Hydrogen Transportation on Lewis

Transportation Results

The off-grid scenario simulated in HOMER for Lewis included a hydrogen filling station where the buffering system excesses are stored for use in a public transportation project on the island. It is proposed in this project that the hydrogen excesses are used to fuel two fuel cell buses (as they are more efficient than HICE and produce zero harmful emissions) on Lewis operating on two of the island’s main commuter routes. These routes pass directly through the majority of the villages on the island making the buses easily accessible to the Lewis residents. A map of the island and the two bus routes in green and red are represented below;

The red route travels from Stornoway bus station to Port of Ness on the north tip of the island via Barvas covering a distance of 26.1miles. Bus timetables from the island indicate the route takes approximately 1 hour each way and the service makes 5 round-trips per day. This equates to 261 miles per day and a maximum annual mileage assuming the buses operate with the same frequency all year round of 95,265 miles.

The green route shown is known as the ‘Stornoway West Side Circular’, which passes through all the main villages on the west side of the island. The route takes around 1 hour and 45 minutes covering 45.16 miles and the current diesel buses in service on the island make 5 round-trips per day. This gives a daily and annual mileage of 225.8 miles and 82,417 miles respectively. The combined annual mileage of the two routes is 177,682 miles. The volume of hydrogen required to cover 177,682 miles per year can be calculated from the average fuel consumptions of a fuel cell powered bus.

Over the year, the Lewis buffering system produces approximately 60 tonnes of excess hydrogen. From background research on the Hyfleet projects, average fuel consumptions from the fuel cell buses was found to range between 2.49 and 4.01 miles per kilogram of hydrogen depending on the routes and terrain the buses were operating over [1]. The lower fuel consumptions were found in buses operating over particularly hilly routes or in heavy traffic locations necessitating a lot of stop-start driving.

Since the roads on Lewis are relatively traffic free when compared to the busy city projects seen in the Hyfleet programmes, it can be assumed that the buses on the island would be capable of achieving better fuel economy than 2.49 miles per kilogram. It was therefore assumed that the buses would achieve a fuel efficiency half way between the high and low consumptions (3.25 miles per kilogram) as an average over the year.

The hydrogen requirements for a fuel cell bus to travel 177,682 miles per year are represented in the table below for the differing fuel consumption figures and energy equivalent in diesel miles per gallon (mpg). (The energy equivalence is calculated from the ratio: 1 UK gallon of diesel = 0.93kg of hydrogen)[2]. The row of numbers presented in bold in the table below highlight the fuel consumption figures used in this project;

Table 1 - Fuel Cell Bus Consumptions

It should be noted that the average fuel consumption for a city diesel bus at 4mpg is very similar to that of the fuel cell buses shown. It was expected since transportation fuel cells operate at efficiencies above 50% that the fuel economy would be better than a diesel bus since their efficiencies are less than half that. However, there is a weight penalty associated with the hydrogen storage systems and hybrid-drive of the new fuel cell buses. In the Australian STEP hydrogen bus project, it was found that fuel cell buses were on average 30% heavier than diesel powered buses and 21% heavier than compressed natural gas (CNG) fuelled buses [3].

From the table above, it was calculated that 149.78kg of hydrogen would be required each day from the buffering system to operate two hydrogen fuel cell buses on the described routes. A daily hydrogen load was introduced into the HOMER simulation for the Stornoway scenario. 152kg is drawn from the main storage tank per day and supplied to a filling station for the buses. The additional 2kg per day provides a small fuel reserve to compensate for varying driving conditions the buses may experience. It is assumed that the hydrogen tanks in the buses have a storage capacity of 45kg, which is the same as used in other hydrogen bus projects. At this capacity, the buses will only have a theoretical range of 146.25 miles, which will necessitate a refill during the day.

The table below shows the litres of diesel and CO2 emissions saved anually and over the project lifetime from replacing the existing diesel buses on the two routes with hydrogen fuel cell buses. (A diesel bus is assumed to emit 1.32kg CO2 per mile);

As mentioned in the transportation section of the website, a fuel cell has an operating efficiency in a bus between 50 and 60%. Taking an average of 55% for the fuel cell conversion efficiency, plus 10% parasitic loses in the hydrogen fuel system and a further 10% loss in the electric drive motor gives an overall efficiency for the bus of 44.6% The buffering system has a an efficiency from the turbines to the filling station of 50.15%. So, combining these efficiencies gives a power generation to wheel efficiency of 22.4%. This compares favourably with the total 'well-to wheel' efficiency of a diesel bus of 17-20%.

In total, the Stornoway buffering system has the potential to sell 55,480kg of hydrogen to a fuel cell bus program per year. This presents a further revenue stream for the system as well as optimising the efficiency of the buffering scheme as less excess electricity is lost over the year. The cost of hydrogen fuel varies significantly depending on the infrastructure in place and the methods used to produce the hydrogen. In the state of California (USA), hydrogen prices are relatively low compared to the prices found in Europe for example. One of the reasons for this is that there are a larger number of hydrogen vehicles on the road in this area so there is more demand for the fuel. As such, hydrogen fuel costs can be as low as $5.00/kg. However, in Europe, prices are generally higher at around 8 Euros/kg (including Government subsidy). At current exchange rates (April 2009, £1 = 1.11 Euros), 1kg of hydrogen would cost £7.20 in mainland Europe. Selling all the available hydrogen from buffering system filling station in the first year would therefore yield an additional £400,000 on top of electricity sales for the Stonoway system.

References:

[1] Experiences from Using Hydrogen in Public Transport, Author: Burkhard Eberwein

[2] Air & Waste Management Association, Author: Heather L. Cooper

[3] Life cycle assessment of the diesel, natural gas, and hydrogen bus transportation systems in Western Australia, Authors: J. Ally and T. Pryor