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Walmart: Solar Electric Auxiliary Power Units (APUs) Case Study

Walmart: Solar Electric Auxiliary Power Units (APUs) Case Study Overview

This case study, provided by Walmart, highlights how solar electric Auxiliary Power Units (APUs) performed in the context of a two-year test carried out in the US with 10 diesel trucks.

Part 1: Baseline Fleet and Technology

This section describes the characteristics of the existing technology in the fleet (i.e. prior to the test). This provides information on the fleet characteristics, duty cycle details, and region of operation for the technology being tested.

Test Name Walmart, Solar Electric APU Case Study

Company Name Walmart Transportation, LLC

Vehicle Type Class 8 Semi Truck with Sleeper Cab and Auxillary Power Unit (APU)

Fuel Type Diesel

Feedstock: Diesel Average Diesel (unspecified source)

Refueling Onsite and over the road. Refueling is primarily done at our Distrubution Centers, but drivers can fuel over the road if needed.

Average Vehicle Miles Traveled (VMT) 120,000 miles

Hours of Operation 3,000 hours

Average Load 38,000 lbs

Maximum Load 44,000 lbs

Length of Haul Weighted Average Distance (WAD) is 102 miles

Return to Base? No

Country United States

City or Region Southern California

Lifespan Average life is 5 years.

Description The majority of tractors in the fleet are sleeper cabs and all sleepers are outfitted with auxillary power units (APUs). Walmart drivers typically take their 10-hour break on-road in the tractor, and the APU eliminates the need to idle the main engine while the truck is parked.


Part 2: Technology and Test Purpose

This section describes the type of technology tested and primary reasons behind the test.

Technology Tested Solar Electric Auxiliary Power Units (APUs): 300 watt solar battery charging system installed on truck’s auxillary batteries.

Test Purpose 1. Evaluate the operational reliability of the equipment e.g. did the units work as intended, how often did they need to be repaired, level of driver satisfaction. 2. Validate the batteries could consistently sustain a normal, uninterrupted hotel load during a driver’s 10-hour break. 3. Validate the batteries could be recharged to 100% state-of-charge (SOC) during a typical day. 4. Understand and quantify the benefits of running the solar application e.g. increased fuel savings, reduced idle time, lower maintenance cost etc.


Part 3: Test Parameters

This section describes the actual parameters used to test the new sustainable technology, including number of vehicles tested, testing timeline, additional training and infrastructure requirements, etc.

Sample Size 10 tractors

Test Start Date 01 December 2015

Test End Date 01 December 2017

Total Miles Tested 1.6 million miles

Total Hours Tested Approximately 125,000 hours total, with 41,000 hours spent on break in the sleeper berth.

Average Load Tested 38,000 lbs

Maximum Load Tested 44,000 lbs

Time to Fuel (in minutes) Unable to determine the amount of time it took to to fully recharge the batteries.

Testing Barriers 1. The data loggers were unable to differentiate the method of recharge (main engine or solar). As a result, we were unable to ascertain if the battery recharge was due to the truck idling or the technology we were testing. 2. Training was provided by the manufacturer to the maintenance team and the initial drivers. While there was some initial follow-up, driver turnover and slip-seating diluted the original training and best practices on how to best manage the solar electric APUs. 3. Only one size of solar panels available.


Part 4: Supporting Services

This section describes the additional supporting services needed for the sustainable technology tested, including details on fuel type, infrastructure requirements, and personnel training.

Fuel Type Electric

Feedstock: Electric 100% Renewable Electricty

Level of Readily Available Infrastructure High

Special Training Requirements 1. Maintenance: repairs, troubleshooting, preventative maintenance 2. Drivers: operation, best practices, troubleshooting


Part 5: Operational Performance

This section describes the key metrics used to measure operational performance of the alternative fuel or technology, benchmarked against the current technology used in the fleet.

Driver Satisfaction Low

Special Training Requirements Driver behavior: • Closing the main cabin curtain while parked to keep the cab cooler in the summer. • Keeping the fan on medium speed instead of turning it on high.

Special Maintenance Requirements • Training for mechanics • Basic troubleshooting training for drivers • Solar panel maintenance and repair

Additional Challenges for Fleet In order to fully recharge the batteries before the next break period, the tractor needed to have at least 5-6 hours of constant engine on time which wasn’t a good match for our duty cycle. Therefore the batteries were rarely fully charged when a driver started their shift.


Part 6: Financial Performance

This section describes company’s expectations for financial and economic performance of the technology, benchmarked against the incumbent fuel/technology. Where noted, minus (-) is savings and plus (+) is additional costs for the fleet.

Fuel Premiums/Savings Percentage Unable to quantify. Summer: Because the units could not sustain a 10-hour hotel load in the extreme heat, the main engine would engage to recharge the batteries eliminating any fuel savings. Winter: Fuel savings was approximately 1 gallon of diesel per day as long as the solar electric APU provided an uninterrupted 10-hour hotel load.

Maintenance Premiums/Savings Percentage Fleet Maintenance estimated the downtime for repairs and service to be about five times more than the diesel units.

Capital Premiums/Savings Percentage There was no savings. The solar electric units were about $3,000 more than a diesel APU.

Estimated Residual Value in US$ There was no residual value on the solar units.

Did You Use Subsidies? No


Part 7: Conclusions

Will You Include this Technology in Your Fleet? No

Additional Comments The solar electric APUs did not provide a 10 hour, uninterrupted hotel load.

Would You Recommend this Technology for Other Fleets or Applications? Yes

Additional Comments Yes or No - This technology might be an option for other fleets if the 10 hour hotel load is not a requirement



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