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Comparison of AC, DC, and AC/DC Bus Configurations for PV Hybrid Systems

Michael M.D. Ross
RER Renewable Energy Research

Dave Turcotte, Sophie Roussin, and Marc-André Fry
CANMET Energy Technology Center-Varennes (Natural Resources Canada)

Full Text Article
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Link to SESCI (Conference)

Acknowledgements:

Research conducted on behalf of the Photovoltaics and Hybrid Systems Program at the CETC-Varennes (Natural Resources Canada) with partial funding from the Panel on Energy Research and Development (PERD).

Citation:

Ross, Michael M. D., Dave Turcotte, Sophie Roussin, and Marc-André Fry. "Comparison of AC, DC, and AC/DC Bus Configurations for PV Hybrid Systems". Proceedings of the 30th Annual Conference of the Solar Energy Society of Canada, Burnaby, B.C., August 20 to 24, 2005.

Abstract:

In North America, the various components of a hybrid photovoltaic system are typically interconnected via an AC/DC bus: the photovoltaic array feeds the battery by a DC connection, the engine-driven generator (genset) charges the battery through a rectifier and powers AC loads directly whenever it is operating, and an inverter supplies the AC loads at other times. Other bus configurations have been promoted in recent years, however. In an AC bus configuration, module or string inverters convert the output of the photovoltaic array to AC, a bidirectional converter connects the battery to the AC bus, and a rectifier feeds DC loads. In a DC bus configuration, a variable speed genset having DC output is used, and an inverter meets all AC loads.

In the present study, PVToolbox, a photovoltaic hybrid system simulation package developed at CETC-Varennes, is used to compare these three bus configurations. PVToolbox is first used to recreate, with very good accuracy, the results of a European study favourable to the AC bus (Gabler and Wiemken, 1998). Then the performance of the AC bus and AC/DC bus are compared over a range of conditions. The annual load is kept constant, but the diuranal and seasonal pattern of variation and the size of the array are varied. Finally, the above two bus configurations are compared to the DC bus.

Gabler and Wiemkin's finding that the AC bus and AC/DC bus have comparable performance is contradicted by this study: if maximum power point tracking is included in both systems, the AC bus system requires about 10 to 18% more electrical energy from the genset than does the AC/DC configuration. The DC bus system, on the other hand, is comparable to the AC/DC bus system, requiring at most 2 to 3% more electricity from the genset. The fuel consumption is 10 to 14% lower in the DC bus system, however, but for a variety of reasons, this study probably overstates the gains of the variable speed genset operation.

The conventional AC/DC bus system offers some compelling advantages compared to either AC or DC bus systems. The genset can satisfy AC loads directly, unlike in DC bus systems, and its fuel consumption suffers only during part load operation. If intelligent dispatch strategy can minimize the time spent at part load, the fuel consumption may be even lower than that of the DC bus system.