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Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 8 of 61 technology, transit agencies are looking for guideposts to assess the effectiveness of each application, the performance characteristics of competing energy storage designs, and estimations on energy cost savings. 2 Study Objectives To address the need of transit agencies to reduce energy use and their need to better understand how energy storage may help, the American Public Transportation Association (APTA) in association with the Electric Power Research Institute (EPRI) developed a group called the Energy Storage Research Consortium, dedicated to accelerating the understanding, coordination and application of energy storage technologies within transit and utilities. From this initiation and publication of an APTA white paper examining the potential of energy storage in transit, a Transit Cooperative Research Project (TCRP) Quick Study was granted with the intent of providing a more concrete examination of energy storage potential utilizing transit agency data and detailed computer simulations. Analyses are performed on varying transit system configurations according to size and frequency of operation, variations in energy storage technologies, and system configuration options. From these analyses, energy storage application guidelines are provided together with results from actual computer simulations showing potential energy savings and energy cost reductions. 3 Potential Benefits of Energy Storage Energy storage technology can be used to address four main principal problem areas, but which application has the better return on investment and how well do these systems perform? To answer this question, the first inclination is to use energy storage to recapture regenerated braking energy. In fact, there has been significant research on this topic for energy storage devices installed directly on vehicles. Advanced train control systems like âCommunication Based Train Control (CBTC)â have the ability to control train operations more closely than conventional systems. CBTC offers greater ability to optimize train operations, and such systems may enhance the usefulness of energy storage devices. On the other hand there has been less research on using energy storage for transit power quality, peak shaving or substation replacement. To analyze the effectiveness of energy storage for capturing a larger share of the regenerative braking energy, many variables must be considered. A detailed analysis is complicated requiring computer simulation of a transit system propulsion power circuit and an iterative solution technique to find an optimum system design. However to gain an order of magnitude estimate we can begin with a few assumptions. First, most rail transit cars built today are capable of using propulsion motors as generators during braking deceleration, which means normally wasted braking energy could be recycled within the system to propel the train-set. Recapturing this energy is done to some degree by transit systems designed today, but there are limitations. The amount of energy recycled during braking is dependent on electrical receptivity. The power generated by a vehicle in braking is automatically distributed to the electrical line of the transit power system in proportion
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 9 of 61 to the electrical receptivity of that line. Receptivity is a measure of the ability of the electrical line to accept the added power and this added power is seen as a voltage rise to the system. As power is added it raises the line voltage, and it is the limit of this voltage that controls the amount of power that can be introduced. Voltage limits are set to protect equipment. Accelerating trains within close proximity of the supplied voltage can draw the added power injected by regenerative braking, but if trains needing power are not present in the same vicinity and if the system voltage limit is exceeded, this added power cannot be used and must be diverted to electrical resistors on the railcar, dissipating the energy as wasted heat. Usually, the allowable distance in which to claim excess braking energy is the track segment measured between propulsion power substations along the alignment that are often placed a mile apart or longer. From propulsion power data collected by transit agencies operating light and heavy rail systems without energy storage, the percentage of braking energy reused by neighboring trains varies, depending on many factors such as age of the system and conditions including train operating density commonly referred to as train headway. Energy storage provides the added capacity to accept additional power distributed to the line system should receptivity limits be exceeded or in the event other trains are unable to utilize excess braking regenerated energy at the time needed. Theoretically, an energy storage system if sized sufficiently can store surplus energy for use at a later time when needed by a local train and improve recapture efficiency to 100 percent within the region between substations affected by an energy storage system. But in reality, when examined from a systems perspective, the amount of improvement is less. Simulation case studies described in following sections show a modest improvement when measured over the entire system. But beyond the regenerative braking application, energy storage has been found of equal or greater importance for addressing other problems in the electrical infrastructure, namely low voltage conditions along the power distribution system, peak power demand costs, and high costs of conventional substation designs. Low voltage of the electrical power system is becoming more of a problem for transit systems, especially those of earlier design. As ridership increases around the country, transit agencies are seeing higher demands placed on their system resulting from the operation of more frequent and longer trains to meet the demand. This added burden is taxing the electrical propulsion power system beyond its intended design. The result is a loss in propulsion power quality and the necessary voltage to power the trains at desired operating envelops. It is the opposite problem of too much voltage in the system of regenerative braking. As voltage drops, supply electric current increases inversely to provide the same level of power. But there are limits placed on electric current because of the need to protect electrical equipment from overheating, and this limit when combined with low voltage results in a reduction of delivered power, thus negatively affecting railcar performance. Transit agencies are looking closely at energy storage to help prop up this low or sagging voltage. From simulation studies discussed later, it appears that voltage sag protection is a significant problem that energy storage systems might fix.
