Comparison of Ultra Capacitors, Hydraulic Accumulators and Batteries Technologies to Optimize Hybrid Vehicle Ecoefficiency

The main objective of vehicle powertrain hybridization is to improve the fuel consumption and environment pollutants impact (Eco-score) without decreasing the vehicle performances and other user satisfaction criteria. The Eco efficiency is a global index which accounts for both environmental impacts and user satisfaction. The Hybrid vehicles ability to overcome these two requirements depends on the optimal choice of their key mechanical, electric or hydraulic components. Some energy storage systems like ultra capacitors and hydraulic system are environmentally friendly and meet some user satisfaction criteria like daily cost, security, short charge/discharge duration and long life service. On the contrary, others like batteries have some times a environmental impact mitigate. The objective of this study is to establish a comparison between HEV (hybrid electric vehicle) and HHV(hybrid hydraulic vehicle) configuration by highlighting the effect of different energy storage systems (batteries, ultra capacitors, hydraulic system), mechanical and electric components sizes (engine, motors) upon the optimized HEV design taking care of both Eco-score and User satisfaction for different driving scenarios. The approach is formulated as a multidisciplinary optimization problem. At first, the HEV and HHV are modeled and simulated using ADVISOR (advanced vehicle simulator) with respect to several driving situations. Then emissions can be determined and the Ecoscore indicator can be calculated. User Satisfaction can be evaluated based on performance criteria extracted from ADVISOR simulation and on simple evaluation tools relying on the state-of-the art of technological information for safety, reliability and daily cost. The design problem is stated as follows: select mechanical, electric or hydraulic components (e.g. engine, motor and energy storage system sizes) to minimize the Ecoscore indicator and to maximize user satisfaction criteria subject to catalogue constraints on the choice of the components. The approach is illustrated on applications dealing with mild parallel hybrid buses. Results show that the hybrid electric buses using ultra capacitors have almost the same performances as those using batteries. However, the preferred choice would go towards super capacities because they have other appreciable properties compared to the batteries: the very high lifetime, high efficiency of charge and discharge and no polluting recyclability. Despite a smaller fuel saving than a HEV, the HHV technology has a chance to compete with HEV because of the hydraulic components low cost (industrial maturity), no pollutants recyclability due to the possible using of water as motor/pump fluid. HHV technology is a possible alternative in niche markets such as urban buses; bin-lorries or heavy urban delivery vehicles.