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International Journal of Technology and Management Research 5 (2016) 14-23
Effects of Energy Storage Systems on Fuel Economy of Hybrid-Electric Vehicles
* Godwin K. Ayetor, Emmanuel Duodu, John Abban
Faculty of Engineering, Koforidua Polytechnic, Koforidua, Ghana. Abstract
Three energy storage systems, namely Nickel Zinc, Nickel Metal Hydride and Lithium ion batteries were simulated on ADVISOR (Advanced vehicle simulator) to determine their impact on fuel economy. ADVISOR, a drivetrain analysis tool developed in MATLAB/Simulink for comparing fuel economy and emissions performance and designed by the National Renewable Energy Laboratory by Ford, GM, and Chrysler was used for the simulations. In choosing the batteries for simulations, only the latest technological advanced batteries of NiZn, Li ion and NiMH were used. The results showed that NiZn battery influence in fuel economy and system efficiency far exceeds the other batteries especially for the combined Powertrain. While a lithium ion battery is seen to be well suited for Parallel and Series powertrains at higher speeds, average values for all drive cycle singles out NiZn as a better performing battery. NiMH showed the worst performance. This confirms NiMH, which is the predominant energy storage system today in the HEV industry, is deficient in advancing the growth of HEV's. Keywords: power trains; hybrid energy storage; hybrid electric vehicle; combined hybrid; parallel hybrid 1. Introduction
All these topologies are somewhat variants of Series and Parallel hybrids. Ever rising crude oil prices and stricter standard Generally, parts of HEV include IC engine, emission regulations have put a lot of pressure on electric motor, battery, power control unit, and automotive manufacturers to produce more fuel reduction gear. For Series and Combined types, efficient and zero emission cars additional generator is required. Combined types . Developing Powertrain have a Power Split device to split traction power systems for automotive vehicles with higher fuel between the engine and electric motor. efficiency and lesser emissions without sacrificing high performance level is an enormous challenge to 1.1. Motivation
the automotive industry . By combining benefits of electric vehicles and Presently hybrid electric vehicles have mainly been conventional vehicles, Hybrid electric vehicles are advertised for their ability to minimize fuel known to produce almost zero emissions, low noise, consumption and eliminate emissions and faster responses hence are more reliable . This has primarily been achieved through electric vehicle (HEV) is described as one with two motor/generator which supplements the IC engine. energy storage systems both of which must provide Fuel savings have been recorded mainly where more propulsion together or independently of the electrical energy is used instead of the IC . The sources of engine. Useful attributes of fuel propulsion have both conventional IC engine or fuel savings and fewer emissions are contradicted by the cells and electric motors. There are approximately 40 overall cost of a hybrid electric vehicle various viable hybrid topologies each having specific . Even though over the entire life advantages and drawbacks . of an HEV the running cost will be lesser, most Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 consumers might not even need a car for that long considered: Energy Density, Power Density, Specific . If HEV's are to become Energy, Specific Power, long life, safety, cost, competitive then drastic reduction in cost must be temperature range, Memory Effect and Recycling considered. Batteries contribute significantly to overall cost of the hybrid vehicle; therefore the need for a lower cost yet efficient battery cannot be 2. Methodology
ADVISOR is an advanced vehicle simulation . It is confirmed that traction battery is the most software developed by the National Renewable critical component of the vehicle and will be the Energy Laboratory to allow studies of advanced most expensive component in most cases vehicles. Three different HEV powertrains have been . To further increase the modelled on ADVISOR in this work. These range of electric motor operation thereby minimizing powertrains include SERIES, PARALLEL and engine use, batteries play a very significant role COMBINED (POWER-SPLIT). Each powertrain is tested with each of the batteries: NiZn, NiMH and Lithium batteries (especially lithium polymer) Lithium ion. The specifications of these batteries have been researched extensively and are were modelled based on their highest performance being considered as the future batteries of HEV. The and current technological advancement. world record for longest distance travelled on a single battery charge was with Lithium-ion Batteries 2.1. Powertrain Specifications
. A special 1999 Mitsubishi coupe using Li-ion batteries covered 2124 km (1330 mi) on a The Toyota Prius vehicle was modelled for each of single charge. However, Lithium batteries boast of the three Powertrains. higher specific energies but very high cost (50% Actual Body Weight=2783 pounds (1398kg)-full tank greater cost than NiMH) coupled with inability to be Vehicle Glider Mass=918kg, Vehicle Cargo recycled . It is incumbent alternative Mass=136kg, Vehicle Coefficient of Drag=0.3 batteries for HEV's are considered. This research Vehicle Frontal Area=1.746m2 , Vehicle Wheel attempts to assess performance of alternative HEV batteries. Nickel Zinc battery is simulated against Centre of Gravity=1.542 from rear axle on empty Lithium and NiMH. Performance is analysed using the criteria of fuel consumption, energy usage, system efficiency and emissions. 2.2. Fuel Converter
1.2. Battery specifications
Fuel Type=Gasoline, Capacity=1.5L Japan Prius Atkinson Cycle Engine HEV require batteries that can be recharged as Maximum Power=43kW at 4000RPM, Peak Torque secondary batteries . To =75 lb-ft at 4000RPM increase the output voltage, its cells are placed in Peak Efficiency=0.39, Weight=137kg. The Torque- series. Battery capacity (Ah) gives speed graph for the fuel converter is shown in Fig.1. indication of how long a battery can give a certain amount of current. For a rating of 3Ah and assuming current is 0.5A, it implies such a battery will be able to deliver 0.5A continuously for 6 hours (360mins). Power Density (kW/L) and Energy Density (kWh/L) are used to answer the question of how much a battery weighs . The higher the values the smaller the batteries will be in volume to deliver energy or power . Specific Energy (kWh/kg) and Specific Power (kW/kg) measure the respective values in relation to the weight. A battery of 2kWh/kg will deliver the same energy as 1kWh/kg but with half the weight of the latter. That is, if Fig. 1: Torque-Speed operation for fuel converter
specific energy is doubled the weight of battery is cut by half. In selecting a battery, the following must be Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 2.3. Traction Motor
Specifications for the Traction motor are as follows: 700-W constant electric load accessories were used Peak Efficiency=0.91 for the entire simulation. Mass=57kg, PRIUS_JPN 30-kW permanent magnet motor/controller 2.5. Parallel Powertrain Architecture
The parallel powertrain architecture used for the
2.4. Generator
modelling on ADVISOR/Matlab is shown in Fig. 2. This applies to only the Series and Combined Hybrid motor/controller Peak Efficiency=0.84 Fig.2: Parallel powertrain as modelled on ADVISOR
2.6. Series Powertrain Architecture
Series powertrain architecture modelled on ADVISOR/ Matlab is as shown in Fig. 3 Fig.3: Series powertrain as modelled on ADVISOR
Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23
2.7. Combined Powertrain Architecture
A model of the combined hybrid electrical vehicle is shown in Fig. 4 Fig. 4: Combined Powertrain as modelled on ADVISOR
3. Results and Analysis
Depending on the power demands of an HEV and the State of Charge of the battery, power output 3.1. Parallel Powertrain (UDDS and NEDC)
must be varied for peak performance and fuel economy. In order to operate in a peak condition, the At an average speed of 19.6mph (31.54kph) the NiZn NiMH gave the least power output resulting in it energy storage system was the most fuel efficient having the lowest fuel economy (54mpg). Same fuel (75mpg) as seen from Fig. 5. The NiZn control economy patterns were recorded for the New system was such that battery output power was more European Drive Cycle (NEDC). than the other batteries at 3511kJ. As battery power output increases fuel economy is improved hence the Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 Fig.5: fuel economy of each battery under Parallel powertrain for UDDS
3.2. Parallel Powertrain Fuel Economy for
(Fig.6). Thereby easing the power output of the Highway Fuel Economy Cycle (HWFEC)
engine and reducing fuel consumption. The control system regulates power delivery and energy storage The pattern for the HWFEC differs considerably based upon the state-of-charges (SoC) of the battery. from UDDS and NEDC. This time best fuel The lowest energy output for the NiZn system show economy (57mpg) was recorded in the Lithium the battery was recharged more often during the energy storage system whose control system allows it cycle compared to the other systems. to give the highest battery power output of 1896kJ ESS ENERGY IN(kJ) Fig.6: fuel economy of each battery under Parallel powertrain for HWFEC
Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23
3.3. Combined Powertrain (UDDS and NEDC)
fuel consumption. 48mpg corresponds to the lowest output of 2215kJ for the NiMH storage system Fuel economy of the NiZn storage system is 71mpg (Fig.7). Again, patterns of fuel consumption were the and it is by far the highest of the energy storage same as for NEDC. systems. Its high power output (3556kJ) ensures that mostly electrical power is used, thereby lessening the Fig.7: fuel economy of each battery under combined powertrain for UDDS
3.3.1. Combined Powertrain Fuel Economy
highest fuel economy of 77 mpg (Fig.8). Even with Results for High Way Fuel Economy
the least energy input of 101kJ, it produced the Cycle (HWFEC)
highest indicating a depleting state of charge for the battery. The patterns remained the same as for the UDDS and NEDC cycle with NiZN again having the Fig.8: fuel economy of each battery under combined powertrain for HWFEC
3.4. Series Powertrain Fuel Economy Results for
the entire cycle. This is followed by Lithium ion and UDDS and NEDC
NiMH respectively. It is also noticed that Nickel Zinc had the least input, but highest output (Fig.9). The best fuel economy favours NiZn storage system whose SoC shows its discharges energy throughout Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 Fig. 9: fuel economy of each battery under Series powertrain for UDDS
3.4.1. Series Powertrain Results for HWFEC
For its high battery power contribution to propel the battery state of charge has to be maintained within a vehicle, Li ion had the highest fuel economy of limit for optimum performance. 55mpg (Fig.10). The control system is such that the Fig.10: fuel economy of each battery under Series powertrain for UDDS
The SoC for Lithium shows that throughout the cycle, the battery was discharged to as low as 0.4 state of charge (Fig.11). Fig.11:. State of Charge (SoE) for Lithium ion under Series Powertrain for HWFEC
Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 NiMH on the other hand operated between 0.76 and is because during recharging the engine powers the 0.68 while recharging for the very first 300s hence wheels as well as charging the battery consuming a the low output and low fuel economy (Fig.12). This Fig.12: State of Charge (SoE) for NiMH under Series Powertrain for HWFEC
Fig.13: State of Charge (SoE) for NiZn under Series Powertrain for HWFEC
3.5. Average Values
For each Powertrain, it is shown from figures 14, 15, 16 that NiZn is the best for fuel economy and overall Combined
system efficiency. For the combined type alone, it achieved an average of 68mpg compared to 54mpg and 53mpg for Li and NiMH respectively. Also for almost all the cycles and for all Powertrains, NiMH showed the worst performance. Fig.15: Battery performance for Combined configuration
Fig.14: Battery performance for Parallel configuration
Ayetor et al /International Journal of Technology and Management Research 5 (2016) 14-23 batteries does not lie exclusively with Lithium batteries as has been portrayed in many research papers. The results show that NiZn battery is the way to go. PowerGenix Company has given proof that in terms of cost, recyclability, environmental impact, specific energy and power. Lithium batteries boast of higher specific energies, but very high cost (50% greater cost than NiMH) coupled with inability to be recycled and the danger it poses to the environment contradicts what the hybrid electric vehicle stands for. It is incumbent alternative batteries for HEV's are considered. PowerGenix NiZn, is by far superior Fig.16: Battery performance for Series configuration
to any of the Lithium batteries. Batteries already contribute tremendously to the cost of HEV cars On the hand, average values for High way economy making them difficult to sell. Considering also that cycle show marginal gains in both fuel economy and hybrid vehicles are sold on the platform of system efficiency for Li battery. Indicating that where environmental safety, NiZn batteries stand a better constant power is needed from the batteries and at chance compared to Lithium batteries. higher speeds, Li ion batteries gives the best Due to their higher power density than batteries, performance (Fig.17). further research should consider the development of electrochemical capacitor or super capacitors as energy storage systems for batteries. References
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