A Technical, Environmental and Financial Analysis of Hybrid Buses Used for Public Transport

Abstract

T his paper presents a technical, financial and environmental analysis of four different hybrid buses operated under Buenos Aires driving conditions. A conventional diesel bus is used as reference and three electric hybrids equipped with different energy storage technologies, Li-Ion, NiMH batteries and double layer capacitors (ultracapacitors), are evaluated, along with a hydraulic hybrid platform which uses high-pressure accumulators as its energy buffer. The operating conditions of the buses are set using real driving GPS data collected from various bus routes within the city. The different vehicle platforms are modeled on AUTONOMIE SA and validated by comparing the obtained fuel consumption results to those reported by local transport authorities and values found in the literature. The embedded energy and CO 2 emissions of each platform are estimated using GREET and the total cost of ownership of each vehicle is calculated and compared to that of the conventional bus. Furthermore, aging models are proposed to evaluate the life duration of the batteries and ultracapacitors. Results show that, independent of the energy storage technology, the fuel economy performance of all hybrids is highly dependent on the size and configuration of the powertrain and energy storage components. When optimized, all hybrids achieve significant fuel consumption reductions compared to a conventional diesel bus, however, the ultracapacitor based system seems to outperform the other technologies. The battery based electric buses achieve similar fuel consumption reductions, but the NiMH based batteries shows a considerably shorter life expectancy. This has a significant impact on both the economic and environmental performance of this vehicle. The life cycle emission analysis shows that, given the high fuel consumption of a conventional bus, the additional embedded CO 2 emissions of the hybrid vehicles are offseted by the achieved reduction of in-service CO 2 emissions due to fuel consumption reductions. Regarding the economic performance of the different platforms, results show that the fuel savings achieved by all hybrids displace the higher capital costs required. Overall, all hybrid buses show a strong potential to reduce both CO 2 emissions and costs, resulting in negative costs of CO 2 abatement.

A probabilistic fleet analysis for energy consumption, life cycle cost and greenhouse gas emissions modelling of bus technologies

“…As a 'stepping stone' towards full electrification, HEBs satisfy current transit service constraints [23] and offer a cost-effective means to meet the challenges of sustainable mobility with minimal disruption [57]. 'Mildhybrid' architectures ( Figure 1c) offer cheaper acquisition costs than BEBs, with significant fuel and emissions reductions compared to diesel buses [58]. An ICE and motor-generator unit (MGU) simultaneously supply tractive power to assist with vehicle acceleration, but propulsion cannot be achieved via MGUs alone, powered by an ESS, i.e.…”

“…ESS replacements are calculated via an energy throughput model which assumes a finite amount of energy can be delivered before the ESS reaches end-of-life conditions (80% of original capacity). Estimations of ESS degradation are adapted from 'Ah-throughput' battery and ultracapacitor aging models used in the analysis of hybrid buses by Orbaiz et al and Carignano et al [58,69]. The model assumes that the cooling of the ESS is sufficient so that any temperature effects on degradation are negligible.…”