Energy Storage · 4 Energy Storage Section 4: Energy Storage Technologies PowerPoint...

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Energy Storage

Energy StorageTechnologies4

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Energy StorageSection 4: Energy StorageTechnologies

Lesson Plan: # 4-0Energy Storage System (ESS): NoneQuestion: HOW is electricity actually generated mechanically?

Learning Objective(s):Students will be able to explain how electricity is mechanically generated by the process of induction.

Student will be able to explain how more or less electricity can be generated by changing select variables of their generators or generating systems.

Activities: See Activities and Lesson Plans section for additional information about each energy storage technology, recommended steps for teaching the technologies, and suggested activities for students.

For information about storage projects and technologies deployed across the globe, the following DOE web site is recommended: http:// www.energystorageexchange.org/

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These are the primary energy storage technologies that are not electrochemical

Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its super-conducting critical temperature.

A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to store electrical energy temporarily in an electric field.

A supercapacitor (SC) (sometimes ultra-capacitor, formerly electric double-layer capacitor (EDLC)) is a high-capacity electrochemical capacitor with capacitance values much higher than other capacitors (but lower voltage limits) that bridge the gap between electrolytic capacitors and rechargeable batteries.

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Energy StorageSection 4: Energy StorageTechnologiesPowerPoint

Electrochemical energy storage (batteries) come in many varieties and forms.

They are primarily defined by the anode, cathode and electrolyte materials used, as well as their specific electrochemistry.

For example, Ni-Cd batteries use nickel hydroxide Ni(OH) 2 for the positive electrode (cathode), cadmium Cd as the negative electrode (anode) and an alkaline potassium hydroxide KOH electrolyte.

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This chart illustrates which energy storage technologies match with various applications and services.

The larger and slower energy storage technologies tend to be best matched with bulk power management.

While smaller and faster energy storage technologies tend to work best for power quality and grid support services.

The adjacent “Applications” slide shows where the various Energy Storage Technologies fit into the scheme of electrical grid / system application. The “Reading for Applications” section provides an explanation as to WHAT the usage categories are and WHY those technologies match the categories. This section should be read by the student when arriving at this point.

Full page images for the adjacent APPLICATIONS chart, as well as for the PERFORMANCE table at the end of this section and COST table in Section 7, can be found in the Activities section of this manual (Section 10). These are available for subsequent student activities.

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Factors such as price, performance, capacity relative to size, and availability in the marketplace will impact the extent to which a battery technology is used. For example, when Tesla completes its Nevada “gigafactory” the number of lithium-ion batteries in the market is expected to double, driving down the cost of lithium-ion batteries across the board, along with the cost of electric vehicles that use them.

A consideration of major importance is the recycle-ability of battery technologies. This factor should be discussed with students. Information on the recycling of batteries can be found in Section 9 (electronic file only).

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Energy StorageSection 4: Energy StorageTechnologiesPowerPoint Lesson Plan: # 4-1

ESS: Hydraulic – Pumped Hydro (Water) Storage (PHS) Question: HOW can water actually store electricity?Scientific Principle: Conservation of Energy: Stored Gravitational Potential Energy (PE = mgh) converted to Kinetic Energy (KE = ½ mv2)

Learning Objective(s):Students will be able to explain how a Pumped Hydro Storage system uses changes in the gravitational potential energy (PE) of a mass of water as it is moved to an increased altitude to store gravitational PE. Students will be able to use the basic gravitational PE equation, PE = mgh, in their explanation.

Students will then be able to explain how the release of that mass of high altitude water converts gravitational PE into kinetic energy, KE, i.e. energy of motion. This KE is, in turn, captured with a turbine to convert the KE into electrical energy as described in the section on electrical induction above.

Students will be able to explain the conversion process from electrical (at the pump) to PE gravitational to KE to electrical, and discuss the efficiency losses that occur during every stage of energy conversion.

Students will be able to explain the advantages, disadvantages, practical benefits and limitations of a PHS system.

See Activities and Lesson Plans section for additional information and student activities.

Pumped hydroelectric energy storage is a large, mature, and commercial utility-scale technology currently used at many locations in the United States and around the world.

Pumped hydro employs off-peak electricity to pump water from a reservoir up to another reservoir at a higher elevation. When electricity is needed, water is released from the upper reservoir through a hydroelectric turbine into the lower reservoir to generate electricity.

Projects may be practically sized up to 4000 MW and operate at about 76%–85% efficiency, depending on design.

Pumped hydro plants have long lives, on the order of 50-60 years.

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CAES systems use off-peak electricity to compress air and store it in a reservoir, either an underground cavern or aboveground pipes or vessels.

When electricity is needed, the compressed air is heated, expanded, and directed through an expander or conventional turbine-generator to produce electricity.

Underground CAES storage systems are most cost-effective with storage capacities up to 400 MW and discharge times of 8 to 26 hours.

Siting such plants involves finding and verifying the air storage integrity of a geologic formation appropriate for CAES in a given utility’s service territory.

CAES plants employing aboveground air storage would typically be smaller than plants with underground storage, with capacities on the order of 3 to 50 MW and discharge times of 2 to 6 hours.

Aboveground CAES plants are easier to site but more expensive to build (on a $/kW basis) than CAES plants using underground air storage systems, primarily due to the incremental additional cost associated with aboveground storage.

Lesson Plan: # 4-2ESS: Pneumatic – Compressed Air Energy Storage (CAES)Question: HOW can air store electricity? Elastic (equivalent) Potential Energy

Scientific Principle: Compressed air creates a high speed, high force jet of air when it is released. This can be demonstrated with a balloon powered car or by releasing an inflated balloon to fly around the room. The higher the pressure in the balloon, the more force propelling the balloon in flight.

Air is a fluid, but it is a compressible fluid, unlike water and other liquids which cannot be compressed. However, the basic fluid principles still apply. If compressed (pressurized) air is released through a tube with some restrictive cross sectional area, it will generate a force. This force can be used to spin a turbine or push a car, etc.

Learning Objective(s):Students will be able to explain how a CAES system works and how it uses compressed air to generate electricity.

Students will be able to explain how compressed air increases the efficiency of the natural gas turbines used to generate electricity in a CAES system.

Students will be able to explain the uses, advantages and disadvantage to building a CAES system for a utility.


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Flywheels store energy in the form of the angular momentum of a spinning mass, called a rotor. The work done to spin the mass is stored in the form of kinetic energy.

A flywheel system transfers kinetic energy into ac power through the use of controls and power conversion systems.

Most modern flywheel systems have some type of containment for safety and performanceenhancement purposes. The containment vessel is often placed under vacuum or filled with a low-friction gas such as helium to reduce the effect of friction on the rotor.

Low speed flywheels are built with steel and rotate at rates up to 10,000 rpm.

More advanced flywheels:• Employ rotating mass made of fiber glass resins or

polymer materials with a high strength-to-weight ratio.

• Employ air or magnetic suppression bearings to accommodate high rotational speed.

• Operate at rotational frequencies in excess of 100,000 rpm with tip speeds in excess of 1000 m/s.

• Have very fast response and ramp rates with the ability to go from full discharge to charge within a few seconds.

Currently 2 kW/6 kWh systems are being used in telecommunications applications.

Lesson Plan: # 4-3ESS: Kinetic – Flywheel TechnologyQuestion: What is a flywheel and how does it store energy?

Scientific Principle: Flywheels store energy in the form of momentum in a rotating wheel or cylinder. Electrical energy can be converted to KE of motion and back to Electrical energy.

Learning Objective(s):Students will be able to explain how a Flywheel energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Flywheel energy storage system for a utility or other customer.


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SMES stores electricity in the form of a magnetic field generated by direct current circulated through superconducting wires.

The geometry of the superconducting coils creates a highly contained electromagnetic field, but relatively little energy is needed to sustain the field.

The energy is released by discharging the coils.

The main component of this storage system is a coil made of superconducting material. Additional components include power conditioning equipment and a cryogenically cooled refrigeration system.

The main advantage of SMES is the very quick response time: the requested power is available almost instantaneously.

SMES is characterized by its high overall roundtrip efficiency (85 % 90 %) and the very high power output which can be provided for a short period of time.

There are no moving parts in the main portion of SMES, but the overall reliability depends crucially on the refrigeration system.

Large SMES systems with more than 10 MW power are mainly used in particle detectors for high-energy physics experiments and nuclear fusion.

Lesson Plan: # 4-4ESS: Electromagnetic – Superconducting Magnetic Energy Storage (SMES)Question: HOW can a magnet store energy?

Scientific Principle: Static (non-moving) INDUCTION: SMES stores energy by inducing an electric field in an existing magnetic field using a current.

Learning Objective(s):Students will be able to explain how a Superconducting Magnetic Energy Storage (SMES) energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a SMES system for a utility or other customer.


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Electrochemical double-layer capacitors (DLC), also known as supercapacitors, are a technology which has been known for 60 years.

Because the charge is stored physically, with no chemical or phase change taking place, the charge/discharge cycle is fast and highly reversible and can be repeated over and over again.

Because of the high surface area and the small thickness of the double layer DLCs can have very high specific and volumetric capacitances.

This enable DLCs to combine a previously unattainable capacitance density with an essentially unlimited cycle life.

DLCs are suited especially to applications with a large number of short charge/discharge cycles, where their high performance characteristics can be used.

Since about 1980 they have been widely applied in consumer electronics and power electronics.

Other applications include use as a UPS to bridge short voltage failures, in medical devices and in hybrid auto mobile applications.

Lesson Plan: # 4-5ESS: Electrical – Double Layer (High Density) Capacitors (DLC)Question: WHAT is a capacitor and HOW does it store energy?

Scientific Principle: Two electrified plates can hold positive and negative charges on them until the plates are connected, at which point they discharge into the circuit for a particular use.

Learning Objective(s):Students will be able to explain how a capacitor works and how it gains, stores and releases energy.

Students will be able to explain how a capacitor can be made to store more or less charges / energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Double Lay Capacitor energy storage system for a utility or other customer.


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Sodium-sulfur (NaS) batteries are a commercial energy storage technology finding applications in electric utility distribution grid support, wind power integration, and high-value grid services.

The active materials in a NaS battery are molten sulfur as the positive electrode and molten sodium as the negative. The electrodes are separated by a solid ceramic, sodium alumina, which also serves as the electrolyte. This ceramic only allows positively charged sodium ions to pass through during discharge. During charging the process is reversed.

NaS battery technology holds potential for use in grid services because of its long discharge period (approximately 6 hours).

Energy density by volume for NaS batteries is 170kWh/m3 and by weight is 117kWh/ton. One manufacturer, NGK, projects its NAS to have a cycle life of 4500 cycles. Resulting in a projected calendar life of 15 years.

NaS batteries have an AC round trip efficiency of 74%.

NaS battery technology has been demonstrated at over 190 sites in Japan. More than 270 MW of stored energy suitable for 6 hours of daily peak shaving have been installed.

Source: Polysulfide Chemistry in Sodium–Sulfur Batteries and Related Systems— A Computational Study by G3X(MP2) and PCM Calculations (pages 3162–3176), Prof. Dr. Ralf Steudel and Dr. Yana Steudel. Article first published online: 16 JAN 2013 | DOI: 10.1002/chem.201203397

Lesson Plan: # 4-6ESS: Electro-Chemical – Sodium Sulfur (NaS) BatteryQuestion: WHAT is a Sodium Sulfur (molten salt electrolyte) Battery and HOW does it work?

Scientific Principle: Sodium Sulfur batteries are based on the chemical storage of electricity using a high temperature fluid molten salt (sodium) electrolyte to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Sodium Sulfur (molten salt electrolyte) Battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Sodium Sulfur (molten salt electrolyte) Battery energy storage system for a utility or other customer.


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Lesson Plan: # 4-7ESS: Electro-Chemical— Sodium Nickel Chloride (NaNiCl) batteryQuestion: WHAT is a Sodium Nickel Chloride (molten salt electrolyte) Battery and HOW does it work?

Scientific Principle: Sodium Nickel Chloride batteries are based on the chemical storage of electricity using a high temperature fluid molten salt (sodium) electrolyte to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Sodium Nickel Chloride (molten salt electrolyte) battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Sodium Nickel Chloride (molten salt electrolyte) battery energy storage system for a utility or other customer.


Sodium-nickel-chloride batteries are high temperature battery devices like NaS.

The electrodes are separated by a ceramic wall (electrolyte) that is conductive for sodium ions but an isolator for electrons.

Therefore, the cell reaction can only occur if an external circuit allows electron flow equal to the sodium ion current.

Cells are hermetically sealed and packaged into modules of about 20 kWh each.

The internal normal operating temperature of 270 °C to 350 °C is required to achieve acceptable cell resistance and must be thermally managed by design features.

Two battery original equipment manufacturer (OEM) suppliers have production facilities operating and are starting to deploy systems in the size range of 50 kW to 1 MW.

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Lesson Plan: # 4-8ESS: Electro-Chemical – Nickel Cadmium (Ni-Cd) BatteryQuestion: WHAT is a Ni-Cd battery and HOW does it store energy?

Scientific Principle: Nickel Cadmium batteries are based on the chemical storage of electricity using a liquid electrolyte to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Nickel Cadmium battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Nickel Cadmium battery energy storage system for a utility or other customer.


NiCd batteries have been in commercial production since 1910.

NiCd batteries use nickel oxide hydroxide and metallic cadmium as electrodes.

NiCd batteries do not excel in energy density or first cost. They do provide simple implementation without complex management systems, while providing long life and reliable service.

All industrial NiCd designs are vented types, allowing gases formed on overcharge to be dissipated but requiring water replenishment to compensate.

Early NiCd cells used pocket plate technology. A design that is still in production today. Sintered plates entered production in the mid 20th Century; followed by fiber plates, plastic bonded electrodes and foam plates.

They are used in aviation, rail and mass transit, backup power for telecoms, engine starting for backup turbines etc.

Using vented cell NiCd batteries results in reduction in size, weight and maintenance requirements over other types of batteries. Vented cell NiCd batteries have long lives (up to 20 years or more, depending on type)

Vented cell NiCd batteries operate at extreme temperatures (from -40 to 70 °C).

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Lesson Plan: # 4-9ESS: Electro-Chemical – Lithium Ion (Li-ion) BatteryQuestion: WHAT is a Li-ion Battery and HOW does it work?

Scientific Principle: Lithium Ion batteries are based on the chemical storage of electricity using a solid state electrolyte to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Lithium Ion battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantages to building and operating a Lithium Ion battery energy storage system for a utility or other customer.


Li-ion battery technology has emerged as the fastest growing platform for stationary storage applications.

Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard since they contain, unlike other rechargeable batteries, a flammable electrolyte and are also kept pressurized. Because of this the testing standards for these batteries are more stringent than those for acid-electrolyte batteries, requiring both a broader range of test conditions and additional battery-specific tests.

The most common types of Li-ion cells are cylindrical and prismatic cell.

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NOTES:

Rechargeable Li-ion batteries are commonly found in consumer electronic products, which make up most of the estimated worldwide production volume of 30 GWh per year. But they are also found in large stationary storage applications.

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NOTES:

The term lithium-ion does not refer to a single electrochemical pairing but to a wide variety of different chemistries.

Li-ion cells do no contain metallic lithium; rather, the ions are inserted into the structure of other materials, such as lithiated metal oxides or phospates in the positive electrode (cathode) and carbon (typically graphite) or lithium titanate in the negative electrode (anode)

Chemistry, performance, cost and safety characteristics vary across LIB types.

Handheld electronics mostly use LIBs based on lithium cobalt oxide (LiCoO2), which offers high energy density, but presents safety risks, especially when damaged.

Lithium iron phosphate (LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety. These battery chemistries are used in stationary storage applications.

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Zinc-air batteries are a metal-air electrochemical cell technology. Metal-air batteries use an electropositive metal, such as zinc, aluminum, magnesium, or lithium, in an electrochemical couple with oxygen from the air to generate electricity. Because such batteries only require one electrode within the product, they can potentially have very high energy densities.

Zinc-air batteries take oxygen from the surrounding air to generate electric current. The oxygen serves as an electrode, while the battery construction includes an electrolyte and a zinc electrode that channels air inside the battery as shown in the Figure.

The Zinc-air battery produces current when the air electrode is discharged with the help of catalysts that produce hydroxyl ions in the liquid electrolyte. The zinc electrode is then oxidized and releases electrons to form an electric current.

Despite advantages such as the low cost of the metal electrode and high power densities metal-air batteries also pose several historical disadvantages such as susceptibility to changes in ambient air conditions, including humidity and airborne contaminants. The air electrode is a sophisticated technology that requires a 3-way catalytic interface between the gaseous oxygen, the liquid electrolyte and the solid current collector. It has been difficult and expensive to make.

Lesson Plan: # 4-10ESS: Electro-Chemical— Zinc Air (Zn-air) BatteryQuestion: WHAT is a Zinc Air Battery and HOW does it work?

Scientific Principle: Zinc Air batteries are based on the chemical storage of electricity using a solid material as it reacts to the oxygen in flowing air (oxidizes) to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Zinc Air battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Zinc Air battery energy storage system for a utility or other customer.


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Electric recharge has been difficult and ineffi- cient with metal-air batteries, with typical round-trip efficiencies below 50 percent.

Zinc-air batteries have up to three times the energy density of Li-ion, its most competitive battery technology.

Unlike lithium-ion, however, Zinc-air batteries neither produce potentially toxic or explosive gases, nor contain toxic or environmentally dangerous components.

Zinc-oxide, which is the main material in a zinc- air battery, is 100-percent recyclable.

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Lead-acid batteries are the oldest form of rechargeable battery technology. Originally invented in the mid-1800s, they are widely used to power engine starters in cars, boats, planes, etc.

All lead-acid designs share the same basic chemistry.

• The positive electrode is composed of leaddioxide, PbO2, while the negative electrode is composed of metallic lead, Pb.

• The active material in both electrodes is highly porous to maximize surface area.

• The electrolyte is a sulfuric acid solution, usually around 37% sulfuric acid by weight when the battery is fully charged.

Lead-acid energy storage technologies are divided into two types:

• Lead-acid carbon

• Advanced lead-acid technologies

Lead-acid carbon batteries include carbon in the negative electrodes to improve power characteristics and to mitigate the effects of partial states of charge.

Lead-acid carbon battery advantages over conventional lead-acid battery:

• Significantly faster recharge rates

• Significantly longer cycle lives in deep discharge applications

• Minimal required maintenance

Lesson Plan: # 4-11ESS: Electro-Chemical – Lead-Acid BatteryQuestion: WHAT is a Lead Acid battery and HOW does it store energy?

Scientific Principle: Lead Acid and all other batteries are based on the chemical storage of electricity using some sort of electrolyte (in this case liquid) to separate charges between positive and negative terminals using chemical reactions.

Learning Objective(s):Students will be able to explain how a Lead Acid battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Lead Acid battery energy storage system for a utility or other customer.


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Advanced Lead-acid batteries improve performance through:

• Carbon-doped cathodes

• Granular silica electrolyte retention systems (GS Yuasa)

• High-density positive active material

• Hilica-based electrolytes (Hitachi).

Some advanced lead batteries have supercapacitor-like features that give them fast response, similar to flywheels or Li-ion batteries.

Traditional vented (VLA) or valve regulated (VRLA) lead-acid batteries are designed for either power or energy applications, not both.

Automotive – Power. Stationary Energy

Lead-acid batteries are the most commercially mature rechargeable battery technology in the world. Applications include:

Automotive, marine, telecommunications,and uninterruptible power supply (UPS)

Lead-acid batteries are among the most recycled products in the world• The lead plates and grids are smelted to purify the lead for

use in new batteries.

• Acid electrolyte is neutralized, scrubbed to remove dissolved lead, and released into the environment.

• Other component parts such as plastic and metal casings are also recycled.

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Vanadium reduction and oxidation (redox) batteries are of a type known as flow batteries.

Vanadium ions remain in an aqueous acidic solution throughout the entire process.

During charging V3+ ions are converted to V2+ ions at the negative electrode through the acceptance of electrons.

Meanwhile, at the positive electrode, V4+ ions are converted to V5+ ions through the release of electrons.

During discharge, the reactions run in the opposite direction, resulting in the release of the chemical energy as electrical energy.

In construction, the half-cells are separated by a proton exchange membrane that allows the flow of ionic charge to complete the electrical circuit.

Both the negative and positive electrolytes are composed of vanadium and sulfuric acid mixture.

The electrolytes are stored in external tanks and pumped as needed to the cells.

The two electrolytes are identical when fully discharged. This makes shipment and storage simple and inexpensive and greatly simplifies electrolyte management during operation.

Lesson Plan: # 4-12ESS: Electro-Chemical – Vanadium Redox Flow Battery (VRFB)Question: WHAT is a Vanadium Redox Flow Battery and HOW does it work?

Scientific Principle: Vanadium Redox (electrolyte) Flow batteries are based on the chemical storage of electricity using two liquid electrolytes flowing past each other and separated by a membrane to create chemical reactions to separate charges between positive and negative terminals.

Learning Objective(s):Students will be able to explain how a Vanadium Redox (electrolyte) Flow battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Vanadium Redox (electrolyte) Flow battery energy storage system for a utility or other customer.


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The cell stack has a useful life of 10 years.

The electrolytes and the active materials they contain do not degrade with time.

Vanadium redox systems are capable of stepping from zero output to full output within a few milliseconds.

The cell stack can produce three times the rated power output provided the state of charge is between 50% and 80%. Systems are rated at 10,000 cycles.

The physical scale of vanadium redox systems tends to be large due to the large volumes of electrolyte required. Sizes range from 50 kW to 1000 kW.

Applications include renewable integration, end user energy management, and telecom applications.

When decommissioning a vanadium redox system,

• The solid ion exchange cell membranes may be highly acidic or alkaline and therefore toxic.

• If possible, the liquid electrolyte is recycled.

• If disposed of, the vanadium is extracted from the electrolyte before further processing of the liquid.

• Research is ongoing to determine the exact environmental risk factors for vanadium.

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Iron-Chromium batteries were pioneered and studied by NASA in the 1970’s and 1980’s and by Mitsui in Japan.

The iron-chromium flow battery is a redox flow battery.

The active chemical species are fully dissolved in the aqueous electrolyte at all times.

During the discharge cycle Cr2+ is oxidized to Cr3+ in the negative half cell and an electron is released to do work in the external circuit.

In the positive half cell during discharge, Fe3+ accepts an electron from the external circuit and is reduced to Fe2+.

These reactions are reversed during the charge cycle when current is supplied from the external circuit.

Hydrogen ions are exchanged between the two half cells to maintain charge neutrality as electrons leave one side of the cell and return to the other side.

Hydrogen ions diffuse through the separator, which electronically separates the half cells.

Lesson Plan: # 4-13ESS: Electro-Chemical— Iron Chromium (Fe-Cr) (electrolyte) Flow Battery

Question: WHAT is an Iron Chromium (electrolyte) Flow Battery and HOW does it work?

Scientific Principle: Iron Chromium (electrolyte flow) batteries are based on the chemical storage of electricity using a two liquid electrolytes flowing past each other and separated by a membrane to create chemical reactions to separate charges between positive and negative terminals.

Learning Objective(s):Students will be able to explain how an Iron Chromium (electrolyte flow) battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating an Iron Chromi- um (electrolyte flow) battery energy storage system for a utility or other customer.


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The standard cell voltage is 1.18 volts and cell power densities are typically 70-100 mW/cm2.

The DC/DC RTE of this battery has been reported in the 70-80%.

Efficiency is enhanced at higher operating temperatures in the range of 105-140 F.

Iron-chromium flow batteries are available for telecom back-up at the 5 kW – 3 hour scale and have been demonstrated at the utility scale.

The iron and chromium chemistry is relatively benign compared to other electrochemical systems

The iron and chromium species have very low toxicity and the dilute water-based electrolyte has a very low vapor pressure.

These factors combine to make the ironchromium redox flow battery one of the safest battery based energy storage systems.

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Lesson Plan: # 4-14ESS: Electro-Chemical— Zinc Bromine Hybrid Flow Battery (ZnBr HFB)

Question: WHAT is a Zinc Bromine Hybrid Flow Battery and HOW does it work?

Scientific Principle: Zinc Bromine Hybrid (electrolyte) Flow Batteries are based on the chemical storage of electricity using two liquid electrolytes flowing past each other and separated by a membrane to create chemical reactions to separate charges between positive and negative terminals.

Learning Objective(s):Students will be able to explain how a Zinc Bromine Hybrid (electrolyte) Flow Battery energy storage system captures, stores and releases energy.

Students will be able to explain the uses, advantages and disadvantage to building and operating a Zinc Bromine Hybrid (electrolyte) Flow Battery energy storage system for a utility or other customer.


Each cell is composed of two electrode surfaces and two electrolyte flow streams separated by a micro-porous film.

The positive electrolyte is called a catholyte; the negative is the anolyte. Both electrolytes are aqueous solutions of zinc bromine (ZnBr2).

The cell electrodes are composed of carbon

plastic and are designed to be bipolar.

This means that a given electrode serves both as the cathode for one cell and the anode for the next cell in series.

Carbon plastic must be used because of the highly corrosive nature of bromine.

The most common factor in degradation and potential failure of Zinc-bromine batteries arises from the extremely corrosive nature of the elemental bromine electrolyte.

Past failure modes have included damaged seals, corrosion of current collectors, and warped electrodes.

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Vendors claim estimated lifetimes of 20 years and long cycle lives.

Operational ac-to-ac efficiencies are estimated to be approximately 65%.

Module sizes vary by manufacturer but can range from 5 kW to 1000 kW.

Variable energy storage duration from two to six hours.

Bromine is a toxic material and should be recovered in the event of a spill or when the unit is decommissioned.

Zinc-bromine is a corrosive and should be handled appropriately.

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NOTES:This table summarizes the performance characteristics of the various types of electrochemical battery technologies.

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