Optimized Electrical Energy Management in Automotive Applications Based on Efficient Battery-Fuel Cells Connection Architectures

The automotive field in the next few years will be characterized by a larger employment of electrical energy, mostly due to the generalized transition, common to the industrial production systems, from traditional shaft and cams based mechanical control systems, toward numerical control based servodrives. The diffusion of technologies such as series or parallel hybrid electric vehicle or drive by wire systems will result in an increasing demand of electric drives that can be estimated in the order of several hundreds of thousands pieces per year. Electrical vehicles equipped with fuel cells are today the object of a special interest due to high efficiency and low pollution features, however, due to cost reasons and fuel cells poor dynamic performance, a power buffer is required in order to support power peaks. Such a power buffer, based on a battery pack or 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 generally parallel connected, through a step–up DC/DC converter. An alternative to the parallel connection of fuel cells and power buffer is given by the series connection, that feature higher efficiency, as the DC bus voltage can be increased, leading to the reduction of currents and power losses. In both cases of series or parallel connection an optimized control of the electrical energy flowing between the fuel cell an the power buffer is required. The paper deals with the optimization of the series connection of a fuel cell generator and a battery pack in a light urban vehicle powered by a 18 kW motor drive. A specific power sharing control strategy has been developed and tested that drives the fuel cell stack to operate very close to the point of maximum efficiency in the most frequent operating conditions, while ensuring acceptable dynamic performance of the vehicle. Furthermore, two different approaches to the design of the step-up converter are presented and compared.