COMPARISION OF SOME STANDARD PV SIMULATION PROGRAMS WITH THE SIMULATION SYSTEM INSEL

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COMPARISION OF SOME STANDARD PV SIMULATION PROGRAMS WITH THE SIMULATION SYSTEM INSEL H.G.Beyer, J.Luther, J.Schumacher-Gröhn Renewable Energy Group, Dept. of Physics, University of Oldenburg D-2900 Oldenburg FRG, P.O. Box 2505, tel.++49-441-798-3544, fax.++49-441-798-3201

ABSTRACT We report on a comparison of four simulation programs: PVF-CHART, PVFORM, SOMES and a specific model of INSEL. The performance figures of a standard system (PV, power conditioner, battery, load) were calculated for two climatic regions. The differences in program outputs were analysed using the block diagram oriented simulation system INSEL. KEYWORDS simulation system, PV system, PVF-CHART, PVFORM, SOMES, INSEL INTRODUCTION Highly flexible simulation tools for the analysis of the energetic behaviour of renewable energy systems have been used with thermal systems for a long time TRNSYS (Klein 1976) is a well known example. In general, there are at least three different categories of simulation programs concerning renewable energy systems: (i) programs that use statistical information to predict the longterm performance of a system - an example using the utilizability concept is PVF-CHART (Klein 1985); (ii) programs that calculate a sequence of states of a predefined system structure (allowing for several system options) in constant time steps - for example PVFORM (Menicucci 1988) and SOMES (Blok 1987); (iii) simulation systems, which give the user great flexibility in modeling different system structures - examples are TRNSYS (Klein 1976) and INSEL (Schumacher-Gröhn 1991). THE SIMULATION SYSTEM INSEL To solve a simulation task on a computer usually means to write source code in a programming language.* Among these are two important classes: (i) algorithmic languages such as FORTRAN, Pascal or C and (ii) simulation languages. The main difference between these languages is, that in case of (i) the user not only has to care for the simulation model, but also for several computer specific problems (the sequence of calculations for instance). In contrast to the algorithmic languages, simulation languages are descriptive, i.e.,these languages provide elements to formulate a problem without giving a concrete algorithmic solution. The sequence of statements is found automatically by the simulation languages compiler and sorting algorithm. Such a program system, together with its simulation language is called a simulation system.

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INSEL is a block diagram oriented simulation system for the interactive simulation of renewable electrical energy supply systems. Blocks, defined by input/output relations can be interconnected to form a model of the system under investigation. Writing an INSEL simulation program means to transform a grafic block diagram into INSELs simulation language, which mainly consists of S-instructions to describe the interconnections of the blocks and P-instructions to define the parameters - for an example see figure 3. When structure and parameter definitions are entered into an input file the model is compiled and executed by INSEL. INSEL is written in ANSI FORTRAN 77 and includes blocks for electrical system components such as photovoltaics, wind turbines, motor/generators, batteries, electrolysis and fuel cells, power conditioning units, load characteristics„etc. Furthermore, there are blocks for file handling, the generation of plots, blocks solving iteration processes, blocks generating synthetic meteorological data and blocks performing parameter variations. A SMALL PV/BATTERY/LOAD-SYSTEM As an application of INSEL, we used the flexible structure of the simulation system to analyse the differences in the results given by some PV simulation programs that were available to us. A typical renewable energy system that may be modeled with such standard programs consists of a PV array, a load, a battery and a power conditioning unit. Because of the modularity of INSEL,it is easy to reproduce the models of the other programs at points of interest. The layout of the simulated system is as follows: The mpp tracked PV array has 40 panels of type AEG PQ10/40, i.e. a nominal power of 1.6kWp, slope is 60°, south oriented. The mean load is assumed to be 100W, the daily load profile is taken from PVFORM. The battery consists of 12 VARTA bloc cells in series, each cell having a nominal capacity of 200 Ah equivalent to a maximum energy content of 4.8kWh. We have chosen two different sites for our analysis, namely Albuquerque, New Mexico (latitude 35.05°N) and Bremerhaven, Germany (53.53°N). The TMYs are used as meteorological data base. It can be seen from figure 1, that most of the radiation data from Bremerhaven are in the range below 200 W/m2, while the Albuquerque data show a relatively smooth distribution. Therefore, it can be expected that part load plays an important role for the system at Bremerhaven. As a typical result figure 2 shows the predicted monthly renewable fraction F of the programs PVF-CHART, PVFORM, SOMES and a corresponding INSEL model - see table 1 for details. Obviously, the main differences appear in months with low insolation, while the differences in summer are comparatively small. The programs calculate F = 1 in July and august with the exception of PVF-CHART. This is due to the fact, that PVF-CHART determines the renewable fraction from statistical correlations, which are based on the longterm behaviour of the system, while the calculation of F in the other three programs is based on a time step simulation using the hourly meteorological data of the TMY of Bremerhaven in this case. As figure 2 shows, the differences in F do not stem from the various models which convert the radiation data from horizontal to the tilted plane. Other possible reasons for the differences in F are (i) the calculation of the PV array output from radiation data, (ii) the programs internal assumptions on the inverter

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PVF-CHART uses as input data monthly averages of daily insolation H, ambient temperature ? a and ground reflectance p. The diffuse radiation Gd is calculated from a correlation by Erbs/Klein/Duffie and is assumed to be isotropic (Liu/Jordan-model). The PV array efficiency is given by a correlation that depends on the hourly insolation on the tilted plane, the ambient temperature Ta and the clearness index &T> while the charge/discharge efficiency is assumed to be constant. PVFORM expects hourly values of direct normal radiation Gbn? global radiation to the horizontal Gh, ambient temperature Ta and wind speed vw. The diffuse fraction of the radiation is treated according to the Perez model. A linear increase of PV efficiency with radiation is assumed for low radiation values, charge/discharge efficiencies of the battery depend on the state of charge Q. SOMES uses hourly values of global radiation, ambient temperature and wind speed. The diffuse fraction is calculated from a correlation by Orgill/Hollands and is assumed to be isotropic. Efficiencies of the battery depend on the power input/output P and the state of charge Q. The applied INSEL-model uses hourly values of global and diffuse radiation plus ambient temperature and wind speed - see also figure 3. The radiation conversion to the tilted plane is based on the Liu/Jordan-model in this case. The mpp of the PV array is calculated from the current/volt age characteristic of the cells using a two diode model. The simulation of the battery is done according to the models of Shepherd and Wood/Crutcher. c h a r a c t e r i s t i c s , ( i i i ) the battery model that i s used and (iv) the characteristics of the charge controller. Table 1 gives a summary of some assumptions made in the programs. We have used the INSEL-model shown in figure 3 to study the influence of the different default models of PV and the power conditioning equipment on the power output. "While PVF-CHART simply assumes a constant inverter e f f i c i e n c y , the e f f i c i e n c i e s of PVFORM and SOMES increase with input power. The predicted values of the dc power production for a mean December day at Bremerhaven are given in figure 4. For comparison, the mean PV array output as calculated by PVF-CHART i s given, too. As may be seen from figure 4, PVF-CHART overestimates the PV power output, especially at low insolation l e v e l s . In addition, PVF-CHART weighs the PV output with a constant inverter efficiency. This approximation does not seem to be f u l l y sufficient under climatic conditions where part load situations frequently occur. These two f a c t s mainly explain the differences in the F-values between PVF-CHART and the time step calculations in figure 2. Finally, figure 5 shows the renewable fraction as a function of the PV array s i z e for the two s i t e s under investigation. While the renewable fraction given by PVF-CHART and PVFORM for the Albuquerque system are nearly i d e n t i c a l , the differences between PVF-CHART and the other two programs are significant for the Bremerhaven system. In particular, t h i s i s true for high values of F. CONCLUSION When calculating integral figures of merit for standard PV systems, PVF-CHART and the time step simulation programs almost y i e l d the same result for favourable

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Fig. » 3. INSEL-model for the calculation of the power output of an mpp-tracked PV system including a dc/dc inverter. Block CLOCK provides the simulation time. The meteorological data Gh, Gdh, T& and Vy, are read from a file by block READ. Block GH2GT converts the horizontal radiation data to radiation on a tilted plane using the Liu/Jordan-model for example. Inputs of GH2GT are the radiation data, the array slope /?, the array azimuth 7, the ground reflectance p and a daylight saving time switch. The macro PVMPP calculates voltage and current in the maximum power point, which are multiplied by block MUL to give the array output power. Finally, different inverter efficiency curves can be used to calculate the power output PDCThe right part of the figure shows the model structure formulated in the simulation language of INSEL. An "S" standing for structure is followed by an arbitrarily chosen block number and the blocks name. Input connections are given by a corresponding block number and the output number of that block, separated by a period. For example, block 5 multiplies the 1 st and 2 n d outputs of block 4, namely PVMPP. Parameters are entered into the input file in a similar manner.

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Fig. 4: Power output of the PV generator/power conditioning subsystem for an average december day at Bremerhaven. The upper curve corresponds to the output of PV F-CHART assuming an inverter efficiency of 1.0. The other curves are calculated with the INSEL-model from figure 3: PV output power (—), PV output and dc/dc inverter from PVFORM (•••)» PV output and inverter from SOMES (—)• The power output curves of INSEL-model are shifted to the right with respect to the PV F-CHART curve. This is due to the fact that the time scale in German TMYs is central european time while PV F-CHART uses local time. For Bremerhaven this means a difference of 30 minutes, approximately. "Average" december day means for PV F-CHART the day at which the extraterrestrial radiation is closest to its monthly mean value. For the time step simulation with INSEL the hourly outputs were averaged over the month to yield a mean daily output pattern. Meteorological data: TMY Bremerhaven.

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Fig.. 5. Renewable fraction as a function of the P V area calculated with PVF-CHART ---, PVFORM • • • and SOMES . The two curves at the left are calculated for Albuquerque, the other three for the system at Bremerhaven. insolation conditions (e.g. Albuquerque). This is not the case for moderate radiation climates where PVF-CHART overestimates the renewable fraction of the system. It follows from analysis with the block diagram oriented simulation system INSEL that the main reasons for this characteristic of PVF-CHART are (i) the assumptions on the efficiency of the PV array at low insolation and (ii) the assumption of a constant efficiency of the inverter even under part load operation. Our analysis shows the importance of a proper modeling of PV generator at moderate insolation levels and of inverters under part load conditions. In this context, the simplicity of testing and implementaion of new component models is one of the main advantages of simulation systems like INSEL. REFERENCES Blok, K. and ter Horst, E. (1987) SOMES - A simulation and optimization model for autonomous energy systems; description and manual Version 1.1. Rijksuniversiteit Utrecht Klein, S.A. and Beckman, W.A. (1985) PVF-CHART User's manual.

Wisconsin - Madison

Klein, S.A. and Beckman, W.A. (1976) TRNSYÇ - A transient system simulation program. ASHRAE Transactions, 82, 623. Menicucci, D.F. and Fernandez, J.P. (1988) User's Manual for PVFORM: A Photovoltaic system simulation program for stand-alone and grid-interactive applications. Sandia Report SAND85-0376-UC-276, Albuquerque, NM. Schumacher-Gröhn, J. (1991) Digitale Simulation regenerativer elektrischer Energieversorgungssysteme. Dissertation, Universität Oldenburg. ACKNOWLEDGEMENTS This work was funded by the German Ministry of Research and Technology (BMFT).