Power conversion specialists Converteam explains how the large capacities possible from the recently developed Vanadium Redox flow batteries are not only well suited to renewable energy storage applications, but can also help power generators cope with large surges in demand.

Eric Lewis, Converteam, UK

Developing viable systems for storing electrical energy is of critical importance to the power industry because one of their key benefits is that they act as instantly usable assets to enable Smart Grids to cope with intermittent sources of power generation such as wind and solar.

To enable the optimal energy store system to be supplied for any project the energy business of Converteam has developed a full range of energy storage solutions, including the use of lithium-ion batteries; super capacitor storage; dynamic flywheel energy storage; and flow cell energy storage.

Flywheel energy storage systems have been specifically designed for applications requiring a large impulse of energy in a very short period of time. It is also possible to use many different types of batteries and super capacitors for long-term energy storage, but the cost is usually unacceptably high.

FLOW CELL ENERGY STORAGE

To make energy storage available over longer time periods Converteam and Prudent Energy, the supplier of the VRB-ESS storage system, are collaborating on the design of a standard range of Vanadium Redox flow cell energy storage systems.

Converteam is an engineering company providing customized designs and systems for converting electrical energy into productive performance based on over 100 years of experience. Prudent Energy is one of a few companies in the world with the capability to deliver commercial MW-class energy storage solutions based on its patented proprietary Vanadium Redox flow cell technology.

This article describes the evolution of the initial flow cell energy storage system with a peak power of 1 MW and an energy storage of 1.5 MW for two hours. The initial design is complete and the systems are under manufacture for installation on AC grids in both China and USA during 2011. A schematic of the flow cell system is shown in Figure 3.

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Figure 1: Sumitomo Electric Industries’ wind farm and electric storage system, which utilizes Vanadium Redox flow cell batteries

The Vanadium Redox flow cell has many advantages including:

  • An electrochemical energy storage system at ambient temperature, operating within the bulk of the fluid, using proton and electron transfer with no deposits forming on electrodes, that is therefore capable of always being on;
  • The same unique element Vanadium used in both reaction half cells – unlike all other designs, which employ different fluids – which prevents cross contamination problems or damage to storage fluids from membrane faults;
  • A fully reversible chemical process based on the different oxidization states of Vanadium;
  • An electrolyte that never wears out, offering the lowest cost of ownership of all energy storage systems;
  • Unlimited deep cycles for over ten years with very fast charge and discharge;
  • Very fast response, of less than 10 milliseconds, between charging and discharging;
  • Required MW rating sets reaction cell size;
  • Required MWh rating sets tank size; and
  • Vanadium is readily available.

The flow cells used are built using a modular system to simplify any repairs that are required, and the flow cells modules are now being installed. A typical unit is shown in Figure 2. The flow cell modules used have a typical operating voltage range of 240–400 volts DC depending on the current flowing and the energy stored – a major factor in selecting the appropriate power conversion technology.

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Figure 2: Vanadium Redox flow cell on test

POWER CONVERSION TECHNOLOGY

A review of available power circuit topologies was conducted in order to select the most appropriate power conversion technology. The use of diode bridge and thyristor bridge circuits was considered but they are unable to meet all the grid code rules and operational rules that apply to a successful flow cell project.

Fortunately the fully proven Active Front End (AFE) wind power system produced by Converteam provided the basis for a fully grid-compliant flow cell power converter system. The AFE input topology uses insulated gate bipolar transistors (IGBT) switching devices inside two fully-rated inverters and is the circuit now used for most advanced wind turbines.

The inverters change the variable frequency generated AC power to DC power at a constant DC voltage and then into fixed frequency AC grid power. The AFE wind power circuit can also be used with other generators of power, like a flow cell, and can also operate with a power flow in either direction.

The IGBT power converter that is normally used is a standard unit rated at 1 MW. The standard 1 MW inverter unit provides a complete AC to DC conversion unit and over 20 units are produced per day for wind turbines, so this is an ideal inverter for use with flow cells.

The flow cell ProNRG converter circuit is the same AC supply circuit as the wind power circuit and the generator inverter is changed to operate as a DC/DC chopper using the same 1 MW inverter module DC inverter.

In the main operating mode the system is either importing, exporting or storing AC power. An external control signal can switch between importing and exporting in one AC mains cycle. There is no limit on the number or depth of the charge and discharge cycles. AC reactive power can be supplied at the same rating as the power rating and is controlled by a separate external control signal. The system is thus able to operate with an AC power factor of ±0.7 per unit. As well as meeting all grid code performance specifications, it can also complete the initial flow cell charging from zero volts DC.

The flow cell system can operate in four different modes: input, output or storing power as defined above; providing grid fault support by injecting MW and MVArs into a fault on an AC grid, as defined in the relevant grid codes; operating as an island generator to feed AC power onto a grid with no other generators connected; and operating in self-sufficient mode to shutdown the flow cell system when the AC supply is lost into a state that has essentially zero loss of stored energy, even for shutdown periods measured in weeks. The shutdown is achieved using the flow cell’s stored energy.

OPERATION OF A FLOW CELL SYSTEM

The Vanadium Redox flow cell technology has been under development for a decade. An early example of an integration of a wind farm and a Vanadium Redox flow cell system, produced by Sumitomo Electric Industries for J-Power in Japan, is shown in Figure 1.

 

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Figure 3: Schematic of the Vanadium Redux flow cell circuit

The system shown is rated at 4 MW rms with a six-hour storage time plus a peak power capability of 6 MW and is used for smoothing out the wind energy coupled to the local AC grid. Despite the success of the Sumitomo Vanadium Redox flow cell batteries the technology was not previously developed for widespread commercial use primarily because of early stage technical cost challenges.

In the future, wind and other renewable energy generation systems will supply a significant part of the energy required to operate AC grid systems. To reduce carbon dioxide emissions, the number of power stations using fossil fuels will also fall, which will reduce the spinning reserve that keeps systems stable by automatically responding to rapid falls in frequency when a very large extra load is applied or a power station trips offline.

The traditional way of providing security is to have one or more fossil fuel grid-connected power stations online but operating at low power to have both increased rotary inertia and the ability to provide extra power on demand. In addition, the present generation of fossil fuel grid-connected power stations were designed to have the ability to produce up to 120 per cent of output power on demand.

With the increasing penetration of wind power and the removal of central fossil fuel power stations these capabilities will be significantly decreased because it is uneconomic to design individual wind turbines that can supply up to 120 per cent of power on demand.

These changes are raising concern in many countries over the stability of AC grid systems with a high penetration of wind power. The use of a dynamic energy storage system can resolve these problems and give a number of benefits when integrated as part of an AC Smart Grid system.

A dynamic energy storage system can act to:

  • Smooth the variations in wind power;
  • Have energy available to compensate for a sudden decrease of wind power;
  • Respond rapidly to an increase in the connected AC grid power;
  • Respond rapidly to a loss of a grid-connected power station;
  • Provide synthetic inertia to increase the stability of an AC grid system;
  • Store energy to permit confident pool price bidding based on forecast wind;
  • Store energy to sell later at a higher pool price;
  • Maximize energy delivered to maximize revenue; and
  • Enable simpler wind turbines to be produced with smaller inverters. 

Smoothing power transients

All AC grid systems can experience very large transients which will test the stability and operating margins of a given AC grid system. The UK’s AC grid was recently hit by a severe transient that progressed from the loss of 345 MW because of the shutdown of a fossil fuel station to the disconnection of large loads to avoid a total system collapse.

One very specific outcome of this transient was the need for smaller generating systems including wind power to have an increased ability to ride through AC grid system transients. This ability is an integral part of the new Vanadium Redox flow cell system being developed.

An essential requirements of an energy storage system is a viable means of providing synthetic inertia. When grid transients occur a conventional AC generator can supply extra energy from its stored inertia, which acts to limit the rate of change of the AC supply frequency.

But to extract the energy an increase must occur in the phase angle, also called the ‘Load Angle’, between the generator’s internal EMF and the AC supply frequency. The new Vanadium Redox flow cell synthetic inertia control uses an algorithm to deliver a rapid boost of power when the AC supply is falling in frequency, and the design can deliver the frequency sensitive power faster than the response of synchronous generators.

Compared with conventional synchronous generators the new Vanadium Redox flow cell/ProNRG energy storage system can continue to deliver the extra power for extended periods of time with no risk of the system pulling out or tripping. These abilities are an essential requirement for a Smart Grid system.

Another essential requirement for an energy storage system is the ability to smooth the power flow from renewable systems to avoid the need for expensive upgrades to transmission systems.

A further requirement of an energy storage system is the ability to carry out a very large number of frequent charge and discharge cycles. The new Vanadium Redox flow cell energy storage system, VRB-ESS, has a response time of less than ten milliseconds and can carry out an unlimited number of charge and discharge cycles within the ability of the stored energy.

Once any given energy storage system is installed the need can arise to increase the amount of storage. For the new Vanadium Redox flow cell energy storage system this is easily achieved at low cost by adding storage tanks and extra storage fluid. The new Vanadium Redox flow cell/ProNRG energy storage system is now being validated and will provide some of the features required to enable Smart Grid systems to be implemented.

The author gratefully acknowledges the contributions of Prudent Energy, Incoteco and National Grid to this article.

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