André Mermoud

The Uc and Uv parameters are not related to the location nor climate, but the mounting mode of the modules (if they are integrated - i.e. back insulated - or not). Now we don't have really well assessed values for these parameters. Please see the Help, available by F1 when you are in the U-definition dialog.

This is indeed a bug: I have forgotten updating these 3 labels (Transposition FT, Loss and Global on coll. plane) in the multi-orientation panel. I have corrected for the next version 6.27, to be released within 2-3 weeks. However these are just values shown as information when choosing an orientation: these are not used anywhere in the simulation.

I don't know, I have never implemented the NREL algorithms. The fact that the differences are rather "symmetrical" around 12H indicate that the problem is probably related to the time definition. The sun progresses by 15°/hour (along its trajectory). Therefore a discrepancy of 3° in the morning may correspond to a very high time discrepancy (3°/15° corresponds to 12 minutes, to be divided by cos(slope of the trajectory at sunrise) ). However in subtropical geometry (trajectory almost "vertical"), I don't understand well how you can get 3 to 5° error at sunrise. The problem of the accuracy will mainly arise if you perform a retro-transposition for getting the GHI and DHI from a measured DNI value. The morning DNI calculated from the GHI and DHI may have a significant error, but without consequence on the incident irradiance on your almost horizontal plane. PVsyst provides the opportunity of specifying a time shift, already at the meteo data import time, which can avoid this problem.

The IEC 61853-1 standard now recommends the measurement of the PV module characteristics under a matrix of different irradiances and temperatures (namely 1100, 1000, 800, 600, 400, 200 and 100 W/m2, for temperatures from 15°C up to 75°C). The low-light efficiency is the basis for the determination of the uncertain parameters of the one-diode model in PVsyst, mainly the Rserie value. These measurements should be performed with high quality flash-test instruments (AAA class). The irradiance attenuation is ensured by calibrated filters. The results are usually provided as ( Imp, Vmp, Isc, Voc ) data sets, for each (Irradiance, Temperature) condition. Let's define: - PmpMeas = Imp * Vmp measured at GMeas = nominal irradiance after filter - PmpSTC = Imp * Vmp measured at GSTC (= 1000 W/m²). The relative efficiency is then (PmpMeas / GMeas) / (PmppSTC / GSTC) - 1 Filter uncertainties One problem is that the filters are not perfect. They may have inaccuracies either in irradiance and in spectral response. Then, an error of 1 % on the irradiance means an error of around 1% on the relative efficiency. For avoiding this problem we can follow the hypothesis that the short circuit current is proportionnal to the irradiance (including spectral mismatch). From experimental basis, this hypothesis is proposed by the Sandia National Laboratories (USA), as a direct measurement of the incident irradiance. In the "theory", it is the basic hypothesis of the one-diode model. During the analysis of the low-light data, a correct estimation of the relative efficiency requires to modify the GMeas nominal response of the filter by using a corrected irradiance GmeasCorr = IscMeas * GSTC / IscSTC Electrical measurement setup Another key point of the measurement of low-light efficiencies is the electrical measurement of the voltage: this should be measured as close as possible of the PV module terminals (so-called "four-wire" measurement). If the voltage is measured at the measurement instrumentation level, the cable resistance will add with the Rserie of the module. As an example, if you have 2 x 5 m of 4mm2 cable (AWG 11), the parasitic resistance will be of 45 mOhm. For a usual 250 Wp modules of 60 cells, this has to be compared to the Rs = 310 mOhm. The addition of this parasitic resistance will induce an error in the modelled low-light efficiency of +0.33% at 800 W/m², +0.69% at 600 W/m², +1.11% at 400 W/m², +1.58% at 200 W/m². These errors are very important, when we usually see (wait) over-efficiencies between +0.3 and max. +1% in the 600 - 800 W/m² range. NB: The current measurement is not affected by voltage drops due to parasitic resistances. Four-wire voltage measurement

Passing from a "normal" calculation version to a comparison calculation is not simple. However we will probably deeply review the Measure-simulation Comparison tool within some months, and I put this suggestion as a topic on which we should pay attention.

PVsyst is not meant for the full study of the grid organization after the PV system, which may take many different forms (one or several transfos or voltages, etc). There are probably specialized software for that. However we wil probably think about this suggestion after our present numerous development priorities.

I already answered: you have to use the tracking frame, with a fixed tilt for the tracker. Now there is no differences in the algorithms, it is always the Transposition model (Hay or Perez). But the "physical" plane orientation is different in both cases.

This is the number of rows of course. It is specified here as the first row has no mutual shadings. Therefore the generic shading factor has to be multiplied by (N-1)/N.

What I name "Shed" or "row" in PVsyst is a physical structure (mechanics) which receives PV modules. In the latest versions, I prefer now the term of "Table". In the "orientation" option, which specifies the orientation of the PV planes (for the application of transposition model), the special option "Unlimited sheds" also performs the calculation of the mutual shadings of a set of "sheds" or "rows". This simple calculation uses the hypothesis that they are unlimited in length (i.e. neglects the edge effects). If you want to define a "realistic" system and take the edge effects ito account, you should use the 3D editor for the evaluation of the mutual shadings. In this case you have to use the normal orientation option "Fixed tilted plane", otherwise the shading effect would be applied twice.

This is not yet possible. The losses defined in stand-alone systems are restricted in the present time. I will update this treatment during the next few months.

You can have a look on our FAQ How is defined the plane orientation?

SMA has not proposed the data for the OND files corresponding to this model for the PVsyst cdatabase.

Nothing has been changed since the beginning of version 6. The update works quite well. However the problem may be some change in your web access. Some proxies or special web access right restrictions may affect the AutoUpdate possibility.

The simulation button is grayed because you have a red warning in the sizing form ("The array is strongly oversized"). Please see the FAQ Can I define a system with highly undersized inverter ?

Sorry, PVsyst only works with Hourly data. It is not able to perform simulations on sub-hourly intervals.

These values are the same if you don't specify Soiling losses. The soiling losses will also be taken into account in the GlobEff value (it is the effective irradiance in the PV modules).

The axis labels of this plot are indeed not correct. I was not aware of that, and will correct for the next version 6.27. The horizontal axis is the instantaneous output power (i.e. for a given hour). The bins accumulate all the energies registered for this power along the full year (or simulation period). I.e. the sum of all hourly energies for which the output power was in this bin range.

The algorithms used in PVsyst are probably a little bit simpler than those of this NREL publication. But the results should nt differ by more than 1-2 minutes. However I will not give all of them here. Perhaps we will give an extended description in the help in the future. In PVsyst, the hourly values are averages over one hour. The time stamp 8:00 concerns accumulations (averages) of instantaneous values from 8:00 to 9:00. Therefore it is natural to compute the solar geometry at the middle of this interval.

OK, done for the next version 6.27.

According to the measurements reported by the specialized laboratories, the maximum power point Pmpp behaves very linearly with the temperature. The muPmpp parameter specified by the manufacturers is the slope of this linear dependence. Now the Pmpp temperature dependency as calculated by the one-diode model is not exactly linear. Until Version 6.25, PVsyst adjusted the slope for getting the correct slope at 25°C (i.e. a tangent adjustment). In these conditions the derive is around -0.03% at 35°C, -0.12% at 45°C and -0.28% at 55°C by respect to the linear hypothesis. The graphics shows that this "error" is almost imperceptible: Pmpp differences (linear - model) acc. to temperature From version 6.26, the slope is adjusted in order to get the measured (linear) value at 25°C and 45°C (i.e. a secant adjustment). This improves the accuracy of the model at the operating temperatures, and results in a diminution of the temperature loss of about -0.1 to -0.2% (of annual yield) with usual PV systems (depending on the climate and the thermal U-factor) The graph shows the zoomed differences: Differences between Pmpp temperature models NB1: The temperature behavior is normally a result of the one-diode model, close to the specifications. However for an exact adjustment of this value PVsyst introduces a slight linear correction of the Gamma value (diode ideality factor) as function of the temperature. NB2: If you are designing plants in very hot climates, you can choose another secant point than 45°C in the Hidden parameters, topic "PV modules", item "Upper temperature reference for muPmpp default". This point should be in the middle range of your operating temperatures along the year, not the highest ! NB2: In some places of the program - e.g. in the specification dialog ("Model parameters" / "Temper. coeff.") - the muPmpp value corresponds to the Manufacturer's requirement (i.e. secant). And in other places (page "Basic data", application of the model), it is the real derivative for a given temperature. Therefore there may be some differences when showing this value.

The program PVsyst is continuously evolving, therefore it is quite unavoidable to get sometimes yield differences between versions. We try to list some of them here (not exhaustive). Version 6.33 with respect to 6.32 Module Layout calculations: - When operating below the VmppMin inverter value, the simulation chooses the second maximum (instead of the voltage limit). - For shed arrangement, the linear shading calculation has been refined, which affects the electrical loss calculation. Version 6.27 with respect to 6.26 - The electrical shading loss calculation has been modified for shed arrangement: there is now a threshold in the shading factor for activating it. - The AC ohmic losses after the inverter (wiring, transfo) are now affected by the power factor (divided by cos(phi)**2). Version 6.26 with respect to 6.25 There were 3 modifications affecting the yield in this version: - The interpretation of the muPmpp value as specified by the manufacturer: see Temperature behavior of the one-diode model . This induces a slight diminution of the temperature loss (around 0.1 or 0.2%). - Concerning crystalline PV modules of the database for which the Rs is not specified by the manufacturer: the default Rs value was previously based on a fixed Gamma value of 1.10. It is now fixed by asking a relative efficiency of -3% at 200 W/m2. This induces a decrease of irradiance losses. - The metal resistivity (copper and alu) have been slightly revised (unperceptibly diminishes the wiring losses). Version 6.09 The shading factor for diffuse now calculated for different tracking positions. => Slight modification with normal systems with shades, but addition of a significant shading loss for Backtracking strategy (see How is calculated the shading loss on diffuse with tracking systems ? Version 6 by respect to V 5 The differences are listed on another post.

Yes, it is the desired behavior. If you have several variants to save, they will obviously create new VCi files. We judged that overwriting the original file would be confusing. However if you "resimulate" a set of old variants (new feature in V 6.26), you have the opportunity of choosing whether you want to create new files or overwrite the existing ones.

It should not recalculate the table if this has already been calculated and nothing has been changed in the shading scene. I will check in the program.

Yes, the simulation with the usual parameters is valid for the first year. In you want to take a degradation into account you can specify it in the parameter "Module Quality Loss". NB: You can specify a degradation rate in the "Financial Balance" tool, which deals with the financial balance over several years, after defining the economical evaluation (inverstment).

In the Hidden parameters, topic "Miscellaneous", you can adjust this limit.