Toshio Hirota

Practical storage and utilization of household photovoltaic energy by electric vehicle battery

In this paper, the authors analyze the household electrical energy balance and self-sustainable consumption of PV-generated energy utilizing the battery of an electric vehicle (EV) parked at home including a practical “vehicle to home” operation. We have estimated typical domestic electricity consumption patterns from actual measurement of various households. Also, typical ranges of solar PV output by random weather changes over a week in different seasons of the year are assumed from solar irradiance measurement and weather patterns. To find weekly charge/discharge cycles, we assumed different driving scenarios for weekdays and weekends. For comparison we have tested several different charge/discharge combinations and various charging schemes. To measure the contribution of an EV as energy storage for a house equipped with a PV panel, the rate of utilization of PV-generated energy, the amount of CO2 reduction at household, and cost reduction are calculated. From the results presented here the authors are convinced that a plug-in electric vehicle can contribute for better utilization of PV-generated electricity.

Analysis of degradation mechanism of lithium iron phosphate battery

The degradation mechanisms of lithium iron phosphate battery have been analyzed with 150 day calendar capacity loss tests and 3,000 cycle capacity loss tests to identify the operation method to maximize the battery life for electric vehicles. Both test results indicated that capacity loss increased under higher temperature and SOC conditions. And also, large increase of internal resistance on the high temperature and high SOC conditions was confirmed by AC impedance tests. The real cycle capacity loss characteristic was derived by subtracting the capacity decrease due to calendar capacity loss during the cycle test from the overall capacity loss characteristic obtained from the cycle test. As a result, it is found that the real capacity loss contains not only structural disorders of electrode but also degradation factors due to the chemical reactions. Characteristics of degradation were quantified with equations based on the chemical kinetics. With this degradation prediction, an operation method was proposed that is compatible with the long life of batteries and the safety driving of a vehicle. As a result, with optimizing the SOC range used in the operation as follows: 30-10% in the warm seasons, 45-25% in the cold seasons, it was found that batteries can last 4 times longer than it used with high SOC range in every season.

Design, manufacture, and environmental sustainability evaluations of advanced electric medium duty bus “WEB for Suntory”

This paper describes the design and manufacture of a highly practical electric medium duty bus called the WEB for Suntory (WEB: Waseda electric bus). The new electric medium duty bus has a 55-passenger capacity, which is significantly greater than the other vehicles in the WEB series. This project prioritized weight reduction and maximization of the passenger compartment space to develop a bus that meets the requirements for short distance transportation and very frequent charging. A new compact powertrain system incorporating the minimum number of lithium-ion batteries was developed. The intended purpose of the bus is to convey passengers on a plant tour along a steep and low-velocity route located at the foot of a mountainous region. The merits of an electric bus make it particularly suited to this usage scenario and route, which require environmentally sustainable transportation. The dynamic performance and environmental sustainability of the bus were evaluated based on long-term actual driving data. The results found that adopting this bus on this route greatly reduced energy consumption and CO2 emissions due to its efficient dynamic performance and energy regeneration system. The bus was charged from the solar power generation system installed at the plant, thereby further reducing CO2 emissions and running costs.

Performance Degradation Prediction and Cell Balance Control Algorithm Construction of Lithium Iron Phosphate Battery

Various studies were conducted from the aspects of both the battery cells and the module with the aim of making more effective use of the battery capacity. As a result, after 77 cycles of driving and rapid charging, the capacity degradation of the battery module as a whole was 7.7 % and it was determined that the cause of this additional 0.7 % degradation of the capacity was the loss of cell balance. In addition, by adopting module balance coefficient (945;) that was proposed as an indicator to show the available percentage of the lowest capacity cell in the module, new cell balancing rules were created that do not significantly affect the convenience of vehicle operation.