A Comparison of Soft-Switched DC-to-DC Converters for Electrolyzer Application

 

ABSTRACT:

 An electrolyzer is part of a renewable energy system and generates hydrogen from water electrolysis that is used in fuel cells. A dc-to-dc converter is required to couple the electrolyzer to the system dc bus. This paper presents the design of three soft-switched high-frequency transformer isolated dc-to-dc converters for this application based on the given specifications. It is shown that LCL-type series resonant converter (SRC) with capacitive output filter is suitable for this application. Detailed theoretical and simulation results are presented. Due to the wide variation in input voltage and load current, no converter can maintain zero-voltage switching (ZVS) for the complete operating range. Therefore, a two-stage converter (ZVT boost converter followed by LCL SRC with capacitive output filter) is found suitable for this application. Experimental results are presented for the two-stage approach which shows ZVS for the entire line and load range.

 KEYWORDS:

  1. DC-to-DC converters
  2. Electrolyzer
  3. Renewable energy system (RES)
  4. Resonant converters

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of a typical RES.

EXPECTED SIMULATION RESULTS:

Fig. 2. Calculated and simulated results for LCL SRC with capacitive output filter for Vin = 40V. (a) Vin,min = 40V, Vo = 60V, and Id = 40 A. (b) Vin,min = 40V, Vo = 60V, and Id = 20 A. (c) Vin,min = 40V, Vo = 60V, and Id = 4A.

Fig. 3. Simulated results for LCL SRC with capacitive output filter for different operating conditions. (a) Vin,max = 60V and Vo = 60V at full-load and half-load conditions. (b) Vin = 40V and 60V, Vo = 40V, and Id = 10 A.

Fig. 4. Simulation waveforms for LCL SRC with capacitive output filter at full-load (2.4 kW) with Vin = 40V and Vo = 60V: inverter output voltage vab ; current through resonant tank inductor iLr ; switch currents (iS 1 –iS 4 ); rectifier input voltage (vrectin ); voltage across and current through output rectifier diode DR1 .

Fig. 5. Simulation waveforms of Fig. 13 repeated for LCL SRC with capacitive output filter at 10% load with Vin = 40V and Vo = 60V.

 CONCLUSION:

A comparison of HF transformer isolated, soft-switched, dc to- dc converters for electrolyzer application was presented. An interleaved approach with three cells (of 2.4kWeach) is suitable for the implementation of a 7.2-kW converter. Three major configurations designed and compared are as follows: 1) LCL SRC with capacitive output filter; 2) LCL SRC with inductive output filter; and 3) phase-shifted ZVS PWM full-bridge converter. It has been shown that LCL SRC with capacitive output filter has the desirable features for the present application. Theoretical predictions of the selected configuration have been compared with the SPICE simulation results for the given specifications. It has been shown that none of the converters maintain ZVS for maximum input voltage. However, it is shown that LCL-type SRC with capacitive output filter is the only converter that maintains soft-switching for complete load range at the minimum input voltage while overcoming the drawbacks of inductive output

filter. But the converter requires low value of resonant inductor Lr for low input voltage design. Therefore, it is better to boost the input voltage and then use the LCL SRC with capacitive output filter as a second stage. When this converter is operated with almost fixed input voltage, duty cycle variation required is the least among all the three converters while operating with ZVS for the complete variations in input voltage and load. A ZVT boost converter with the specified input voltage (40–60 V) will generate approximately 100V as the input to the resonant converter for Vo = 60V. Therefore, we have investigated the performance of a ZVT boost converter followed by the LCL SRC with capacitive output filter. It was shown experimentally that the two-stage approach obtained ZVS for all the switches over the complete operating range and also simplified the design of resonant converter.

REFERENCES:

[1] A. P. Bergen, “Integration and dynamics of a renewable regenerative hydrogen fuel cell system,” Ph.D. dissertation, Dept. Mechanical Eng., Univ. Victoria, Victoria, BC, Canada, 2008.

[2] D. Shapiro, J. Duffy, M. Kimble, and M. Pien, “Solar-powered regenerative PEM electrolyzer/fuel cell system,” J. Solar Energy, vol. 79, pp. 544–550, 2005.

[3] F. Barbir, “PEM electrolysis for production of hydrogen from renewable energy sources,” J. Solar Energy, vol. 78, pp. 661–669, 2005.

[4] R. L. Steigerwald, “High-frequency resonant transistor DC-DC converters,” IEEE Trans. Ind. Electron., vol. 31, no. 2, pp. 181–191, May 1984.

[5] R. L. Steigerwald, “A Comparison of half-bridge resonant converter topologies,” IEEE Trans. Power Electron., vol. 3, no. 2, pp. 174–182, Apr. 1988.

 

 

 

Analysis and Design of High-Frequency Isolated Dual-Bridge Series Resonant DC/DC Converter

ABSTRACT:

Bidirectional dual-bridge dc/dc converter with high frequency isolation is gaining more attentions in renewable energy system due to small size and high-power density. In this paper, a dual-bridge series resonant dc/dc converter is analyzed with two simple modified ac equivalent circuit analysis methods for both voltage source load and resistive load. In both methods, only fundamental components of voltages and currents are considered. All the switches may work in either zero-voltage-switching or zero-current-switching for a wide variation of voltage gain, which is important in renewable energy generation. It is also shown in the second method that the load side circuit could be represented with an equivalent impedance. The polarity of cosine value of this equivalent impedance angle reveals the power flow direction. The analysis is verified with computer simulation results. Experimental data based on a 200 W prototype circuit is included for validation purpose.

 KEYWORDS:

  1. Analysis and simulation
  2. Dc-to-dc converters
  3. Modeling
  4. Renewable energy systems
  5. Resonant converters

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

 image001

Fig. 1. Hybrid renewable energy generation system with battery back-up function.

EXPECTED SIMULATION RESULTS:
image002

 Fig. 2. Output power versus phase-shift angle φ. (a) F = 1.1, M = 0.95,

and different Q. (b) F = 1.1, Q = 1, and different converter gain M.

image003

Fig. 3. Operation in charging mode (Vi = 110 V, Vo = 100 V), simulated waveforms of vAB and vCD , resonant current iS , resonant capacitor voltage vCs , output current before filter capacitor io for output power (a) Po = 200W, (b) Po = 100 W, and (c) Po = 20 W.
image004

Fig. 4. Operation in regeneration mode (Vi = 110V, Vo = 100 V). Simulated waveforms of vAB and vCD , resonant current iS , resonant capacitor voltage vCs , output current before filter capacitor io for output power Po = −200 W.

image005

Fig. 5. Full-load test results (Vi = 110 V, Vo = 100 V). (a) From top to bottom vAB (100V/div), vCD (100V/div), is (2A/div). (b) vC (100V/div). (c) Primary switch current (1A/div). (d) Secondary switch current (1A/div).

image006

Fig. 6. (a) Half-load test results (Vi = 110 V, Vo = 100 V): from top to bottom: vAB (100 V/div), vCD (100 V/div), is (2 A/div), primary switch current (1 A/div), secondary switch current (1 A/div). (b) 10% load condition test results (Vi = 110 V, Vo = 100 V): from top to bottom: waveforms of (a) repeated.

image007

Fig. 7. Output current of secondary converter under different load levels (Vi = 110 V, Vo = 100 V). (a) 200 W (2A/div). (b) 100 W (2A/div). (c) 20 W (1A/div).

CONCLUSION:

In this paper, a HF isolated dual-bridge series resonant dc/dc converter has been proposed, which is suitable for renewable energy generation applications. Two modified ac equivalent circuit analysis methods were presented to analyze the DBSRC. First method used was voltage-source type of load, whereas, second method uses a controlled rectifier with resistive load. It was shown that an equivalent impedance could be used to represent the secondary part circuit in the case of resistive load to include the bidirectional feature. Detailed analysis has been presented for both the methods. Same results were obtained from both the methods. ZVS turn-ON for primary-side switches and ZCS turn-OFF for secondary-side switches could be achieved for all load and input/output voltage conditions. Design procedure has been illustrated by a 200Wdesign example. Through the SPICE simulation and experimental results, the theoretical results have been verified.

In the DAB converter, performance of the converter is heavily dependent on the leakage inductance of the transformer (used for power transfer and should be as small as possible) [15], [19], whereas, in the DBSRC, leakage inductance is used as part of resonant tank. If the DAB converter is used for application with wide input/output voltage variation, ZVS of primary-side converter may be hard to achieve [19]. DBSRC has low possibility of transformer saturation due to the series capacitor (that can be split as mentioned earlier). The disadvantage of DBSRC is the size of resonant tank (additional capacitor), which brings extra size and cost. Further work is required to compare the DAB converter with the DBSRC for such applications. In the future, more study will be done based on the DBSRC. Efforts will focus on modifications to realize ZVS on the secondary side to reduce the switching losses further. With all two quadrant switches replaced with four-quadrant switches [23], the converter could be controlled as an ac/ac electronic transformer, which can be used in doubly fed induction generator (DFIG) based wind generation system. For high-power applications, multicells of the converter may be used to meet high power density requirements.

REFERENCES:

[1] L. H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie, S. Munk-Nielsen, H. Bindner, P. Sørensen, and B. Bak-Jenseen, “Conceptual survey of generators and power electronics for wind turbines,” Risø Nat. Lab., Roskilde, Denmark, Tech. Rep. Risø-R-1205(EN), ISBN 87-550-2743-8, Dec. 2001.

[2] N. Kasa, Y. Harada, T. Ida, and A. K. S. Bhat, “Zero-current transitions converters for independent small scale power network system using lower power wind turbines,” in Proc. IEEE Int. Symp. Power Electron., Electric Drives, Autom. Motion 2006, May 23–26, pp. 1206–1210.

[3] J. Lai and D. J. Nelson, “Energy management power converters in hybrid electric and fuel cell vehicles,” Proc. IEEE, vol. 95, no. 4, pp. 766–777, Apr. 2007.

[4] H. Tao, A. Kotsopoulos, J. L. Duarte, andM. A.M. Hendrix, “Multi-input bidirectional dc-dc converter combining dc-link and magnetic-coupling for fuel cell systems,” in Proc. 40th IEEE IAS Annu. Meet., Oct. 2005, vol. 3, pp. 2021–2028.

[5] F. Z. Peng, H. Li, G.-J. Su, and J. S. Lawler, “A new ZVS bidirectional dc–dc converter for fuel cell and battery application,” IEEE Trans. Power Electron., vol. 19, no. 1, pp. 54–65, Jan. 2004.

A Comparison of Soft-Switched DC-to-DC Converters for Electrolyzer Application

ABSTRACT:

An electrolyzer is part of a renewable energy system and generates hydrogen from water electrolysis that is used in fuel cells. A dc-to-dc converter is required to couple the electrolyzer to the system dc bus. This paper presents the design of three soft-switched high-frequency transformer isolated dc-to-dc converters for this application based on the given specifications. It is shown that LCL-type series resonant converter (SRC) with capacitive output filter is suitable for this application. Detailed theoretical and simulation results are presented. Due to the wide variation in input voltage and load current, no converter can maintain zero-voltage switching (ZVS) for the complete operating range. Therefore, a two-stage converter (ZVT boost converter followed by LCL SRC with capacitive output filter) is found suitable for this application. Experimental results are presented for the two-stage approach which shows ZVS for the entire line and load range.

KEYWORDS:

  1. DC-to-DC converters
  2. Electrolyzer
  3. Renewable energy system (RES)
  4. Resonant converters.

 SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:
image001

 Fig. 1. Block diagram of a typical RES.

 EXPECTED SIMULATION RESULTS:
image002

Fig. 2. Simulation waveforms for LCL SRC with capacitive output filter at full-load (2.4 kW) with Vin = 40V and Vo = 60V: inverter output voltage vab ; current through resonant tank inductor iLr ; switch currents (iS 1iS 4 ); rectifier input voltage (vrectin ); voltage across and current through output rectifier diode DR1 .

image003

Fig. 3. Simulation waveforms of Fig. 13 repeated for LCL SRC with capacitive output filter at 10% load with Vin = 40V and Vo = 60V.

image004

Fig. 4. Experimental waveforms obtained for two stage converter cell (see Fig. 15) at full-load (2.4 kW) with Vin = 40V and Vo = 60V. (a) Voltage vSW across drain-to-source of boost switch (SW) and gating signal vg to the boost switch. (b) Inverter output voltage vab and current through resonant tank inductor iLr . (c) Rectifier input voltage vrectin and current through parallel inductor Lt , iLt . (d) Rectifier input voltage vrectin and secondary current isec . Scales: (a) vSW (40V/div) and vg (10V/div). (b) vab (100 V/div) and iLr (20A/div) (c) vrectin (100 V/div) and iLt (20A/div). (d) vrectin (100 V/div) and isec (20A/div).

image005

 Fig. 5. Experimental waveforms of Fig. 17 repeated for Vin = 40V and Vo = 40V at Id = 10 A. Scales: (a) vSW (40V/div) and vg (10V/div). (b) vab (40V/div) and iLr (20A/div). (c) vrectin (100 V/div) and iLt (20A/div). (d) vrectin (100 V/div) and isec (20A/div).

image006

Fig. 6. Experimental waveforms of Fig. 17 repeated for Vin = 60V and Vo = 40V at Id = 10 A. Scales: (a) vSW (40V/div) and vg (10V/div). (b) vab (40V/div) and iLr (20A/div). (c) vrectin (100 V/div) and iLt (20A/div). (d) vrectin (100 V/div) and isec (20A/div).

 CONCLUSION:

 A comparison of HF transformer isolated, soft-switched, dc to- dc converters for electrolyzer application was presented. An interleaved approach with three cells (of 2.4kWeach) is suitable for the implementation of a 7.2-kW converter. Three major configurations designed and compared are as follows: 1) LCL SRC with capacitive output filter; 2) LCL SRC with inductive output filter; and 3) phase-shifted ZVS PWM full-bridge converter. It has been shown that LCL SRC with capacitive output filter has the desirable features for the present application. Theoretical predictions of the selected configuration have been compared with the SPICE simulation results for the given specifications. It has been shown that none of the converters maintain ZVS for maximum input voltage. However, it is shown that LCL-type SRC with capacitive output filter is the only converter that maintains soft-switching for complete load range at the minimum input voltage while overcoming the drawbacks of inductive output filter. But the converter requires low value of resonant inductor Lr for low input voltage design. Therefore, it is better to boost the input voltage and then use the LCL SRC with capacitive output filter as a second stage. When this converter is operated with almost fixed input voltage, duty cycle variation required is the least among all the three converters while operating with ZVS for the complete variations in input voltage and load. A ZVT boost converter with the specified input voltage (40–60 V) will generate approximately 100V as the input to the resonant converter for Vo = 60V. Therefore, we have investigated the performance of a ZVT boost converter followed by the LCL SRC with capacitive output filter. It was shown experimentally that the two-stage approach obtained ZVS for all the switches over the complete operating range and also simplified the design of resonant converter.

REFERENCES:

[1] A. P. Bergen, “Integration and dynamics of a renewable regenerative hydrogen fuel cell system,” Ph.D. dissertation, Dept. Mechanical Eng., Univ. Victoria, Victoria, BC, Canada, 2008.

[2] D. Shapiro, J. Duffy, M. Kimble, and M. Pien, “Solar-powered regenerative PEM electrolyzer/fuel cell system,” J. Solar Energy, vol. 79, pp. 544–550, 2005.

[3] F. Barbir, “PEM electrolysis for production of hydrogen from renewable energy sources,” J. Solar Energy, vol. 78, pp. 661–669, 2005.

[4] R. L. Steigerwald, “High-frequency resonant transistor DC-DC converters,”IEEE Trans. Ind. Electron., vol. 31, no. 2, pp. 181–191, May 1984.

[5] R. L. Steigerwald, “A Comparison of half-bridge resonant converter topologies,” IEEE Trans. Power Electron., vol. 3, no. 2, pp. 174–182, Apr. 1988.