Does PVsyst treat Bifacial systems ?

[André Mermoud]

Since the version 6.60, we have implemented a model for the simulation of bi-facial systems.

This model is for the moment restricted to shed-like systems, with the hypothesis of "unlimited sheds". We will develop models for other configurations later on.

This tool becomes available in the software as soon as you choose a bi-facial PV module.

In PVsyst, Bi-facial PV modules are characterized by one only parameter, the bifaciality factor, which is the ratio of the STC efficiency of the rear side with respect to the front side.

For such a model, we have to evaluate the irradiance available on the ground, a reflexion parameter named "albedo", and the reemission to the back side of the PV array.

This evaluation is done numerically, based on 2 basic hypothesis: the diffuse irradiance is isotropic, and the re-emission from a ground point is also isotropic.

From V 6.64, the diffuse irradiance from sky on the rear side will also be taken into account. However this is a little contribution with reasonable tilts (less than 20°).

Irradiance on the ground

The irradiance available on the ground is calculated taking the shading of the PV system into account:

- the beam component between the rows is obviously dependent on the sun's position,

- for each point of the ground, the diffuse effectively reaching this point is calculated using the isotropy hypothesis of the diffuse (view factor of the sky). The diffuse distribution on the ground is independent on the sun's position. It is only related to the geometry of the system.

Irradiance on the rear side

The irradiance on each ground point is reemitted in all directions according to the Albedo factor. Again, using the isotropy hypothesis of the re-emission , we can calculate the fraction which is re-emitted to the PV array, and the quantity lost to the sky (view factor). The luminous energy available for each ground point is the sum of the diffuse component, only dependent on the geometry, and the beam component when this point is illuminated by the beam (depending on the sun position). We should emphasize here that the ground irradiance and reemission is dependent on the position below the array.

NB: We have recently found a conceptual error in the Bi-facial model. Up to the V 6.63, we had considered the exchange between the ground and the rear side of the PV modules as an energy, when it is indeed an irradiance. This means that the reemitted energy to the rear side of the collectors should be renormalized by the involved surfaces. That is, the rear side irradiance should be multiplied by Pitch/Coll. width (1/GCR), which gives a factor of 2 or more! This has been corrected in the version 6.64.

We have indeed: Energy(rear) = Albedo * ViewFactor * Energy(Ground)

i.e. Irrad(Rear) * Coll area = Albedo * ViewFactor * Irrad(Ground) * GroundArea

=> Irrad(Rear) = Albedo * ViewFactor * Irrad(Ground) * GroundArea / CollArea

Therefore Please use the version 6.64 and forget the results of the previous versions 6.60 .. 6.63 !

Other perturbations

We should take several perturbations of this simple model into account:

- a shading loss, which is characterized by a simple factor (could be the structures, the eventual junction box if on the rear side, etc )

- the rear side irradiance is not uniform, there is therefore a mismatch between sub-modules, leading to electrical losses. PVsyst doesn't have any reliable value to propose for this parameter in the present time,

- The PV module may eventuially have some transparency (spacing between the cells - or even the modules): this will contribute to the ground irradiance.

Now PVsyst provides a calculation framework - established with simple and well-defined hypothesis - which may be applied all over the year.

This is a reasonable physical model, but we don't have any validation.

Comparison with other studies

However comparisons with existing experimental systems and publications is very difficult as:

- Most of the experimental setups are made of one or two modules, above an "illimited" ground area. Therefore receving light from a big illuminated environment.

The model in PVsyst supposes a realistic installation with Unlimited sheds, i.e. usual rows spacing and no additional albedo contribution from the edges of the system.

- The results are often available for some specific conditions, not over a full year measurement.

- The albedo conditions may be not realistic with respect to real systems (specific surfaces, variability when wet, ageing, etc).