Renewable and Sustainable Energy Reviews 79 (2017) 293–303 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Laboratory exercises of photovoltaic systems–Review of the equpment, methodology, trials and results MARK Sasa M. Skokoa, Rade M. Ciricb, ⁎ a b The Secondary School "Mihailo Pupin", Novi Sad, Serbia The Higher Education Technical School of Professional Studies in Novi Sad, Serbia A R T I C L E I N F O A BS T RAC T Key words: Renewable energy sources Characteristics of photovoltaic cells Experiments Off-grid photovoltaic systems Laboratory exercises The laboratory for renewable energy sources of the Higher Education Technical School of Professional Studies in Novi Sad (Serbia) contains the equipment regarding solar and thermal techniques, based on the implementation of solar panels, heat pumps and a pellet stove. Photovoltaic systems include the implementation of static photovoltaic panels, trackers and inverters for power supply of autonomous consumers. In addition, the laboratory is equipped with modern systems of internal and external lighting, a wind generator with accompanying inverter, elements of smart installations and fuel cell systems for laboratory measurements. This paper presents experiences in the implementation of the photovoltaic systems in the educational process through laboratory exercises. The specification of the laboratory equipment, the methodology of work, as well as the electrical schemes of experiments of open circuits and short circuit, recording of electricity-voltage characteristics, tracking the maximum power point, serial and parallel connection and shading of photovoltaic cell are presented. The second group of the trial is performed on the "off grid" photovoltaic system set in the schoolyard and connected to the appropriate inverter with the autonomous group of consumers. The paper presents the results of laboratory measurements, issues for discussion, evaluation of the performance and conclusions. 1. Introduction At present, the impact on the environment represents a significant factor in the process of reviewing the connection of new production units to the electric power system. Particular attention is being paid to the development of technologies and procedures that would allow better use of solar, wind and hydro energy, bioenergy, geothermal energy and similar [1,2]. Willingness of local communities to pay for renewables is observed to be correlated to socioeconomic charactersitics including eduaction, interest in environmental issues and knowledge of renewable energy sources [3]. The need for renewable energy education and training at all levels is globally recognized. During the last three decades a large number of countries have initiated academic programmes on renewable energy technologies and related aspects [4]. During the last decade the modern technology in the domain of renewable energy sources is also introduced in Serbia [5–11]. The key link in the application of renewable energy sources consists of engineers and technicians of different profiles, who obviously need new knowledge and skills. The question is how to harmonize the curriculum of the higher education institutions with the ever growing ⁎ Corresponding author. E-mail address: ciric@vtsns.edu.rs (R.M. Ciric). http://dx.doi.org/10.1016/j.rser.2017.05.070 Received 22 February 2016; Received in revised form 20 April 2017; Accepted 17 May 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved. requests of industry and open markets in the electric power sector. The curriculum of renewable energy sources at the higher education institutions world-wide is based mainly on theoretical units, numerical calculations and computer simulations of operating conditions of the grid with distributed generators. The course Small power plants and renewable energy sources, is attended by the students of the Higher Education Technical School of Professional Studies in Novi Sad (HETSPS), Serbia, at the specialist studies at the Electrical Engineering Department. The course was introduced in 2012 [12]. The goal of the course is to enable students for project management in the field of application of renewable energy sources. The course was designed to last one semester in accordance with the Bologna Declaration, and it bears 8 ECTS, with two hours of lectures and two hours of exercises per week, or the total of 70 h. The pre-examination obligations consist of the elaboration and defense of a seminary paper in class and laboratory exercises, while the exam is written consisting of theoretical questions and calculation tasks. The paper presents exercises conducted in the Laboratory for renewable energy sources (LRES) with photovoltaic systems in the educational process of future professional engineers specialists at the Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric panels are used, while for the secondary warming the heat pump or pellet stove are optionally used. The automation complies with the parameters of the system and operates the entire system. This system allows introduction with the placement and installation of thermaltechnical systems, monitoring and adjustment of system parameters, measuring of the characteristic temperatures in the system, and evaluation of obtained thermal power. HETSPS. The equipment in the LRES allows exercises in practical measuring in groups of three students. The objective of performing the presented exercises is to make the future specialists acquainted with the basic topology of photovoltaic (PV) modules, their networking and commissioning, monitoring of the basic parameters of the PV plant, processing of measurement results and their interpretation and making appropriate conclusions. Consequently the educational process is significantly improved since the students can verify theoretical principles on the physical models of renewable sources. The list of laboratory equipment is presented, the methodology of work is described, as well as the electrical schemes, experiments, measurement results, issues for discussion and conclusions. The above mentioned contents establish a correlation with other educational subjects at the study programme of Electrical Power Engineering such as, Fundamentals of Engineering 1, Distribution and Industrial Systems, Electrical Measurement, Electrical Installations and Electrical Machinery. 2.2. Wind generator Through laboratory trials, the students in the first phase get acquainted with the basics of construction of wind generators, principle of work of asynchronous wind generators, synchronous generators with permanent magnets on the rotor, as well as low power wind generators for the implementation in the household. The experiments consist of three parts: 1. The connection of wind generators to the electronic system for charging accumulation batteries and accompanying inverter with their own load ("the off-grid" mode); 2. Testing of the wind generator prototype of low power direct current in the mode of load for different speeds of rotation and degrees of load (determination of ''MPP-Maximum Power Point'', experiment of the adjustments per power). During the trial, an experiment of an asynchronous machine in the generator mode of work is also performed, as well as the measurement of relevant sizes, and then appropriate conclusions are made. 3. Monitoring of the system in the period of windy days is usually realized through additional seminary papers. 2. Laboratory for renewable energy sources The equipment installed in the LRES provides exercises on different systems. Solar and thermal-technical systems are based on the implementation of solar panels, a thermal pump and a pellet stove; Photovoltaic systems include the implementation of static PV panels, tracker and inverters for the power supply of autonomous consumers. Besides, there are modern systems for external and internal lighting and a wind generator with accompanying inverter and autonomous consumer group. Elements of the smart installation enable familiarization with the home automation system including installations, remote control and setting. Fuel cells systems for laboratory measurements provide the demonstration of basic technological processes. The recording of the housing facilities by a thermovision camera allows the introduction of the future professional engineers with the basis of energy efficiency of the facilities. Fig. 1 shows features of the laboratory for renewable energy sources. The exercises are conducted as a joint demonstration or by groups of three students in the presence of the course lecturer, teaching assistant and laboratory assistant. Fig. 2 shows the appearance of the wind generator. 2.3. Efficient lighting From the aspect of energy efficiency and electricity saving, special attention is paid to modern lighting sources for internal and external lighting and basic elements of "smart" electrical installations. Students get acquainted with the characteristics of energy-saving light sources, and the following experiments are performed: 2.1. Thermal-technical system • Within the laboratory, a system composed of the equipment placed both outside and inside the facility has been installed. Outside is a system with six solar collectors for water warming. In the facility, there is a pump of the solar collector with related automatics, accumulation tank of 1000 l, with two converters. The first converter is connected to the solar collectors, and the second is connected to the thermal pump and pellet stove. For the primary warming of water in the tank, solar • Fig. 1. Laboratory for renewable energy sources (LRES). Measuring voltage, electricity and power, lux of conventional light sources for internal lighting (incandescent lamps), as well as modern light sources (energy-saving bulbs, LED lamps); statistical comparison of results is conducted and appropriate conclusions are made. Electrical measurements on the system of light sources for external lighting, measurement of the harmonic light source in cold and warm conditions. The comparison of the results of measurements on standard bulbs for public lighting (sodium, mercury) in relation to the results obtained with LED lamps for public lighting. Fig. 2. Wind generator on the school roof. 294 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric Apart form the relatively high cost, the low PV module conversion efficiency is another factor that restricts the wide usage of PV systems [24]. Therefore, a power converter embedded with the capability of maximum power point tracking (MPPT) integrated with the PV system is essential for the application of this technology. Ref. [25], and [26] provide the comprehensive review of the available MPPT techniques, both in the uniform insolation and partial shaded conditions. Increasing the efficiency of the PV panel with the use of phase change material (PCM) is elaborated in [27]. 3.2. Basic exercises Introduction to the operation of PV systems is divided into two parts. The main part represents the introduction to PV basic features, while the other part refers to the introduction into the realistic PV systems with DC/AC inverters and autonomous load. Within the framework of the fundamental measurements on PVcells in the laboratory conditions, the students get acquainted with the characteristics and the way of work of the cells and modules in the trials carried out through five laboratory exercises: Fig. 3. Demo model of the system with fuel cells. 2.4. Fuel cells Electrical energy can be generated through the application of the fuel cell hydrogen and oxygen from the air. Fig. 3 shows the appearance of laboratory devices for testing basic technological processes with fuel cells based on the technology of hydrogen. Within laboratory experiments the students learn basic chemical processes occurring in the fuel cells, as follows: • • 1. Determination of open circuit voltage and short circuit current of photovoltaic cell depending on the volume of light; 2. Measuring of open circuit voltage and short circuit current of photovoltaic cell depending on the intrusion angle of the light rays; 3. Measuring of open circuit voltage and short circuit current of serial, parallel and combined link of solar cells; 4. Determination of the point of maximum power of solar cells (''MPPMaximum Power Point'', experiment of the adjustments per power) for different values of the lux at the surface of the photovoltaic cell; and 5. Experiment of shading a solar cell. Operating mode during which distilled water is decomposed to oxygen and hydrogen through the electricity, and Operating mode in which by reverse chemical reaction, the incorporation of oxygen and hydrogen is performed, where water is obtained and electrical voltage on the electrodes of the fuel cell. In the following text, the equipment, methodology of work, measurement results and conclusions made on the basis of the described experiments with PV cells are described in detail. 3. Photovoltaic modules 3.1. Theoretical base 3.3. Laboratory equipment The theoretical base for the preparation and implementation of the exercise applied in the laboratory can be found in ref. [13–27]. A simulation model for modeling the PV system power generation and performance predicition is described in [14]. Over the three decades, a number of methods for determining fundamental (DC) parameters of solar cells have been developed [15]. The current-voltage characteristics of a PV module can be reproduced by modeling the PV panel as an equivalent circuit made of linear and non-linear components [16]. Paper [17] analyses the PV equivalent circuit considering electrical characteristics and environmental conditions, including varying temperature and non-uniform solar irradiance due to partial shading. In [17] a comprehensive designing process of solar photovoltaic water pumping system, a standalone PV system and a grid connected PV system are presented. An accurate computational technique for the two-diode PV model using differential evolution optimization technique is presented in [19], while the parameter estimation of PV cells is presented in [20]. A comparison of two PV array models, the five parameter model and the Sandia Array performance model are presented in [21]. A problem of the mismatch loss due to differences between single modules in the PV array is analyzed in [22]. Results show that the maximum power, the maximum power voltage and open circuit voltage are preferably represented by Burr probability density function and normal distribution function. Prediction of I-V characteristics for a PV panel by combining single diode model and explicit analytical model is presented in [23]. Fig. 4 shows demo models of PV cells used for basic laboratory reviews and the spotlight for achieving different levels of brightness in laboratory conditions. In Fig. 6 the equivalent scheme of the PV cell is shown. (Fig. 5). Laboratory equipment contains the following elements: • • • • • • • • • Four PV cells 0.5 V /1.0/0.5 W with 4 mm bushings [28], Two measuring devices (universal measuring instrument with a rechargeable 9 V - battery), One luxmeter with a rechargeable 9 V, Battery charger for charging 9 V - batteries, Set of resistors on the aluminum holder (7 resistors /1 diode), Halogen source of radiation 200 W (with a spare halogen bulb), Conductors for measuring: a. 2 x red conductor of 100 cm length, b. 2 x blue conductor of 100 cm length, c. 3 x red conductor of 50 cm length, d. 3 x blue conductor of 50 cm length, Measuring pole, and Hood to cover the PV cells. 3.4. Determination of PV characteristics of open circuit and short circuit The aim of the exercise is to introduce basic modes of operation of solar cells, while the task of the exercise is as follows: 1. Study the way of performing the laboratory exercise; 2. Connect the equipment according to the schematic diagrams in Figs. 6 and 7; 3. Measure the open circuit voltage U0 and short circuit current ISC for 295 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric Fig. 4. Photovoltaic cells used for the basic laboratory experiments: a) appearance of photovoltaic cells; b) example of setting the exercise with the accompanying spotlight. Rs Is When performing experiments, students can make the following observations: I I0 Rp U • • Rload Fig. 5. Equivalent scheme of PV cells. 3.5. Determination of the open circuit voltage and short circuit current of PV cell depending on the angle of the light beam + S U0 230V V The power of solar cells depends not only on the intensity but also on the incident angle of solar radiation. If more vertical light beams fall on the solar module, losses due to reflection will be less. The mobility of the PV module is technically feasible but requires certain costs. Therefore, it is important how the solar modules will be installed on the roof. The best is that the panels are placed exactly on the Southside and at an angle corresponding to the precise setting place. The aim of this exercise is to introduce the functional dependence of voltage and current of PV cells on the angle of incident radiation α, and the task of the exercise is to: L Fig. 6. Schematic diagram of the open circuit experiment. + S Isc 230V For low intensities of brightness the voltage of a cell reaches the maximum value, and The current increases linearly with the increase of light intensity. Students should notice this linear relationship and, if necessary, perform interpolation of the measuring points. A - 1. Study the way of performing the laboratory exercise; 2. Connect the equipment according to the schematic diagrams in Fig. 9; 3. Measure the open circuit voltage U0 and short circuit current Isc of the cells for different values of the angle of light emission; and 4. Draw the diagram U0 = f(α) based on the results of measurements. L Fig. 7. Schematic diagram of the short circuit experiment. different brightness values on the surface of the solar cell; and 4. Draw the diagrams U0 = f(E), and ISC = f(E) based on the measurements results. The course of the exercises is as follows: 1. Place the sample solar cell on the intended surface with the indicated angles of rotation α; 2. Illuminate the solar module with 6000 lx, which corresponds to the distance of the radiation source from solar cells L =42 cm; 3. Measure the open circuit voltage and short circuit current for the corresponding angle of incident angle of light rays simulating a rotating panel; Enter the measurement results in the table and repeat the process until the angle is 90°; 4. The measuring range for the voltage is 20 V/ DC, and 10 A/DC for the circuit; 5. Draw the diagrams U0 = f(α), ISC = f(α) based on the results of measurements. The course of the exercises is as follows: 1. Pay attention during the measurement to the uniformity of the brightness of modules; 2. Measure the voltage and current separately, first in the open circuit and then in the short circuit for the same amount of brightness; 3. The measuring range for the voltage is 20 V/ DC, and for the circuit 10 A/DC; 4. Enter the measured values in the table and draw the corresponding graph of voltage and current. In Fig. 8a and b the layout characteristics obtained from the open circuit and short circuit experiments performed on one photovoltaic cell are presented. Table 1 presents the results of measurements of voltage in the open circuit U0 and short-circuit current Isc of the photovoltaic cells. In Table 2 the measurement values of the open circuit voltage and short circuit current of photovoltaic cells, depending on the angle of incidence of radiation are given, and Fig. 10 shows the diagrams U0 = f(α), ISC = f(α). 296 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric b) a) Voltage U0 [V] Current Isc [mA] 0.52 1200 1000 0.50 800 0.48 600 0.46 400 0.44 200 U0 = f(E) Isc = f(E) 0.42 0 0 2000 4000 6000 8000 10000 12000 14000 16000 E [Lux] 0 2000 4000 6000 8000 10000 12000 14000 16000 E [Lux] Fig. 8. Basic measurements on the photovoltaic cells: а) open circuit voltage U0, b) short circuit current Isc. Table 1 Results of measurements in the open circuit and short-circuit of photovoltaic cells. Е [Lux] L [cm] U0 [V] Isc [mA] 2000 74.5 0.44 150 4000 54.5 0.48 310 6000 42 0.5 450 8000 39 0.51 580 10000 35 0.51 720 12000 31.5 0.51 880 Table 2 Open circuit voltage and short circuit current for the corresponding angle of incident angle of light rays. 14000 29.5 0.51 990 α ( °) U0 (V) ISC (mA) 15 0.49 450 30 0.48 410 45 0.47 350 60 0.46 260 75 0.43 140 Open circuit voltage is in a wide range independent of the angle of incidence irradiation; Short circuit current changes by the cosine curve. However, when the angle of irradiation is 90°, zero current is not achieved because of the ambient light. 1. Study the course of the laboratory exercises; 2. Connect the equipment according to the schematic diagrams in Fig. 11; 3. Measure the open circuit voltage and short-circuit current in series, parallel and mixed connection of solar cells; and 4. Make conclusion remarks based on the results of measurements. 3.6. Testing of the series, parallel and combined link of PV cells The course of the exercises is as follows: PV modules are connected in series to increase the voltage of the solar generator, and if necessary, to give a large current, the solar modules are connected in parallel. For devices that are connected to the network, the solar modules are often connected as mixed-serial and parallel, causing the inverter to bring higher voltages and higher currents and thus higher power. 1. Perform the following measurement with illumination of 6000 lx, keeping the distance from the source of light and the solar cell about 42 cm; place solar cells in a semicircle, to make each cell be illuminated with the same intensity. а) b) L S U0 230V 90 0.42 40 The aim of the exercises is to show basic effects of serial, parallel and mixed connection of PV cells, and the task of the exercise is as follows: When performing experiments, students can make the following observations: • • 0 0.49 460 V α Fig. 9. Determination of the characteristics of the open circuit voltage of PV cell depending on the angle of the light beam: а) principal scheme, b) hardware. 297 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric a) b) Voltage Uo [V] 0.6 Current Isc [mA] 500 Uo = f(α ) Isc = f(α) 0.5 400 0.4 300 0.3 200 0.2 100 0.1 0.0 0 0 20 40 60 80 100 ο Angle α [ ] 0 20 40 60 80 100 o Angle α [ ] Fig. 10. Open circuit voltage and short circuit current for the corresponding angle of incident angle of light beam. 2. Measure the voltage and current one after another in accordance with the following electric schemes; • • • • The measured values of the open circuit voltage and short circuit current for series and parallel wiring of solar cells are shown in Table 3. The worksheets contain the questions for discussion: What is achieved by the series link of solar cells? What is achieved by parallel connection of solar cells? What is achieved by the mixed connection of solar cells? Based on the results presented in the table students should conclude that the mixed connection results in twice the value of current and voltage, which actually represents a four-fold increase in power. Some of the student observations are: Measurements were made using two universal measuring instruments, one set up to measure the current, and the other to measure the voltage, providing more Scheme 1: single solar cell; Scheme 2: two cells in series connection; Scheme 3: two cells in parallel connection; Scheme 4: two cells in series connection linked all together in parallel. 3. The measuring range for the voltage is 20 V/ DC, and 10 A/DC for the circuit. b) a) + + S U0 230V V А V U0 Isc А Isc + - - L d) c) + + - - U0 V + + U0 Isc V - + + - - Isc А А - Fig. 11. Schemes for testing of the series, parallel and combined connection of solar cells. а) single cell, b) series link, c) parallel link, d) mixed wiring. 298 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric Table 3 Open circuit voltage and short circuit current of the series, parallel and mixed link of the PV cells. U0 (V) ISC (A) Scheme 1 Scheme 2 Scheme 3 Scheme 4 0.48 0.48 0.99 0.47 0.49 0.90 0.79 0.87 Table 4 Determination of the MPP for 6000 lx. Brightness 6000 lx Resistance R1 = 0 Ω U1 (V) I1 (A) P1 (W) accurate results. The measurement of the short circuit currents should be performed quickly in order to avoid overheating of the cells. When reconnecting the solar cells, it is needed to switch off the light source that is rapidly heated. With the increase of the cell temperature, the cell power decreases by 0.5% /°C. 0.04 0.47 0.019 R3 = 0.39 Ω 0.2 0.44 0.088 R4 = 0.82 Ω 0.32 0.36 0.115 R5 = 2.7 Ω 0.43 0.15 0.065 Brightness 2000 lx Resistance R1 = 0 Ω U2 (V) I2 ( A) P2 (W) PV cell produces DC voltage that is converted in the inverter DC/AC to the alternating voltage. The point of maximum power (MPP) is a point at which the solar cell produces maximum power. To achieve the highest degree of efficiency of the cell, the inverter must at all times, depending on the load and illumination, work at the point of maximum power. The MPP of a solar cell for different lighting can be simulated through the measurement of current and voltage for different resistors in the circuit, keeping the constant brightness. Individual measurement results are plotted through the previously calculated power (P= UI). The aim of the exercise is the experimental verification of the principle of adjustments by power of the PV cell. The task of the exercise is as follows: R3 = 0.39 Ω R4 = 0.82 Ω R5 = 2.7 Ω R6 = 5.6 Ω R7 = 15 Ω 0.07 0.15 0.0105 0.13 0.15 0.0195 0.28 0.1 0,028 0.35 0.06 0,021 0.38 0.02 0.0076 0.01 0.16 0.0016 R2 = 0.18 Ω 0.04 0.15 0,006 The measured voltage and current values in determining the operating point of maximum power at 6000 lx and 2000 lx are given in Tables 4 and 5, respectively. Fig. 13 shows the current and MPP of the photovoltaic cell. The students can perform the following observations based on the measured values: • • • A MPP exists for each intensity of brightness; The PV cell has a MPP for the appointed load; and To achieve the maximum power of the PV cell, the inverter has to adjust its working point to the current values of sunlight. 3.8. Shading of PV cells The PV module is usually made of a serial/parallel connection of individual cells. In case one of the PV cells is in the shadow, it cannot generate current and voltage and acts as a consumer in relation to other cells. This results in an increase in the PV cell operating temperature since the electrical power of the shaded cell is converted into the heat. The consequence is the overheating of the shaded PV cell and destruction of its structure known as the hot-spot effect. The problem is solved by setting the appropriate anti-parallel “bypass” diodes. Due to significantly less resistance of the diodes in the conducting state, the current is redirected through the bypass diodes. The aim of the exercise is the experimental testing the effect of shading of PV cells. The task of the exercise is as follows: The course of the exercises is as follows: Lighten the PV cell with 6000 lx; Distance of the light source L =42 cm; Measure the intensity of the light source and adjust the cell brightness; Connect resistors R1 to R7 successively one after another into the PV cell circuit; Measure the current and voltage one after another; The measuring range for the voltage is 20 V /DC and current 10 A /DC; Calculate the power based on the measured values of voltage and current (P= U·I); Repeat the experiment with the brightness of 2000 lx, at the distance 1. Study the course of the laboratory exercise; 2. Connect the equipment according to the schematic diagrams in b) а) А + S I U V 0.45 0.08 0.036 R7 = 15 Ω 0.47 0.03 0.014 of the light source L =74 cm. 1. Study the course of the laboratory exercise; 2. Connect the equipment to the schematic diagrams in Fig. 12; 3. Measure the voltage and current of solar cells for different values of resistors that are available (R1 to R7); and 4. Draw a diagram on the basis of the measured power as a function of the voltage of the PV cells P= f(U) and determine the MPP. 230V R6 =5.6 Ω Table 5 Determination of the MPP for 2000 lx. 3.7. Determination of the PV maximum power point • • • • • • R2 = 0.18Ω 0.12 0.46 0.055 R1 -R7 L Fig. 12. Determination of maximum power point of solar cells: а) principle scheme, b) hardware. 299 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric a) b) Current I1 [A], I2 [A] 0.5 Power P1[W], P2 [W] 0.14 I2 = f(U) I1 = f(U) P1 = f(U) 0.12 P2 = f(U) 0.4 0.10 0.3 0.08 0.06 0.2 0.04 0.1 0.02 0.0 0.00 0.0 0.1 0.2 0.3 0.4 0.0 0.5 0.1 0.2 0.3 Voltage U [V] 0.4 0.5 Voltage U [V] Fig. 13. Diagram of the measured current and power for the experiment of MPP determining: а) I1=f(U), I2=f(U); b) P1= f(U), P2= f(U). а) - + - + - c) + - + - + + b) - ‘’Bypass’’ diode V V А А Fig. 14. Schemes for performing the experiment of shading of the PV cell: а) regular system, b) system with shaded PV cell, c) hardware. • • • Table 6 Measurement of voltage and current in the experimental PV cell shading. Measured variable Three cells in the series link Shading a single cell without a diode Shading a single cell with diode U0 (V) ISC (A) 1.46 0.46 1.01 0.02 0.98 0.3 The measured voltage and current values in the case of normal operation, one PV cell shaded, and the shaded PV cell with bypass diode, at 6000 lx brightness are shown in Table 6. Fig. 14; 3. Measure the open circuit voltage and short circuit current of the PV cell system in three cases: all three cells are directly exposed to the light rays; one of the three PV cells is shaded; and one of the three PV cells is shaded and there is an adequate bypass diode; 4. Comment on the results and draw conclusions based on the results of measurements. 3.9. Conclusion remarks about DC exercises The values measured by laboratory assistants may differ from the measured values of students, because: • • The course of the exercise is as follows: • • • Measure the current and voltage of the link one after another; The measuring range for the voltage is 20 V/DC and current 10 A/DC; Evaluate the effect of the shaded PV cell in the serial link with and without the bypass diode; and Comment on the results and draw conclusions based on the results of measurements. Connect the PV cell in the serial link; Lighten the PV cell with 6000 lx; Distance of the light source is L =42 cm; Measure the intensity of the light source and adjust the cell brightness; • Specified intensity of PV cell illumination cannot be accurately set, Measured values of voltage and currents depend on the PV cell temperature; by brightening PV cells by halogen source of light, the cell is heated; and finally PV cells and measuring equipment have a certain tolerance. However, deviations of the measured values have not a significant impact on the conclusions reflected. 300 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric Fig. 15. School PV system: а) with a single ax tracking sun position. b) static PV panels. Fig. 16. Block diagram of the school PV system with inverter and own loading. in Fig. 16, the test unit of the inverter system in the laboratory is in Figs. 17, and 18 shows the layout application interface for inverter monitoring. 4.1. Real PV system exercises The following experiments are performed on the test unit: 1. Continuous monitoring of electricity production from the PV system, and data recording on hourly, daily, weekly and monthly bases; 2. Setting the appropriate PV system mode through the control panel using the technical guidance; 3. Measurement of power quality parameters on several consumer groups using the professional measurement unit METREL®; and 4. Comparative analysis of the behavior of the static PV panels in relation to the PV system with tracking of the sun position. Fig. 17. Test unit of inverter system in the school laboratory. The input voltage of the PV system is shown in Fig. 19a, while the AC active power is presented in Fig. 19b. Battery voltage and output inverter voltage are given in Fig. 20a and b, respectively. 4. Photovoltaic system with the inverter The second group of the trial is performed on the off-grid PV system set in the schoolyard and connected to the appropriate inverter DC/AC of 2000 VA with the autonomous group of consumers. Through this group of experiments the students learn the basic PV system installation, connection and commissioning of PV systems of low power. By using an appropriate software application for monitoring system parameters the students learn the possibilities of setting the inverter mode. The PV system with a single ax tracking the sun position is shown in Fig. 15a, and b shows the static PV panels in the schoolyard. The block diagram of the school PV system with inverter and loading is presented 4.2. Safety and health at work Having in mind that the experiments on the PV system test station are carried out at mains voltage 230 V, regardless of the fact that the PV system is not connected to the distribution grid, it is necessary to strictly take care of occupational safety and health measures for teachers, laboratory technicians and students in accordance to the regulations [29,30]. Accordingly, the following protection measures are applied in the Laboratory for renewable energy: 301 Renewable and Sustainable Energy Reviews 79 (2017) 293–303 S.M. Skoko, R.M. Ciric Fig. 18. Application interface for inverter monitoring. Fig. 19. Parameters monitoring: а) PV input voltage UPV (V), b) AC output active power PAC (W). Fig. 20. 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Despite the significant potential for the application of renewable energy, as well as the feed-in tariff, due to the unfavorable loans and complicated procedure for obtaining permission to connect DG to the grid, the number of implemented renewable energy sources in Serbia is negligible. As a result, Serbia lacks practical knowledge, good practice and experience in the construction and operation of renewable energy sources. It is clear that investing in human resources in the field of renewable energy sources is of great importance. The objective of this paper is to present the authors’ experience in implementation of the laboratory exercises on the PV systems, performed in the teaching process at the specialist studies at the HTSPSN, Serbia. Special attention was paid to the comprehensiveness of content with the aim to familiarize future engineers with basic principles and technologies of modern photovoltaic systems. By careful selection of experiments and demonstration exercises in the laboratory the authors tended to cover all relevant activities in the field of photovoltaic technology. By applying the proposed laboratory exercises students can check all the theoretical principles taught within the subject Small power plants and renewable energy sources. Furthermore, they can notice and comment on the differences of measured and expected results. The authors strongly believe that by performing proposed laboratory exercises the overall process of electrical engineering education, consisting of teaching theoretical units, practicing numerical calculations and performing computer simulations, is significantly improved. The general impression is that the students showed a high motivation to work in the Laboratory for Renewable Energy Sources and were well prepared for carrying out practical experiments. The discussions and comments of the students stated in their reports indicate they understand the principles of work and have adopted the practical knowledge of modern photovoltaic systems. Therefore, it is not surprising the average grade regarding laboratory exercises is 8.72 out of 10, earned in the 2015/2016 academic year. It can be concluded that Laboratory for Renewable Energy Sources in the HETSPS in Novi Sad is of great importance for the educational process since besides laboratory exercises it is used for various student research activities and preparation of seminars and undergraduate thesis. Also, the experience shows positive student feedback as well as employers’ satisfaction regarding the acquired knowledge and skills necessary for engineering tasks in the field of renewable energy sources. Acknowledgment This paper is part of the Reseach project No. III 42009: 2011–2016 Integrated Energy Networks, supported by the Ministry of Science and Technology of Republic of Serbia. The authors are grateful to Branka Petrovic for lectoring the manuscript. References [1] Masters GM. Renewable and Efficient Electric Power Systems. John Wiley and Sons 303