Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 10 of 61 Discussions in following sections analyze these problem areas in addition to the use of energy storage devices to replace conventional electrical power substations and reduce peak power demand. 4 Transit Agency Needs Assessment To focus the investigation on the potential benefits of storage and select appropriate transit system designs for computer simulation, the study team interviewed various U.S. transit agencies to ascertain their needs. The team composed a survey outline that posed questions regarding energy costs, operational problems, transit system age, ridership change, expansion plans, and availability of propulsion power data. Candidate transit agencies were identified and site visits conducted from which selections of transit agencies were made for further simulation studies The study team selected Los Angeles County Metropolitan Transit Authority, Sacramento Regional Transit Authority, New York City Transit, Washington Metropolitan Area Transportation Authority, and Long Island Railroad. Other agencies considered included the Regional Transportation District of Denver, Houston Metro, and Miami-Dade. From detailed on-site meetings with selected agencies, the project team discovered that all were experiencing problems with voltage sag. And generally, all transit systems interviewed were considering the use of energy storage to address a range of problems. Table 4â1 highlights the spectrum of needs across the selected rail agencies. It is noteworthy to point out that New York City Transit in cooperation with NYSERDA (New York State Energy Research and Development Authority), is testing a long-term battery for overnight storage of electrical energy in an effort to reduce high utility charges associated with daytime refueling of Compressed Natural Gas (CNG) buses. The stored energy, which would be captured at night when electrical rates are lower, would be used to operate CNG fuel compressors that fuel busses at peak power demand times in morning hours. Choosing the best energy storage device (ESD) for applications like those listed in Table 4â1, will require detailed knowledge of energy storage device characteristics. Energy storage technology has been under development since the dawn of time beginning with storing potential energy in a raised weight to todayâs complex electro-chemical reactions and advanced mechanical flywheels. Selecting the optimal energy storage technology for a specific need is dependent on a number of distinguishing characteristics of energy storage systems. Consideration must be given to the amount of energy that must be stored and for how long, the rate at which the energy storage device can be charged and discharged, cycle life and long-term durability. The types of systems that can meet a particular demand could be highly varied ranging from mechanical flywheel devices, to electro-chemical batteries and electro-chemical capacitors. Obtaining further information on power charge and discharge cycle characteristics which affect ESD life is needed for a thorough analysis, but a simple breakdown by application can be helpful as a guide to selecting the best technology. One method used to provide such delineation is a plot of power density vs. energy density, often referred to a Ragone Plot.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 11 of 61 Power and energy can be viewed as characteristics that depict the rate of energy delivery and the amount of energy stored, respectively. Some devices store large amounts of energy efficiently but may be unable to charge or discharge this energy over a short time span. The ability of an ESD to discharge and charge quickly may be desirable if there is a need to correct power quality problems often associated with electrical line voltage drop. Voltage sags can occur on a time scale of only seconds, especially for electrical utilities. In such a case, electrical energy from an ESD would need to be quickly injected into the electrical supply line or a transit system third-rail to correctively elevate the sagging voltage. The distinction between energy density and power density can also be seen through an electric car analogy. Energy density and power density can be associated with the range of an electric car and its acceleration, respectively. By simply viewing an ESD Ragone plot together with general knowledge about the characteristic propulsion load demand of a rail system alignment, a preliminary selection of an appropriate storage technology can be estimated. A typical Ragone plot rendition is represented in Figure 4â 1. More detailed plots of current ESD technology are obtained through the Electric Power Research Institute or from government sources such as the U.S. Department of Energy and its various research laboratories such as Sandia National Laboratories, Argonne National Laboratories, or Idaho National Laboratories among others. Another example of the tradeoff made between the relative benefits of specific energy versus specific power is the difference in headway scheduling between heavy rail (subway) and light rail systems. A heavy rail transit system may operate many trains at short headways of two- minutes versus a light rail system with headways of 5, 15 or even 20 minutes. Each rail system may require a different energy storage characteristic to meet the intended demand. Charge and discharge cycling rates within a corridor of interest or as measured between electrical substations along the alignment are also dependent on the number of vehicle station stops or in the case of light rail systems, also the number of urban traffic stops. Each stop introduces the opportunity for an ESD discharge or charge cycle potentially affecting the need for rapid response. Selecting an ESD from knowledge of energy storage characteristics and system load demand also requires selection of ESD capacity (kWh). Energy storage capacity affects the time of discharge as a function of power level. An example of this relationship is seen in Figure 4â2, showing two curves of different ESD energy capacity (kWh) and their respective power delivery response times. From this discharge characteristic information, candidate energy storage technologies and sizing specifications begin to emerge.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 12 of 61 Table 4-1 Energy storage application by mode and transit property Energy Storage Application Agency Rail Mode Location B ra ki ng En er gy R ec ap tu re Po w er Q ua lit y â V ol ta ge S ag Pe ak P ow er R ed uc tio n En er gy St or ag e Su bs ta tio n Comments Los Angeles County Metropolitan Transportation Authority Light Rail Los Angeles, California Yes Yes Yes Yes LACMTA expansion is considering utilizing energy storage to reduce number of power substations. LACMTA also won a large TIGGER grant to evaluate energy saving technologies including energy storage. Sacramento Regional Transit Light Rail Sacramento, California Yes Currently demonstrating a battery energy storage system installed on a weak power section of alignment. Washington Metropolitan Area Transportation Authority Heavy Rail Washington, DC Yes Yes Yes WMATA has selected candidate sites for a battery energy storage system and has FTA funded support. New York City Transit Authority Heavy Rail New York City, New York Yes Yes Currently demonstrating a battery substation Long Island Rail Road (LIRR) Heavy Rail Long Island, New York Yes Under contract to install mechanical flywheels. Metro-North Railroad Commuter Rail Northeast Corridor Yes Studies previously conducted and data available for energy storage modeling
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 13 of 61 Figure 4-1: Ragone Chart for energy storage device Figure 4-2: Sustainable discharge rate for energy capacity 4.1 Vendor data summary Detailed information on specific energy storage device operational data for use in subsequent computer modeling was obtained from energy storage vendors participating in the APTA/EPRI Energy Storage Research Consortium. Figures 4â3 through 4â5 summarize the variation in performance measures as a function of energy and power availability, discharge and charge times, and number of charging cycles capable over the life of the device. From this data and other generalizations regarding charge and discharge rate, device efficiency, and charging current limitations among others, simulations were performed of energy storage device performance as part of this transit propulsion system modeling study. Each data point in the figures represents a vendor supplied characteristic from which verification could be made against modeling assumptions. Fuel Cells EC Capacitor Batteries El ec tro ly tic C ap ac ito r Flywheel
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 14 of 61 Energy/Power Chart Based on Vendor Data 1 10 100 1,000 10,000 100 1,000 10,000 Power (kW) En er gy (k W h) Figure 4-3: Energy/power for energy storage device Charge/Discharge Cycle Life Based on Vendor Data 1,000 10,000 100,000 1,000,000 10,000,000 100 1,000 10,000 Power (kW) N um be r o f C yc le s Figure 4-4: Cycles/power for energy storage device
