Improving the DC-DC Power Conversion Efficiency in a Solid Oxide Fuel Cell System
Hiltunen, Jani (2019-11-29)
Väitöskirja
29.11.2019
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Sähkötekniikka
Kaikki oikeudet pidätetään.
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In reference to IEEE copyrighted material which is used with permission in this thesis, the IEEE does not endorse any of Lappeenranta-Lahti University of Technology LUT's products or services. Internal or personal use of this material is permitted. If interested in reprinting/republishing IEEE copyrighted material for advertising or promotional purposes or for creating new collective works for resale or redistribution, please go to http://www.ieee.org/publications_ standards/publications/rights/rights_ link.html to learn how to obtain a License from RightsLink.
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-445-6
https://urn.fi/URN:ISBN:978-952-335-445-6
Tiivistelmä
The solid oxide fuel cell (SOFC) is a promising technology for combined heat and power generation as it provides low local emissions, high efficiency, and fuel flexibility. However, the unique electrical characteristics of the SOFC present challenges for power conversion efficiency and system reliability. This doctoral dissertation addresses these challenges through the design and modulation of the DC-DC converter.
Safe and reliable operation of an SOFC requires a power conversion unit (PCU) that is capable of interfacing between the different voltage levels and controlling the output current of the fuel cell. The challenge is that the output voltage of an SOFC is dependent on the reactants feed and load current. This voltage-current dependence creates a need for a PCU capable of efficient power conversion with a wide voltage conversion ratio. Moreover, the SOFC is vulnerable to sudden changes in the load and reactants feed, which may arise in a case of an emergency shutdown of the SOFC system. The impacts of an unexpected shutdown can be reduced by applying reverse bias current to the fuel cell during the emergency shutdown. This directs interest to the research of bidirectional power converters and efficiency improvement for a wide voltage conversion ratio.
In this doctoral dissertation, two DC-DC converter topologies and their use in an SOFC system are studied—the objective of this work is to enable efficient bidirectional DC-DC power conversion under varying load conditions. The converter topologies studied are the current-fed resonant push-pull (RPP) and the dual active bridge (DAB). The traditional RPP topology is well suited for SOFC applications but is not capable of bidirectional operation. The DAB topology, however, is bidirectional by nature, but its conversion efficiency is heavily dependent on the input-output voltage conversion ratio and load
current.
In this doctoral dissertation, the use of an RPP converter as a bidirectional converter is demonstrated. The power conversion efficiency of the DAB converter is improved by developing a variable-frequency modulation method. Further, the origin of the phase drift phenomenon is determined, a simple phase drift compensation method is developed, and a method for online efficiency maximization of the DAB converter is introduced.
Safe and reliable operation of an SOFC requires a power conversion unit (PCU) that is capable of interfacing between the different voltage levels and controlling the output current of the fuel cell. The challenge is that the output voltage of an SOFC is dependent on the reactants feed and load current. This voltage-current dependence creates a need for a PCU capable of efficient power conversion with a wide voltage conversion ratio. Moreover, the SOFC is vulnerable to sudden changes in the load and reactants feed, which may arise in a case of an emergency shutdown of the SOFC system. The impacts of an unexpected shutdown can be reduced by applying reverse bias current to the fuel cell during the emergency shutdown. This directs interest to the research of bidirectional power converters and efficiency improvement for a wide voltage conversion ratio.
In this doctoral dissertation, two DC-DC converter topologies and their use in an SOFC system are studied—the objective of this work is to enable efficient bidirectional DC-DC power conversion under varying load conditions. The converter topologies studied are the current-fed resonant push-pull (RPP) and the dual active bridge (DAB). The traditional RPP topology is well suited for SOFC applications but is not capable of bidirectional operation. The DAB topology, however, is bidirectional by nature, but its conversion efficiency is heavily dependent on the input-output voltage conversion ratio and load
current.
In this doctoral dissertation, the use of an RPP converter as a bidirectional converter is demonstrated. The power conversion efficiency of the DAB converter is improved by developing a variable-frequency modulation method. Further, the origin of the phase drift phenomenon is determined, a simple phase drift compensation method is developed, and a method for online efficiency maximization of the DAB converter is introduced.
Kokoelmat
- Väitöskirjat [1072]