Designing systems with bifacial modules

[pv_n00b]

There is no standard for this so I am wondering what people are doing to get the Isc value for a bifacial module. I have seen standard proposals that use 1.2 and 1.3 times the front side Isc rating and then there is the option of doing a system analysis with SAM or PVSyst and using the maximum estimated Isc value. I'm leaning towards using 1.3*Isc of the front side. It's fairly conservative, but there might be some extreme cases of gain where it goes over 1.3.

 

[ggunn]

There is no standard for this so I am wondering what people are doing to get the Isc value for a bifacial module. I have seen standard proposals that use 1.2 and 1.3 times the front side Isc rating and then there is the option of doing a system analysis with SAM or PVSyst and using the maximum estimated Isc value. I'm leaning towards using 1.3*Isc of the front side. It's fairly conservative, but there might be some extreme cases of gain where it goes over 1.3.

Doesn't the manufacturer supply Isc numbers? Why wouldn't you just use them?

 

[Carultch]

Doesn't the manufacturer supply Isc numbers? Why wouldn't you just use them?

They do. However, because of back surface irradiance, the Isc values will be greater than when STC applies only on the front surface. Design with ordinary modules doesn't require you to think about this. The manufacturers commonly give you test data of Voc, Vmp, Imp, and Isc with 10% backlighting, 20%, etc, all on the assumption that 1 standard sun of irradiance is available on the front side. The OP's question is how to know which one of the backlighting test data tables should be used, when sizing everything else connected to the bifacial module? I would design by the highest backlighting that the manufacturer tests, for a worst-case scenario.

I'd like to see examples of typical backdrop surfaces that are known to produce each percentage of backlighting. Such as white TPO roof produces X% backlight, parking lot with cars below produces Y%, green landscaping is Z%, gravel/stone/concrete is XX%, etc.

 

[electrofelon]

IMO the NEC is overly conservative, so, personally, I wouldnt sweat it too much. Besides, in practice it seems like having a 30-40% higher ISC would be unlikely to make a difference for anything,

 

[pcanning87]

We use the current calculation methodology from NEC 2017. PVSYST gives you backside irradiance so we run the 8760 and look at the max irradiance (front plus back) at any hour and use that as our basis. Also applicable to pre-2017, we just apply the same logic to get an Isc multiplier.


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