Keep track of greenhouse gases with electric trains

Keep track of greenhouse gases with electric trains

According to the International Energy Agency (IEA), rail is one of the most efficient and lowest emitting modes of transport. In a foreword to the IEA’s report The Future of Rail, executive director, Dr Fatih Birol, believes that rail is neglected in public debates about future transport systems, losing out due to a focus on cars and aeroplanes.

Worldwide, 75% of rail passenger movements and half of rail freight relies on electricity. It is, argues the IEA, positioned to take advantage of the rise of renewable energy.

Rail accounts for 8% of the world’s motorised passenger movements and 7% of freight transport, yet uses just 2% of the world’s transport energy demand, the report says.

The report says that while the global rail energy demand has remained relatively constant, since 2000 it has fallen in Europe and Japan, increased in Russia, China and India, and stayed relatively constant in North America.

If all rail services were to be carried by air and road, transport-related greenhouse gas (GHG) emissions would be 1.2bn tonnes of CO2-equivalent (GtCO2e) per year higher.

Planes, trains and automobiles

Approximately 0.3% of CO₂ emissions from fossil fuels come from rail, compared to around 2% for global aviation emissions, says the report. Emissions from trains vary, depending on if they are powered by diesel or electricity, as well as how that electricity is generated. Electric trains can reduce emissions compared with diesel-powered trains only if the power generation mix is not dependent on fuels with high carbon content, such as coal.  

According to the IEA, environmental life-cycle assessments show that the rail projects best able to reduce greenhouse gases are those that minimise the need for large amounts of steel, iron and concrete in construction, have a high passenger or freight throughput and help to shift away from other modes of transport with even higher carbon intensities, such as car, lorries and aviation.

Figure 1: The EU has 54% electrified rail in the same year or over 217,000km (approximately 135,000 miles) of useable railway lines

Metro and light rail networks operate in most of the world’s major cities, and networks are growing. The report also makes the comparisons of efficiency, noting that passenger trains are less energy efficient in the US and Europe than in Asia, primarily due to lower occupancy.

Korea, Japan, Europe, China and Russia have rail networks which are more than 60% electrified. In Asia, Korea has around 85% and Europe has some countries strongly committed to electric railways.

According to Statistica, in 2017, Switzerland was the only European country where all railway lines in use were electrified. Another European champion is Luxembourg which had the highest share of electrified railway systems, at 95.3%. The EU as a whole has 54% electrified rail in the same year or over 217,000km (approximately 135,000 miles) of useable railway lines.  

North and South America have less than 5% rail electrification. Rail is the primary transport mode in India and its rail activity is set to grow more than any other country, says the IEA. It is constructing the country’s first high-speed line from Ahmedabad to Mumbai.

High-speed rail is particularly important as it offers an established low-carbon alternative to short-distance flights, says the IEA. The overall impact on greenhouse gas emissions of a new high-speed rail line depends on passenger behaviour and operational practices, but a new high-speed line can produce “almost immediate net CO2 benefits” by reducing air and car journeys and reduce aviation transport on the same routes by as much as 80%, says the IEA.

On urban routes, rail can reduce emissions, but depends on whether it attracts commuters who would otherwise use a car, as well as the emissions intensity of its power supply.

Freight expansion

Rail freight has risen steadily over the past 20 years and continues to expand in most countries, says the IEA. However, other forms of surface freight, such as lorries, are expanding faster, it adds. The US and China each account for about 25% of global rail freight activity and Russia about 20%, says the report. In some countries, freight transport outweighs passenger rail. In the US, for example, around 93% of kilometres travelled by train are for freight.

Rail uses around 90% less energy than trucks per unit of freight and freight trains in Russia and China are the most energy efficient due to high loading and electrification, adds the IEA.

“If railways in Europe and eastern Asia are anything to go by, an electrified rail network is inevitable,” says Steve Hughes, managing director of REO UK. “What’s crucial is to do it right,” he adds.

Hughes believes there are many reasons that businesses and passengers are not jumping on board electric trains, just yet.

“The political wind has been blowing towards complete rail electrification for decades now without significant progress, so it’s clearly not a straightforward problem to tackle. If it’s to be done, it needs be done right,” he says.

He believes ensuring the selection of components for electric trains need to be carefully considered.

Despite the reduction in CO₂ emissions for electric trains, Hughes argues the only reason diesel trains have survived is that over 40% of the UK’s rail network remains un-electrified, “meaning only diesel trains are able to run these vast swathes of rail”.

To ensure the infrastructure stands the test of time, original equipment manufacturers (OEMs) must select the right electrical components, optimised in terms of application and cost, says Hughes.

Kinetic energy

A 150-tonne locomotive travelling at 40m/second (around 90mph) carries 240m joules of kinetic energy, equivalent to about 72kg of TNT. When coming to a stop at a platform, this huge amount of energy is simply discarded through the braking process.

“This is where an important energy-saving benefit of electrified trains becomes apparent,” says Hughes. “Instead of wasteful rheostatic, air or friction braking, regenerative braking captures the kinetic energy of the speeding train and turns it into electricity”.

The captured energy can either be reintroduced to the grid or stored. Locally, batteries or super-capacitors hold the charge for use when required. This adds costs to the train’s maintenance and construction, as well as increasing the weight of the train.

Reintroducing the power to the grid is a better approach for large-scale electrified rail. In mature, well implemented systems, with trains consistently stopping and moving off, spare energy pushed onto the grid by braking vehicles is immediately co-opted for use elsewhere.

When switching between braking and driving, pushing current to the grid for braking and pulling current from the grid for driving, the switching of loads causes pulses of voltage on the grid. Rectifying and inverting circuits on-board the train has the same effect.

If unaccounted for, these transient pulses can create dramatic voltage spikes in the network. The resulting disturbances can have a range of negative effects, from simply blowing fuses and damaging voltage-sensitive equipment, to causing long, unshielded stretches of overhead lines to act like colossal radio transmitters, explains Hughes.

These events can be addressed by electromagnetic-compatible (EMC) devices take the incoming power signal - direct or alternating - and strip out the unwanted higher frequencies to then transmit clean and consistent voltage from their outputs.