Efficient Load Sharing Operation through Series Connection of Batteries and Fuel Cell Stack

Electrical vehicles equipped with fuel cells (FC) are today the object of a special interest due to their high efficiency and low pollution features. However, due to cost reasons and poor dynamic performance, a power buffer is required in order to support power peaks. Such a power buffer, based on a battery pack or a super-capacitor bank, also allows the braking energy to be recovered, improving the global efficiency of the vehicle. The FC generator and the power buffer are usually connected in parallel through a DC/DC converter. Since the rated voltage of a fuel cell stack for light vehicle applications is about 48V, while the rated voltage of the battery pack ranges from 200 to 300V, a step-up converter is required. An alternative to the parallel connection of fuel cells and power buffer through a step up converter is given by the series connection of the two power sources. In this case, a higher efficiency can be obtained as the DC bus voltage can be increased, leading to the reduction of currents and power losses. Moreover, a portion of the generated power can be directly transferred from the fuel cell to the propulsion drive rather than processed from the step up converter and then delivered to the propulsion drive or stored in the power buffer. The series connection of the two power sources sets some relevant problems, either in terms of the selection structure of the step-up converter, either in terms of the development of a suitable control of the power flows. In the paper, the series connection of the fuel cell generator and the power buffer in a light urban vehicle powered by a 18 kW motor drive is discussed and two different approaches to the design of the step-up converter are presented and compared.