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Fuel cells as auxiliary power units for range extender electric vehicles
Abstract
Powertrains electrification resulted in a wide range of hybrid vehicle architectures. Fuel consumption of these powertrains strongly relies on the energy converters performance, as well as on the energy management strategy deployed on-board. This paper investigates the fuel cell as energy converter for electrified vehicles. The serial hybrid range extenders architecture is considered. Two main types of fuel cells are considered for range extenders modules: proton exchange membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC). The solid-oxide fuel cell offers the possibility to use liquid fuels as carrier of hydrogen: bio-ethanol or biogas. The functional mechanisms of the coupling of the renewable energy vectors to the fuel cell converters are described. The article investigates the potential of energy consumption savings of a serial hybrid electric vehicle (SHEV) using both the PEMFC and the SOFC as energy converter operating as auxiliary power unit (APU) instead of the conventional internal combustion engine (ICE). Both FC APU are integrated in the developed SHEV powertrain. Energy consumption simulations are performed on WLTP cycle using dynamic programing as global optimal energy management strategy.
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Within the EU-FCH JU funded project DESTA the partners AVL, Eberspächer, Topsoe Fuel Cell, Volvo and Forschungszentrum Jülich are collaborating since January 2012 to demonstrate the first European Solid Oxide Fuel Cell Auxiliary Power Unit (SOFC APU) on a heavy duty truck. The APU is used to provide the truck with electrical energy during vehicle standstill to power comfort functions like A/C, TV, refrigerator & PCs. To facilitate these functionalities typically, the main diesel engine is idled which causes significant emissions and operating cost. The APU is used with conventional diesel fuel and provides about 3 kW of electrical power at 30% electrical efficiency. To enable the usage of conventional fuel, SOFC technology is used.
Main advantage of the SOFC stack technology is the use and operation with synthesis gas generated out of conventional fuels. Hence DESTA SOFC APU systems can be operated with conventional road Diesel fuel. Within the APU system, the Diesel fuel is reformed to a hydrogen and carbon monoxide containing synthesis gas. The SOFC technology allows the conversion of this hydrogen and also carbon monoxide with oxygen of the air directly into electricity. This creates a significant benefit in comparison to other fuel cell technologies like PEM or HT PEM. This process also allows significantly higher electrical efficiencies than conventional solutions like a diesel gen-set. Additionally the fuel cell APU is extremely silent and doesn’t emit other harmful emissions like NOx, THC or PM.
Petroleum as main raw material to production of practical fuels in car engines, is not source of renewable energy. Besides, in result of fuels combustion produced from petroleum (petrol, diesel oil)are emitted pollutants to earth's atmosphere. Therefore in automobile concerns are realized intensive development works, aiming to diminution of fuel consumption and restrict of issue pollutants. Alternative sources of energy to drive cars, which would be able to take place with success of petroleum derivative fuels also are searched. Fuel cells are doubtless of these solutions, which can in nearest future to revolutionize drive systems in car vehicles. In article kinds of fuel cells and their principle of operation has been described. Physical-chemistry parameters of these cells are also compared. In recapitulation prognosis for fuel cells in context development of motorization in the future has been formulated.
Solid oxide fuel cells (SOFCs) have received attention in the transport sector for use as auxiliary power units or range extenders, due to the high electrical efficiency and fuelling options using existing fuel infra structure. The present work proposes an SOFC/battery powered vehicle using compressed natural gas (CNG), liquefied natural gas (LNG) or liquefied petroleum gas (LPG) as fuels. A model was developed integrating an SOFC into a modified Nissan Leaf Acenta electrical vehicle and considering standardized driving cycles. A 30 L fuel tank and 12 kW SOFC module was simulated, including a partial oxidation fuel reformer. The results show a significant increase of the driving range when combining the battery vehicle with an SOFC. Ranges of 264 km, 705 km and 823 km using respectively CNG, LNG and LPG compared to 170 km performed by the original vehicle were calculated. Furthermore, a thorough sensitivity analysis was carried out.
Significant research efforts have been invested in the automotive industry on hybrid electrified powertrains in order to reduce the dependence of passenger cars on oil. Electrification of powertrains resulted in a wide range of hybrid vehicle architectures. The fuel consumption of these powertrains strongly relies on the energy converter performance, as well as on the energy management strategy deployed on board. This paper investigates the potential of fuel consumption savings of a series hybrid electric vehicle using a gas turbine as an energy converter instead of the conventional internal-combustion engine. An exergo-technological explicit analysis is conducted to identify the best configuration of the gas-turbine system. An intercooled regenerative reheat cycle is prioritized, offering higher efficiency and higher power density than those of other investigated gas-turbine systems. A series hybrid electric vehicle model is developed and powertrain components are sized by considering the vehicle performance criteria. Energy consumption simulations are performed over the Worldwide Harmonized Light Vehicles Test Procedure driving cycle using dynamic programming as the global optimal energy management strategy. A sensitivity analysis is also carried out in order to evaluate the impact of the battery size on the fuel consumption, for self-sustaining and plug-in series hybrid electric vehicle configurations. The results show an improvement in the fuel consumption of 22–25% with the gas turbine as the auxiliary power unit in comparison with that of the internal-combustion engine. Consequently, the studied auxiliary power unit for the gas turbine presents a potential for implementation on series hybrid electric vehicles.
The fuel savings of plug-in hybrid electric vehicles strongly rely on the energy management strategy deployed onboard. For the current mass-produced plug-in hybrid electric vehicles, notably the Toyota Prius, the energy management strategy is a rule-based type, which is configured to optimize instantly the fuel consumption without taking into consideration the upcoming driving patterns of the given route schedule. Hence, it operates the vehicle first in the electric mode over a predefined all-electric range and then in the charge-sustaining mode. The energy consumption results are seen to be far from optimal when compared with global optimization strategies with prior knowledge of the scheduled route, such as dynamic programming. Hence, this study presents the methodology to optimize the rule-based energy management strategy for real-time implementation in the Prius plug-in hybrid electric vehicle, using dynamic programming as the global optimization routine. The optimization process takes into account the desired trip profile selected by the driver on the vehicle’s onboard Global Positioning System and linked to a traffic management system. A basic rule-based energy management strategy, which emulates the vehicle performance and the energy consumption, has been set first using on-road measurement data logging. As a second step, the dynamic programming optimization routine was applied to the model, assuming a repeated New European Driving Cycle as the scheduled route. The results obtained for the behaviours of the powertrain components under optimal control are evaluated and used to update the operating energy management rules of the basic controller. Finally, an optimized rule-based controller is proposed by coupling between the dynamic programming and the basic rule-based controller, followed by an evaluation of the energy consumption and the powertrain efficiency of the three investigated control strategies.
Among the general problematic of the HEV power trains, the most critical point is the determination of the power-split ratio between the mechanical and the electrical paths, known as the energy management strategy (EMS). Many EMS are proposed in the literature, and can be grouped in two categories: the local optimization EMS and the global optimization EMS. The local optimization category corresponds to the EMS based on human expertise and the knowledge of the power train components efficiency maps. Thus, the local optimization EMS manages the power train operations by referring to predefined rules. The drawback of such strategies is that it brings an instantaneous fuel consumption optimization, and does not fully optimize the fuel consumption over the whole trip. Therefore, additional fuel savings are still possible. This paper presents an overall optimized predictive EMS for the Toyota Hybrid System (THS-II) power train of the Prius. The proposed EMS is based on Dynamic Programming (DP), where the prior knowledge of the route is required in order to predetermine the power-split ratio and optimize the fuel consumption for the whole predicted route. The DP EMS proposed for the THS-II power train is designed with a very short computation time, intended to be implemented in real-time applications. The potential of this DP-controller in reducing fuel consumption on regulatory cycles are computed and compared to a rule-based controller and to the Prius published fuel consumption results. Finally, the fuel reduction enhancements of the DP-controller are computed for real road tests achieved on a MY06 Prius in Ile-de-France, by comparing to the associated observed consumption measurements.
In this study, a comprehensive performance analysis of a transportation system powered by a PEM fuel cell engine system is conducted thermodynamically both through energy and exergy approaches. This system includes system components such as a compressor, humidifiers, pressure regulator, cooling system and the fuel cell stack. The polarization curves are studied in the modeling and compared with the actual data taken from the literature works before proceeding to the performance modeling. The system performance is investigated through parametric studies on energy, exergy and work output values by changing operating temperature, operating pressure, membrane thickness, anode stoichiometry, cathode stoichiometry, humidity, reference temperature and reference pressure. The results show that the exergy efficiency increases with increase of temperature from 323 to 353 K by about 8%, pressure from 2.5 to 4 atm by about 5%, humidity from 97% to 80% by about 10%, and reference state temperature from 253 to 323 K by about 3%, respectively. In addition, the exergy efficiency increases with decrease of membrane thickness from 0.02 to 0.005 mm by about 9%, anode stoichiometry from 3 to 1.1 by about 1%, and cathode stoichiometry from 3 to 1.1 by about 35% respectively.
Auxiliary power units (APUs), i.e. devices designed to provide additional onboard power in vehicles, are believed to be an important entry point for fuel cell (FC) technology into commercial markets. Three technologies are under consideration for this market: solid oxide fuel cells (SOFCs), proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). By using the concept of total addressable, potential and capturable markets, this paper discusses the opportunities and challenges of fuel cell auxiliary power units (FC APUs). A number of conclusions can be drawn. As FC APUs do not offer increased fuel efficiency when meeting electrical demand while the main engine is used for propulsion (as opposed to idling), applications on board transit vehicles such as buses seem limited. Potential markets begin to open up in vehicles that either have a very large electricity demand due to many high-energy onboard functions, such as luxury limousines, or that require electrical power whilst stationary. Examples in the latter category include law enforcement vehicles, recreational vehicles, and most importantly heavy duty trucks. Volume and start-up time seem to be the major technical challenges hindering the market penetration of FC APUs. However, whilst the functional benefits of FC APUs over existing technologies are limited, the former must also be able to compete on a cost basis. The intense activities of APU manufacturers suggest a confidence either in the potential for cost reductions or in the consumers’ willingness to pay. Similarly, the involvement of a number of big truck manufacturers casts doubt on the extent to which incumbents are taken by surprise by competence-destroying, disruptive and radical technologies like FC APUs.
The well-known compatibility of the SOFC with hydrocarbon fuels makes the solid oxide fuel cell a strong contender for transportation applications. Gasoline and diesel will continue to be the dominant fuels for decades to come. The 500 million automobiles, the existing fuel distribution system and the economic power of oil and automobile companies combined form a strong inertia in favour of liquid fuels and internal combustion engines. The chances are slim that hydrogen will soon appear at gasoline stations to power "PEMobiles", i.e. vehicles equipped with polymer electrolyte fuel cells.
A SOFC concept based on microtubular cells is presented. Mechanical tests and modelling show viability for fast thermal cycling in air for both zirconia and ceria electrolyte tubes. To combine advantages of both these materials, the concept includes a thin double layer electrolyte, with zirconia on the fuel side and ceria on the air side. This allows to use active cobaltite cathodes. The surface reaction mechanism of oxygen reduction on the cobaltite/ceria system was analysed and shown to be determined by dissociative adsorption rather than bulk incorporation. Cells based on this concept were fabricated, at present in an easier accessible planar configuration, for demonstration purposes, and shown to feature high power density at low operating temperature (0.8 W cm−2 at <800°C).
This paper introduces a generic dynamic programming function for Matlab. This function solves discrete-time optimal-control problems using Bellman's dynamic programming algorithm. The function is implemented such that the user only needs to provide the objective function and the model equations. The function includes several options for solving optimal-control problems. The model equations can include several state variables and input variables. Furthermore, the model equations can be time-variant and include time-variant state and input constraints. The syntax of the function is explained using two examples. The first is the well-known Lotka-Volterra fishery problem and the second is a parallel hybrid-electric vehicle optimization problem.
A feasibility study and techno-economic analysis for a hybrid power system intended for vehicular traction applications has been performed. The hybrid consists of an intermediate temperature solid oxide fuel cell (IT-SOFC) operating at 500–800 °C and a sodium–nickel chloride (ZEBRA) battery operating at 300 °C. Such a hybrid system has the benefits of extended range and fuel flexibility (due to the IT-SOFC), high power output and rapid response time (due to the battery). The above hybrid has been compared to a fuel cell-only, a battery-only and an ICE vehicle. It is shown that the capital cost associated with a fuel cell-only vehicle is still much higher than that of any other power source option and that a battery-only option would potentially encounter weight and volume limitations, particularly for long drive times. It is concluded that increasing drive time per day decreases substantially the payback time in relation to an ICE vehicle running on gasoline and thus that the hybrid vehicle is an economically attractive option for commercial vehicles with long drive times. In the case where the battery has reached volume production prices at £70 kWh−1 and current fuel duty values remain unchanged then a payback time <2 years is obtained. For a light delivery van operating with 6 h drive time per day, a fuel cell system model predicted a gasoline equivalent fuel economy of 25.1 km L−1, almost twice that of a gasoline fuelled ICE vehicle of the same size, and CO2 emissions of 71.6 g km−1, well below any new technology target set so far. It is therefore recommended that a SOFC/ZEBRA demonstration be built to further explore its viability.