REGENERATIVE braking has the potential to generate important efficiency gains and cost reductions for metro networks. When a train brakes entering the station, its traction motors act as generators, providing power that can be fed back to the third rail or overhead contact system. This energy can be used to provide a power boost for another train accelerating out of the station. However, unless the second train is accelerating from the platform at the same time the first train is braking, the regenerated power is dissipated as heat through the train’s brake resistors to prevent causing an overvoltage on the dc bus.
Each year, thousands of kWh of energy are lost through wasted heat from braking regeneration. Typical metro headways range from around three minutes in the peak to five minutes at other times. Assuming a line has 12 stations, there would be 2880 stops and starts during a 20-hour day. This means even a relatively modest reduction of electrical usage per stop and start can have a beneficial effect on the bottom line.
The key to successful optimisation of rail regeneration is to provide a local energy storage capability that can capture and store energy produced by braking systems, and deliver it on-demand to reduce the power required for an accelerating train. In a typical application, the energy storage unit is connected to the dc bus in parallel with two traction power rectifiers. Regenerated power from the braking train is fed through the third rail or overhead electric line to charge the energy storage unit. When another train sets off from the station, the storage unit discharges the regenerated energy and sends it to the accelerating train. This would also permit the braking train and accelerating train to be one and the same.
In addition to reducing the amount of energy dissipated through brake resistors, a flywheel-based regeneration system can stabilise the traction power system voltage by eliminating voltage sags and peaks which commonly occur when trains accelerate and brake. The energy storage unit charges during peaks and discharges during sags, keeping the voltage within operating tolerances. This can help protect other systems from damage. It can also supplement low voltage on the traction power at a fraction of the cost of adding new traction power substations.
There are three basic components in a rail regeneration energy storage and recovery system:
- a power conversion subsystem, and
- measurement and controls.
The storage element is the most costly and the most critical to efficiency and lifespan.
There are three choices when it comes to energy storage technology: batteries, supercapacitors, and flywheels. Battery-based systems are expensive, in terms of both capital expenditure and lifespan operating costs. Moreover, current lithium-based battery technology cannot produce a charge or discharge duration repetitively to satisfy a typical metro duty cycle of two minutes or less. For this reason, battery-based systems are normally oversized for the application. Batteries also need more space, must be replaced periodically and require environmental controls.
Supercapacitors - electrolytic double-layer capacitors - are capable of faster repetitive charge-discharge cycles. While they are less expensive than batteries and do not require oversizing to meet the demands of rail regeneration applications, they have limitations. Supercapacitors have a relatively short lifespan and their effectiveness decays over time, so they must be replaced every few years.
Flywheel-based energy storage technology is proven and mature and provides a low-risk, low-cost solution. Flywheels have a high level of reliability, durability and availability, can operate continuously with two-minute headways without compromising product life. They also provide the lowest life-cycle cost, including installed costs and maintenance.
The factors influencing the viability of a flywheel-based Wayside Energy Storage System (Wess) installation are essentially the same as for any other energy storage technology. The starting point is a study to determine actual energy recapture needs, desired return on investment, capital expenditures, operating costs, maintenance requirements, space restrictions and other considerations, based on the line’s headways, traffic density, utility costs, and other factors.
Vycon has extensive practical experience in flywheel energy storage systems, with a global installed fleet of more than 1200 deployed sites, which have accumulated over 26 million operating hours and 19 million discharge/recharge cycles. Applied in both regenerative energy and critical back-up power applications, where failures are unacceptable.
In terms of reliability, Vycon’s flywheel energy storage systems are used for UPS backup in mission-critical applications such as hospitals, data centres, utilities and military installations, where failures are unacceptable. They are designed for better than 99.9999% reliability.
Vycon has now turned its attention to the metro rail market, and has developed a new flywheel energy storage and delivery unit specifically to meet the unique requirements of rail braking regeneration.
The Vycon flywheel system stores kinetic energy in the form of a rotating mass, and is designed for high-power short-discharge applications. Patented technology used within the flywheel system includes a high-speed motor generator and contact-free magnetic bearings that levitate and sustain the rotor during operation. Flywheel systems can accelerate and decelerate at extremely high rates, enabling them to charge and discharge energy in seconds. Their low losses and high reliability make them suitable for demanding duty-cycle applications. A typical flywheel can handle over a million deep-discharge cycles without affecting performance or life of the storage element.
The flywheel uses permanent magnet-biased magnetic bearings and runs in a vacuum. At idle speeds, power losses are near zero and the permanent magnetic bearings are maintenance-free. The only recurring maintenance requirements for a typical installation would be to check the air filters, typically every six months, depending on the cleanliness of the location, and topping up oil on the vacuum pump, which is usually carried out annually. Self-diagnostics provides a warning if any maintenance is required.
The footprint of the flywheel unit is approximately 1x1m. The recommended installation location is at a traction power substation, but it can be installed in the station itself. The modular design means any number of flywheels can be installed at the same location, or distributed in available spaces in groups. During installation, access is required from the top of the unit, but access is only required at the front of the unit for maintenance.
The first Vycon rail regeneration system has been installed by Los Angeles Metro in a project funded by the Federal Transit Administration (FTA) through the Transit Investment Generating Economic Recovery (Tiger) programme. The system has accumulated 200,000 hours and
1 million cycles during normal train operations and has reportedly generated significant operating cost savings.
Services on the metro Red and Purple lines are operated by six-car trains, which run at speeds of up to 105km/h with five-minute headways on weekdays. To reduce energy usage, Los Angeles Metro installed a Vycon flywheel Wess at the traction power substation (TPSS) at Westlake/ MacArthur Park station, and the system was commissioned in August 2014.
The Wess has a 2MW installed capacity for 15 seconds, or 8.33kWh, and can be expanded to 6MW for 15 seconds, or 25kWh. The 2MW installation consists of four flywheel modules (FWMs), each containing four flywheel units (FWUs). The 16 FWUs are connected in parallel and commanded to charge and discharge in sync. Wess can turn individual FWUs off or on independently and can operate with any number of FWUs active. In normal operation, the FWUs run between 10,000rpm with zero available energy and 20,000rpm with maximum available energy.
Initial tests during the first year of operation revealed savings of 10-18% in traction power consumption at Westlake TPSS. Wess has saved an average of 1.6MWh per day during the week and 1.5MWh per day at weekends.
The design lifespan for a flywheel Wess is around 1 million cycles or 20 years. After that, the system can be re-inspected and recertified to extend its life. This compares very favourably to the lifespan of a battery or supercapacitor-based Wess.
The Los Angeles metro installation has been continuously optimised by collection and analysis of data, to improve the rate of return for the customer. In addition to plain energy storage and recovery, the system has also been optimised to enable peak power saving and bus voltage stabilisation, and to switch between the support modes automatically.