Hengbing Zhao | University of California, Davis (original) (raw)

Papers by Hengbing Zhao

Proton Exchange Membrane fuel cell (PEMFC) technology is one of the most attractive candidates fo... more Proton Exchange Membrane fuel cell (PEMFC) technology is one of the most attractive candidates for transportation applications due to its inherently high efficiency and high power density. However, the fuel cell system efficiency can suffer because of the need for forced air supply and water-cooling systems. Hence the operating strategy of the fuel cell system can have a significant impact on the fuel cell system efficiency and thus vehicle fuel economy. The key issues are how the fuel cell back pressure and air flow through the fuel cell are controlled. One approach is fixed back pressure control. The other is optimum varying back pressure control. In both cases, the air flow stoichiometry is optimized. In this paper, a dynamic forward-looking vehicle model with a dynamic fuel cell system model is employed. The effects of different fuel cell system operation modes and different power split strategies on fuel economy of fuel cell hybrid vehicles are simulated. The simulation results...

Most vehicles presently use batteries for energy storage, but there are vehicle designs in which ... more Most vehicles presently use batteries for energy storage, but there are vehicle designs in which ultracapacitors alone or in combination with batteries can increase the efficiency of the vehicle and in addition lead to significantly longer battery cycle life. Ultracapacitors can be used alone in charge sustaining hybrid vehicles (HEVs) if the energy storage requirement is less than 150Wh. Simulations show that when ultracapacitors are used alone in HEVs, the roundtrip efficiency of the energy storage is 95-98% and the engine efficiency in on/off operation can be maintained near the peak efficiency value. Vehicle simulations were also run for plug-in hybrid vehicles (PHEVs) using advanced batteries with high energy density (>300 Wh/kg). The simulations were run with the batteries alone and in combination with ultracapacitors. Simulation results for the electric useage (Wh/mi) and all-electric range and fuel economy (mpg) for the PHEVs using batteries in combination with ultracapac...

Hybridization of fuel cells with additional batteries or ultracapacitors in a fuel cell vehicle r... more Hybridization of fuel cells with additional batteries or ultracapacitors in a fuel cell vehicle reduces electrical and mechanical stresses on fuel cells and improves the overall drive train efficiency over a standard drive cycle. This paper primarily analyzes hybrid fuel cell vehicles with different drive train arrangements, and compares projected fuel economies of hybrid fuel cell vehicles with improved conventional vehicles and hybrid electric vehicles at three points in the future: 2015, 2030, and 2045. The key points addressed are as follows: drive train arrangements, control strategies, and the influence of energy storage sizing on vehicle fuel economy. The study shows that fuel cell vehicles having ultracapacitors coupled with fuel cells via a DC/DC converter with load-leveling control is the best approach in term of improving fuel economy and mitigating the stress on the fuel cell. Power-assist control is well suited for fuel cell-battery hybrids in terms of fuel economy impr...

The use of ultracapacitors in plug-in hybrid vehicles (PHEVs) with high energy density lithium-io... more The use of ultracapacitors in plug-in hybrid vehicles (PHEVs) with high energy density lithium-ion and zinc-air batteries is studied. Simulations were performed for various driving cycles with the PHEVs operating in the charge depleting and charge sustaining modes. The effects of the load leveling of the power demand from the batteries using the ultracapacitors are evident. The average and the peak currents from the batteries are lower by a factor of 2-3.

This paper is concerned with the use of ultracapacitors in hybrid vehicles in place of batteries.... more This paper is concerned with the use of ultracapacitors in hybrid vehicles in place of batteries. In the case of the mild, charge sustaining hybrid, the ultracapacitors would replace a lithium or nickel metal hydride battery: for a stop-start micro-hybrid, the capacitors would be used in combination with a lead-acid battery with the capacitors starting the engine, accepting energy during regenerative braking, and providing accessory loads during relatively short stop periods. Test data are shown for the performance of advanced carbon/carbon and hybrid lithium ultracapacitors indicating higher energy density (more than 2X) than that of commercially available carbon/carbon cells from Maxwell and NessCap. The advanced devices showed no sacrifice in high power capability in order to achieve the higher energy density. Simulations of mid-size passenger cars using the advanced ultracapacitors in micro-hybrid and charge sustaining hybrid powertrains were performed using the Advisor vehicle ...

In this paper, various alternative hybrid vehicle powertrains that are being considered by auto c... more In this paper, various alternative hybrid vehicle powertrains that are being considered by auto companies are evaluated based on simulation studies performed at the Institute of Transportation Studies, University of California-Davis. The following hybrid powertrain arrangements have been considered: a. Single-shift, parallel (Honda) b. Single-planetary, dual-mode (Toyota/Prius) c. Multiple-planetary, dual-mode (GM) d. Multiple-shaft, dual-clutch transmission (VW and Borg-Warner) e. Series – range extended EV (GM Volt) The primary strategy in all the options considered was to operate the engine only in the high efficiency part of its map and to lose as little as possible of the gain by losses in the energy storage unit and the electric machines. The simulations indicated that there are in general not large differences in the fuel economies predicted using the various powertrains for the same vehicle and battery. The fuel economy improvements were large in all case – 80-100% for the F...

gates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel... more gates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel cell durability.

Journal of Power Sources, 2015

A Fuel cell-LiFePO 4 battery hybrid powertrain using a direct parallel structure. Fuzzy control b... more A Fuel cell-LiFePO 4 battery hybrid powertrain using a direct parallel structure. Fuzzy control based energy management strategies. Energy management for adjusting and stabilizing the DC bus voltage. Application of ultracapacitors for protecting Fuel cells and LiFePO 4 batteries.

Transportation Research Part D: Transport and Environment, 2013

Non-electrification efficiency-improving technologies and powertrain technologies for reducing th... more Non-electrification efficiency-improving technologies and powertrain technologies for reducing the heavy-duty truck fuel consumption are studied. The study indicates that improvements in engine efficiency, aerodynamic drag and rolling resistance will benefit fuel economy significantly over the day drive and over-the-road highway driving cycles; 6–13% in fuel savings can be expected from each technology. Hybridization can achieve fuel saving of 16% and is financially attractive for the day drive cycle. Compared to the baseline Class 8 conventional trucks, an improvement of 20–22% and 28–50% in fuel economy by 2020 can be expected using non-electrification efficiency-improving and a combination of non-electrification and hybrid technologies. Fuel economy improvements of a factor of four to five can be achieved by hybridizing the heavy-duty trucks used on ocean ports.

Journal of Power Sources, 2009

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other app... more Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

IEEE Transactions on Power Electronics, 2004

An accurate nonlinearity compensation technique for voltage source inverter (VSI) inverters is pr... more An accurate nonlinearity compensation technique for voltage source inverter (VSI) inverters is presented in this paper. Because of the nonlinearity introduced by the dead time, turnon/off delay, snubber circuit and voltage drop across power devices, the output voltage of VSI inverters is distorted seriously in the low output voltage region. This distortion influences the output torque of IM motors for constant V/f drives. The nonlinearity of the inverter also causes 5th and 7th harmonic distortion in the line current when the distributed energy system operates in the grid-connected mode, i.e., when the distributed energy system is parallel to a large power system through the VSI inverter. Therefore, the exact compensation of this nonlinearity in the VSI inverter over the entire range of output voltage is desirable. In this paper, the nonlinearity of VSI inverter output voltage and the harmonic distortion in the line current are analyzed based on an open-loop system and a -load. By minimizing the harmonic component of the current in a -axis and -axis synchronous rotating reference frame, the exact compensation factor was obtained. Simulations and experimental results in the low frequency and low output voltage region are presented.

The paper studies a series-parallel hybrid powertrain configuration for the medium-duty plug-in h... more The paper studies a series-parallel hybrid powertrain configuration for the medium-duty plug-in hybrid trucks and Volt-like passenger cars. The series-parallel hybrid combines the features of the parallel hybrid and the series hybrid. Series-parallel hybrid powertrains with pre- and post-transmission configuration for the plug-in hybrid medium-duty trucks were modeled and compared with a conventional diesel and a mild/full parallel hybrid with pre-transmission configuration to explore the greatest possible benefit of fuel economy by powertrain hybridization. A control strategy for the series-parallel hybrid vehicle was developed, where the electric motor and the engine can work individually or together, depending on the speed and the power required for driving the vehicle and the state-of-charge (SOC) of the battery. The simulations were performed over the urban drive, highway drive, urban heavy duty drive, and the local parcel delivery drive cycles. The simulation results show that series-parallel are well suited to medium duty parcel delivery vehicle applications within the range of 50-100 miles. The Volt-like PHEV utilized a gasoline engine and the vehicle fuel economies were compared for the series-parallel and single-shaft approaches for various city and highway driving cycles.

Journal of Power Sources, 2006

Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portab... more Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portable power electronics and automotive applications due to their potentially high-energy density, simple storage, and distribution of the fuel. The concentration of methanol in the fuel circulation loop of a DMFC system is an important operating parameter, because it determines the electrical performance and efficiency of the system. The methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor for the purpose of maintaining optimal power and efficiency in the DMFC system. Various methods of sensing methanol concentration have been proposed over the past decade. This paper reviews these methanol concentration sensors for DMFCs, which are generally classified into two groups: electrochemical and physical. The construction and operating principles of each sensor, as well as its advantages and disadvantages, are described. The sensorless methods for controlling the methanol concentration are introduced briefly. Finally, the perspective on the future of methanol concentration sensors is discussed. Crown

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

Class 8 trucks using various powertrains and alternative fuel options have been analysed to deter... more Class 8 trucks using various powertrains and alternative fuel options have been analysed to determine their fuel economy, greenhouse gas emissions, and economic attractiveness at the present time (2013) and in the future. This was done by modelling the vehicles and simulating their operation on day, short haul, and long haul driving cycles. The economic attractive was determined by calculating the differential vehicle cost of each powertrain option and the corresponding breakeven alternative fuel price needed to recover the additional cost in a specified payback period with a fixed discount rate. The baseline vehicle was a diesel engine truck of the same weight and road load using $4/gallon diesel fuel. The use of some of the powertrains resulted in an energy saving and others resulted in higher energy consumption, but compared to the conventional Class 8 diesel trucks, conventional LNG-CI trucks, LNG-SI and LNG-CI hybrids, battery electric trucks, and fuel cell trucks can reduce CO 2 emission by 24-39% over the day drive cycle and 12-29% over the short haul and the long haul drive cycles.

2014 IEEE International Electric Vehicle Conference (IEVC), 2014

This paper is concerned with supercapacitors (electrochemical capacitors) and their applications ... more This paper is concerned with supercapacitors (electrochemical capacitors) and their applications in electric drive vehicles in place of or in combination with batteries. The electric drive vehicles considered are hybrid vehicles and fuel cell vehicles. The first section of the paper presents recent test data for advanced proto-type devices. The data for the new carbon/carbon device from Skeleton Technologies showed an energy density of 9 Wh/kg and 95% efficient power capability of 1730 W/kg. Both of these characteristics are significantly better than those of commercially available devices. Test data are shown for a hybrid supercapacitor from Yunasko that has an energy density greater than 30 Wh/kg and a 95% efficient power capability of 3120 W/kg. This device has the best performance of any supercapacitor device tested at UC Davis to date. Various vehicle applications of supercapacitors have been reviewed in detail. Simulation results are presented for light duty vehicles using supercapacitors in place of lithium batteries in hybrid and fuel cell vehicles. It was found in all cases that the vehicles using the supercapacitors had the same as or better performance than those using batteries and in general were more efficient. The cost of supercapacitors compared to lithium batteries was discussed briefly. It was shown that when one recognizes that the energy stored in the capacitors is less than 1/10 that in the batteries for hybrid applications, the price of supercapacitors needs to decrease to about .5- 1 cent Farad for capacitors to be cost competitive with high power batteries at $500-700/kWh. In addition, there is a good possibility that the life of the capacitors would be equal to that of the hybrid vehicles.

Sensors and Actuators A-physical - SENSOR ACTUATOR A-PHYS, 2005

Microfuel cell systems require microvalves that are chemically tolerant of hydrogen, have thermal... more Microfuel cell systems require microvalves that are chemically tolerant of hydrogen, have thermally insensitive activation mechanisms, and tight geometric constraints. A planar bulk-micromachined piezoelectric valve for portable fuel cell system is proposed. The actuating diaphragm and the valve seat are etched on silicon wafers. A piezoelectric bimorph disc is glued to the actuating diaphragm to lift the diaphragm and open the valve. The pressure differential between the valve chamber and the outlet and the initial pressure caused by the deformed tethers are employed to increase the sealing force. The actuating device with four Z-tethers attached to the actuating diaphragm is thermally insensitive and can reduce thermally induced deflection at the center and avoid clamping effect at circumferential edge. The proposed actuating diaphragm with Z-tethers shows good thermal stability and excellent actuating performance through the finite element method (FEM) analyses. The prototype valve of 20 mm in diameter and 2 mm thick was fabricated and tested as a regulator. Using compressed air, the flow characteristics for the prototype valve were measured at different inlet pressure and elevated air temperature. The design, fabrication, simulation and characterization of the prototype valve are described.

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other app... more Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been extensively developed in recent decades. Besides the fuel cell stack, air and fuel control, and thermal and water management are major challenges in the fuel cell vehicle development. The air supply system can have a major impact on overall system efficiency. In this report, a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. The model is scalable so that it can be used to simulate the operation of an arbitrary size (power) fuel cell. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

Class 8 trucks using various powertrains and alternative fuel options have been analysed to deter... more Class 8 trucks using various powertrains and alternative fuel options have been analysed to determine their fuel economy, greenhouse gas emissions, and economic attractiveness at the present time (2013) and in the future. This was done by modelling the vehicles and simulating their operation on day, short haul, and long haul driving cycles. The economic attractive was determined by calculating the differential vehicle cost of each powertrain option and the corresponding breakeven alternative fuel price needed to recover the additional cost in a specified payback period with a fixed discount rate. The baseline vehicle was a diesel engine truck of the same weight and road load using $4/gallon diesel fuel. The use of some of the powertrains resulted in an energy saving and others resulted in higher energy consumption, but compared to the conventional Class 8 diesel trucks, conventional LNG-CI trucks, LNG-SI and LNG-CI hybrids, battery electric trucks, and fuel cell trucks can reduce CO2 emission by 24-39% over the day drive cycle and 12-29% over the short haul and the long haul drive cycles.
The breakeven fuel price was calculated for all the powertrain/fuel options. The economic results indicate that at “today’s” differential vehicle costs, none of the alternative powertrains/fuels are economically attractive except for the LNG-CI engine in the long-haul application (VMT=150,000 miles) for which the DGE cost is 2.98/DGEandtheLNGcostis2.98/DGE and the LNG cost is 2.98/DGEandtheLNGcostis1.70/LNG gallon. If the differential costs of the alternative powertrains are reduced by ½, their economics is improved markedly. In the case of LNG-CI engine, the breakeven fuel costs are 3.42/GDE,3.42/GDE, 3.42/GDE,1.96/LNG gallon for the long haul applications (VMT= 150,000 miles) with payback periods of 2-3 years. This makes LNG cost competitive at 2013 prices of diesel fuel and LNG. The fuel cell powered truck is also nearly cost competitive at VMT= 150,000 miles, but this requires a fuel cell cost of less than 25/kW.Hybridizingisnotattractiveexceptfortheconventionaldieselvehicleoperatingonthedaycycle(somestopandgooperation)forwhichthebreakevendieselpriceisabout25/kW. Hybridizing is not attractive except for the conventional diesel vehicle operating on the day cycle (some stop and go operation) for which the breakeven diesel price is about 25/kW.Hybridizingisnotattractiveexceptfortheconventionaldieselvehicleoperatingonthedaycycle(somestopandgooperation)forwhichthebreakevendieselpriceisabout2/gallon at ½ today’s differential vehicle costs. The regulated exhaust emissions from the LNG-CI engines will meet the same standards (EPA 2010) as the new diesel engines and use the same exhaust emission technology.

An intelligent energy management approach for a solar powered EV charging station with energy sto... more An intelligent energy management approach for a solar powered EV charging station with energy storage has been studied and demonstrated for a level 2 charger at the University of California-Davis West Village. The approach introduces solar PV electrical energy forecasting and EV charging demand projection to optimize the energy management of the charging station. The percentage of cloud cover is extracted from a weather forecast website for estimating the available PV electrical energy. A linear fit of the historical EV charging load from the same day of the week over the previous six weeks is employed for extracting the charging pattern of the workplace EV charging station. Both simulations and actual operation show that intelligent energy management for a charging station with a buffer battery can reduce impacts of the EV charging system on utility grids in terms of peak power demand and energy exchange, reduce grid system losses, and benefit the charging station owner through the Time-of-Use rate plans.

Proton Exchange Membrane fuel cell (PEMFC) technology is one of the most attractive candidates fo... more Proton Exchange Membrane fuel cell (PEMFC) technology is one of the most attractive candidates for transportation applications due to its inherently high efficiency and high power density. However, the fuel cell system efficiency can suffer because of the need for forced air supply and water-cooling systems. Hence the operating strategy of the fuel cell system can have a significant impact on the fuel cell system efficiency and thus vehicle fuel economy. The key issues are how the fuel cell back pressure and air flow through the fuel cell are controlled. One approach is fixed back pressure control. The other is optimum varying back pressure control. In both cases, the air flow stoichiometry is optimized. In this paper, a dynamic forward-looking vehicle model with a dynamic fuel cell system model is employed. The effects of different fuel cell system operation modes and different power split strategies on fuel economy of fuel cell hybrid vehicles are simulated. The simulation results...

Most vehicles presently use batteries for energy storage, but there are vehicle designs in which ... more Most vehicles presently use batteries for energy storage, but there are vehicle designs in which ultracapacitors alone or in combination with batteries can increase the efficiency of the vehicle and in addition lead to significantly longer battery cycle life. Ultracapacitors can be used alone in charge sustaining hybrid vehicles (HEVs) if the energy storage requirement is less than 150Wh. Simulations show that when ultracapacitors are used alone in HEVs, the roundtrip efficiency of the energy storage is 95-98% and the engine efficiency in on/off operation can be maintained near the peak efficiency value. Vehicle simulations were also run for plug-in hybrid vehicles (PHEVs) using advanced batteries with high energy density (>300 Wh/kg). The simulations were run with the batteries alone and in combination with ultracapacitors. Simulation results for the electric useage (Wh/mi) and all-electric range and fuel economy (mpg) for the PHEVs using batteries in combination with ultracapac...

Hybridization of fuel cells with additional batteries or ultracapacitors in a fuel cell vehicle r... more Hybridization of fuel cells with additional batteries or ultracapacitors in a fuel cell vehicle reduces electrical and mechanical stresses on fuel cells and improves the overall drive train efficiency over a standard drive cycle. This paper primarily analyzes hybrid fuel cell vehicles with different drive train arrangements, and compares projected fuel economies of hybrid fuel cell vehicles with improved conventional vehicles and hybrid electric vehicles at three points in the future: 2015, 2030, and 2045. The key points addressed are as follows: drive train arrangements, control strategies, and the influence of energy storage sizing on vehicle fuel economy. The study shows that fuel cell vehicles having ultracapacitors coupled with fuel cells via a DC/DC converter with load-leveling control is the best approach in term of improving fuel economy and mitigating the stress on the fuel cell. Power-assist control is well suited for fuel cell-battery hybrids in terms of fuel economy impr...

The use of ultracapacitors in plug-in hybrid vehicles (PHEVs) with high energy density lithium-io... more The use of ultracapacitors in plug-in hybrid vehicles (PHEVs) with high energy density lithium-ion and zinc-air batteries is studied. Simulations were performed for various driving cycles with the PHEVs operating in the charge depleting and charge sustaining modes. The effects of the load leveling of the power demand from the batteries using the ultracapacitors are evident. The average and the peak currents from the batteries are lower by a factor of 2-3.

This paper is concerned with the use of ultracapacitors in hybrid vehicles in place of batteries.... more This paper is concerned with the use of ultracapacitors in hybrid vehicles in place of batteries. In the case of the mild, charge sustaining hybrid, the ultracapacitors would replace a lithium or nickel metal hydride battery: for a stop-start micro-hybrid, the capacitors would be used in combination with a lead-acid battery with the capacitors starting the engine, accepting energy during regenerative braking, and providing accessory loads during relatively short stop periods. Test data are shown for the performance of advanced carbon/carbon and hybrid lithium ultracapacitors indicating higher energy density (more than 2X) than that of commercially available carbon/carbon cells from Maxwell and NessCap. The advanced devices showed no sacrifice in high power capability in order to achieve the higher energy density. Simulations of mid-size passenger cars using the advanced ultracapacitors in micro-hybrid and charge sustaining hybrid powertrains were performed using the Advisor vehicle ...

In this paper, various alternative hybrid vehicle powertrains that are being considered by auto c... more In this paper, various alternative hybrid vehicle powertrains that are being considered by auto companies are evaluated based on simulation studies performed at the Institute of Transportation Studies, University of California-Davis. The following hybrid powertrain arrangements have been considered: a. Single-shift, parallel (Honda) b. Single-planetary, dual-mode (Toyota/Prius) c. Multiple-planetary, dual-mode (GM) d. Multiple-shaft, dual-clutch transmission (VW and Borg-Warner) e. Series – range extended EV (GM Volt) The primary strategy in all the options considered was to operate the engine only in the high efficiency part of its map and to lose as little as possible of the gain by losses in the energy storage unit and the electric machines. The simulations indicated that there are in general not large differences in the fuel economies predicted using the various powertrains for the same vehicle and battery. The fuel economy improvements were large in all case – 80-100% for the F...

gates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel... more gates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel cell durability.

Journal of Power Sources, 2015

A Fuel cell-LiFePO 4 battery hybrid powertrain using a direct parallel structure. Fuzzy control b... more A Fuel cell-LiFePO 4 battery hybrid powertrain using a direct parallel structure. Fuzzy control based energy management strategies. Energy management for adjusting and stabilizing the DC bus voltage. Application of ultracapacitors for protecting Fuel cells and LiFePO 4 batteries.

Transportation Research Part D: Transport and Environment, 2013

Non-electrification efficiency-improving technologies and powertrain technologies for reducing th... more Non-electrification efficiency-improving technologies and powertrain technologies for reducing the heavy-duty truck fuel consumption are studied. The study indicates that improvements in engine efficiency, aerodynamic drag and rolling resistance will benefit fuel economy significantly over the day drive and over-the-road highway driving cycles; 6–13% in fuel savings can be expected from each technology. Hybridization can achieve fuel saving of 16% and is financially attractive for the day drive cycle. Compared to the baseline Class 8 conventional trucks, an improvement of 20–22% and 28–50% in fuel economy by 2020 can be expected using non-electrification efficiency-improving and a combination of non-electrification and hybrid technologies. Fuel economy improvements of a factor of four to five can be achieved by hybridizing the heavy-duty trucks used on ocean ports.

Journal of Power Sources, 2009

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other app... more Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

IEEE Transactions on Power Electronics, 2004

An accurate nonlinearity compensation technique for voltage source inverter (VSI) inverters is pr... more An accurate nonlinearity compensation technique for voltage source inverter (VSI) inverters is presented in this paper. Because of the nonlinearity introduced by the dead time, turnon/off delay, snubber circuit and voltage drop across power devices, the output voltage of VSI inverters is distorted seriously in the low output voltage region. This distortion influences the output torque of IM motors for constant V/f drives. The nonlinearity of the inverter also causes 5th and 7th harmonic distortion in the line current when the distributed energy system operates in the grid-connected mode, i.e., when the distributed energy system is parallel to a large power system through the VSI inverter. Therefore, the exact compensation of this nonlinearity in the VSI inverter over the entire range of output voltage is desirable. In this paper, the nonlinearity of VSI inverter output voltage and the harmonic distortion in the line current are analyzed based on an open-loop system and a -load. By minimizing the harmonic component of the current in a -axis and -axis synchronous rotating reference frame, the exact compensation factor was obtained. Simulations and experimental results in the low frequency and low output voltage region are presented.

The paper studies a series-parallel hybrid powertrain configuration for the medium-duty plug-in h... more The paper studies a series-parallel hybrid powertrain configuration for the medium-duty plug-in hybrid trucks and Volt-like passenger cars. The series-parallel hybrid combines the features of the parallel hybrid and the series hybrid. Series-parallel hybrid powertrains with pre- and post-transmission configuration for the plug-in hybrid medium-duty trucks were modeled and compared with a conventional diesel and a mild/full parallel hybrid with pre-transmission configuration to explore the greatest possible benefit of fuel economy by powertrain hybridization. A control strategy for the series-parallel hybrid vehicle was developed, where the electric motor and the engine can work individually or together, depending on the speed and the power required for driving the vehicle and the state-of-charge (SOC) of the battery. The simulations were performed over the urban drive, highway drive, urban heavy duty drive, and the local parcel delivery drive cycles. The simulation results show that series-parallel are well suited to medium duty parcel delivery vehicle applications within the range of 50-100 miles. The Volt-like PHEV utilized a gasoline engine and the vehicle fuel economies were compared for the series-parallel and single-shaft approaches for various city and highway driving cycles.

Journal of Power Sources, 2006

Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portab... more Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portable power electronics and automotive applications due to their potentially high-energy density, simple storage, and distribution of the fuel. The concentration of methanol in the fuel circulation loop of a DMFC system is an important operating parameter, because it determines the electrical performance and efficiency of the system. The methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor for the purpose of maintaining optimal power and efficiency in the DMFC system. Various methods of sensing methanol concentration have been proposed over the past decade. This paper reviews these methanol concentration sensors for DMFCs, which are generally classified into two groups: electrochemical and physical. The construction and operating principles of each sensor, as well as its advantages and disadvantages, are described. The sensorless methods for controlling the methanol concentration are introduced briefly. Finally, the perspective on the future of methanol concentration sensors is discussed. Crown

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

Class 8 trucks using various powertrains and alternative fuel options have been analysed to deter... more Class 8 trucks using various powertrains and alternative fuel options have been analysed to determine their fuel economy, greenhouse gas emissions, and economic attractiveness at the present time (2013) and in the future. This was done by modelling the vehicles and simulating their operation on day, short haul, and long haul driving cycles. The economic attractive was determined by calculating the differential vehicle cost of each powertrain option and the corresponding breakeven alternative fuel price needed to recover the additional cost in a specified payback period with a fixed discount rate. The baseline vehicle was a diesel engine truck of the same weight and road load using $4/gallon diesel fuel. The use of some of the powertrains resulted in an energy saving and others resulted in higher energy consumption, but compared to the conventional Class 8 diesel trucks, conventional LNG-CI trucks, LNG-SI and LNG-CI hybrids, battery electric trucks, and fuel cell trucks can reduce CO 2 emission by 24-39% over the day drive cycle and 12-29% over the short haul and the long haul drive cycles.

2014 IEEE International Electric Vehicle Conference (IEVC), 2014

This paper is concerned with supercapacitors (electrochemical capacitors) and their applications ... more This paper is concerned with supercapacitors (electrochemical capacitors) and their applications in electric drive vehicles in place of or in combination with batteries. The electric drive vehicles considered are hybrid vehicles and fuel cell vehicles. The first section of the paper presents recent test data for advanced proto-type devices. The data for the new carbon/carbon device from Skeleton Technologies showed an energy density of 9 Wh/kg and 95% efficient power capability of 1730 W/kg. Both of these characteristics are significantly better than those of commercially available devices. Test data are shown for a hybrid supercapacitor from Yunasko that has an energy density greater than 30 Wh/kg and a 95% efficient power capability of 3120 W/kg. This device has the best performance of any supercapacitor device tested at UC Davis to date. Various vehicle applications of supercapacitors have been reviewed in detail. Simulation results are presented for light duty vehicles using supercapacitors in place of lithium batteries in hybrid and fuel cell vehicles. It was found in all cases that the vehicles using the supercapacitors had the same as or better performance than those using batteries and in general were more efficient. The cost of supercapacitors compared to lithium batteries was discussed briefly. It was shown that when one recognizes that the energy stored in the capacitors is less than 1/10 that in the batteries for hybrid applications, the price of supercapacitors needs to decrease to about .5- 1 cent Farad for capacitors to be cost competitive with high power batteries at $500-700/kWh. In addition, there is a good possibility that the life of the capacitors would be equal to that of the hybrid vehicles.

Sensors and Actuators A-physical - SENSOR ACTUATOR A-PHYS, 2005

Microfuel cell systems require microvalves that are chemically tolerant of hydrogen, have thermal... more Microfuel cell systems require microvalves that are chemically tolerant of hydrogen, have thermally insensitive activation mechanisms, and tight geometric constraints. A planar bulk-micromachined piezoelectric valve for portable fuel cell system is proposed. The actuating diaphragm and the valve seat are etched on silicon wafers. A piezoelectric bimorph disc is glued to the actuating diaphragm to lift the diaphragm and open the valve. The pressure differential between the valve chamber and the outlet and the initial pressure caused by the deformed tethers are employed to increase the sealing force. The actuating device with four Z-tethers attached to the actuating diaphragm is thermally insensitive and can reduce thermally induced deflection at the center and avoid clamping effect at circumferential edge. The proposed actuating diaphragm with Z-tethers shows good thermal stability and excellent actuating performance through the finite element method (FEM) analyses. The prototype valve of 20 mm in diameter and 2 mm thick was fabricated and tested as a regulator. Using compressed air, the flow characteristics for the prototype valve were measured at different inlet pressure and elevated air temperature. The design, fabrication, simulation and characterization of the prototype valve are described.

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other app... more Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been extensively developed in recent decades. Besides the fuel cell stack, air and fuel control, and thermal and water management are major challenges in the fuel cell vehicle development. The air supply system can have a major impact on overall system efficiency. In this report, a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. The model is scalable so that it can be used to simulate the operation of an arbitrary size (power) fuel cell. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

Class 8 trucks using various powertrains and alternative fuel options have been analysed to deter... more Class 8 trucks using various powertrains and alternative fuel options have been analysed to determine their fuel economy, greenhouse gas emissions, and economic attractiveness at the present time (2013) and in the future. This was done by modelling the vehicles and simulating their operation on day, short haul, and long haul driving cycles. The economic attractive was determined by calculating the differential vehicle cost of each powertrain option and the corresponding breakeven alternative fuel price needed to recover the additional cost in a specified payback period with a fixed discount rate. The baseline vehicle was a diesel engine truck of the same weight and road load using $4/gallon diesel fuel. The use of some of the powertrains resulted in an energy saving and others resulted in higher energy consumption, but compared to the conventional Class 8 diesel trucks, conventional LNG-CI trucks, LNG-SI and LNG-CI hybrids, battery electric trucks, and fuel cell trucks can reduce CO2 emission by 24-39% over the day drive cycle and 12-29% over the short haul and the long haul drive cycles.
The breakeven fuel price was calculated for all the powertrain/fuel options. The economic results indicate that at “today’s” differential vehicle costs, none of the alternative powertrains/fuels are economically attractive except for the LNG-CI engine in the long-haul application (VMT=150,000 miles) for which the DGE cost is 2.98/DGEandtheLNGcostis2.98/DGE and the LNG cost is 2.98/DGEandtheLNGcostis1.70/LNG gallon. If the differential costs of the alternative powertrains are reduced by ½, their economics is improved markedly. In the case of LNG-CI engine, the breakeven fuel costs are 3.42/GDE,3.42/GDE, 3.42/GDE,1.96/LNG gallon for the long haul applications (VMT= 150,000 miles) with payback periods of 2-3 years. This makes LNG cost competitive at 2013 prices of diesel fuel and LNG. The fuel cell powered truck is also nearly cost competitive at VMT= 150,000 miles, but this requires a fuel cell cost of less than 25/kW.Hybridizingisnotattractiveexceptfortheconventionaldieselvehicleoperatingonthedaycycle(somestopandgooperation)forwhichthebreakevendieselpriceisabout25/kW. Hybridizing is not attractive except for the conventional diesel vehicle operating on the day cycle (some stop and go operation) for which the breakeven diesel price is about 25/kW.Hybridizingisnotattractiveexceptfortheconventionaldieselvehicleoperatingonthedaycycle(somestopandgooperation)forwhichthebreakevendieselpriceisabout2/gallon at ½ today’s differential vehicle costs. The regulated exhaust emissions from the LNG-CI engines will meet the same standards (EPA 2010) as the new diesel engines and use the same exhaust emission technology.

An intelligent energy management approach for a solar powered EV charging station with energy sto... more An intelligent energy management approach for a solar powered EV charging station with energy storage has been studied and demonstrated for a level 2 charger at the University of California-Davis West Village. The approach introduces solar PV electrical energy forecasting and EV charging demand projection to optimize the energy management of the charging station. The percentage of cloud cover is extracted from a weather forecast website for estimating the available PV electrical energy. A linear fit of the historical EV charging load from the same day of the week over the previous six weeks is employed for extracting the charging pattern of the workplace EV charging station. Both simulations and actual operation show that intelligent energy management for a charging station with a buffer battery can reduce impacts of the EV charging system on utility grids in terms of peak power demand and energy exchange, reduce grid system losses, and benefit the charging station owner through the Time-of-Use rate plans.