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Limitations in PVsyst Simulation Encountered while Evaluation of PV Energy Prediction

Satish Pandey & Rakshita Mhatre

Mahindra Susten Pvt. Ltd, AFL House, Marol Maroshi Road, Andheri, Mumbai-400019, India 27thDecember 2017

Abstract

The main scope of this article is to highlight the various limitations encountered while using the renowned PV energy predication software used for simulation in the solar Industry. PVsyst is a PC software package used generally for the study, sizing, simulation and data analysis of complete PV systems. While practically using this software we encountered some limitations which could alter the predictions and thus affect the design of our system at an early stage & project viability / bankability. The paper looks at providing the Industry at large some opportunity areas to further improve the existing product or look at developing a new product in the space to overcome the stated limitations.

Keywords:

1. Tarry Calculations 6.Customization of IAM
2.Pitch in Backtracking 7. Variation in POA from Meteo file imported.
3.Language Barrier 8. Issue encountered entering DC Ohmic loss
4.Nullifying temperature coefficient 9. Incorrect GHI/POA gain for tracker.
5.Utilization of PVsyst 10. Time taken for Simulation.

1. Introduction

As per Bloomberg New Energy Finance, Renewable energy sources are set to represent almost three quarters of the $10.2 trillion the world will invest in new power generating technology until 2040, thanks to rapidly falling costs for solar and wind power, and Solar taking $2.8 trillion and seeing a 14-fold jump in capacity.
[Reference: https://about.bnef.com/blog/global-wind-solar-costs-fall-even-faster-coal-fades-even-china-india/]

PVsyst is a PC software package used globally for the study, sizing, simulation and data analysis of complete solar PV System. A lot of financial decisions are taken basis the simulation report which is given as an output by the software.

While using this software, we came across certain limitations which if not overcome can alter the prediction given by software. The limitations are encountered across parameters mentioned below.

In this article we are trying to highlight some of the limitations which we came across during the use. As the software matures and the usage increases, we will publish more papers detailing the same.

2. Limitations Observed

2.1 TArray Calculations.

Limitation Observed: According to PVsyst, TArray is the average module temperature during operation but it is not the actual value according to our cross calculations.

Problem: This creates problem during analysis while comparison of actual vs predicted irradiance weighted Module surface temperature.

Solution Proposed: The actual PVsyst Predicted module temperature can be calculated by taking the sum TArray * EArrNom for each hour, and then divide by the sum of all EArrNom.

Recommended Action: PVsyst Reports should consider Irradiation Weighted module temperature as default column in ‘Balance and Main result Table ‘of final generated report.

2.2 Pitch in Backtracking.

Limitation Observed: According to PVsyst, the pitch and collector width are indeed basic parameters and are less fundamental for non-backtracking tracker systems so they do not mention them on the final generated report.

Problem: This creates a problem while reviewing the report as we cannot be sure of the pitch considered for non-backtracking tracker systems.

Solution Proposed: These parameters should be mentioned in the report. To be considered in the next version of PVsyst.

2.3 Improper language for error message.

Limitation Observed: Improper language for error message detected with updated version.

Problem: This creates problem during accessing the software.

Solution Proposed:-They have fixed the error that we have reported. The correction will be available in next PVsyst release (v6.6.8).

2.4 Nullifying the temperature coefficient in module .PAN file.

Limitation Observed: No option to nullify the temperature coefficient in module .PAN file.

Problem: This creates problem if we try to analyze the performance of plant by nullifying the temperature losses

Solution Proposed: This is not possible directly, however we have the opportunity of explicitly specifying the Array temperature in our Meteo data, and use this value during the simulation. We can specify a value of 25°C for each hour of the year.

For this we have to export hourly Meteo data on a CSV file, add a column for the TArray, and reimport these data using "Databases > Import ASCII Meteo data".

2.5 Calculate the utilization of PVsyst.

Limitation Observed: To calculate the utilization of PVsyst by the License holders per month.

Problem: We cannot monitor the utilization of purchased software by authenticated License holders.

Solution Proposed: There is no way of doing such statistics with PVsyst. There should indeed be a methodology for the same. We request the feature to be considered for the next version.

2.6 Customization of IAM in .PAN file

Limitation Observed: Regarding Customization of IAM in .PAN file of Module.

Problem: This limits in assuming losses as per user requirement and also does not give the predictions according to User Input.

Solution Proposed: There are indeed problems with IAM in the version 6.66, the IAM was not read in the internal database. This was the main motivation for creating a new version 6.67, where it is corrected. However in the version 6.67, the IAM is now not well read in the *.PAN files of our personal database. Therefore if we want to perform simulations with our own PAN files with customized IAM, we have to revert to the version 6.66.

2.7 Variation in POA from Meteo file imported.

Limitation Observed: Importing the ASCII Meteo file, the value of GlobInc (POA) gives always 1% higher value e.g. if the actual sum is 1988.63 however when we import the ASCII file the POA value comes to be 2007.3

Problem: This leads problem in giving the predictions according to User requirements/Inputs also it affects the gain of GHI over POA which is over predicted and thus can affect validation and financial modelling of the project.

Solution Proposed: Data is retro transposed according to hay model, whatever the selection in the preference menu. This behavior was associated with daily data.

We can observe it by:
- Selecting Hay as the model
- Importing our Meteo file
-Checking the transposition in Meteo Tables & Graph. We will thus get the correct sum.

This Correction has been brought and it will be available in the next V 6.68 release.

2.8 Issue encountered entering DC Ohmic loss.

Limitation Observed: Regarding issue encountered entering DC Ohmic loss.

Problem Created: For projects having two sub-arrays, while entering the value of DC Ohmic loss at STC we are getting incorrect values for global wiring resistance moreover If we enter value as 1.8% at STC in one subarray and apply it to all subarrays we get 151.1% at STC in the other sub array and thus wrong values for DC Ohmic losses in final generated report even when define some value in other losses and coming back to Ohmic losses the value of DC Ohmic loss is varying.

Solution Proposed: It is noticed that the dialogue for 'Ohmic losses' is sometimes mixing up the sub-arrays. In fact, when you enter the 'Ohmic losses' tab, the selection box shows sub-array#1, but in fact the last sub-array is being displayed. When we explicitly select the sub-array before making any changes, the correct sub-array is displayed. This behavior is fixed for the next PVsyst version 6.68

2.9 Incorrect GHI/POA gain for tracker.

Limitation Observed: In case of SAT, actual POA gain over GHI is not equal to predicted by PVsyst. It is observed that Perez model always over predicts SAT POA gain over GHI by 3% -5 %( absolute) w.r.t actual.

Problem: This impacts validation of performance and can affect the financial modelling of the project

Recommended Action: Diffuse and direct component calculation algorithms should be better predicted.

Conclusion

It is observed that various limitations which we came across can impact the predictions or limit our analysis. Similarly while performing simulations for projects of higher DC capacity like 55-60MWp takes around 20 minutes for the simulation to run which reduces the operational efficiency of the teams. These are just a few improvements steps which we wanted to highlight to contribute towards a better and more Solar world. we will look forward for collaborators and other publishers as well to contribute with their findings with reference to this paper to make PVSyst simulations more accurate and closer to reality. Since, generation or performance ratio (PR) Predicted by PVsyst play most prominent role in Solar Industry globally as the financial viability of projects is based on them and such limitations can alter/impact the analysis of actual plant performance benchmarking and financial modeling of project.