Michele Oliosi Posted 10 hours ago Posted 10 hours ago Sub-hourly simulation Release note: Sub-hourly Simulation for Grid-Connected and Standalone systems The main feature of PVsyst 8.1 is the possibility to run sub-hourly simulations. It is therefore now possible to model a PV system with more accuracy. Several physical models have been adapted to this new time scale, as detailed in the next section. The granularity at which it is possible to simulate a project is directly that of the weather data file (.MET). To support this feature, it is therefore also possible in PVsyst 8.1 to import sub-hourly weather time series to generate sub-hourly weather data files. At this time, sub-hourly files can either be created by using the custom weather import or using the Meteonorm 9.0 DLL and choosing the `minute` option. All weather data files, independently of their time step, can also be used to run hourly simulations. A radio button, present both in the main project window as well as in the "Advanced simulation" window, allows you to easily switch from one granularity to the other. Sub-hourly simulations will correspondingly generate sub-hourly time series tables when using the "Output file" functionality. Hourly simulations retain the advantage of being much faster, since fewer steps need to be simulated. It is therefore recommended to run hourly simulations during iterative design phases. Sub-hourly simulations, which take longer to run, can be left for more final evaluations. Generally, small differences may appear between hourly and sub-hourly simulations. This is inevitable, since several physical models are nonlinear and are therefore sensitive to averaging in time. For the most part, sub-hourly models lead to small changes that can be considered small gains in accuracy. However, some models have required more work to ensure the accuracy of the simulation at all time scales (see section below) despite more important differences between hourly and sub-hourly calculations. Note also that the largest discrepancy, i.e., clipping effects, is still addressed in hourly simulations (since version 8.0) with underlying sub-hourly weather data with a dedicated correction. Improved physical models With the new possibility to run sub-hourly simulations, several models had to be adapted. The three main changes concern models that did not preserve sufficient accuracy when applied at sub-hourly scales. Release notes: Thermal model: PV module temperature is now computed using a transient thermal model for both hourly and sub-hourly simulations Weather data, custom file: new model to estimate diffuse horizontal irradiance from global horizontal irradiance. Dedicated coefficients for sub-hourly simulation are used in order to reduce the discrepancies with hourly simulations Transposition model: sub-hourly horizontal diffuse irradiance data with the Perez model now uses coefficients adapted to each time step. These adapted coefficients aim at reducing the discrepancies between sub-hourly and hourly simulations Each of these changes is detailed here below. Transposition model The default model used in PVsyst, the Perez model, relies on several weather data indicators (sky brightness, sky clearness, sun zenith angle), associated with a set of empirical coefficients, to decompose the horizontal diffuse irradiance into components that are to be transposed with different geometric rules. This decomposition leads to effective discrepancies when comparing corresponding hourly and sub-hourly time series. When studying an ensemble of sites across the globe, this granular discrepancy translates to a systematic discrepancy, averaging half a percent in terms of PV system yield. Since the empirical coefficients used in PVsyst (and in several other simulation software) have been fitted with hourly and quarter-hourly data, the sub-hourly results of the Perez diffuse decomposition using these coefficients can be considered biased. For this reason, a new set of coefficients, which corrects the bias observed over 113 sites worldwide, is used by PVsyst when running sub-hourly simulation with the Perez model. Note that the direct irradiance transposition also leads to discrepancies between hourly and sub-hourly simulations. While the exact cause is not yet understood, since the transformation is purely geometric, the sub-hourly simulation can be considered more accurate in this regard. Decomposition model When importing custom weather-data with only global horizontal or only plane-of-array irradiance, PVsyst must estimate the diffuse horizontal irradiance. This was done until now using Erbs model. This model, based on a single clearness indicator, is considered to have sufficient accuracy for hourly simulation. However, at shorter timescales, it fails to capture fast phenomena such as cloud enhancement and results in a systematic discrepancy compared to real measurements. To reduce this discrepancy, we selected a more flexible model from the literature, ENGERER2, whose additional indicators (clear-sky clearness, cloud-enhancement index, ...) allow one to represent sub-hourly dynamics more accurately. Similar to the adaptation of the Transposition model, we refitted the model coefficients to reduce the bias observed on the previously mentioned 113 sites dataset. Compared to Erbs, this new computation successfully mitigates the bias for all sub-hourly timescales. It also mitigates a small residual bias of the previous model at the hourly timescale. This small difference will slightly affect all irradiance loss components (IAM loss, shading losses, ...) affected by the decomposition of irradiance into components. Transient Thermal model In PVsyst 8.1, two layers of thermal models are used for the PV module: First, as in previous version, the thermal balance model outputs the steady-state temperature, represented by the PVsyst variable TArrSS. Second, a thermal inertia model uses the previously calculated steady-state temperatures to compute the actual transient temperature, represented by the PVsyst variable TArray. The user can modify the heat transfer coefficients and the mass of the module (affecting the thermal inertia) in the detailed losses and .PAN file menus, respectively. For hourly simulations of conventional PV systems, thermal inertia affects the production E_Grid by about ~0.1-0.3% annually. In particular settings, it can have a stronger impact. For example, if an inverter is designed to operate close to its over-voltage limit, production can be affected by 1-3 % annually, as the temperature change induced by thermal inertia mainly impacts the PV array voltage.
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