How to Run PV Capacity Tests with Bifacial Panels

Introduction

Bifacial panels can be accounted for in ASTM Photovoltaic (PV) Capacity Tests through a modification of the measured irradiance input used in the multilinear regression analysis.

While the currently published version of the ASTM 2848 standard does not natively address bifacial systems, the common practice consists in calculating and utilizing the total effective irradiance ( $E_{\text{TOTAL}}$ ) which incorporates both front and rear irradiance contributions.

Calculation of Total Effective Irradiance

The Total Effective Irradiance ( $E_{\text{TOTAL}}$ ) is calculated as the sum of the front Plane of Array ( $E_{\text{POA}}$ ) irradiance and the rear Plane of Array ( $E_{\text{R-POA}}$ ) irradiance, adjusted by the module's bifaciality factor and structural shading:

$E_{\text{TOTAL}}=E_{\text{POA}}+E_{\text{R-POA}}\ast \varphi \ast (1-SSF)$

where:

• $E_{\text{POA}}$ is the front-side POA irradiance
• $E_{\text{R-POA}}$ is the rear-side POA irradiance
• $\varphi$ is the bifaciality factor of the module (typically 70%-95%)
• $SSF$ is the rear Structural Shading Factor. It represents the production loss percentage due to shading caused by the structural elements of the mounting system (typically 3%-10%)

Important note when working with PVsyst

In the exported data from PVsyst, the column GlobBak represents the rear POA irradiance that already accounts for the structural shading losses. Therefore the calculation of the Total Effective Irradiance ( $E_{\text{TOTAL}}$ ) for the modeled data simplifies to

$$E_{\text{TOTAL}}=E_{\text{POA}}+E_{\text{R-POA}}\ast \varphi$$

If you use the Heliotest app for your capacity test, this adjustment is handled automatically—no manual calculation is needed.

Application in the ASTM 2848 Capacity test

The ASTM E2848 capacity test standard requires performing multilinear regressions that relate system output power ( $P$ ) to weather conditions. The standard defines the regression formula using the POA irradiance ( $E_{\text{POA}}$ ) as the primary variable:

$P=E_{\text{POA}}\times (a_1+a_2{E}_{\text{POA}}+a_3{T}_{\text{amb}}+a_4{V}_{\text{wind}})$

where:

• $T_{\text{amb}}$ is the ambient temperature
• $V_{\text{wind}}$ is the wind velocity
• $a_1,a_2,a_3,a_4$ are the linear regression parameters

For bifiacial systems, the Total Effecitve Irradiance ($E_\text{TOTAL}$) replaces the front POA irradiance $E_{\text{POA}}$ in the above formula. With this modification, the standard can be applied as usual. Separate multilinear regressions are performed for both measured and modeled data, and the results are compared at a common reporting condition to compute the Capacity Test Ratio (CTR).

For a comprehensive walkthrough of the PV capacity testing procedure, refer to How to Perform ASTM Capacity Testing: A Practical Guide.