Introduction
A range extended hybrid energy storage system combines two or more storage technologies with an auxiliary generator. This topology uses the high energy density of batteries, the high power density of supercapacitors or flywheels, and the unlimited runtime potential of a generator. Each component handles different portions of the load profile. The supercapacitor manages millisecond to second scale power spikes. The battery manages minute to hour scale energy delivery. The generator manages hour to day scale recharge. Injet HanCang constructed a two hundred fifty kilowatt range extended hybrid system for a port crane application and a one hundred kilowatt system for a rapid charging station. This article describes the architecture, control strategy, and measured outcomes from both installations.
Component Selection for Hybrid Operation
A range extended hybrid energy storage system requires careful selection of each storage technology. The battery should have high energy density and moderate power capability. Injet HanCang selects lithium iron phosphate battery cells with a design that prioritizes cycle life over power density. The battery operates at a maximum C rate of one. The supercapacitor should have very high power density and low internal resistance. Injet HanCang selects supercapacitor cells with a specific power of ten kilowatts per kilogram and a round trip efficiency of ninety seven percent. The generator should be sized to match the average load after battery and supercapacitor contributions are removed. In the port crane application, the average load after peak shaving was sixty kilowatts, so a sixty kilowatt diesel generator was selected. The battery size was determined by the requirement to operate for two hours without generator support, resulting in a one hundred twenty kilowatt hour battery bank. The supercapacitor bank was sized to handle the crane peak power of four hundred kilowatts for five seconds, resulting in a two megawatt bank.
Control Hierarchy for Three Component Systems
The control system for a range extended hybrid energy storage system operates on three time scales. At the millisecond scale, the supercapacitor responds to load transients through a droop control loop. The supercapacitor voltage is monitored at one kilohertz, and power is adjusted to maintain voltage within a narrow band. At the second scale, the battery supplies power to maintain the DC bus voltage and to recharge the supercapacitor. At the minute scale, the generator starts if battery state of charge falls below thirty percent and supercapacitor voltage is below ninety percent of nominal. Injet HanCang implemented this hierarchy using a field programmable gate array for the supercapacitor loop and a programmable logic controller for the battery and generator. The measured response time to a five hundred amp load step was two milliseconds from the supercapacitor and forty milliseconds from the battery.
Port Crane Installation Test Results
The port crane application provided the most demanding test of the range extended hybrid energy storage system. The crane operates with a highly variable load profile. Lifting a container requires four hundred kilowatts for five seconds. Lowering a container regenerates two hundred kilowatts for five seconds. The cycle repeats approximately every two minutes. Injet HanCang conducted a four hour test recording one hundred twenty lift cycles. The range extended hybrid system supplied all peak power from the supercapacitor. The battery supplied average power of sixty kilowatts. The generator started once during the test, when battery state of charge reached twenty eight percent after fifty minutes of continuous operation. The generator ran for twenty minutes, recharging the battery to fifty five percent. Total fuel consumed was one point eight liters. Without the hybrid storage, a generator only solution would have required a four hundred kilowatt generator idling between lifts, consuming an estimated fifteen liters of fuel over the same period.
Efficiency Measurements Across Power Levels
The overall system efficiency of a range extended hybrid energy storage system varies with power level. Injet HanCang measured efficiency at three power levels using the port crane installation. At low power, defined as under fifty kilowatts, the battery supplied directly with ninety two percent efficiency. The supercapacitor was bypassed because its leakage current would reduce efficiency at low power. At medium power, defined as fifty to two hundred kilowatts, the battery supplied with ninety one percent efficiency and the supercapacitor remained idle. At high power, defined as above two hundred kilowatts, the supercapacitor supplied with ninety seven percent efficiency and the battery contributed the remaining power through the DC DC converter. The generator supplied energy at thirty three percent efficiency when running. The weighted average efficiency of the complete system, from all sources to load over the four hour test, was eighty four percent. This compares to sixty three percent for a generator only system and seventy six percent for a battery only system with the same peak load capability.
Battery Cycle Life Extension Through Hybridization
The supercapacitor reduces battery exposure to high C rate pulses, which are a primary cause of accelerated aging. In the port crane test, the battery saw a maximum C rate of point eight C, whereas the load demanded C rates up to six point seven C. Injet HanCang projected battery cycle life based on the measured current profile from the four hour test. Without supercapacitor hybridization, the battery would have experienced eight thousand high rate cycles per year at C rates above two. With hybridization, the battery experiences four hundred moderate rate cycles per year at C rates below one. The projected battery replacement interval extends from two years to seven years. Supercapacitor lifetime, based on voltage and temperature data from the test, exceeds fifteen years for the tested duty cycle. Capacitance degradation measured after the four hour test was zero point zero five percent.
Rapid Charging Station Application Data
A second range extended hybrid energy storage system was installed at an electric vehicle rapid charging station. The station has a grid connection limited to one hundred kilowatts but serves vehicles that can accept up to three hundred kilowatts. The range extended hybrid system uses a one hundred kilowatt hour battery, a one megawatt supercapacitor bank rated for thirty seconds, and a fifty kilowatt generator. Injet HanCang collected data over three months of operation. The station served four hundred twenty vehicles during the period. The supercapacitor handled all inrush current during vehicle connection, which reaches three hundred kilowatts for ten seconds. The battery supplied the sustained charging power of one hundred kilowatts for the remainder of each charging session. The generator started on twelve occasions when battery state of charge fell below thirty percent due to back to back charging sessions without grid recharge time. Total generator operating hours were six hours. Total fuel consumption was eighteen liters. The grid connection was never exceeded. The average vehicle charging time was twenty two minutes, compared to forty five minutes without the storage system.
Economic Analysis for High Power Applications
A range extended hybrid energy storage system has higher upfront cost than a simple battery system due to the supercapacitor and its power electronics. However, for applications with frequent high power peaks, the total cost of ownership may be lower. Injet HanCang performed an economic analysis for the port crane application over a ten year horizon. The battery only system required a one megawatt battery to handle the four hundred kilowatt peaks, costing four hundred thousand dollars. Battery replacement at year three and year six added three hundred thousand dollars each, totaling one million dollars over ten years. The hybrid system used a one hundred twenty kilowatt hour battery costing seventy thousand dollars, a two megawatt supercapacitor bank costing forty thousand dollars, and a sixty kilowatt generator costing twenty thousand dollars. Battery replacement at year seven added seventy thousand dollars. Total component cost over ten years was two hundred thousand dollars. The hybrid system saved eight hundred thousand dollars in component costs compared to the battery only system. Generator fuel and maintenance added five thousand dollars per year, or fifty thousand dollars over ten years, resulting in a net saving of seven hundred fifty thousand dollars.
Practical Limitations and Design Constraints
Range extended hybrid systems are more complex to control and commission than single storage systems. Injet HanCang observed that tuning the control parameters for the port crane installation required three weeks of on site adjustment. The supercapacitor bank requires a balancing circuit to maintain voltage uniformity across cells. The balancing circuit adds approximately fifteen percent to the supercapacitor system cost. The generator must be sized carefully to avoid excessive start stop cycles. A start stop cycle more frequent than once per hour reduces generator life. The port crane installation limited generator starts to a maximum of six per day through control logic. The rapid charging station installation had no such limit because generator starts occurred only on twelve occasions over three months. Sites with expected generator starts exceeding two per day should consider a larger battery to reduce generator cycling.
Conclusion
The range extended hybrid energy storage system combines battery, supercapacitor, and generator capabilities into a single coordinated unit. Test data from Injet HanCang’s port crane prototype show an eighty four percent weighted efficiency, a battery life extension from two years to seven years, and a fuel saving of ninety percent compared to a generator only system. The economic analysis for the port crane showed a net saving of seven hundred fifty thousand dollars over ten years compared to a battery only system. The rapid charging station data confirmed successful operation with four hundred twenty vehicles served and no grid limit violations. These results indicate that range extended hybrid storage is economically viable for high peak power, variable load applications such as cranes, rapid chargers, and industrial lifting equipment.