Technical Performance Modeling of Hybrid and Plug-In Hybrid Electric School Buses Using ADVISOR

Abstract

The hybrid electric vehicle is changing the automotive market at an impressive rate. While not as highly publicized, the transit bus market is being transformed at an equally great rate. As these markets move forward, the school bus market remains largely unchanged. As an unchanged market, there is still the opportunity to optimize a hybrid vehicle platform for school buses. This study models an existing class C school bus and investigates the potential of both series and parallel hybrids to reduce fuel consumption and emissions. The primary focus of this study is to investigate the potential benefits of adding an electric grid interconnection to hybrid electric school buses, allowing them to add to the hybrid potential to a pre-charged battery pack. These vehicles are known as plug-in hybrids. The school bus models shown in this paper were generated in a Matlab/Simulink-based program developed by NREL called ADVISOR. ADVISOR is used by vehicle manufacturers as a tool to experiment with different vehicle configurations. In this study both a generic series hybrid and a generic parallel hybrid are generated and used in both charge-sustaining and charge-depleting scenarios with varying sizes of battery packs to increase the “grid energy.” The results of each model are presented by fuel economy and emissions reductions taking into account the power plant emissions and electricity costs. The results of the study show that by adding a plug-in connection to existing hybrids, significant savings can be achieved, both in fuel costs and in overall emissions. By analyzing the emissions at the power plant level and at the vehicle level we show that emission of NOx, Particulate Matter and Carbon Dioxide can all be reduced while saving on fuel costs. This study also shows that some models of traditional hybrid can be operated as plug-in models with little or no change to the system to gain significant benefit from the initial charge.