at 29 W/m² for row-like big power plants). 

- For domes, a manufacturer has measured the U-value on several installations  (height about 40 to 70 cm above the ground): 

Uc =  27  W/m²·k,            Uv = 0   W/m²·k / m/s: 

Now if reliable hourly wind velocity data are present in the data, we don't have any reliable measured data with wind.

PVUSA proposes the following thermal correlation, widely used for the free-standing (open-rack) situations when wind speed data are available.

Uc =  25  W/m²·k,            Uv = 1.2  W/m²·k / m/s 

This corresponds to our default 29 W/m2, when the average wind velocity is 3.3 m/sec (rather usual in continental - not coast situations).