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EPRI Proprietary Licensed Material Figure 3 Construction of a VRB cell stack (Courtesy SEI) 1.4. Technology Attributes Capacity The capacity of a battery energy storage system (BESS) is measured in both maximum power level (kW) and energy storage capability (kWh). In the case of the VRB, these two system ratings are independent of each other. In principle, the battery stack and PCS capabilities determine the kW rating, while the electrolyte concentration and storage tank dimensions determine the amount of energy that can be stored. For a given power level, the incremental cost of energy storage is based primarily upon the cost of additional electrolyte storage. The VRB technology favors applications having a high kWh/kW ratio, applications requiring several hours of storage. Most VRB systems fielded to date are capable of discharging at maximum design power for a period of 4-10 hours. Space requirements The main components of the VRB include the storage tanks, pumps and plumbing, cell stacks, and power conversion equipment. Footprint and volumetric space requirements scale with system ratings and can be very site-specific. For example, in one project, the tanks and stacks were located on separate floor, increasing the height requirement, but decreasing the footprint. In another project, tanks were made from rubber bladders that could be folded and passed through confined passageways and then expanded and installed in an unused underground office basement area. One study [Corey, 2002] estimated the size of a 2.5 MW/10 MWh VRB system to be 12,000 – 17,000 sq. ft. This was significantly larger than the 5,000 – 7,000 sq. ft. footprint estimated for the other technologies included in the study, sodium/sulfur and zinc/bromine. These results would suggest that the VRB is more suited to locations in which space is not a primary constraint. Maintenance requirements Without extended field experience, the system maintenance requirements are not well established. However, the primary maintenance items would be annual inspections, and the electrolyte pump bearings and impeller seals would need to be replaced at intervals of about every five years. As necessary, smaller parts, such as electronic boards, sensors, relays, and fuses would be replaced. Life The critical system component is the cell stack, which can degrade in performance over time and require replacement or refurbishment. At 100 charge/discharge cycles per year, it is expected that the cell stack would have a life of 10 – 15 years. However, the tanks, Vanadium Redox Battery 10
EPRI Proprietary Licensed Material plumbing, structural elements, power electronics, and controls would have longer useful lifetimes. It is possible to replace only the stacks, and keep the remainder of the system in place. Efficiency Several losses must be accounted for in characterizing the VRB performance: • Transformer losses. Most utility scale and industrial PCSs are designed with outputs around 480 VAC. To connect to utility distribution voltages, a transformer must be installed resulting in losses of a few percent. Even for nonutility systems, isolation transformers are installed to prevent DC injection into the AC grid. • PCS losses. Whether charging or discharging, power flow through the PCS is subject to losses related to voltage drops across the switching devices. PCS throughput efficiency depends somewhat on load and PCS design, but is typically in the 92-96% range. • Battery DC losses. The energy to charge the battery is typically 20% greater than the energy delivered during discharge. Internal battery losses include voltaic losses such as ionic flow resistance and coulombic losses such as cell-to-cell shunt currents (stray ionic flow through the stack manifold). Actual DC losses depend on rate of charge and discharge (the system is slightly more efficient at lower rates). • Pumping losses. Pumping power is a relatively constant auxiliary load that is drawn whenever electrolyte must be supplied to the stacks, i.e., during charge and discharge. In some applications such as backup power, it is possible to charge the battery, then turn the pumps off for long periods of time. The actual efficiency penalty for pumping depends upon the operation of the pumping, the frequency of cycling, and the pump design. At the 250 kW Stellenbosch demonstration in South Africa (see below), four pumps each drew 2.2 kW (8.8 kW total). The “round trip” (“turnaround”) efficiency – including transformer losses during charge, PCS losses during charge, battery DC losses, PCS losses during discharge, transformer losses during discharge, and pumping losses – is on the order of 70%. Response Time The battery can is capable of transitioning from zero output to full output in microseconds – virtually instantaneously – provided the stacks are primed with reactants. However, the power electronics respond within milliseconds, and the response time of the controls and communications (sensing the load requirements and signaling the PCS to take action) can be even longer. Where response time is important, the control system must be programmed to keep the pumps on and electrolyte flowing through the stacks. This requirement imposes a small performance penalty due to the constant auxiliary losses of the pumps. If response time is not critical, such as in peak shaving applications, then the stacks can be drained and the Vanadium Redox Battery 11
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