Energy Storage News

Vanadium Redox Flow Batteries for Large Scale Storage of Electricity

Vanadium redox flow batteries are a potential low cost (relatively speaking at least) electrical energy storage technology. In flow batteries liquid electrolytes flow past solid electrodes which are separated by an ion exchange membrane which allows one of the charged species to move back and forth between the two electrodes (one direction of movement corresponds to discharge and the other charging). In pure flow batteries it is claimed that the solid electrodes inert and do not directly take part in the chemical reaction taking place in the two half cells, so that they are not consumed or degraded (or at least degraded only very slowly) by the chemical reaction occurring in the liquid electrolytes. For this reason flow batteries can potentially have very long cycle life compared to other kinds of batteries.

However, one problem with flow batteries is that the ion exchange membranes are not completely impermeable to the species of ions which are supposed to be separated so that over time some amount of cross contamination of the electrolytes in the two half cells occurs, thus degrading performance. This degradation can be fixed by periodically replacing the electrolyte which adds to the ongoing maintenance cost of the batteries.

Vanadium flow batteries get around this cross contamination problem by using different charge states of vanadium ions in the two half cells. In this case cross contamination slightly degrades the energy efficiency but does not lead to performance degradation.

Below is information on three different companies which are currently developing vanadium flow batteries.

Prudent Energy (formerly VRB Power) is a manufacturer of Vanadium flow batteries. Below is some information about their product taken from their web site.

The VRB Energy Storage System (VRB-ESS) is an electrical energy storage system based on the patented vanadium-based redox regenerative fuel cell that converts chemical energy into electrical energy. Energy is stored chemically in different ionic forms of vanadium in a dilute sulphuric acid electrolyte. The electrolyte is pumped from separate plastic storage tanks into flow cells across a proton exchange membrane (PEM) where one form of electrolyte is electrochemically oxidized and the other is electrochemically reduced. This creates a current that is collected by electrodes and made available to an external circuit. The reaction is reversible allowing the battery to be charged, discharged and recharged.

The principle of the VRB is shown in more detail in Figure 1 - it consists of two electrolyte tanks, containing active vanadium species in different oxidation states (positive: V(IV)/V(V) redox couple, negative: V(II)/(III) redox couple). These energy-bearing liquids are circulated through the cell stack by pumps. The stack consists 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.

The VRB-ESS employs vanadium ions in both half-cell electrolytes. Therefore, cross-contamination of ions through the membrane separator has no permanent effect on the battery capacity, as is the case in redox flow batteries employing different metal species in the positive and negative half-cells. The vanadium half-cell solutions can even be remixed bringing the system back to its original state.

Model: Mark III kW-Class VRB-ESS™
Performance Characteristics
Open circuit voltage 49.0 V to 57.0 V
Maximum charge voltage 49.0 V to 57.0 V
Minimum voltage on discharge 59.0 V
Maximum charge current 42 V
Maximum discharge current (continuous) 140 A
Maximum discharge current (< 120s) 125 A
Continuous power at the end of discharge 175 A
Continuous power at the end of discharge 6.0kW
Rated capacity 20kWh ~ 40kWh (500Ah ~ 1000Ah)
System Efficiency (Full SOC cycle) 65%
Response Time < 1 ms
Charge-to-discharge duration ratio 1.6:1 (Adjustable, can be 1:1 at additional cost)
Cycle Life (100% DOD) > 10,000 cycles
Service Life 100,000 hours
Duty cycle 0%~100%

On Prudent Energy's projects and installation page nine projects involving the installation of their batteries are listed. The most recent installation date is 2006. All of these installations occurred during the original incarnation of this company as VRB energy.

The battery efficiency is listed as 65% but they do not say whether this efficiency applies to DC or AC operation. For the moment I will make the optimistic assumption that this number is the AC efficiency. It should be compared to 75% efficiency for NGK insulators sodium sulphur (NAS) battery. The efficiency cost penalty is given by:

Ceff = Base Generation Cost * eff/(1-eff)

Therefore Ceff for NGK and Prudent energy is 0.33 and 0.54 times base generation cost respectively. For example if these batteries were storing energy from offshore wind at US $0.15/kWh then the efficiency penalty would be US $0.05 and $0.08 respectively. If the efficiency quoted is for DC operation the efficiency cost difference will be higher.

The advertised cycle life of >10,000 is a big improvement over 4500 cycles for NAS batteries. However, realizing this cost advantage requires taking a long term investment view. As I have mentioned before large scale utility storage is a hard market to enter. The up front costs are high and the benefits take a long time to be realized. Unless a battery technology has performance track record it is hard to persuade power companies to risk the investment dollars. I did not find any mention of prices on Prudent Energy's web site.

Cellennium is company based in Thailand which is developing Vanadium flow batteries. They list a single battery installation at an eco-village in Thailand in November of 2008, with no sign of any new projects since that time. However, they claim to have a novel architecture for their battery which will improve performance over other vanadium battery designs. Here is some information about this architecture taken from their web site:

Electrolyte Flow

The electrolytes are fed through the stack of cells in series instead of in parallel as in conventional designs.

Cell and Stack Orientation

The cells are place horizontally in a vertical stack with the electrolyte flow upwards. The stack is compact and structurally stable.

Cell Components

The cells are formed from molded structural components designed for speedy "LEGO" style leak-tight assembly. Channels in the moldings provide paths for the flow of the electrolytes between the cells (shown schematically as external channels in Fig. 3).

Power Conversion

The Squirrel technology includes novel, highly efficient "induction-less" methods of charging the battery with any DC or AC input and delivering any DC or AC output free from harmonics. This breakthrough technology will enable cost-competitive means to convert various forms of electricity very efficiently.

Bypass Currents

Electrical bypass currents can flow in the channels distributing the electrolytes to the cells. In a conventional battery with electrolytes fed to the cells in parallel these currents span not only individual cells but also the whole stack. Because the channels must be wide to feed the electrolytes uniformly to many cells at once the bypass resistances are low. Moreover, the bypass potentials range from the voltage of a single cell to the voltage of the whole stack. Consequently, relatively large currents can occur. These currents reduce the electrical storage efficiency of the battery.

A second undesirable effect of the bypass currents in batteries with parallel electrolyte flow is corrosion of the electrodes, which are normally made of carbon. This occurs when there is a high current density in the region of an electrode near a channel carrying a bypass current at a sufficiently high voltage.

In the Squirrel architecture with electrolytes flowing in series through the cells the effects of bypass currents are negligible. The voltage across the channels is never more than that of one cell and wide channels are not needed so they have high electrical resistances. There are no bypass currents between any non-adjacent cells in the stack and the conditions for corrosion of the electrodes do not occur.

Variations in Operating Conditions between Cells

In parallel feed designs it is difficult (or impossible) to ensure that all the cells receive the electrolytes at the same flow rate. Since all the cells carry the same electrical current, the electrolyte in a cell with a lower flow rate than average will be raised to a higher state of charge than the electrolyte in the other cells. This electrolyte is then partly discharged by mixing with the electrolyte from other cells and there is a loss of efficiency.

In the most extreme case an undetected blockage in one cell during charging will cause the evolution of hydrogen and oxygen gases and a large pressure difference across the membrane. There will be an explosion if the membrane breaks and the gases are ignited electrically. Another cause of lost efficiency related to variations in the flow is variation in the degree of polarization due to the formation of concentration gradients within cells.

In the Squirrel series feed design all cells receive the electrolytes at the same flow rate. Losses due to the mixing of electrolytes at different states of charge cannot occur. If there is a blockage during charging the whole flow is reduced or stopped and the charging current must be cut for safety. This can be done with a single pressure switch - a much simpler operation than monitoring many cells at once in a parallel feed system. If it happens that hydrogen and oxygen gases are produced by accident, then the Squirrel vertical stacking arrangement allows the gases to flow naturally upwards (in the same direction as the pumping of the electrolyte) where they can be released through one-way valves.

Number of Cells per Stack

In conventional vanadium battery designs the problems described above associated with parallel feed limit the size of the stack to not more than 20-30 cells. In the Squirrel series feed design, on the other hand, more than 100 cells can be stacked together without ill effects to give simple and compact high-voltage units.

Electrolyte Flow Rate and Pumping Energy

In conventional parallel feed designs the rate of flow of the electrolytes is typically 20 times the rate actually required for charging and discharging. The purpose of this is (a) to minimize variations in the flow rate between cells and guard against individual cells becoming blocked, and (b) to maintain sufficient mass transfer at the electrodes and reduce polarization losses. The large flow rate demands for pumping at least 10% of the total power. Furthermore, since in a single pass through the stack each portion of the electrolyte is charged or discharged in only one cell, the electrolytes must be recirculated through the stack many times for complete charging or discharging.

In the Squirrel series feed design all the cells in the stack have the same electrolyte flow rate. There is no need to pump the electrolytes faster than the rate required for charging and discharging. The cells have a low flow resistance, and the total power needed for pumping at the optimum rate is only 1% of the total power. Volumetric pumps are used to keep the flow rate steady. However, although the total flow rate is low, the flow through individual cells is high because the whole of the electrolyte flows through each cell. Consequently, mass transfer at the electrodes is good and polarization effects are small.

Total Stack Voltage

In parallel feed designs the total voltage across the stack depends on the state of charge of the electrolytes. For example, in a stack of 20 cells the total voltage is 22V in the fully discharged state and 32V in the fully charged state. This large variation in the voltage creates serious difficulties in many applications.

In the Squirrel series feed design the problem does not occur. If fully discharged electrolytes are fed into the bottom of a stack, the operating voltage of the first cell is 1.1V. The flow rate and charging current can be adjusted so that the electrolyte leaves the top of the stack fully charged and the operating voltage of the last cell is 1.6V. The total voltage is the average voltage per cell times the number of cells. In a stack of 100 cells the total voltage will be constant at 135V throughout the whole process. To prevent mixing of the charged and discharged electrolytes each electrolyte tank can be divided into two separate tanks or compartments (not shown in Fig. 3), one for the charged electrolyte and the other for the discharged electrolyte.

In spite of Cellenium's detailed description of the design advantages of their incarnation of the vanadium flow battery, they do not appear to be selling anything, so presumably there are still some technical barriers to cost effective manufacturing yet to be surmounted.

Golden Energy Fuel Cell is a Chinese manufacturer of vanadium redox flow batteries. Below is some information about their products taken from their web site:

Founded on Aug 8, 2003, GEFC developed from Zheng Zhong Discovery, the private laboratory of Prof. Zheng Zhong De, GEFC Chairman & CEO. GEFC is the world leader to produce perfluorinated IEM by solution casting process. Our products, with the main technical data surpassing the like products of USA and Japan, are sold to over 20 countries and highly appreciated by their users.

With world's leading VRB technology, GEFC enjoys huge advantages in technology, product and cost with respect to perfluorinated IEM, VRB electrode, VRB electrolyte and VRB stack structure. GEFC has begun to successively launch the VRB products used for solar power, wind power, peak shaving, distributed power, base station, UPS/EPS, EV etc.

GEFC-125V200A2h-VRB Vanadium Redox Battery
Rated Voltage 125 V
Rated Current 200 A
Rated Power 25 kW
Maximum Power 100 kW
Rated Time 2 h
Rated Energy Efficiency 72%
Single Cell Number 100
Electrode Area 2349 cm2
Stack Weight 570 kg
Stack Size 70 x 70 x 140 cm
Electrolyte Weight 1400 kg x 2
Electrolyte Volume 1000 L x 2
Electrolyte 2 M VOSO4
Environment Temp. - 20 ~ 50 ?
Limited Charge Voltage 160 V
Limited Discharge Voltage 100 V
Operating Life 20 years
Store Life Limitless

GEFC-50V50A2h-VRB Vanadium Redox Battery
Rated Voltage 50 V
Rated Current 50 A
Rated Power 2.5 kW
Maximum Power 10 kW
Rated Time 2 h
Rated Energy Efficiency 75%
Single Cell Number 40
Electrode Area 625 cm2
Stack Weight 80 kg
Stack Size 36 x 36 x 62 cm
Electrolyte Weight 140 kg x 2
Electrolyte Volume 100 L x 2
Electrolyte 2 M VOSO4
Environment Temp. - 20 ~ 50 ?
Limited Charge Voltage 64 V
Limited Discharge Voltage 40 V
Operating Life 20 years
Store Life Limitless

In addition to complete batteries GEFC sells battery stacks without the liquid electrolyte system. Standard models listed in their web site are:

Model Rated Power
GEFC-50V50A-VRB Vanadium Redox Battery Stack 2.5KW
GEFC-50V200A-VRB Vanadium Redox Battery Stack 10KW
GEFC-125V200A-VRB Vanadium Redox Battery Stack 25kW

GEFC claims that they will produce customized battery stacks as well. Looking over their news items for the last six months I found the following sales of VRB batteries:

Sale Date
Spain bought GEFC-50V50A2h-VRB 2009-12-22
England bought GEFC-12.5V50A2h-VRB 2009-12-14
State Grid bought GEFC-12.5V10A3h-VRB 2009-11-25
Beijing bought GEFC-12.5V10A,12.5V10A8hVRB 2009-9-29
Tsinghua Univ. bought GEFC-50V50A2h-VRB 2009-8-28
Ireland bought GEFC-12.5V10A4h-VRB 2009-7-1

Two of these sales were of the 2.5KW battery, but the others have smaller power ratings, including two at only 125Watts. I am not sure what people are doing with these lower power systems. Perhaps they are just experimenting to understand the performance characteristics. I like the idea of selling small systems which allow customers to get experience with the product without investing a lot of money.

GEFC does not give a lot of details about what differentiates their batteries other vanadium flow batteries except that they use perfluorinated ion exchange membranes. Improved membranes might improve efficiency or they might lower the cost per kW. Flow batteries have the virtue that the power production function and the energy storage function are in some sense separate. The amount of energy storage depends on the size of the electrolyte tanks, and power production depends on the details of the solid electrodes and the ion exchange membrane. Batteries are power generation devices as well as energy storage devices so that cost per kW and cost per kWh are both important parameter in determining the economics.

GEFC's quoted efficiencies of over 70% are greater than Prudent Energy's claim of 65%, but since neither company specifies whether their efficiency number applies to AC operation or DC operation, it is impossible to know if these numbers represent an apples to apples comparison

Vanadium is a comparatively abundant element with crustal atomic abundance of 75,000 ppb (compared to 22,000 ppb for copper). Its chief commercial use at present is as an important ingredient in steel alloys. What would happen to vanadium prices if vanadium batteries became a major component of a global energy storage system is not clear.

January 22, 2010

Energy Storage News

rogerkb [at] energystoragenews [dot] com