March 21, 2018

New onboard batteries lighten the load

Written by  Benedikt Vogel
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Swiss Federal Railways’ (SBB) desire to reduce the energy consumption of its vehicle fleet has led to the development of a new lighter and more efficient onboard battery. Benedikt Vogel, on behalf of the Swiss Federal Office of Energy (SFOE), explores the intricacies of the SBB New Battery Technology project.

WHILE Switzerland’s entire 3200km network of standard and narrow-gauge lines is electrified at 15kV ac, each of Swiss Federal Railways’ (SBB) approximately 3500 passenger coaches, multiple units or locomotives which use the railway are also fitted with one or more batteries.

These batteries feed the vehicle’s onboard network in emergencies and during short supply interruptions, which can occur in normal operation or during shunting at stations. During these incidents, the electrical system supplies the energy required for lighting, doors and customer information systems as well as guarantees the operation of the magnetic rail brakes.

SBB mainly uses 36V lead-acid batteries, each weighing 334kg, for these tasks. Across its entire vehicle fleet, this totals around 2000 tonnes of battery equipment and is contributing significantly to energy consumption.
SBB thus initiated a programme to begin replacing these heavy lead-acid batteries with new technologically-advanced batteries which are lighter, more energy-efficient and cheaper over their lifecycle.

SBB batteriesAs well as improving performance, the new battery must be installed without requiring special adaptations to the existing railway vehicles. This was a major challenge facing the SBB New Battery Technology project, which was launched in mid-2014.

SBB is working on the initiative in partnership with BFH-CSEM Energy Storage Research Centre in Biel, which was created in 2015 as a joint institution of the Bern University of Applied Sciences (BFH) and the Center Suisse d’Electronique et de Microtechnique (CSEM). The Swiss Federal Office of Energy (SFOE) is supporting the project as part of its pilot and demonstration programme.

Around three-and-a-half years after the project launched, a functional prototype battery is now available. The new battery satisfies both the technical requirements of a modern SBB battery while providing various implementation options.

The prototype model consists of 11 lithium iron phosphate battery cells (LiFePO4). Each cell is equipped with a circuit connection board, which among other things, can determine the temperature of the cells. Control and monitoring takes place via the central battery management system (BMS).

The prototype model tolerates a charging voltage of 39-45V. This flexibility is necessary because of variations in the charging voltage of the onboard electrical system and differences in the charging characteristics of lead batteries depending on the ambient temperature. The prototype battery weighs 110kg, which compares with the 334kg weight of the lead-acid battery. The 60-litre volume is also half that of the older model.

The new battery is initially designed for retrofitting in older railway vehicles which use a 36V onboard system. In the long-term, however, it could be used in an adapted form in newer railway vehicles, which use onboard systems that are fed by a 110V supply. Indeed, a corresponding project is already underway with an industrial partner, which incorporates the insights gained from the 36V prototype model.

The new battery is designed to supply power sporadically to wagons and, to a limited extent, support everyday operations. For example, a Stadler Flirt low-floor EMU, which is used for regional services, requires around five minutes of energy, or about 570Wh, when the train is brought into service in the morning. During the day, the battery is typically used 18 times in very brief instances, namely when the train passes a section without power supply such as neutral sections, where the onboard systems draw approximately 150Wh per minute from the battery. When decommissioning the train in the evening, the battery supplies another 1800Wh for 20 minutes of use.

The battery is recharged after each load. And while an electric car battery must have a maximum energy density, this is not required of the SBB battery because of the very specific performance profile.

Robustness

The robustness of the lithium-iron phosphate technology used in the prototype reduces the potential for damage to occur from strong vibrations. It also enables the unit to work reliably in temperatures as low as -20°C (see below). However, it is important that the battery is not fully charged when in use in warm and ambient temperatures, especially when new. The storage capacity was chosen to be similar to the previous lead acid battery to guarantee three-hour emergency operation of the vehicle even under the most adverse circumstances. For the functional battery prototype, this corresponds to a capacity of 6.5kWh when new.

The control and monitoring of the prototype is provided by a specially-developed battery management system (BMS). Among other things, the BMS ensures that the battery is recharged as needed and regulates the charging process so that the battery can provide at least 2kWh of energy at any time. This is required according to the demands of emergency operation in the event of a power failure by, for example, ensuring that onboard lighting is available for three hours.

Thanks to this optimised control and the low electrical load, the battery is forecast to have a lifespan of at least 12 years, which compares with five to seven years for a lead-acid battery. “Since the new battery has twice the service life, the higher initial costs can be easily compensated,” says BFH development engineer, Mr Christian Vögtli.

Electricity supply for an onboard network is usually sourced from overhead catenary. In Switzerland, 15kV of alternating current is transformed at various stages before being rectified into about 42V of direct current.

Permanently connected

The lead acid batteries used in railway vehicles are permanently connected to the electrical system and are therefore always exposed to a charge voltage. The new battery is different because it can temporarily disconnect from the electrical system, for example, when it is fully charged. This enables the battery voltage to fall after the charging process, which increases the life of the battery cells.

For this purpose, a specially controlled semiconductor switch was developed which ensures the battery is ready for use at any time. For example, if the onboard network malfunctions due to a power interruption, the switch can control the energy flow into the onboard network immediately and without active intervention. The semiconductor switch can also enable charging to take place according to seasonal requirements.

In addition, the battery prototype developed in Biel offers a new option for maintenance. Current batteries must be checked every six months - for outgassing, for example - and their expiry date must be verified visually. This could become superfluous in the future as the BMS automatically analyses the state of health of the battery and periodically sends the values via GSM to the service centre. Batteries would then no longer be replaced as a precaution, as is the case today, but only if their state of health no longer meets the requirements.

Work is continuing to optimise the prototype battery with developers striving to replace its fire-resistant steel hutch to further reduce weight. This is not at the expense of the battery’s safety credentials as fire protection remains the highest priority for battery developers, especially because lithium-ion batteries were cited as the cause of several notable fires in recent years.

“The problem with these incidents was rarely down to the design of the cells alone, but an over-exhausted and overloaded system,” Vögtli says. “The fire risk is manageable in our case since the electrical load and performance of the system is not critical. The BMS primarily provides operational reliability since the lithium-iron phosphate cells, with their comparatively moderate energy density and high intrinsic safety, are extremely stable even in the event of mishandling, which also mitigates the risk of fire.”

The new battery is one of several measures which SBB is exploring to cut power consumption and operating costs over the medium-term. In 2012, SBB’s board of directors approved a programme to increase energy efficiency with the goal of permanently saving 600GWh of energy per year by 2025. This corresponds to 20% of SBB’s total energy consumption - or the equivalent of the annual usage of 150,000 four-person households. If SBB is successful, its trains will only use renewable energy in the future and would no longer source power from nuclear plants.

Originally, SBB planned to build several prototype batteries before field testing and producing the battery itself. However, the railway has now decided against this plan. “On the one hand, the increase in knowledge of technical issues would be disproportionate to the investment in a pilot operation,” says Mr Ueli Kramer, project manager with SBB. “On the other hand, SBB strives to procure modular products under favourable market conditions, instead of acting as a manufacturer.”

As a result, the prototype will serve as a technical reference for the procurement of a new battery that meets the required specifications from the open market. SBB is planning to equip around 2200 older coaches and locomotives with the new batteries in the medium term, and Kramer says this will begin with an initial order in early 2018.

“With the prototype model, we were able to develop the necessary knowledge, gain important experience and thus develop the specifications for a railway-compatible system,” Kramer says. “Each manufacturer can use the best available technologies in their products and adopt approaches and ideas from the prototype model in order to achieve the specified requirements.”

SBB, then, is not the only railway operator which could benefit from its battery development programme. It might soon be used by other railways looking to reduce energy consumption, both in Switzerland and around the world.

 

Defying low temperatures

IN order to ensure a long life, lithium-iron phosphate batteries must be charged gently, especially when exposed to temperatures of -20°C and below.

BFH developers have pursued several approaches to overcome this challenge. For example, special battery cells have been chosen that tolerate low temperatures, as shown by laboratory tests of the BFH-CSEM Energy Storage Research Centre. In addition, the use of an electric cell heater was tested where a 700W/h heat input through the poles is able to increase the temperature of the battery mass by 20 Kelvin. Finally, the prototype model contains a dc-dc converter, which enables a controlled, slowed and gentle charge of up to 800W, independent of the fixed charging current settings of the onboard electrical system.

Not all of these precautions are necessary for the operation of the battery in low temperatures. Indeed, the project managers say it is possible to maintain the desired service life and availability with two of the three precautions. How the battery successfully harmonises low-temperature chargeability with longevity will be demonstrated during the bidding process.

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