UltraCaps win out in energy storage

Richard Hope explains how double-layer capacitors have broken through the economic barriers to capturing braking energy on a Mannheim light rail vehicle

REGENERATIVE BRAKING is widely practised, but there have to be other trains around to absorb the surplus power being fed back into the catenary or third rail. Processing the output from trains and pushing it back into the local grid is possible with an AC power supply, but very expensive with DC traction. Too often, power produced by traction motors in braking mode ends up heating resistor banks.

The elegant alternative is to store the braking energy on the train. This not only avoids the electrical complications of regenerating through the traction power supply network. It reduces the rated power requirement of that network by lopping demand peaks during acceleration, saves energy by reducing losses in the catenary or conductor rail, and by limiting voltage drop it allows substations to be further apart.

NiMH batteries have the necessary energy storage density in terms of kWh/kg, and are slightly more expensive, but their life in terms of charge/discharge cycles in no way matches the LRV requirement for 2million cycles over 10 years. Flywheels have been tried but never caught on for several reasons.

A decade ago, few would have considered seriously using capacitors for this purpose, but electrical engineers realised that the energy storage capacity of double-layer capacitors was developing so rapidly that using them to boost the power of trams, metro trains and even DMUs for periods of a few minutes was becoming feasible.

UltraCap is one such product. A paper presented last month by Dr Michael Steiner and Dr Johannes Scholten of Bombardier at the World Congress on Railway Research in Montréal showed that their energy storage density had more than tripled since 1998 to reach 6 kWh/kg today. Equally important is the maximum power that can be drawn for traction; over the same period this has increased sevenfold to 6 kW/kg.

Mitrac trials successful

To demonstrate the practical value of using UltraCaps in this way, Bombardier equipped a light rail vehicle with a roof-mounted Mitrac Energy Saver. City transport operator MVV has been operating the LRV in normal service on the Mannheim Stadtbahn network since September 2003 (Fig 1).

This LRV has two powered bogies, each with two motors. A Mitrac unit was connected in parallel to the DC link of the traction inverter producing variable voltage and frequency AC for the two motors of one bogie. The other bogie is supplied conventionally so that the energy performance of the two halves of the tram could be compared.

In addition to the UltraCap capacitors, the Mitrac unit contains a bi-directional step-down IGBT chopper which regulates charge and discharge of the capacitors according to the requirement to store or export energy (Fig 2). Mitrac weighs about 450 kg, and the external dimensions are 1900 mm long, 950 mm wide and 455 mm deep.

The other half of the LRV regenerates braking energy into the line when it is receptive, or into braking resistors if the line voltage is too high. So the comparison of energy saved with and without Mitrac directly compares regeneration into the line with regeneration into the UltraCaps. It does not just compare energy storage on the LRV with no regeneration at all, as would be the case with a diesel-electric train.

This explains why the average energy saving of around 30% demonstrated by the Mitrac half of the Mannheim LRV for most of the year, compared to its conventional twin, dropped to around 27% in the cold winter of early 2004. Steady loads such as point heaters and heating inside other trams had made the line more consistently receptive to regenerated energy.

Peak load reduced

The energy saving is not just limited to the LRV itself. As Fig 3 shows, there is a 50% reduction in the current drawn during acceleration, which not only reduces losses in the overhead wire and the supply system generally, but also cuts electricity tariffs which normally include a maximum demand element.

Note that the line voltage drop is also halved in Fig 3. This feature can be exploited either by boosting acceleration of the LRV, thus cutting journey time, or by economising in the power supply system when building a new line, perhaps cutting out a substation. Or where traffic is rising Mitrac would enable an existing power supply to cope with a more frequent service.

Drawbacks to energy storage include an increase of about 2% in the weight of an LRV, and also the space required unless the unit is roof-mounted. As for reliability, Bombardier reports that there has not been a single failure of an UltraCap since the Energy Saver trials started more than 30 months ago.

Spanning a gap

Another advantage of energy storage is that the vehicle can still move under its own power where there is a break in the continuity of the overhead contact wire. The Mannheim LRV was consistently able to travel through a simulated 500m 'gap' reaching speeds up to 26 km/h with the pantograph lowered, despite the fact that it has only one Mitrac unit fitted rather than a normal pair.

This has a number of operational advantages, such as reaching a convenient place to stop when power is lost, coping with ice on the contact wire, and moving within depots or workshops.

There are also cases where planning authorities object to overhead wires in front of historic buildings. This is not a trivial matter. Where a complex tram junction is proposed in an old city square, for example, the visual impact of overhead wires can become a make-or-break issue, determining whether or not a new tram route is allowed to go ahead.

Metros can benefit too

As part of Bombardier's development project, the potential energy savings were modelled for a modern European metro. The general conclusion was that the proportion of total energy saved is likely to be less on a metro than for trams or LRVs, but still very substantial.

One reason for this is that regeneration rates are typically much higher on a metro, provided the third rail or overhead wires are coupled through between substations, because there are more trains to absorb the braking energy. On the other hand, the opportunity to reduce supply losses is much greater because the trains are heavier and the currents in the third rail or overhead wires much higher.

If the sections fed by each substation are not coupled through, storing braking energy on the train saves significantly more energy because more of the braking energy is lost in the resistor grids instead of being used productively by other trains. This is evident from the range of energy savings potentially gained from using Mitrac under three circumstances:

• LRV or tram: 18% to 31%
• Metro coupled: 14% to 21%
• Metro not coupled: 17% to 29%

Two more points are worth noting. First, the distance between stops has a big effect on the energy saved. In the tram/LRV case, with a maximum speed of 50 km/h on level track the energy saving is 33% where the average distance is 500m, but only 20% at 2 km, other things being equal.

Secondly, although the metro simulation was in tunnel, there is no mention in Bombardier's paper of energy that is consumed by fans and other ways of removing heat from metros. This would certainly boost the total energy saved by on-train storage devices significantly in hot climates, where much of the world's metro construction is taking place.

Diesels can also benefit

Bombardier has also modelled a three-car articulated diesel-electric vehicle designed for a maximum speed of 120 km/h and local services with stops at 6 km intervals. This spacing is longer than the LRV and metro examples, but against that, for diesel powered trains on-board energy saving offers the only possibility to recover any braking energy.

The modelled vehicle has two 315 kW diesel power packs under the floor of the end cars, each with a power converter on the roof. The centre car has a Mitrac Energy Saver on the roof which can store 4·4 kWh. It weighs much less than a third diesel power pack, but can effectively deliver a 50% increase in output, and therefore acceleration.

The user can take the energy storage benefits in two ways, or combine them:

• he can run the service faster through higher acceleration, maybe reducing his fleet size for delivering the same timetable as well as winning more passengers, and still use 10% less fuel;
• or he can cut his fuel bill by up to 35% and provide the same service as he would with only two power packs.

And it should not be forgotten that a 35% fuel saving means emissions will be reduced by a similar amount.

Return on investment

All this is fine, but what is the impact on the bottom line? Bombardier reckons that in the case of the modelled three-car DMU the initial investment in a Mitrac Energy Saver would be recovered in two to fouryears, depending on the way it is used. This estimate assumes that diesel costs €0·95/litre, and that passengers value their time saved at €5 an hour.

As energy costs rise, the benefits of on-train energy storage also go up. After more than two years of proven service in Mannheim, the time has surely come for operators to try installing double-layer capacitors on a fleet basis.

• Fig 1. This LRV in Mannheim has one 860 Wh Mitrac Energy Saver (inset) on the roof of the leading section supplying two of the four traction motors
• Fig 2. The Mitrac unit and the IGBTs that control it are connected in parallel with choppers supplying VVVF to the traction motors
• Fig 3. Calculations show that an LRV fully fitted with Mitrac halves both the current drawn and the consequent line voltage drop when it accelerates on full power for over 20 sec

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