EFFICIENT ENERGY STORAGE SYSTEMS USING VANADIUM REDOX

BATTERIES FOR ELECTRICITY TRADING. FOSSIL FUEL REDUCTION

AND ELECTRICITY POWER COST SAVINGS FOR CONSUMERS

TECHNICAL FIELD

The present invention describes energy storage systems incorporating vanadium redox flow batteries (VFB) to store renewable energy or off-peak electricity from the grid to power a load during times of high electricity prices, low solar insolation or low wind speed The vanadium redox flow battery is integrated into a system employing wind turbines, photovoltaic arrays, wave generators, diesel generators or other power generating equipment, with or without an inverter to convert the DC power of the battery to AC power, or the AC power from the generator to DC power needed to charge the battery.

The VFB may be used in a wind-diesel grid for the purpose of reducing diesel fuel consumption and reducing greenhouse gas emissions from the operation of diesel generators dunng periods of low wind speed. In the case of a grid connected system, the VFB may be used for the purpose of power arbitrage, allowing electricity retailers to purchase power from the grid when the spot price is low and sell power to the grid when the electricity spot price is high. In this system, the VFB system is connected to the grid via an intelligent control system that has a link to electricity spot price information. When the spot price is low, the control system uses grid power to charge the VFB. When the electricity spot price is high, the control system sells power to the grid from the VFB.

The VFB can also be employed to provide back-up power in remote area power systems using wind turbines and solar arrays. By installing sufficient electrolyte volume in the VFB equivalent to 2-10 days of storage, the system may operate without the need for a diesel generator to meet power needs during extended periods of cloud cover and/or low wind speed. The VFB-wind-PV system is typically set up to provide DC power for telecommunications equipment in

remote areas, but may also be connected to a DC-AC inverter to power AC loads for remote off-grid houses and farms.

In the case of individual residential, commercial or industrial users, a VFB system is used in conjunction with a clock and prθ-εβt timer that switches on the charge to the VFB using off-peak electricity from the grid. A further grid connected application of the VFB is its use to provide emergency back-up power for IT, computer and server rooms in the event of loss of AC power from the grid or other power source. In each system, the VFB is connected to an intelligent control system that includes charging and discharging rate monitoring, as well as cell and stack voltage measurements to determine when to cut off charging to protect the cell from overcharge and to minimise pumping energy losses by setting the optimal electrolyte flow-rate into each stack.

BACKGROUND

Two types of redox flow batteries have been described that use vanadium electrolytes in both half-cells. The All-Vanadium Redox Flow Battery (V-VFB), described in the following patents: Australian patent 575247, AU 696452, AU 704534, US patent 6143443 and US patent 6562514, uses vanadium sulphate solutions in both half-cells and the cell employs the V(II)AZ(III) couple in the negative half-cell and a V(IV)AZ(V) couple in the positive half-cell. The Vanadium Bromide Redox Battery (VZBr) ₁ described in PCT/AU02/01157, PCT/GB2003/ 001757 and PCTZAU2004Z000310. Both batteries employ a vanadium electrolyte solution in both half-cells, but in this case the cell employs a vanadium bromide electrolyte with the V(II)ZV(III) couple in the negative half- cell and a BrVBr ₃ ^' or Br7CIBr ₂ ^' couple in the positive half-cell electrolyte. These systems share the same common feature of eliminating cross-contamination of the two half-cell electrolytes by diffusion through the membrane that separates each half-cell in the cell-stack. Since there is no cross-contamination, the electrolytes have an indefinite life, allowing low life-cycle costs to be achieved. A recent advance in the development of a new low cost membrane for the V-

VFB and V/Br, as described in PCT application "Improved Membranes and Electrolytes for Redox Cells and Batteries" , PCT/AU2006/000856, filed June 2006, has now allowed the production of stacks and batteries at a price that will allow them to enter markets for applications that were previously regarded as uneconomical. These applications include power arbitrage by electricity wholesalers and individual residential and commercial consumers who are able to purchase off-peak electricity to charge the battery for use during times of high electricity tariffs.

The term VFB is used in this specification to collectively describe V-VFB and V/Br cells and batteries. The VFB may be integrated into an energy system incorporating photovoltaic arrays, wind turbines, wave generators, diesel generators or other power generation equipment. The power generation system may include one or more wind turbine generators and a vanadium redox battery to store energy from the wind turbines during periods of low electricity demand and supply power to the grid during times or high demand. Patent application, WO2007001416 "Power Generation System Incorporating a Vanadium Redox Battery and a Direct Wind Turbine Generator", describes the integration of a V- VFB to a DC-wind turbine. Most large wind farms use AC wind turbines however and the power from the wind farm is fed to the grid when it is generated. If this occurs during periods of low demand, the wind generators must be shut down, wasting significant amounts of energy. Patent application WO2006086015, "Method for Retrofitting Wind Turbine Farms" refers to a V- VFB used to absorb excess energy from wind turbine generators to ensure that a pre-determined rating limit is not exceeded. The method involves replacing the wind turbine generators having a collective energy output with replacement wind turbine generators having an increased collective energy output, wherein the collective energy output exceeds the rating limit. In most instances however, there is no need to replace existing wind turbine generators to gain benefits from energy storage. By installing a VFB storage system in conjunction with the wind farm, energy generated by the wind farm during periods of low demand, can be stored in the VFB for release back to the grid at times of high demand. VFB systems can also be installed in new wind farms to maximise the

renewable energy penetration and reliability of supply. By supplying the grid only during times of high demand, wind farm owners can negotiate higher prices for their generated power. Similar financial benefits can also be achieved by using a VFB energy storage system for power arbitrage by electricity traders. To optimise the operation of such a wind energy storage or power arbitrage system, therefore, a controller is incorporated between the VFB energy storage system and the grid that is linked to local electricity spot prices. When the electricity spot price drops below a pre-set value, the VFB is charged from grid power through an AC-DC inverter. When the spot price exceeds a pre-set value, the VFB discharges its stored energy to the grid through a DC-AC inverter.

The power generation system may also include one or more wind turbine generators and one or more diesel fuel generators. Patent application WO2006088509 "Vanadium Redox Battery Energy Storage and Power Generation System Incorporating and Optimising Diesel Engine Generators describes a VFB system interfaced to a control system with one or more wind turbines and one or more diesel generators, whereby the control system manages the operation of the generators and VFB to control system stability, system frequency and voltage. In the present invention however, the control system manages the vanadium redox battery's absorption and power generation and switches on the diesel generator or generators only when the state-of-chargβ of the VFB has dropped below a pre-set limit. This allows diesel fuel consumption to be reduced and carbon dioxide emissions to be minimised.

The power generation system may include a photovoltaic or solar array and a vanadium redox battery to match the power generation with load demands or to store solar energy for use at night or on cloudy days The solar array provides DC power that may be used to charge the vanadium redox battery. Generated DC power may also be used for power distribution and, if required, supplemented by DC power from the vanadium redox battery. The power

generation system interfaces with a control system to optimize performance and efficiency.

According to the prior art described in patent applications WO2007001416, WO2006086015 and WO2006088509, the vanadium redox battery employed in each of the applications and systems described, employs a vanadium electrolyte in both half-cells that employs a sulphuric acid supporting electrolyte that excludes HCI. In the present invention, a Vanadium Bromide Redox Cell (V/Br) is integrated to a DC wind-turbine to store energy for periods of low wind speed. The V/Br uses a vanadium bromide solution in a supporting electrolyte of HBr and HCl. Unlike the V-VFB that excludes HCI ₁ the V/Br uses a HBr/HCI supporting electrolyte that allows a higher vanadium electrolyte concentration, offering a higher energy density. The higher solubility of vanadium bromides compared with vanadium sulphates, also allows the V/Br to withstand greater temperature ranges, typically -5 ⁰ C to 50 ⁰ C compared with 10 ⁰ C to 40 ⁰ C for the V-VFB. The V/Br can therefore be integrated to a DC or AC wind turbine with or without a solar array to store energy for remote area power systems for telecommunications equipment in climates with extreme temperature ranges between night and day with no risk of vanadium ion precipitation that occurs with the V-VFB.

This invention also relates to vanadium redox battery energy storage systems and associated automated control systems to optimise performance. According to WO2006076059 "System and Method for Optimising Efficiency and Power Output from a Vanadium Redox Battery Energy Storage System" a control system for a VFB is described that uses an open-circuit voltage and temperature measurement to calculate the anolyte and catholyte pump speeds required. Such as system requires the need to incorporate expensive flow metres to determine the flow-rate and feed-back to the controller. Alternatively, pressure transducers may be used to indirectly determine flow-rate, but the variation In electrolyte density and viscosity as a function of SOC, makes this a very inaccurate method for flow-rate determination. The present invention

describes a method for controlling the electrolyte flow-rate without the need for a flow-metre or pressure transducer.

Throughout the specification the term redox cell may also be referred to as a redox battery. The Vanadium Bromide Redox Cell may be referred to as the

Vanadium Bromide Redox or Battery or the V/Br cell or battery. The ions Br ₃ ^" or

CIBr ₂ ^" produced in the positive half-cell electrolyte of the V/Br during charging, are referred to as polyhalides. The V-VFB and the V/Br are collectively referred to as vanadium redox batteries (VRBs) or vanadium flow cells or batteries (VFBs).

SUMMARY OF THE INVENTION

According to a first aspect of this invention there is provided a energy storage system for storing low cost electricity from the grid for use during times of high electricity prices, comprising: a VFB, an AC-DC converter connected between the VFB and the AC power supply, a control system connected to the VFB, a controller programmed to start charging the VFB during times corresponding to off-peak electricity tariff prices and to discharge the VFB during times of high electricity prices, a DC-AC inverter connected between the VFB and the load.

In a second aspect of the invention, the controller includes a clock and a timer to determine when to turn on charging from the AC power source and when to discharge to the load. The timer is set according to the local off-peak and peak electricity tariff periods.

In a third aspect of this invention, the VFB is connected to the main electricity grid and the controller is connected to a computer that is linked to electricity spot market prices. When the spot prices are below a pre-set value, the VFB takes power from the grid to charge up the vanadium solutions. When the spot price is above a pre-set value the VFB is discharged and the energy stored in the charged VFB solutions is sold back to the grid.

In a fourth aspect of this invention, a VFB is integrated into an energy or power generation system incorporating a photovoltaic array and one or more wind turbines to supply energy to a load. The VFB of the fourth aspect employs a large electrolyte volume in each half-cell corresponding to 3-10 days of storage, eliminating the need for a diesel generator or the grid for back-up power. The power generation system interfaces with a control system to optimize performance and efficiency. The control system monitors the charging and

discharging current and thθ battery voltage to determine when to turn the pumps on or off so as to minimise energy consumption from the pumps.

In a fifth aspect of this invention, a V/Br is integrated to a DC or AC wind turbine to store energy during periods of low wind speed in climates of high temperature extremes in the range -5 ⁰ C to 50 ⁰ C that are beyond the operating limits of the V-VFB. The V/Br is interfaced to a control system to optimise performance and efficiency.

In a sixth aspect of this invention, a V/Br is integrated to a solar array to store energy to power a load at night or during periods of low solar irradiation, in climates of high temperature extremes in the range -5°C to 5O ⁰ C that are beyond the operating limits of the V-VFB. The V/Br is interfaced to a control system to optimise performance and efficiency.

In the first to forth aspects, the VFB is defined as comprising: a positive half cell containing a positive half cell solution comprising a supporting electrolyte selected from H ₂ SO ₄ , HBr or HBr/HCI mixtures and one or more ions selected from vanadium (III), vanadium (IV), vanadium (V) and polyhalide; a negative half cell containing a negative half cell solution comprising a supporting electrolyte selected from H ₂ SO ₄ , HBr or HBr/HCI mixtures and one or more vanadium ions selected from vanadium (II), vanadium (III) and vanadium (IV);

In the fifth and sixth aspects of this invention, the V/Br is defined as comprising: a positive half cell containing a positive half cell solution comprising a supporting electrolyte selected from HBr or HBr/HCI mixtures and one or more ions selected from vanadium (III), vanadium (IV), vanadium (V) and polyhalide; a negative half cell containing a negative half cell solution comprising a supporting electrolyte selected from HBr or HBr/HCI mixtures and one or more vanadium ions selected from vanadium (II), vanadium (III) and vanadium (IV);

In each of the first to sixth aspects the VFB includes a control system that may incorporate an open circuit cell in both the input and output electrolyte lines to the celt stack or stacks. By monitoring the input and output open-circuit potentials, the control system can calculate the conversion per pass and use this to adjust the electrolyte flow-rate needed for the particular state-of charge and charge or discharge current. This allows the pumping energy requirement to be minimised, thereby increasing the overall energy efficiency of the VFB, without the use of pressure transducers or expensive and inaccurate flow metres.

DESCRIPTION OF THE INVENTION

This Invention relates to vanadium redox battery energy storage systems and associated automated control systems to optimise performance, minimise fossil fuel consumption or maximise financial returns to the user. World-wide environmental concerns combined with new technological advances and increasing demand for electricity have made solar and wind power plants viable alternative power options to conventional coal or gas-fired power plants. Energy storage systems, such as rechargeable batteries, are an essential requirement for remote power systems that are supplied by wind turbine generators or photovoltaic arrays. Energy storage systems also enable energy arbitrage for selling electricity during peak tariffs periods and buying power during off peak times. Vanadium redox energy storage systems promise to be inexpensive and possess many features that provide for long life, flexible design, high reliability, and low operation and maintenance costs.

Due to their scalability, VFB energy storage systems are accessible to both small electricity consumers as well as to electricity retailers and wholesale traders. With widespread installation of timed electricity meters in residential, commercial and small industrial properties, an opportunity is arising for individuals and small companies to purchase power from the grid during off- peak periods at night, store this electricity in an energy storage device such as

a VFB and then use this stored energy to power their appliances during periods of high electricity tariffs. A VFB is ideal for such applications since it can be designed to provide anything from 1 to 20 or more hours of storage, allowing consumers to chose their optimal storage capacity for their specific requirements. Factors that influence the efficient operation and power output of a vanadium redox energy storage system include electrolyte flow rates, internal temperatures, pressure, charging and discharging times. By connecting the VFB to an intelligent battery controller, the VFB can also be designed for automatic operation with minimal input from the user. A system and method for optimizing the efficiency of a vanadium redox energy storage system using the vanadium redox battery of this invention is therefore also disclosed. The battery control system is designed to eliminate any cell overcharge that could destroy the conducting plastic electrode substrates, reduce the risk of vanadium salt precipitation at temperatures above 40 and below 10 ⁰ C and minimise parasitic losses associated with electrolyte pumping by determining when to turn the pumps on or off.

In more sophisticated control systems, the optimal electrolyte flow-rate can be determined and set without the need for expensive flow-metres. This can be achieved by Incorporating an open-circuit cell at both the inlet and outlet of each cell stack in order to determine the state-of-charge (SOC) of the electrolytes entering and exiting from the cell stack. The difference in SOC measured (δSOC) is then fed back to the controller that adjusts the electrolyte pump speeds so as to maintain the desired conversion per pass.

The power output from a wind turbine generator or set of wind turbine generators varies as a function of wind speed. Since wind speed is variable in nature, this reduces the availability and reliability of the wind energy. As the power output cannot be guaranteed, its value is therefore discounted. Vanadium redox batteries can increase power availability and enhance the value and price that can be charged for wind energy. The use of a vanadium bromide redox battery of this invention to provide a stable and constant power output from a wind turbine generator is therefore an advantage over prior art in

that it allows a lower cost, longer life, smaller footprint and wider temperature range compared with other redox cell energy storage systems, including the V- VFB.

The vanadium redox battery energy storage system (VBES) of this invention includes all sizes of vanadium redox batteries in both absolute KVA rating and energy storage duration in hours. The VBES includes reservoirs to store the vanadium electrolyte, an energy conversion mechanism defined as a cell or cell stack, a piping and pumping flow system, and a power conversion system (PCS). The VBES is in electrical communication with a control system that monitors important parameters and controls the performance of the VBES components in such a manner as to maximise cycle life and optimise efficiency and safe operation. In order to optimize the overall performance of the V/Br and V-VFB, the present invention employs a control system that uses algorithms with control logic that allow the VBES to operate in an automatic mode to ensure safe operation and maximum cycle life. The control system includes upper stack and cell voltage limits that are used to limit charging and prevent cell overcharge that could irreversibly damage the positive electrodes, reducing cell life. In the case of the V-VFB, the electrolyte temperature is also monitored and the upper and lower open-circuit voltage limits are reset accordingly to ensure that thermal precipitation of V(V) does not occur at elevated temperatures in the positive half-cell electrolyte, and to prevent precipitation of V(II) or V(III) ions in the negative electrolyte at temperatures below 5 ⁰ C. If the controller detects an electrolyte temperature outside this range, it instructs the VFB to undergo emergency discharge, either electrically or chemically, or shed power across an emergency discharge load until a safe state-of-charge is achieved. The state-of-charge of the VFB can be monitored using an open circuit cell, or by monitoring changes in the conductivity of each half-cell electrolyte. Another control parameter is the full or partial remixing of the anolyte and catholyte solutions to compensate for any changes in solution composition or electrolyte reservoir levels due to crossover of electrolyte or differential diffusion of ions across the membrane. This involves simple gravity equalisation by opening for a preset time, a valve connecting the anolyte and

catholytθ tanks. A similar electrolyte partial mix can also be used for the chemical emergency discharge procedure if the electrolyte temperature goes above or below the safe operating limits of the battery, thereby avoiding undesirable precipitation of vanadium compounds at high SOC limits and temperature extremes.

In the prior art, WO2006076059 describes a control system for a V-VFB that determines when heat exchangers are turned on to cool the anolyte and catholytθ solutions if the temperature exceeds the preset limit. The use of a heat exchange system is avoided in the present invention by using partial electrolyte mixing as an emergency discharge method to bring the SOCs of the anolyte and catholyte down to a safe level for the corresponding temperature. The electrolyte mixing is allowed to continue until the measured SOC falls below the safety level for the corresponding temperature.

In a further example, the power generation system includes one or more DC

^• wind turbine generators) connected to a V/Br. The higher energy density and wider operating temperature range of the V/Br compared with the V-VFB allows operation in extreme climates and permits a smaller foot-print that reduces land and housing costs. The DC wind generator may be in direct communication with the VBr and the rectifier to provide DC power to the distribution system or V/Br. Alternatively, the DC wind generator may be in indirect electrical communication with the V/Br. The power generation system further includes a controller that is in electrical communication with the V/Br and the DC wind turbine generator to control their respective operation. The controller manages the performance of the V/Br and the wind turbine generator in such a manner as to maximise cycle life and safe operation.

In order to compensate for wind speed fluctuations, diesel fuel generators may also be incorporated into the power supply system. This is particularly useful for remote area power supplies (RAPS) where a link to an extended grid is not available for back-up power in the event of long periods of little or no wind.

Description of Diagrams.

Figure 1 illustrates a vanadium redox flow cell employing a cast perfluorinated cation exchange membrane (1) to separate the positive and negative half-cells. Each half-cell includes a porous graphite felt or matte as the negative (2) or positive (3) flow-through electrode, each making electrical contact with a conducting substrate or current collector (4 and 5). The negative and positive electrolyte half-cell solutions are stored in separate external reservoirs (6 and 7) and pumps 8 and 9 are used to pump the electrolytes through the corresponding half-cells where the charge-discharge reactions occur. For practical applications, a number of cells are connected electrically in series using bipolar electrodes in a cell stack to provide the desirable stack voltage. The area of each bipolar electrode determines the current that can be delivered or accepted by the stack during discharge and charge respectively.

Figure 2 illustrates the layout of a system incorporating a VFB, a wind turbine and a solar array to provide power to a DC load.

Figure 3 illustrates the use of a VFB in a grid connected application wherein the VFB Is connected to a controller/timer that determines when to use low-cost or off-peak power to charge the electrolytes and when to discharge the VFB to power the AC or DC loads during periods of high electricity prices for the purpose of reducing electricity costs to the consumer.

Figure 4 illustrates the use of a VFB connected to the grid via an intelligent controller that is linked to spot electricity price data to determine when to buy power from the grid to charge the VFB and when to sell stored power to the grid during periods when the electricity spot price is high.

Figure 5 illustrates a VFB energy storage system that is used as an emergency back-up power source for protecting IT Servers and computer systems from power outages.

Figure 6 illustrates the arrangement of the inlet and outlet open-circuit cells used to monitor changes in state-of-charge through the cell stack for the purpose of adjusting electrolyte flow-rate so as to minimise pumping energy losses.

Figure 7 shows a plot of the difference between inlet and outlet open circuit cell potential values as a function of inlet state-of charge for different values of differential state-of-charge between the inlet and outlet electrolyte solutions. For a set differential state-of charge across the cell stack, the calculated values of δEcen can be used to adjust the pump speed and control the electrolyte flow- rates. ^'

MODE OF OPERATION

A vanadium redox cell or battery is electrically connected to one or more suitable power sources selected from grid power, an AC or DC wind generator, a photovoltaic array, diesel generator or wave generator, and uses excess power from the renewable source or off-peak electricity to charge the two half- cell electrolytes for the purpose of using or reselling the stored energy during times of low wind speed, low solar irradiation or high electricity prices. The VFB is connected to a battery controller that manages the operation of the VFB so as to prevent overcharge or extreme temperature operation that could damage the electrodes or electrolyte. The control system monitors electrolyte temperature and cell open-circuit potential or electrolyte conductivity to determine if an emergency discharge is required to bring the system state-of- charge to a level that would prevent precipitation of vanadium ions that could block the electrolyte channels and reduce cell capacity. This eliminates the need to use a refrigeration or cooling system to control electrolyte temperature. The control system further monitors the charging or discharging currents and stack voltage to determine when to turn the electrolyte pumps on and when to turn them off in order to reduce parasitic energy losses.

A power generation system that includes a VFB may be used in grid connected or an off-grid application where the system is isolated from other generator stations and serves as a Remote Area Power System (RAPS). The VFB provides a direct current to a coupling circuit and an inverter to convert the direct current to alternating current. The power generation system can include one or more wind turbine generators that are each in communication with a step up transformer. The power generation system for a RAPS application may further include a photovoltaic array and one or more diesel fuel generators. The power generation system for both the grid connected and off-grid application further includes a control system that interfaces with the VFB ₁ wind turbine generators and fuel generators to control their respective operation.

Example 1

A vanadium redox energy storage system is used to store energy in an off-grid telecommunications relay station or mobile phone tower powered by photovoltaic arrays and a DC-wind turbine. By using a large electrolyte volume for the vanadium redox battery equivalent to 2-5 days of storage, it is possible to operate the system with no diesel generator back-up, providing a fully standalone power system for the telecommunications equipment in the remote location. Figure 2 Illustrates the equipment layout for a remote DC load powered by a DC wind-turbine and PV array. Any excess power from the wind- turbine or PV array is used to charge the VFB. At night or on cloudy days, the. equipment is powered by the wind-turbine. If the wind speed is too low however, the VFB discharges, providing power to the DC load. Depending on the location and the corresponding solar irradiation and average wind speed, the volume of the electrolytes in each half-cell is chosen to ensure that sufficient energy storage capacity will always be available to continuously power the load without the need for a permanent diesel generator connected to the system.

The battery, controller is designed to ensure efficient operation of the VFB by switching off all charging once the stack voltage reaches the pre-set upper limit and minimising the parasitic energy losses from the electrolyte pumping by deciding when to turn the pumps on and off. When the pumps are off, energy

can be stored or generated through the electrolyte contained within the battery stack. To ensure that the stack is always full of electrolyte and does not drain out when the pumps are in the off position, the stack is situated such that the electrolyte levels in the reservoirs are always above the stack. For example, if the charge or discharge current to and from the VFB is greater than 10 Amps, the pumps remain off and the charging and discharging reactions use the solutions inside the cell stack only. Once the control system detects the stack voltage going above 1.5 V per cell during charging or below 1.2 V per cell during discharging, the pumps are turned on for 3-5 minutes to replenish the electrolytes in the cell stack and then switched off again until the current and/or voltage limits are exceeded. To eliminate the need for electrolyte level detectors in each tank, overflow tubes are Installed in the tanks to allow the electrolyte to automatically flow from one tank to the other, compensating for any electrolyte imbalance that occurs during normal operation of the system.

To prevent damage of the positive electrodes during cell overcharge, electrical connection is made to each cell in the stack and the control system monitors the voltage of each individual cell. As soon as the voltage of any individual cell exceeds a pre-determined cut-off value, the charging current is switched off to ensure that no cell goes into overcharge. This avoids problems associated with monitoring overall stack voltage where the averaging effect can disguise the presence of an early on-set overcharging cell, leading to delamination of the bipolar electrode and irreversible damage to the cell, affecting the operation of the entire stack.

The same system can also be used to power an isolated house or farm that has no connection to the electricity grid. If the system uses AC loads, a DC-AC inverter is connected between the DC power sources and the load to convert the power from the wind turbine, PV array and VFB into AC power for the load. Where the house or farm has excessive power demands that cannot be met by the wind-turbine, PV array and VFB alone, one or more diesel generators may be connected to the system for back-up power purposes. If the energy stored in the VFB system is depleted during periods of extended cloud cover with little or

no wind, the diesel generator can be switched on to charge up the battery and power the household or other loads until the PV arrays and wind generators can begin to generate sufficient power for the system.

Example 2.

A VFB energy storage system is used to store energy in grid-connected residential, industrial or commercial buildings during off-peak periods and supply power to the load during peak times. Figure 3 illustrates the use of a VFB energy storage system that incorporates a control system and timer that determines when the VFB will be charged using off-peak electricity and when it would be discharged to supply the load during periods of peak electricity tariffs. The timer is adjusted to coincide with the local electricity retailer's rated peak and off-peak tariff periods so as to enable the customer to save on their daily electricity power costs. The VFB system is sized according to the individual customer's daily power requirements. Typically, a residential customer may install a 5 kW VFB with up to 6-8 hours of storage that would supply up to 30-40 kWh of power during peak periods.

Example 3. A VFB energy storage system can be used by electricity traders for the purpose of power arbitrage by using grid power to charge the battery when the electricity spot price is low and supplying power to the grid when the spot price is high as illustrated in Figure 4. The VFB energy storage system is connected via an intelligent controller may be linked to instantaneous electricity spot market price information that allows it to determine when to purchase power from the grid to charge the VFB and when to sell power to the grid from the VFB. The selected purchase and selling prices used to trigger charging or discharging are set according to calculated values that provide a profit margin to the electricity trader.

Example 4.

A VFB energy storage device is used as an emergency back-up power source for protecting IT Servers and computer systems from power outages. The

system components are illustrated in Figure 5. When an interruption to the AC power from the grid occurs, the VFB automatically cuts in allowing uninterruptible power to the IT Server or computers. The VFB may be interfaced to a control system that monitors AC power and decides when to switch across to the VFB back-up power system.

Example 5.

A power generation system incorporating a vanadium flow battery energy storage device involves a VFB connected to a wind-farm comprising a multitude of AC or DC wind turbines connected to the main grid. Power from the wind turbines is used to charge the VFB during times of low electricity demand from the grid or low electricity prices. During periods of high demand or high electricity prices, the VFB discharges its stored energy to the grid to supplement power from other sources and maximise renewable energy penetration.

Example 6

A V/Br is connected via an AC-DC converter, to a wiπd-diesel grid comprising one or more wind turbines and one or more diesel generators. A control system connected to the V/Br, wind turbines and diesel generators to optimise the operation of each component.

[Example 7

A VFB is connected to a control system that incorporates an open-circuit cell at both the inlet and outlets of the cell stack as illustrated in Figure 6. For a VFB employing 2 M vanadium solutions in each half-cell, the stoichiometric flow-rate, F ₃ is given by:

Current (Amps)

Fs = 3.2 x SOC ml per minute per cell

Thus, if a current of 10 Amps is drawn from the battery, the stoichiometric flow- rate (minimum flow-rate needed for full conversion of electro-active species between the inlet and outlets of the cell), would be 3.1 ml/min per cell if the

battery is fully charged (SOC = 1.0), but 31 ml/miπ per cell when the battery is only 10% charged (SOC = 0.1). In order to minimise pumping energy requirements in flow-cells, the electrolyte flow-rate may be varied with SOC and current, however, this would require the need to install an in-line flow-metre in each of the anolyte and catholytθ lines, increasing the expense of the VFB system. An alternative approach that does not require pressure transducers or flow metres for pump control, incorporates an open-circuit cell at both the inlet and outlets of the cell stack. The open circuit cell potential at the inlet would be given by:

Similarly, the open-circuit cell potential at the outlet would be:

Where [VIN] and [V _ou t] are the vanadium ion concentrations in the inlet and outlet lines respectively.

If x = state-of-charge of inlet electrolytes, SOCIN y = state-of-charge of outlet electrolytes, SOCOUT z = fraction conversion per pass δSOC = y-x = SOCOUT - SOC _(N

then, E ^IN cβiι = E ^o ceiι ■ (2RT/nF) In [(1-x)/x]

and E ^IN ouτ = E°ceii - (2RTYnF) In [(1-y)/y]

Also, E ⁰⁰¹ _O61 I = E ⁰ _C6 H - (2RTVnF) In {[(1-x) + xz]/[x - xz]}

The electrolyte flow-rate can therefore be adjusted by setting particular conversion per pass to calculate the required outlet open-circuit cell potential. Alternatively a desired state-of-charge differential is set between the outlet and inlet electrolytes, δSOC, and this is used to calculate the corresponding outlet open-circuit cell potential, this being fed to the controller to allow it to keep adjusting the pump speed in order to maintain the calculated δEoβii value between the inlet and outlet open-circuit cell potentials. Thus, if a differential SOC value of 0.1 is required, for an inlet electrolyte SOC of 100% or 1.0, the outlet electrolytes would have a SOC of 90% or 0.9 under discharge mode. Alternately, under charge mode, for an inlet SOC of 0.1 or 10%, the outlet SOC would be 0.2 or 20%. The open-circuit potential values at the inlet and outlet to the cell stack would then be 1.287 V and 1.329 V respectively, giving a AE ₀₆ II value of 0.042 V. δEcβn values have been calculated for different δSOC conditions and these are plotted as a function of the inlet electrolyte SOC in Figure 7. Depending on the stack design and cell geometry, an optimal flow- rate would be required for minimal concentration polarisation and maximum voltage efficiency. Once the flow-factor has been determined for the particular stack design, the corresponding δSOC value can be determined and this is entered as a set value in the control system. The controller would then monitor the inlet open circuit cell potential and determine the required AE^H value or outlet open circuit cell potential needed to provide the state-of-charge differential. By feeding back this value to the pump controller, the need for a mass flow-metre for flow-rate control is thereby eliminated.

Example 8.

When operating a vanadium flow cell with 2 M vanadium electrolytes, the control system needs to continuously monitor the electrolyte temperatures to ensure that they are maintained between 15 and 35 ⁰ C so as to avoid possible thermal precipitation of V2O5 in the positive half-cell and electrolyte lines at elevated temperatures, and the precipitation of V ²⁺ or V ³⁺ sulphates at the lower temperatures. Earlier studies have shown that by maintaining the state-of- charge in the range 20 to 80%, it is possible to extend the temperature range to

10 to 40°° for without precipitation. Earlier approaches for preventing precipitation have involved the use of a heat exchanger or a refrigeration system to maintain the temperature within the required range. This involves additional expensive equipment however, so an alternative approach is to monitor the temperature and limit the upper and lower states-of-charge so as to avoid going into unsafe operation. While it is possible to reset the upper and lower SOC limits during charging and discharging respectively, what happens if the electrolytes are already at a high SOC when the temperature exceeds the safe limit? If the battery does not undergo normal discharge over a short time- frame, there is a danger of precipitation, leading to blockages in the electrolyte channels and pipes. In the present system, the controller is set to monitor the temperature and the electrolyte SOC and if an unsafe operating condition is detected, the control system instigates an emergency discharge procedure that will bring the electrolyte SOC to a safe level for the corresponding temperature. This may involve dumping energy through an emergency discharge load, in the present example, however, a VFB was charged to an SOC of 95% and then subjected to heating to increase the electrolyte temperature from 34 ⁰ C to an unsafe 42 ⁰ C. When this condition was detected by the control system, a valve between the 2 half-cell electrolyte lines was opened, allowing the electrolytes to rapidly mix and self-discharge until the SOC dropped to 50% SOC, at which precipitation is reduced to a safe level.