Spatially differentiated comparison of diesel and electric buses
Spatially differentiated energy and environment comparison of diesel and electric buses
Damon Honnery1, Robbie Napper2, Ilya Fridman2, Patrick Moriarty2
1Department of Mechanical and Aerospace Engineering, Monash University
2Department of Design, Monash University
Abstract
Urban transport increasingly faces the twin global challenges of oil depletion and climate change. Two additional concerns in most cities are local air and noise pollution. Electric public transport vehicles could potentially help alleviate all four problems. Here we compare the energy, climate, and air and noise pollution implications of diesel and electric vehicle (EV) buses. Full fuel cycle energy and CO2 emissions per km of route were modelled, with the number of stops per km varied to represent inner and outer urban routes. We find that per km of route, EV buses hold an advantage over diesel buses on full fuel cycle energy use. But given current means of electricity production, minimum CO2 emissions of both vehicle types are almost identical, although these occur at a slightly lower average speed for diesel than for EV buses. We also find significant air and noise pollution benefits of EV over diesel buses, especially for inner city routes. This advantage is expected, because the higher density of population for the inner route magnifies the health risks of air and noise pollution. Finally, the results are generalised to aid decision-making about the benefits of electric buses in other cities, both in Australia and overseas.
1. Introduction
Transport, whether passenger or freight, faces the twin challenges of global depletion of conventional oil and global climate change. These two problems are not independent, since increasing reliance on non-conventional petroleum will greatly increase the overall CO2 emissions from petroleum-fuelled vehicles (Moriarty Honnery 2013). For urban passenger transport, two additional problems are air and noise pollution, while passenger comfort is of concern for all bus occupants.
A number of studies have compared diesel and electric vehicle (EV) bus energy usage for urban and suburban duty cycles (Ercan et al. 2015, Kamiya et al. 2010, Lajunen 2014). These studies concluded that battery EV buses consumed less energy than diesel buses during urban and suburban operation. When differentiating EV bus energy use between urban and suburban operation in Oporto, Portugal, Perrotta et al. (2014) found that more energy was used on urban routes with greater frequency of stops and lower average driving speeds. This finding supports the importance of considering route characteristics when calculating energy use, which is largely dependent on average driving speed, stop frequency and acceleration.
A comprehensive Life Cycle Analysis would include the energy embodied in the manufacture of the vehicles, including the batteries. For cars in Australia, Sharma et al. (2013) found that the embodied energy for EV cars was larger than for conventional cars. However, the results are highly dependent on relative battery weight and performance. The present study is restricted to the full fuel cycle (FCC) only, so embodied energy has not been included in this analysis.
This paper first presents energy and greenhouse gas comparisons for EV and diesel buses, based on a model developed in the Appendix. For diesel buses, actual operating parameters on Melbourne inner and outer routes were used; for EV buses, the relevant parameters were obtained from published data. The subsequent sections then examine in turn air pollution, noise pollution and passenger comfort for diesel compared with EV buses, with a further distinction between inner and outer suburban operation. The final section first explores the limitations of cost comparisons, then looks at how comparative CO2 emissions might change as fossil fuel use is reduced, and lastly gives the main findings of the paper.
2. Bus energy and greenhouse gas comparisons
Only the simpler bus parameters are discussed in this section. Values for the more technical parameters, such as for acceleration and road load, are given in the Appendix.
2.1. Vehicle parameters
The vehicle parameters given in Table 1 are based on existing diesel buses used on both inner and outer Melbourne routes. Vehicle frontal area was modelled on data supplied by Volgren Australia for their Optimus low-entry 12.5 m route bus. Vehicle mass was calculated at a fully laden 16 000 kg for both diesel and EV buses, in accordance with the maximum allowable load on Victorian roads (Public Transport Victoria 2015).
Electric motor and diesel engine power values were obtained from manufacturer data sheets (ZF 2015; Volvo 2011). Acceleration constants were referenced from Bradley and Associates (2006) for diesel and Choi et al. (2012) for EV buses. Factors of 0.25 and 0.70 were used for powertrain efficiency of diesel and EV buses respectively based on assessment of recent studies (eg Gupta et al. 2016). The EV value of 0.70 includes energy losses during battery charging. Regenerative braking is an advantage that EVs hold over diesel buses. It depends on two variables: powertrain efficiency and the battery’s capacity for energy acceptance—a capacity that significantly reduces the rate of energy recovery in short high-power scenarios (Perrotta et al. 2012). A conservative regenerative factor of 0.5 was modelled for the EV bus, representing battery energy acceptance capacity as well as some systems losses.
Table 1: Vehicle parameters specification
Parameter / EV bus / Diesel busBus frontal area (m2) / 7.0 / 7.0
Vehicle mass (kg) / 16 000 / 16 000
Max. bus engine power (kW) / 250 / 213
Motor and powertrain efficiency / 0.70 / 0.25
Regeneration factor / 0.5 / 0.0
Accessory power demand (kW) / 10 / 10.0
FCC MJ/(tank or power point MJ) / 2.94 / 1.16
kg CO2 per FCC MJ / 0.087 / 0.073
Air-Conditioning (A/C) is regarded as a key aspect of quality bus service in Australia (Hensher Prioni 2002) and the top environmental factor for passenger comfort (Kogi 1979). Accessory power demand was modelled at 10 kW, representing nominal A/C power (Spheros 2013) and other system components including doors, lighting and power steering that were assumed electric across both vehicles. This accessory power value represents an estimated average between different climate conditions, as peak A/C power alone may be greater than 10 kW on hot days.
2.2. Full fuel cycle energy comparisons
Bus fuel consumption is usually presented for direct energy use, for example litres of diesel per 100 km. But for a fairer comparison we must convert diesel and electricity use to a full fuel cycle (FCC) or a ‘well-to-wheels’ basis. For diesel, this means following the energy needed to find and develop the oil fields, pump the oil and deliver it to the refinery, refine the crude, transport the diesel to the fuel station, and finally pump it into the fuel tank. Needless to say, such an analysis is very difficult; diesel used in Australia may be refined from both local and imported crudes, or even refined overseas. Torchio Santarelli (2010) calculated that this input energy was 16% of the refined diesel energy in the tank for the European Union. We will also use this figure, giving the primary FCC energy values of 1.16 times the diesel fuel use (Table 1).
A difficulty arises when converting EV electricity consumption to primary energy, since the Victorian grid is part of a wider network incorporating NSW, SA, and Tasmania. Because this extended grid accounts for most electricity generation in Australia, the Australian average data are used. Using the statistical data from the Department of Industry and Science (2015), and assuming 6% transmission and distribution losses (World Bank 2016a), the full fuel cycle energy use is 2.94 times the electricity input to the bus at the charging point (Table 1).
Figure 1 shows how FCC MJ/km varies with both average bus speed over a route and with the number of stops per km (N) ranging from 0.5 to 8.0. For comparison, two commercial bus routes selected from Melbourne, Victoria to represent urban (Route 235) and suburban (Route 900) duty cycles had scheduled stops and traffic lights measured from timetables at 5.41 and 2.45 per km respectively. With R=0.5 an EV bus will consume less FFC energy than a diesel bus (R=0) for the same number of stops per km (i.e. for the same value of N), and the difference between the vehicles increases with increasing average speed.
Figure 1: Variation of FCC energy (MJ/km) with average speed for different numbers of stops/km (N)
Taking N=2 as an example, minimum EV bus FFC energy is 20% lower than diesel. Minimum FFC energy also occurs at higher average speed for the EV than for the diesel bus. For both vehicles, as average speed reduces, energy consumed ultimately becomes independent of stop numbers, because vehicle accessory load dominates FFC energy consumption. As average speed increases, vehicle acceleration dominates FFC energy consumption for high stop numbers, with cruise dominating for low stop numbers. (It takes more energy to increase speed by one km/h at high speeds than at low speeds.)
In Figure 2, N has been fixed at 2.0, but R allowed to vary from 0 to 0.9, with the diesel bus included for comparison. The R=0.9 curve highlights the advantage of regenerative braking: the energy curve is much lower, and the minimum energy value occurs at a higher average speed than for lower or zero R values.
Figure 2: Variation of FCC energy (MJ/km) with average speed for different values of Regeneration Factor (R)
2.3. Greenhouse gas comparisons
Greenhouse gas emissions other than CO2 from bus transport (mainly minor amounts of nitrous oxide and methane) can be converted to CO2-equivalent emissions, and so allow direct comparisons to be made. However methane has a short atmospheric half-life, so the period over which climate forcing needs to be integrated should be specified when determining CO2-equivalent. However, given given both the minor effect of non-CO2 gases in transport emissions, and data availability problems, only CO2 comparisons have been made.
For diesel, we have used the IEA data (IEA 2015b) of 20 kg C per GJ of diesel fuel. As with primary energy, 16% has been added to the diesel value to allow for upstream CO2 costs. Using the same IEA data set (IEA 2015b) and the Department of Industry and Science (2015) energy statistics, CO2 emissions for each MJ of Australian electricity used were calculated as 0.087 kg (Table 1).
Figure 3 shows the resulting emission curves as average route speed increases. Since for both diesel and EV buses, emissions per km are a simple multiple of FCC energy per km, the curves are similar in shape to those of Figure 1. However, because of the relatively higher emission factor for grid electricity compared with diesel (Table 1), differences between the vehicles are less than that seen for FFC energy. At low average speed, the diesel bus shows a slight advantage over the EV bus, but as average speed increases, the diesel bus produces relatively higher levels of CO2. Importantly, there appears to be very little difference in the minimum CO2 between the two vehicle types, although the minimum occurs at lower average speed for the diesel bus than EV bus. For example, at N=2, minimum CO2 emissions occurs at 18.2km/h for the EV bus, and 16.1 km/h for the diesel bus.
Figure 3: Variation of FCC CO2 (kg CO2/km) with average speed for different numbers of stops/km (N)
3. Air pollution comparisons
Fine particulate emissions from diesel vehicles are increasingly recognised as a serious health problem. In France, which has the highest percentage of diesel-powered road vehicles in Europe, the Mayor of Paris announced in late 2014 that diesel cars would be banned from Paris after 2020 (Penketh, 2014). Lawal et al. (2016, p.92) reported that ‘Epidemiological studies have shown a consistent positive correlation between exposure to particulate matter (PM) and increased mortality largely due to increased rates of cardiovascular morbidity and mortality.’ Diesel vehicles are important contributors to PM in cities. In the European Union, the ‘enhanced environment-friendly vehicle’ (EEV) directive (European Union 2005) requires diesel vehicles to meet limits of 2.0 g/kWh for NOx and 0.02 g/kWh for fine particulate emissions.
Electric vehicles produce no particulates in city streets from combustion (although brake linings and tyres on both vehicle types also contribute some particulates). Even particulate emissions from fossil fuel power stations are largely controlled by electrostatic precipitation. The stop-start driving conditions for urban diesel buses aggravate their particulate emissions. As expected, per km of route, inner city diesel buses emit more particulate emissions than outer city diesel buses, because emissions are disproportionately produced during the acceleration phase. A further problem for such buses is that, per km travelled, their particulate pollution affects far more people than would an outer area route bus—the result of both higher residential and day-time workforce population densities.
4. Noise pollution comparisons – external to vehicle
Noise pollution is now recognised as a major health problem in cities, and one that is far harder to solve than urban air pollution. Münzel et al. (2014, p. 829) reported that noise not only causes annoyance and disrupts sleep, but can have more serious consequences, especially ‘increased incidence of arterial hypertension, myocardial infarction, and stroke.’ It is not necessarily the sound intensity (measured in dB) as such—loud natural sounds from birds, for example, do not appear to stress people, in contrast to vehicular or industrial noise (Bohannon, 2008).
Ross Staiano (2007, p. 2) compared the noise levels generated by diesel and electric buses for a transit project in Maryland, USA. For electric buses, noise contributors include ‘electric motors, auxiliary equipment such as cooling fans and air conditioning and the tire/pavement interaction.’ For diesel buses noise sources include ‘the exhaust system, radiation of the engine block, the cooling system (especially fans), air intake components and tire/pavement-interaction noise.’ As might be expected from these lists, electric buses were much quieter than diesel buses. The authors (p. 1) reported that ‘buses with purely electric propulsion systems were found to have noise impacts extending only about one-third the distance as those for conventional diesel buses.’
The noise pollution benefits for electric buses were, however, dependent on bus speeds. At speeds above about 65 km/h, ‘noise levels of all technology buses are expected to be relatively similar as noise from the tire/pavement-interaction begins to dominate emissions’ (Ross & Staiano 2007, p. 7). In the inner area route, where speeds are low, engine noise will dominate for diesel buses, giving electric buses a clear advantage. Road noise caused by tyre-road interaction becomes more significant as speed increases. The throttle of a diesel bus under acceleration increases noise, until a cruising throttle is reached at which point engine noise decreases and road noise becomes more apparent. Road noise is similar for diesel and electric drive, with only small decreases predicted in a more lightweight electric bus (Fridman et al. 2013). At 0 km/h the noise difference between diesel and electric buses is also expected to be significant, as diesel buses typically operate at an idle of some 600 rpm. As with air pollution, a given noise level will impact far more people in the more densely populated inner area of the city.