XV International PhD Workshop OWD 2013, October Effective Utilization of Photovoltaic Energy Using Multiphase DC/DC converters

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1 XV International PhD Workshop OWD 2013, October 2013 Effective Utilization of Photovoltaic Energy Using Multiphase DC/DC converters Ján Perduľak, Technical University of Košice Abstract This article describes effective utilization of photovoltaic energy using a novel concept of multiphase DC/DC converters. These new concepts allow effective utilization of energy from photovoltaic solar module. The effective utilization of energy is ensured by adding five parallel legs to the conventional single phase DC/DC converters. Appropriate algorithm of switches control allows taking the photovoltaic output energy one of these six parallel legs in every moment. The simulation and the laboratory models were built and the simulation and experimental results were obtained to verify the theoretical properties of multiphase DC/DC converters. 1. Introduction The photovoltaic has a dominant position in using of renewable sources of energy in these days. Photovoltaic ensures the direct conversion of light into electricity. Problem is in the low energy efficiency of photovoltaic systems and in the low efficiency of photovoltaic cells. The conversion efficiency is moving around 5% for a-si to 25% - 30% for GaAs [1]. These problems can be solving by two ways. Firstly, it s necessary to develop and to apply the photovoltaic cells with high conversion efficiency in practice. This is from technological and financial point of view very difficult. The second way, more realistic, is to develop the converters which should be able to utilize whole maximal output energy from photovoltaic module under the all weather conditions. This way is from technological and financial point of view very friendly not only for future utilization in practice but also in terms of availability to the general public. P PV(MPP) form PV module at the different operating conditions. These statements explain the following example, Fig.1. Impinging sun energy P inpv on surface of PV module is converted to electric energy. The efficiency of conversion of solar energy to electric energy η PV is low resulting from material and physical properties of PV module [odkaz na lit]. The output PV energy P outpv is equals the input energy to the dc-dc converter P incon. The role of dc-dc converter is to ensure that the maximum output PV energy, P PV(MPP), will be taken from PV module and then the maximum energy usability of photovoltaic generator λ E at the different operating conditions. Other words, we are able to take as much energy as the dc-dc converter allows. Let's assume the bad operating conditions as it is shown on Fig.1 b). It can be seen that the open circuit voltage V OC and optimum operating voltage V MPP of PV module are smaller. Let's assume that the boost converter is used in PV system. Under the bad operating conditions we are unable to set the MPP point using boost converter. We are unable to take the maximum power output P FV(MPP) form PV module. This is because boost converter can`t takes the optimum operating current I MPP from PV module under the optimum operating voltage V MPP. The energy usability of photovoltaic generator λ Ε is for bad operating conditions very low. 2. Problem with effective utilization of photovoltaic energy There are two big problems with effective utilization of photovoltaic (PV) energy. First of them is the conversion of sun energy to electric energy by the PV module. The second is in the possibilities of effective utilization of maximum output power Fig.1. PV system under different operational conditions. 145

2 It can be seen that the efficiency of energy conversion of PV energy η Ε and the energy usability of photovoltaic generator λ Ε are very low under the bad operating conditions. It is because we are unable to set the MPP operating point with using boost converter and then utilizing the whole potential PV output energy. 3. Simulation and experimental verification of theoretical assumptions Two types of dc-dc converters were selected to verify the theoretical assumptions. a) Boost converter, b) BROSKOV`s converter it is a new type of dcdc converter. It belongs to family of buck-boost converter. Simulation models of boost and BROSKOV s converter created in program OrCAD Capture CSI are shown in Fig.2. The dc source V PV was used to simulate PV module under the different operating conditions. Another dc source was used as a load Z to simulate the battery with nominal voltage V BAT = 14V. The dc source V PV simulate PV module with characteristic properties: open-circuit voltage - V OC = 17V, short-circuit current - I SC = 9A, optimum operating voltage - V MPP = 12V, optimum operating current - I MPP = 8A. Simulation parameters: switching frequency - f S = 50 khz, inductance - L = 100 uh, battery voltage - V BAT = 14V. To make simulation more realistic the resistor R WIRE represents resistance of a wire, resistor R BAT represents resistance of a battery, resistor R SPV represents series resistance of solar cell and inductance L WIRE represents inductance wire were added to simulation model of boost and BROSKOV s converter. The control structures of converters were created in program MATLAB-SimuLink. To implement control structures created in MATLAB to converters created in OrCAD Capture CSI the SLPS (SimuLink PSpice) interface was used. Simulation models of control structures are shown in Fig.3. The duty cycle z is calculated in z_cal block for whole range of operating conditions from V PV = 4V (simulation bad weather conditions) to V PV = 12V (simulation ideal weather conditions). After the duty cycle z is calculated the PWM signal is generated by PWM_block for switching transistor. Fig.3. simulation models of control structures of boost converter (up) and BROSKOV s converter (down). The simulation results are shown in Fig 4 and Fig. 5. Fig.4 and Fig. 5 show overview, the output converter current i A(t) (current before entering to the filter), photovoltaic current i PV(t) and duty cycle z at different value of photovoltaic voltage U PV. It can be seen that the MPP point is reached only for good operating conditions, other words for good weather conditions when V PV <12V 8V> in case of boost converter and when V PV = 12V in case of BROSKOV s converter. Converters take maximal energy from the PV module and so energy usability of PV generator λ Ε is maximal. Fig.2. Simulation models of boost converter (up) and BROSKOV s converter (down) Fig.4. Simulation results boost converter. Overview (upper part) and detail view (lower part). 146

3 When the PV voltage V PV decreases (deteriorating weather conditions) the converters are unable to set the operating point near the MPP and thus ensure the maximal energy usability of PV generator λ Ε. It can be seen that for whole range of input voltage V PV the converters work in discontinuous current mode (DCM) as can be seen in Fig.4 and Fig. 5. for boundary conditions at the V PV = 12V and V PV = 4V. The efficiency of energy conversion of PV energy η Ε is low for whole range of input voltages V PV. The oscillograms, shown in Fig. 6 and Fig. 7, fully confirm the simulation results and theoretical assumptions. The oscillograms show the dc source voltage V PV (= input converter voltage = output PV voltage), gate voltage V GS, dc source current i PV(t) and the output converter current i A(t) flow to the load Z. It can be seen that the MPP point is reached only for ideal operating conditions when V PV = 12V. When the dc source voltage V PV decreases under 6V the converter is unable to set the operating point near the MPP even if the maximal duty cycle z is set. Fig.5. Simulation results BROSKOV s converter. Overview (upper part) and detail view (lower part). To verify the theoretical assumptions and simulation results the laboratory models of converters with controls were built. The dc regulated source was used to simulate the different operation conditions of PV module. The dc regulated source simulate PV module with same characteristic properties as were used in the case of simulation. The 12V battery was used as a load Z in both cases. 4. Improving utilization of photovoltaic energy The Section 3 point to the wasteful, uneconomic and inefficient utilization of photovoltaic energy when the efficiency of energy conversion of PV energy η Ε and energy usability of PV generator λ Ε are very low for whole range on operation conditions. To avoid these serious deficiencies it is necessary increase the efficiency of taking PV energy from PV module for whole range of operation conditions. Even if there are bad operation conditions the maximum PV energy has to be taken form PV module. We have to ensure that the converter will be work in CCM mode regardless of its pulse mode. One of the solutions to abovementioned drawbacks is using multiphase energy drawn. To do it is using the proposed concept of multiphase converters shown in Fig.8, [2], [3]. Fig.8. the proposed topology of multiphase boost converter and multiphase BROSKOV s converter. Fig.6. Experimental results boost converter. Fig.7. Experimental results BROSKOV s converter. The main idea is very simple. When we taking energy form PV module by one or more legs of multiphase converter (MPC) the rest of legs delivery PV energy to the load Z. The number of legs which are delivering PV energy to the load Z depends on value of duty cycle z which is sets to the same value for all legs. Theoretically it is possible to taking the PV energy by all legs of MPC at one moment. The control scheme of MPC depends from number of phases because the different phase shift is needed. Even if operation conditions or more precisely weather conditions become worse we are able to set the MPP point by these multiphase converters. The energy usability of PV generator λ Ε and efficiency of 147

4 energy conversion of PV energy η Ε are maximal for whole range of operation conditions with using the proposed multiphase converters. The another important fact is that the converters work in CCM in comparison with conventional single phase boost converter (SPBoC) and single phase BROSKOV s converter (SPBrC) for whole range of operation conditions. The cooperation between phases starts within period T so the PV energy is continuously delivering to the load Z during the whole period T. Minimum number of phases is one. In this case we get the conventional single phase converter. The theoretical maximum numbers of phases are infinity. Selection of right number of phases is very important. We have to achieve compromise between complexity, dimensions, weight and economic aspect of design. 5. Simulation and experimental verification of theoretical assumptions The simulation models of multiphase boost converter (MPBoC) and multiphase BROSKOV s converter (MPBrC) were created in program OrCAD Capture CSI to verify the theoretical assumptions, Fig. 9 and Fig. 10, [4]. The MOSFET transistors with N-channel and P-channel were used as switches. The dc sources V PV were used to simulate PV modules under the different operating conditions. Another dc sources V BAT were used as load Z. The dc sources were used to simulate the battery with nominal voltage V BAT = 14V. The dc sources V PV simulate PV modules with the same characteristic properties as were used in the case SPBoC and SPBrC. Simulation parameters: switching frequency - f S = 50 khz, inductance - L 1 L 6 = 100 uh, battery voltage - V BAT = 14V. The simulation models of proposed MPBoC and MPBrC are shown in Fig. 9 and Fig. 10. Fig.9. Simulation model of MPBoC. Fig.10. simulation model of MPBrC. To make simulation more realistic the resistor R WIRE represents resistance of a wire, resistor R BAT represents resistance of a battery, resistor R SPV represents series resistance of solar cell and inductance L WIRE represents inductance wire were added to simulation models of MPBoC and MPBrC. The six phases was chosen as a compromise between the economics and energy aspects. The control structure of MPBoC and MPBrC created in program MATLAB-SimuLink is shown in Fig. 11 and Fig. 12. Fig.11. simulation models of control structures of MPBoC. Fig.12. simulation models of control structures of MPBrC. The duty cycle z is calculated in z_cal block for whole range of operating conditions from V PV = 4V to V PV = 12V. After the duty cycle z is calculated the PWM signal is generated by PWM_block for switching transistors. The phase shift of control signals is ensured by PS_block. Błąd! Nie można odnaleźć źródła odwołania.fig.13 and Fig. 14 show energy usability of PV generator λ Ε of MPBoC and MPBrC under different operational conditions. The results of multiphase converters are much better in comparison with single-phase converters. Fig.13 and Fig.14 show overview, the output converter current i A(t), photovoltaic current i PV(t) and duty cycle z at different value of photovoltaic voltage V PV. The multiphase converters work: 148

5 a) in MPP for whole range of photovoltaic voltage VPV. Converter takes maximal energy PPV(MPP) from the PV module even if bad operational conditions occurs. The energy usability of PV generator λε is maximal for whole range of operational conditions. b) In DCM for whole range of photovoltaic voltages VPV from 4V to 12V. This is because another five phases are added to the conventional single-phase converters and appropriate algorithm of switches control is sets. The PV energy is continuously taken from PV module and delivered to the load Z within the whole period T. The current ia(t) is sum of currents in particular legs which deliver the PV energy to the load Z. The efficiency of energy conversion of PV energy ηε is maximal for whole range of input voltages VPV. c) With ripple current ia(t) moreless in comparison with single-phase converters. Fig.15. Laboratory model of MPBoC with control. Fig.16. Laboratory model of MPBrC with control. The dc regulated source was used to simulate the different operation conditions of PV module. The 12V battery was used as a load Z. The dc source VPV simulate PV module with same characteristic properties as in the previous cases The following oscilograms fully confirm the theoretical assumptions and simulation results. Fig.13. MPBoC - simulation results. Fig.14. MPBrC - simulation results. To verify the theoretical assumptions and simulation results the laboratory models of MPBoC and MPBrC with control were built as it shown in Fig.15 and Fig.16, [5], [6]. The experimental results of multiphase converters were compared with experimental results of single-phase converters [7], [8], [9], [10]. The switching frequency fs was set to 50 khz and the inductance in particular L1 to L6 was set to 100 uh. Fig.17. Experimental r esults MPBoC. 149

6 Oscilograms shown in Fig. 17 and Fig. 18 describe comparison of properties of laboratory models of multiphase converters (MPCs) (left part of Fig. 17 and Fig. 18) and single-phase converters (SPCs) (right part of Fig. 17 and Fig. 18). The following general conclusions can be draw: MPCs work in comparison with SPCs : avoid this situation means to begin economical use of solar energy. This can be done by using of proposed topologies of MPBoC and MPBrC. These converters effective utilization of PV energy and so the efficiency of energy conversion of PV energy η Ε and energy usability of PV generator λ Ε are extremely growing. a) In MPP for whole range of dc source voltage V PV. The energy usability of dc source λ Ε is maximal for whole range of operational conditions. b) In DCM for whole range of dc source voltage V PV. c) With ripple current i A(t) moreless in comparison with SPCs. d) The MPCs provides more variable duty cycle z. Fig.18. Experimental results MPBrC. 6. Conclusion The uneconomic and wasteful utilization of PV energy is with using of single-phase dc/dc converters was shown. This is the major factor in long-term energy recovery and high cost of PV modules. In other words, the time for which we produce the equivalent amount of electricity proportionate to the investment of the financial capital to PV system plant is at the present time and in the local geographical conditions is significant. To ACKNOWLEDGMENT The paper has been prepared under support of Slovak grant projects KEGA No. 005TUKE- 4/2012. Bibliography [1] Fundamental of photovoltaic materials National Solar Power Research Institute, Inc 12/12/98, p.s.10. [2] D. Kováč, I. Kováčová, Patent application No titled as, Multiphase boost DC/DC converter. [3] D. Kováč, I. Kováčová, Patent application No titled as, Multiphase boost DC/DC converter with constant slope falling off of inductance current. [4] D. Kováč, I. Kováčová,, Modeling and measuring of the electronic circuits. Košice: Elfa, 1996, 96 pages. [5] D. Kováč, I. Kováčová, J. Perduľak, T. Vince, J. Molnár, Patent application No titled as, Pulse generator for multiphase boost converter. [6] D. Kováč, I. Kováčová, J. Perduľak, T. Vince, J. Molnár, Patent application No titled as, Analogue pulse generator for multiphase step-up DC/DC converter. [7] D. Kováč, I. Kováčová,, EMC Aspect as Important Parameter of New Technologies. In: New Trends in Technologies: Control, Management, Computational Intelligence and Network Systems, Publisher: Sciyo, November 2010, pp , ISBN [8] D. Kováč, I. Kováčová,, Multiphase boost Power transistor MOSFET and IGBT (in Slovak), Košice: Elfa,1996, 117 pages. [9] D. Kováč, I. Kováčová,, Safeguard circuits of power semiconductor parts. In: Acta Electrotechnica et Informatica. Vol. 3, No. 3 (2003), pp , ISSN [10] D. Kováč, I. Kováčová, J. Oetter, Applied electronics: Rules for exercises I. (in Slovak), Košice: Akris, 2001, 94 pages, ISBN