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Can I import POA measurements ?

André Mermoud

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PVsyst is only able to perform simulations starting from the Horizontal irradiance GlobHor. This is motivated by the fact that we only avail of models for estimating the Diffuse irradiance in the horizontal plane (the diffuse irradiance is used namely for transposition model and shading or IAM calculations).

Therefore starting from Plane irradiance (POA) requires first to perform a retro-transposition for getting the horizontal global and diffuse irradiances, which will lead to the specified POA after transposition. This uses the Liu-Jordan (or Erbs) model for the evaluation of diffuse.

When using these data, the simulation should retrieve the exact input POA value.

If this is not the case, it indicates that there was a problem during the import process. The usual problem is a time shift between the measurement time and the PVsyst time.

If the measurements are not accumulated at the exact time (for example between 9:00 and 10:00 for the PVsyst 9:00 step), the solar geometry is not computed at the right time (middle of the interval) and the transposition my lead to very high errors in the morning or the evening, i.e. when the sun is low on the horizon (as the POA irradiance is divided by sin Hsol). This may lead to very high erroneous horizontal Global, which will be limited to the clear day model.

In these cases a Time shift correction should be applied, already during the import process.

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