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Systems involving concentrating devices are not treated in whole generality in PVsyst. Some specific features have been implemented from version 4.2, for evaluating especially high-concentrating systems. But their accuracy is not quite established.

The general study of concentration systems involves a detailed description of the irradiance distribution, which cannot be available using the present treatment of the Meteo, nor the limited information included in the meteo database (site database).

Namely high concentrating performances require a good knowledge of the beam component. Then accurate models for achieving this evaluation would involve parameter like turbidity, water and aerosol contents of the atmosphere, which are not defined in our database.

As an example, the eruption of the Mt. Pinatubo in 1991, had little effects on the global irradiance (less than 2%) but induced a very high loss of beam component: the pure direct was scattered by aerosols, resulting in a strong halo around the sun, up to 30% during almost 2 years. This had dramatic consequences on the productivity of the high concentration thermal plants all around the earth [Molineaux 1996].

In PVsyst, the simulation of concentrating systems has to deal with 2 aspects:

Acceptance of the Diffuse component

The higher the concentration, the lower the acceptance angle, which implies a limited acceptance of the diffuse component.

The maximum achievable Concentration Ratio CR is related to the acceptance half-angle θ  as:

CRmax (1axis)  = 1 / sin θ

CRmax (2axis)  = 1 / sin² θ                         (Kreith and Kreidler, 1978, p 248).

In the present state, PVsyst is only able to treat 2-axis high concentration systems.

General treatment of low concentration systems (especially 1-axis parabola or "Compound Parabolic Concentrator" CPC)  would imply a very detailed description of the optical system, as well as a good knowledge of the irradiance distribution by any weather conditions, and would result in inhomogeneous irradiance on the PV receiver, which is very difficult to take into account when there is more than one cell.

Therefore, concentration system parameters are only proposed in the 2-axis tracking dialog, where you have to define:

- The diffuse fraction to be taken into account in the simulation  (usually near 0 with high concentration).

- The acceptance angle for a full efficiency (half-opening angle, i.e. the angle between incident and optical axis).

- The limit angle at which the efficiency falls to 0. Besides tracking errors (which cannot be taken into account), this will be useful when the array is reaching its tracking mechanical limits. The simulation will perform a linear decrease between the acceptance and limit angles, and will accumulate the corresponding tracking loss.

Electrical behavior of the concentration device

In a 2-axis high concentrating system, the PV sensitive device is usually a single multi-junction cell of some few cm², with very high efficiency. This receives a flux of the order of  500x suns (50 W/cm²), and therefore works in a domain where our PV one-diode model is not well attested.  We can just notice that the logarithmic behavior of the open-circuit voltage with irradiance  - which is a straightforward result of the model - favors the efficiency at these running conditions.

The device is mounted on a heat spreader (passively cooled), which ensures an acceptable operating temperature, of the order of 80°C or less.

Electrical modeling in PVsyst

We did not develop a specific model in PVsyst for such a configuration.

But some manufacturers of concentrating devices use to give performance data for their whole component - including concentrators - in a  comparable way to usual photovoltaic modules. That is, when a set of concentrators with their PV devices is assembled as a module, they give the usual parameters  Isc, Vco, Imp and Vmp, referenced to the irradiance on the aperture area  (sometimes under 850 W/m² instead of 1000 W/m² ). The I/V curve is very sharp, with an excellent fill factor.

Then,we do the hypothesis that the standard one-diode model applies to this system, even though it has no real physical meaning. This is motivated by the sharpness of the I/V characteristics, and by the fact that our model allows to set a customized temperature coefficient, as required by the manufacturer. Therefore, power behavior according to irradiance and temperature that necessary for the simulation process - should be close to the reality.

In this phenomenological model, any optical aberrations are neglected.

NB:  With respect to standard systems, such high concentration systems suffer of two main loss sources, the negligible diffuse acceptance  (diffuse is of the order of 30% even in most sunny regions, 40 to 50% in the middle Europe climates) and the full loss when reaching the tracking limits of the heliostats. These appear of course on the PVsyst Loss diagram.

On the other hand, in the system definition, the heat loss factor Kc should be set according to the effective sensor temperature, reached under nominal irradiance and which should be specified by the manufacturer (some equivalent of the NOCT data).