This paper provides a detailed comparative analysis of optimal sizing of battery-only, ultracapacitor-only, and battery-ultracapacitor hybrid energy storage systems (ESSs) for a plug-in electric city ...bus (PECB). It is shown how the configuration affects the optimal size of the ESS. The problem of optimal sizing of the storage system for the PECB is formulated and solved for a specific set of vehicle parameters, battery, and ultracapacitor cell data, as well as a specific daily drive cycle. A modified particle swarm optimization algorithm is used to solve the optimal sizing problem. The optimization platform developed by the authors and used for this study is highly flexible and accepts different component data, vehicle parameters, and drive cycles as the input for determining the optimal sizes of the storage system.
Research work on the energy efficiency of vehicles is driven, among other things, by limits related to fuel consumption carbon dioxide emissions. This also applies to city buses, where fuel ...consumption averages between 25 and 30 dm3 per 100 km, which can be converted into approximately 87 kg CO2 per dm3. This article therefore presents the results of a study of the total efficiency of the power train of a city bus, taking into account the internal combustion engine, transmission, hydrokinetic clutch, and tyre friction on the rollers. The test object was a 12-metre city bus equipped with diesel engines and an automatic gearbox. The tests were carried out on a chassis dynamometer by implementing the World Harmonized Vehicle Cycle (WHVC). The WHVC driving test is a synthesis of the vehicle's on-road speeds and consists of three stages: Urban, Rural and Motorway. During the tests, the fuel consumption, vehicle speed and power generated at the wheels of the bus were recorded. From this, efficiency was calculated as the ratio of the power measured at the wheels of the bus to the power contained in the fuel supplied to the engine. Efficiency was shown to range from 5 to 22%.
To understand the roles of different transport modes in the spread of COVID-19 pandemic across Chinese cities, this paper looks at the factors influencing the number of imported cases from Wuhan and ...the spread speed and pattern of the pandemic. We find that frequencies of air flights and high-speed train (HST) services out of Wuhan are significantly associated with the number of COVID-19 cases in the destination cities. The presence of an airport or HST station at a city is significantly related to the speed of the pandemic spread, but its link with the total number of confirmed cases is weak. The farther the distance from Wuhan, the lower number of cases in a city and the slower the dissemination of the pandemic. The longitude and latitude coordinates do not have a significant relationship with the number of total cases but can increase the speed of the COVID-19 spread. Specifically, cities in the higher longitudinal region tended to record a COVID-19 case earlier than their counterparties in the west. Cities in the north were more likely to report the first case later than those in the south. The pandemic may emerge in large cities earlier than in small cities as GDP is a factor positively associated with the spread speed.
•This paper explores the factors influencing the COVID-19 spread speed and spatial pattern.•Frequencies of flights and HSTs are significantly associated with the number of cases.•The farther the distance from Wuhan, the lower the number of cases and dissemination speed.
This paper evaluates the lifecycle costs and carbon dioxide emissions of different types of city buses. The simulation models of the different powertrains were developed in the Autonomie vehicle ...simulation software. The carbon dioxide emissions were calculated both for the bus operation and for the fuel and energy pathways from well to tank. Two different operating environment case scenarios were used for the primary energy sources, which were Finland and California (USA). The fuel and energy pathways were selected appropriately in relation to the operating environment. The lifecycle costs take into account the purchase, operating, maintenance, and possible carbon emission costs. Based on the simulation results, the energy efficiency of city buses can be significantly improved by the alternative powertrain technologies. Hybrid buses have moderately lower carbon dioxide emissions during the service life than diesel buses whereas fully-electric buses have potential to significantly reduce carbon dioxide emissions, by up to 75%. The lifecycle cost analysis indicates that diesel hybrid buses are already competitive with diesel and natural gas buses. The high costs of fuel cell and battery systems are the major challenges for the fuel cell hybrid buses in order to reduce lifecycle costs to more competitive levels.
•Alternative powertrains can significantly improve energy efficiency of transit buses.•Operating environment has an important impact on the lifecycle costs of buses.•Diesel hybrid buses are already cost effective solution for public transportation.•The cost of fuel cell technology is the major challenge for fuel cell hybrid buses.•Fully-electric buses have potential to significantly reduce carbon dioxide emissions.
•Dynamic programing approach is used to optimize the hybrid energy storage system.•Components sizes and the system control strategy are optimized simultaneously.•The life cycle cost of the system is ...rapidly reduced initially with SC increases.•Four control rules are extracted from the DP results to obtain an on-line strategy.•The on-line strategy performance can be further improved by increasing the SC usage.
This paper utilizes the dynamic programming (DP) approach to deal with the integrated optimization problem for deriving the best configuration and energy split strategies of a hybrid energy storage system (HESS), including a battery and a supercapacitor (SC), for an electric city bus. Within the optimization process, a preset cost function is employed to evaluate the HESS life cycle cost based on a dynamic degradation model of the LiFePO4 battery, which is initially proposed by us. For system hybridization, the battery size is optimized according to the requested minimal mileage, while the optimal configuration of the SC pack (i.e., the numbers of the SC modules in series and parallel) is determined using the DP approach. It is shown that the life cycle cost of the HESS initially decreases rapidly with the addition of SCs, though the rate of this reduction decreases as the amount of SC increases. The HESS candidates occurring in the transition area can therefore be regarded as the best solutions. For the energy split strategy, several control rules can be extracted from the DP results, and a near-optimal rule-based strategy is proposed in this paper. When compared to the battery-only configuration, the HESS controlled by the rule-based strategy can reduce 47% and 60% of the ESS life cycle cost along the typical China Bus Driving Cycle and the Urban Dynamometer Driving Schedule, respectively. This paper also proves that a well-tuned rule-based strategy, which can be easily implemented in a vehicle, presents rather good performance when compared to the DP approach. In addition, the proposed strategy performance can be further improved by increasing the SC usage.
In this paper, the control strategy of a fully-active hybrid energy storage system, which uses two bi-directional DC/DC converters to decouple supercapacitor and battery pack from the DC bus, is ...proposed based on a 5th-order averaged model. Three control objectives, the battery and supercapacitor currents as well as the DC bus voltage, are regulated by using the two DC/DC converters. A Lyapunov-function-based controller is proposed to regulate the DC bus voltage to its reference value. In addition, a sliding-mode controller is designed to control the battery and supercapacitor currents to their reference values. The battery current reference is generated by the energy management strategy, while the supercapacitor current reference is generated by the Lyapunov controller to ensure DC bus voltage regulation. Simulation and experimental results show that the proposed control method has satisfactory performance, including robust tracking and a smooth transition when the load power varies in the large range.
•The control strategy of the hybrid energy storage system is designed.•A Lyapunov-function-based controller is proposed to regulate the DC bus voltage.•A sliding-mode controller is designed to control battery/supercapacitor currents.•The proposed control method is verified by simulation and experimental results.
•Different control strategies compared for battery/supercapacitor hybrid energy systems.•A dynamic battery degradation model is adopted to evaluate the battery capacity loss.•Fuzzy-logic and ...rule-based controllers perform better along certain driving cycles.•The comparison result is validated by the dynamic programing result.•The system life cycle cost is dramatically reduced when supercapacitors are adopted.
This paper deals with the real-time energy management strategies for a hybrid energy storage system (HESS), including a battery and a supercapacitor (SC), for an electric city bus. The most attractive advantage deriving from HESSs is the possibility of reducing the battery current stress to extend its lifetime. To quantitatively compare the effects of different control strategies on reducing battery degradation, a dynamic degradation model for the LiFePO4 battery is proposed and validated in this paper. The battery size is optimized according to the requested minimal mileage, while the size of SC is optimized based on the power demand profile of the typical China Bus Driving Cycle (CBDC). Based on the optimized HESS, a novel fuzzy logic controller (FLC) and a novel model predictive controller (MPC) are proposed and compared with the existing rule-based controller (RBC) and filtration based controller (FBC), after all the controllers are tuned to their best performance along the CBDC. It turns out that FLC and RBC achieve the best performance among the four controllers, which is validated by the DP-based result. Furthermore, about 50% of the HESS life cycle cost is reduced in comparison with the battery-only configuration. In addition, the controllers are also compared along the New European Driving Cycle (NEDC), which represents another normalized driving cycle. The results show that the RBC, MPC, and FLC achieve a similar performance, and they reduce about 23% of the HESS life cycle cost when compared to the battery-only configuration. The RBC and FLC are regarded as the best choices in practical applications due to their remarkable performance and easy implementation.
•A new battery/supercapacitor energy storage system is proposed in this paper.•A novel dynamic battery capacity fade model is employed in system optimization.•The system cost and the battery capacity ...loss are simultaneously minimized.•The battery degradation is reduced rapidly with the initial increase in SC usage.•Candidates appear in the inflection area can be regarded as the optimal solutions.
This paper proposes a semi-active battery/supercapacitor (SC) hybrid energy storage system (HESS) for use in electric drive vehicles. A much smaller unidirectional dc/dc converter is adopted in the proposed HESS to integrate the SC and battery, thereby increasing the HESS efficiency and reducing the system cost. We have also included a quantitative battery capacity fade model, in addition to the theoretical HESS model proposed in this paper. For the proposed HESS, we have examined the sizing optimization of the HESS parameters for an electric city bus, including the parallel and series number of the battery cell and the SC module. Considering the constraint of requirement on minimal mileage, the optimization goal is to simultaneously minimize (i) the total cost of the HESS and (ii) the capacity loss of a LiFePO4 battery over a typical China Bus Driving Cycle. The simulation result shows that these two objectives are conflicting, and trades them off using a non-dominated sorting genetic algorithm II. Finally, the Pareto front including optimal HESS parameter groups has been obtained, which indicates that the battery capacity loss can be reduced rapidly when the SC cost increases within the range from 10 to 40 thousand RMB.
•A reconstructed fuel cell life-prediction model is proposed to simplify the voltage analysis in a fuel cell city bus.•A new classified method is used to define two typical operating conditions to ...analyze the performance degradation.•Compared with traditional method, a better performance degradation prediction is obtained in this study.
Life prediction is a significant and difficult topic for a proton-exchange membrane fuel cell stack, especially a commercial fuel cell stack. This paper proposes a reconstructed fuel cell life-prediction model to estimate the fuel cell lifetime adopted in a city bus. Considering the temperature fluctuation and sensor errors, the voltage model is separated into three parts to simplify the fitting process, and the validation results show that the proposed degradation model is credible and robust. Furthermore, the 14-day training data show that the deviation of the predicted voltage is less than 1%. In this study, the most important innovation is separating the time into different categories. Compared to the traditional method without classification, the proposed model obtains an improved predicted deviation of 50–100%.
In this paper, four different semi-active hybrid energy storage systems (HESSs), which use both super-capacitors (SCs) and batteries, are compared based on an electric city bus running the China Bus ...Driving Cycle (CBDC). The SC sizes of the different HESS topologies are optimized by using the dynamic programming (DP) approach, based on a dynamic degradation model of the LiFePO sub(4) battery. The operation costs of different HESSs, including the electricity and the battery degradation costs over a whole CBDC, are minimized in the optimization process. Based on the DP results, near-optimal control strategies of different HESSs for on-line uses are proposed. Finally, the four HESS topologies are comprehensively compared from different aspects, including operation cost, initial cost, and DC bus voltage variation. Simulation results show that all HESS topologies have their merits and drawbacks, and can be used in different applications with different requirements. In addition, about 50% of the operation cost of the energy storage system is reduced by the semi-active HESSs when compared to the battery-only topology. Thus the effectiveness of adopting the SC in the HESS is verified.
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