Software tool for evaluation of electrical energy produced by photovoltaic systems

Software tool for evaluation of electrical energy produced by photovoltaic systems Goran Dobric, Željko Durišic and Zlatan Stojkovic Faculty of Electrical Engineering, University of Belgrade, Belgrade, Serbia E-mail: zstojkovic@etf.rs Abstract This paper presents a software tool for evaluation of electrical energy generated by photovoltaic (PV) systems. The software was developed using the MATLAB software package and contains the elements of an expert system. The software is designed for engineers who are involved in photovoltaic system design and for students (undergraduates and Masters) of electrical engineering who, using the software, adopt theoretical assumptions and solve practical engineering problems. The structure, organization and software capabilities are illustrated within the example of the design of an actual photovoltaic system. Keywords electrical energy; photovoltaic systems; software Environmental pollution and global warming are the first to be mentioned among the problems that should be addressed during this century. The current trend of energy production and consumption in the world is not doing any good in addressing these problems and presents the main cause of the greenhouse effect 1, acid rain and other negative global and local impacts on health and the environment. Aside from the aforementioned problem of environmental pollution, the dynamics of fossil fuel exploitation will lead to the exhaustion of their reserves in the near future. This presents an additional incentive for increasing the share of renewable energy sources within global energy consumption. The European Union (EU) position on the problems of environmental pollution is reflected in decisions on the obligations of the EU countries to reduce environmental pollution and increase energy efficiency 2. According to these decisions, EU countries are obliged to take certain actions by 2020, in order to reduce emissions of greenhouse gases by 20%, increase energy efficiency by 20% and increase the share of renewable energy consumption in the EU by 20%, all in relation to the levels in 1990. As a solution to meeting the growing demand for energy and reduction of environmental pollution, many governments were forced to promote the construction of power plants that use renewable energy sources through corresponding subsidies. This policy has led to the popularization and increasing share of renewable energy sources within overall electrical energy generation. This paper relates only to the use of solar energy for photovoltaic power generation systems. Table 1 shows the trend of growth in the installed capacity of photovoltaic systems in the world from 2000 to 2009. 3 Designing photovoltaic systems entails a series of calculations, more or less complicated, which are implemented in order to form an energy efficient system at a given location. The energy efficiency of photovoltaic systems is influenced by numerous factors including irradiation (radiation power per unit area) at the panel http://dx.doi.org/10.7227/ijeee.49.4.3

384 G. Dobric, Ž. Durišic and Z. Stojkovic Year TABLE 1 Installed capacity of photovoltaic systems (MW) in the world from 2000 to 2009 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 EU 189 286 429 628 1,334 2,341 3,309 5,279 10,338 15,943 USA 139 168 212 275 365 479 624 831 1,173 1,650 China 19 30 45 55 64 68 80 100 145 305 Japan 318 452 637 860 1,132 1,422 1,708 1,919 2,149 2,633 ROW 763 825 913 1,000 1,044 1,051 1,235 1,422 1,870 2,347 Total 1,428 1,762 2,236 2,818 3,939 5,361 6,956 9,550 15,675 22,878 surface, atmospheric conditions (temperature, precipitation, atmospheric contamination), orientation of the panel, etc. Based on measurements of horizontal irradiation and temperature, collected at the desired location, it is possible, with a group of equations 4, to estimate the electrical energy generated by a photovoltaic system at the same location during the same period. Without using a PC, the entire procedure of photovoltaic system design and evaluation of electrical energy generation, with respect to all the influential parameters based on actual measurements, would be long and arduous, almost impossible. Hence the idea for developing the software for photovoltaic system design in actual exploitation conditions. An accurate evaluation of electrical energy generation is essential for further cost-benefit and other economic analysis. A more accurate estimate is obtained by taking into account as many influencing factors as possible, based on actual measurements taken over as long a period as possible (at least one year). This may lead to the need to process over 100,000 bits of measured data, which justifies the efforts made to develop software that allows quick and easy estimation of PV electrical energy generation. Using MATLAB 5 7, software that estimates electrical energy generation of a gridconnected photovoltaic system was developed. The software uses actual measurement data of temperature and horizontal irradiation with an arbitrary time interval and arbitrary resolution of the data. The software is user-friendly and follows a logical sequence of calculations used for photovoltaic system design. Software tool description Basic information A photovoltaic system is formed from the database of photovoltaic modules and inverters which is contained within the software. The database can be expanded by entering in the basic information on modules and inverters which can be found in the catalogues of manufacturers on the market. Tables 2 and 3 show the basic information on photovoltaic modules and inverters which is contained in the database and is needed for database expansion. The software allows calculation for an arbitrarily oriented surface (flat roof, building facade, open space, etc.) and also provides the opportunity to consider the

Software for PV systems 385 TABLE 2 Basic information on photovoltaic modules Manufacturer Model Material Kyocera KC-120-1 Multicrystal P DC (W) 120 V m (V) 16.9 I m (A) 7.1 V OC (V) 21.5 I SC (A) 7.45 Length (mm) 1425 Width (mm) 662 η (%) 12.9 TABLE 3 Basic information on inverters Manufacturer Model Sunny Boy SB2000 P AC (W) 2000 V AC (V) 198 251 V MPPT range (V) 125 500 V ul max (V) 500 I ul max (A) 10 η max (%) 96 optimal solution of panel orientation in order to increase energy efficiency. The software also takes into account the effects of reflection with the possibility of seasonal (monthly) changes in the reflection coefficients. The relatively complex calculation of insolation on the panel surface and conversion efficiency is performed, while accounting for the effect of efficiency changes influenced by the change of temperature of the panel, the effect of panel surface contamination and the effect of mismatched modules. Based on actual measurement data of horizontal irradiation and ambient temperature, as well as the characteristics of the modules and inverters selected from the database, the calculation of electrical energy production is performed. The output interface provides graphical and numerical presentation of the results. This allows close monitoring of the production profile of electrical energy per month and total electrical energy generation for the specified photovoltaic system. The software provides high-quality data for further cost-benefit analysis and also allows for a sensitivity analysis to be conducted. After completion of the calculation, it is possible for a report to be automatically generated in the form of a PDF file. The report includes site information (longitude and latitude, reduction due to contamination of the panel surface, monthly reflection coefficients, etc.), the characteristics of the photovoltaic system (module and inverter

386 G. Dobric, Ž. Durišic and Z. Stojkovic characteristics, their number and type of connection, reduction due to module mismatch, etc.) and evaluated monthly and total electrical energy. The software was tested using actual measurement data of horizontal irradiation and ambient temperature for several locations in Serbia and Bosnia and Herzegovina. The software is distinguished by its simplicity in use and very descriptive graphical interface. It could also have a very successful commercial future in photovoltaic system design. Further expansion of the software is reflected in the implementation of cost-benefit analysis and in the design of a complete photovoltaic system including conductors, switches, fuses, etc. Software structure As stated above, the software was developed using the MATLAB software package and is named PVP (PhotoVoltaicProject). The software is characterized by its userfriendliness and follows a logical sequence of calculations used for photovoltaic systems design. The main program contains a series of subprograms, with their own graphical user interface, which are used for performing various activities. Activities which can be performed using this software are: 1 Measurement loading; 2 Defining exploitation conditions; 3 Module selection and forming the panel; 4 Inverter selection; 5 Calculation of electrical energy; 6 Generating a report. Each of the activities will be briefly explained. The appearance of the program and its application is provided further on in the text through an example of the design of an actual structure. Measurement loading The software provides the opportunity to browse through all files in order to find the measurement data. The measurement data must be organized in a TXT file containing four columns: date, time, temperature and irradiation. The columns may occur in any order. The appearance of a properly organized data file is shown in Fig. 1. Defi ning exploitation conditions In order to evaluate electrical energy generated in actual exploitation conditions, after the measurement data has been loaded, it is necessary to define these exploitation conditions. The software considers four values: reflection coefficients, latitude of the site, longitude of the site and percentage reduction due to contamination of the panel surface, as indicators of the exploitation conditions. Reflection coefficients are defined monthly as they change during the year. The typical reflection coefficient for a grass surface is 0.2, and for a snow-covered area 0.8.

Software for PV systems 387 Fig. 1 The appearance of properly organized measurement data. Module selection and panel formation The software provides the opportunity to select a photovoltaic module from an existing database. The information on the module contained in the database is shown in Table 2. As stated above, the software provides the opportunity to expand the database. Modules are selected by manufacturer and model under names that can be found in catalogues. There are three ways to form the photovoltaic panel: defining the number of selected modules, defining the area of the panel, defining total power of the panel. The software allows for the input of percentage reduction due to module mismatch, as well as selection of one of four types of panels: two-axis tracking device, one-axis tracking device, manually adjustable tilt angle, fixed orientation. In order to better understand the differences between these four types, Fig. 2 shows position angles between the Sun and a photovoltaic module. The angles shown in Fig. 2 are: Σ tilt angle; β altitude angle of the Sun; Φ S azimuth angle of the Sun; Φ C azimuth angle of the module.

388 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 2 Position angles between the Sun and a photovoltaic module. Depending on the selected type, the corresponding calculation of insolation on the panel surface is conducted. The total horizontal irradiation (I H ) is part of the measurement data and is equal to the sum of direct (I BH ) and diffuse (I DH ) components of horizontal irradiation. The software calculates the total irradiation on the panel surface using the following equations: IBC = IBH cosθ (1) sin β I I DC RC + = IDH 1 cosσ 2 = IH 1 cosσ ρ 2 (2) (3) IC = IBC + IDC + IRC (4) where: cosθ = cosβ cos( ΦS ΦC ) sin Σ+ sin β cosσ (5) I BC is direct irradiation component on the panel surface, I DC is diffuse irradiation component on the panel surface, I RC is reflected irradiation component on the panel surface. In the case of a two-axis tracking device, tilt and azimuth angles of the panel are: Σ []= 90 β [] (6) Φ C = Φ (7) S In the case of a one-axis tracking device, tilt and azimuth angles of the panel are: Σ []= 90 β[]+ δ []+ Σ[] L[] (8) eff

Software for PV systems 389 Φ C = H (9) where: H[]= 15 HoursBeforeSolarNoon (10) δ []= 360 23. 45 sin ( n 81) (11) 365 n day of the year. Panel types 3 and 4 are without Sun-tracking devices. In that case the panels have a fixed orientation. The difference between types 3 and 4 is that type 3 panels can be manually adjusted during the year. The software asks for tilt angle input in the case when type 2 is selected, or tilt and azimuth angles in the case when types 3 or 4 are selected. However, the software offers the optimal tilt angle estimation in order to achieve maximum PV system efficiency. In that case, the software does not ask for the mentioned angle input, but instead calculates the optimal angles. Due to the short duration, the optimal tilt angle estimation is carried out using multiple iterations of changing the tilt angle value. When the optimal angle is found, the iterative procedure stops (Fig. 3). Inverter selection Similar to module selection and panel creation, the software offers manufacturers and models of inverters from an existing database. The inverter selection list contains only the inverters that can meet all of the output requirements of the panel (power, voltage and current) as inverter inputs, considering the effect of panel temperature on the change of the mentioned requirements. The estimation of panel temperature is based on ambient temperature measurement data according to one of the equations (12) and (13), depending on whether or not the module database provides NOCT (Normal Operating Cell Temperature). T cell NOCT [ C] 20 kw = Tamb + ( ) I 2 08. m (12) T cell kw I 2 = T m amb + γ 2 kw m 1 where: T cell is panel temperature, T amb is ambient temperature, I is irradiation measurement data, γ is proportionality factor that depends somewhat on wind speed and how well ventilated the modules are when installed (typical values range between 25 C and 35 C). When selecting an inverter, the software provides all possible combinations of series-parallel connections of the modules for each inverter. In order to form a corresponding panel, the software itself provides the capability to change the number of modules set by the user within a range of ±10%. (13)

390 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 3 Algorithm of the PVP software.

Software for PV systems 391 Calculating generated electrical energy Once the inverter is selected, the software calculates the electrical energy of the created PV system over a time period that depends on measurement data, taking into account the temperature influence on the efficiency of the system. The temperature influence is taken into account using the standard power temperature coefficient Kp = 0.005 W/ C. Calculations of the power of the system, electrical energy and capacity factors are made using the following equations: P [ kw]= P [ kw] η η η ( 1 0, 005 ( T [ C] 25) ) (14) AC DC zap neup inv cell W kwh I PAC kw day = [ ] CF = P AC W kwh day h [ kw] 24 day C kwh 2 m day kw 1 2 m (15) (16) where: P AC is the a.c. power of the system (inverter output), P DC is the d.c. installed power of the panel, η zap is efficiency due to contamination of the panel surface, η neup is efficiency due to mismatch of the modules, η inv is efficiency of the inverter, W is daily generated electrical energy, CF is system capacity factor. Generating a report The software provides the opportunity to automatically create a report that contains all information on the project design: name of measurements, exploitation conditions settings, characteristics of the selected module, number of modules, number of series and parallel connections, characteristics of the selected inverter and number of inverters, average day temperature and irradiation diagrams, evaluation of monthly and annually produced energy and capacity factors. When generating a report, the software asks for the designer name, the project name and the location name. These entries are also contained in the report. The appearance of a report is shown further on in the text as an example of a complete PV system design. Example of a complete design In the text below an example of the complete process of designing a photovoltaic system using the PVP software is presented. For this purpose, the measurement data of temperature and horizontal irradiation in Bavaniste, Serbia (44.75 N; 21.08 E) is used. After starting the program the main window, shown in Fig. 4, opens. The main window contains three menus File, Language and Load. The File menu contains two commands Open and Close that are used for opening reports and closing the program. The Language menu allows changing the language of the

392 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 4 The main window. program into English or Serbian. The Load menu allows expanding of the existing module and inverter database. This example starts with expanding the module database by loading a new module. Load Module opens the window shown in Fig. 5. By entering the values and pressing OK the module is successfully loaded. An inverter can be loaded following a similar procedure. The first step in designing the PV system is loading measurement data. Pressing the Measurement loading button opens the window shown in Fig. 6. Pressing the Browse button opens the browser in order to find the TXT file containing measurement data. By selecting the file, the measurement data is automatically loaded. The loaded measurement file can be opened by pressing the button Open. It is necessary to do so in order to check the file structure which needs to be set in the Measurement structure panel. By setting the structure it is necessary to adjust the columns of date, time, temperature and irradiation in the same order they appear in the measurement file, as well as to format the date and time (yyyy/mm/dd, mm.dd.yyyy., etc.). Pressing the button Load confirms all the settings. If the measurement structure setting does not match the actual structure of the file the software displays an error. Pressing the button Measurement check checks the horizontal irradiation data in order to find any irregularities (negative irradiation error or irradiation greater than 1000 W/m 2 warning). If any irregularities were found, the user is informed about which rows of the data contain these irregularities. Figure 6 shows the window with loaded measurement data and set measurement structure.

Software for PV systems 393 Fig. 5 Module database expansion. The second step, after loading measurements, would be defining the exploitation conditions. Pressing the button Exploitation Conditions opens the window shown in Fig. 7. The following parameters should be defined: reflection coefficients for each month (capability to easily set all the coefficients to the same value by pressing the button Set); latitude and longitude of the site; percentage reduction due to contamination of the panel surface. Figure 7 shows the window with entered values for Bavaniste, the location where the measuring was carried out. Pressing the button OK loads the settings. The third step of designing PV systems using PVP would be module selection and forming the panel. Pressing the button Module Selection opens the window shown in Fig. 8. Three panels can be noticed in the window: Panel, Tracking and Orientation. In the panel Panel it is possible to select manufacturer and model of any module contained in the existing database. In this example, Shell SP150, the module that was loaded at the beginning (Fig. 5), is selected. As stated above, there are three ways to form a photovoltaic panel: by defining the number of selected modules, by defining the area of the panel and by defining total power of the panel. In this example an 8 kw panel is formed. It is also possible to enter the percentage reduction due to module mismatch. In the panel Tracking, one of the four panel

394 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 6 Measurement load window. types is selected: two-axis tracking, one-axis tracking, manually adjustable tilt angle and static panels. In this example, a fixed oriented panel is formed. In the panel Orientation, the tilt and azimuth angles are defined or the maximum efficiency option is selected. In the case of maximum efficiency, the optimal angles are estimated. In this example, the maximum efficiency option is selected. Pressing the button OK loads the settings. The fourth step, after module selection, would be inverter selection and defining the number of series and parallel connections in the panel. Pressing the Inverter Selection button opens the window shown in Fig. 9. Similar to module selection, it is possible to select the manufacturer and model of any inverter contained in the existing database. For the selected inverter, the software offers all possible combinations of series and parallel connections in order to meet input requirements of the inverter. In this example, a three-phase inverter Xantrex PV 10, 3f is selected. 11 series and 5 parallel connections are created which makes 55 modules, one more than the 54 suggested modules. As before, pressing the button OK loads the settings. After inverter selection, press the button Calculation to obtain estimates of the electrical energy and capacity factors. Figure 10 shows the main window. The differences between windows in Figs 4 and 10 are obvious. After measurement loading, the software displays the temperature and horizontal irradiation diagrams for each month. Months can be selected from the list on the right of the diagrams. If the diagrams are to be used outside the software, it is possible to open them by pressing the button Use Diagrams. Also,

Software for PV systems 395 Fig. 7 Exploitation conditions setting window. Fig. 8 Module selection and panel creation window.

396 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 9 Inverter selection window. Fig. 10 The main window after the design is completed.

Software for PV systems 397 Fig. 11 Project information input window. at the bottom of the main window, the name of the measurements, selected module and inverter with some of their characteristics and the total estimated energy and capacity factor are shown in the table. The purpose of this table is only to provide insight into the current project. Complete results can be seen by creating a report. The software creates a report in the form of a PDF file. Pressing the button Report opens the window shown in Fig. 11. After pressing the OK button, it is necessary to define the path and the name of the report to be saved. The report contains information on exploitation conditions, selected module and created panel with all of the characteristics. It also contains information on the selected inverter with all of the characteristics, tables and diagrams of an average day temperature and irradiation, and the monthly and total estimated electrical energy and capacity factors. Some parts of the report are shown in Figs 12 to 14. Educational aspect of the software tool application The software presented in this paper is used as an educational tool at the University of Belgrade, Faculty of Electrical Engineering, within the course Renewable energy sources. Students can choose this subject during their final year of Undergraduate or Master s studies. Students are introduced to the theoretical assumptions of photovoltaic system design and to the software itself and they learn how to use the software within PV system design. Application of the software for writing term papers within the course Renewable energy sources has been met with a positive response from the students. The students are very pleased with the simplicity of the software usage and modularity of the software itself. They are also pleased with the report creation which shortens the time needed to complete the project. A short

398 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 12 Report. questionnaire was filled in by the students and the software got an overall average mark of 8.9 (1 to 10). Using the software, students, performing different comparative analyses based on actual measurements, are introduced to all the factors that influence PV system efficiency. Students are introduced to the realistic PV system potential and the theoretical basis of design is complemented with actual problems. Based on feedback from the students who have used this software, valuable suggestions for further expansion and improvement of the software have been collected. It was suggested that more detailed descriptions of the modules and inverters be added, with pictures if possible. The idea is to attach documents to each module and inverter which the user would be able to open and view more detailed descriptions. Another suggestion was to expand the software to the level of a complete project, defining cables, switches, fuses, etc. The upgrade of the software to meet the needs of Masters studies would be the addition of cost-benefit analysis, payback time evaluation and comparative analysis between realistic conditions and clear sky energy production.

Software for PV systems 399 Fig. 13 Report. Conclusion This paper describes a software tool for designing photovoltaic systems. The software takes influential factors into account in order to estimate the electrical energy of PV systems with as much accuracy as possible. The structure and organization of the software easily and quickly provide optimal designing solutions. The software tool is developed using the powerful MATLAB software package. User orientation and expert system elements enable users who are not experts in this field to use the software. This software tool is used as an educational tool within the course Renewable energy sources. It enables users to solve practical problems in the field of photovoltaic system design in a very sophisticated way. Users learn how to perform research using the appropriate procedures, how to find out what the problem is and how to determine the appropriate solution.

400 G. Dobric, Ž. Durišic and Z. Stojkovic Fig. 14 Report. Work continues on the expansion of the software tool which aims to take into account new methods and ideas that will assist in the further optimization of PV designing solutions. That includes economic cost-benefit analysis of proposed solutions. Acknowledgement The authors would like to thank the Ministry of Science and Technological Development of the Republic of Serbia which made this work possible. The third author would like to thank the Alexander von Humboldt Foundation, Bonn, FR Germany, for its support for his scientific research work. References 1 V. Quaschning, Understanding Renewable Energy Systems (Earthscan, London, 2005), pp. 10 15. 2 The European Parliament and the Council of the European Union, Directive 2009/28/EC, Official Journal of the European Union, (2009). 3 European Photovoltaic Industry Association, Global market outlook for photovoltaics until 2014 (2010). 4 G. M. Masters, Renewable and efficient electrical power systems (Wiley, Hoboken, NJ, 2004), pp. 385 604.

Software for PV systems 401 5 J. H. Mathews and K. D. Fink, Numerical Methods Using MATLAB, 4th edn (Pearson, London, 2004). 6 S. T. Karris, Numerical Analysis Using MATLAB and Spreadsheets, 2nd edn (e-book, 2004, available from www.orchardpublications.com). 7 A. Gilat, Introduction to MATLAB 7 with Examples, 2nd edn (Wiley, Hoboken, NJ, 2005, transl. Mikro Knjiga).

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