Giving Rail the Green Light
You might have heard that taking the train is the 'green' choice. It is. Rail takes far less energy to transfer a passenger from A to B than air or road. In the European Union, rail transports around 6% of all passenger journeys by distance travelled, but only accounts for 2% of the total transport energy! That is pretty good going, but there's still plenty of opportunity to make our railways greener.
Running train services in the UK takes the equivalent of 12 Tera Watt hours (TWh) of electrical energy every year. That's roughly the same as boiling half a million kettles a million times each. In fact, meeting the EU target of a 6% reduction in energy would save enough electricity for every UK resident to have an extra 435 cups of tea annually! Unfortunately though, it isn't current policy to save energy in order to supply commuters with free cups of tea.
Where is the energy 'used'?
Before we think about saving energy, we need to know where it is 'used'. Basic physics dictates that energy cannot be created or destroyed, but is transformed from one form to another. Energy is considered 'used' once it is converted into a non-reusable form - such as heat, light, noise or vibration.
Trains themselves use energy in lots of ways: to supply heating, lighting and cooling on board; to overcome the aerodynamic resistance to motion; and in less than perfect power electronics in the conversion chain. But one of the biggest uses might surprise you...
Up to half the energy of a train can be used just in stopping! Traditional brakes are a bit like the brakes on your bike, physically stopping the wheels with brake discs. The movement energy, which had first been converted from either electrical or chemical energy, gets wasted as heat in the brakes as the train slows down.
Fortunately, there is a better way. Dynamic braking is a technology that allows the recovery of a vehicle's braking energy as electricity. Instead of applying physical brakes, the electric motors act in reverse so that the force opposes the direction of rotation, slowing the train down and generating electric power. In some cases using dynamic braking can recover over a third of the total energy!
Normally, the extra electricity, known as regenerated energy, satisfies any immediate requirements on board the vehicle, powering comfort functions such as lighting, heating and cooling. But the destination of any surplus depends on the type of railway.
In electrified networks, the extra electricity can be returned to the overhead line, or third rail, to be used by other trains. While this sounds great in theory, it requires a bit of thought in practice.
The national grid is a three phase alternating current (AC) supply system, carried in three wires. AC means that the supply voltage changes over time in the shape of a sine wave. Another supply type is direct current, or DC which provides a fixed voltage a bit like a battery. Electrical systems are connected to the electricity grid by one of three possible wire combinations, essentially meaning that there are three supplies. These supplies are really high voltage AC, and are usually converted to the appropriate type and voltage to power things. Typically, railways run on 25kV AC or between 600V-3000V DC.
As mentioned earlier, the demand from the rail network is huge, so if it were connected to just one of the three 'supplies' it would massively unbalance the grid! To overcome this, the railway is split into electrically isolated 'sections', connected to one of the three.
This means that if a train uses regenerative braking, the electricity it produces can only power trains within the same section. If there are no other trains in the section, the electricity is wasted - it heats up banks of resistors to prevent the line voltage from rising too high.
In non-electrified networks, diesel trains also use dynamic braking to prevent wear of the wheels and rails. But since there is nowhere for the excess electricity to go, it is diverted into resistors too.
How can we use it?
1. Timetable optimisation
There is one really easy way to ensure that we benefit from the regenerated energy in electrified networks: optimise the timetable so that when a train brakes there is another train to absorb the energy. This is particularly effective for metro systems, like London Underground, because there are lots of trains, all stopping and starting frequently. Under these conditions, even without optimisation, there is the possibility of energy exchange between vehicles, but the efficiency of doing so can be improved. Optimised timetables for metros in Beijing, Madrid and Xi'an provide estimated energy savings of between 5% and 15%!
2. Reversible substations
One big limitation for regeneration on DC railways is that the electricity has to power a train close by, because there is nowhere else for it to go. The sections are powered by rectifier substations which only allow current to flow in one direction. But if those substations were to be 'reversible' the electricity could be returned to the national grid. In theory this means that all of the recovered energy could be used! Installation of this technology is expensive, but it has been done on both the Rotterdam and Bilbao metros, saving between 10% and 20%. The benefit of reversible substations is more significant in off peak conditions, when there is less chance of direct exchange between trains on the network. Substations on AC railways allow the regenerated energy to return to the grid without any special equipment.
3. Energy Storage
Both methods above assume that the energy has to be used immediately - but why not store it?
On board systems allow vehicles to store their braking energy until they need it later on. This has significant potential for diesel-electric vehicles especially, as they normally have to waste any regenerated electricity. However, there is a trade-off between storing all of the available energy and developing a system small and light enough to fit into a vehicle.
For electric railway networks, wayside energy storage is another option. Storage units can be positioned at fixed locations along the route, to supply energy to trains when they need it. Wayside systems aren't really constrained by size and, in theory, can be as large as you want.
Batteries are the most familiar storage technology and there certainly are many kinds of batteries available: lead-acid, nickel metal hydride, lithium-ion and so the list goes on. But batteries are not the only option for rail. There are high speed mechanical flywheels, which store energy in a rotor in a vacuum, and special capacitors known as 'ultra' or 'super' capacitors. These work in a similar way to the capacitors you'd find in electric circuits, storing energy in an electrostatic field. All of these technologies have their pros and cons, which make storage design challenging. For each case the engineers have to decide which type, technology and size of unit is required.
Despite the difficulties in managing regenerated energy, operators are implementing all these methods with successful results, right now. They are also trying to save energy in a number of other ways, e.g., with more efficient equipment, eco-friendly driving, and better traffic management, to name just a few. The next time you want to make the 'green' choice, take the train. Railways really are driving us towards a more sustainable future.