NaNiCl Batteries for a PEM Range Extender-Based EV

Long term petrol price increases and climate changes have created in the automotive industry a new competitive market based on the growth of more sustainable technologies. Recently, several automotive manufacturers have promoted interest in full electric vehicles (EVs) and hybrid vehicles (HEVs) because both types of means of transportation could be run comparable with traditional fueled vehicles but are more cheaper to maintain and environmentally friendly [1-3]. Most of all, HEVs are attracting interest due to great potential to achieve higher fuel economy and a longer range with respect to pure electric mode but still depend entirely on petroleum to charge the battery pack. Within a national project CNR TAE Institute has developed a zero emission hybrid EV based on PEM Fuel Cell technology able to increase the range with respect to the same vehicle in EV configuration. The vehicle selected for the prototype realization is a city bus of 44 passengers capacity and a drive motor of 85 kW (rated power) already tested with NaNiCl batteries (Rated Energy: 21.2 kWh) in pure electric mode [4-5]. The characteristics of this battery, in fact, make it an excellent solution both for HEV and for EV, particularly in terms of energy density. A typical NaNiCl module exhibits an energy density of 120 Wh/kg which is 3-4 times higher than conventional lead-acid batteries and 2-3 times higher than nickel-metal hydride batteries. The ZEBRA cell has a central positive electrode mainly consisting of Nickel and sodium chloride plus some additives and a liquid electrolyte contained within a "?" alumina tube electrolyte. The electrolyte is in molten state (NaAlCl) and in solid state, "?" alumina, which provides fast transport of sodium ions and ensures the electrical insulation between anode and cathode. The cell works in a range of temperature between 270-350 °C and during charge sodium ions formed in the central positive electrode moves through the wall of the beta alumina tube to form the liquid sodium negative electrode which is contained by a square section mild steel case. NaNiCl cells offer high specific energy, maintenance free operation, immunity to ambient temperature conditions (constant performance in critical operating environments (-20 °C to + 60 °C), no gassing and zero self-discharge. About safety the Sodium-Nickel technology has proven to present relatively low intrinsic risks during normal operation. Additional safety is provided by the integrated Battery Management Interface (BMI) which issues warning signals when the operating conditions exceed the limits and disconnects the battery if they persists to dangerous conditions. The selected model is the Z5-557-ML3X-38 (216 cells, Rated Energy: 21.2 kW h). In Fig. 1 the batteries installed behind the driver's seats and in the rear of the vehicle are shown. Experimental data shown that 20h 48min. is the time needed to warm-up the battery up to 250 °C, starting from 25 °C, and 12h 21min. is the time needed to charge the battery from 20% SoC up to 100% SoC. The battery charge begins at about 230 C. The on board chargers regulate the charge of the battery through a Pulse With Modulation (PWM) managed by BMI. The slow charge, about 12 h, is achieved with currents from 5 A to 0 A, corresponding to SoC from 20% to 100%. At the end of charging phase, the battery shows an open circuit voltage of 565.7 V, even if, after a relaxing time period, it reaches the 558 V open circuit voltage. The optimal level of hybridization has been evaluated on the basis of several aspects using simulation models and consists of a low power fuel cell of 5kW and a reduced number of batteries [6-8]. The PEM system, that has been installed on the bus, is fed by hydrogen stored in 2 tanks (compressed at 200bar) containing about 4,8 kg of hydrogen each [9]. The FC system is connected to the dc-bus via DC/DC step-up that converts the stack voltage for batteries recharging. The battery pack is connected in parallel on the same dc-bus. The Range Extender architecture used for the developed city bus is expected to be more popular in next years with development of battery and FC technology and support of the automotive industry. Fuel Cells and Batteries achieve an optimal synergy but a basic premise remains the possibility of implementing appropriate control policies through the use of suitable power converters. This work reports data coming from on road tests where the behavior of NaNiCl batteries was analyzed in different conditions (climb, downhill, long route with deep discharge, regenerative breaking).