Pvsyst Tracker Simulation

[Ezilda Newnam]

Inversion 7, you should always do separate variants for fixed and tracker.

We would like to publish version 8 during the first half of next year, but this is still uncertain, and we cannot yet give a more precise release window, unfortunately.

This is not possible currently (not possible to have trackers and fixed in the same simulation in version 7).

I would advise running both variants without grid limitation. You can define an output file ( _file.htm) for each of them.

Then combine the output powers for each simulation step. You can then apply a grid limitation in another tool, e.g. in excel.

JC: For a lab environment, the most important thing is to have a stable albedo. Gravel is a very stable surface and easy to control for vegetation. We have simulated higher albedos with various synthetic surfaces as well. A stable albedo allows us to fit models which can then be used to predict yields with varying albedos. It is best to think of albedo in the real world as a site-dependent time series, changing seasonally and also with precipitation and snow.

KM: As the albedo decreases, the relative advantage of 1MIP over 2MIP also decreases. The reason for this trend is that the optical advantage of the 1MIP to 2MIP pertains to the rear-side optics (for the tested configurations).

KL: Yes, when the system is not backtracking. But the view factor does not increase proportionally with the ground cover ratio (GCR), and so there is a crossover point in terms of economics.

KL: The bifaciality of a PV module is measured per IEC TS 60904-1-2. Basically, Standard Test Conditions peak power on backside / Standard Test Conditions peak power on frontside. In modeling, you do not use different efficiencies for the front, back and integrated. The module efficiency stays constant, and it is the total effective irradiance that gets calculated. Total effective irradiance = Frontside Plane of Array irradiance + Bifaciality * Backside Plane of Array irradiance.

KL: The best practice would be to make an actual albedo measurement at the site over a year. A fallback solution would be to use albedo values as estimated by various satellite-based services, such as PVGIS.

KL: For a 2MIP setup with fixed array row spacing, having zero gap between the modules will give you more energy. A 2MIP configuration with a gap between the modules is just not an economically viable setup. You lose more energy from prolonged backtracking than any energy gain from reduced shading loss.

KL: It really depends on each case. Currently, the biggest sources of uncertainty are (1) module characteristics and (2) albedo. If these are known, the accuracy of the energy calculated by PVsyst for a given weather data can be well below 0.5%.

KL: It depends on the module bifaciality. Widely available bifacial modules are 65% bifacial, and so the back side will produce about 35% less for the same irradiance. There are modules that are more than 70% bifacial (LG n-type, for example).

KL: The testing did not include 2MIP trackers. Our approach was to gain confidence in 3D ray tracing and 2D view factor modeling (PVsyst) via testing on our 1MIP tracker, and then extend the ray tracing and modeling to 2MIP cases.

KL: East-west gap over the torque tube on 2MIP tracker actually reduces the energy production, because you backtrack more and longer. The best strategy for 2MIP tracker is to not have any gap.

JC: This is a complicated question. We did not measure small area backside irradiance in this project. We used pyranometers. Small area backside irradiance is probably best handled through modeling.

KL: The mismatch loss factors that we determined from the ray tracing and SPICE model are smaller than what has typically been suggested for PVSyst, partly because the ground-reflected rays come in at high angles, creating soft shadows.

KL: Wiring management is important, and in the CFV testing, a lot of care was taken to make sure that the wiring shading was not a significant factor. Offering a utility-scale solution is something we plan to do in near future.

KM: Yes. The incident spectrum from the sun was estimated using published spectral models, the spectral albedo of the ground (and torque-tube) was taken from a NASA database, and the spectral response of the modules was determined from ray tracing and device simulation.

KL: The bifaciality of an n-PERT module is in the 70-75% range. The bifaciality of a p-PERC module is in the 65-70% range. The n-PERT module we tested had higher series resistance and lower temperature coefficient too, which also helped the energy yield.

KL: Yes, we used a GCR of 35.1% for both 1P and 2P simulations. The height of the torque tubes was set to 1.6 m for 1MIP and 2.4 m for 2MIP (which are typical of commercial systems). Since the 2MIP configuration has two times the module length but not two times the height, it has a lower aspect ratio, and hence more light is lost to the sky.

KL: I recommend carrying out PVsyst + economics studies to determine the optimum DC:AC ratio. My first guess would be nominal monofacial DC:AC ratio minus the expected (unclipped) bifacial gain.

KL: PV Lighthouse found that the structural shading loss is not so different for 2MIP and 1MIP. We studied the no-gap case, because we found that a 2MIP configuration with a module gap suffered more losses from the prolonged backtracking than any energy gain from the reduced structural shading.

KL: There is no standardized method. We calculated the bifacial gain by first calculating the daily specific yields of bifacial arrays and a monofacial array and by looking at their ratios.

KL: The best practice would be to make an actual albedo measurement at the site over a year. A fallback solution would be to use albedo values as estimated by various satellite-based services such as PVGIS. NSRDB also offers albedo values.

KM: The results for the PVSyst inputs are on Slide 26 of the webinar and they are also given in the white paper. These results are specific to the particular site (in West Texas) and system configuration. PVSyst inputs for other sites and configurations can be determined with SunSolve.

PVcase Ground Mount tool has a new functionality allowing it to work with terrain-following trackers. Recently we organized a webinar on the feature hosted by Guy Atherton, PVcase Ground Mount Product Manager. Below we give you the answers to the most frequently asked webinar questions, that we hope will shed more light on how to optimally deploy our new feature.

PVsyst will interpret each of those sections as a separate orientation, so the number of orientations in your PVsyst simulation will be large. Moreover, our PVcase Yield tool takes orientations into account much more quickly and easily, so you should be fine in this case.

It depends on how you want to compare. We have the heatmap, which compares the existing and proposed surfaces graphically. We have the spot levels, which allow you to put actual points that tell you the difference between the two surfaces. We also have the export points, which export the .csv of the proposed terrain. So the comparison we have at the moment is limited to the heatmap and spot levels. You can also compare surfaces in the section view and front view in AutoCAD. Otherwise, you have to use another surface-based tool, such as AutoCAD Civil3d.

SunSolve Yield uses an optical raytracing framework that was first created by PV Lighthouse in 2015. Since then, it has been tested by many researchers at the leading PV companies and institutes. Combined, they have published over 100 academic papers that apply and validate their SunSolve simulations of PV cells, modules and systems.

For example, LONGi compared simulations and measurements of their world-record solar cell published in Nature Energy [1]. As evident in the figure below, they demonstrated that their simulated EQE (edge of blue area) agreed with their measurements (red symbols) at all wavelengths. This gave them high confidence in their SunSolve loss analysis.

As another example, FTC Solar and PV Lighthouse found that SunSolve Yield accurately represented the optics of a solar tracker [2, 3]. The graphs below plot the irradiance at the front and rear of a bifacial 1P tracking system on a sunny day. The blue symbols plot measurements from pyranometers installed at the axis of the torque tube and in the plane of array, and the orange symbols plot the irradiance calculated at the same location by ray tracing.

As demonstrated by the graphs (and by the low MBE and CRMSE), there was very close agreement between simulation and experiment. In addition to predicting the general rise and fall of the irradiance, the ray tracing matched second-order effects caused by two complications: (i) a conservative backtracking algorithm that allowed light to fall between rows early and late in the day, and (ii) light reflecting from the front side of neighbouring modules onto the rear POA detector.

SunSolve Yield contains the option to include advanced thermal models that extend the standard Faiman model used in most software. These models were evaluated in separate collaborative studies with FTC Solar and 5B. The advanced models were found to reduce the error in the simulated module temperature by a factor of 2 or 3.

For example, the figure below plots the predicted and measured module temperature Tm from a study with FTC Solar on single-axis trackers [3]. The results for Model 5 illustrate the best fit that can be achieved with the standard Faiman model; it amounts to an uncertainty in the predicted Tm of 6.6 C. The various advanced thermal models greatly reduce that uncertainty; the uncertainty in Tm of the most advanced model was just 2.8 C. (The quoted error represents the 95% confidence interval.)

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