Electrochemical Energy Storage & EDLCs

Electrochemical Energy Storage (EES)

• EES systems are vital for the transition to sustainable energy.
• They store energy from renewable sources and power electric vehicles and portable devices.
• The EES market is projected to expand rapidly.

Electrochemical Double Layer Capacitor (EDLC)

Structure

• EDLCs comprise two electrodes, an electrolyte, and a separator.
• Electrodes use high surface area materials like activated carbon, carbon nanotubes, or graphene.
• Electrolytes can be aqueous, organic, or ionic liquid solutions with ions.
• Separators prevent electrical contact but allow ion transport.
• Energy is stored through ion accumulation at the electrode-electrolyte interface.
• Applying voltage causes ions to migrate to the electrode surface forming an electrical double layer (EDL).
• The EDL includes an inner Helmholtz layer and an outer diffuse layer.
• Capacitance is directly proportional to electrode surface area and inversely proportional to electrode distance.

Performance Parameters

• Specific capacitance is measured in F/g or F/cm², and determined by the formula $$C = \frac{Q}{V} = \frac{I \times t}{V}$$. In this formula: C is capacitance, Q is charge stored, V is voltage, I is current, and t is time.
• Energy density is measured in Wh/kg or Wh/L and determined by the formula $$E = \frac{1}{2}CV^2$$.
• Power density is measured in W/kg or is W/L and determined by the formula $$P = \frac{V^2}{4R}$$, where R is the equivalent series resistance (ESR).
• Cycle life refers to the number of charge-discharge cycles before significant performance degradation
• Rate capability reflects the ability to deliver high power at high charge-discharge rates.

Advantages

• High power density.
• Fast charge-discharge rates.
• Long cycle life.
• Wide operating temperature range.
• Safe and environmentally friendly.

Disadvantages

• Lower energy density compared to batteries.
• Voltage decays linearly during discharge.
• High cost compared to conventional capacitors.

Batteries

Introduction

• Electrochemical devices that convert chemical energy into electrical energy.
• Consist of one or more electrochemical cells, each containing two electrodes (anode and cathode) and an electrolyte.
• During discharge, the anode undergoes oxidation, releasing electrons that flow to the cathode, where reduction occurs.
• Classified into primary (non-rechargeable) and secondary (rechargeable) types.

Main Components

Electrodes (Anode and Cathode)

• Provide the site for electrochemical reactions (oxidation and reduction) to occur.
• Anode: Where oxidation occurs, releasing electrons.
• Cathode: Where reduction occurs, accepting electrons.

Electrolyte

• Contains free ions that conduct ionic current between the anode and cathode.
• Can be liquid, solid, or gel.

Separator

• A porous membrane that prevents physical contact between the anode and cathode, while allowing ion transport.

Current collectors

• Conductive materials that provide an electrical connection between the electrodes and the external circuit.

Performance Parameters

• Cell Voltage (V): Potential difference between the cathode and anode.
• Capacity (Ah or mAh): The amount of charge a battery can deliver.
• Specific Energy (Wh/kg): Energy stored per unit mass.
• Energy Density (Wh/L): Energy stored per unit volume.
• Specific Power (W/kg): Power delivered per unit mass.
• Power Density (W/L): Power delivered per unit volume.
• Charge-Discharge Efficiency (%): Ratio of energy delivered during discharge to energy required for charging.
• Cycle Life: Number of charge-discharge cycles before significant performance degradation.
• Coulombic Efficiency (%): Ratio of total charge extracted during discharge to total charge injected during charging.

Advantages

• High energy density.
• Relatively high power density.
• Long cycle life (for some types).

Disadvantages

• Lower power density than EDLCs.
• Slower charge-discharge rates than EDLCs.
• Limited cycle life (for some types).
• Safety concerns (e.g., thermal runaway, flammability).
• Environmental concerns (e.g., disposal of toxic materials).

Fuel Cell

Introduction

• Converts chemical energy of a fuel and an oxidant into electricity.
• Consists of an anode, a cathode, and an electrolyte.
• At the anode, the fuel is oxidized, releasing electrons that flow to the cathode, where the oxidant is reduced.
• Requires a continuous supply of fuel and oxidant to operate.

Main Components

Anode

• Where the fuel is oxidized, producing electrons; made of a porous material with a catalyst.

Cathode

• Where the oxidant is reduced, consuming electrons; typically made of a porous material with a catalyst.

Electrolyte

• Conducts ions between the anode and cathode; can be liquid, solid, or membrane-based.

Bipolar Plates

• Separate individual fuel cells, provide channels for gas distribution, and remove heat.

Performance Parameters

• Cell Voltage (V): Potential difference between the cathode and anode.
• Current Density (A/cm2): Current produced per unit area of the electrode.
• Power Density (W/cm2): Power produced per unit area of the electrode.
• Fuel Utilization (%): Percentage of fuel converted into electricity.
• Efficiency (%): Ratio of electrical energy produced to the chemical energy of the fuel consumed.
• Durability: Ability to maintain performance over time.

Advantages

• High efficiency.
• Low emissions when using hydrogen as fuel.
• Quiet operation.
• Scalability.

Disadvantages

• High cost.
• Fuel storage and distribution challenges.
• Durability concerns.
• Sensitivity to fuel impurities.
• Slow start-up time for some types.

Comparison of EES Technologies

• EDLCs have low energy density, high power density, fast charge/discharge rate, long cycle life, high efficiency, and medium cost, and are used in hybrid vehicles and energy storage.
• Batteries have high energy density, medium power density, slow charge/discharge rate, medium cycle life, medium efficiency, and medium cost, and are used in portable electronics and electric vehicles.
• Fuel Cells have high energy density, medium power density, continuous charge/discharge rate, long cycle life, high efficiency, and high cost, and are used in stationary power and transportation.

Flywheel Energy Storage (FES)

Basic Principle

• Stores energy by accelerating a rotating mass (flywheel) to high speed and maintaining energy as kinetic energy
• The amount of energy stored is proportional to its moment of inertia and the square of its rotational speed.
• Energy extracted by slowing it down to drive a generator or mechanical system.

Main components

Rotor (Flywheel)

• Rotating mass stores kinetic energy.
• Made of high-strength materials such as steel, composites, or carbon fiber.

Bearings

• Support the rotor and allow it to spin with minimal friction.
• Can be mechanical, magnetic, or hybrid bearings.

Motor/Generator

• Converts electrical energy into mechanical energy (motor) or converts mechanical energy back into electrical energy (generator).

Vacuum Enclosure

• Reduces air friction and aerodynamic drag to improve efficiency and reduce energy losses.

Power Electronics

• Control the flow of energy between the flywheel and the electrical grid or load.

Performance Parameters

• Energy Storage Capacity (Wh or kWh): Max energy stored at maximum speed.
• Power Output (W or kW): Rate at which energy can be transferred.
• Charge/Discharge Rate: Speed at which the flywheel is accelerated or decelerated.
• Efficiency (%): Ratio of energy extracted to energy used to charge it.
• Cycle Life: Number of charge-discharge cycles before performance degrades.
• Self-Discharge Rate: Rate at which the flywheel loses energy when idle.

Advantages

• High power density.
• Fast charge-discharge rates.
• Long cycle life.
• High efficiency.
• Environmentally friendly.

Disadvantages

• Low energy density.
• Self-discharge losses.
• Complex control systems.
• Safety concerns related to high-speed rotation.

Compressed Air Energy Storage (CAES)

Basic Principle

• Stores energy by compressing air and storing it in underground caverns, tanks, or pipelines.
• During discharge, the compressed air is released, heated, and expanded through a turbine to generate electricity.
• Used for grid-scale energy storage, providing peak shaving, load leveling, and frequency regulation services.

Main Components

Air Compressor

• Compresses ambient air to high pressure.
• Multi-stage compressors with intercooling typically used for increased efficiency

Air Storage Reservoir

• Stores the compressed air at high pressure using underground caverns (e.g., salt domes, depleted gas reservoirs), above-ground tanks, or pipelines.

Air Turbine

• Expands the compressed air to drive a generator and produce electricity.

Heat Exchanger

• Heats the compressed air before it enters the turbine to improve efficiency and power output.

Generator

• Converts the mechanical energy from the turbine into electrical energy.

Performance Parameters

• Energy Storage Capacity (MWh or GWh): Max energy stored at maximum pressure.
• Power Output (MW): Rate at which energy can be delivered to the grid.
• Charge/Discharge Rate: Speed at which the air storage reservoir can be filled or emptied.
• Efficiency (%): Ratio of energy delivered to energy used to compress the air.
• Cycle Life: Number of charge-discharge cycles before performance degrades.
• Storage Duration: Length of time the CAES system can store energy.

Advantages

• Large-scale energy storage capacity.
• Long storage duration.
• Relatively low cost compared to other technologies.
• Potential for integration with renewable energy sources.

Disadvantages

• High Geographic limitations.
• Environmental concerns (air emissions and land use).
• Efficiency losses due to compression and expansion processes.
• Dependence on fossil fuels for some types of CAES systems.

Pumped Hydro Energy Storage (PHES)

Basic Principle

• Stores energy by pumping water from a lower reservoir to an upper reservoir during low electricity demand.
• During high electricity demand, water is released to generate electricity.
• Mature technology for grid-scale energy storage, providing peak shaving, load leveling, and ancillary services.

Main Components

Upper Reservoir

• Stores water at a high elevation; natural or man-made.

Lower Reservoir

• Stores water at a lower elevation; natural or man-made.

Pump/Turbine

• Pumps water to the upper reservoir and generates electricity on release.

Penstock

• Carries water between the reservoirs.

Generator

• Converts the mechanical energy from the turbine into electrical energy.

Performance Parameters

• Energy Storage Capacity (MWh or GWh): Max energy stored at its maximum water level.
• Power Output (MW): Rate at which energy is delivered to the grid.
• Charge/Discharge Rate: Speed to fill/empty the upper reservoir.
• Efficiency (%): Ratio of energy delivered to energy used to pump the water.
• Cycle Life: Unlimited.
• Storage Duration: Length of time the system can store energy.

Advantages

• Large-scale energy storage capacity.
• Mature and reliable technology.
• Long lifespan.
• Low operating costs.
• Ancillary services for grid stabilization.

Disadvantages

• Geographic limitations.
• High capital costs.
• Environmental impacts (aquatic ecosystems and land use).
• Long construction times.
• Potential for water losses due to evaporation.

Comparison of Mechanical Energy Storage Technologies

• FES has low energy density, high power density, fast charge/discharge rate, long cycle life, high efficiency, and medium cost, and is used in grid stabilization and transportation.
• CAES has medium energy density, medium power density, medium charge/discharge rate, long cycle life, medium efficiency, and medium cost, and is used in grid-scale energy storage and peak shaving.
• PHES has high energy density, medium power density, slow charge/discharge rate, unlimited cycle life, medium efficiency, and high cost, and is used in grid-scale energy storage and peak shaving.