Energy management and sizing of fuel cell hybrid propulsion systems

E. Tazelaar

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Our dependency on road transportation of people and goods is huge. Unfortunately, this transportation is mainly fed by fossil fuels, with as accompanying disadvantages undesired local and global emissions and politically less desired dependencies. Electric propulsion systems can help to cover the disadvantages of fossil fuel based transportation. Still, battery based, grid charged electric vehicles will not provide an overall solution. Charging takes time and puts a signicant demand on the local electric infrastructure. In addition, such vehicles have a modest range. Therefore, our future mobility is expected to be driven by a more diverse palette of energy carriers, including hydrogen. Hydrogen can be provided by renewable sources and fueling a vehicle with hydrogen takes little time. As the energy density of hydrogen is large compared to batteries, also fuel cell hybrid vehicles provide a signicant longer driving range compared to battery only electric vehicles. In addition, local production of hydrogen by electrolysis with electricity from the local grid, provides new possibilities for demand side management. A fuel cell hybrid propulsion system consists of a fuel cell system to convert hydrogen into electricity, a battery to cover variations in the demand for electric power, a DC/DC converter to match the battery and the fuel cell stack voltages, and an electric motor with inverter to propel the vehicle. Apart from the fuel cell stack, the fuel cell system itself comprises a hydrogen tank to store hydrogen under pressure, a control valve to reduce the hydrogen pressure, a recirculation pump to reuse the surplus of hydrogen leaving the fuel cell stack, a humidier to prevent the fuel cells from dehydration, an air compressor to supply oxygen to the stack and a cooling system to keep the stack at the right operating temperature. The fuel consumption of such a fuel cell hybrid propulsion system is determined by the operation of the vehicle, the management of powers from fuel cell stack and battery, and the sizes of the fuel cell stack and battery. The operation of the vehicle depends on its purpose, its dimensions, the trac conditions and how the driver drives the vehicle. Although how to drive the vehicle is up to the driver, the resulting demand for electric power can still be characterized. This study suggests to characterize the power for traction as a normal distribution. Both simulations and measurements support this assumption, but conclusive evidence is not part of this study. The energy management system denes the management of powers from fuel cell stack and battery when driving the vehicle. This study concludes a close to optimal operation of fuel cell stack and battery is achieved, when the point of operation of the fuel cell stack is chosen equal to the average power demand, and when all variations from this average power demand are distributed over fuel cell stack and battery according to the ratio in their internal losses. The real-time implementation of this energy management strategy determines the value of the average power demand on the measured changes in the amount of energy stored in the battery. The resulting feedback loop provides a robust solution against disturbances, model errors and slowly changing parameters due to issues related to aging. The proposed solution to the energy management problem is validated on a full-scale test facility and demonstrated on a fuel cell hybrid delivery van. The found analytic solution to the energy management problem enables the derivation of an expression for the third factor in fuel consumption; the sizes of fuel cell stack and battery. This derivation makes use of the previously mentioned characterization of the power demand as a normal distribution. The resulting relation expresses the fuel consumption in terms of: properties of the driving cycle such as average speed, parameters of the vehicle such as weight and air resistance, and the number of fuel cells and battery cells. The optimum sizes of fuel cell stack and battery are derived from this relation. The resulting expression shows that, to obtain a minimum fuel consumption, the optimum size of the fuel cell stack does not match the average power demand, nor the maximum power demand, two choices commonly observed for prototypes currently available. Under ordinary condition, its optimum size is located between these two extremes. The sensitivity of the fuel consumption on the fuel cell stack size around this optimum value is limited, which makes the choice for the fuel cell stack less critical. The corresponding optimum size of the battery is the smallest size for which fuel cell stack and battery still can provide the maximum in the power demand and absorb the regenerated power when decelerating the vehicle. The optimum sizes found for the fuel cell stack and the battery are veried through brute force computer simulations. Based on an existing fuel cell hybrid vehicle, for dierent driving cycles, the fuel consumption for all feasible combinations of fuel cell stack size and battery size are derived by simulation. The results match the results of the proposed method. This not only substantiates the proposed sizing method, but also supports a normal distributed power demand as suitable characterization when optimizing the sizes of fuel cell stack and battery in terms of a minimum fuel consumption. An important contribution of the study is the analytical approach of both the energy management problem and the sizing problem. This analytical approach provides a solution to the sizing problem. Moreover, it provides an understand ing how parameter values aect the fuel consumption. For example, a reduction of the specic weights of battery cells and fuel cells oers the best opportunity for additional fuel savings, followed by a reduction of the auxiliary power and a reduction of the internal resistance of battery cells and fuel cells. The observation the fuel consumption is not very sensitive against variations in the fuel cell stack size around its optimum value, can help manufacturers of fuel cell stacks and systems to reduce the number of products over a certain power range. Consequently, the turnover per product will increase and production costs per product will reduce. This will help fuel cell hybrid propulsion systems to faster contribute to the desired reduction of our dependency on fossil fuels and to a further reduction of unwanted (local) emissions.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Department of Electrical Engineering
  • van den Bosch, Paul, Promotor
  • Veenhuizen, Bram, Copromotor
  • Kessels, John T.B.A., Copromotor
Award date15 Apr 2013
Place of PublicationEindhoven
Print ISBNs978-90-386-3364-0
Publication statusPublished - 2013

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