In-depth Inquiry on BIPV Aging, Temperature Modeling, and Loss Equations

[Zahra]

Hello PVsyst community,

I am working on a research project focused on the long-term aging and performance degradation of Building-Integrated Photovoltaic (BIPV) systems. To accurately model this, I need a more detailed understanding of PVsyst's internal calculations.

I have already reviewed the official documentation, but I have a few specific questions that I'm hoping for some clarity on:

1. Temperature Modeling for BIPV: I understand that PVsyst does not have a detailed building model for BIPV. The documentation suggests importing a custom temperature time series. Could you please provide more insight into the specific equations used in the internal thermal model for BIPV (or non-ventilated modules)? For example, are there any specific heat transfer coefficients or thermal resistance parameters that can be adjusted to better simulate the higher operating temperatures of BIPV modules?

2. Losses and Aging Equations: I need to understand how the software models long-term performance losses. Could you elaborate on the mathematical models or equations used for

Thank you in advance for any documentation, white papers, or detailed explanations you can provide. Your insights will be invaluable to my research.

[Auriane Canesse]

Hi Zahra,

1. PVsyst has only one temperature model as described here:
   https://www.pvsyst.com/help/project-design/array-and-system-losses/array-thermal-losses/index.html?h=temperature#thermal-model
   Our recommendation is using Uc = 15 W/m²·k for modules with a fully insulated backside (eg. PV roof tiles). But BIPV has a very wide range of PV designs so no we do not have recommendations for BIPV in general. If you import cell temperature measurements they will be used directly in the simulation insteand of using the temperature model.
2. For the long tem losses, the degradation factor is applied to Imp/Vmp. The one diode model is fitted again to compute the IV curve of the modules and these are used in the simulation. The mismatch factor on the other hand is an effective correction. Individual modules are expected to degrade at slightly different rates, hence increasing the mismatch losses over time. For each module, a degradation loss is drawn from a gaussian distribution (defined in the ageing tool). The modules are assumed to degrade at that rate over the years an the mismatch factor is computed from that. The mismatch factor is added to the array losses.

Unfortunately, we do not have white papers describing these 2 points, only  the help pages.

[Zahra]

On 8/8/2025 at 4:38 AM, Auriane Canesse said:

https://www.pvsyst.com/help/project-design/array-and-system-losses/array-thermal-losses/index.html?h=temperature#thermal-model
Our recommendation is using Uc = 15 W/m²·k for modules with a fully insulated backside (eg. PV roof tiles). But BIPV has a very wide range of PV designs so no we do not have recommendations for BIPV in general. If you import cell temperature measurements they will be used directly in the simulation insteand of using the temperature model.

Thanks for respond, How about Uv? is there anyway to model it more accurate?

[Auriane Canesse]

By default we recommend Uv=0 as this coefficient is trickier to measure and varies even more from one PV installation to the next. There can be a large difference between the wind speed from a data provider (measured at 10m above ground, not always close by) and local conditions at the module level.

But yes to be more accurate you simply need to measure this coefficients on your installation.
https://www.pvsyst.com/help/project-design/array-and-system-losses/array-thermal-losses