Overview of the Grid-Saver Fast Energy Storage System

Transcription

1 Overview of the Grid-Saver Fast Energy Storage System November 17, Introduction and Background Transportation Power, Inc. ( TransPower ) has developed a battery energy storage system called Grid-Saver, which provides a modular, flexible, low-cost energy storage option for commercial and utility-scale energy storage needs. TransPower is a California corporation whose mission is to become a leading supplier of power generation, energy storage, and power control technologies for the green economy. TransPower s primary products are: Components to enable large vehicles such as trucks and buses to operate on battery-electric or hybrid-electric power; and Large battery systems for stationary energy storage applications TransPower s business strengths include its product diversity and its vertically integrated business model, as illustrated in Figure 1. TransPower offers total system solutions that convert large vehicles to run on electric power and that can store large amounts of energy for stationary uses. But the Company also designs and manufactures a range of specific components that can be sold independently, or packaged into intermediate subsystems. This enables TransPower to support its customers with a greater degree of flexibility than most competitors in the electric vehicle and stationary energy storage markets. Grid-Saver Figure 1. TransPower vertically integrated business model. capitalizes on these strengths. It is a modular system that meets can meet peak discharge requirements from the hundreds of kilowatts to the tens of megawatts, with hundreds of kilowatt-hours to tens of megawatt-hours of total energy storage, using common building blocks. 2 System Design TransPower s Grid-Saver design utilizes a highly modular, scalable battery configuration employing large format lithium iron phosphate (LiFePO 4 ) battery cells, which TransPower has 1

2 found to be the best-value lithium-ion batteries available anywhere in the world, combining a safer chemistry, high energy density, good stability, and low cost. The specific cell product used as a basic building block in most Grid-Saver systems is a 180 ampere-hour (Ah) cell manufactured by China Aviation Lithium Battery (CALB), one of the world s largest battery manufacturers. The cells are installed into modules that are easy to handle, 24 cells to a module, and that can be arranged in series strings to achieve a wide variety of operating voltage requirements. 11 such modules connected in series will provide 871 VDC nominal, compatible with standard 480 VAC three-phase transformers. Each series string is typically connected to an Inverter-Charger Unit (ICU), a proprietary inverter product developed by a sister company, EPC Power Corp. A typical Grid-Saver configuration sized to fit inside a standard 40-foot trailer would have the following specifications: Four strings of CALB 180 Ah cells per trailer. 871 VDC nominal and kwh energy storage per string, kwh per trailer. One inverter-charger unit (ICU) rated at 250 kw per string, providing 1 MW per trailer (expandable to 1.33 MW if required). TransPower s energy storage approach is to combine low-cost cells with the revolutionary new ICU and a sophisticated battery management system (BMS) in a well-engineered integration concept, resulting in a system with a lower cost of energy than competing systems, but that also offers high performance and long operating life possibly as long as years. The configuration summarized above is a proven architecture TransPower has spent more than $2.6 million developing and testing since late 2011, in a product development effort supported with a $2 million grant from the California Energy Commission (CEC). As Grid-Saver battery modules are derived from similar modules originally developed for electric vehicle (EV) applications, the total investment TransPower has made in related R&D is actually significantly greater than $2.6 million, and Grid-Saver customers benefit from the sophistication of the battery integration concept enabled by these investments. 2.1 Module Design Figure 2 shows two different battery module products developed by TransPower for stationary applications. The module on the left is the version discussed above, containing 24 CALB cells. The module on the right uses higher power cells supplied by EnerDel, which is an alternative option. TransPower is very comfortable with the CALB cells for energy storage applications for a number of reasons, including high stability and low internal resistance. Figure 3 shows a discharge curve for these cells provided by CALB, and Figure 4 shows the internal resistance of the CALB cells as a function of temperature. Figure 2. Grid-Saver stationary energy storage modules. 2

3 Figure 3. Discharge curve for CALB 180 Ah cell. Figure 4. Internal resistance of CALB 180 Ah cell as a function of temperature. Under our CEC Grid-Saver contract, TransPower performed extensive testing of CALB cells and modules using these cells for more than two years, creating a valuable data base and experience in using such batteries. Initial testing was achieved with an AV-900 battery tester located in TransPower s advanced battery testing and integration laboratory. Subsequent testing of a complete prototype system was conducted at the U.S. Department of Energy s Sandia National Laboratory (discussed below). Figure 5 shows testing of a Grid-Saver battery module with the AV900. Data from testing is synthesized using TransPower s proprietary battery control unit (BCU). As discussed in more detail below, the BCU provides voltage and temperature information on each individual cell. A sample read-out of battery data from the BCU is shown in Figure 6. Figure 5. Module testing with AV-900 battery tester. Figure 6. Sample of data read-out from TransPower battery control unit. 3

4 2.2 Battery Management The BCU reads and displays data collected using an Orion BMS supplied by Ewert Energy Systems. The BCU/BMS combination continuously monitors the temperature and voltage of each and every cell in the Grid-Saver system, allowing operators to view these data in real time and providing warnings of cell conditions outside of normal conditions. As indicated in Figure 6, the BCU employs a graphical user interface, custom designed by TransPower, which is intuitive and easy to interpret. The system is also designed to shut down a string or take other protective actions when necessary. The BMS also utilizes a simple passive cell balancing technique to help keep cell voltages equalized. As part of its EV R&D and manufacturing, TransPower has worked with other commercially-available BMS products and recently developed its own BMS which has unique features that may be beneficial in future stationary applications. However, for Grid-Saver systems TransPower is currently recommending the Orion BMS because it is inexpensive, has shown itself to be reliable and adequate for stationary applications, and is the system with which TransPower currently has the most experience in stationary system utilization. Figure 7 is a close-up photo of a Grid-Saver module using CALB cells, showing with a little more clarity how the Orion BMS sensors are wired. The green sheet covering the cells is a printed circuit board designed by TransPower with embedded circuitry for low-current BMS electrical and data connections. Higher current connections between cells are handled with bus bars bolted to the cell terminals, right below the printed circuit board. Figure 7. Close-up view of proposed battery module. 4

5 2.3 Inverters The inverter used in the Grid-Saver system is an advanced technology device that TranPower believes offers an unprecedented combination of low cost, high efficiency, and compact design. TransPower achieved development of this new product, called an Inverter- Charger Unit (ICU), by tapping into the pioneering power electronics capabilities of a partner company, EPC Power Corp. EPC was founded at about the same time TransPower initiated operations in 2010, and its principals are seasoned power electronics engineers who designed high-power inverters for the U.S. Navy while formerly employed with General Atomics. As with TransPower s battery module R&D, with the ICU the Company has been able to leverage funding from both EV and stationary R&D efforts to develop a superior product and achieve a significant amount of testing to validate ICU performance. For stationary applications, TransPower and EPC developed an ICU rated at 250 kw, six of which were manufactured and validated under the CEC-funded Grid-Saver project (Figure 8). The ICU is an extraordinarily compact device, with a power density of greater than 2 MW/m 3, offering 100% kva capability at a remarkably high efficiency of greater than 98%. The compact geometry is achieved with liquid cooling and other advanced features, but without inflating costs. ICUs are typically priced today at less than 30 cents/watt, and in larger production volumes TransPower and EPC can offer this product for less than 20 cents/watt. For the higher power applications, the ICU output can be increased from 250 kw to 333 kw, an upgrade that requires software modifications and little to no hardware changes. 2.4 Integrated Systems Various integrated systems can be built using modules installed into trailers. First, the modules described in Section 2.1 are Figure 8. Six ICUs built and tested during the Grid-Saver project. manufactured and integrated into racks, then electrically connected to form 871 VDC strings of 11 modules, each string connected to a an ICU. Figure 9 is a DC schematic of one such batteryinverter string, showing how 11 modules are connected to an ICU, which is then connected to a standard 480 VAC, three-phase transformer. In the 1 MW prototype system built by TransPower and tested at Sandia, four of these battery-icu strings were arranged in parallel and installed into a 45-foot climate-controlled trailer. 5

6 500 ohm Inverter Enclosure Module 11 BMS uh AC Grid 480V 400A fusing LCL Input Filter 450 uf 75 uh 24 cell battery modules nominal 79.2 volts each VDC, 400A fuses Figure 9. DC schematic of battery-icu string. Any number of such trailers can be utilized to meet the specific requirements different installations. As an example, eight trailers with ICU s upgraded to 333 kw would be able to provide a MW peak discharge and 5,369 kwh of total energy. Figure 10 is a photo of a 50% scale version of the system in a typical trailer, partially complete during installation into a 20-foot trailer in Figure MW prototype Grid-Saver system installed in 45-foot trailer. 6

7 As this system uses the current 250 kw variant of the ICU, the four battery-icu strings in this trailer provide 1 MW of peak discharge while storing more than 600 kwh of total energy. The two ICUs along with their cooling lines are not visible in the photo due to their extraordinarily compact size. The ICUs are located at the bottom of each rack structure below the battery modules shown in the photo. This and a smaller 500 kw prototype system were subjected to numerous tests throughout 2014, successfully demonstrating basic battery-icu functionality, charge-discharge capability, and the ability to move charge from one string to the other. The 1 MW system shown above was transported to Sandia National Laboratory for several months of testing which was initiated in July The Sandia testing validates the complete functionality required for each such trailer to meet our stated specifications, essentially validating the proposed architecture at the trailer level. Figure 11 is an AC schematic for one possible layout of this four-string Grid-Saver system, with ICUs upgraded to 333 kw each, showing power aggregated from each four trailers via a 480VAC bus and routed to a 480 VAC 13 kva transformer rated at 5 MVA. This is just one possible approach to achieving a 13.8 kva grid connection. As an example of an alternative approach, a single 2 MVA transformer could be utilized at each trailer. This would likely increase the transformer expense, but could provide added redundancy in the event of a transformer failure KV Xfmr Transmission Power 480VAC Bus 1.3 MW Module 1.3 MW Module 1.3 MW Module 1.3 MW Module 1.3 MW Module 1.3 MW Module 1.3 MW Module 1.3 MW Module KV Xfmr Transmission Power Figure 11. AC schematic illustrating possible layout of a 10 megawatt system. The proposed system also includes system-level controls, and will be configured to interface with a customer s SCADA or to respond to signals from a utility of ISO. 3 Validation of a Megawatt Grid-Tied Energy Storage System As discussed above, TransPower s 2014 tests included testing of a two-string, 500 kw Grid- Saver and a four-string, 1 MW system. Following satisfactory tests of the prototype 500kW 7

8 system at its Poway battery testing facility, TransPower fabricated a four string system and installed it into a 45-foot refrigerated trailer. Figure 12 is an outline drawing of how the major components are installed into the trailer and connected to the grid using the orange wires (600 volt welding cable rated for 350 amperes). 480V, 3ph, 30A 208V, 3 Ph. 30A 480V, 3ph, 300A InvD CALB String D CALB String E InvE CALB String C InvC InvF EnerDel String Figure 12 - Megawatt capable Energy Storage System with ICUs positioned on Sandia ESTP with AC hookup in orange. Figure 13 shows the completed system on the Sandia ESTP (Energy Storage Test Pad) with the AC connection switchgear in the foreground, and Sandia Program Director David Rosewater at the local control station. The ESTP system is controlled from a remote building, so these cabinets are generally closed and unmanned. Figure MW Grid-Saver system in trailer at Sandia National Laboratory. 8

9 The megawatt sized trailer system was moved to the Sandia ESTP in June, commissioned in July, and has been operated since August For the first 2-3 months of system testing, tests were performed in accordance with the scope of work of the Grid-Saver contract funded by the California Energy Commission. These validation exercises included validation of the capability of megawatt level operation, demonstration that the four strings could be successfully and conveniently controlled by the Grid-Saver Control Unit software, and evaluation of the energy storage capability of the system. Following completion of these contract tasks, Sandia engineers continued to test the system to meet U.S. Department of Energy research goals, with minimal oversight from TransPower. Additional testing performed by Sandia included simulated grid frequency regulation at up to 800kW, which expanded the demonstrated capabilities of the Grid- Saver system into key areas such as frequency regulation. The 1M Grid-Saver system is the largest battery system tested by this Sandia group. 4 Summary of Grid-Saver Benefits TransPower s design approach for the Grid-Saver system offers many advantages, including: Scalability. A single trailer system can be acquired to validate a system configuration for less than $1.5 million. Expansion to much larger systems can be achieved by employing additional trailers identical in design. Low Risk. TransPower has already built 66 battery modules and 6 ICUs and has tested the battery energy storage system at power levels to one megawatt in conjunction with Sandia Laboratories. Flexibility. TransPower is agnostic with respect to many of the major components utilized in the Grid-Saver system, and is willing to work with customers to modify the proposed system design if the customer has a preference for use of specific components or wants to try different approaches. Low Cost. The proposed system and price work out to approximately $1,000/kW of peak power delivered, representing a highly cost-effective solution. Future cost reductions are possible with higher volume purchases and incorporation of lower cost components as they become available. Adaptability to new technologies. TransPower s philosophy is to provide integrated systems that can be rapidly updated to utilize new products as they become available. TransPower customers can thus be among the first to realize the benefits of new technologies or cost reductions. In addition to utilizing flexible system architectures and controls that can be adapted to new products as they appear on the market, TransPower is continuously developing new technologies and products of its own related to energy storage. One example is TransPower s own proprietary battery management system (BMS), which TransPower has been developing since 2012 and which is presently undergoing final testing on a 215 kwh battery pack installed on an electric Class 8 truck. TransPower has partnered with its ICU development partner EPC to develop customized BMS sensing and balancing circuitry, which TransPower provides new levels of functionality. Figure 14 is a photo one of the battery modules on this truck with its lid removed, showing the batteries installed into the enclosure. The BMS sensing-balancing boards developed by EPC are the green printed circuit board mounted directly to the bus bars connecting the cells. 9

10 Figure 14. New TransPower-EPC BMS sensing-balancing boards installed on Class 8 electric truck. The new TransPower-EPC BMS may offer several major advantages over competing BMS products, including: Greater processing capability, enabling more accurate measurements of cell voltage. This is expected to help improve balancing of cells, which can extend vehicle operating range and battery life. High-current continuous, active charge shuffling, enabling energy from more fully-charged cells to be transferred continuously to lesser-charged cells. Most competing BMS products, including the Orion BMS, achieve balancing through passive charge dissipation, which drains energy from higher-charged cells but which simply reject this energy in the form of heat, rather than transferring the charge to lower cells. The active approach of TransPower s BMS will eliminate this energy waste and improve the efficiency of the system. The TransPower BMS is also designed to perform cell balancing with 6 amps of current, a significantly faster rate than competing BMS products. Bolt-on feature, enabling BMS sensor boards to be bolted directly to the bus bars connecting cell terminals. Other competing BMS products require the routing of wires from cell terminals to the BMS boards. Eliminating this wiring may reduce assembly time and maintenance issues relating to the possibility of wires becoming damaged or disconnected. 10

11 Higher voltage tolerances, enabling the BMS to be utilized with higher voltage architectures than competing BMS products. TransPower is available to evaluate the potential benefits of new innovations such as this BMS for its customers energy projects. For information contact: Michael Simon, President & CEO/(858) or Paul Scott, VP, Advanced Technologies/(858) TransPower Danielson Street, Suite A Poway, California