Hydrogen Buses: Modelling and Analysing Suitability from an Operator and Environmental Perspective

Doyle, D., Harris, A., Douglas, L., Early, J., & Best, R. (2020). Hydrogen Fuel Cell Buses: Modelling and Analysing Suitability from an Operator and Environmental Perspective. In 2020-04-14 Hydrogen Fuel Cell Buses: Modelling and Analysing Suitability from an Operational and Environmental Perspective 2020-01-1172 (Technical Papers ). SAE International. https://www.sae.org/publications/technical-papers/content/2020-01- 1172/ Published in: 2020-04-14 Hydrogen Fuel Cell Buses: Modelling and Analysing Suitability from an Operational and Environmental Perspective 2020-01-1172

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Abstract 2030 [2]. In the UK, Aberdeen City Council are working towards a hydrogen economy through the H2 Aberdeen initiative, who join London in placing orders for the world’s first double-deck FCEBs to In response to the need to decrease greenhouse gas emissions, the add to their existing single-deck fleet [3–5]. Chile and Norway are current trend in the transport sector is a greater focus on alternative pursuing the production of hydrogen from renewable sources, with powertrains. More recent air quality concerns have seen controlled some predictions for future prices reaching as low as $1.60/kg for zero emission zones within urban areas. As a result, there is growing Chile and $2.20/kg for Norway [6]. Australia and Saudi Arabia have interest in hydrogen fuel cell electric buses (FCEBs) as a zero local also been identified as potential producers [6]. emission vehicle with superior range, operational flexibility and refuelling time than other clean alternatives e.g. battery electric buses (BEBs). This paper details the performance and suitability analysis of The objective of this paper is to initially establish the modelled a proposed FCEB, using a quasistatic backwards-facing capabilities of a future production Wrightbus FCEB with recent fuel Simulink powertrain model. The model is validated against existing cell and battery technology. Next, the suitability of the FCEB and prototype vehicle data (Mk1), allowing it to be further leveraged for corresponding BEB as local zero emission options are compared, predictions of an advanced future production vehicle (Mk2) with next along with a comparison of single and double-deck variants. Case generation motors and fuel cell stack. The modelled outputs are used studies from the UK and Chile are considered. The paper will for a comparison of the FCEB performance to an equivalent BEB on compare the vehicles from two perspectives: industry standard drive cycles, as well as several real bus routes generated through data logging activities. The suitability of FCEB vs 1. The bus operator – considering vehicle range, passenger BEB from an operator usage perspective is thus analysed in different carrying capabilities, operational cost/efficiency and level use cases, with case studies from the UK and Chile. Both single-deck of service. and double-deck vehicle types are considered. Modelled FCEB and 2. Environmental – considering a well-to-wheel assessment BEB outputs are further utilised in a comparative well-to-wheel based on geographical location and fuel production method. assessment, highlighting that the relative suitability of each from an environmental perspective is sensitive to geographical and fuel Mahmoud et al [7] comprehensively review literature on alternative production method pathways. The paper concludes with a discussion powertrain buses considering operational, environmental and of the environmental and societal benefits of deploying hydrogen economic factors. From an economic perspective, the total cost of buses to reduce local health damaging pollutants and alleviate energy ownership of FCEBs and BEBs was widely uncertain in literature. security concerns via the introduction of a feasible and sustainable Figures used throughout the review are taken from other studies transport alternative. (simulation or operational data) and often averaged for fairness. Whilst overall fair, it may not represent a direct comparison in all Introduction circumstances, depending on the type of study used. In addition, advances in relevant technology since the review may set new benchmarks influencing the operational and environmental aspects of Global commitments to decrease greenhouse gas emissions have led the study. to a shift to alternative powertrains in the transport sector. In addition to this, stricter controls on air quality within cities has seen the More recently, Correa et al [8] compare alternative powertrain buses introduction of zero emission zones, requiring vehicles with full zero from an energy and environmental perspective using characteristic emission capabilities. As a result, there is growing interest in vehicle parameters from the simulation software, ADVISOR. For the hydrogen fuel cell electric buses (FCEBs) as a zero local emission drive cycles considered, the road is assumed to be flat. Several recent vehicle with superior range, operational flexibility and refuelling time studies consider an environmental assessment of FCEBs, for than other clean alternatives e.g. battery electric buses (BEBs). This operation in Taiwan and the United States [9,10]. Neither study also is illustrated in increased investment through projects such as analyses BEBs as a comparative zero emission vehicle and also do JIVE/JIVE2, which are deploying nearly 300 FCEBs and refuelling not consider operational requirements. infrastructure in Europe by the early 2020s [1]. Currently, FCEBs and BEBs make up 2 % and 19 % of the European bus market respectively, with those figures predicted to rise to 10 % and 52 % by This paper adds to the current literature firstly by demonstrating the capabilities of a future production FCEB availing of the most recent Page 1 of 11

01/11/2016 fuel cell and battery technologies available. Technology in this area is Table 1. Vehicle parameters. developing rapidly, and this work will model predicted performance with state-of-the-art production components. Working with FCEB (Mk2) BEB Wrightbus to develop a model which is representative of a production intent vehicle powertrain avoids using generic parameters. This work Singe- Double- Single- Double- is of a similar process to existing work at Queen’s University Belfast deck deck deck deck in conjunction with Wrightbus [11–15]. All powertrain parameters and control logic are known and implemented. Additionally, the GVW (kg) 18,000 19,500 18,000 19,500 hydrogen consumption of the model is validated with a prototype vehicle. Passenger Capacity 72 100 72 100

Whilst many previous studies investigate the environmental impacts Battery Capacity 53.96 339.18 of FCEBs and compare with BEBs, fewer also incorporate an (kWh) operational perspective. This work considers both, and is the first opportunity for a comparison of the two vehicle types designed with Hydrogen Capacity production intent on a common, modular platform with shared @350 bar and 15ºC 30.88 26.78 N/A N/A components. Adding to this, the difference between single and (kg) double-decks is considered. Real bus routes are simulated for which vehicle utilisation is known, to allow consideration of the level of service possible. Finally, the use of real bus routes allows the influence of the road gradient on the overall consumption to be Modelling accounted for. The vehicles are modelled as quasi-static backwards-facing Methodology powertrain models within Simulink/MATLAB. Drive cycles consisting of velocity and elevation profiles form the model input, This section gives an overview of the vehicles modelled, modelling with power-flow working backwards from the required method, validation efforts and the drive cycles considered. velocity/elevation change at each timestep to the battery. Lateral motion was neglected. This modelling methodology neglects Two different local zero emission bus technologies are considered: transients and thus cannot be used for detailed vehicle control and FCEBs and BEBs. For each, single-deck and double-deck options are response, but gives a good prediction of macro quantities such as modelled. The first model is of a prototype Wrightbus FCEB (Mk1) energy and fuel usage. Component specifications and efficiency maps [17], for which detailed vehicle parameters and driving test data are implemented based on OEM datasheets and laboratory test data exists for validation. This validated model is then leveraged to predict where available. Figures 1 and 2 show the vehicle architecture for the the performance of an intended future production FCEB (Mk2), FCEB and BEB. Both are identical with the exception of the fuel cell which is used for the comparative analysis to the BEB. Mk2 avails of stack, and an associated brake resistor to dissipate excess current next generation technology compared to Mk1, available for during significant regenerative braking events. production use now. As well as a different tractive motors, a new fuel cell offering improvements in size, reliability and total cost of ownership is used [18]. Battery chemistry changes from lithium- titanate (LTO) to lithium-nickel-manganese-cobalt-oxide (NMC) which is less expensive and offers a higher specific energy. Changes Fuel Cell to the power electronics mean that despite the decrease in specific Brake power of the NMC cells, the charging current limit of the battery DC/DC Resistor

increases, allowing greater energy recuperation through regenerative

braking.

Battery Inverter Motor

Diff Axle/ The BEB is an overnight charging variant from the same manufacturer and based on the same 10.6 m platform as the FCEB DC/DC Mk2. Thus, the motor, axle, chassis and body are consistent between vehicles. The battery also uses the same NMC cells as FCEB Mk2, with a larger quantity to provide the additional capacity required in Auxiliaries the absence of a . The increase in battery mass from FCEB to BEB is offset by the mass of the fuel cell, support structure Figure 1. FCEB model schematic. and auxiliary systems, as well as the hydrogen tanks, associated fittings and enclosures/crash-worthy structure. The commonality of components in each vehicle provide an opportunity to compare FCEBs and BEBs consistently and fairly. Table 1 lists the vehicle parameters used throughout. As the Mk1 FCEB is used only for model validation and not for the comparative analysis, it has been omitted.

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01/11/2016 results, allowing them to be simulated within the model [19]. Table 3 compares the actual LEB test results for BEBs from other

manufacturers with the values predicted by the model. Note that the

simulated values represent predictions for the modelled Wrightbus Battery Inverter Motor

Diff BEB during an LUB drive cycle at the same vehicle mass as the LEB Axle/ test vehicle. Thus, they are not intended to validate the Wrigthbus DC/DC BEB model specifically, but indicate that it is giving results in line with other BEBs. The two results show different levels of accuracy, which is to be expected as the powertrain parameters were not known Auxiliaries and were thus simulated with the Wrighbus BEB powertrain. It is likely that when the modelled BEB performs the same test the accuracy will increase as the assumptions are reduced. Figure 2. BEB model schematic.

Table 3. BEB model predicted energy usage compared to measured usage Validation during LEB accreditation for other BEBs.

Extensive validation of the models is difficult for several reasons. LEB Test Model Test Mass Energy Energy The BEB and FCEB Mk2 are future production vehicles and Vehicle Error therefore there is no physical vehicle for testing. FCEB Mk1 is a (kg) Usage Usage physical prototype of a double-decker, however only limited test data (kWh/km) (kWh/km) is available. Furthermore, the added uncertainty of the HVAC power 7900 usage during testing must be eliminated for validation of the model, 13,471 0.847 0.789 -6.85 % and thus only test data with HVAC turned off can be used. EV Quantifying the HVAC load adds uncertainty as it is dependent on BYD eBUs the weather during the test (ambient temperature, level of sunlight 13,868 0.831 0.802 -3.50 % and humidity), initial cabin temperature, bus velocity profile, 12m passenger loading/unloading and the frequency and amount of time the bus doors were open; amongst other factors. This leaves one test run for the validation of predicted macro quantities with the known condition of zero HVAC load, listed in Table 2. This test can easily Drive Cycles be repeated for future tests as the route (and all stops) is known, and the velocity profile of the test route was recorded. Repeatability of Several different drive cycles are used in this paper to illustrate the the test can be achieved through either following the same route or performance of the FCEB under various conditions. The first set of recreation on a rolling road. drive cycles are industry standard and act as a performance benchmark to demonstrate the capabilities of the FCEB and BEB. Table 2. FCEB Mk1 model predicted hydrogen consumption compared to The first is the LUB cycle used in the validation section of this report, measured consumption during a test. which is the test for the LEB scheme. The LUB cycle is a modification of the previous standard cycle (MLTB) to include a FCEB Mk1 FCEB Mk1 rural phase in addition to the outer/inner city phases, typical of UK Model Error Test operation [20]. Next are the three synthetic “Standardised On-Road Prediction Test” (SORT) cycles developed by the UITP Bus Committee specifically for comparing energy consumption between different Hydrogen Usage 6.58 6.68 +1.52 % buses [21]. SORT 1,2 and 3 represent heavy urban, urban and (kg/100km) suburban respectively.

In addition to this, four drive cycles have been created from data logging of real bus routes to indicate real-world performance. The Although the result is promising, it is only a validation of one route drive cycles consist of both the outbound and return journeys, without HVAC, and only for the FCEB Mk1 vehicle. However, it is therefore capturing the full characteristics of the route. Two relatively assumed that because the FCEB Mk2 model is derived from Mk1, flat urban routes from London (UK), a moderately hilly suburban that it will be similarly accurate. Additionally, the commonality of route from Santiago (Chile), and a steep ascent then descent in components and the use of the same modelling techniques throughout Rancagua (Chile). This final route serves as transport for workers to the models minimises the opportunity for variation. Creation of the and from a mine with an elevation change of almost 1500 m, and FCEB Mk1 model was for validation purposes only. For the therefore represents a very challenging route for zero emissions remainder of this paper, any values attributed to the FCEB refer to capable vehicles. Table 4 lists the drive cycles with some metrics to the Mk2 vehicle, as it is on the same platform at the BEB. characterise them. Relative positive acceleration (RPA) is a measure of the power required for all accelerations (ignoring decelerations) in Although there is no physical BEB for validation, the Office for Low the drive cycle, normalised by the cycle length [22]. It is essentially a Emission Vehicles/Low Carbon Vehicle Partnership in the UK measure of the aggressiveness of acceleration, and influences operates a Low Emission Bus (LEB) scheme offering financial fuel/energy consumption. Relative slope is the elevation range incentives against the purchase of LEB accredited vehicles. Vehicles (maximum – minimum) normalised by the cycle length. Trips per day undergo a specified drive cycle (LUB cycle) and their emissions is a record of how many times the buses used for data logging were performance compared to a baseline set by diesel buses. Test required to complete the route in one day to achieve the desired level certificates are published online along with the test conditions and of service. Page 3 of 11

01/11/2016 Table 4. Summary of drive cycles created from data logging of real bus To allow a direct comparison of the FCEB and BEB, the routes. consumption values in Table 5 are converted to a maximum range for each vehicle and drive cycle (plotted in Figure 3), taking in to Average Relative Stops per account the percentage of the battery capacity which is not used for RPA (m/s) Speed Slope km battery life considerations. (km/h)

Urban - 0.257 0.649e-3 19.2 2.76 London 1

Urban - 0.214 1.09e-3 14.7 4.46 London 2

Suburban - 0.185 1.95e-3 25.6 2.56 Santiago

Hilly - 0.111 15.9e-3 28.3 0.110 Rancagua

Results & Discussion

This section details the modelled vehicle performance on industry standard drive cycles, followed by a comparison of each vehicle operating on real bus routes from both an operator and environmental perspective. Each of the industry standard and the real bus routes from Table 4 were simulated in the vehicle models. For each simulation, the vehicle was assumed to be at 50 % of it’s passenger capacity, and with no HVAC power requirement due to the difficulties in accurately quantifying it. All per-passenger values in this paper treat the vehicle as half-laden. Note that all figures given Figure 3. Predicted vehicle range on four industry standard bus routes. are predicted values from modelling efforts and may differ from actual fuel/energy consumption. Real Bus Routes – Operational Perspective

Industry Standard Drive Cycles Table 6 shows the simulated hydrogen/energy usage per km for each vehicle whilst driving on the four real bus routes. As a better Table 5 shows the simulated hydrogen/energy usage per km for each representation of real-world conditions than the previous drive vehicle whilst driving on the four industry standard drive cycles. The cycles, these are the results which will form the analysis for the results shown here are for demonstration of the vehicle’s capabilities remainder of the paper. under known operating conditions. Table 6. FCEB and BEB hydrogen/energy usage for four real bus routes. Table 5. FCEB and BEB hydrogen/energy usage for four industry standard drive cycles. FCEB Hydrogen Usage BEB Energy Usage

(kg/100km) (kWh/km) FCEB Hydrogen Usage BEB Energy Usage

(kg/100km) (kWh/km) Single- Double- Double- Singe-deck deck deck deck Single- Double- Double- Singe-deck deck deck deck London 1 6.95 7.49 1.08 1.12

LUB 5.63 6.03 0.891 0.928 London 2 6.11 6.30 0.948 0.988

SORT 1 6.30 6.57 0.985 1.03 Santiago 5.87 6.15 0.880 0.945

SORT 2 5.67 6.13 0.905 0.955 Rancagua 6.69 6.96 0.844 0.879

SORT 3 5.38 5.94 0.863 0.915 Average 6.41 6.73 0.938 0.983

Average 5.74 6.17 0.911 0.956

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01/11/2016

Efficiency & Operating Cost

Naturally, the single-deck variants of each vehicle consume less hydrogen/energy per km than their equivalent double-decks due to their smaller mass and reduced drag – around 5 % on average. However, when expressed on a per-passenger basis, this trend is reversed. The FCEB single-deck is on average 32 % less efficient per passenger-km then the double-deck. Similarly, the BEB single-deck is 33 % less efficient than the double-deck.

Using the average per-passenger values in Table 6, the sensitivity of the vehicle operating cost to the price of hydrogen/electricity can be determined. Figure 4 shows the cost delta incurred per 100- passenger-km by using a double-deck FCEB over a BEB. The cost delta is plotted as a function of the hydrogen and charging electricity prices (positive values indicate that the FCEB is more expensive to operate). Figure 5 shows the same analysis for the single-deck variant. Both variants show an operational cost premium for FCEBs under current circumstances, with California highlighted on the graph for reference [23,24] . However, as hydrogen production increases, costs will go down. If costs reach the levels predicted in [6] FCEB operational costs could reach parity to BEBs. Note that the operational cost premium of the double-deck FCEB is less sensitive to the price of hydrogen than the single-deck. Figure 5. Single-deck FCEB vs BEB: operational cost delta per passenger-km as a function of hydrogen/electricity price.

Range & Passenger Carrying Capability

As in the previous section, the consumption values in Table 6 are converted to a maximum range for each vehicle and drive cycle and plotted in Figure 6.

Figure 4. Double-deck FCEB vs BEB: operational cost delta per passenger- km as a function of hydrogen/electricity price.

Figure 6. Predicted vehicle range on four real bus routes.

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01/11/2016 As expected, for both vehicles the single-deck variant has a larger circumstances. For three of the four real bus routes logged for this average range than the double-deck (21 % for FCEB and 5 % for work, the frequency of operation per bus is known. The buses used BEB) due to the lesser mass of the single-deck. were diesel hybrids, and thus the frequency of their operation will be representative of normal levels of service for that route. Service Whilst carrying the same number of passengers as the BEB, the information is listed in Table 7. FCEB on average has a 90 % larger range for the single-deck, and 65 % larger for double-deck. Table 7. Total time and distance covered per route each day.

The FCEB and BEB expected range follows the same trend for the Total Trips per day Total Time first three drive cycles but diverge for the Rancagua route. Despite Distance (km) this route being an intensive hill ascent, it is the route with the largest range for the BEB whilst being second to worst case for the FCEB. London 1 7 23:02:30 215 This is a result of the much larger battery in the BEB which can accept larger currents than the FCEB. Thus, the regenerative braking London 2 7 13:59:04 173 energy recuperation potential is higher and the BEB can take better advantage of the steep descent on the return journey of the drive Santiago 6 17:36:00 300 cycle. The FCEB must activate the brake resistor or use a greater proportion of friction brakes to avoid overloading the battery. Although BEBs have a smaller energy storage on board for the ascent, in severe ascent/descent drive cycles this can be compensated for with an increase in efficiency from regenerative braking. Figure 8 shows how the modelled vehicles compare to the current level of service, in terms of the number of full trips they are capable Although the single-deck variants have a greater absolute range, this of before running out of charge or fuel. On all three routes, both trend is again reversed when factoring in the number of passengers. variants of the FCEB can comfortably provide the same level of Looking at the range in passenger-km (Figure 7), the single-deck service as with the existing diesel fleet with no additional vehicles performs worse (13 % for FCEB and 25 % for BEB). Thus, despite needed. The quick refuelling time of around 10 minutes [7] is useful the double-deck vehicles having a smaller absolute range, they are for quick turnaround times on routes with a high degree of vehicle more range efficient considering passenger-km. The trend is less utilisation. pronounced for the FCEB as the single-deck variant has increased hydrogen storage capacity.

Figure 8. Number of trips possible for each vehicle on three real bus routes, Figure 7. Predicted vehicle range on four real bus routes, normalised by compared to the number of trips required for parity with existing service passenger loading. levels.

Level of Service The BEB is only capable of meeting the required range on the “London 2” cycle, indicating that in terms of maintaining service levels, BEBs are best suited to lighter duty/shorter routes. It is close Ideally for operators, new clean vehicle technologies would be able to providing parity of service on “London 1”, however factoring in to replace their existing fleet of diesel vehicles without any change to the distance to the bus depot, future battery degradation and a factor the level of service provided or fleet size required. Due to the range of safety it is unable to provide the 7 trips without topping up the and infrastructure restrictions, this is unlikely to be the case in most Page 6 of 11

01/11/2016 battery. Additionally, the bus used for the “London 1” route has a GHG emissions per passenger-km for the real-world UK and Chile very high degree of utilisation, not leaving enough time for overnight based drive cycles have been modelled using a WTW approach, with charging. Based on a depot charging rate of 100 kW [25], a full GHG emissions are quantified as a carbon dioxide equivalent value, charge of the BEBs considered in this study would take 2 hours and CO2e (Figures 9 & 10). The impact that the CO2e intensity of the 22 minutes leaving it unsuitable for routes with such high utilisation. electricity supply has on both WTT hydrogen emissions from WE These results are in line with the International Renewable Energy and subsequent GHG emissions per passenger-km is demonstrated. Agency who state that fuel cells are most suited to heavy duty Assumptions used in this calculation are detailed in the Appendix. In vehicles, long distances and high utilisation, while the opposite is true the interest of a fair comparison, the WTW impacts of BEB for the BEB [26]. Many larger buses fall in to both categories by technologies are also displayed. Both Figures 9 & 10 show that from having shorter distances than most heavy-duty applications, but the WTW perspective, FCEBs have a higher environmental burden potentially higher utilisation. than their BEB counterparts if hydrogen produced by WE comes from the same electricity grid that charges the BEBs. On a per The large range and low refuelling times of FCEBs result in the most passenger-km basis, GHGs from double deck vehicles are lower than attractive option for replacing a diesel fleet with the goal of their single deck counterparts. Notably, hydrogen produced via SMR minimising disruption to current service levels. In many cases, to has a lower GHGs than hydrogen produced via WE using each maintain service levels with a fleet of overnight charging BEBs will countries’ current electricity grid. necessitate a surplus of vehicles at an increased capital expenditure. A full total cost of ownership assessment is required to determine if These results are intended to illustrate various environmental impact the additional BEBs required outweighs the increased operating and ranges of low emission technologies and does not mean hydrogen as capital costs of FCEBs. a transport fuel should be discarded. For instance, to create a new export commodity, Chile is pursuing a clean hydrogen economy. The introduction of either FCEBs or BEBs as a local zero emission Chile aims to leverage abundant solar electricity resources to produce option will require the addition of supporting infrastructure. The cost low cost hydrogen, enabling the transition to lower carbon fuels and and availability in providing this will also factor into the individual diversifying the economy [6]. Electricity generated via sustainable or operator’s decision on which technology to adopt. Opportunity renewable sources combined with zero tailpipe emission technologies charging BEBs offer another alternative, however these have major is seen as a clear pathway for reducing the environmental burden of infrastructure modification requirements [7]. transport and increasing energy security [30]. For locations with an unreliable or high emission factor grid electricity supply, the prospect Environmental Perspective of importing clean hydrogen for an FCEB fleet offers the opportunity of reliable low overall emissions where BEBs could not.

Reducing greenhouse gas (GHG) emissions and improving local air 2000

quality are key motivations behind the uptake of alternative 0 H2 via WE emission factor (gCO2e/kgH2) Axis Axis powertrain technologies [27]. Operators seeking to identify suitable Title 0 9814 19628 29442 39256 49070 low emission powertrains often compare vehicle emissions using life 100

cycle assessment (LCA) methods. Well-to-wheels (WTW) analysis is

e e

e e 2 90 2

seen as the dominant LCA method for comparing alternative vehicle

e/pkm) 2 technologies and is widely used for policy support in road transport CO [28]. WTW analysis focuses on the life cycle processes of the energy 80

carrier (e.g. diesel, hydrogen, electricity) used to propel the vehicle intensity

70 km (gCO km

during operation. The WTW phase can be further broken down into CO grid UK - the well-to-tank (WTT) and tank-to-wheel (TTW) phases. The TTW intensity 60 stage considers the on-board energy conversion to drive the vehicle based on the lifetime distance travelled, fuel energy required and electricity Wind

vehicle efficiency [29]. The vehicles assessed in this paper produce 50 via SMR via

zero TTW air quality or GHG emissions.

40 2 H

The WTT stage includes the recovery or production of the feedstock 30 for the energy carrier and subsequent energy conversion, delivery, and storage. The production of the energy carrier as a 20 transport fuel will require a primary energy input, either from a fossil fuel feedstock or a renewable energy source, and result in the direct 10 or indirect production of GHG emissions. Vehicles powered by electricity or hydrogen produced via fossil fuel sources could have 0

the unintended consequence of producing higher emissions from a passenger per emissions GHG WTW 0 200 400 600 800 1000 WTW perspective than internal combustion engine counterparts [30]. Electricity grid emission factor (gCO2e/kWh) London #1-BEB-SD London #1-BEB-DD Hydrogen is typically produced from one of two methods: steam London #1-FCEB-SD London #1-FCEB-DD methane reforming (SMR) and the electrolysis of water (WE). 48% London #2-BEB-SD London #2-BEB-DD London #2-FCEB-SD London #2-FCEB-DD of global hydrogen sources are produced via SMR, WE accounts for 4% and the rest is a result of fossil oil and coal gasification [26]. For WE, the energy demand required to split water is high and it is Figure 9. WTW GHG emissions sensitivities of UK based cycles. therefore imperative to use electricity generated from renewable sources to produce low WTW emissions.

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01/11/2016 2000 H via WE emission factor (gCO e/kgH ) 4. Whilst carrying the same number of passengers as the BEB, the

0 2 2 2 FCEB had a 90 % larger range for the single-deck, and 65 % Axis Axis Title 0 9814 19628 29442 39256 49070 larger for double-deck. 100 5. For the routes considered, only the FCEB had the required range

to maintain current levels of service even on longer routes with e e

90 2 higher vehicle utilisation. The BEB was capable on one route

e/pkm) 2

indicating suitability for light duty/shorter routes. These findings

80 intensity e

2 agree with the International Renewable Energy Agency [26].

grid grid CO

70 CO

km (gCO km intensity

- A WTW analysis based on the modelled outputs for the four real bus

60 routes (from Chile and the UK) showed:

Chile SIC Chile Solar PV PV Solar 50 1. On a per passenger-km basis, GHGs from double deck vehicles

are lower than their single deck counterparts

via SMR via

40 2 2. FCEBs have a higher environmental burden than their BEB H counterparts if hydrogen produced by water electrolysis comes 30 from the same electricity grid that charges the BEBs. For locations with a high emission factor grid, importation of 20 hydrogen from a renewable source can mitigate this.

10 The optimum local zero emission bus depends on many factors and is specific to each situation. Consideration should be taken to whether 0 current levels of service can be met, and whether the additional

WTW GHG emissions per passenger per emissions GHG WTW 0 200 400 600 800 1000 capital cost of a FCEB and current higher operating costs outweigh Electricity grid emission factor (gCO2e/kWh) the potentially larger fleet of BEBs required for parity of service. Santiago-BEB-SD Santiago-BEB-DD Santiago-FCEB-SD Santiago-FCEB-DD Environmental impacts are highly sensitive to geographical location Rancagua-BEB-SD Rancagua-BEB-DD and the emission factor of hydrogen/grid electricity supply. A future Rancagua-FCEB-SD Rancagua-FCEB-DD work recommendation to help quantify the above considerations is a full life cycle assessment including total cost of ownership, Figure 10. WTW GHG emissions sensitivities of Chile based cycles. considering the modelled outputs discussed here.

Summary References

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PV Photovoltaic Contact Information RPA Relative positive Sir William Wright Technology Centre, 50 Malone Road, Queen’s acceleration University Belfast, Belfast, BT9 5BS, United Kingdom SIC Sistema Interconectado email: [email protected] Central

Acknowledgements SMR Steam methane reforming SORT Standardised on road test The authors would like to thank and acknowledge Wrights Group Ltd., as well as the Engineering and Physical Sciences Research Council Grant EP/S036695/1, for funding this research. TTW Tank-to-wheels

Definitions/Abbreviations WE Water electrolysis WTT Well-to-tank BEB Battery electric bus WTW Well-to-wheels CO2e Carbon dioxide equivalent

FCEB Fuel cell electric bus

GHG Greenhouse gas

HVAC Heating, ventilation and cooling

LCA Life cycle assessment

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01/11/2016 Appendix

For the WTW emission calculations, the following assumptions are used:

• Electricity demand per kg hydrogen produced via water electrolysis is calculated as 49.07 kWh/kg. This assumes a lower heating value of hydrogen of 120.1 MJ/kg [31], an electrolysis process efficiency of 70% [32] and a compression efficiency of 97% [31]. Values are assumed the same regardless of geographical location. • Emission factor of hydrogen produced via steam methane reforming is based on average EU conditions [31], calculated as 13.8 kgCO2e/kgH2. • UK grid emission factor calculated as 316 gCO2e/kWh [33]. Chilean Sistema Interconectado Central (SIC) grid emission factor is taken as 336 gCO2e/kWh [34]. The SIC grid provides electricity to the central part of the country, including Santiago and Rancagua. Both emission factors are based on latest publicly available values. • Solar photovoltaic (PV) and wind based electricity emission factors are median life cycle GHG emissions values reported by selected electricity supply technologies by the Intergovernmental Panel on Climate Change [35]. Values are assumed the same regardless of geographical location.

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