If all Swedish buses and coaches were to be electrified, the total energy use would decrease and the shift from diesel and gas fuels to electricity would lead to an increase of the electric energy use by 1.2% compared to the total energy use today.
We know that the energy efficiency increase a lot when buses and coaches becomes electrified. This is the case both when “tank-to-wheel” and when “well-to-wheel” is studied. The reason is that buses and coaches have big engines to cope with their duty cycle that involves a lot of starts and stops. For example, a typical bus engine for a standard 12m bus has about 250 hp (horse power maximum output) while the average power outtake in a city duty cycle is about 50 hp. The average power-load is hence only 20% of the maximum power. The low average load leads to low efficiency. Or, to make a long story short, the average efficiency of the diesel engine in a bus duty cycle is about 20-30%. An average load below 30% is hence the case even when the efficiency in the most efficient load point can go beyond 45%.
|Before proceeding to the impact on the electricity production and distribution I therefore want to firmly underline that the total energy use will in almost every real world situation lead to a decrease in total energy use.|
In reality, since the life-time of a bus is more than 10 years, the paradigm shift will take place during a decade and there will be a lot of time to adapt the electricity production and the distribution chain to the new needs.
Nevertheless, as an example I have below looked-up some key data for my home country, Sweden, to answer the question:
What would the impact on the electric production and grid be if all buses and coaches in Sweden were to become plug-in or electric over-night?
Would the electric production capacity be sufficient?
Would the electric grid be capable of dealing with the increased power requirement?
To start I want to understand the relationships in terms of how many households does an electric bus correspond to? According to the Swedish Institute Elforsk, a household uses between 4 000 and 25 000 kWh electricity per year, depending on weather it is an average apartment or a private house with electric heating. In order to make a comparison I have assumed a yearly mileage of 55 000 km for a city bus with an average power use of 1.6 kWh per km. This number accounts for higher energy use at winter conditions but assumes that a biofuel is used for heating the bus compartment; else the energy use will be higher.
For the charging station I have assumed a high utilization of about 60%. This would be a worst case to relate to. One bus uses the same amount of electricity as between 4 and 22 households, depending on which type of household we compare to. Buses mostly give service to between 500 and 5 000 persons per day. One charging station will be able to serve several buses. It will transfer the same amount of electricity as used by between 15 and 100 households. It seems that the ball-park figures are reasonable.
Next, I want to understand the total energy use for buses, if they were to be electrified. The total number of registered buses and coaches were 14 203 (2012). This number includes all buses from 3.6 ton minibuses, to 40 ton 24m tall bi-articulated buses. Most buses in Sweden are between 12 and 14.5 meters. I have not found any reliable data for the average mileage. I have therefore used some rough assumptions to put us in the right ball park. Some school buses and spare buses are barely used while some few long distance coaches may travel more than 300 000 km / year. A guestimate gave me 70 000 km per year as an average for all buses and coaches. Since we now deal with all buses and coaches I also added some margin to the electricity use. An educated guess gives us: 2 kWh/km as a rough number to cover all sorts of traffic. This number does not include electricity for heating the compartment in the winter. This is for practical reasons assumed to be done with a biofuel.
The calculated energy use for all buses and coaches was about 2 109 kWh per year. As this number has no meaning for an old chemical engineer as myself. I want to relate it to the daily use of the citizens in Sweden. Again, I end up with a lot of numbers. The buses and coaches will use a fraction of the electricity used today in Sweden; 1.2% to be precise. Note that the production and use does not ad-up, since Sweden export electricity, in recent years.
In Sweden households use 23% of the electricity that is produced. Most of the electricity is used by industry (35%) and 24% is used by “Society services” (railway, street lights, hospitals etc). Buses are used mostly where people live (households) and where people work (Industry and at offices). It seems that theoretically, if there is a limitation in the electric distribution chain it would be close to the households. The relative increase of electricity use, if all electricity for the buses would be pulled from the part of the grid supporting households only, would increase the load on the grid by in average 5.3%. Provided that peak loads can be secured not to overlap in time for households and buses, we can conclude that the electric grid will not become a limitation in the distribution chain. This statement is only valid on a “city level”. Certainly, there may be local effects but in general it seems that the electric grid in Sweden will not pose a limitation for the electrification of the buses.
Next question: Is the production capacity of electricity a limitation to supply energy to electric buses and coaches?
There are of course several complex questions about spare capacity and peak capacity and the availability over time. For this a much more detailed analysis is required. Still some relations to the maximum capacity may give some indication for weather we are close to the production capacity limit or not. Again stressing that the analysis is an over simplification it is concluded that the electrification of all buses and coaches in Sweden will “eat” about 1.2% of the spare production capacity (it is a coincidence that both relative increase of electricity use and the relative use of spare capacity ends up at the same 1.2%).
Which conclusions can be drawn for Sweden?
1, On the national level there is sufficient production capacity of electricity to supply all buses and coaches with electricity, this as a yearly average.
2, On the city level (this is however not possible to claim on street or block level), the electric grid does not pose a limitation to distribute electricity to the buses.
3, Locally and time analysis requires a more detail data and analysis.