DC Distribution for Fuel Cell Power Generation

Application of fuel cell technology in a DC distribution system compared to an AC distribution system not only provides an environmental impact but could also prove to be more efficient, reliable, and less costly. We are conducting a study that will compare and demonstrate the asset of moving from an AC distribution system to a DC distribution system with fuel cells.

The increasing demand for energy and government regulations along with health concerns over the increasing amount of pollution have placed an emphasis on generating power that is both energy efficient and has low environmental impact. This has led to the development of many sources of green power including fuel cells, wind turbines, and photovoltaics. Unfortunately photovoltaics and wind turbines are location limited seeing that the correct amount of light and wind are not available in all locations. Fuel cells do not suffer from the same shortcomings. Instead fuel cells, can be placed in any location, are reliable, quiet, and clean, and have high efficiency ratings. In particular, the solid oxide fuel cell (SOFC) has efficiency capabilities of 65% which can be increased further to 85% when combined with other power generation technologies. These efficiencies exceed the ratings of many of the conventional power generation technologies that exist.

Yet, the power electronics necessitated to connect SOFC to the power grid can add significant cost and reduce efficiency. The fuel cell by its simple nature is a DC source. Connection of this source to the power grid requires the use of a DC-DC converter to first boost the voltage to the correct level and an inverter to convert the DC power to AC for connection to the power grid. Although, technology has allowed the inverter to achieve relatively high efficiencies with significant control of the output, a DC power system might have a significant advantage with SOFCs over AC. The original founders of AC, George Westinghouse and Nikola Tesla originally developed AC for transmission purposes, since no method existed for stepping the voltage up or down in DC. However with the development of switching devices and the DC-DC converter, HVDC has made a comeback and is showing many advantages over traditional AC. These advantages include easier control of power flow, lower losses, higher transmission capacity, and lower cost due to fewer lines. Hence, a reevaluation of the current power infrastructure is needed to assess the gains that can be achieved with using a DC distribution system with fuel cells.

The University of Tennessee (UT) has partnered with the Power Electronics and Electric Machinery Research Center (PEEMRC) of Oak Ridge National Laboratory (ORNL) to conduct a study that will examine the differences in an AC and a DC distribution system with fuel cells. This study, funded by DOE’s Solid-State Energy Conversion Alliance (SECA) program, will compare the power flow, efficiency, reliability, and cost of the current AC grid to that of a similar DC grid with fuel cells.

Some important questions to be answered by this study are:

  • Are some of the distribution components compatible between AC and DC? The hope is that although a transition from AC to DC distribution would require significant changes, some components could remain thereby providing a financial reward.
  • Which type of distribution provides the most benefit, centralized or distributed? Distributed generation is already known to provide voltage support, loss reduction, and improved reliability, but further gains could also be possible.
  • Are there significant amounts of loads that already exist that would benefit from a DC distribution system? A compelling amount of electronic devices in the office use DC such as desktop computers, laptops, and hard drives and employ an adapter to rectify the AC from the power outlet.

Although this study is in the early stages, an AC model of the ORNL grid is already completed and a DC model is now under development in the power systems software SKM. Formation of the DC model constitutes the replacement of the current AC components within the AC model with equivalent DC components. With the completion of both models, analysis on the differences of AC and DC with fuel cells will commence.

The result of this study should provide insight into the possibility of fuel cell generation and DC distribution on the ORNL campus and present ORNL the pros and cons to move towards a completely fuel cell powered campus.

We would like to acknowledge the generous help from Lynn J. Degenhardt and Pierre Boheme of ORNL in allowing us to use the SKM software and the new AC substation model in SKM in development of our own DC substation model.

References:

  1. G. K. Andersen, C. Klumpner, S. B. Kjter, F. Blaabjerg, “A New Green Power Inverter for Fuel Cells,” 33rd Annual Power Electronics Specialists Conference, Vol. 2, 2002, page(s) 727-733.
  2. K. Rajashekara, “Hybrid-Fuel Cell Strategies for Clean Power Generation,” IEEE Transactions on Industry Applications, Vol. 41, June 2005, page(s) 683-689.
  3. J. R. Angelle, W. E. Simon, “Fuel Cell Power Generation for Residential and Commercial Applications with Waste Heat Recovery,” Proceedings of the 2002 ASEE Gulf-Soutwest Annual Conference, 2002.
  4. Buffalo and Erie County Historical Society, http://ah.phpwebhosting.com/h/tesla/tesla.html
  5. N. M. Kirby, M. J. Lukeft, L. Xu, W. Siepmann, “HVDC Transmission for Large Offshore Windfarms,” Seventh International Conference on AC-DC Power Transmission, Nov 2001, page(s) 162-168.
  6. L. Wermers, “HVDC Light: A New Technology for a Better Environment,” IEEE Power Engineering Review, Aug 1998, page(s) 19-20.
  7. P. P. Barker, R. W. De Mello, “Determining the Impact of Distributed Generation on Power Systems: Part 1- Radial Distribution Systems,” IEEE Power Engineering Society Summer Meeting, Vol. 3, 2000, page(s) 1645-1656.

Submitted by: Michael Starke and Burak Ozpineci, Power Electronics & Electric Machinery Research Group

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