Electric Bus Proposal

Overview:

The ever-increasing dependence on fossil fuels has led to a drastic increase in greenhouse gas (GHG) emissions, specifically in the transportation industry. The implementation of electric buses (EB) globally has shown to be a promising solution to this issue due to zero tailpipe emissions. Nevertheless, the usage of electric vehicles (EV) will lead to a subsequent demand in energy production. Therefore, the viability of EBs depends on the source of energy. Various countries, such as Norway, have held strong decarbonizing goals with minimal dependence on GHG emitting energy sources, thus, proving to be a suitable location for the implementation of EBs on a large scale. A simulation-based on the macroscopic version of the conservation of momentum is used to analyze the viability of the chosen design for this pursuit. The simulation is used to make design decisions on the propulsion configuration, specifically commercially available electric motors and needed gear reductions. Subsequently, the configuration is used to determine the proper battery chemistry. Here, the three most common battery chemistries for EB were analyzed: lithium iron phosphate (LFP), lithium nickel manganese oxide (NMC), and lithium titanate (LTO). Individual cylindrical cells will be used to make a battery pack which will be sized based on the motor power draw, charging strategy, and approximated auxiliary power draw estimates. Finally, all decisions are analyzed against a diesel transit bus to determine its viability. As a result, the EB proved to be almost twice as efficient, in terms of miles per gallon diesel, and almost $400,000 cheaper.

Design Requirements:

European regions, such as the Nordic, have over 80% of energy production coming from GHG free sources, as shown in the figure below [1]:

With such a high dependence on CO2-free sources, this region is suitable to meet energy requirements from electric vehicles. Bus route 26 in Tromsø, Norway, is chosen to model the design. Furthermore, the following weather, elevation, and average speed data was collected for the location [2,3]:

ParametersValues (Avg.)
Min Temperature [°C (°F)]-4.0 (25)
 Max Temperature [°C (°F)]12 (54)
Temperature [°C (°F)]4.0 (40)
Max Wind Speed [m/s (mph)]4.3 (9.6)
Min Wind Speed [m/s (mph)]2.3 (5.1)
Wind Speed [m/s (mph)]3.3 (7.4)
Direction [rad (deg)]0.96 (55)
Min Snowfall [in (ft)]0.10 (8.3e10-3)
Max Snowfall [in (ft)]1.7 (0.14)
Snowfall [in (ft)]0.80 (0.07)
 Pressure [kPa (mb)]100 (1000)
Average temperature, wind speed, wind direction, pressure, and snowfall accumulation
Elevation and Average Speed for Route 25

The route is roughly 10,250 m (6.37 miles) with 27 stops [4]. Moreover, the average round-trip time is estimated to be 82 minutes and the average diesel transit bus makes 12 round-trips per day [5] with an average fuel economy of 4.2 miles/gallon of diesel [6]. Here, these values will be used to determine the viability of the EB compared to a diesel transit bus based on the results obtained from the simulation.

Propulsion:

The typical propulsion system for EBs consists of one traction motor connected straight to the differential powered by the battery pack. Power density, efficiency, reliability, and cost are important factors to consider when choosing the right motor. Since regenerative braking will be utilized in the design, DC brushless motors will not be considered. To choose the right motor, the maximum torque experienced by the vehicle needs to be found. To do this, the curb weight of the with a max passenger load of 6120 kg, rolling resistance due to snow chains, and the lowest average temperature was simulated over the driving profile. Moreover, a maximum acceleration of 1.4715 m/s^2 and deceleration of 1.8639 m/s^2 was set into the system to avoid any irrational driving. As a result, a peak torque of 30, 272 Nm is needed from the motors. Therefore, multiple motors in combination with gear reduction is needed to achieve this value. When further analyzed, the only type of motors that can be efficiently coupled are EMRAX 348 MV.  To achieve this same result, the Proterra EB will need to produce roughly 16,156 Nm of torque. Thus, the bus will utilize three EMRAX 348 MV motors with a gear reduction of 10:1 to ensure the peak torque is met while also allowing it to reach its maximum speed of 75 km/hr. Moreover, three RMS PM150DZ motor controllers with a continuous motor current of 225 Arms will be used to regulate revolutions and the amount of current going into the motor. A detailed analysis of the three motors considered can be found in the following table with 1 being most expensive and 5 being the cheapest [7]:

ParametersYASA-750R [8]TM4 SUMO HP HV 1000 [9]EMRAX 348 MV [10]
Peak Torque [Nm]7909651000
Peak Power [kW]200205300
Voltage [VDC]700900800
Continuous Torque [Nm]400745500
Max Speed [rpm]325040004000
Mass [kg]3711240
Efficiency [%]>95>9592-98
Cost [-]431
Summary of AC Permanent Magnet Motors Considered for the Design

Battery Technology:

LIBs have been primarily used for small scale applications such as consumer electronics. When considering them for electrical vehicles, factors such as cycle life, specific energy, and cost need to be considered. The three main types of chemistries looked into were lithium-iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), and lithium titanate (LTO). A thorough comparison is shown in the following table:

ParametersLFPNMCLTO
ModelANR 26650 M1B [11]INR-26650-5000 [14]GWL LTO1865-13 [11]
Voltage [VDC]3.30 [11]3.60 [14]2.40 [11]
Capacity [Ah]2.50 [11]5.00 [14]1.30 [11]
Max Cycle life [Cycles]1,000 [11]1,500 [14]20,000 [11]
Specific energy [Wh/kg]108 [11]190 [14]78 [11]
C-rate1C [11]1C [14]5C [11]
Dimensions- diameter x height [mm]26 x 65.15 [11]26.9 x 65.5 [14]18.7 x 65.3 [11]
Mass [g]76 [11]95 [14]40 [11]
SOC range [%]20 – 80 [11]20 – 80 [14]10 – 100 [11]
Γ1.0310 [11]0.9891 [13]0.9414 [11]
α°1.0063 [11]1.0530 [13]1.0998 [11]
β°0.3189 [11]2.6969 [13]2.0744 [11]
Cost [$/kWh]360 [11] [12]264 [12]899 [11] [12]
Considered 18650s Cell Comparison

The cells charge rate is assumed to be at its maximum and the Haussmann and Depcik model coefficients (Γ, α°,β°) were found through literature. Subsequently, the overall price of the battery pack will depend on its size and the needed kWh found from the simulation results. Here, some type of thermal management system will be used to ensure the battery cells are within their respective operating temperature range to allow for longer cycle life and higher efficiency.

There are three prominent methods of thermal management. Air can be used either passively or forced to regulate battery pack temperature. In this case, the pack can be cooled by the general wind moving around the vehicle and heated using present exothermic reactions. When it comes to large scale applications, typically air cooling is not feasible and will not be used for the Proterra EB. Another method utilizes dielectric oil to regulate the battery pack temperature. The battery pack is flooded with the oil which is pumped out to a heat exchanger system. This method is more efficient than air cooling and is used in multiple types of high-power applications. The third most common method for thermal management, utilizes a water-based coolant that passes through the battery pack [15].

For this design, the Tesla patented cooling method is considered [16]. However, the liquid being used will be Paratherm LR fluid instead of glycol. Paratherm is less dense, has double the thermal conductivity, and can operate at extreme temperatures [17, 18]. Moreover, the thermal management system will be regulated by a battery management system (BMS).

A BMS will be used to protect the batteries, calculate state of charge (SOC), and monitor general health/safety of the battery pack. Over-charging or over-discharging can decrease the lifetime and stability of the battery pack. The BMS regulates this by constantly monitoring each battery in the pack and determining how much current can safely go into and out of it. These stand as limits and are sent to other electronic components in the vehicle. Subsequently, the BMS calculates SOC by monitoring how much energy goes into and out of the pack. Finally, the BMS monitors health/safety of the battery by constantly checking for weak connections or shorts [19]. The BMS will be in contact with all safety devices in the battery pack, be able to alert the driver, cut-off connections to the battery pack if necessary, and ensure all cells in the battery pack are at one voltage. To meet the above constraints, an Orion BMS, priced around $1200 [20], will be used for the design.

In this design, the battery pack will experience large amounts of current during the charging phase. Therefore, a thorough comparison is shown in the following figure between compatible charging infrastructures with different cell chemistries [21]:

CaseCharging MethodMax Power [kW]Cost [$]Maintenance Cost [$/year]
DC – LFP  Plug-in7539,000500
 DC – NMCPlug-in7539,000500
Inductive OC – LTOInduction200266,00013,000
Conductive OC – LTOPantograph375313,00013,000
Charging Infrastructure and Battery Chemistry Combination Scenarios

Aside from the power drawn from the motor and the maximum power from the different charging system, the overall power drawn from auxiliary system will also be used to size the battery pack.

Auxiliary Components:

In a normal powered vehicle, a 12 VDC battery is charged through a DC-to-DC converter with the approximate power draw shown below [22]:

Auxiliary systemsPart of traction battery energy
HVAC30-35%
Power steering5%
Braking5%
Miscellaneous (lights, media, locks etc.)5%
Approximate Power Draw of Auxiliary Systems in an EV

The values shown above are for an electric car but the values can be slightly adjusted to fit the needs of an electric bus. A greater power consumption will be assumed for the HVAC system due to the low temperatures in Tromsø, and the size of the vehicle. Two HVAC systems were considered for the design. System 1 consists of a R134a rood top air-conditioner with an electric compressor and an electric resistance heater. While system 2 consists of a CO2 roof top air conditioner with an electric compressor and an additional heat pump mode [22]. To choose the appropriate system, an approximate acquisition, energy, maintenance, financing, and total LCC costs over a 12-year operating period with an inflation rate of 1.9% is conducted. The results are shown in the table [23]:

CostsSystem 1System 2
Acquisition$18,700$31,000
Energy$24,900$12,400
Maintenance$6,200$7,400
Financing$6,200$11,100
LCC$56,000$61,900
12 Year Analysis Comparison

In conclusion, the Proterra EB will utilize system 1 for its HVAC system and an approximate cost of $4666.67/year will be used for the final cost analysis.

Results/Comparison:

Two types of simulations were ran to determine the feasibility of the proposed design. The first type of simulation is utilized to determine the proper battery chemistry and charging strategy. The second, used GREET to determine the total amount of CO2 emissions that will be experienced with the design. Results from the two processes will be used to determine the feasibility of the proposed electric bus when compared to a diesel counterpart.

Based on specification of the Proterra Catalyst, an analysis of different battery chemistry/charging infrastructure over a 20-year period including maintenance costs and battery pack replacements was conducted and is summarized in the tables below:

CaseDC LFP (Plug-in)DC NMC (Plug-in)OC Inductive LTO (Induction)OC Conductive LTO (Pantograph)
Pack Size [kWh]5045044075
Pack Voltage [VDC]853850849849
MPDe7.40277.63337.89467.8383
kWh/mi5.08404.93044.76724.8014
Mass [kg]4,6702,650515960
Charge Time [hours]6.726.720.200.20
Charges/Day11105
Cost [$]181,440133,05635,96067,425
Round Trips/day12121011
20-year Battery Chemistry/Charging Infrastructure Analysis
ParametersTimes replacedTotal maintenance costs [$]Economy [$/mi]
DC LFP (Plug-in)5 10,0003.21
DC NMC (Plug-in)310,0001.49
OC Inductive LTO (Induction)0260,0001.17
OC Conductive LTO (Pantograph)0260,0001.33
20-Year Evaluation of Battery Pack and Charging Strategy

NMC and LFP battery packs were increased by 20% to account for degradation and their maximum DOD being 80%. While LTO packs were increased by 10% with a maximum DOD of 90%. Here, the Proterra EB is expected to travel 2,000 miles every month, resulting in 480,000 miles over 20 years. Based on the max cycle life, the LFP based battery pack needs to be replaced every 33 months; the NMC is replaced every 50 months; the LTO based chemistries are not expected to be replaced along this period. Factoring in maintenance costs, OC Inductive LTO charging was found to be the cheapest strategy with an approximate price of $1.17/mile and will be used in the design.

Therefore, the bus will be designed with a 40 kWh LTO battery pack using a conductive OC strategy. To obtain the needed pack voltage, 355 cells will be used in series to total 852 VDC. Furthermore, the ORION BMS is capable of supporting 180 cells in series, thus, two units will be used for this design. Finally, the following table compared the proposed design to its diesel counterpart:

ParametersDieselProposed EB
Acquisition$      550,000.00$      810,000.00
Propulsion$        94,080.00$        20,160.00
Brake$        26,880.00$          6,720.00
HVAC$          6,720.00$        17,270.00
Carbon Tax$      122,453.88$        45,637.79
Fuel$   1,058,080.00$      432,480.38
Charging StationNA$      266,000.00
Total$  1,858,213.88$  1,598,268.18
Comparison Between Diesel and Electric Bus

As predicted, electric buses produced more than half as less carbon than its diesel counterpart. Here, GHG emission values were approximated from results obtained from the software, GREET simulations. Furthermore, values from GREET were based on U.S energy sources. Since Norway has less dependence on GHG emitting sources, the cost due to carbon taxation is expected to be lower due to a lower net emission of GHG. Subsequently, low efficiency and high fuel costs lead to over a 2 to 1 difference in fuel costs between the two buses. Finally, the cost of a charging station was factored into the analysis due to the low amounts of charging infrastructure.

In conclusion, the Proterra EB proved to be a proper substitute for a diesel transit bus. Furthermore, the need for EB in the transportation industry further increases due to increasing GHG emissions. Nevertheless, the viability of EB depends on the location. Here, Tromsø, Norway proved to be a suitable location due to strong political support as well as its dependence of GHG free energy sources. To validate this claim, a simulation was done based on a macroscopic scale on the conservation of momentum along a bus route in the municipality, route 26. As a result, the Proterra EB will contain three EMRAX 348 MV motors, each with its own RMS PM150DZ controller, a 40 kWh LTO battery pack, a CO2 based HVAC system, and carbon fiber chassis and battery pack enclosure. Moreover, it was shown the Proterra EB was almost twice as efficient, in terms of MPDe, and almost $400,000 cheaper over a 28-year period than a diesel transit bus. Finally, it was assumed that if the Proterra EB can accomplish the worst case then it will be able to go through any other scenario. Therefore, the Proterra EB can be assumed to be even more efficient and have an even lower cost than the diesel transit bus.

A further detailed report of the project, design consideration, and analysis can be received upon request.

Acknowledgements:

I would like to thank Dr. Chris Depcik for his help with the project.

Sources:

  1. IEA, “https://www.iea.org/news/nordic-region-offers-valuable-lessons-for-rapid-ev-deployment-worldwide,Accessed December 2019
  2. Weather Spark, “https://weatherspark.com/y/84211/Average-Weather-in-Tromsø-Norway-Year-Round,” Accessed November 2019
  3. World Weather, “https://www.worldweatheronline.com/lang/en-us/tromso-weather-averages/troms/no.aspx,” Accessed November 2019
  4. VTT Technical Research Centre of Finland LTD, “Planning of electric bus systems,” September 4, 2017
  5. Moovit, “https://moovitapp.com/index/en/public_transit-line-26-Norway-1679-1102880-687570-0,” Accessed November 2019
  6. NREL Transforming Energy. “NREL Fuel Cell Bus Analysis Finds Fuel Economy to be 1.4 Times Higher than Diesel,” Accessed December 2019, https://www.nrel.gov/news/program/2016/nrel-fuel-cell-bus-analysis-finds-fuel-economy-to-be-14-times-higher-than-diesel.html
  7. Simpson, Tyler, “Design and Simulation of a Battery Electric Bus for the Nordic Urban Transportation Market,” 2019
  8. Yasa, “https://www.yasa.com/yasa-750/,” Accessed November 2019
  9. Dana, “https://www.danatm4.com/products/direct-drive-electric-powertrain/sumo-hp/,” Accessed November 2019
  10. Emrax, “https://emrax.com/products/emrax-348/,” Accessed November 2019
  11. Depcik, Christopher, et al., “Electrifying Long-Haul Freight-Part II: Assessment of the Battery Capacity,” SAE International, January 7, 2019
  12. Becker, Jan, et al., “Dimensioning and Optimizing of Hybrid Li-ion Battery Systems for Evs,” June 28, 2018
  13. O’Malley, Ryan; Liu, Lin; Depcik, Christopher, “Comparative study of various cathodes for lithium ion batteries using an enhanced Peukert capacity model,” Journal of Power Sources, 396(p621-631), August 2018
  14. RTD Vapor, “VAPCELL INR 26650 5000MAH 20A”, Accessed November 2019, “https://www.rtdvapor.com/vapcell-inr-26650-5000mah-20a/
  15. Avid Technology, “What is the Best Electric Vehicle Battery Cooling System,” Accessed November 2019, https://avidtp.com/what-is-the-best-cooling-system-for-electric-vehicle-battery-packs/
  16. Insideevs, “https://insideevs.com/news/328909/tesla-or-gm-who-has-the-best-battery-thermal-management/,” Accessed November 2019
  17. Prinsloo, Arno, “Class 8 Electric Semi-truck,” 2018
  18. Paratherm Heat Transfer Fluids, “https://www.paratherm.com/heat-transfer-fluids/paratherm-lr-low-range-heat-transfer-fluid/,” Accessed November 2019
  19. Hu, Rui, “Battery Management System for Electric Vehicle Applications,” Thesis
  20. Orion BMS, “https://www.orionbms.com/general/how-it-works/,” Accessed November 2019
  21. G hlich, Dietmar, et al. “Design of Urban Electric Bus Systems,” 2018
  22. Idaho National Laboratory “EV Auxiliary Systems Impacts,” Accessed October 2019
  23. Ghlich, Dietmar, et al., “Economic assessment of different air-conditioning and heating systems for electric city buses based on comprehensive energetic simulations,” Department Methods for Product Development and Mechatronics Technische Universitat Berlin, 2015  

Published by Nikhil Biju

I am a battery engineer looking to use my modeling and professional skills to make an impact in the electric vehicle industry.

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