A Novel Hybrid Control Strategy for Maximising Regenerative Braking Capability In a Battery-Supercapacitor Energy Storage System

EVO Electric has designed, built and tested the DuoDrive hybrid system based on proprietary axial flux motor technology, and installed it in a London Taxi Cab. The DuoDrive switchable series/parallel hybrid system has demonstrated a 60% improvement in fuel economy compared to a conventional taxi when operated over an urban drive cycle. As with many hybrid vehicles, a large part of this improvement is attributed to effective and efficient recapture of braking energy. The amount of braking energy that can be recovered, and the efficiency with which it can be returned to the road will therefore have a significant impact on the overall fuel economy of the vehicle. One factor that limits the amount of energy that can be recovered is the allowable charge rate of the battery, as braking events are usually high power and in a hybrid vehicle the battery size is generally small. The vehicle described in this paper has an energy storage system comprised of high power ultra-capacitors and a high energy lithium ion battery connected through a DC/DC converter. This allows efficient, high power transfer under regenerative braking and acceleration, and similarly efficient energy storage over longer timescales. Managing the power flow through the DC/DC converter and therefore the ultra-capacitor voltage, is a key control parameter that affects the efficiency of the overall system. This paper presents the energy storage system layout and demonstrates how different DC/DC converter control strategies can affect the system energy efficiency.