Software tool for evaluation of electrical energy produced by photovoltaic systemsGoran Dobric’, Zeljko Durisic’ and Zlatan Stojkovic’Faculty of Electrical Engineering, University of Belgrade, Belgrade, SerbiaE-mail: [email protected]
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.
Environmental pollution and global warming are the fi rst 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 effect1, 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 refl ected in decisions on the obligations of the EU countries to reduce environ-mental pollution and increase energy effi ciency2. 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 effi ciency 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 envi-ronmental 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 genera-tion systems. Table 1 shows the trend of growth in the installed capacity of photo-voltaic 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 effi cient system at a given location. The energy effi ciency of photovoltaic systems is infl uenced by numerous factors including irradiation (radiation power per unit area) at the panel
surface, atmospheric conditions (temperature, precipitation, atmospheric contamina-tion), 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 equations4, 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 infl uential 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 evalu-ation of electrical energy generation is essential for further cost-benefi t and other economic analysis. A more accurate estimate is obtained by taking into account as many infl uencing 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 justifi es the efforts made to develop software that allows quick and easy estimation of PV electrical energy generation.
Using MATLAB5–7, software that estimates electrical energy generation of a grid-connected photovoltaic system was developed. The software uses actual measure-ment 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 informationA 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 infor-mation 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 (fl at roof, build-ing facade, open space, etc.) and also provides the opportunity to consider the
TABLE 1 Installed capacity of photovoltaic systems (MW) in the world from 2000 to 2009
optimal solution of panel orientation in order to increase energy effi ciency. The software also takes into account the effects of refl ection with the possibility of sea-sonal (monthly) changes in the refl ection coeffi cients. The relatively complex cal-culation of insolation on the panel surface and conversion effi ciency is performed, while accounting for the effect of effi ciency changes infl uenced 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 profi le of electrical energy per month and total electrical energy generation for the specifi ed photovoltaic system. The software provides high-quality data for further cost-benefi t 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 fi le. The report includes site information (longitude and latitude, reduction due to contamination of the panel surface, monthly refl ection coeffi cients, etc.), the characteristics of the photovoltaic system (module and inverter
characteristics, their number and type of connection, reduction due to module mis-match, 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 Herze-govina. The software is distinguished by its simplicity in use and very descriptive graphical interface. It could also have a very successful commercial future in pho-tovoltaic system design.
Further expansion of the software is refl ected in the implementation of cost-benefi t analysis and in the design of a complete photovoltaic system including conductors, switches, fuses, etc.
Software structureAs stated above, the software was developed using the MATLAB software package and is named PVP (PhotoVoltaicProject). The software is characterized by its user-friendliness 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 Defi ning 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 briefl y 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 loadingThe software provides the opportunity to browse through all fi les in order to fi nd the measurement data. The measurement data must be organized in a TXT fi le containing four columns: date, time, temperature and irradiation. The columns may occur in any order. The appearance of a properly organized data fi le is shown in Fig. 1.
Defi ning exploitation conditionsIn order to evaluate electrical energy generated in actual exploitation conditions, after the measurement data has been loaded, it is necessary to defi ne these exploita-tion conditions. The software considers four values: refl ection coeffi cients, latitude of the site, longitude of the site and percentage reduction due to contamination of the panel surface, as indicators of the exploitation conditions. Refl ection coeffi cients are defi ned monthly as they change during the year. The typical refl ection coeffi cient for a grass surface is 0.2, and for a snow-covered area 0.8.
Module selection and panel formationThe software provides the opportunity to select a photovoltaic module from an exist-ing 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 data-base. Modules are selected by manufacturer and model under names that can be found in catalogues. There are three ways to form the photovoltaic panel:
• defi ning the number of selected modules,• defi ning the area of the panel,• defi ning 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:
Depending on the selected type, the corresponding calculation of insolation on the panel surface is conducted. The total horizontal irradiation (IH) is part of the measurement data and is equal to the sum of direct (IBH) and diffuse (IDH) components of horizontal irradiation. The software calculates the total irradiation on the panel surface using the following equations:
I IBC BH= ⋅⎛⎝⎜⎞⎠⎟
cos
sin
θβ
(1)
I IDC DH= ⋅ +⎛⎝⎜
⎞⎠⎟
1
2
cosΣ (2)
I IRC H= ⋅ ⋅ −⎛⎝⎜
⎞⎠⎟ρ 1
2
cosΣ (3)
I I I IC BC DC RC= + + (4)
where:
cos cos cos sin sin cosθ β β= ⋅ −( )⋅ + ⋅Φ Φ Σ ΣS C (5)
IBC is direct irradiation component on the panel surface, IDC is diffuse irradiation component on the panel surface, IRC is refl ected 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 ΦC S= (7)
In the case of a one-axis tracking device, tilt and azimuth angles of the panel are:
n – day of the year.Panel types 3 and 4 are without Sun-tracking devices. In that case the panels have
a fi xed 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 effi ciency. 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 selectionSimilar 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 TNOCT C
IkW
mcell amb= + °[ ]−( )⋅ ⎡
⎣⎢⎤⎦⎥
20
0 8 2. (12)
T TI
kW
mkW m
cell amb= + ⋅⎡⎣⎢
⎤⎦⎥
⎛
⎝
⎜⎜⎜
⎞
⎠
⎟⎟⎟
γ2
21 (13)
where: Tcell is panel temperature, Tamb is ambient temperature, I is irradiation meas-urement 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 cor-responding panel, the software itself provides the capability to change the number of modules set by the user within a range of ±10%.
Calculating generated electrical energyOnce 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 infl uence on the effi ciency of the system. The temperature infl uence is taken into account using the standard power temperature coeffi cient 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 T CAC DC zap neup inv cell[ ] = [ ]⋅ ⋅ ⋅ ⋅ − ⋅ °[ ]−( )( )η η η 1 0 005 25, (14)
WkWh
dayP kW
IkWh
m daykW
m
AC
C⎡⎣⎢
⎤⎦⎥
= [ ]⋅
⎡⎣⎢
⎤⎦⎥2
21
(15)
CFW
kWh
day
P kWh
dayAC
=
⎡⎣⎢
⎤⎦⎥
[ ]⋅24 (16)
where: PAC is the a.c. power of the system (inverter output), PDC is the d.c. installed power of the panel, ηzap is effi ciency due to contamination of the panel surface, ηneup is effi ciency due to mismatch of the modules, ηinv is effi ciency of the inverter, W is daily generated electrical energy, CF is system capacity factor.
Generating a reportThe 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
program into English or Serbian. The Load menu allows expanding of the existing module and inverter database. This example starts with expanding the module data-base 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 fi rst 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 fi nd the TXT fi le containing measure-ment data. By selecting the fi le, the measurement data is automatically loaded. The loaded measurement fi le can be opened by pressing the button Open. It is necessary to do so in order to check the fi le structure which needs to be set in the Measure-ment 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 meas-urement fi le, as well as to format the date and time (yyyy/mm/dd, mm.dd.yyyy., etc.). Pressing the button Load confi rms all the settings. If the measurement structure setting does not match the actual structure of the fi le the software displays an error. Pressing the button Measurement check checks the horizontal irradiation data in order to fi nd any irregularities (negative irradiation error or irradiation greater than 1000 W/m2 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.
The second step, after loading measurements, would be defi ning the exploitation conditions. Pressing the button Exploitation Conditions opens the window shown in Fig. 7. The following parameters should be defi ned: refl ection coeffi cients for each month (capability to easily set all the coeffi cients to the same value by pressing the button Set); latitude and longitude of the site; percentage reduction due to con-tamination 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 defi ning the number of selected modules, by defi ning the area of the panel and by defi ning 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
types is selected: two-axis tracking, one-axis tracking, manually adjustable tilt angle and static panels. In this example, a fi xed oriented panel is formed. In the panel Orientation, the tilt and azimuth angles are defi ned or the maximum effi ciency option is selected. In the case of maximum effi ciency, the optimal angles are esti-mated. In this example, the maximum effi ciency option is selected. Pressing the button OK loads the settings.
The fourth step, after module selection, would be inverter selection and defi ning 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 combina-tions 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 tempera-ture 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,
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 fi le. Pressing the button Report opens the window shown in Fig. 11. After pressing the OK button, it is necessary to defi ne 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 dia-grams 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 fi nal year of Undergraduate or Master’s studies. Students are introduced to the theoretical assumptions of pho-tovoltaic 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
questionnaire was fi lled 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 infl uence PV system effi ciency. Students are introduced to the realistic PV system potential and the theo-retical basis of design is complemented with actual problems.
Based on feedback from the students who have used this software, valuable sug-gestions 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, defi ning cables, switches, fuses, etc.
The upgrade of the software to meet the needs of Masters studies would be the addition of cost-benefi t analysis, payback time evaluation and comparative analysis between realistic conditions and clear sky energy production.
This paper describes a software tool for designing photovoltaic systems. The soft-ware takes infl uential 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 fi eld 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 fi eld of photo-voltaic system design in a very sophisticated way. Users learn how to perform research using the appropriate procedures, how to fi nd out what the problem is and how to determine the appropriate solution.
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-benefi t analysis of proposed solutions.
Acknowledgement
The authors would like to thank the Ministry of Science and Technological Develop-ment 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 scientifi c research work.
References
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(2010).4 G. M. Masters, Renewable and effi cient electrical power systems (Wiley, Hoboken, NJ, 2004), pp.
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
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