Krishna emphasizes that to be fully effective, the WESS must also have the power capacity to peak-shave the high-power demands required to accelerate trains from passenger stations. These peak-power loads can often require up to 6 megawatts of power for only a few seconds at unscheduled times and can place a burden on the utility. Therefore, utility charges typically include costly peak-power demand surcharges, which are determined by the time period(s) with the highest energy usage. Each electric utility sets a time period for measuring power usage; typically, it is one or two intervals of five-, 15- or 30-minute durations. Peak-power demand charges are set by the period with the highest energy usage in a billing cycle, which is typically one month. Power-demand charges during peak hours of the peak season can be as much as five times higher than charges during base periods. The peak-power demand is calculated by dividing the energy used in the peak period by the duration. The units are kWh/h, or just kW. “Some transit operators report that their utility bills are difficult to understand based on the various surcharges and demand charges. But it is often the case that the peak-power demand costs are higher than their total energy use costs,” notes Krishna.
Energy Storage Technologies
In looking for the optimum clean energy storage solution for LACMTA, Krishna concludes that flywheel systems would be the best solution. Table 1 compares the flywheel, supercap and battery technologies. The most effective technology for this application will require the ability to cycle (charge and discharge) up to once every two minutes while maintaining its capacity to absorb all of the regenerated energy created by a train. In the case of the LACMTA Westlake station, current requirements will require more than 100,000 cycles per year and over the next 10 years this number is expected to increase by 25 percent. “The flywheel is an attractive technology, as it is most efficient for high-power discharges in short bursts of approximately 15 to 30 seconds. This fits nicely into the required output of the traction power application. The flywheel is also impervious to cycle life and can, therefore, cycle indefinitely without wear and with high reliability,” says Krishna.
The problem with most existing energy storage technologies is that they have a limited life, dependent on the number of charge-discharge cycles they must produce. Most technologies are also temperature sensitive and require heavy maintenance. Chemical batteries, such as the familiar lead-acid batteries, have long been the accepted method to store energy in most applications. However, if a battery system must charge and discharge for each train arriving and leaving a passenger station over a full day and over an entire year, the life of the batteries is severely reduced. One idea is to increase the energy capacity of the battery system so that discharges are only partial and, therefore, their useful life is extended. The difficulty here is that there is an increase in the number of batteries, resulting in an increase in initial cost, increased maintenance and added reliability problems. The recharge time required in this application, which is 15 to 30 seconds to absorb the regenerated energy, is also much too fast for traditional batteries to absorb. A WESS demonstration site is in operation in Sacramento, Calif. This site supports voltage fluctuations on the light rail system using lead-acid batteries. The site has shown the benefit of a WESS in eliminating the voltage sag problem, but has also shown that lead-acid batteries cannot charge using the short duration, high-power input of the train’s regenerative energy.
Supercapacitor or ultracapacitor technology is also an option for this application. Although great strides have been achieved in lowering cost and improving cycle life of supercapacitors, they have not yet shown that they can support the 20-year life expectancy and harsh environment of the traction power application.
The next step is to install and demonstrate the flywheel technology in a traction power application. Based on existing data from flywheels used in critical backup power applications and Vycon’s experience with installations in heavy cycling shipyard mobile cranes, Krishna believes that the flywheel can provide the benefit of high cycling without degradation and long life with high reliability and minimal maintenance.
Although busy with his new consulting firm, Krishna keeps in close contact with his former LACMTA colleagues. He expects that the Wayside Energy Storage Substation will be in operation by June 2012. He hopes that the results of his dedicated effort will convince other transit operators and communities to follow his lead and help improve the efficiency of our power grids. Can we afford to throw away free energy over the next 50 years? Krishna says emphatically, “No!”