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Fast charging beats battery swapping for underground electric haul trucks

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We co-authored a paper that summarizes the results of battery swapping versus fast charging for the hardest-working vehicles underground. It turns out the fast charging provides a 48% savings over battery swapping when productivity rates are equal.

Investigation of Fast Charging and Battery Swapping Options for Electric Haul Trucks in Underground Mines

Md Ahsanul Hoque Rafi1, Student Member, IEEE, Robert Rennie2, John Larsen2, and Jennifer Bauman1, Member, IEEE
1Electrical and Computer Engineering, McMaster University, Hamilton, ON, Canada
2 MEDATECH Engineering Services, Collingwood, ON, Canada

Abstract— Diesel-based vehicles are prevalent in underground mines all over the world. These mines require extensive ventilation in part due to toxic gas and heat emissions from diesel engines. Thus, mining is a prime candidate for electrification. This research performs an investigation of battery swapping and fast charging for purely battery-powered underground haul trucks, focusing on productivity and cost. The results show that battery swapping with the largest feasible battery size (348 kWh pack) gives 2.8% more productivity than the best fast charging option (600 kW charge rate with 228 kWh battery pack), but costs 65% more over the five-year time span considered. With a 228 kWh pack, battery swapping gives equal productivity as fast charging, but costs 48% more.

I.  Introduction

A revolution in the mining industry is beginning to unfold. For the last sixty years, diesel-powered mining vehicles have dominated the industry. Prior to this, electric mine locomotives powered by extensive underground electrical cabling systems were used [1]. The transition to diesel mining vehicles allowed haul trucks to move more easily around the mine regardless of the location of the cabling systems. However, underground diesel mining vehicles produce dangerous emissions, including particulate matter, nitrogen oxides, and sulfur dioxide. A large retrospective cohort study of over 12,000 workers from 8 mines in the United States found an increased risk of lung cancer death with increasing levels of exposure to diesel exhaust [2, 3]. Extensive ventilation systems are used to move heat and exhaust emissions to the surface, but health risks remain as evidenced by [2] and [3]. Once removed from the depths of the mine, these emissions contribute to environmental damage at the surface [4, 5]. Furthermore, the massive ventilation infrastructure needed to remove emissions from kilometers below the ground is costly and requires enormous amounts of electricity to run the machinery. As today’s mines must continue to dig deeper in the face of depleting resources, the associated ventilation costs continue to grow.

The recent advancements in lithium-ion battery technology have led to batteries that have high enough energy and power densities to make battery-powered electric mining vehicles not only feasible but also economically advantageous. For underground mines, the primary economic benefit stems from a reduction in ventilation costs. As an example of the potential savings, Goldcorp recently piloted a connected tracking system in its Éléonore mine in northern Canada, allowing ventilation systems only to be turned on in the areas of the mine where employees were present. The ventilation requirements were reduced from 1.2 million cubic feet per minute of air to about 0.65 million cubic feet per minute, resulting in an annual savings of $1.5 to $2.5 million [6]. Goldcorp has recently announced plans to operate its Borden Lake mine with only electric mining vehicles, meaning it will be the world’s first electric mine. Goldcorp predicts a reduction of 5,000 tons of CO2 per year and 50% less ventilation needs compared to a baseline diesel underground mine [7]. The economic advantages paired with improved health, safety, and environmental benefits make mining a prime sector for electrification.

Haul trucks, which transport the ore from the rock face to the dump point, use the most energy out of the underground mining fleet and are thus a prime candidate for electrification, and the focus of this research. Another benefit of electrifying haul trucks is that they can capture regenerative braking energy when travelling downhill, unlike a diesel haul truck. However, a major challenge is determining the optimal battery recharging scheme, with the main two options being fast charging and battery swapping. It is important to minimize truck downtime for recharging since mine operators strive to maximize daily ore production, which is directly related to the operation of the haul trucks. Thus, both daily production rates and costs should be considered when evaluating these two charging methods.

Fast charging requires costly electrical infrastructure and generates losses and heat, but can reduce charging downtime with higher charging rates. Battery swapping usually requires costly swapping infrastructure to be installed in the mine and can triple the number of battery packs which must be purchased, but reduces the charging losses since the batteries can be slowly charged, and can achieve high effective charge rates of 2.6 MW [8]. To reduce infrastructure costs, Artisan Vehicles has developed a haul truck with a built-in mechanism to lift, drop, and swap the batteries in about 15 minutes, eliminating the need for a gantry crane in the charging bay, though a crane-based system is currently the more common approach. Battery swapping has been investigated for light-duty passenger vehicles: Tesla offered battery swapping in California in 2013, but the project failed due to lack of participation from EV owners [9]. Better Place operated battery swapping stations in Israel and Denmark, but the company went bankrupt in 2013, mainly because vehicle manufacturers did not cooperate [10]. Furthermore, a Swedish start-up company, Powerswap AB, has recently revived the concept by developing robotic arms that swap batteries of parked cars autonomously [11]. For mining vehicles, there is no problem of participation from drivers, but compatibility issues between different vehicle manufacturers is still a major concern.

It is clear that the choice of fast charging or battery swapping will depend on many parameters. While previous research has investigated electrified mining vehicles, none has addressed this critical design decision. For example, [12] evaluates the potential benefit of hybrid haul trucks powered by both diesel and electrical trolley lines in terms of up-grade speed, travel time, and fuel consumption. Reference [13] discusses hybrid powertrain configurations with various energy storage systems for storing regenerative energy but does not discuss charging options for the storage systems. Furthermore, [14] discusses the design considerations of battery storage systems for heavy duty vehicles, where the battery is charged from an overhead catenary and battery power is only used when the catenary is not available. This research performs a comprehensive analysis of fast charging and battery swapping on purely electric, battery-powered underground haul trucks, by considering three different fast charging rates (200 kW, 400 kW, and 600 kW) and multiple battery sizes for each charging rate. Section II describes the modeling framework, Section III presents the simulation results on truck productivity with different charging scenarios, and Section IV presents an estimated economic analysis of the two charging methods. Finally, Section V concludes the paper.

II.  Modelling Framework

A. Haul Truck Model

A forward-looking vehicle model of a 40-tonne battery powered haul truck model is created in MATLAB/Simulink, based on the MJ-A42 underground haulage truck from GHH Fahrzeuge. Fig. 1 shows a block diagram of the three top-level blocks in the model: driver, controller, and vehicle plant.

The driver is modeled as a PI feedback loop so that the driver torque request ensures that the simulated vehicle speed closely follows the speed reference of the provided drive cycle. The controller block generates the motor torque demand and friction brake demand subject to the driver torque request and the motor speed-torque limits. In the plant model, the vehicle speed in m/s at the next simulation step, vchas(t+1), is calculated from the force out of the wheel block and the chassis aerodynamic losses, as shown in (1), where m is vehicle mass in kg, ρair is air density, A is vehicle frontal area in m3, and Cd is the estimated coefficient of drag.

  (1)

The force out of the wheel block is calculated from the torque into the wheel block (τin_wheel, positive for propulsion), the friction braking torque (τfriction_brake, negative for braking), and the rolling resistance losses, as shown in (2), where rwheel is the

Fig. 1. Block diagram of electric haul truck model

wheel radius in meters, μ1 and μ2 are rolling resistance coefficients, and g is gravitational acceleration.

  (2)

The torque into the wheel block is equal to the motor output torque (τmotor) multiplied by the final drive ratio (rfd) and the final drive efficiency (ηfd), as shown in (3). The motor speed in rad/s is calculated using (4).

  (3)

  (4)

The truck model has a dual traction motor system with one motor each at the front and rear axle, where each motor model is based on DANA TM4’s Sumo HD 3500 [15]. The motor and inverter are modeled as a lumped 2-dimensional efficiency table, with inputs of motor speed and motor torque as shown in Fig. 2. The main model parameters are shown in Table I. Where precise values were unavailable, estimates have been used. For the 40-tonne truck, the maximum fill rate is estimated at 90%, meaning the loaded truck mass is 71 tonnes.

Fig. 2. Motor and inverter efficiency curve

TABLE I: Haul Truck Model Parameters

Parameter

Symbol

Value

Truck Empty Mass

m

35,000 kg

Truck Loaded Mass

m

71,000 kg

Drag Coefficient

Cd

0.32

Frontal Area

A

4.86 m2

Rolling Resistance

µ1

0.006

Rolling Resistance

µ2

0.0001

Wheel Radius

rwheel

0.834 m

Final Drive Ratio

rfd

43.50

Final Drive Efficiency

ηfd

98.7 %

The motor/inverter block calculates the DC input current required from the battery using (5) during traction, where Vbatt is the battery terminal voltage and Paccessory is the electrical accessory use, which increases from a low baseline value when raising the dump box for a dumping event. Note that current out of the battery is assumed positive. During charging, the current into the battery is calculated using (6), where Pcharging is the power from the charging outlet. The battery model uses the battery current and an initial state-of-charge (SOC) value to determine the SOC and battery terminal voltage at the next simulation step. The SOC is the integral of the battery current divided by the total battery capacity (Cbatt) as shown in (7). The battery open circuit voltage (Vbatt_oc) is determined from a look-up table using the current SOC value. The terminal voltage (Vbatt) is calculated from Vbatt_oc and the internal battery resistance Rbatt, as shown in (8). The battery subsystem is modeled based on the AKASOL AKASYSTEM 15 AKM 46 NONO NMC lithium-ion battery module [16] with nominal voltage 666 V.

  (5)

  (6)

  (7)

  (8)

Multiple battery sizes are investigated because the impacts of fast charging and battery swapping will depend on the battery size. The size is varied by considering parallel modules, where non-integer values of parallel modules are permitted so that the current offering of a manufacturer do not impact the overall comparison. Two different battery sizes are investigated for each fast charging power level, and all selected sizes are investigated for battery swapping. The upper limit on battery size is set such that the battery pack mass is about 10% of the vehicle payload (40 tons), which equates to a 348 kWh pack. The lower battery size is set such that the 2.5 C continuous rate limit is not exceeded for each charging level. This means that a 248 kWh pack, 160 kWh pack, and 110 kWh pack are required for 600 kW, 400 kW, and 200 kW charging rates, respectively. For the 600 kW and 400 kW charging rates, it is the charging power that reaches 2.5 C, and for the 200 kW pack, it is the discharging power while driving that reaches 2.5 C, and thus sets the 110 kWh size. The starting battery SOC is selected for each pack such that the maximum SOC reached is 95%, even after a downhill segment of regenerative energy capture.

B. Drive Cycle

A typical drive cycle is created for the analysis, consisting of 4 unique segments from the dump point to the rock face (i.e., loading area), and then a repeat of these four segments in the opposite direction while loaded, back to the dump point. Fast charging occurs at the dump point. Since a fast charging station can be moved as the mine operations progress to lower levels, it is assumed that the station is always on the operating level. Battery swapping infrastructure is more difficult to move (with gantry crane removal and re-installation) and thus it is assumed at times to be close to the dump point, and at other times, it may be up to 1 km from the dump point, as operations move to lower levels of the mine. Thus, this study assumes an average extra distance to travel to the swapping area of 500 m on a 5.8 km cycle loop. Fast charging is set to occur after each complete round-trip (to improve battery lifetime by limiting battery depth-of-discharge), and battery swapping is set to occur when the truck cannot complete another full cycle, and assuming the SOC never dips below 20%. Each segment has a constant grade and top speed, as well as stop-and-go times and acceleration rates, as shown in Table II.  Fig. 3 shows the side view of the physical route, including the first 4 segments that are completed unloaded and the return 4 segments that assume a 36 tonne load.

The speed versus time drive cycle was estimated based on logged data of a small underground utility truck as shown in Fig. 4, because logged data was not available for an electric haul truck. The haul truck cycle was created to have lower top speeds, as shown in Fig. 5, in consultation with mining experts. The mine is assumed to operate in two shifts every day, of 12 hours each. In each shift, vehicles are assumed to operate for 75% of the time, and during the other 25% of the time, the mine is closed for explosions, lunch break, and shift change.

TABLE II: Route To The Loading Area (Dump Point)

Segments

1 (8)

2 (7)

3 (6)

4 (5)

Grade (%)

0 (0)

-15 (15)

0 (0)

-5 (5)

Driven Distance (m)

400

500

1000

1000

Avg. Top Speed (km/h)

17 (15)

12 (8)

17 (15)

15 (12)

Avg. Time Stopped (s)

10

10

10

10

Avg. Time Driving (s)

200

200

200

200

Acceleration Rate (m/s2)

1.3

1.3

1.3

1.3

Deceleration Rate (m/s2)

1.1

1.1

1.1

1.1

Fig. 3. Side view of the physical route with four segments
Fig. 4. Logged speed profile of an underground utility truck
Fig. 5. Created speed profile of an underground haul truck (8 segments)

III.  Simulation Results

Figs. 6 and 7 show the simulated front motor torque and battery current during traction with a 348 kWh battery pack. These figures show regenerative braking occurring on the downhill segments evidenced by the negative motor torque and negative battery current. Fig. 8 shows simulated battery SOC for the fast charging scenarios while the truck drives from the dump point to the rock face and back (8 segments), and then charges back to the initial SOC. Table III summarizes the simulation results and ore calculations. The total ore transported during a shift is calculated based on the traction time, refuelling time, and loading/dumping/maneuver time. Table III shows that for a given charging rate, the use of a smaller battery gives a slightly faster charging time because less energy must be replenished during charging since the truck is lighter, and thus uses less energy while driving, compared to its heavier-battery counterpart. However, the most critical factor in maximizing ore transported per shift is using a high charging rate, and the battery size has a minor effect. Also, if the 600 kW charge rate is used, there is no benefit to productivity (tonnes of ore transported per shift) of using the larger 348 kWh battery compared to the 228 kWh battery.

Fig. 9 shows simulated battery SOC for the battery swapping scenarios and Table IV summarizes the simulation results and ore calculations. In terms of productivity, battery swapping is only beneficial if large battery packs are used to reduce the number of swaps required per shift.

Fig. 6. Simulated front motor torque for 348 kWh battery truck

Fig. 7. Simulated battery current for 348 kWh battery truck

Fig. 8. Simulated battery SOC results for fast charging scenarios
Fig. 9. Simulated battery SOC results for battery swapping scenarios

TABLE III: Summary of Fast Charging Scenario Results

Charging Rate (kW)

Battery Size (kWh)

Energy Used Per Shift (kWh)

Energy Used Per Day (kWh)

Traction Time (min)

Charging Time (min)

Loading, Dumping, & Maneuvering Time (min)

Total Cycle Time

(min)

Completed Cycles Per Shift

Transported Ore Per Shift (tonnes)

600

348

403.52

807.04

27.50

2.79

9.00

39.29

13.74

494.78

600

228

400.92

801.84

27.50

2.77

9.00

39.27

13.75

495.03

400

348

388.32

776.64

27.50

4.17

9.00

40.67

13.27

477.99

400

160

382.67

765.33

27.50

4.10

9.00

40.60

13.30

478.82

200

348

349.95

699.90

27.50

8.27

9.00

44.77

12.06

434.22

200

110

346.29

692.59

27.50

8.17

9.00

44.67

12.08

434.88

TABLE IV: Summary of Battery Swapping Scenario Results

Battery Size (kWh)

Cycles Completed Between Swaps

Energy Used Per Shift (kWh)

Energy Used Per Day (kWh)

Traction Time (min)

Swapping Time (min)

Loading, Dumping, & Maneuvering Time (min)

Total Cycle Time

(min)

Completed Cycles Per Shift

Transported Ore Per Shift (tonnes)

348

10

410.80

821.61

276.90

15

90.00

381.90

14.14

509.03

228

6

395.94

791.87

166.92

15

54.00

235.92

13.73

494.40

160

4

382.94

765.87

111.98

15

36.00

162.98

13.25

477.11

110

3

363.52

727.03

84.49

15

27.00

126.49

12.81

461.06

Only battery swapping with 228 kWh to 348 kWh pack sizes gives productivity similar to fast charging with 600 kW. Battery swapping with the 228 kWh battery gives virtually the same productivity as 600 kW fast charging with the 228 kWh battery. Battery swapping with the 348 kWh battery gives 2.8% higher productivity than 600 kW fast charging with the 228 kWh battery, yet also uses 2.5% more energy due to the larger mass of the truck (excluding the extra energy used to operate the swapping crane). In mining operations, both productivity and cost are critical factors, thus the next section examines the cost implications of the two charging methods.

IV. Economic Analysis

Though an economic analysis of installing fast charging stations or battery swapping stations is difficult to do precisely due to the many variables and partially unknown costs involved, this paper performs an estimated economic analysis for one case to highlight the issues which must be considered in any formal analysis. The mine case examined uses five dump trucks and three loaders for a typical 2,500-3,000 tonne/day mine – thus, eight trucks will need fast charging or battery swapping. Other support trucks (bolters, graders, utility trucks, etc.) are assumed to use slow charging connections (~50 kW) available throughout the mine and are excluded from the economic analysis. Furthermore, the analysis only considers costs that differ between the two charging strategies, i.e., battery costs rather than the entire truck cost. The focus is on charging infrastructure costs, battery costs, and operating costs.

For fast charging, only the 600 kW charging rate with 228 kWh battery back is considered in the economic analysis, since it resulted in the highest productivity rate. One 600 kW off-board charger is assumed, as it can service all eight trucks assuming the trucks are evening spaced throughout the drive cycle. Industrial grade lithium-ion NMC cells with a liquid-cooled pack are considered for the fast charging case [16]. Based on quotes from several suppliers, the cost of this pack is estimated using $850/kWh to be $193,800 USD.

For battery swapping, both the 228 kWh and 348 kWh packs are considered as the former gave productivity equal to 600 kW fast charging, and the later gave the maximum productivity of the cases considered. For battery swapping, three sets of batteries are required: one set is on the driving trucks, one set is charging (at 150 kW), and one set is charged and cooling/waiting. Since slower charging is used, a charger is required for each truck, thus eight 150 kW chargers are required. The simulation results use the lithium-ion NMC battery model for both charging types, so as to isolate the differences in productivity between the charge types. However, it is likely that a lower cost air-cooled lithium iron phosphate battery would be used for battery swapping systems. Based on discussions with multiple providers of lithium iron phosphate packs, the cost is estimated at $380/kWh. For fast charging and battery swapping, the three loaders are assumed to use a 228 kWh battery, of NMC and iron phosphate types, respectively.

Table V summarizes the infrastructure and battery costs for the scenarios considered. The rough dimensions of each station type have been designed, and excavation costs are estimated at $320/m3 based on calculations of labour, fuel, and equipment. Cable bolts are used to support the opening, and shotcrete is sprayed on the walls to support the space and prevent loose rock from falling. Since the fast charging station is easier to move, it is assumed that it moves to the production level at the time, where three levels are assumed for the 5 year time frame considered. Since it is a more temporary station, shotcrete is not required for the main station area. The battery swapping station is considered more permanent due to the crane, and thus requires shotcrete reinforcement to allow it to last for the 5 years considered. However, the excavation and cable bolt cost for the fast charging station is added for every new level.

    The 150 kW chargers are approximated at $120,000 each based on discussions with suppliers, which includes underground installation, which is significantly higher than above-ground installation costs. The 600 kW fast charger cost of $1 million is also based on discussions with suppliers, and includes the high-power connector and underground installation. All costs shown are in USD.

TABLE V: Charging Infrastructure and Battery Costs

600 kW Fast Charging

Battery Swapping

Description

L×W×H (m)

Cost for 1 Level ($)

Cost for 3 Levels ($)

L×W×H (m)

Cost for 228 kWh Case ($)

Cost for 348 kWh Case ($)

Excavation for battery charging/storage area

24×2×5

76,800

230,400

22×2.5×6

105,600

105,600

Cable bolts, 1.8m×1.8m pattern

24×7

15,600

46,800

22×7

14,300

14,300

Shotcrete, 2”-4” nominal depth

not required

0

0

22×7×6

39,700

39,700

Load center station (with shotcrete for swapping)

8×4.5×4.5

153,100

153,100

8×4.5×4.5

166,600

166,600

Switch gear station (with shotcrete for swapping)

4×4.5×4.5

91,600

91,600

4×4.5×4.5

98,300

98,300

Battery swapping crane, 5 tonne, installed

0

0

180,000

180,000

150 kW battery chargers, 8@$120,000 each, installed

0

0

960,000

960,000

600 kW battery charger, 1@$1 million, installed

1,000,000

1,000,000

0

0

Batteries

1,550,400

1,550,400

2,079,360

2,763,350

Engineering (10% of total excluding battery costs)

133,700

152,190

156,500

156,500

Total Infrastructure and Battery Costs

3,021,200

3,224,490

3,800,360

4,327,850

TABLE VI: Operational Costs

600 kW Fast Charging

Battery Swapping

228 kWh

228 kWh

348 kWh

Haul truck charging energy used per day (kWh)

801.84

791.87

821.61

Electricity rate ($/kWh)

0.10

0.10

0.10

Yearly haul truck charging electricity cost for 360 days ($)

28,866

28,507

29,578

Yearly labor cost for battery swap operator ($)

0

298,000

298,000

Total operational costs for

5 haul trucks and 3 loaders for five years ($)

1,010,463

2,487,899

2,514,665

    Table VI shows the estimated operational costs for a 5 year time span. Only operational costs directly related to battery charging or swapping are considered in this analysis: electricity for truck charging and battery swapping operator labor costs. Since the loader trucks were not modeled in this study, their electricity usage is approximated as 2/3 of that of a haul truck with a 228 kWh battery pack. For fast charging, the truck operators can easily perform the fast charge event themselves and thus no additional labor is required. For the battery swapping station, a station operator is required to move around the uncharged, charged, and cooling batteries to ensure a battery is ready whenever a truck pulls in for a swap. The management of these 24 battery packs requires an operator for each 12 hour shift, amounting to roughly 298,000 per year.

Fig. 10. Fast charging and battery swapping costs and productivity

    The results are summarized in Fig. 10. For the scenarios analyzed, battery swapping with 228 kWh packs is 48% more costly than fast charging over the 5 year period considered, but gives virtually the same productivity as fast charging with 228 kWh packs. The battery swapping option with the largest feasible battery size, 348kWh, gives 2.8% higher productivity than the best fast charging option (228 kWh), but at a cost of $2.764 million more than fast charging, a 65% increase. Much of the higher costs related to battery swapping result from having to purchase multiple sets of batteries (even for lower cost cell types) and labor for battery swapping management.

V. Conclusions and Future Work

    This research developed an electric mine truck modeling framework and analyzed the productivity and costs of multiple battery sizes for fast charging and battery swapping. It was found that only the highest fast charging rate (600 kW) gave productivity results comparable to the battery swapping scenarios with large batteries. Overall, the battery swapping option with the largest feasible battery gave 2.8% higher productivity than the best fast charging option, but cost 65% more than the fast charging option over the 5 year period considered. Thus, 600 kW fast charging of underground electric haul trucks has been found to be a cost-effective method that also gives high productivity. Future work will consider other drive cycle patterns and battery lifetime implications of the charging strategies.

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