Cost-Effective Capacity Testing: Transposing GHI for Sites with Multiple Arrays and Bifacial Modules

Introduction

For PV capacity tests conducted on small to medium size installations with multiple array orientations, placing a POA (Plane of Array) pyranometer on every orientation can be prohibitively expensive. In this context the ASTM E2848 test procedure can be adapted to use GHI (Global Horizontal Irradiance) sensors and apply transpositions to obtain the POA irradiances for each array. This article covers the full workflow from GHI measurement to final weighted average POA irradiance.

Essential measurements

• For monofacial systems, at least one GHI sensor is required
• For bifacial systems, an additional back-facing GHI sensor must be added
• Optionally but recommended: a DHI (Diffuse Horizontal Irradiance) sensor can significantly improve the accuracy of the irradiance transpositions

Transposition Process

The measured subhourly GHI data can be imported into estabished PV modeling tools such as PVsyst, NREL's System Advisor Model (SAM) or Sandia's PVLIB. These tools use tranposition models such as the Perez-Ineichen or Hay's model to calculate the POA irradiance for a given array orientation.

Since PVsyst uses the Perez-Ineichen model by default, Heliotest uses the same model to ensure consistency during the PV Capacity test. Additionally, when DHI measurements are provided, Heliotest uses them to improve the transposition accuracy.

Combining Multiple POAs into a Single Representative Value

For systems with multiple array orientations, the ASTM E2848 standard requires a single representative POA irradiance to fit the regression model. Therefore, a weighted average POA irradiance is calculated based on the number of modules per orientation and their STC rating.

For more details on this step, please refer to PV Capacity Testing for Sites with Multiple Array Orientations.

Special Consideration: Bifacial Systems

For sites with bifacial panels, the rear POA irradiance can be estimated as follows, provided that a rear GHI sensor has been installed in addition to the front GHI sensor

• start from the weighted average front POA irradiance $E_{\text{POA-front}}$
• then, to obtain the back POA irradiance, scale it based on the ratio of back GHI to front GHI measured data and factor in the bifaciality factor $\varphi$ and structural shading loss $SSF$ (more detail on $\varphi$ and $SSF$ here).

$$E_{\text{POA-back}}=E_{\text{POA-front}}\times \frac{GH{I}_{\text{back}}}{GH{I}_{\text{front}}}\times \varphi \times (1-SSF)$$

Note that this equation applies to the measured data only. For modeled data from PVsyst, the front POA (GlobInc) and and back POA (GlobBak) are directly provided and GlobBak already accounts for the structural shading loss. Therefore only the bifaciality factor must be factored in to obtain the equivalent $E_{\text{POA-back}}$:

$$E_{\text{POA-back}}=GlobBak\times \varphi$$

Finally, for both measured and modeled data, the total bifacial POA irradiance is obtained as

$$E_{\text{POA}}=E_{\text{POA-front}}+E_{\text{POA-back}}$$

The so obtained $E_{\text{POA}}$ for the modeled and measured data can then be used to apply the ASTM E2848 standard as usual.

Potential Enhancement

If there is one predominant array orientation in the system, installing a dedicated POA sensor for this array may be a worthwhile economic trade-off. Note however that this feature is currently not supported by the Heliotest App.