Driving and Filling Personal Vehicles – The Questions of Energy and Power - Density ( A Fast Filling Station for the Compressed Air Car ) A. Rufer, S. Lemofouet, M. Habisreutinger, M. Heidari, A. Leuba EPFL, Ecole Polytechnique Fédérale de Lausanne CH 1015 Lausanne, Switzerland
Abstract: In the context of the development of zero emissions personal vehicles, compressed air cars have been proposed, as alternative solutions to electric solutions, with a small energy content leading to a limited range, but with interesting properties regarding aging phenomena, costs, and possibility to be recharged rapidly. This paper discusses the questions of energy – and power-density of alternative vehicles like electric and compressed air solutions. It proposes and describes a concept of a fast filling station recently developed. The system uses a local reservoir that is precharged by an isothermal compression machine. The fast transfer itself uses a low dissipation nozzle limiting the transfer flow rate. Then, in order to recover the heat produced during the fast pressure rise in the vehicle’s reservoir, a dedicated recirculation system is activated in order to reach finally the higher air mass-density in it, corresponding to the best energy content on board. Simulation results are presented, and an experimental setup is described.
Keywords: electrical vehicule, compressed air car, fast filling, car autonomy, recirculation
1. Introduction The fact that humankind passed the 150% threshold in its use of the planet’s resources available each year has only alerted a limited number of individuals. The years 2010 and 2011 however, with the two significant events of the Gulf of Mexico and Eastern Japan with severe environmental impacts related to the use of fossil and nuclear resources have been the trigger of many interrogations. From the scientific community to the political world, there is no doubt that fundamental questions need to be answered, namely that of the unlimited and unconsiderated energy consumption per capita . Many sectors of the energy consumption are concerned, but the individual transportation occupies a front place in the list of problematic energy users. After several decades of hesitations following the first oil crisis, the automotive industry finally presented 2010 the first for mass production designed electric car. Significant progress in the energy density of electrochemical accumulators, together with reliable and performant electric propulsion systems using modern semiconductor devices and permanent magnet synchronous motors have contributed to this important developpement.
Alfred Rufer, firstname.lastname@example.org
The concept of the electric cars with limited range in comparison with the conventional gasoline and diesel cars has basically not been contested due to the fact that these cars are destinated to specific use. The question of the electric energy resource has been addressed, but maybe not quantitatively, with its compatibility with renewable sources, and said benefits of its integration in smarts grids with concepts like V2G. Electric vehicles playing the role of the needed energy storage capacity with their integration in systems with distributed and non constant resources , . One additional argument used in favour of the electric car is its capability to be fed from a zero carbon source like nuclear power stations. The reality of the unforeseenable security limit of those generation means has interrupted for a while any euphory in the field of existing and planned nuclear stations. In the same category of zero emission vehicles, alternative solutions have been studdied like liquid nitrogen  or compressed air propulsion systems . This last system has been presented as soon commercially available solution for several years, but its manufacturing company has not yet passed the status of prototypes. Compressed air cars have of course very limited range due to the poor energy content of its pressurised reservoir. But they can be envisaged as a solution for specific application segments like factory areas, airports, mail delivery where known repetitive tracks represent the dominant part of the use. Together with the energy content of the cars, the refill time must be considered, as a result of the possible power density of the storage system. This paper will present several aspects related to alternative vehicles like electric cars and compressed air cars. The performances of a fast refill system are analysed and compared to the properties of electric vehicles. Fast refill is reached by the use of a larger local air reservoir from which the car is refilled. A non dissipative nozzle assumes the maintaining of a limited airflow during the transfer, and a dedicated recirculation equipment allows to recover the heat produced by the fast increase of the pressure in the car reservoir. Simulation results of the fast filling process will be presented, together with practical experimentation measured on a small scale demonstration plant.
2. Various energy quantities of personal vehicles Compressed air cars have been generating many discussions about their autonomy, especially in the context of the range announced by its manufacturer. Especially the comparison with classical cars has produced severe criticism. In order to get a better picture for comparison and reference, a comparison is given in Figure 1, where different vehicles (also low range) are presented with their on-board energy content. First, a classic gasoline vehicle is represented, where the fuel energy density is given as 42.5 kWh/l. So a 50 liter reservoir contents nearly 600 kWh when full. The second example represents a high performance electric sports car where 400 kg of Li-ion batteries with 132 Wh/kg are used. The quantity of energy of the fully charge battery is here equal to 53 kWh that is less than 10% of the content of the gasoline car. The next case shown in the figure corresponds to a small electric car with 200 kg of lead-acid batteries having 30 kWh/kg. The calculation of the quantity of the on-board energy gives in this case only one percent of the case with gasoline. The compressed air car is the next example in the figure with a calculated energy of 7.8 kWh, stored in a pressurised air reservoir of 200 liters under 300 bar. This value corresponds to the potential energy that can ideally be extracted from this volume when isothermal expansion is used.
Figure 1 : Energy quantities of different reservoirs in individual vehicles The last example corresponds to a world record in autonomy, obtained by the Swiss Federal Institute of Technology of Zurich during the 2005 Shell Marathon . The reached range was equal to nearly 5400 km with only an equivalent energy quantity of one liter gasoline. The runner was propulsed by a fuel cell system and electric motor.
3. The energy and power density : the vehicle autonomy and filling time While the energy density of an accumulator defines the range of a given vehicle, one must consider also the possible or allowed power density of the same accumulator in order to define the possible time needed for refilling. Generally, the refill time of a vehicle is expected to be short in comparison with the possible time of driving, as it is of evidence for classical gasoline cars. The associated parameter characterising the possible short time refill is defined as an equivalent power density. As an example, for a classical gasoline car, the refill time is depending on the gasoline flowrate provided by the petrol-stationâ€™s pump. In practice it is equal to 38 l/min, and leads with a density of 0.83 kg/l to a huge instant equivalent power of 22 MW ! Figure 2 shows the significant differences between an electric car where the charge power density is approximately equal to the discharge power density and leads to drive time and refill time of similar lengh. An example like the gasoline car is represented in the lower curve of fig. 2, where the much higher power density of the refill process shortens the refill time significantly versus the driving time.
Figure 2 : Driving and refilling times of electric and gasoline car (not at scale).
4. The limitations of electric vehicles As already discussed, the charge time of an accumulator is related to its energy density. As an inverse definition, for a given energy capacity of an accumulator, the resulting power level of its recharge can be easily calculated, taking into account that the power level is kept constant during the whole charge process. From a very simple model that includes a constant voltage source and a series resistor for the battery, the needed charge power is calculated and represented in fig. 3. As an additional parameter, the battery series resistor has been considered. This figure indicates that the power level needed for a predefined charge time is not really influenced by the value of the series resistor. The power needed for the charge of a 25 kW battery reaches the 1000 kW range when the charge is realised within several minutes. Related to the important current flowing inside the battery while the fast charge, the internal loss affect dramatically the energy efficiency of the charge process. Also with the internal resistor as parameter, fig. 3b represents the efficiency in dependency of the charging time. This time, the value of the internal resistor plays an important role.
Figure 3a : Power demand as a function of the charge time
Figure 3b : Efficiency of the charge process as a function of the charge time.
5. The compressed air car : A concept for performant fast filling In the introduction paragraph, zero emission vehicles have been shortly discussed as alternative solutions to classical cars. The technology of compressed air belongs to that family, but have a very limited range due to the poor energy content of its pressurised reservoir. The energy amount has been evaluated in the second paragraph compared to usual solutions. Due to their property of zero emissions, together with the possibility to be fed from renewable sources, compressed air cars can be envisged as a solution for specific application segments like factory areas, airports, mail delivery where known repetitive tracks represent the main part of the use. Regarding the real autonomy of compressed air cars, divergent meanings and pretentions have been formulated about the presented prototypes. When the range of 200 km indicated by the manufacturer seems to be exagerated, real values of around 50 km would already allow to cover several interesting applications.
Together with the energy content of the cars, the refill time must be considered, as a result of the possible power density of the storage system. In reality, the shorter the driving range the faster the refill process should be. The concept proposed in this paper comprises a local storage reservoir that has a significantly larger capacity than the carâ€™s reservoir. This oversising is necessary in order to obtain in the car reservoir a not to low pressure level after the filling as was previousely in the local reservoir, and to be able to fill more than one car before refilling it. This type of design can be compared with the design of classical CNG stations. It must be said that it is important to refill the local reservoir with a performant compression system, in order to realise this prefilling under nearly isothermal conditions, in order to achieve good energy efficiency of the whole system. The structure of the fast filling system is shown in Fig. 4, where the main components are the precompression stage, the local reservoir, and the reservoir of the car.
Figure 4 : The structure of the fast filling station The fast air transfer of the air from the local reservoir to the car is obtained with the help of a nozzle, where the air flowrate is limited without dissipation, reaching sonic conditions. This elements allow to fast fill the car reservoir under good energy efficiency, but this process presents the drawback of rising the temperature in the receiver due to fast rise of the pressure. After transfer with heating up the air in the car, there is the problem of cooling down this air after leaving the filling station, that would lead to a significant reduction of the pressure. The main consequence is that the possible range would be reduced. In order to ÂŤ better fill Âť the car reservoir, a recirculation system is proposed, with which the heated air in the car is brought back in the local reservoir after the equalisation of the pressures. Such a recirculation of the heated air corresponds to replacing the hot air by cold one, and needs only to compensate the circulation loss if the pressures are really stabilised. The recirculation of the air brings the temperature in the car reservoir down to nearly atmospheric temperature, and increases the air density in the car reservoir as shown in fig. 6 and 7.
Figure 5 : Evolution of the temperature in the car (dotted line) and in the local (full line) reservoirs. The filling lasts from 0 to 23 s, and is followed by the recirculation process. The filling time is adapted to the low volume chosen for the demonstration equipment, with an air flowrate that causes a similar elevation of the temperature as in the nominal case.
Figure 6 : Evolution of the air density in the car (dotted line) and in the local (full line) reservoirs. The filling time is adapted to the low volume chosen for the demonstration equipment using with an air flowrate that causes a similar elevation of the temperature as in the nominal case.
6. Experimental verification In order to verify experimentally the properties of the fast filling, the reduced scale set-up illustrated by the scheme of fig. 8 has been built. The local reservoir of 25 liters is filled at 300 bar with the help of a diving compressor. The car reservoir has a volume of 5 liters and is fed through the Laval nozzle. The recirculation of the heated air from the car to the local reservoir is achieved through an air-driven gasbooster. This equipment has been chosen for the experimentation because it has to be operated under high pressure (against the atmospheric air), even if its property of pressurisation is not needed.
Figure 7 : Scheme of the experimental set-up The complete demonstration system is shown in Fig. 8, where the small-scale reservoirs can be shown, as well as the recirculation equipment. The experimental set-up is completed by valve control electronics, and also instrumented with pressure, temperature and flow-rate measuring equipments. An interface to a host computer uses a local DSP-based control and acquisition board. The volumes of the reservoirs are scaled at 1: 40, leading to a stationary reservoir model of 25 liters, and a car reservoir model of 5 liters. In the demonstration system, the filling time is varied from 1 to 5 min, by using different nozzles.
Figure 9 : Experimental set-up
Figure 10 shows the flow-rates of the fast filling, as well as the pressures of the stationary and of the car’s reservoir during fast filling. In the model, the maximum pressure has been limited to 200bar for security reasons and because of the properties of the recirculation equipment.
Figure 10: Flow-ratse and pressures during fast filling. The verification of the principle of recirculation is underway, but needs an appropriate temperature measuring sensor.
7. Conclusions This paper discusses the questions of energy – and power-density of alternative vehicles like electric and compressed air solutions. It proposes and describes a concept of a fast filling station recently developed at EPFL’s LEI. The system uses a local reservoir that is pre-charged by an isothermal compression machine. The fast transfer itself uses a low dissipation nozzle limiting the transfer flow-rate. Then, in order to recover the heat produced during the fast pressure rise in the vehicle reservoir, a dedicated recirculation system is activated in order to reach finally the higher air mass-density in it, corresponding to the best energy content on board. Simulation results are presented, and an experimental setup is described.
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