Dr Stuart Hillmansen explains about how much energy we really need for a train, and how we can decarbonise this energy.
We have heard from our aerodynamic team about the importance of air resistance on train energy consumption, and in today’s blog we are going to get down to business to delve deeper into truly understanding the factors which affect exactly how much energy trains use and need to undertake their tasks. I’ll be splitting this blog up into three sections, and stick around for them all because the last is definitely the best.
Section 1. Fundamentals and basic physics
Our group in BCRRE are specialists in all aspects of rail traction and propulsion systems, and we demonstrate this through our research and application-focused projects. Our most notable recent success was the partnership with Porterbrook and the supply industry to deliver the HydoFLEX train. We used our deep knowledge of the fundamentals to develop a concept design for the propulsion system which we implemented – and it worked! The first time the train actually moved everyone on the team was quite emotional but the rational engineering ones amongst us knew exactly how the train would work when fed electricity produced by a fuel cell. It all comes down to basic physics.
As an example, to illustrate this in my lectures on railway traction, I often pose the question to the audience, “Have any of you ever pushed (or pulled) a train?” Now I must say that I am generally aware of the health and safety practices at railway depots (and pushing trains is kind of a bit of a grey area in terms of H&S) but I am always pleasantly surprised by the number of people who have actually had the opportunity, and admit to pushing a railway vehicle. “Why,” you might ask, “is this interesting?” Well first of all trains are very heavy, so you might think that you have to push really hard to get things going, but due to the low rolling resistance, it is actually much easier than you think to move a train, and with a bit of a push a few people can normally get the train to glide along quite smoothly.
This low rolling resistance means 2 things. Firstly it means that the energy which is put into the train by the traction system gets transferred very efficiently into kinetic energy and therefore train motion. Secondly, it means that in braking, the drag force is low meaning that trains fitted with regenerative braking can return much of that hard-earned energy to the network, or possibly stored in an energy storage system.
Section 2. Electricity, and cheap energy
A train is a powerful machine and each vehicle in a train may need a peak of 600 kW of installed train power to deliver the traction requirement. If the train is on the electric network, then this power comes from the grid and Network Rail, with its massive buying power, can procure electricity at relatively low cost. The exact price will vary according to the time of day, and also the location, but for this exercise, let’s assume the price is around 10 pence per kWh.
A high speed train with around 10 cars would need a peak power of around 6 MW of traction power but it does not use this power all the time, just in the latter stages of acceleration, and the average will probably be around 2-3 MW. Estimating ball-park energy use is therefore easy: you just need to multiply by the time in hours to get the answer in MWh. A MWh is 1000 kWh, and would cost about £100. This means that the 10 car train would cost somewhere around £200 pounds per hour to run. That may sound expensive, but the train could have 400 plus passengers and therefore the cost per person per hour is really quite low – and a very low percentage of the ticket price.
But how powerful does a train need to be? It is actually very easy to estimate how much power needs to be installed in the vehicle, so I am going to give you a simple method to work out how much power typical trains require. Please put your own calculations in the comments. It is very simple, just multiply the train’s Mass by its initial acceleration by its top speed and then divide by 3. It works really quite well for most trains. Needless to say you need to use SI units, and then you’ll get the answer in Watts.
Section 3. Our long term vision: the 40 year plan
On my next birthday I will be 45, so decided that we would create a vision for how the railway traction system may develop over a timescale which can easily facilitate a complete transformation in propulsion, and would also allow me to look back in this blog when I reach the young age of 85.
Our group firmly believes that, through the effective and progressive expansion of the electrification network, we can rapidly achieve a net zero carbon railway. The majority of the high performance and high frequency routes can be operated through continuously electrified lines: the economics work out for this. Services which run on parts of the network which do not meet the business case for electrification will need to be delivered with trains which have some autonomous capability. For short distances, where a train might need to operate in an autonomous mode for 1-2 hours per day, then a battery solution would be possible. For longer times spent on non-electrified sections of the network then Hydrogen bi-mode is a viable solution, and then for fully autonomous operations Hydrogen is still just about viable although there will be challenges in achieving a long range. Ultimately there is no doubt that we can realise the vision of a fully electric zero carbon (at the point of operation) railway, where electric drives dominate, and the electricity either comes from a fuel cell, a battery, or directly from the infrastructure.