Modeling Stationary Power for Heavy-Duty Trucks: Engine Idling vs. Fuel Cell APUs

Line-haul truck engines are frequently idled to power hotel loads (i.e. heating, air conditioning, and lighting) during rest periods. Comfortable cabin climate conditions are required in order for mandatory driver rests periods to effectively enhance safety; however, the main diesel engine is an inefficient source of power for this conditioning. During idle, the diesel engine operates at less than 10% efficiency, consuming excess diesel fuel, generating emissions, and accelerating engine wear. One promising alternative is the use of small auxiliary power units (APUs), particularly fuel cell-based APUs. The Institute of Transportation Studies (ITS-Davis) developed an ADVanced VehIcle SimulatOR (ADVISOR)-based model to quantify the costs and benefits of truck fuel cell APUs. Differences in accessories, power electronics, and control strategy between the conventional engine idling and APUequipped systems are analyzed and incorporated into the model. A case study of a diesel-reformer solid oxide fuel cell (SOFC) APU system retrofit on a Class 8 linehaul truck is presented here. We sized a SOFC for heavy duty truck APUs to have a power output of 4 kW (net) electric. The modeled SOFC system was found to have an efficiency of about 30-35% efficient and consume about 0.14-0.17 gallons of diesel fuel per hour. The fuel savings resulting from replacement of avoidable idling are estimated at 3-8% of total vehicle energy use. For trucks that idle 10 hours or more daily, overall fuel consumption would be reduced by about 5-12%.