Potential of bifacial panel on angled roof

Dear Paul,

I am trying to model PV yields of a panel setup on an angled roof. One question that I need to get answered is whether it makes sense to use bifacial panels and colour the roof behind the panels white (i.e. increase the albedo of the roof).
From the SAM documentation and from my simulations I learned that the Ground Clearance Height has a considerable impact on the yields of bifacial panels.

However, one thing is unclear to me: is SAM using the GCH as a constant value for the distance of the solar panel to the background for the entire solar panel (as would be the reality in a roof-mounted setup) or is the GCH used as an input to calculate the average distance from the panel to the ground (as would be correct in case of a ground or rack-mounted setup). Is SAM able to evaluate the potential of bifacial panels on roof-mounted setups in a feasible manner or is the functionality of calculations on bifacial panels yet limited to ground or rack-mounted setups?

Thanks in advance!

Best regards Jaap
 

Hi Jaap,

SAM's bifacial module calculations do not differentiate between a roof and the ground, so "ground" clearance height could be the distance between the array and the ground for a ground-mounted array or between the array and the roof for a roof-mounted array.

However, SAM assumes that the ground (or roof) is horizontal, except for a special case: For one-axis tracking with backtracking and/or linear self shading, you can use the terrain slope and azimuth inputs to change the ground slope from the horizontal. (Self shading input is on the Shading and Layout page. Tracking, backtracking, and terrain slope inputs are on the System Design page.)

As you suggest, you can use the albedo input to represent the reflectiveness of different roof colors.

SAM uses a set of geometric equations to determine shadows on the ground (or roof) and irradiance incident on the rear of the array. You can see those equations here: github.com/NREL/ssc/blob/82d9c9ebb98b556018e02ffa8464f53528be31d4/shared/lib_irradproc.cpp#L2648 .

For a bifacial array on a horizontal roof, choose the following options on the Module page:

• For temperature correction, if you are using the NOCT method, set Mounting standoff to "Ground or rack mounted." If you are using the heat transfer method, set Mounting configuration to "Rack." This assumes that air can circulate behind the array to help cool the modules.
• For bifacial, set Ground clearance height to the distance between the array and the roof. (The distance between bottom of modules and the roof for a fixed array, or the distance between the tracking axis and the roof for one-axis tracking.)

Best regards,
Paul.