*5.2. TCO Variations for Di*ff*erent Timetables*

The TCO model can help analyse how the timetable influences the costs of different types of buses. In Figure 13, the cost of three types of buses are compared for one bus route, but with three different timetables of varying bus traffic density. In this analysis, the bus traffic density is varied in the same proportions over the whole day. Thus, the ratio between peak and off-peak traffic remains the same, and the start and end of the traffic periods also remain the same in the three compared cases.

**Figure 13.** The cost per km for end-stop-charged and biogas (CBG) buses with varying bus traffic density for a whole day. (Note that the *y*-axis starts at 18 SEK/km, so the driver cost is more than half the TCO).

In Figure 13, the cost per km for biogas buses is found to be the same, irrespective of the number of departures per day. Since conventional buses do not share any infrastructure on the bus route, doubled bus traffic density results in all costs increasing by a factor of two. Thus, the cost per trip (km) for biogas buses is not changed when the bus traffic density is changed by the same factor over the whole day. Electric buses with chargers at the end-stops instead become increasingly expensive per trip (km) as the bus traffic density is lowered. This is caused by the investment in grid connection and chargers being shared by fewer and fewer buses as the bus traffic density decreases. This can be seen as the yellow and blue parts in the bar-chart growing as the bus traffic density is lowered. The increase in total cost per km is only in the order of 5%, but it represents an important difference compared to conventional buses, as the electric buses are relatively more cost effective at a higher bus traffic density. The two different end-stop-charging strategies both change cost in a similar way, as the bus traffic density influences their respective TCO in a similar way.

Another important timetable parameter is the ratio of departures per hour in the peak and off-peak periods. In the following, we analyse the effect of keeping the peak bus traffic density constant and varying the off-peak traffic. This will give a different result to that when we vary the bus traffic density over the whole day, since the number of buses is mainly determined by the bus traffic density during the peak times and not so much by the off-peak traffic. Therefore, an increased traffic off-peak can be expected to reduce the cost per km. The results of varying the off-peak bus traffic density is shown in Figure 14. For all three cases, the timetable has 12 departures per hour in the peak times. The difference is in the number of departures off-peak which are either the same (12) two thirds (8) or one third (4) of the number of departures in the peak times. The number of departures during the evening has been set to 50% of the off-peak departures per hour.

**Figure 14.** The cost per km for end-stop-charged and biogas (CBG) buses with varying bus traffic density off-peak. (Note that the *y*-axis starts at 18 SEK/km, so the driver cost is more than half the TCO).

According to Figure 14, the cost per km for biogas buses is found to be lower the higher the bus traffic density is off-peak. The main reason for this reduction is that the same number of buses are driving more trips and, thus, the bus depreciation is divided among more trips (km). Another contributing mechanism is that the number of times the bus drives to and from the depot decrease as more and more of the buses drive the whole day without a midday break in the depot. This is reflected as a lower driver cost per trip (km) and lower fuel consumption per trip (km).

The electric buses also reduce their cost per trip (km) if the departures per hour off-peak is increased. They have the same reason for reducing the costs as the biogas buses, but on top of that, they have charger depreciation and grid fees which are also divided by more and more km in traffic. Therefore, the electric buses have even more of a reduction to their TCO when the off-peak traffic is increased than the biogas buses have. In Figure 14, the dotted line shows that biogas buses have lower or the same TCO as the electric buses at four departures per hour off-peak. However, the dashed line shows that the electric buses are significantly cheaper than the biogas buses at 12 departures per hour during the whole day. Generally, we can conclude that electric buses with end-stop charging will be most cost effective for routes which have a high amount of and constant traffic during the whole day, but it is not possible to determine one type of bus which is always the cheapest.

Now that we have looked at the TCO with some different timetables, we can see that it seems that the whole idea of not charging during the peak times in order to reduce the number of buses manages to achieve the expected reduction in the number of buses; but in these cases, this does not lead to a lower TCO than charging for the whole day, mainly due to a more expensive battery. The authors of this paper hypothesized that the costs for these two charging strategies would change differently with timetable variations, leading to EndStop2 having a lower TCO for some timetables, but so far it seems that the TCO for EndStop1 and EndStop2 follow each other well. This illustrates the value of having a rather detailed TCO model, as there are often several second order effects which may influence the TCO.
