System Components
The VRB Energy Storage System (VRB-ESS®) is designed with elegant simplicity – providing ground-breaking advantages with a modular framework and customizable controls.
Cell Stacks
VRB® cell stacks are 7 kW-rated sealed devices that consist of many cells, each of which contains two half-cells that are separated by a membrane. In the half-cells, electrochemical reactions take place on inert carbon felt, polymer composite electrodes from which current is used to charge or discharge the battery.
When charged electrolyte fluids pass through the cell stack, different ionic forms of vanadium react, resulting in a balancing electron flow into an external circuit (DC current) — thus completing an electrochemical path for discharge. Forcing current into the stack from an external source reverses the process and recharges electrolyte in the stack, with fluids then pumped back into the reservoirs.
The highly consistent and measurable characteristics of VRB® cell stacks allow modular systems to be assembled in large parallel and series configurations, suitable for use in many industrial applications requiring megawatts of power. After over 20 years of R&D – much of which was and continues to be pioneered by the Prudent team – the VRB® is the flow battery technology of choice for both small and grid-scale applications. Delivered under standard handling, the cell stacks have no special site requirements other than an adequate footprint.
Electrolyte & Storage Tanks
Energy is stored in Prudent Energy systems in a working fluid called the electrolyte. An electrolyte is any substance containing atoms or molecules that have positive or negative electric charges. These charged atoms or molecules are called ions, and the amount of a charge on an ion is its oxidation state. In a battery, electrolytic fluids are where the energy is stored. So when separate electrolytes – each containing ions in different oxidation states – come together to exchange charges, the result is an electric current. This process is known as ‘reduction-oxidation' or a redox reaction, which is what gives Prudent Energy's vanadium redox battery its name. The electrolyte in the VRB-ESS® is composed of a proprietary blend of sulfuric acid and emulsified vanadium particles.
Storage tanks are used to contain the positive and negative electrolyte. These tanks are typically a double-wall, self-supporting fiberglass type with an internal PVC lining. They are easily transported and supported on-site. Each tank provides its own secondary containment, following best-practices design in spill management and regulatory compliance. Tanks are factory-assembled and transported to the site.
Power Management
Power Conversion System (PCS)
The Power Conversion System (PCS) charges and discharges the battery, while providing enhanced power quality, voltage support and frequency control to the local grid. The PCS converts AC current and DC current in order to connect the battery system to the AC grid. Prudent Energy's PCS has a sophisticated, fast acting, multi-quadrant, dynamic controller with proprietary control algorithms, and is capable of switching output across the full range of the device (i.e., from absorbing full power to exporting full power within cycles). The PCS also functions on a reactive power basis and in any combination of both real and reactive power requirements.
The intelligence within the inverter is integrated into the overall control system. Therefore, the PCS is easily reprogrammed (on site or remotely) and adjusted for any changes in site requirements or settings required by the operator. The PCS is connected either in a series (isolated load) mode or in a shunt configuration with static transfer switch option for UPS functionality.
With Prudent's PCS capable of delivering real power (watts) and/or imaginary power (volt-ampere reactive, or "VARs"), the system provides not only power smoothing but also ancillary services such as voltage regulation and VAR support. All Prudent power electronics undergo extremely rigorous testing before they are integrated into a complete installation.
Control System
The VRB-ESS® is controlled by a Programmable Logic Controller (PLC) and a Human Machine Interface (HMI) for effective power management. Together they constitute a full Supervisory Control and Data Acquisition (SCADA) system.
One of the key advantages of our PLC system is its ability to control the times and rates of charging the VRB-ESS®. For example, the PLC can be fed real-time data on prices. Based on the allowable maximum demand, state of charge, and price of off-peak versus peak energy, the PLC will decide when, how long, and how quickly to re-charge the VRB-ESS®. This control can be dynamic and can be optimized based upon the situation.
The PLC is integrated with the rest of the system through standardized communications inputs, control signals and power supplies. It may be dialed up or accessed through the Internet and has multiple security layers to limit access to its various functions, while providing tailored reports and alarm functions for remote control and monitoring.
Prudent Energy's SCADA can seamlessly integrate with other SCADA systems, such as a utility SCADA, wind turbine generator SCADA, or PV system SCADA. With such total system control, we provide a very high level of cyber-security and a ready-made interface with smart grid technologies. What's more, each VRB-ESS® control system is customized with automated (or manual) features based on existing infrastructure and planned asset purchases to manage total capital and operating costs.
Balance of Plant
The balance of components required for the installation of a VRB-ESS® consists of PVC pipes for connection between the electrolyte storage tanks and the cells stacks, and pumps to circulate the electrolyte through the system. Where required, HVAC units are included to ensure that electrical equipment is not exposed to extreme ambient temperatures. In cold climates, heat exchangers may be added to maintain the operating temperature of the electrolyte. Our systems typically make maximum use of local parts, supplies and labor to construct and operate installations.