Batteries On Class 777 Trains
In this article on Railway Gazette, which is entitled Merseyrail Class 777 arrives in Liverpool, there is this sentence.
There is space under one vehicle to house a battery weighing up to 5 tonnes within the axleload limit.
This matter-of-fact sentence, draws me to the conclusion, that these trains have been designed from the start to allow future battery operation.
Batteries are not an add-on squeezed into a design with great difficulty.
Battery Capacity
Energy densities of 60 Wh/Kg or 135 Wh/litre are claimed by Swiss battery manufacturer; Leclanche.
This means that a five tonne battery would hold 300 kWh.
Note that Vivarail find space for 424 kWh in the two-car Class 230 train, I wrote about in Battery Class 230 Train Demonstration At Bo’ness And Kinneil Railway, so it would appear that Stadler aren’t being over ambitious.
Kinetic Energy Of A Full Class 777 Train
The weight of a full Class 777 train is calculated as follows.
- Basic empty weight – 99 tonnes
- Battery weight – 5 tonnes
- 484 passengers at 80 Kg – 38.72 tonnes
Which gives a total weight of 143.72 tonnes.
Intriguingly, the weight of a current Class 507 train is 104.5 tonnes, which is 500 Kg more than an empty Class 777 train with a battery!
If these weights are correct, I suspect Stadler have used some very clever lightweight design techniques.
For various speeds, using Omni’s Kinetic Energy Calculator, this weight gives.
- 30 mph – 3.6 kWh
- 40 mph – 6.4 kWh
- 50 mph – 10.0 kWh
- 60 mph – 14.4 kWh
- 70 mph – 19.5 kWh
- 75 mph – 22.4 kWh
Note.
- The average speed between Bidston and Wrexham General stations on the Borderlands Line is under 30 mph
- The operating speed on the Wirral Line is 70 mph
- The operating speed on the Northern Line is 60 mph
- The maximum speed of the trains is 75 mph.
Every time I do these calculations, I’m surprised at how low the kinetic energy of a train seems to be.
How Small Is A Small Battery?
One battery doesn’t seem enough, for a train designed with all the ingenuity of a product with quality and precision, that is designed to out-perform all other trains.
This is another paragraph from the Railway Gazette article.
According to Merseytravel, ‘we want to be able to prove the concept that we could run beyond the third rail’. By storing recovered braking energy, the batteries would help to reduce power demand and the resulting greenhouse gas emissions. All of the Class 777s will be fitted with small batteries to allow independent movement around workshop and maintenance facilities.
I am not quite sure what this means.
It would seem strange to have two independent battery systems in one train.
I think it is more likely, that the smaller battery can be considered the primary battery of the train.
- After all in the depot, it looks after the train’s power requirement.
- Does it also handle all the regenerative braking energy?
- Is it used as a secondary power supply, if say the power is low from the electrification?
- Could it be used to move the train to the next station for passenger evacuation in the event of a power failure?
I wonder if the power system is a bit like the average battery-powered device like a lap-top computer, smart phone or hybrid car.
- The electrification and the regenerative braking charges the battery.
- The battery provides the traction and hotel power for the train.
When the five tonne battery is fitted, does the train’s control system move power between the two batteries to drive the train in the most efficient manner?
I’ll return to factors that define the size of the small battery.
The small battery must be big enough for these purposes.
- Handling regenerative braking at the operating speed.
- Recovering a full train to the next station.
- Keeping a train’s systems running, during power supply problems.
- Moving a train around a depot
As the lines leading to depots are electrified, the train can probably enter a depot with a battery fairly well-charged.
As the new Class 777 trains have a maximum operating speed of 75 mph, I would suspect that the small battery must be able to handle the regenerative braking from 75 mph, which my calculations show is 22.4 kWh with a full train. Let’s call it 30 kWh to have a reserve.
Using Leclanche’s figures, a 30 kWh battery would weigh 500 Kg and have a volume of just under a quarter of a cubic metre (0.222 cubic metre to be exact!)
I suspect the operation of the small battery through a station would be something like this.
- As the train runs from the previous station, the power from the battery will be used by the train, to make sure that there is enough spare capacity in the battery to accommodate the predicted amount of energy generated by regenerative braking.
- Under braking, the regenerative braking energy will be stored in the battery.
- Not all of the kinetic energy of the train will be regenerated, as the process is typically around eighty percent efficient.
- Whilst in the station, the train’s hotel services like air-conditioning, lights and doors, will be run by either the electrification if available or the battery.
- When the train accelerates away, the train’s computer will use the optimal energy source.
The process will repeat, with the battery constantly being charged under braking and discharged under acceleration.
Lithium-ion batteries don’t like this cycling, so I wouldn’t be surprised to see dome other battery or even supercapacitors.
A Trip Between Liverpool and Wrexham Central in A Class 777 Train With A Battery
The train will arrive at Bidston station with 300 kWh in the battery, that has been charged on the loop line under the city.
I will assume that the train is cruising at 50 mph between the twelve stops along the twenty-seven and a half miles to Wrexham Central station.
At each of the twelve stops, the train will use regenerative braking, but it will lose perhaps twenty percent of the kinetic energy. This will be two kWh per stop or 24 kWh in total.
I usually assume that energy usage for hotel functions on the train are calculated using a figure of around three kWh per vehicle mile.
This gives an energy usage of 330 kWh.
But the Class 777 trains have been designed to be very electrically efficient and the train is equivalent in length to a three-car Class 507 train.
So perhaps a the calculation should assume three vehicles not four.
Various usage figures give.
- 3 kWh per vehicle-mile – 247.5 kWh
- 2.5 kWh per vehicle-mile – 206 kWh
- 2 kWh per vehicle-mile – 165 kWh
- 1.5 kWh per vehicle-mile – 123.8 kWh
- 1 kWh per vehicle-mile – 82.5 kWh
Given that station losses between Bidston and Wrexham Central could be around 24 kWh, it looks like the following could be possible.
- With a consumption of 3 kWh per vehicle-mile, a Class 777 train could handle the route, but would need a charging station at Wrexham Central.
- If energy consumption on the train could be cut to 1.5 kWh per vehicle-mile, then a round trip would be possible.
It should also be noted that trains seem to do a very quick stop at Wrexham Central station of just a couple of minutes.
So if charging were to be introduced, there would need to be a longer stop of perhaps eight to ten minutes.
But the mathematics are telling me the following.
- The Class 777 train has been designed to weigh the same empty as a current Class 507 train, despite carrying a five tonne battery.
- If power consumption can be kept low, a Class 777 train with a battery can perform a round trip from Liverpool to Wrexham Central, without charging except on the electrified section of line between Liverpool and Bidston.
- Extra stops would probably be possible, as each would consume about 2 kWh
I feel that these trains have been designed around Liverpool to Wrexham Central.
Conclusion
Wrexham Central here we come!
Other routes are possible.
- Hunts Cross and Manchester Oxford Road – 27 miles
- Ormskirk and Preston – 15 miles
- Headbolt Lane and Skelmersdale – 6 miles
- Ellesmere Port and Helsby – 5 miles
- Kirkby and Wigan Wallgate – 12 miles
Chargers will not be needed at the far terminals.
The Anonymous Widower 