Hybrid Train Power with Diesel Locomotive and Slug Car–Based Flywheels for NOx and Fuel Reduction

An energy-storage flywheel consists of a large inertia wheel sharing a common shaft with a motor generator (MG) set and with magnetic bearings to support the entire rotating assembly. Flywheels mounted on a special slug car are charged from the local utility grid and from regenerative-braking events. Usage of these power sources reduces fuel consumption and the related NOx emission by the locomotivemounted Diesel generator sets (DGS). The flywheel-supplied power can replace the DGS-supplied power in one or more of the eight fixed power settings (notches), plus idle and reverse, which are common to most locomotives either for line-haul or switchyard service. The slug cars have separate traction motors to be driven by the flywheel systems so that the flywheel power and DGS power are electrically and physically decoupled. A system model is presented that includes the train dynamics coupled with the electromechanical models for the flywheels and traction motors. The modified Davis equation is employed in the train model to account for windage and other losses. A novel, feedback-based flux-weakening control of the flywheel’s motor generator current-torque and speed-back electromotive force (emf) gain is employed to increase the charge capacity, depth of discharge, and regenerative-braking efficiency for the flywheels. The simulation results show significant costand emissions-reduction potential for the proposed hybrid DGS–flywheel locomotive power system in line-haul and switcher service. DOI: 10.1061/(ASCE)EY.1943-7897.0000081. © 2012 American Society of Civil Engineers. CE Database subject headings: Emissions; Environmental issues; Energy storage; Rail transportation; Fuels. Author keywords: Green technology; Emissions reduction; Energy storage; Railroad; Locomotive; Flywheel; Smart technology; Regenerative breaking; Energy harvesting; All electric; Hybrid power. Introduction and Background Energy storage is an important enabling technology for developing clean and renewable energy resources and for energy-harvesting processes. The objective of this manuscript is to present a novel approach for the usage of energy-storage flywheels for harvesting and reusing braking energy and for providing a portable power source that derives energy from the electrical grid. Grid-generated power is obtained with far less NOx emissions than are produced by the portable Diesel engine generator sets on locomotives. A flywheel-powered locomotive was developed by Maschinenfabrik Oerlikon of Switzerland (1958) for the National Coal Board (NCB) at Seaton Delaval to be a fireand emissionfree candidate for underground applications. The four-wheel, 150-kw locomotive was powered solely by two flywheels. In 1979, the U.S. Federal Railroad Administration (FRA) sponsored research on a switcher locomotive powered by flywheel energy storage (AiResearch Manufacturing Company of California 1979). The 4.5-kW · h flywheel model employed for the investigation had a calculated parasitic loss of 7.22 kWattributable to the use of roller bearings and a planetary gear train between the flywheel and MG. This flywheel design would dissipate 100% of its energy within 38 min. In comparison, modern flywheels are supported on magnetic bearings, have motor generators that are integral with the flywheel shaft, and have highly efficient power-conversion systems, yielding energy losses between 5% and 10% over a 24-h period, while operating in an idle mode. The University of Texas at Austin Center for Electromechanics (UT-CEM) developed an advanced locomotive propulsion system (ALPS) flywheel with a design energy storage of 130 kW · h and peak power of 2 MW for hybrid locomotive power as part of the next-generation high-speed rail program (Thelen et al. 2003; Caprio et al. 2004). The hybrid power system consisted of a gas turbine–driven generator, along with a flywheel to supply supplemental power from the turbine while climbing grades or for other peak power demands. This enabled optimization of the 3,730-kw gas turbine performance design for a steady operating point without the constraint of peak power demand. Research has also been conducted on incorporation of flywheels in passenger cars and buses for brake-energy harvesting and for supplemental peak power (Brockbank and Greenwood 2009; Hawkins et al. 2003; Huang and Wang 2007). The National Aeronautics and Space Administration (NASA) (Kenny et al. 2005) has built a series of advanced energy-storage flywheels for potential satellite applications. The usage of high-speed flywheels for integrated power and attitude control system (IPACS) on satellites (Park et al. 2008) has also been investigated. The use of a hybrid train power system is proposed, consisting of contributions from a bank of flywheels that provide propulsion power for a slug car, and Diesel generator sets, which provide propulsion power for the locomotives. A conventionally powered slug car has traction motors (TMs) that derive their power from the Diesel generator sets on the locomotives, through electrical cabling, and are used to increase traction force by dividing it between the locomotive and the slug car. In the proposed approach, the slug car Ph.D. Candidate, Vibration Control and Electromechanics Laboratory, Dept. of Mechanical Engineering, MS 3123, Texas A&M Univ., College Station, TX 77843-3123. Professor, Vibration Control and Electromechanics Laboratory, Dept. of Mechanical Engineering, MS 3123, Texas A&M Univ., College Station, TX 77843-3123 (corresponding author). E-mail: [email protected] Samsung Corp., Seoul, South Korea. Note. This manuscript was submitted on July 1, 2011; approved onApril 2, 2012; published online on April 11, 2012. Discussion period open until May 1, 2013; separate discussions must be submitted for individual papers. This paper is part of the Journal of Energy Engineering, Vol. 138, No. 4, December 1, 2012. © ASCE, ISSN 0733-9402/2012/4-215-236/$25.00. JOURNAL OF ENERGY ENGINEERING © ASCE / DECEMBER 2012 / 215 traction motors are powered by, and deliver regenerative-braking power to, a bank of flywheels located on the slug car. An alternative approach is to carry the flywheels in a freight car with a cable connection to the locomotive traction motors. The effectiveness of the proposed system is evaluated by simulation utilizing average power duty cycles for line-haul and switcher service, published by the EPA (Sierra Research Inc. 2004). In the report, the main generator in a locomotive is driven by a Diesel engine, which consumes on average 220 L of Diesel fuel per hour in line-haul service and 76 L=h in switchyard service. The generated electricity powers a set of TMs that exert torques on the locomotive wheels through geared transmissions located between the TMs and the locomotive’s wheels. The wheels then propel the train forward or in reverse. The TMs may also act as traction generators (TGs), in which case the TG shafts exert braking torques on the train wheels. This torque brakes the train, while the electric currents that are generated in the TGs are dissipated as waste heat in resistor banks. The emission measurements from locomotives are made at each notch setting, and the average emissions for the locomotive are computed from an assumed duty cycle (representing normal locomotive operation in the field). The average NOx emissions for tier-2 class Diesel engine locomotives are approximately 12.7 kg=h for line-haul and 3.5 kg=h for switchyard service. As a reference, 24-h=day, 365-days=year service yields NOx emissions of 122 t=year for line-haul and 33.8 t=year for switchyard service. The present paper investigates the feasibility of reducing train NOx emissions by utilizing a portable electrical energy source (PEES) such as an onboard bank of flywheels that are charged from the (low NOx) local utility grid at train stations, from the energy produced by braking the train (i.e., regenerative braking), or from an onboard Diesel engine that runs more efficiently. The PEES will drive TMs on their corresponding slug cars, which are separate from Diesel locomotive ones, and supplement or replace driving power produced by the Diesel engines. The U.S. annual NOx emission rate for grid electricity as of 2007 is 0.8 kg=MW · h, as released in the EPA’s eGRID2010 files (EPA 2010). The NOx emissions for average Diesel engine notches can vary between 8.93 and 16.58 kg=MW · h, according to EPA line-haul and switcher data (Sierra Research Inc. 2004), which is up to 20 times that of grid electricity. The grid electricity cost paid by the railroad is approximately US 11.17 1⁄4 kW · h, as released by the U.S. Energy Information Administration (2010). At a Diesel price of US 0.925 1⁄4 L (US 3.5 = gal:), the Diesel unit energy price varies between US 0.221 1⁄4 kW · h and US 0.916 1⁄4 kW · h depending on the different power-level efficiencies derived from EPA’s report (Sierra Research Inc. 2004). The Diesel fuel cost is approximately 2–8 times the cost of grid electricity, on an equivalent energy basis. In fairness to Diesel power, its upper cost limit corresponds to very low power level, which consumes a relatively small amount of fuel. These comparisons support the usage of PEES to reduce emissions and operating costs. Flywheel energy-storage systems (FESS) have no chemical battery disposal issues, are more robust for temperature extremes, are similar to batteries in energy density, and can operate with lower depths of discharge and longer life than batteries (Wilson and Fausz 2006). This is the motivation for the use of flywheels, over batteries, in the proposed approach. The three primary components of a flywheel are a rotating inertia wheel that stores kinetic energy, an MG that provides power conversion between electrical and mechanical (kinetic) forms, and a magnetic suspension (bearing) system that supports the weight of the spinning flywheel with minimal drag. The flywheel rotating assembly is typically supported by a five-axis magnetic bearing system that reacts to the flywheel loads. Patented technology developed by Palazzolo et al. [“System and method for controlling suspension using a magnetic field,” U.S. Patent No. 6,323,614B1 (2001); “Fault tolerant homopolar magnetic bearings,” U.S. Patent No. 7,429,811 (2008)] improves the fault tolerance of the magnetic bearings and the stability of the magnetically levitated, spinning flywheels. The rotating assembly is housed in a vacuum vessel to minimize windage losses. The flywheel MG receives electrical energy from the utility grid or from the TGs during regenerative braking to spin up the flywheel increasing its rotational kinetic energy. The flywheel MG converts the stored rotational kinetic energy of the flywheel into electrical energy to power the traction motors that propel the train. Fig. 1 depicts the proposed system structure, which contains the following: • Diesel locomotives, which have alternating current/direct current (ac/dc) traction motors. • Slug cars, which have their own dc traction motors physically separated from traction motors on Diesel locomotives. The slug cars do not have Diesel generators and are often filled with dummy weight load (concrete) to increase their traction. • Flywheel assemblies, which replace the dummy weight on slug cars. They are used to collect regenerative-braking energy and provide propulsion energy when needed.