Use and validation of PVSYST, a user-friendly software for PV-system design

Proceedings Chapter

Reference

Use and validation of PVSYST, a user-friendly software for PV-system design

MERMOUD, André

Abstract

PVSYST is a PC software package for simulation, sizing and data analysis of complete PV-systems, with a user-friendly graphic interface under Windows. It is oriented towards engineers, architects and researchers. With its numerous graphical outputs, immediately available on screen, printer or in documents, it is also a powerful tool for education. PVSYST 1.0 simulates grid-connected systems in great detail (stand-alone and other systems are foreseen in near future). It includes large libraries of meteorological and geographical data, as well as commercially available components. An important feature of PVSYST is the detailed treatment of near shading, with a CAO tool to construct the 3D representation of the PV system environment. Other graphical tools are proposed for visualising solar geometry, PV panel or field behaviour under partial shading, mismatch effects, planes of different orientations, and quick meteo yields on tilted planes. In version 2.0, a new part will allow importation of measured data and perform detailed comparisons with simulated values, which will be a powerful instrument for data analysis of real [...]

MERMOUD, André. Use and validation of PVSYST, a user-friendly software for PV-system design. In: Freiesleben, Werner. Thirteenth European Photovoltaic Solar Energy Conference . Bedford : H.S. Stephens, 1995.

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13th European Photovoltaic Solar Energy Conference, Nice, 23 - 27 October 1995

Use and Validation of PVSYST,

a user-friendly software for PV-system design.

Dr. Andre M

Group of Applied Physics and

Centre Universitaire d'Etude de Problemes de l'Energie University of Geneva

4, eh. de Conches, CH 1231 Conches (Geneva) - Switzerland Tel. (+41) 22 789 13 11 - FAX (+41) 22 347 86 49.

PVSYST is a PC software package for simulation, sizing and data analysis of complete PY-systems, with a user- friendly graphic interface under Windows. It is oriented towards engineers, architects and researchers. With its numerous graphical outputs, immediately available on screen, printer or in documents, it is also a powerful tool for education. PYSYST 1.0 simulates grid-connected systems in great detail (stand-alone and other systems are foreseen in near future). It includes large libraries of meteorological and geographical data, as well as commercially available components. An important feature of PYSYST is the detailed treatment of near shading, with a CAO tool to construct the 30 representation of the PY system environment. Other graphical tools are proposed for visualising solar geometry, PY panel or field behaviour under partial shading, mismatch effects, planes of different orientations, and quick meteo yields on tilted planes. In version 2.0, a new part will allow importation of measured data and perform detailed comparisons with simulated values, which will be a powerful instrument for data analysis of real systems. This feature has been used to carry out validations of the simulation with seven well-measured systems, which gave very good results.

1. - Objectives and approach

The purpose of this software is to provide solar engineers, researchers and architects a versatile, didactic and easy-to-use tool for sizing, understanding and analysing PY-systems. But PYSYST is more than a simple simulation software; it proposes various pedagogic graphical tools to help feeling complex behaviours like solar geometry, clear day radiation, components and PY-fields under perturbing conditions (cell shading, mismatch, hot-spot, etc.), partial shadings of near objects.

Moreover it includes all meteorological and component databases necessary to simulations.

We first present the major features of the basic version PVSYST 1.0, which is distributed since the beginning of 1995.

Then we will study validations of the simulation process, which were performed thanks to long-term measured data of seven Swiss installations. Special developments were performed in the software to allow detailed comparisons between measured and simulated values at several stages of the system. These extensions will make of PVSYST V2.0 a powerful instrument for analysing measured data.

The development of PYSYST was supported by the Swiss Federal Office for Energy (OFEN/BEW).

2. - Description of PVSYST, Vl.O

PYSYST Yl.O runs on IBM-PC compatibles, with a graphic user-interface under Windows (3.0 or upper). A user's manual and some contextual help windows guide the user in the dialogues and parameters interpretation. This version is written in French only, and restricted to grid-connected PY systems. It has been described in details in ref. [I] and [2].

The software is made up of four main parts: meteorological data processing and library, component definitions, PY-system simulations and general solar tools.

2.1. - Meleo data

Simulations are based on hourly meteorological data, which may be supplied under a great variety of ASCII formats, thanks to a flexible programmable format interpreter. However, if hourly data are not available, PYSYST is equipped with a synthetic hourly data generator from monthly average of global horizontal radiation and temperature. For the analysis of real system data, the use of global insolation measured in the collector plane is also possible.

The package includes DRY data files for 15 locations in Switzerland, and about hundred European geographical locations with climatic data (global horiz. and temperature) in monthly values. Insertion of new locations is straightforward. Quick computations of meteo data may be obtained on a monthly basis, taking horizon, tilted plane, shed shadings or incidence angle modifier into account.

2.2. - Components

Up to now, components necessary to grid-connected systems are restricted to PY panels and inverters. PY-panels are modelled thanks to the one-diode model, which is established with the basic parameters available from manufacturer or own measurements (Isc, Yoc, Impp, Ympp at any reference conditions Gref and Tref, current temperature coefficient and shunt resistance [3]). A set of graphics shows the resultant behaviour of the I/V model with respect to incident irrndiation, cell temperature, serie and shunt resistances.

Inverters are characterised by some basic parameters (power and voltage limits, cut or limiting behaviour in case of overload, MPP or fixed voltage operation), and an efficiency curve given by two to ten points. The package includes a library of commercially available components: about hundred PV- panels, and data (from manufacturer or measured by independent institutes, [4],[5]) of more than 15 inverters in the range I to 100 kW.

2.3. - Simulation and shadings

The simulation process of PYSYST treats in detail the main perturbing effects identified during the careful measurement of real systems (ref. [J]): far shadings (horizon limit), near shadings (partial shadings of neighbouring objects), incidence angle modifier, thermal behaviour of PY-panels, wiring resistance (with a computing sheet for estimating it from sections and lengths of wires), panel mismatches of the PY-field (with a built- in graphical tools for establishing the involved parameter), dynamic models of the system components. The PY-field may be fixed, seasonally adjustable, tracking one or two axes. The load may be defined in a versatile way, taking monthly, weekly, or power-probability distributions user's energy needs into account.

Results are provided in on-line tables and graphics for a choice among more than 60 variables involved in the simulation, in monthly, daily or hourly values. As in all dialogues and displays in PYSYST, energy and irradiation units may be chosen by the user al any time.

An important and original feature of PYSYST is the detailed treatment of near shadings, with a simplified CAO built- in tool to construct a JO-representation of the PY system environment. Shadows may be visualised for any time of the year or sun position. A table of the loss shading factor is established according to sun heights and azimuths. An iso-shading diagram superposes the seasonal shading effects on the sun trajectories plot, giving a synthetic sight of the seasonal and time-of-day shading perturbations. The energetic impact is then evaluated through the simulation, distinguishing between beam and diffuse irradiation components.

2.4. - General solar tools

Other useful tools are provided for engineers: graphics and tables of over 40 solar parameters at any place in the world (solar geometry on horizontal, tilted or tracking planes, clear day model, Incidence Angle Modifier effects, shed mutual shadings, etc.); graphics of the electrical behaviour of PY-panels (or fields) with reversed polarisation, dispersion of the cells/panels parameters, partial shading on one cell in a panel or a field, "hot spot" phenomena and by-pass protection diode analysis.

2 - The next version PVSYST 2.0

This new issue, which should be available at the end of 1995, will include some additional features: a data analysis module to perform direct comparisons of measured data with simulation, the treatment of other types of systems - especially stand-alone with batteries -, improvements of the shading calculations taking module cabling into account, and a lot of other minor contributions.

The interface and user's manual will be translated into English, and perhaps in German.

We describe here the measured data analysis and comparisons tool, which has been developed and perfected in the frame of our detailed validation program with experimental data of several different installations.

2.1. - Import of measured data

Data analysis involves the reading of data files, which should be performed easily for any format, as long as one ASCII line contains one time record. The measurement rate may be in hourly or sub-hourly time steps. The user first decides which data are to be read (among parameters usually recorded on such measurements: horizontal and/or plane global radiations, ambient and panels temperatures, electrical power/currents at the PY- field and the output of the inverter, and so on). If the solarimeter is placed in the collector plane, the plane irradiation is converted into horizontal corresponding irradiations (global and diffuse) suited for the input to the simulation model.

Then the user has to define general formatting conditions (separator, date/hour representation, order of each parameter field, units, etc.). Automatic sequencing possibilities are provided

for reading daily or monthly files according to their name. Sub- hourly data are accumulated and averaged in hourly values. A new internal hourly data file is produced, which may still be mnnipulntcd (transfer of one or several parameters from a file to another, merging, cutting of data).

The contents of the new internal data file may be visualised and checked on tables and graphics in hourly, daily or monthly values. Graphics of time evolution, histograms, ordered values, as well as specific graphics like normalised system performances (performance ratio PR, Yr, Ya and Losses), constitute a complete set of measured data presentation, avoiding the use of other heavy softwares.

Measured inverter efficiency may be displayed and directly compa1ecJ to a pn:cJi:fini;cJ component curve, showing the actual inverter behaviour by respect to technical data sheet. At this stage, modifying the existing component, the user may build-up a new profile corresponding exactly lo his specific device.

Mesure de l'etticacile IN/OUT onduleur

Onduleu' SOLAR MAX JO, don"h• l•bncenl Eueur eu1 etr\c.ecll.6 MO'J •1DS7 el Slgma-11 04%

l(IO Eueur sur EOulOnd Moy.•181 6 el Sigm.-13 38 W

1.-.cro

Puinenu d'cnltCe (E!chemp) IW) 15000

Fig 1. -Measured and parametrised inverter efficiencies (parametrisation according to manufacturer) 2.2. -Comparisons with simulation

These data are then included in a simulation process, and comparisons between measured data and simulated results may be asked for at any stage of the PV system, i.e. for any measured parameter. Accumulations may be submitted to specific conditions (for example chosen hour-of-day cuts, or limits on diffuse to global ratio for selecting strong direct or pure diffuse data sets, and so on).

Several comparison graph types are then displayed for hourly, daily or monthly measured and simulated values: direct comparisons, differences or ratios as a function of time-of-year or time-of-day. These allow to immediately check the general behaviour of the system, by respect to the simulation parameters, as well as to identify the deviant measurements. The exact date of any displayed data point may be shown by simple clicking on it with the mouse.

On the comparison graphs, the user may eliminate undesirable data (for example failures of the system, snow on collectors, etc.), directly by mouse or by defining limit criteria on simulated, measured, differences, or ratios of data points.

Statistical estimators are applied for remaining good data, to characterise the quality of the system in good running conditions:

the MBE (mean bias error, that is the average difference between measured and simulated values) and the RMSE (root mean square error, the standard deviation of these differences).

The eliminations may be transferred (marked) into the original data file, in order to be taken into account in subsequent simulation/accumulation processes.

The comparison plots of PVSYST are a very sensitive tool for the supervision of system running. System parameters may be checked with great precision, and misrunning conditions are quickly identified. As an example, fig 2 shows that minor perturbations on the system could be immediately detected and identified during the data acquisition.

Field Energy OutpUt, Daily Values

To11111 :348 po1"h, Oetly ewrege 10 OU5'11 kWliljour Meas • Mode.I Error : ewcir --5 4, rm•-1'1197 %

.

O~L._Jo_n_._F_e_b.._M_e_•.__~-,-'-M-o-y.._Ju-ne_.__J~--'-A-u-g.._S_op_.__Oo-1.._Now __ ,__O_eo~

Yur 19941

Fig 2. -Example or defect: blowing of one or two fuses in collector branches (among 5 branches).

3. - Validations

Detailed validations of the complex simulation process imply careful comparisons between well-measured data and simulation at different stages of the system, in order to assess the validity of the main algorithms used: solar radiation models (diffuse component estimation, transposition in collector plane, shadings), field electrical behaviour with all perturbations, field temperature, inverter conversion. These analyses should be performed in hourly values to identify all deviations, even at the limit running conditions.

For this purpose we disposed of measured data of seven Swiss installations, ranging from 0.4 to 100 kWc (about one year each). But a lot of them suffer of different measurement or running problems, making very difficult to distinguish between data acquisition inaccuracies and model deviations.

The main conclusions of our checks are summarised on table 1. First of all, the irradiation models (Liu and Jordan correlation for estimating the diffuse fraction, and Hay model for the transposition) are tested as almost all sites were equipped simultaneously with horizontal and plane irradiation measurements. Results are in good agreement with other similar studies (ref [7],[8]) for most sites, but EIV calibrations seem to be very bad. The more sophisticated model of Perez [8] was tried, too, but it did not lead to significantly better results in absence of well-measured diffuse component.

The cell temperature derives from a very simple thermal balance model, with a K loss-factor set-lo (20+6 • v,.ind) [W/m\:], measured on nude-modules in sheds (or 29 W/m¹k without wind measurements). This value should be reduced for rear-covered collectors. This model gives good results, quite sufficient for its use as input to the PV characteristics of the field. With c~stalline silicium, the temperature scnsivity of the field at MPP is about -0.8%/°C.

PV Field temperature

~ ⁴⁰

~

~

. ...

i:

"

"°

~

..

:i; •O

0 10 •o

Slmuletd VlllUH ('"CJ

Fig 3. - Field Temperature Modelisation

Fig 4 gives an example of the field energy output comparison plots for one of the best systems: it att~ts tha~ the field modelling is able 10 reproduce the system behaviour wnh a remarkable precision. Other systems show minor discrepancies, which could be attributed to different error sources: sensor calibrations (especially of pyranometers), panel parameters, inverter efliciency.

LESO-Sheds: Field Enerl1)' Output. Hourly Values

~

"l 2000

~

" _{~ ISCIO}

~ :i; 10:0

100J 1500 3500

Simu1•1ed .,.1ues IWJ

Fig 4. -Field output measurements-simulation comparison PV field energies are reproduced with a satisfactory MBE of some few percents; but the hourly RMSE results, particularly low on some systems (LESO-sheds, ElV), attest that the PV- field is well-suited to reproduce the PV behaviour in any external conditions.

The LESO-USSC amorphous field is a noticeable exception. Comparisons performed on pure diffuse and. s.trong beam selections of data show clearly the enhanced sens1V1ty of this technology to the diffuse radiation: MBE is 0.5 % for points above 70% of beam, but I 12 % (!) with more than 75% of diffuse. Fig Sb indicates that this should be considered as a spectral consequence, since even high direct points at l~w level lie very close to the reference line. Unfortunately, this effect cannot be taken into account in PYSYST, as we don't dispose of reliable models relating the spectral composition to the available meteo data (global irradiation).

Finally, the inverter calculation is closely related to the actual efficiency profile of the device. Besides the running limitations (power and voltage threshold or overloads), modelling is straightforward and discrepancies should not be attributed to simulation inaccuracies. This is the reason why we used adjusted inverter efficiency curves in this test, u~ing the analysing tool mentioned above to match the measured inverter behaviour. This involves that simulation performances at global AC output are quite similar to the field DC ones.

4. - Conclusions

PVSYST is a new powerful tool for PY·e.ngineeri_ng a~d training. Models used have been evaluated m detail w11h experimental data of several systems, and seem. to reproduce their effective behaviour very well. But the .aim of such a software is not only to predict absolute energy yields of systc~s, which are subject to high uncertainties due to meteo~olo~cal conditions and component parameters i~determinat1ons.

Nowadays, especially with the bu_ilding integrauon, PY-systems are built in situations which deviate more and mo~e .from the optimum; so that the interest of such a tool, by quantify.mg every perturbing effect independently, allows to evaluate the impact of different construction choices.

Installation N 13 Marzili LESO-Sheds SIG EIV LESO-LRE LESO-USSC

Site (Switzerland) Domal-Ems Berne Lausanne Genc\'c Sion Liusannc Lausanne

•'ield: type Anti-noise-wall Sheds Sheds Sheds Sheds Facade Demosile

Tilt angle 45 ° 35 ° 45 ° 35 ° 45 ° 90° 28°

Azimulh 25 ° East 37 ° Easl South 9

°

Installed power I04 kWc 25.6 kWc 12kWc 7.6kWc 3.2 kWc 3 kWc 0.45 kWc

Field area 967 m² 170 m² 111.6 m² 61.5 m² 31.7 m² 28.6 m² 8.2 m²

Collectors: nmnur. Kyocera BP Solar Solarex Arce-Solar Photowatt Flagsol USSC (USA)

Type LA36l J48 BP495-Saturn MSX60 M55 BPX 4751Kl Oplisol/LESO

Nominal Power STC 48Wc 95 We 60Wc 51 We 48\Vc 250\Vc

Measured Power STC 48Wc 88 We 55.1 We 50.8 We 40.3 We 17 We

Measurements lspra lspra TISO lspra TISO Manufacturer LESO

Technolol!v SI-ooh• SI-mono SI-oolv SI-mono SI-ooh· Sl-p0lv a-Si:H tandem

Irrad. transposition:

MBE (lranspos.-meas.) 2.8% -0.9% -6.0% -2.2% 9.3 % -11.3 %

RMSE {daily val.) 5.5% 3.2% 9.6% 2.9% 5.5% 7.7%

_ItMSE (hourlr val.~ 11.7% 7.8% 15% 5.1% 10.4 % Jl.4 %

Coll. Tem11erature model

Wind velocity measuremen No No No Yes No No No

J< factor (input param) 29 W/m²K 29 W/m²K 29W/m²K 20 + 6 v, ... 29 W/m²K BW/m²K 23 W/m²K

MBE {simul-mcasure) -0.J

•c

·c

•c

·c

•c

•c o.o•c

RMSE (hourlv val.) 2.1

•c

•c

·c

•c

•c

PV-Field DC energy (field #3)

Simul. Base: Plane irrad. Plane irrad. Plane irrad. Horiz. irrad. Plane irrad. Horiz. lrrad. Plane irrad

MBE (simul-meas.) 5.6% 1.0% -0.7% 0.7% 3.1 % 1.4 % -13.6%

RMSE {daily values) 8.7% I0.0% 2.2% 5.0% 3.4 % 10.8% 8.9%

RMSE lhourlv values) 11.0% 15.5% 5.2% 9.8% 6.5% 17.7% 13.4 %

Global system AC output

MBE (simul-meas.) 5.5% 1.0% -0.7% 1.9% 2.8% 2.7% -12.8%

Rl\1SE (monthly values) 5.4% 4.5% 1.1% 1.7% 1.2% 8.2% 7.5%

RMSE (daily values) 8.3% 9.9% 2.4% 5.3% 3.3 % 12.7% 8.5%

RMSE (hourlv values) 10.8% 15.3% 5.5% 9.7% 6.5% 19.0% 12.5%

Table I. - Summary or the seven installation: comparisons simulation-measurements.

(A positive MBE indicates that the simulation overestimates the measurements).

0

USSC, Fiold Output Energy, beam/global< 0.25

Total 1493 points, Hourly 1wt1r•;e 2J 38 W Meu •Model Enor ; iwer, •22.89, rms • 10 94 W Me1111 • Model Error : ~r. •112 JI, rm1 - 53,67 %

so 100 150 300 350 450

USSC, rield Output Energy, beam/global• 0.7

Tol11I 792 po1nl11, Hourly •ver1111e 296 021 W

<IO Men • ~odel Error : ever •I 36, rms • 19 701 W

Mou • Mod11I Error : llWr, -0 47. rm& • 6 66 %

IOC

.. . ^.

S1mu1eted values fW]

3ilo

Fig 5. - Comparisons for the LESO-USSC amorphous field, for pure diffuse and high beam selections.

Rererences

[ l] A. Merrnoud. PVSYST: A user-friendly software fo PV- systcms simulation.

I 2th European Photovoltaic Solar Energy Conference, Amsterdam, 11-15 April 1994.

[2) A. Mcrrnoud. PVSYST: logiciel pour systemes PV COMPLES - Actes du colloque international, 29-30 sept 1994, Perplgnan.

[3] T.U. Townsend, S.A. Klein, W.-A. Beckman, Solar Energy Laboratory, Univ. of Wisconsin.

Simplified Performance Modcling of Direct-Coupled Photovoltaic Systems.

Solar '89 Conference, Denver-Colorado, 19-22 june 1989.

[4) H. Habcrlin, H.R. Rothlisberger; Vcrgleichsmessungcn an Photovoltaik-Wcchsclrichtem. Schlussbericht des BEW- Projectes EF-REN(89) 045,

OFEN/BEW. 3003. Bern. 1993.

. .. .

·

[5] J.M. Servant, J.C. Aguillon: Essais d'ondulcurs PV raccordes au reseau EdF.

GENEC-Cadarache, 1993.

[6] P. Schaub, 0. Guisan, A. Mem1oud. Evaluation of the different losses Involved in Two Photovoltaics Systems

2th European Photovoltaic Solar Energy Conference, Amsterdam, 11-15 April 1994.

[7] R.R. Perez, P. Incichcn, E.L. Maxwell, R.D. Seals, A. Zelenka Dynamic Global to Direct Jrradiance Conversion Models.

ASHRAE Transactions, Vo! 98, Part I, #3578, 1992.

[8] R. Perez, P.lneichen, R. Seals, J. Michalsky, R. Stewart.

Modcling Daylight Availability and lrradiance Component from Direct and Global lrradiance. Solar Energy 44, no 5, pp 271- 289, 1990.