Photovoltaic Efficiency of a Grid Connected 10 kwp System Implemented in the Brasov Area

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1 2010, 12th International Conference on Optimization of Electrical and Electronic Equipment, OPTIM 2010 Photovoltaic Efficiency of a Grid Connected 10 kwp System Implemented in the Brasov Area Alexandru Enesca a,*, Mihai Comsit a, Ion Visa a, Anca Duta a a The Centre: Product Design for Sustainable Development, Transilvania University, Eroilor 29, , Brasov, Romania aenesca@unitbv.ro Abstract- The paper presents an experimental study concerning the energy production of a 10kW polycrystalline photovoltaic system implemented in Brasov area. The variation of the energy production was studied correlated with three parameters: ambient temperature, PV module temperature and solar radiation. The variations in the PV module temperature as result of the ambient temperature were also investigated. I. INTRODUCTION Photovoltaic power generation has emerged as a very important non-conventional energy source and in the past decade the technology of photovoltaics has evolved and matured to become an economical alternative to other power sources. The demand for high power and high efficiency resulted in the use of high-speed switching charge controllers for solar array power conditioners, [1-3]. Numerical models have become indispensable tools for the design of any kind of efficient solar cells. They have largely contributed to our understanding of cell operation and are necessary for future cell improvements. Analytic models of solar cells have been used since the earliest days to improve understanding of the operation and to provide guidance for their design. As our understanding has increased, so has our need increased for more complex models to provide adequate descriptions of their functionality and carefully constructed numerical models provide the needed help, [4, 5]. The paper reports on the influence of the climatic condition in the Brasov area on the energy production of 10kW polycrystalline photovoltaic system. was choose as target month considering two aspects: (1) as monitoring has proved, this month has the highest monthly energy production and (2) spring season consist on successive warm and cool days which allows to study the photovoltaic conversion behavior in a large temperature scale. II. EXPERIMENTAL The investigations were made on a 10 kwp polycrystalline photovoltaic (Figure 1) grid connected system, implemented in the Transilvania University of Brasov. The photovoltaic system contains: - three photovoltaic arrays, each containing of 16 modules (QCells, ); - two temperature sensors (Platin SENSOR Sunny Sensor Box); - total solar radiation sensor (ISET Sunny Sensor Box); - one wind sensor (Sunny Sensor); - three invertors (Sunny Boy 3300TL HC); - data logger (Sunny Boy Control Plus). Each photovoltaic array is characterized by the following parameters: nominal power, P N = 3200 W; maximum current, I max = 7.8A; short circuit current, I SC = 8.4A and open circuit voltage, V OC = 647V. The acquisition system consists of a data logger (Sunny Boy Control Plus) and dedicated software (Sunny Data Control 3.93). The climatic parameters were recorded each 15 minutes, and the values obtained during day time (from 8.00 a.m. to 6.00 p.m. local time) were used in this study. Fig. 1. Photovoltaic 10 kwp polycrystalline grid connected system III. III.1. Energy production RESULTS AND DISCUSSIONS In Figure 2 the distribution of the energy production during each month in 2009 and the detailed energy production for each are presented. The energy production in is kwh which correspond to 13.27% of the total energy production in 2009 (around kwh) /10/$26.00 '2010 IEEE 1146

2 Fig. 2. Energy production in 2009 and detailed energy production in each weeks Compared with other months, including summer season, has the most favorable climatic potential for photovoltaic conversion in the Brasov area. Also, the distribution of the energy production in shows that the last week of the month offered conditions for the highest conversion efficiency. To understand this behavior, a detailed discussion for each of the weeks is presented below. III.2. Influence of climatic condition on the first week of In Figure 3, the variation of the module s temperature is presented as function of the atmospheric temperature. The difference between the module and atmospheric temperature varies from 20 C in up to 40 C in Fig. 3. The atmospheric and module temperature in the first week of These values were correlated with the total solar radiation (see Figure 4) and the results show a remarkable correlation between the solar radiation intensity and the temperature on the module, while a similar dependence of the atmospheric temperature can not be strictly observed. The highest energy production (43.18 kwh) was obtained on , when the maximum T mod (module temperature) was 52 C, the maximum T atm (atmospheric temperature) was 21.3 C and the highest R s (total solar radiation) was 815 W/m 2. In the R s was significantly higher (967 W/m 2 ) compared with the values recorded in but the energy production was lower (41.25 kwh). The reason can be found in the value of T mod which is also larger that in 07.04, rising up to 54.1 C. By increasing the T mod it is expected to increase the thermal 1147

3 disorder in the polycrystalline silicon material with direct consequences on the generation and transportation of the charge carriers. Fig. 5. The atmospheric and module temperature in the second week of Fig. 4. The total solar radiation and the module temperature in the first III.3. Influence of climatic condition on the second The dependence between the module temperature and the energy production is even more relevant in the second when the minimum T mod (see Figure 5) of the month was registered (in ) and when the energy production was about kwh, corresponding to a total R s maximum value of only 395 W/m 2 (compared to 846 W/m 2, in when the obtained energy was kwh, Figure 6). The difference can be again related to the thermal disorder due to the T mod value which decreases from 54 C (in 10.04) down to 23.6 C (in 14.04). The working temperature in the module influences the vibration movement into the silicon crystal. A higher working temperature will induce thermal (at atomic level) and mechanical (extension) stress into the material. These vibrations are higher in polycrystalline compared with monocrystalline structures being mainly located in the interfacial regions. At the interface between two crystalline structures, the thermal expansion and vibration will differently occur, considering that these processes depend on the material composition and structure. The thermal vibration will influence the generation of the charge carrier during the photovoltaic 1148

4 conversion and the thermal expansion will influence the charge carrier transportation into the material. Fig. 6. The total solar radiation and the module temperature in the second III.4. Influence of climatic condition in the third week of From and a constant evolution of energy production was recorded due to relative similar climatic conditions with T mod about 50 C (see Figure 7) and R s of 950 W/m 2 (see Figure 8). The total energy production in the third week was kwh which represent 26.87% of the total amount of energy obtained in. The data presented in Figure 7 and 8 also outline the fact that the atmospheric temperature has low influence on the module s temperature compared with the total Fig. 7. The atmospheric and module s temperature in the third week of solar radiation. This is the result of a cumulative heating of the module, able to store the IR part of the solar spectrum or even to act as a solar-thermal converter using the UV-VIS part from the total radiation; obiviously this part of radiation is no longer used in the photovoltaic conversion, supplementary decreasing the efficiency. Each time the total solar radiation varies, a similar variation can be observed in the module s temperature. These observations are not valid when T atm and T mod are compared, due to the long period of time needed to impose significant changes into the atmospheric temperature. It is therefore important to notice that in designing a PV array the specific radiation data of the implementation area are important. 1149

5 Fig. 8. The total solar radiation and module s temperature in the third III.5. Influence of climatic condition on the fourth week of The last is the most efficient in terms of energy production with a quantity of kwh, representing 28.54% of the total amount of energy produced in. It is also the higher quantity of energy produced in one week in A similar behavior as in the first and the second week can be found in this case, as exemplified in a comparative approach for and 25.04: in the energy production Fig. 9. The atmospheric and module temperature in the fourth week of was kwh, when T mod was 45.6 C (see Figure 9) and the R s was 1000 W/m 2 (see Figure 10). Comparatively, in the R s was lower (623 W/m 2 ) but the energy production increased at kwh. These data support the need of accurate measurements on the PV module, correlated with the solar radiation, because atmospheric temperature is not a reliable design data. The evaluation of the photovoltaic conversion during a long period of time can be made considering supplementary direct (atmospheric temperature, wind speed and direction) and indirect (module temperature, geographical location. etc.) climatic parameters. 1150

6 mechanical (extension) stress, mainly located at the interface of polycrystalline silicon structures. As result, the highest energy production corresponds to the lowest module temperature (which is not directly related the atmospheric temperature) and not to the highest total solar radiation. Therefore, accurate design data require monitoring on the spot and further studies must also be correlated with the spectral solar radiation profile. ACKNOWLEDGMENT This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD) Post-Doctoral Studies, financed from the European Social Fund and by the Romanian Government under the contract number POSDRU REFERENCES [1] N. Amin, K. Sopian and M. Konagai, Numerical modeling of CdS/CdTe and CdS/CdTe/ZnTe solar cells as a function of CdTe thickness, Sol. Energ. Mat. Sol. C., vol. 91, pp , [2] P. Balraju, M. Kumar, Y.S. Deol, M.S. Roy and G.D. Sharm, Photovoltaic performance of quasi-solid state dye sensitized solar cells based on perylene dye and modified TiO 2 photo-electrode, Synthetic Met., vol. 160, pp , [3] H. Bayhan and A. Sertap, Study of CdS/Cu(In,Ga)Se 2 heterojunction interface using admittance and impedance spectroscopy, Sol. Energ., vol. 80, pp , [4] O. Breitenstein, J. Bauer, A. Lotnyk and J.-M. Wagner, Defect induced non-ideal dark I-V characteristics of solar cells, Superlattices Microst., vol. 45, pp , [5] M.P. Deshmukh and J. Nagaraju, Measurement of CuInSe 2 solar cell AC parameters, Sol. Energ. Mat. Sol. C., vol. 85, pp , Fig. 10. The total solar radiation and module s temperature in the fourth IV. CONCLUSIONS The paper presents in a comparative approach the influence of different climatic parameters on the energy production of a 10kWp photovoltaic grid connected system implemented in Brasov area. The analysis results were developed based on a three-years long monitoring and are exemplified on experimental data obtained in 2009 when the highest value of the yearly energy production was registered. The graphical representation of atmospheric temperature, module temperature and total solar radiation (direct and diffuse) shows that the photovoltaic conversion is strongly influenced by the thermal events recorded in the module. When the module temperature increases, the energy production decreases due to the thermal (at atomic level) and 1151