This section describes the fuel/technology combinations analyzed and the major cost assumptions used in the base case analysis for each; the sources of all major assumptions are noted.
Many of the cost assumptions used in this analysis are based on data reported by the National Renewable Energy Laboratory's (NREL) Advanced Vehicle Testing Activity. Seven recent NREL reports were reviewed, which covered three fuel cell bus deployments, two diesel hybrid-electric bus deployments, and two natural gas bus deployments. Other assumptions are based on data reported in the Federal Transit Administration's National Transit Database, and discussions with vehicle and technology manufacturers and transit maintenance managers.
The five fuel/technology combinations analyzed here represent the most common existing and emerging options for powering U.S. transit buses. Currently approximately 82% of U.S. transit buses are powered by diesel engines and 15% are powered by natural gas engines2. Hybrid-electric drive is also growing in popularity as an alternative to standard propulsion for buses, with over 1,600 diesel hybrid buses in service in 2007 and almost 900 more on order3.
Fuel cells are an emerging technology for buses. To date only small scale demonstration fleets have been put into service, and there are currently eight fuel cell transit buses operating in California and Connecticut4.
The five fuel/technology combinations chosen for analysis do not represent the only options currently in service or under development. They were chosen to be illustrative of available options and to demonstrate the utility of the life cycle cost model used. Other fuel/technology combinations that could have been analyzed using the model include gasoline hybrid-electric propulsion, and internal combustion engines operating on hydrogen fuel.
Table 2 shows the major elements of the propulsion system assumed to be included on each of the bus types analyzed. All other bus systems are assumed to be identical.
Both the Diesel and Diesel Hybrid buses are assumed to operate on standard on-highway diesel fuel, which since late 2006 has been "ultra-low sulfur diesel" (ULSD) with less than 15 parts per million sulfur.
CNG buses are assumed to operate on natural gas which is delivered to and stored on the vehicle in compressed form at maximum pressures of 3,600 pounds per square inch (standard in the transit industry).
The engines used in the Diesel, Diesel Hybrid, and CNG buses are assumed to be compliant with 2007 EPA emissions standards for new heavy-heavy duty engines.
Both Fuel Cell and Fuel Cell Hybrid buses are assumed to operate on hydrogen gas which is delivered to and stored on the vehicle in compressed form at maximum pressures of 5,000 pounds per square inch (standard for current fuel cell buses).
The following describes the sources of the major cost assumptions used in the analysis for each fuel/technology combination.
For this analysis buses are assumed to be assigned to a notional 100-bus depot facility, which is a typical size for many U.S. transit operations. To maximize necessary depot and fueling investments it is assumed that all buses assigned to the depot will be of the same type.
Depot personnel assignments for a 100-bus depot are assumed to be as follows:
Note that in the model these personnel assignment numbers are only used to calculate training costs.
Bus mechanics are assumed to have a fully-loaded labor rate of $50/hour. This is consistent with the data used to determine average bus maintenance costs, as discussed in Section 1.2.2 below. Bus operators are also assumed to have a fully-loaded labor rate of $50/hour and managers are assumed to have a fully-loaded labor rate of $75/hour.
The assumptions used in this analysis for diesel fuel and natural gas commodity costs were taken from the U.S. Department of Energy's Clean Cities Alternative Fuel Price Report for March 2007. That report shows that in March 2007 the average price of diesel fuel at 333 public gas stations surveyed was $2.63/gallon (and it ranged from an average of $2.48/gallon on the Gulf Coast to $2.96/gallon on the West Coast). Compressed natural gas was also sold at 123 of the same stations, and it's price averaged $2.17/diesel-equivalent gallon (ranging from $1.56/DEG in the mid-west to $2.83/DEG in New England).
Three of four U.S. transit agencies currently operating fuel cell buses report that the cost of producing and delivering compressed hydrogen to their buses ranges from $4.26/kg to $9.06/kg (see Table 4). This is equivalent to $4.81 - $10.23/DEG6. This analysis assumes that compressed hydrogen will cost $6.70/kg, or $7.57/DEG.
Capital Cost Share is assumed to be 80% for the federal government and 20% for a local match. This is typical for capital funding provided by the Federal Transit Administration.
Annual inflation is assumed to be 2.3% for fuel and 2.3% for labor and materials (including bus overhaul costs). This is in line with current market expectations for long-term inflation, as calculated by the difference in the yields of long-term nominal U.S. treasury notes and treasury inflation-protected securities (TIPS)7.
A 5% discount rate is used for net-present-value calculations. This includes the expected inflation noted above plus a 2.7% "real discount rate" to account for risk return on invested capital. This risk return value is equivalent to the current rate of return on treasury inflation-protected securities 8.
The analysis also assumes that no programmed overhauls will be performed within two years of retirement of any bus. This precludes the model from assuming that a major investment will be made in any bus just prior to retirement.
In this analysis the useful life for all buses is assumed to be 12 years. This is the minimum in-service age at which transit agencies which use federal funds for bus purchase can retire buses, per FTA rules, and is a standard widely used in the transit industry for planning and financial analysis.
To determine appropriate assumptions for annual mileage per bus, and average in-service speed, data on bus operations reported to the National Transit Database9 was analyzed. This data is summarized in Table 3. As shown, for over 42,000 buses operated by 374 U.S. transit agencies the average in-service speed in 2005 was 12.4 mph, and the average annual mileage was 32,602 miles per bus. These assumptions were used in the analysis for all bus types.
Assumptions about average fuel economy for Diesel and CNG buses were also taken from the NTD data. As shown in Table 3 predominantly diesel fleets (>75% of reported fuel use diesel) report significantly higher average fuel economy than predominantly CNG fleets (>75% of reported fuel use NG) - 3.2 MPG versus 2.4 MPG. The analysis used these average values for Diesel and CNG bus fuel economy. High and low values were entered as +/- 20% of these averages, to account for variability from fleet to fleet. For both predominantly diesel and predominantly NG fleets in the NTD database, average fuel economy data covering approximately 80% of reported buses is within +/-20% of the total fleet average. These assumptions are also in agreement with data reported by NREL for operations with similar average speed (~12 mph) see Table 4 and Table 5.
The model calculates basic annual bus maintenance costs based on $/mile cost factors for propulsion system-related and non-propulsion-related maintenance. To determine appropriate assumptions for these maintenance cost factors, and for Hybrid and Fuel Cell bus average fuel economy, seven NREL bus evaluation reports were reviewed. The data from these reports is summarized in Tables 4 and 5.
As shown in these tables non-propulsion related maintenance costs for most of the buses covered by these analyses ranged from $0.23 - $0.54/mile10. For this analysis we assumed that all buses would have non-propulsion related maintenance costs of $0.40/mile +/- $0.15/mile.
With the exception of both CNG and hybrid buses at NYCT total propulsion-related maintenance costs for diesel, natural gas, and hybrid buses in these studies ranged from $0.06 - $0.20/mile. A direct comparison of natural gas and hybrid bus costs to diesel bus costs at the same agency indicates that both natural gas and hybrid buses have the same, or only marginally higher, propulsion-related maintenance costs as diesel buses. For this study we assumed that diesel buses have propulsion-related maintenance costs of $0.15/mile +/- $0.05/mile. Both CNG and Hybrid buses were assumed to have propulsion-related maintenance costs $0.01/mile higher than diesel buses.
Propulsion-related maintenance costs reported by NREL for fuel cell buses were much more variable. At AC Transit reported $/mile costs for propulsion-related maintenance were actually lower for the fuel cell buses than for the comparison diesel buses, while at Sunline they were almost three times higher, and at VTA they were almost 12 times higher ($2.38/mile).
At both AC Transit and Sunline virtually all propulsion-related maintenance during the study period was done by the manufacturer under warranty and is not included in the reported costs. VTA took greater responsibility for fuel cell bus maintenance and their reported costs are likely more representative. Based on availability and reliability statistics for the AC Transit and Sunline fuel cell buses it is clear that they too required significantly more maintenance than the comparison diesel buses during the study period.
Despite requiring more maintenance the actual $/mile costs reported for VTA fuel cell buses are somewhat misleading because these buses only accumulated one fifth the mileage of the comparison diesel buses during the study period. For this analysis we used a conservative, forward-looking assumption of $1.00/mile +/- $0.25/mile for propulsion-related maintenance costs for both Fuel Cell and Fuel Cell Hybrid buses.
Assumptions about Diesel Hybrid, Fuel Cell, and Fuel Cell Hybrid fuel economy were also taken from the NREL data. As shown in Table 5 the Diesel Hybrid buses operated by KC Metro had 21 27% better fuel economy than the comparison diesel buses, on a duty cycle very similar to the one chosen for this analysis (~12.4 mph). The Diesel Hybrid buses operated by NYCT had even higher relative fuel economy (36% better than diesel and 88% better than CNG), but on a much slower duty cycle (6.2 6.5 mph) which is advantageous to hybrid buses. For this analysis we assumed that Diesel Hybrid buses will have 25% better fuel economy than Diesel buses.
As shown in Table 4 the Fuel Cell buses operated by VTA had 12% worse fuel economy than the comparison diesel buses (miles per diesel equivalent gallon, MPDEG); this is the assumption that was used for this analysis. As shown in Table 4 the Fuel Cell Hybrid buses operated by AC Transit had 55% better fuel economy (MPDEG) than the comparison diesel buses and the Fuel Cell Hybrid buses operated by Sunline had 149% better fuel economy than the comparison CNG buses . This analysis assumes that Fuel Cell Hybrid buses will get 60% better fuel economy than diesel buses and 112% better fuel economy than CNG buses. The fuel economy assumptions used in the analysis for all bus types are shown in Table 6.
The model calculates the cost of brake relines separately from base $/mile maintenance costs because hybrid propulsion systems have been shown to significantly extend brake reline intervals due to regenerative braking. In addition, CNG and Fuel cell buses are typically up to 25% heavier than diesel buses due to the greater weight of the gaseous fuel system and other components, which reduces reline intervals since the braking system needs to do more work to stop the bus.
Table 7 contains the values used in the analysis for front and rear reline interval, front and rear reline material cost, and front and rear reline labor hours for Diesel buses. These assumptions are based on an informal poll of maintenance staff at six transit agencies conducted by the author in 200411. For all other bus types the brake reline material costs and labor hours are assumed to be the same as for Diesel buses.
For CNG buses brake reline intervals are assumed to be 10% shorter (worse) than for Diesels due to the greater bus weight. For Fuel Cell buses brake reline intervals are assumed to be 15% shorter.
Given that significant numbers of hybrid buses have not been in service for more than a few years, hard data on brake life does not yet exist. However, anecdotal evidence from several maintenance managers with hybrid experience indicates that brake lining life on hybrids may be more than double brake lining life on conventional buses. This is consistent with in-use fuel economy results for hybrids. A 20% reduction in fuel use for a hybrid bus implies that the braking system is recapturing about half the energy normally dissipated in braking, and that therefore the braking system is only doing about half the work that it would on a conventional bus12, which implies that the bus should only require relines half as often. This analysis uses a conservative assumption of a 75% increase in reline interval for Diesel Hybrid buses and a 60% increase in reline interval for Fuel Cell Hybrid buses (the difference is due to the greater weight of fuel cell hybrids).
The model also allows a user to specify up to five different "technology-specific" maintenance costs, over and above base propulsion-related costs, in order to better evaluate the differences between technologies. In this analysis only one technology-specific maintenance item was included - diesel particulate filter cleaning - which is applicable to Diesel and Diesel Hybrid buses.
Diesel particulate filters (DPF) are required on all 2007 model year and later diesel engines, to reduce emissions of particulate matter. DPFs must be removed periodically to have accumulated ash removed. This ash accumulates as engine lubricating oil is burned in the cylinder, since inorganic components of the oil can not oxidize out of the filter along with collected carbon. The actual cleaning interval will depend on duty cycle and how much oil the engine burns. However, most filter manufacturers recommend a base cleaning interval of once per year. This annual interval is the assumption used in this analysis.
Based on the author's experience at New York City Transit, the cost of this annual cleaning is $300 to $400 per bus. This includes two hours for removal/replacement of the DPF and a third-party cleaning fee of $200 - $300 per DPF. The model applies this annual DPF cleaning cost to Diesel buses and Diesel Hybrid buses.
All hybrid-electric propulsion systems use an energy storage sub-system to act as a load leveler during vehicle operation (supplying peak electrical power and absorbing electrical power during braking). There are a number of different energy storage technologies commercially available, including lead-acid batteries, nickel-metal hydride batteries, sodium/nickel chloride batteries, lithium ion batteries, and ultra-capacitors. Different manufacturers have made different commercial decisions about which battery technology to supply with their hybrid drive systems13. Some battery technologies require periodic maintenance, while others do not14. To provide a consistent comparison this analysis assumes that both Diesel Hybrid and Fuel Cell Hybrid buses will be equipped with either nickel-metal hydride or lithium-ion batteries, neither of which require regular maintenance. It is the author's judgment, based on current commercial developments, that these are the most likely energy storage technologies to be used for future hybrid bus deliveries in 2008 and beyond.
Operator labor rates were assumed to be $50/hr for all bus types, equivalent to labor rates for bus mechanics.
To determine average vehicle purchase costs for Diesel, CNG, and Diesel Hybrid buses data was gathered from the American Public Transportation Association 2006 Transit Vehicle Database15. Table 8 summarizes this data on the weighted average price for 35-ft and 40-foot buses purchased for delivery in 2005 and 2006. The 2006 values for 40-ft buses were used in the analysis for the purchase cost of Diesel, CNG, and Diesel Hybrid buses.
In this analysis both Fuel Cell and Fuel Cell Hybrid buses are assumed to cost $3.2 million each. This is consistent with pricing reported by NREL for the three most recent fuel cell bus deliveries (see Table 4).
In order to maintain their buses in service for twelve years or more most transit agencies regularly overhaul them. The life cycle cost model used for this analysis allows the user to separately specify overhaul costs and overhaul intervals (in miles or hours of operation) for the following six bus sub-systems:
The technology-specific overhaul categories are designed to allow the user to separately identify items such as hybrid battery system replacements, which is the only technology-specific overhaul category used in this analysis.
For all bus types the analysis assumes that a Bus Overhaul will happen at 200,000 miles (6 years, or mid-life of the bus) and cost $50,000. Table 9 contains the values used in this analysis for the cost and interval of engine/powerplant and transmission/drive system overhauls and hybrid battery replacement for the different bus types. These assumptions on Diesel and CNG engine and transmission overhauls are based on an informal poll of maintenance staff at six transit agencies conducted by the author in 200410. The assumptions for hybrid drive system overhaul, hybrid battery replacement, and fuel cell powerplant overhaul are based on discussions with system manufacturers and review of manufacturer literature.
Given that large numbers of hybrid buses have not been in service long enough to reach expected system overhaul intervals the assumptions about hybrid drive system overhauls used in this analysis have a significant amount of uncertainty. For a series hybrid system the primary activity during hybrid drive system overhaul will be replacement of the traction motor and generator bearings. As relatively simple electric machines they should be able to go for at least twice as long as a standard automatic transmission before an overhaul is required, and bearing replacement is relatively inexpensive.
The assumed reduced cost of engine overhaul for Diesel Hybrid buses compared to Diesel buses is due to the fact that hybrid systems can use smaller and less expensive medium-duty diesel engines that would normally be installed in a pick-up truck, as opposed to the heavy-heavy duty diesel engines typically installed in Diesel transit buses.
During a Fuel Cell powerplant overhaul the major activity will be a complete replacement of the fuel cell stacks. The assumption used in this analysis of a 10,000 hour replacement interval and $100,000 replacement cost for fuel cell stacks is a forward-looking assumption.
The model used for this analysis allows the user to specify variable overhaul costs and variable overhaul intervals throughout a bus' life. For example, one could assume that as Fuel Cell technology matures fuel cell powerplant overhaul intervals will increase (i.e. fuel cell stacks will become more durable) and replacement cost will decrease, within the life time of a bus.
For this base case analysis all overhaul costs and intervals were assumed to be constant. No sub-systems for any bus type were assumed to have variable overhaul intervals or costs.
The assumptions used in this analysis for the cost of CNG fuel station installation, and depot changes required for CNG buses, is taken from the Transit Costs 1.0 model developed for the U.S. Department of Energy by TIAX, LLC16. This model assumes that CNG fuel stations have a fixed cost of $200,000 and a variable cost of $800 per standard cubic foot per minute (scfm) station capacity. The required scfm capacity of the station is based on the number of buses, the amount of fuel each bus will use every day, the maximum allowable fill time per bus, and the total available fueling hours per day at the bus depot. Station scfm is calculated using equations 1 and 2.
Assuming 100 assigned buses, a six minute "fast fill" for each bus, and six to eight hours per day available for fueling, two CNG fueling nozzles will be required. Assuming 33,000 annual miles per bus and CNG bus fuel economy of 2.4 MPDEG, the fuel station will need to have a capacity of 1,850 scfm, rounded up to 2,000 scfm. The cost of the CNG fuel station will therefore be $1.8 million. This does not include any costs for extending natural gas lines to the location of the CNG fuel station. Depending on current installed capacity of the local natural gas utility these costs can be significant, but are unique to each facility location.
Facility design for compressed natural gas operations generally requires installation of a building methane detection system and additional building ventilation for gas purging, as required. It also requires that all potential ignition sources (including standard electrical fixtures and conduit) not be located within 18-24 inches of ceiling level, and that the building roof structural design not allow for dead pockets at ceiling level where released gas could collect without being purged by the building's ventilation system. Many existing facilities built for diesel vehicles require modifications to both HVAC and electrical systems when CNG buses are introduced.
Transit Costs 1.0 assumes that these CNG facility requirements have a fixed cost of $100,000 plus a variable cost of $2,500 per bus if buses will be stored out doors and $4,000 per bus if they will be stored in doors. This results in a cost of $350,000 - $500,000 for CNG facility modifications for a 100-bus fleet.
Diesel and Hybrid buses use diesel fuel. They require the installation of a diesel fuel storage system with dispenser(s) and do not require any other special building systems17. Based on the author's experience at MTA New York City Transit the cost of diesel fuel stations are generally approximately one tenth the cost of CNG fuel stations which can handle the same number of buses. This analysis therefore assumes that the cost of a diesel fuel station that can accommodate 100 buses will be $180,000.
Because hybrid systems incorporate a significant number of batteries, this analysis also assumes that the bus depot will require modifications/expansion of its existing battery room to accommodate Diesel Hybrid and Fuel Cell Hybrid buses. The assumption used for the cost of these modifications is $20,000.
The model also assumes that CNG, Diesel Hybrid, Fuel Cell, and Fuel Cell Hybrid buses will require the installation of an overhead crane at the maintenance facility, since all of these bus types usually incorporate more roof-mounted equipment than standard Diesel buses. The assumption used for the cost of this crane is $25,000.
Given the limited U.S. experience with Fuel Cell buses and hydrogen fueling infrastructure it is more difficult to determine appropriate assumptions for the cost of installing a hydrogen fuel station and modifying a depot to handle hydrogen-fueled buses. Fueling station costs will also depend on the method used for fueling.
NREL reports that VTA purchased their hydrogen fuel station, which is designed to handle a maximum of six buses, for $640,000. The VTA fuel station stores liquid hydrogen which is then vaporized and compressed onto the buses.
Sunline and AC Transit both chose to create hydrogen on site using a natural gas reformer. NREL reports that Sunline purchased, for $750,000, a commercial unit that can create and store up to 9 kg/hr of hydrogen at 5,000 psi.
Other researchers have estimated the cost of hydrogen fueling infrastructure in the context of analyses of the "transition costs" to a hydrogen economy. All of these analyses are based on conversion of privately-owned public gas stations to hydrogen operations to service a relatively small number of light-duty fuel cell cars. Their estimates range from $800,000 to over $5 million for the construction of a single hydrogen station capable of producing and dispensing between 24 kg and 3,000 kg per day or hydrogen. The analyses which evaluated the cost of both small (< 100 kg/day) and large (>1,000 kg/day) stations generally assumed large economies of scale, with the relative capital cost per unit of capacity (daily kg) falling by 50% or more as station size increased from 100 to 1,000+ kg/day.
Based on the fuel economy assumptions used in this analysis a Fuel Cell bus would consume 0.40 kg hydrogen/mile and a Fuel Cell Hybrid bus would consume 0.22 kg/mile. In this analysis all buses are assumed to travel approximately 100 miles/day, so that each Fuel Cell bus would consume 40 kg/day of hydrogen, and a fleet of 100 Fuel Cell buses would consume 3,400 kg/day18. Each Fuel Cell Hybrid bus would consume 22 kg/day of hydrogen, and a fleet of 100 Fuel Cell Hybrid buses would consume 1,870 kg/day.
Table 10 shows the projected capital costs of hydrogen fuel stations this large, based on the cost of the VTA and Sunline fuel stations, and based on the other published cost estimates discussed above. For each projection the published cost estimate was multiplied by a scaling factor based on the required volume (kg/day) to service 100 buses, compared to the station volume used to develop the estimate. When scaling estimates based on small stations, total costs were reduced by 50% to account for economies of scale. Based on these projected estimates, the base case assumes that a hydrogen fuel station sized to accommodate 100 Fuel Cell buses would cost $3.5 $7.0 million, and one sized to accommodate 100 Fuel Cell Hybrid buses would cost $1.7 - $4.0 million. These assumed costs are two to four times greater than the assumed base case cost of a CNG fuel station.
The same types of modifications required at a depot to safely handle natural gas are also required to handle hydrogen. Unlike for natural gas, however, the building codes relevant to hydrogen are not well developed at this time. This has lead to a wide range of facility modification costs for the fuel cell bus demonstration projects implemented to date. For example, VTA reports spending $4.4 million on facility modifications to handle three fuel cell buses, while AC Transit reports spending $1.5 million for the same number of buses, and Sunline reports spending only $50,000 to accommodate one fuel cell bus (see Table 4). For this analysis we assumed that the cost of facility modifications to accommodate a 100-bus fleet of Fuel Cell or Fuel Cell Hybrid buses would be double the costs to accommodate the same number of CNG buses or $700,000 - $1,000,000.
This analysis assumes that all infrastructure investments will have a useful life of 20 years.
For all infrastructure investments (fuel station, depot modifications) this analysis assumes that the annual cost of operations and maintenance would be 5% of installed capital costs.
This analysis assumes that bus mechanics will require an average of 20 hours each of initial training on Diesel buses and five hours of annual refresher training, while bus operators will require two hours of initial training and no annual refresher training.
The analysis assumes that bus mechanics will require more training, both initial and annual, for Diesel Hybrid, CNG, Fuel Cell, and Fuel Cell Hybrid buses, due to unfamiliarity with these systems. Incremental initial and annual CNG and Fuel Cell training requirements for bus operators and managers are primarily for safety training related to natural gas and hydrogen fuel. All of the training assumptions used in the analysis are shown in Table 11.
2 American Public Transportation Association. 2006 survey data. <http://www.apta.com/research/stats/bus/power.cfm>
3 2006 APTA survey and discussion with bus manufacturers.
4 These buses are operated by the Alameda Contra Costa Transit District (3), the Santa Clara Valley Transportation Authority, the Sunline Transit Agency (1), and Connecticut Transit (1)
6 Assuming 128,400 btu/gallon for diesel and 113,628 btu/kg for hydrogen = 1.13 kg/diesel gallon.
7 See information from the Federal Reserve Bank of Cleveland <http://www.clevelandfed.org/research/inflation/TIPS/index.cfm>
8 See Daily Treasury Real Long Term rates as calculated by the U.S. Treasury. <http://www.ustreas.gov/offices/domestic-finance/debt-management/interest-rate/real_yield_historical.shtml>
9 Federal Transit Administration, 2005 National Transit Database, Tables 17 and 19. <http://www.ntdprogram.com/ntdprogram/pubs.htm>
10 The exceptions were both hybrid and CNG buses at NYCT whose costs were similar, but higher than at other agencies and fuel cell buses at VTA, which had significantly higher costs than the comparison diesel buses.
11 The agencies polled included: Dallas Area Rapid Transit, Dallas, TX, Toronto Transit Commission, Toronto, ON, Washington Metropolitan Area Transit Authority, Washington, DC, MTA New York City Transit, Brooklyn, NY, Coast Mountain Bus Company, Vancouver, BC, Los Angeles County Metropolitan Transportation Authority, Los Angeles, CA.
12 On a typical transit bus approximately 20% of the energy supplied by the engine is used to operate accessory loads, and 80% is supplied to the bus wheels. Of the energy supplied to the bus wheels, approximately one half (40% of the total) is dissipated as friction between the tires and the road, and half (40% of total) is dissipated in the brake system. Assuming that all of the fuel savings from a hybrid bus comes from energy recovered through regenerative braking, a 20% savings implies that the brake system in only dissipating half the energy that it would on a standard bus.
13 The three leading U.S. heavy-duty drive system suppliers all use different technologies. BAE Systems Controls currently supplies commercial hybrid systems with lead-acid battery packs, but recently announced that they would switch to lithium-ion batteries beginning in 2008. Allison Electric Drives supplies commercial systems with nickel-metal hydride battery packs, while ISE has recently supplied systems using both ultra-capacitors and sodium/nickel chloride batteries.
14 Lead-acid batteries used in a hybrid system typically require twice-yearly "conditioning" charging to reverse negative plate sulfation. Sodium/nickel chloride batteries operate at approximately 260C, and often must be plugged into grid electrical power to maintain this temperature if the bus will not be used for an extended period. The other battery technologies do not require regular maintenance or charging in a hybrid application.
15 American Public Transportation Association, Transit Vehicle Database, May 2006, www.apta.com/references/info/pubs
16 Kassoy, E.; Kamakate, F.; Leonard, J.; TIAX LLC, Transit Costs1.0; September 2003; Developed under contract to U.S. Department of Energy; www.eere.gov/afdc/apps/toolkit/docs/Mod09b_Transitcost.xls
17 While building codes have specific requirements for facilities that will house diesel fueled vehicles, most bus facilities are, or would be, designed for the use of diesel fuel absent the introduction of natural gas or hydrogen vehicles. The cost of diesel fuel design is therefore assumed to be included in the base facility costs and the cost of CNG- and hydrogen-specific systems included in the model is for the incremental cost of designing for these operations.
18 This calculation assumes that only 85 buses out of 100 will be in service each day.