Improved solar system with maximum power point tracking
2018; Institution of Engineering and Technology; Volume: 12; Issue: 7 Linguagem: Inglês
10.1049/iet-rpg.2017.0618
ISSN1752-1424
AutoresChih‐Chiang Hua, Yi‐Hsiung Fang, Cyuan‐Jyun Wong,
Tópico(s)Solar Radiation and Photovoltaics
ResumoIET Renewable Power GenerationVolume 12, Issue 7 p. 806-814 Research ArticleFree Access Improved solar system with maximum power point tracking Chih-Chiang Hua, Corresponding Author Chih-Chiang Hua huacc@yuntech.edu.tw Department of Electrical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this authorYi-Hsiung Fang, Yi-Hsiung Fang Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this authorCyuan-Jyun Wong, Cyuan-Jyun Wong Department of Electrical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this author Chih-Chiang Hua, Corresponding Author Chih-Chiang Hua huacc@yuntech.edu.tw Department of Electrical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this authorYi-Hsiung Fang, Yi-Hsiung Fang Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this authorCyuan-Jyun Wong, Cyuan-Jyun Wong Department of Electrical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, TaiwanSearch for more papers by this author First published: 22 March 2018 https://doi.org/10.1049/iet-rpg.2017.0618Citations: 24AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract This study presents an improved solar system with maximum power point tracking (MPPT). A digital signal processor is used to control the converter with the proposed control; thus, the system can implement MPPT independently for each solar panel whether it is under shading, various irradiation conditions or with faulty solar cells. A modified MPPT method is designed to reduce the power oscillation significantly, thus the power loss is reduced. With the proposed scheme, most of the solar panel available output power is delivered to the load only through a diode, reducing the converter power loss. In addition, the converter only handles part of the solar panel output power. Therefore, the system cost is reduced due to the low-power rating converter. A laboratory prototype is constructed and tested to evaluate the effectiveness of the proposed system. The proposed photovoltaic (PV) system and the PV balancer are compared through simulations and experiments under various shading conditions. The experimental results show that the proposed PV system can produce more output power with high efficiency. 1 Introduction Nowadays, increasing attentions has been put on the environmental issues due to severe natural disasters caused by global warming. Some standards have been set to limit the carbon emissions. The traditional thermal power plants are getting unpopular. Therefore, renewable energies, such as hydro-energy, wind energy, solar energy, biomass energy, geothermal energy, tidal energy, and ocean thermal energy, have been developed rapidly in recent years. The solar energy is one of the most common renewable energies, but it has some disadvantages. The first is the low conversion efficiency of about 20%. The second is the variations of the output power with weather variations including partial shading conditions, and the non-linear characteristics. Therefore, the maximum power point tracking (MPPT) technology is necessary for the solar systems, and a DC–DC converter is usually required to stabilise the output voltage for the applications. In recent years, solar modules integrated with converters have been developed. Fig. 1 shows the common topologies for the module integrated converters [1, 2]. To meet the requirements for high-power applications, the converters are connected in series or in parallel. As shown in Fig. 1a, the DC–DC converters are connected in series [3], and the DC–DC converters are connected in parallel as shown in Fig. 1b [4, 5]. The figures show a single solar panel connected to a converter. For the connection to the solar arrays in literatures [6, 7], a more complex MPPT is required for the solar systems. Since the P–V curve may have multiple peaks under various weather conditions, the solar arrays may not operate at the true maximum power point (MPPs). Although the MPPT can be achieved independently for each module, the converter must handle the full power of the solar panel. Therefore, the power rating and efficiency requirements of converter are usually high. Fig. 1c shows the photovoltaic (PV) balancer topology [8, 9]. The converters are integrated into a module and connected to a common DC bus. In this topology, the voltage differences among the modules can be reduced or eliminated by the proper control. In addition, the power rating of converter can be reduced. However, the converter only compensates for the voltage difference between the DC bus and the modules rather than adjusts the output voltage to achieve MPPT. In the PV balancer topology, the converter output voltage can only maintain a fixed voltage. Even the illumination changes, the variation of the maximum power point voltage of the solar panel is little. However, the operation at the non-optimal operating point results in the power loss. Under the mismatched conditions, for example one of the solar panels generates maximum power and the others generates less power due to shaded conditions, the converters of Figs. 1a and b may not be able to generate enough power to provide the minimum input voltage of inverter, thus the solar panels under shaded conditions may not produce the maximum power. Fig. 1Open in figure viewerPowerPoint PV systems (a) Cascaded DC–DC converter, (b) Parallel DC–DC converter, (c) PV balancer Based on the aforementioned discussions, the proposed solar system is designed to implement MPPT independently for each solar panel. In addition, the converter only needs to compensate for a portion of the load power. The proposed system has all of the advantages of the aforementioned architectures, which includes high efficiency, MPPT, and low-power rating. Section 2 explains the proposed circuit and its expansion. In Section 3, the proposed MPPT method is introduced. Section 4 shows the analysis of the circuit operation of the proposed system. Section 5 gives the discussion and the simulation results of the proposed system. Section 6 presents the experimental results, and finally the conclusions and future research. 2 Proposed MPPT system Fig. 2a shows the proposed improved PV system with MPPT. The simplified circuit is shown in Fig. 2b. The major difference between the proposed PV architecture and the PV balancer architecture is that the PV balancer only compensates for the voltage difference between the DC bus and each solar panel. The proposed PV architecture can achieve MPPT for the system and it is easily expanded for high-power applications as shown in Fig. 2c. In the proposed circuit, all of the solar panel outputs will not affect each other. In other words, each solar panel can operate at its MPP's. As a result, the proposed system can maximise the total energy extraction under various irradiation conditions or even partial shading conditions. Fig. 2Open in figure viewerPowerPoint Proposed PV system (a) Proposed improved PV system with MPPT. (b) Simplified circuit of single unit. (c) Expansion of the proposed circuit The proposed PV architecture consists of a single solar panel and a converter, and the tracking method can be as simple as that in [10-12]. The proposed architecture can operate with a number of solar arrays in series. In the case of non-uniform illumination, more than one power peak will appear for the PV system. Thus, a complex MPPT is required for the system to search for global MPP's, like the systems in [13, 14]. The circuit operation and design are the same as that with a single solar panel. With the proposed circuit, the power loss is reduced since most of the power only flow through a diode. The proposed converter only needs to deal with part of the load power; thus, the cost of the converter can be reduced. If the load power demand is less than the solar panel power, the system charges the battery. If the solar panel power is not enough to supply the load, the battery discharges. The proposed system can be applied to the electric vehicle battery charging or home-type smart micro-grid applications. 3 MPPT Fig. 3 shows the flowchart of the proposed MPPT algorithm. To verify that the proposed architecture can function properly, an improved perturbation and observation MPPT is designed for the system. First, the initialisation is set, including the duty cycle D, the adjusting step size d, and the previously measured solar panel power P(k − 1). Next, the present solar panel voltage VPV(k) and the present solar panel current IPV(k) are measured, and the current solar panel power P(k) is calculated. ΔP = P(k) − P(k − 1) (power variation) and ΔVPV = VPV(k) − VPV(k− 1) (voltage variation) are obtained. If ΔP and ΔVPV are greater than zero, D is adjusted to increase the solar panel voltage. If ΔP is greater than zero and ΔVPV is less than zero, D is adjusted to reduce the solar panel voltage. However, if ΔP is less than zero and ΔVPV is greater than zero, D is adjusted to reduce the solar panel voltage and d is multiplied by 0.5 to approach the MPP with less oscillation. If ΔP is less than zero and ΔVPV is less than zero, D is adjusted to increase the solar panel voltage and d is multiplied by 0.5. In this way, the proposed system with MPPT can produce a stable output power. Fig. 3Open in figure viewerPowerPoint System block diagram 4 Analysis of the proposed circuit The proposed system has three operating states: Pload > Ppv state, Pload = Ppv state, and Pload < Ppv state, the analysis of system operation under steady state is as follows. The circuit operation of the Pload > Ppv state is the same as that of the Pload = Ppv state. In Mode 1 (t0–t1), the switch is turned off and the diode is turned off, the battery provides power to the primary side of the transformer, and provides power to the load through the DC bus with the capacitor CO as shown in Fig. 4a. In Mode 2 (t1–t2), the switch is turned off and the diode is turned on, the primary side of the transformer releases energy to the secondary side of the transformer, and the solar panel provides power to the load and charges the capacitor CO. A small current flows into the battery through the DC bus as shown in Fig. 4b. In Mode 3 (t3–t4), the switch and the diode are turned off, the battery provides power to the load through the DC bus with the capacitor CO as shown in Fig. 4c. The key waveforms are shown in Fig. 5a. Fig. 4Open in figure viewerPowerPoint Operation modes (a) Mode 1 (three states), (b) Mode 2 (three states), (c) Mode 3 for Po ≧ Ppv state, (d) Mode 3 for Po < Ppv state Fig. 5Open in figure viewerPowerPoint Waveforms of the proposed circuit (a) Po ≧ Ppv state. (b) Po < Ppv state In Pload < Ppv state, the operations of Mode 1 and Mode 2 are the same as that of the first two states. In Mode 3 (t3–t4), the switches and the diode are turned off, the capacitor CO provides power to the load and provides a small current to the battery through DC BUS as shown in Fig. 4d. The waveforms are shown in Fig. 5b. 5 Simulation results In order to evaluate the performance of the proposed solar system, PowerSim is used to simulate the proposed system and the PV balancer, and the simulations of PV system under four illumination conditions (300, 500, 700, 1000 W/m2) are carried out and compared. The parameters of the solar panels and the converter are listed in Table 1. In later experiments for the system, several halogen lamps are used to simulate the light source for the performance tests. Table 1. Solar panel and converter parameters Material Polysilicon Solar panel parameters Pmpp 20 W Vmpp 23.41 V Impp 0.854 A Voc 26.67 V Isc 0.99 A Converter parameters Vin 29.2 V Np：Ns 24：8 Lm 350 μH frequency 20 kHz Vout Vin–Vmpp The simulation parameters of solar panel are shown in Table 2. The proposed circuit and the PV balancer show similar steady-state performance under illumination of 1000 W/m2. However, the PV balancer may not operate at the MPP's for other illumination conditions. Simulation results for the case of 1000 W/m2 are shown in Fig. 6a The reference voltage is the voltage used to control the PWM in the proposed system, and the adjustment steps are getting smaller to achieve convergence, at which the solar panel operates at MPP. The proposed solar system can perform MPPT under uniform or shading conditions, thus, the proposed system can generate more power under illumination of 700, 500 or 300 W/m2, as shown in Figs. 6b and c. As can be seen in simulation results, the output power of the proposed system is higher than that of the PV balancer, and the oscillations are reduced significantly. The Vmpp of the solar panel does not change a lot with the illumination, thus the output power variation is not obvious. The output power difference will be obvious for a PV system of higher power rating. To evaluate the expansion of the proposed architecture, the scheme of Fig. 2c is used for simulation under three different illumination conditions, and the simulation results are shown in Fig. 7. It can be seen that the results are similar to that of the single architecture. In addition, each solar panel can operate at its MPPs, i.e. the solar panels do not interfere with each other, even in the case of shading conditions or different illumination conditions. Table 2. Solar panel simulation parameters Material Polysilicon Pmpp 8.29 W Vmpp 18.11 V Impp 0.458 A Voc 21.49 V Isc 0.543 A Fig. 6Open in figure viewerPowerPoint Simulation results of the system (a) Simulation results at 1000 W/m2. (b) Simulation results at 700 W/m2. (c) Simulation results at 500 W/m2. (d) Simulation results at 300 W/m2 Fig. 7Open in figure viewerPowerPoint Simulation results for the expansion of the proposed architecture 6 Experimental results To evaluate the system performance, several halogen lamps and air conditioning equipment are used to create a testing environment of 25°C and different illumination conditions for the experiments in the laboratory. Four different illumination conditions (1000, 700, 500 and 300 W/m2) are provided and arranged to make eight different cases as shown in Table 3. The characteristic curves and the MPP's of the solar modules might be different, Tables 4–6 list the measured data for the solar modules. Fig. 8 shows the measured V–I curves and the P–V curves for PV3. Table 3. Different cases with three PV modules Case PV1, W/m2 PV2 W/m2 PV3 W/m2 1 1000 1000 1000 2 700 700 700 3 500 500 500 4 300 300 300 5 1000 700 700 6 1000 700 500 7 700 500 300 8 500 300 300 Table 4. Measured data for PV1 under different illumination Voc, V Isc, A Vmpp, V Impp, A Pmpp, W Illumination W/m2 21.19 0.530 18.13 0.399 7.23 1000 21.12 0.337 18.04 0.272 4.90 700 21.04 0.241 18.32 0.191 3.49 500 20.91 0.171 17.82 0.133 2.37 300 Table 5. Measured data for PV2 under different illumination Voc, V Isc, A Vmpp, V Impp, A Pmpp, W Illumination W/m2 21.31 0.596 18.09 0.485 8.77 1000 21.39 0.386 18.03 0.312 5.62 700 21.14 0.279 17.95 0.232 4.16 500 20.96 0.175 18.23 0.151 2.75 300 Table 6. Measured data for PV3 under different illumination Voc, V Isc, A Vmpp, V Impp, A Pmpp, W Illumination W/m2 21.49 0.543 18.11 0.458 8.29 1000 21.53 0.400 18.55 0.320 5.87 700 21.41 0.269 17.88 0.246 4.39 500 21.06 0.168 17.82 0.147 2.60 300 Fig. 8Open in figure viewerPowerPoint Measured PV curves (a) Measured I–V curves, (b) Measured P–V curves A microcontroller TMS320F335 digital signal processor (DSP) is used to sense the solar panel voltage and current through the voltage sensor and current sensor. The DSP is used to calculate the new duty cycle to adjust the converter output voltage and control the solar panel operate at the MPPs. The programme flowchart used in the DSP is shown in Fig. 3. With the proposed MPPT method, the operating point of the solar panel will gradually converge to the MPP, and the oscillations is reduced significantly when the system operates close to the MPP, thus a stable maximum output power can be obtained. Fig. 9a shows the block diagram of proposed system. Experimental equipment setup is shown in Fig. 9b. The comparison between the proposed PV architecture and PV balancer architecture is based on the experimental results. The measured data of single architecture are recorded in Tables 7 and 8 and illustrated as shown in Fig. 10a. The blue histograms and curves of Fig. 10a are the measured results of the proposed architecture, and the green ones are the measured results of the PV balancer. The results show that the proposed architecture can operate at the MPP's, and its output power and the system efficiency η are 3% higher than that of the PV balancer. Table 7. Measured data of the proposed architecture (single) under different illuminations Illumination W/m2 VPV, V IPV, A PPV, W Pload, W η, % 1000 18.19 0.463 8.42 8.02 95.24 700 19.10 0.307 5.87 5.62 95.73 500 18.71 0.235 4.40 4.21 95.68 300 18.10 0.140 2.54 2.44 96.08 Table 8. Measured data of the PV balancer architecture (single) under different illuminations Illumination W/m2 VPV, V IPV, A PPV, W Pload, W η, % 1000 18.11 0.460 8.33 7.62 91.42 700 18.08 0.316 5.71 5.21 91.29 500 18.08 0.234 4.23 3.92 92.56 300 18.07 0.138 2.49 2.32 92.87 Fig. 9Open in figure viewerPowerPoint The proposed system (a) Block diagram of proposed system, (b) Experiment setup Fig. 10Open in figure viewerPowerPoint Measured results of the system (a) Results of the proposed system and the PV balancer under different illuminations (single architecture). (b) Measured results of the two expansion architectures in Cases 1–4. (c) Measured data of the two expansion architectures in Cases 5–8 The measured results for the expansion architectures of the proposed circuit and the PV balancer under uniform illumination are listed in Tables 9 and 10 and shown in Fig. 10b. It is obvious that the proposed expansion architecture makes each solar panel operate at MPPs. However, the PV balancer expansion architecture only operates at MPP's in Case 1, and it produce less power than the proposed system in Cases 2–4. The proposed system also operates at high efficiencies. Table 9. Measured data of the proposed expansion architecture in Cases 1–4 Case PPV1, W PPV2, W PPV3, W Pload, W η, % 1 7.21 8.73 8.31 22.91 94.41 2 4.91 5.64 5.86 15.71 95.72 3 3.49 4.19 4.37 11.62 96.31 4 2.34 2.76 2.58 7.42 96.31 Table 10. Measured data of the PV balancer expansion architecture in Cases 1–4 Case PPV1, W PPV2, W PPV3, W Pload, W η, % 1 7.19 8.71 8.32 22.31 92.05 2 4.83 5.54 5.71 14.92 92.65 3 3.39 4.01 4.28 10.81 92.53 4 2.30 2.68 2.53 7.02 93.23 Similar results can be observed under non-uniform illumination conditions. The PV balancer can achieve MPPT at illumination of 1000 W/m2; however, it cannot achieve MPPT under other illumination conditions. The proposed architecture can control the solar panel to operate at the MPP's, thus producing maximum power under various illumination conditions. The measured results of the two expansion architectures are listed in Tables 11 and 12 and shown in Fig. 10c. It is obvious that the proposed architecture produces higher output power and operates at higher efficiencies than the PV balancer under different illumination conditions. Table 11. Measured data of the proposed expansion architecture in Cases 5–8 Case PPV1, W PPV2, W PPV3, W Pload, W η, % 5 7.19 5.61 5.86 17.71 94.87 6 7.20 5.62 4.42 16.42 95.11 7 4.91 4.13 2.55 11.12 95.83 8 3.47 2.76 2.57 8.51 96.63 Table 12. Measured data of the PV balancer expansion architecture in Cases 5–8 Case PPV1, W PPV2, W PPV3, W Pload, W η, % 5 7.17 5.51 5.75 17.21 93.34 6 7.12 5.60 4.32 15.82 92.75 7 4.79 3.93 2.49 10.51 93.66 8 3.33 2.65 2.49 8.01 94.50 The differences of the output power between the two architectures will be more significant for a high-power PV system. The observed differences are caused by the smaller solar system used in the experiments. The proposed PV architecture and the PV balancer are operated in discontinuous conduction mode, and the waveforms are shown in Figs. 11a and b, respectively. Fig. 11c shows the tracking waveforms of the proposed architecture. It can be seen that the proposed architecture can achieve an effective and stable MPPT. The experimental results agree with the simulation results. Fig. 11Open in figure viewerPowerPoint Tracking waveforms (a) Waveforms of the proposed architecture, (b) Waveforms of the PV balancer circuit. (c) Tracking waveforms of the proposed architecture From the experimental results, very slight differences are observed on the output voltages and currents of panels even if a solar panel is tested with the same light irradiance and temperature. According to the measurements, most of the power differences for the solar panel are within ± 0.08 W. The differences are considered insignificant and negligible. The errors between the simulation and the experimental results are shown in Figs. 12a and b. From the figures, it is observed that the errors are less than ±2.5% for the proposed architecture and less than ±2% for the PV balancer. The errors between simulation and experimental results due to the solar panels and control circuits are insignificant. Fig. 12Open in figure viewerPowerPoint The errors and efficiency of the system (a) Errors of the proposed architecture (single architecture). (b) Errors of the PV balancer (single architecture). (c) Converter efficiency of the proposed architecture The converter efficiency of the proposed architecture is shown in Fig. 12c. The converter efficiency is calculated for the converter after the DC bus and the solar panel are removed from the system. Since the converter requires only partial power to compensate for the voltage difference between the solar panel and the DC bus, thus, the power loss is reduced. The system efficiency η is higher than the converter efficiency. For example, the converter efficiency is 85% and the output power of the solar panel is 10 W, then the system efficiency is 95%. The converter only needs to handle the power of 3.33 W, and most of the power only goes through the diode, thus the power loss can be reduced. Based on the experimental results, the proposed solar system can achieve effective MPPT under different illumination conditions and operate at high efficiencies. 7 Conclusion An improved solar system architecture with MPPT has been proposed in this paper. A DSP microcontroller is used to make each PV module operate at its MPP's whether it is under shaded conditions or regular conditions. The experimental results show that each solar panel operates at its MPP's, which are different, this confirms that each panel works independently. The proposed architecture can achieve MPPT and produce more power with high efficiency than the PV balancer. Although the power improvement is not significant, the measured results show that the proposed architecture is effective with high efficiency. 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