Control System
The VRB-ESS is controlled by a Programmable Logic Controller (PLC) and a Human Machine Interface (HMI). One of the key functions of the PLC system is to control the times and rates of charging the VRB-ESS. For example the PLC can be fed real time data on prices and based upon the allowable maximum demand, state of charge and price of off-peak versus peak energy, it will decide how quickly to re-charge the VRB-ESS, when and for how long. This can be dynamic and can be optimized based upon the situation. It is integrated with the rest of system through standardized communications inputs, control signals and power supplies. It may be dialed up or accessed through the Internet. It has multiple security layers to limit access to its various functions and provides a tailored reporting and alarming function for remote control and monitoring.
The PCS role is to charge and discharge the battery and to provide enhanced power quality, voltage support and frequency control to the local grid. It has a sophisticated, fast acting (DSP), multi-quadrant, dynamic controller with proprietary control algorithms, which is capable of switching the output the full range of the device i.e. from absorbing full power to exporting full power within cycles. It does the same on a reactive power basis and in any combination of the real and reactive power requirements. The intelligence within the Inverter is integrated with the overall control system. It is therefore capable of being easily reprogrammed (on site or remotely), to adjust for any changes in site requirements, or settings that the operator requires adjusted. The PCS is connected either in a series (isolated load) mode or in a shunt configuration with static transfer switch option for UPS functionality.
Energy is stored by chemical changes to a working fluid called the Electrolyte. In redox flow batteries the fluids contain dissolved species that can be electrochemically oxidized or reduced to store the energy. The Electrolyte in the VRB-ESS is known as an "Aqueous Solution of Sulphates of Vanadium". It is made up of Sulphuric Acid, and emulsified vanadium particles.
Storage tanks are used to contain the positive and negative Electrolyte. The 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 for the purposes of achieving "best-practice" design for spill management and regulatory compliance. Tanks are a sub-assembly item factory assembled and transported to site.
The Cell Stacks are self-contained sealed devices that consist of many cells, each of which contains two half-cells that are separated by a membrane. In the half-cells, the electrochemical reactions take place on inert carbon felt, polymer composite, electrodes from which current may be used to charge or discharge the battery.
When charged electrolyte solution is allowed to flow through the stack, ionic transfer between the different forms of vanadium ions across a separating membrane will result in a balancing electron flow into an external circuit (DC current) and so complete the electrochemical path for discharge. Forcing current into the stack from an external source reverses the process and recharges electrolyte in the stack, which is then pumped back into the reservoirs.
Each Cell Stack has a nominal rating of 50kW peak for large units.
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 to ensure electrical equipment is not exposed to extreme ambient temperatures and heat exchangers to maintain the operating temperature of the Electrolyte are included. In cold climates the heat exchangers are not required. Utility step up transformer, breaker and associated protection is included as an option.