Energy analysis of electric vehicles using batteries or fuel cells through well-to-wheel driving cycle simulations
Introduction
The environmental impact of energy production, conversion and final use is more and more influencing our life, and the consensus about the necessity to limit carbon dioxide emissions is widely increasing. Moreover, energy is becoming an increasingly expensive commodity, and even in absence of climate change issues, all the energy intensive processes need to explore new technologies able to reduce their consumptions. One of the sectors featuring the most energy-burning processes is transportation, typically covering 30–35% [1] of the primary energy needs of most industrialized countries, with a large majority of consumption related to road transportation: as a matter of fact the average power consumption of transportation is typically comparable to the maximum electricity power demand of a national power grid.1
For this reason, road transportation of passengers through vehicles is one of the sectors where R&D activities are more important, generally aiming to reduce the pollutant emissions and the energy consumptions of cars. It is well known that the current widespread technology is based on reciprocating internal combustion engines (ICE), directly driving the vehicle wheels through a gearbox, but the majority of car manufactures are developing a number of new solutions which make use of electric drives experimented through prototypes intended for medium or long term applications and in some cases already on the market. The proposed solutions typically cover three general categories:
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pure battery electric vehicles (BEV), where a battery stores energy previously taken from the electric grid, and the battery powers an electric drivetrain, which includes an electric motor driving the car wheels;
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fuel cell powered electric vehicles (FCEV), where a fuel cell generates onboard the electricity needed to power an electric drive; the fuel cell is fed with hydrogen, either coming from a tank (filled with hydrogen produced elsewhere), or produced onboard through a dedicated fuel processor, using gasoline, bio-ethanol or other liquid fuels;
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hybrid electric vehicles (HEV), integrating in several possible ways the use of ICE, batteries and/or fuel cells, which together generate the electricity required to power an electric drive.2
This paper explores the energy saving potentials of two of these technologies, BEV and FCEV, aiming to assess the advantages given by their possible future introduction on the market, where they could integrate or substitute the current dominating ICE technology.
After a preliminary survey of available battery and fuel cell technologies [2], it is proposed a comparison of two technologies based respectively on the use of state-of-the-art Li-ion batteries and Polymer Membrane Electrolyte fuel cells (PEM).
Both BEV and FCEV are often regarded as the only long term complete solution to the problem of pollution in urban areas, as well as to the problem of CO2 emissions, thanks to the use of clean energy vectors like electricity and hydrogen: in principle, the electricity used by BEVs or the hydrogen used by FCEVs (at least in the option where it is not produced onboard) could be generated by clean and CO2-free processes, using renewable sources or nuclear energy or fossil energy with CO2 capture and storage techniques.
Several examples of vehicles corresponding to the three categories have been demonstrated or put into the market in recent years. Among many available examples, we may recall here:
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BEV intended for urban mobility, like the Think project recently endorsed by GE, featuring the use of Li-ion or sodium batteries in a 250 kg pack, powering a 30 kW drivetrain to a top speed of 100 km h−1 and 200 km driving range [3];
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BEV sport vehicles intended for high performances, like the Tesla roadster, a two seat car featuring a 185 kW electric drivetrain powered by 150 Ah Li-ion battery, with a top speed of 200 km h−1 and a driving range of about 300 km, with regular production scheduled for starting in late 2008 [4];
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FCEV running with hydrogen stored onboard, like the Class B Mercedes Benz prototypes, which after several previous versions (starting from the NECAR projects in the late 90 s with Ballard PEM fuel cells) have reached a 400 km driving range with gaseous hydrogen stored at 700 bar and a 100 kW PEM drivetrain, and expect production in 2010 [5]; or the Honda FCX, already marketed in small fleets in California, featuring 300 km driving range with a 150 litres compressed hydrogen tank, top speed of 150 km h−1 and a 78 kW fuel cell drivetrain [6]; or the recent FCHV-adv of Toyota which can travel up to 830 km on a single hydrogen fuelling thanks also to an optimized use of onboard batteries and regenerative braking [7].
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FCEV systems running with liquid fuels like gasoline, ethanol, methanol or natural gas through an onboard fuel processor, like the Star™ project, developed by Nuvera fuel cells with Renault and other automakers, featuring up to 200 kWth (LHV) and 80% energy efficiency [8].
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HEV with conventional gasoline ICE, like the well known Toyota models which are established on the market since several years (the Prius, and Lexus high performance models) [9]; or recent projects like the GM Chevrolet Volt, where the driving range in all-electric mode is extended up to 60 km through a 16 kW h−1 Li-ion battery pack and a 120 kW electric drivetrain [10].
The approach followed by the work is twofold.
- (1)
Initially, for both technologies it is assessed a number of energy pathways where primary energy sources are converted into electricity (for the BEV) or hydrogen (for the FCEV), calculating their efficiency in terms of well-to-tank (WTT), tank-to-wheel (TTW) and ultimately well-to-wheel (WTW) energy balances. The analysis yields the primary energy consumption for each kWh of energy given at the vehicle wheels.
The pathway study compares different possible feedstocks, choosing the most plausible possibilities and analyzing the associated efficiencies. Feedstocks considered are, for BEV: coal, natural gas, renewable energy and the average mix of the Italian electric park. Regarding hydrogen for FCEV, coal, natural gas and renewables are taken into account. Finally, for FCEV, other considered on-board fuels are gasoline, natural gas, methanol and ethanol (biofuel).
This approach is often referenced in literature [11], [12] and offers an immediate comparison among different solutions, although results are affected by numerous assumptions regarding the energy conversion chain that have to be evaluated carefully. However, such analysis gives an idea of the energy performances only at “nominal” operating conditions of the drivetrain, i.e. does not take into account the effects of a real use, featuring variable loads and the necessity to sustain a rather long (up to several hundreds km) driving range.
- (2)
The second approach used in this work considers the necessity to sustain a realistic driving cycle and range. In this case, the vehicle and drivetrain specifications change, heavily influencing both the vehicle weight (for instance, the weight of the batteries required by a BEV varies, and the power and weight of the fuel cell system also varies) and the energetic performances of the well-to-tank comparison. These effects are taken into account by simulating the vehicle power and energy demand under standardized driving cycles, for a variable driving range, with homogeneous specifications for the vehicle performances.
Within these hypothesis, it is possible to recalculate the WTT balances and evidence realistic efficiency scenarios for each technological solution. The analysis is based on the simulation of ECE-EUDC standardized driving cycles, which constitutes a widely used reference and are normally used to qualify vehicle consumptions and emissions in the European Union.
Moreover, aiming to reproduce the power features of current average market cars, it is also assumed that the power capacity of all vehicles must be able to sustain a standard “high performance” driving cycle, for which we have assumed the US06 cycle, normally used to test vehicles under aggressive driving conditions.
The simulation of driving cycles points out that one of the advantages, which can be achieved by adding a significant battery electricity storage onboard a vehicle, is the possibility to recover energy during vehicle deceleration, and to give it back during acceleration (except for the losses related to the charge/discharge cycle). For this reason, in the FCEV case it is also discussed the possibility to add a significant battery storage to the vehicle powertrain.
The conclusions of the work offer an interesting and original insight on the comparison between the BEV and FCEV technologies, through a realistic simulation of primary energy consumptions and CO2 emissions under reference driving cycles.
The comparison shows equilibrium as well as advantage or disadvantage areas depending on the vehicle driving range, and indicates the possibility to reach CO2 emissions well below those of current vehicles and ultimately to fit the perspective of clean or zero-emission vehicles (ZEV).
Section snippets
Well-to-wheel analysis on different energy pathways
The well-to-wheel (WTW) analysis evaluates the total primary energy consumption yielded by the vehicle for each kWh of energy given at the vehicle wheels, comprising all the steps covered by the well-to-tank (WTT) conversion path and subsequently by the tank-to-wheel (TTW) onboard energy conversion. The analysis depends on the considered energy pathway, and results are influenced by the numerous assumptions made to evaluate the efficiency of each passage in the energy conversion chain. Aiming
Driving cycle simulation
The simulation of the driving cycle is carried out with reference to standardized cycles, namely the ECE-15 (part one, urban) and EUDC (part two: “extra-urban driving cycle”) used by European legislation to assess car consumptions and emissions [42].
Because of low acceleration and rather low maximum speed, the peak power demand generated by ECE-EUDC cycles is low and does not correctly reproduce the features of current average market cars. For this reason, it is also assumed that:
- (1)
all vehicles
Effect of a regenerative braking
As already mentioned, one of the most interesting ways to improve the vehicle efficiency under real driving cycles is the introduction of a battery able to store energy during braking, recovering part of the vehicle kinetic energy. This option can be easily adopted in a BEV, where the vehicle is equipped with a battery, while in the case of a FCEV the installation of a dedicated battery pack is required. In this case, the battery size has been calculated after an optimization process in order
Conclusions
This work discusses the energy and environmental balances for electric vehicles using state-of-the-art Li-ion batteries (BEV) or PEM fuel cells (FCEV), through the methodology of the well to wheel (WTW) analysis, applied to ECE-EUDC driving cycle simulations. Well to wheel balances are carried out considering different scenarios for the primary energy feedstock (renewable energy, coal, natural gas) and for the energy conversion chain, which in the case of FCEV may rely on gaseous or liquid
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