Deutsche Bahn

Are battery powered trains a realistic alternative to network electrificati

Mike Squires: "Rechargeable propulsion batteries can provide a source of energy to drive a train’s traction motors. ..."

Mike Squires,  ESG Rails Electrical Engineering Manager discusses: "Rechargeable propulsion batteries can provide a source of energy to drive a train’s traction motors. Propulsion battery technology is long established and has been in use since the 1930’s, however it is only in the last few years that this technology has been considered a serious contender for passenger rolling stock.

Bombardier announced a new project at last month’s Innotrans exhibition in Germany, trialling a battery - electric version of a Talent 3 Electric Multiple Unit (EMU), one of many similar trials that have been recently undertaken or are currently running globally. Clearly there is renewed global interest in this technology. It is therefore appropriate to consider whether trains fitted with propulsion batteries really are a serious alternative to diesel or network electrification.

The case for battery powered units

Rail propulsion battery technology has improved greatly over the past 20 years. Conventional battery technology not only added huge weight to trains and required considerable maintenance, but these vehicles generally had no means of re-charging on the move, which severely limited their range. For example, London Underground Limited (LUL) have used battery locomotives to haul Infrastructure trains for many years, but usually only just have sufficient range on a single charge to make a single return run to the worksite.

The latest lithium-ion based battery technologies are now much better suited to the duty required for propulsion applications. The power densities available with this type of battery technology make it feasible to install battery packs of sufficient capacity on a multiple unit that can give a range of several tens of kilometres on a single charge.

In the case of a hybrid battery / EMU arrangement, the propulsion batteries can be recharged on the move, drawing power from overhead lines when running on electrified sections. If sufficient range can be achieved when running on battery power alone, a hybrid / battery EMU certainly lends itself to “bridging” non-electrified routes between electrified lines, or making a return run on a branch line linked to an electrified main line. Typically in this situation, the conventional solution would be to simply run a Diesel Multiple Unit (DMU), perhaps over a largely electrified route, just so that it can serve a few non-electrified sections!

A hybrid battery / EMU would be highly advantageous on these partially electrified routes, eliminating the need to run a DMU for these services. For example, the lightly used Matlock branch in Derbyshire need not be electrified along with the rest of the Midland Main Line, thus saving considerable cost. A hybrid battery / EMU based on such as the Bombardier Class 379 prototype (discussed later) could comfortably make a return run on this branch line on battery power alone.

Another prime example where a hybrid / battery EMU would be extremely useful, is on the line passing through Farringdon station in London. Such a unit would be able to eliminate the current use of conventional dual voltage stock, which have to stop in the platform and manually switch from the 25kV ac overhead catenary to 750V dc third rail traction supply when heading south. This could even lead to the complete removal of the short section of third rail electrification north of the Thames.

Other Considerations

EMUs fitted with propulsion batteries will of course incur a higher initial purchase price and, depending on the specific operational requirements, may also require considerable additional infrastructure to be installed to facilitate propulsion battery recharging. The limitations on maximum achievable range means that a wholly battery powered EMU is not yet a completely viable alternative to diesel on all non-electrified routes.

In spite of the advantages outlined previously, hybrid battery / EMUs are still likely to be heavier than a conventional EMU, simply because of the additional equipment fitted (propulsion battery pack, charging control equipment and associated thermal management systems). Special precautions also need to be taken with regard to propulsion battery containment and safe maintenance practices, as the terminal voltages are in the order of 700V. Furthermore, the batteries themselves contain some hazardous materials, which may present an increased fire hazard to the unit. The battery containment solution therefore has to be thought of as more of a sealed “tank”, this requires careful design to maintain adequate heat dissipation. The resulting additional weight of the unit may have both a negative impact on performance and incur higher track access charges. Therefore in some cases, entire route electrification may prove to be a better option.

Electrification remains expensive and can take years to complete. The UK network is very complex, with many branch lines stemming from mainline routes. Whilst the case for electrifying the core sections may be clear, electrifying all the branch lines may prove economically inviable. Unfortunately, this situation could still result in diesel trains continuing to operate over large sections of electrified networks for many years because of the need to also serve the non-electrified branch lines.

The UK is investing a significant amount into route electrification, which will eventually result in 50% of the network becoming electrified and 80% of passenger kilometres delivered by electric passenger trains. Electric trains are generally more reliable than their diesel counterparts, require less maintenance, are quieter and produce far less direct CO2 and pollution. However, as 50% of UK routes will remain non-electrified, hybrid / battery EMUs have the potential to be part of the solution by providing a more eco-friendly, ‘go anywhere’ alternative to DMUs, on at least some of the non-electrified routes. These units will take traction power from the overhead line when on electrified sections, running as a 25kV EMU, whilst simultaneously recharging the propulsion battery pack. The units would then seamlessly switch to the propulsion batteries when on the non-electrified, branch lines.


What is being done in the UK to develop this technology?

In January/February 2015, a trial of the first UK battery hybrid train for 50 years was conducted. A prototype, known as an independently powered, electric multiple unit (IPEMU) entered trial passenger service. The unit consisted of a converted Bombardier Electrostar Class 379 4-car, 25kv 50Hz EMU, that had been retrofitted with a lithium ion magnesium phosphate propulsion battery pack for off catenery use. This trial was deemed to be a success and has strengthened the case for similar IPEMUs running in the UK.

However, in spite of the positive outcome of the trial, there remain some issues which may deter potential industry investors, such as uncertainties over propulsion battery life, and the need to further develop some details of the prototype design to production standard, such as the propulsion battery containment and thermal management. All stakeholders in the concept now need to develop the case further and determine the cost of developing a new generation of battery powered vehicles versus conversion of current stock.

How can ESG Rail help?

ESG can help determine the best rolling stock for a particular operation, whether retrofit, new build, diesel, hybrid, electric battery or electric.

ESG understands railway rolling stock and can assist manufacturers with development issues, such as the physical integration of propulsion battery technology into rail vehicle design. In addition, ESG can utilise the specialist battery expertise of its parent company DB Systemtechnik (DBST). DBST has already replaced diesel drives on trains with hybrid versions, accommodating lithium ion batteries and is developing an Eco Train using a VT642 unit, which includes amongst its many innovations, the latest battery technology.