Optimized Energy Management for FuelCell-SuperCap Hybrid Electric Vehicles VPP Track 4: Energy Storage Components/Systems Julia Schiffer, Oliver Bohlen, Rik W. De Doncker, Dirk Uwe Sauer Institute for Power Electronics and Electrical Drives ISEA RWTH Aachen University Aachen, Germany sf@isea.rwth-aachen.de Kyun Young Ahn Research & Development Division for Hyundai and Kia Motors Company 104, Mabuk-Ri, Guseong-Eup, Yongin-Si, Gyeonggi-Do, 449-912, Korea ahnky@hyundai-motor.com Abstract— Combining a fuel cell (FC) as primary power source with a supercap (SC) as a buffer for high power demands is a promising approach for future hybrid electric vehicles (HEV). The objective of an energy management is to minimize the hydrogen consumption and to assure power availability at any time. A simulation environment incorporating models of the FC and SC stacks and the kinetic state of the vehicle allow the detailed analysis and comparison of control strategies. Control strategies that operate the fuel cell most efficiently and take best advantage of the supercap can save more than 20% hydrogen fuel. Keywords - energy management, fuel cell, supercap, hybrid electric vehicle I. INTRODUCTION A promising approach for future, environmentally friendly vehicles is an electrical propulsion system powered by a fuel cell. Hydrogen or methanol can be generated from regenerative energy sources and a fuel cell can directly convert this chemically stored energy into electrical power aboard. However, fuel cells are limited in their dynamics with regard to increasing the output power and can not reuse energy from regenerative braking. These drawbacks can be compensated by including a double-layer capacitor module (supercap), which can deliver and accept peak power. In such a hybrid system, power is not only delivered from the primary power source, the fuel cell, to the propulsion system, but can be buffered in the supercap storage. This gives an additional degree of freedom for the energy management and allows for optimization. II. ENERGY MANAGEMENT A. Purpose of the energy management The general purpose of the energy management is to minimize the fuel consumption while assuring optimal power availability at any time. The vehicle demands power for acceleration and to compensate for all the electrical and mechanical losses. The power for acceleration is accumulated in the kinetic energy of the vehicle and can be recovered partly during deceleration if the generator is used for regenerative braking. In all situations, the fuel cell (FC), the supercaps (SC) and the mechanical brakes together must be able to cope with the energy and power attributed to driving (acceleration or deceleration), losses and other electric loads like radio or air conditioner (aux): () () () () () () t P t P t P t P t P t P aux loss accel brake SC FC + + = + + (1) The fuel cell is a power source, losses, auxiliaries and mechanical brakes are power sinks and supercaps and driving power can be both power source or sink. The acceleration power P accel can be derived from the change in the kinetic energy of the vehicle. The power needed for a constant acceleration a at an initial velocity v 1 with v(t) = v 1 + a t is () ( ) () a t v m t a v a m t P ⋅ ⋅ = ⋅ + ⋅ ⋅ = vehicle 2 1 vehicle accel (2) Driving losses P loss are mainly caused by friction of the wheels and in the gear and by air drag: () ( ) 3 2 1 loss v f v f P + = (3) where f 1 represents losses caused by friction of the wheel, which are linear functions of velocity and f 2 represents losses caused by air drag, which is a cubic function of velocity. The functions f 1 and f 2 have been modeled in detail according to fundamental mechanical equations [6]. Losses that are generated in the fuel cell and the supercaps are implicitly considered by the efficiency characteristics and impedance models, respectively, of these devices (cf. III.B). The power and energy needed for supplying the electric loads depends on the number of loads and their individual energy demand, which is typically independent from the vehicle speed. 0-7803-9280-9/05/$20.00 ©2005 IEEE. 716