Q.1 Which energy storage technology stores energy by raising water to a higher elevation?
Lithium‑ion battery
Pumped hydro storage
Flywheel
Supercapacitor
Explanation - Pumped hydro uses excess electricity to pump water uphill; when needed, water flows back through turbines to generate power.
Correct answer is: Pumped hydro storage
Q.2 What is the typical round‑trip efficiency of a lithium‑ion battery system?
60‑70%
70‑80%
85‑95%
95‑100%
Explanation - Lithium‑ion batteries have high round‑trip efficiencies, usually between 85% and 95%, due to low internal losses.
Correct answer is: 85‑95%
Q.3 Which storage technology has the highest power density but relatively low energy density?
Lead‑acid battery
Pumped hydro
Supercapacitor
Flow battery
Explanation - Supercapacitors can charge/discharge rapidly (high power density) but store less total energy compared to batteries.
Correct answer is: Supercapacitor
Q.4 In a vanadium redox flow battery, the energy is stored in:
Solid electrode plates
Electrolyte solutions
Compressed gas
Mechanical springs
Explanation - Redox flow batteries store energy in liquid electrolyte that circulates through the electrochemical cell.
Correct answer is: Electrolyte solutions
Q.5 Which factor primarily determines the cycle life of a lead‑acid battery?
Depth of discharge
Operating temperature
Electrolyte viscosity
Magnetic field strength
Explanation - Frequent deep discharges greatly reduce the number of cycles a lead‑acid battery can deliver.
Correct answer is: Depth of discharge
Q.6 Compressed air energy storage (CAES) stores energy by:
Compressing air in underground caverns
Heating water in solar collectors
Electrolyzing water to produce hydrogen
Spinning a massive flywheel
Explanation - CAES uses electricity to compress air, which is stored in large caverns and later expanded through turbines to generate power.
Correct answer is: Compressing air in underground caverns
Q.7 Which storage technology is most suitable for providing short‑duration, high‑power support to the grid?
Pumped hydro
Lithium‑ion battery
Thermal molten‑salt storage
Hydrogen fuel cells
Explanation - Lithium‑ion batteries can respond within milliseconds and deliver high power for short periods, making them ideal for frequency regulation.
Correct answer is: Lithium‑ion battery
Q.8 The term "self‑discharge" in batteries refers to:
Loss of capacity due to temperature
Energy lost while the battery is idle
Decrease in voltage under load
Increase in internal resistance over time
Explanation - Self‑discharge is the phenomenon where a battery loses stored charge even when not connected to a load.
Correct answer is: Energy lost while the battery is idle
Q.9 Which of the following is a major environmental concern associated with lead‑acid batteries?
High greenhouse‑gas emissions during use
Toxic lead leakage into soil and water
Radioactive waste generation
Excessive noise pollution
Explanation - Improper disposal or leakage of lead‑acid batteries can contaminate the environment with lead, a toxic heavy metal.
Correct answer is: Toxic lead leakage into soil and water
Q.10 A flywheel stores energy primarily in the form of:
Electrochemical potential
Thermal heat
Kinetic energy of a rotating mass
Potential energy of compressed fluid
Explanation - Flywheels store energy by accelerating a rotor to high speeds; the kinetic energy is released by slowing the rotor.
Correct answer is: Kinetic energy of a rotating mass
Q.11 Which storage technology can also produce hydrogen as a by‑product when integrated with electrolyzers?
Lithium‑ion battery
Compressed air energy storage
Pumped hydro storage
Power‑to‑gas (hydrogen) system
Explanation - Excess electricity can be used to electrolyze water, generating hydrogen that can be stored and later reconverted to electricity.
Correct answer is: Power‑to‑gas (hydrogen) system
Q.12 The main advantage of a thermal molten‑salt storage system in solar‑thermal plants is:
Very high round‑trip efficiency (>95%)
Ability to store energy for several days with low loss
Instantaneous power delivery
Zero material cost
Explanation - Molten‑salt can retain heat for long periods, allowing solar‑thermal plants to generate electricity after sunset.
Correct answer is: Ability to store energy for several days with low loss
Q.13 Which parameter is most directly affected by the depth of discharge (DoD) in a battery system?
Voltage ripple
Cycle life
Self‑discharge rate
Thermal conductivity
Explanation - Higher DoD generally reduces the number of charge/discharge cycles a battery can sustain.
Correct answer is: Cycle life
Q.14 In a grid‑connected renewable system, what role does a Battery Energy Storage System (BESS) play in "peak shaving"?
Increasing the overall generation capacity
Storing excess energy during low demand and discharging during high demand
Converting AC to DC power
Providing reactive power support only
Explanation - Peak shaving uses stored energy to reduce the amount of power drawn from the grid during demand peaks.
Correct answer is: Storing excess energy during low demand and discharging during high demand
Q.15 Which of the following battery chemistries is known for having the lowest cost per kWh but also the lowest energy density?
Lithium‑ion
Nickel‑metal hydride (NiMH)
Lead‑acid
Sodium‑sulfur (NaS)
Explanation - Lead‑acid batteries are inexpensive but heavy and have lower energy density compared to newer chemistries.
Correct answer is: Lead‑acid
Q.16 What is the primary safety concern when operating large lithium‑ion battery banks?
Lead poisoning
Thermal runaway
Explosion due to hydrogen gas
Corrosion of metal casings
Explanation - If a cell overheats, it can trigger a chain reaction causing rapid temperature rise and fire.
Correct answer is: Thermal runaway
Q.17 Which storage method uses phase‑change materials (PCMs) to store energy?
Compressed air energy storage
Thermal energy storage
Flywheel storage
Supercapacitor storage
Explanation - PCMs absorb or release latent heat during phase transitions, enabling compact thermal storage.
Correct answer is: Thermal energy storage
Q.18 A battery's "C‑rate" is a measure of:
Capacity degradation over time
Charge/discharge current relative to its rated capacity
Cost per kilowatt hour
Temperature coefficient
Explanation - A 1C rate means charging or discharging the battery in one hour; 2C would take half an hour, etc.
Correct answer is: Charge/discharge current relative to its rated capacity
Q.19 Which storage technology can be directly integrated with wind turbines to smooth out short‑term power fluctuations?
Pumped hydro storage
Lithium‑ion battery
Thermal molten‑salt storage
Hydrogen fuel cell
Explanation - Batteries can respond quickly to the rapid changes in wind power output, providing short‑term smoothing.
Correct answer is: Lithium‑ion battery
Q.20 In a sodium‑sulfur (NaS) battery, the electrolyte is in which physical state during operation?
Solid
Liquid
Gas
Plasma
Explanation - NaS batteries operate at high temperature (~300 °C) where the sodium and sulfur are molten, and the electrolyte is a liquid ceramic.
Correct answer is: Liquid
Q.21 Which of the following is NOT a typical metric used to evaluate an energy storage system?
Energy density
Power density
Round‑trip efficiency
Magnetic permeability
Explanation - Magnetic permeability is not a standard performance metric for energy storage technologies.
Correct answer is: Magnetic permeability
Q.22 What does the term "grid‑scale" storage generally refer to?
Storage devices used in personal electronics
Energy storage systems with power capacity above 1 MW
Battery packs that fit inside a smartphone
Any storage that uses solar energy
Explanation - Grid‑scale typically means large installations capable of providing megawatt‑level power to the utility grid.
Correct answer is: Energy storage systems with power capacity above 1 MW
Q.23 The main advantage of a flow battery over conventional solid‑state batteries is:
Higher energy density
Scalable energy capacity independent of power rating
Lower self‑discharge rate
Zero maintenance requirements
Explanation - In flow batteries, energy is stored in liquid electrolytes, allowing capacity to be increased simply by adding more electrolyte.
Correct answer is: Scalable energy capacity independent of power rating
Q.24 In the context of renewable energy, "load shifting" is achieved by:
Increasing generation capacity
Storing excess energy and using it during high‑demand periods
Building more transmission lines
Reducing the size of the solar panels
Explanation - Load shifting moves electricity consumption from peak to off‑peak times using stored energy.
Correct answer is: Storing excess energy and using it during high‑demand periods
Q.25 Which storage technology typically requires a large underground cavern or aquifer for operation?
Flywheel
Pumped hydro
Compressed air energy storage
Lithium‑ion battery
Explanation - CAES stores compressed air in large caverns or porous rock formations underground.
Correct answer is: Compressed air energy storage
Q.26 The term "depth of discharge" (DoD) is expressed as:
Percentage of total capacity that has been used
Voltage drop per ampere
Temperature rise per hour
Number of cycles per year
Explanation - DoD indicates how much of a battery's total capacity is drawn during discharge.
Correct answer is: Percentage of total capacity that has been used
Q.27 Which technology is best suited for seasonal energy storage (months‑long duration)?
Supercapacitor
Lithium‑ion battery
Pumped hydro storage
Flywheel
Explanation - Pumped hydro can store large amounts of energy for long periods, making it ideal for seasonal storage.
Correct answer is: Pumped hydro storage
Q.28 What is a primary advantage of using hydrogen as an energy storage medium?
Instantaneous power delivery
Very high energy density per unit mass
Zero energy loss during conversion
No need for compression
Explanation - Hydrogen has high gravimetric energy density, making it attractive for long‑duration storage despite conversion losses.
Correct answer is: Very high energy density per unit mass
Q.29 Which of the following storage systems typically suffers from high self‑discharge rates?
Lead‑acid battery
Lithium‑ion battery
Supercapacitor
Pumped hydro storage
Explanation - Supercapacitors lose stored charge relatively quickly when idle, especially at higher temperatures.
Correct answer is: Supercapacitor
Q.30 The term "levelized cost of storage" (LCOS) is analogous to LCOE for generation. It represents:
Total capital cost only
Cost per kWh stored over the system’s lifetime
Efficiency loss during discharge
Maximum power output of the storage system
Explanation - LCOS spreads all costs (capital, O&M, replacement) over the total energy delivered to give a $/kWh figure.
Correct answer is: Cost per kWh stored over the system’s lifetime
Q.31 A battery management system (BMS) in a lithium‑ion pack primarily monitors:
Weather forecasts
Cell voltage, temperature, and state‑of‑charge
Grid frequency only
Fuel consumption
Explanation - The BMS ensures safe operation by tracking each cell’s critical parameters and balancing them as needed.
Correct answer is: Cell voltage, temperature, and state‑of‑charge
Q.32 Which storage technology can also provide ancillary services such as voltage regulation and reactive power support?
Flywheel
Pumped hydro
Lithium‑ion battery
Compressed air energy storage
Explanation - Power electronics associated with battery inverters can supply reactive power and help regulate voltage.
Correct answer is: Lithium‑ion battery
Q.33 The main disadvantage of using molten‑salt thermal storage with solar‑thermal plants is:
Low round‑trip efficiency
Requirement for high‑temperature materials
Inability to store energy for more than a few hours
Excessive noise generation
Explanation - Operating at >500 °C demands specialized, costly materials that can withstand corrosion and thermal stress.
Correct answer is: Requirement for high‑temperature materials
Q.34 Which of the following statements about battery cycle life is TRUE?
Cycle life is independent of temperature.
Deeper discharges increase the total number of cycles.
Higher charge rates always extend cycle life.
Shallow cycles (low DoD) generally increase cycle life.
Explanation - Operating batteries within a small DoD reduces stress and extends the number of usable cycles.
Correct answer is: Shallow cycles (low DoD) generally increase cycle life.
Q.35 Which storage method is based on converting electricity into chemical energy using water electrolysis?
Compressed air storage
Hydrogen power‑to‑gas
Flywheel storage
Supercapacitor storage
Explanation - Electrolysis splits water into hydrogen and oxygen; hydrogen can later be reconverted to electricity.
Correct answer is: Hydrogen power‑to‑gas
Q.36 What is the typical energy density (Wh/kg) range for modern lithium‑ion batteries?
10‑30 Wh/kg
50‑100 Wh/kg
150‑250 Wh/kg
300‑500 Wh/kg
Explanation - Advances in electrode materials have pushed Li‑ion energy density into the 150‑250 Wh/kg range for commercial cells.
Correct answer is: 150‑250 Wh/kg
Q.37 Which storage technology can provide both long‑duration energy and grid‑scale power without the need for water resources?
Pumped hydro storage
Compressed air energy storage
Flywheel storage
Lithium‑ion battery
Explanation - CAES does not require large water reservoirs, unlike pumped hydro, making it suitable for arid regions.
Correct answer is: Compressed air energy storage
Q.38 When integrating a BESS with a solar PV plant, the primary purpose of the inverter is to:
Convert DC from batteries to AC for the grid
Increase the voltage of the solar panels
Store heat generated by the batteries
Measure solar irradiance
Explanation - Inverters change the DC output of batteries (and PV) into grid‑compatible AC power.
Correct answer is: Convert DC from batteries to AC for the grid
Q.39 Which factor most directly affects the power rating (kW) of a battery system?
Energy density
Cell internal resistance
Depth of discharge
Thermal expansion coefficient
Explanation - Lower internal resistance allows higher currents, thus higher power output.
Correct answer is: Cell internal resistance
Q.40 A major challenge for large‑scale hydrogen storage is:
Low gravimetric energy density
High self‑discharge rate
Need for cryogenic temperatures or high pressures
Inability to be transported
Explanation - Hydrogen must be stored as a compressed gas or liquefied, requiring energy‑intensive infrastructure.
Correct answer is: Need for cryogenic temperatures or high pressures
Q.41 The term "state‑of‑charge" (SoC) in a battery is analogous to:
Battery temperature
Battery voltage only
Percentage of remaining capacity
Number of cells in series
Explanation - SoC expresses how much charge remains in the battery as a percentage of its total capacity.
Correct answer is: Percentage of remaining capacity
Q.42 Which of the following is a typical characteristic of a sodium‑ion battery compared to lithium‑ion?
Higher energy density
Lower cost due to abundant sodium
Higher operating voltage
Zero thermal runaway risk
Explanation - Sodium is more abundant and cheaper than lithium, making sodium‑ion batteries potentially lower‑cost despite lower energy density.
Correct answer is: Lower cost due to abundant sodium
Q.43 In an electric grid, "frequency regulation" can be provided by:
Long‑duration thermal storage only
Fast‑responding batteries or flywheels
Large hydroelectric dams exclusively
Solar panels without storage
Explanation - Frequency regulation requires rapid power adjustments; batteries and flywheels can respond within seconds or less.
Correct answer is: Fast‑responding batteries or flywheels
Q.44 Which storage technology is most commonly used for uninterruptible power supply (UPS) applications in data centers?
Lead‑acid battery
Pumped hydro
Flywheel
Compressed air
Explanation - Lead‑acid batteries are reliable, inexpensive, and have proven performance for short‑duration UPS needs.
Correct answer is: Lead‑acid battery
Q.45 A major disadvantage of using supercapacitors for grid energy storage is:
Slow charge time
Low cycle life
High self‑discharge
Requirement of hazardous chemicals
Explanation - Supercapacitors lose stored energy relatively quickly when idle, limiting long‑term storage capability.
Correct answer is: High self‑discharge
Q.46 Which parameter is most important when sizing a battery to provide 4 hours of backup for a 500 kW load?
Power rating (kW)
Energy capacity (kWh)
Voltage rating (V)
Self‑discharge rate
Explanation - Energy capacity determines how much energy can be supplied; 500 kW × 4 h = 2000 kWh.
Correct answer is: Energy capacity (kWh)
Q.47 The term "grid‑forming inverter" refers to an inverter that can:
Only convert AC to DC
Create a stable AC grid voltage from a DC source
Increase the frequency of the grid
Store energy as heat
Explanation - Grid‑forming inverters can establish voltage and frequency, enabling microgrids to operate without a utility grid.
Correct answer is: Create a stable AC grid voltage from a DC source
Q.48 Which storage technology typically uses a rotating magnetic field to store energy?
Flywheel
Supercapacitor
Pumped hydro
Lithium‑ion battery
Explanation - Flywheels store kinetic energy in a rotating mass; some designs use magnetic bearings to reduce friction.
Correct answer is: Flywheel
Q.49 In a battery pack, "cell balancing" is performed to:
Equalize the state‑of‑charge among cells
Increase the total voltage
Reduce the temperature of the pack
Change the chemical composition
Explanation - Balancing prevents over‑charging or deep‑discharging individual cells, extending pack life.
Correct answer is: Equalize the state‑of‑charge among cells
Q.50 Which storage technology can be directly coupled with a nuclear power plant to provide load‑following capability?
Lithium‑ion battery
Pumped hydro storage
Thermal molten‑salt storage
Flywheel
Explanation - Molten‑salt can store heat from the reactor and release it when needed, enabling flexible output.
Correct answer is: Thermal molten‑salt storage
Q.51 A key advantage of using a hybrid storage system (e.g., battery + supercapacitor) is:
Lower overall cost than any single technology
Combining high energy density with high power density
Eliminating the need for power electronics
Reducing the physical footprint to zero
Explanation - Hybrid systems leverage the strengths of each component: batteries for energy, supercapacitors for power.
Correct answer is: Combining high energy density with high power density
Q.52 Which factor primarily determines the maximum discharge current a battery can safely deliver?
Battery's physical size
Cell chemistry and internal resistance
Color of the battery casing
Ambient humidity
Explanation - Low internal resistance and appropriate chemistry allow higher currents without overheating.
Correct answer is: Cell chemistry and internal resistance
Q.53 The term "energy‑only market" in electricity markets often requires which type of storage?
Long‑duration storage for seasonal shifts
Fast‑response storage for price spikes
No storage, only generation
Only thermal storage
Explanation - Energy‑only markets reward storage that can capture high price spikes by quickly charging/discharging.
Correct answer is: Fast‑response storage for price spikes
Q.54 Which storage technology is most affected by temperature extremes, requiring active cooling or heating?
Flywheel
Lithium‑ion battery
Pumped hydro
Compressed air
Explanation - Li‑ion performance and safety degrade at high/low temperatures, necessitating thermal management.
Correct answer is: Lithium‑ion battery
Q.55 In a pumped‑hydro system, the "head" refers to:
The amount of water stored
The vertical distance between the upper and lower reservoirs
The flow rate of water
The efficiency of the turbine
Explanation - Head determines the potential energy available for conversion to electricity.
Correct answer is: The vertical distance between the upper and lower reservoirs
Q.56 Which energy storage technology can be easily scaled by adding more identical modules in parallel?
Pumped hydro
Lithium‑ion battery
Flywheel
Compressed air
Explanation - Battery modules can be paralleled to increase capacity without redesigning the entire system.
Correct answer is: Lithium‑ion battery
Q.57 What is the main purpose of a "grid‑forming" battery inverter in an islanded microgrid?
To regulate battery temperature
To provide a stable voltage and frequency for loads
To store excess solar energy
To compress air
Explanation - Grid‑forming inverters set the reference voltage/frequency, allowing the microgrid to operate independently.
Correct answer is: To provide a stable voltage and frequency for loads
Q.58 Which storage technology typically has the longest discharge time (hours to days) without significant efficiency loss?
Supercapacitor
Flywheel
Pumped hydro
Lithium‑ion battery
Explanation - Pumped hydro can discharge over many hours with modest efficiency penalties, making it suitable for long‑duration storage.
Correct answer is: Pumped hydro
Q.59 The main environmental advantage of using flow batteries over traditional lead‑acid batteries is:
Higher energy density
No heavy metals in the electrolyte
Zero water consumption
Lower operating temperature
Explanation - Flow batteries typically use vanadium or organic electrolytes, avoiding toxic lead contamination.
Correct answer is: No heavy metals in the electrolyte
Q.60 Which of the following storage technologies can directly provide inertia to a power system?
Lithium‑ion battery
Flywheel
Compressed air energy storage
Supercapacitor
Explanation - Flywheels store kinetic energy that can be released quickly, mimicking mechanical inertia.
Correct answer is: Flywheel
Q.61 In a renewable‑integrated grid, "dispatchability" of an energy storage system means:
It can generate power only when the sun shines
It can be controlled to supply power on demand
It is limited to a fixed output
It cannot be used for peak shaving
Explanation - Dispatchable resources can be turned on/off or modulated according to grid operator requests.
Correct answer is: It can be controlled to supply power on demand
Q.62 Which storage technology typically requires the use of rare earth magnets in its construction?
Pumped hydro
Flywheel
Lithium‑ion battery
Thermal molten‑salt
Explanation - High‑speed flywheels often use magnetic bearings with rare‑earth magnets to reduce friction.
Correct answer is: Flywheel
Q.63 The efficiency loss in a battery energy storage system is primarily due to:
Mechanical friction
Electrochemical overpotentials and internal resistance
Wind turbulence
Solar panel shading
Explanation - Voltage drops during charge/discharge caused by internal resistance and reaction kinetics reduce efficiency.
Correct answer is: Electrochemical overpotentials and internal resistance
Q.64 Which storage option is most suitable for providing backup power to remote off‑grid telecom towers?
Pumped hydro
Lithium‑ion battery
Flywheel
Compressed air
Explanation - Compact, high‑energy‑density batteries are ideal for remote sites where space and maintenance are limited.
Correct answer is: Lithium‑ion battery
Q.65 When comparing two battery technologies, one with 90% round‑trip efficiency and another with 70%, the higher efficiency results in:
Higher energy losses during charge/discharge
More usable energy per unit of stored energy
Longer cycle life
Lower cost per kWh
Explanation - Higher round‑trip efficiency means less energy is lost, delivering more electricity to the load.
Correct answer is: More usable energy per unit of stored energy
Q.66 Which of the following best describes a "Hybrid Renewable Energy System" (HRES)?
A single type of renewable generation with no storage
Multiple renewable sources plus energy storage working together
A conventional fossil‑fuel plant with backup diesel
A grid without any renewable sources
Explanation - HRES combines, e.g., solar, wind, and storage to improve reliability and efficiency.
Correct answer is: Multiple renewable sources plus energy storage working together
Q.67 In a grid‑scale battery, the term "power rating" (MW) is determined by:
Total energy stored (MWh)
Maximum continuous discharge current
Number of cells in series
Battery's self‑discharge rate
Explanation - Power rating is the product of voltage and the maximum sustainable current output.
Correct answer is: Maximum continuous discharge current
Q.68 Which storage technology can be directly coupled with a wind turbine to provide "synthetic inertia"?
Supercapacitor
Flywheel
Lithium‑ion battery
Pumped hydro
Explanation - Flywheels can release kinetic energy quickly, mimicking the inertia of rotating turbines.
Correct answer is: Flywheel
Q.69 A major drawback of using large‑scale lithium‑ion batteries for seasonal storage is:
Low round‑trip efficiency
High self‑discharge over months
Limited cycle life for deep cycles
Inability to operate at low temperatures
Explanation - Lithium‑ion batteries lose charge over time, making them unsuitable for months‑long storage without recharging.
Correct answer is: High self‑discharge over months
Q.70 Which factor most influences the cost of a pumped‑hydro storage project?
Cost of solar panels
Availability of suitable topography and water resources
Price of lithium
Number of wind turbines
Explanation - Pumped hydro requires specific site conditions (elevation difference, water), which dominate cost.
Correct answer is: Availability of suitable topography and water resources
Q.71 The term "grid‑forming battery" most closely relates to:
A battery that can set voltage and frequency for an islanded grid
A battery that only stores energy without conversion
A battery that can only discharge, not charge
A battery that works only with AC power
Explanation - Grid‑forming batteries can create a reference grid, enabling operation without an external utility.
Correct answer is: A battery that can set voltage and frequency for an islanded grid
Q.72 Which energy storage technology typically has the highest specific power (kW/kg)?
Lithium‑ion battery
Supercapacitor
Pumped hydro
Flow battery
Explanation - Supercapacitors can deliver large power bursts relative to their mass.
Correct answer is: Supercapacitor
Q.73 When evaluating storage for a solar farm with high midday generation and low night demand, the most appropriate storage choice is:
Flywheel
Lithium‑ion battery
Pumped hydro (if site permits)
Thermal molten‑salt storage
Explanation - Batteries efficiently store midday solar energy and release it during evening peaks.
Correct answer is: Lithium‑ion battery
Q.74 The primary reason why lead‑acid batteries are still widely used in stationary applications is:
Highest energy density
Low upfront cost and proven reliability
Zero maintenance requirements
Fast charging capability
Explanation - Lead‑acid batteries are inexpensive and have a long history of use despite lower performance.
Correct answer is: Low upfront cost and proven reliability
Q.75 Which technology uses a chemical reaction between zinc and bromine in liquid electrolyte to store energy?
Flow battery
Lithium‑ion battery
Supercapacitor
Compressed air
Explanation - Zinc‑bromine flow batteries store energy in dissolved zinc and bromine species.
Correct answer is: Flow battery
Q.76 In the context of energy storage, the term "energy density" is measured in:
kW
V
Wh/kg
A
Explanation - Energy density expresses the amount of stored energy per unit mass.
Correct answer is: Wh/kg
Q.77 Which storage technology is most suitable for providing ancillary services that require rapid response (<1 s) and short duration (seconds to minutes)?
Pumped hydro
Lithium‑ion battery
Thermal molten‑salt
Compressed air
Explanation - Battery inverters can react within milliseconds, ideal for fast ancillary services.
Correct answer is: Lithium‑ion battery
Q.78 The main cause of capacity fade in lithium‑ion batteries over many cycles is:
Electrolyte evaporation
Growth of solid electrolyte interphase (SEI) layer
Loss of magnetic field
Water ingress
Explanation - Repeated cycling builds up SEI on the anode, reducing active lithium and capacity.
Correct answer is: Growth of solid electrolyte interphase (SEI) layer
Q.79 Which storage technology can be used to store surplus wind energy as potential energy in the form of elevated masses?
Pumped hydro
Gravity‑based energy storage
Supercapacitor
Lithium‑ion battery
Explanation - Mass‑based systems lift heavy weights using excess power and release them to generate electricity later.
Correct answer is: Gravity‑based energy storage
Q.80 When designing a storage system for frequency regulation, which characteristic is most critical?
High energy density
Low self‑discharge rate
Fast response time
Large physical footprint
Explanation - Frequency regulation requires near‑instantaneous power adjustments.
Correct answer is: Fast response time
Q.81 The term "grid‑connected" in the context of renewable energy storage means:
The system operates only in isolation
The storage can exchange power with the utility grid
The system uses only diesel generators
The system is underground
Explanation - Grid‑connected systems can charge from and discharge to the grid as needed.
Correct answer is: The storage can exchange power with the utility grid
Q.82 Which storage technology typically requires the use of a turbine‑generator set for power conversion?
Flywheel
Pumped hydro
Supercapacitor
Lithium‑ion battery
Explanation - Water released from the upper reservoir drives a turbine that turns a generator.
Correct answer is: Pumped hydro
Q.83 In a battery pack, the "nominal voltage" is defined as:
Maximum voltage during charging
Voltage when the battery is fully discharged
Average voltage of a single cell under normal conditions
Voltage of the inverter
Explanation - Nominal voltage is the typical operating voltage of a cell or pack during use.
Correct answer is: Average voltage of a single cell under normal conditions
Q.84 Which storage technology is most likely to be affected by seismic activity due to its underground nature?
Compressed air energy storage
Lithium‑ion battery
Supercapacitor
Flywheel
Explanation - CAES caverns are underground and can be impacted by earthquakes.
Correct answer is: Compressed air energy storage
Q.85 A battery with a high C‑rate capability is best suited for:
Slow, long‑duration discharge
Fast charging and high power applications
Thermal storage
Water purification
Explanation - High C‑rate indicates the battery can be charged/discharged quickly without damage.
Correct answer is: Fast charging and high power applications
Q.86 Which energy storage method can be directly coupled with a nuclear reactor's waste heat for electricity generation?
Lithium‑ion battery
Molten‑salt thermal storage
Compressed air
Supercapacitor
Explanation - Molten‑salt can absorb high‑temperature waste heat and later drive a turbine.
Correct answer is: Molten‑salt thermal storage
Q.87 In a grid‑scale battery, the term "availability" refers to:
The percentage of time the battery is ready to deliver power
The total energy stored
The number of cells in series
The depth of discharge
Explanation - Availability measures reliability and uptime of the storage system.
Correct answer is: The percentage of time the battery is ready to deliver power
Q.88 Which technology can store electricity as a chemical fuel that can be transported via pipelines?
Lithium‑ion battery
Hydrogen fuel cell
Flywheel
Supercapacitor
Explanation - Hydrogen can be compressed or liquefied and moved through pipelines for later reconversion.
Correct answer is: Hydrogen fuel cell
Q.89 A major advantage of using a battery management system (BMS) is:
Increasing the battery's weight
Ensuring safe operation and extending lifespan
Reducing the battery's voltage
Eliminating the need for cooling
Explanation - The BMS monitors cell health, balances cells, and protects against over‑charge/discharge.
Correct answer is: Ensuring safe operation and extending lifespan
Q.90 Which storage technology can be directly integrated with a solar‑thermal plant without converting electricity?
Pumped hydro
Molten‑salt thermal storage
Lithium‑ion battery
Compressed air
Explanation - Molten‑salt stores thermal energy from solar concentrators directly as heat.
Correct answer is: Molten‑salt thermal storage
Q.91 The term "grid‑forming" vs. "grid‑following" inverter distinguishes:
Whether the inverter can operate without a grid reference
The color of the inverter housing
The type of battery used
The size of the inverter
Explanation - Grid‑forming inverters set voltage/frequency, while grid‑following sync to an existing grid.
Correct answer is: Whether the inverter can operate without a grid reference
Q.92 Which storage technology is most affected by the "peukert effect"?
Lithium‑ion battery
Lead‑acid battery
Flywheel
Supercapacitor
Explanation - The Peukert effect describes how higher discharge rates reduce usable capacity, prominent in lead‑acid cells.
Correct answer is: Lead‑acid battery
Q.93 The main benefit of a "modular" battery system is:
Higher self‑discharge rate
Scalability and easy maintenance
Lower voltage output
Increased weight
Explanation - Modules can be added or replaced without redesigning the entire system.
Correct answer is: Scalability and easy maintenance
Q.94 Which storage method uses the electrochemical reaction of hydrogen and oxygen in a solid‑oxide fuel cell to produce electricity?
Lithium‑ion battery
Hydrogen fuel cell
Supercapacitor
Compressed air
Explanation - Solid‑oxide fuel cells convert stored hydrogen into electricity via electrochemical reactions.
Correct answer is: Hydrogen fuel cell
Q.95 When evaluating a storage technology for a microgrid on a remote island, the most important consideration is:
High energy density
Low capital cost only
Robustness and low maintenance
Ability to provide inertia
Explanation - Remote sites need reliable, low‑maintenance solutions due to limited service access.
Correct answer is: Robustness and low maintenance
Q.96 Which storage technology can achieve round‑trip efficiencies above 90% and is often used for short‑term grid services?
Pumped hydro
Lithium‑ion battery
Flywheel
Compressed air
Explanation - Flywheels have very low electrical losses, resulting in high efficiencies for brief power bursts.
Correct answer is: Flywheel
Q.97 What is the primary reason for using a DC‑DC converter in a battery‑based storage system?
To change AC to DC
To match battery voltage to the inverter’s input range
To increase the battery’s temperature
To store thermal energy
Explanation - DC‑DC converters step the voltage up or down to suit the downstream inverter or load.
Correct answer is: To match battery voltage to the inverter’s input range
Q.98 Which storage technology typically requires a "state‑of‑charge window" to avoid degradation?
Lithium‑ion battery
Flywheel
Supercapacitor
Pumped hydro
Explanation - Operating Li‑ion cells within a limited SoC range (e.g., 20‑80%) prolongs life.
Correct answer is: Lithium‑ion battery
Q.99 The term "dispatchable renewable" most accurately describes:
A solar panel without storage
Wind turbine with integrated battery storage
Hydroelectric dam without turbines
Geothermal plant that cannot be controlled
Explanation - Adding storage makes intermittent renewables controllable, i.e., dispatchable.
Correct answer is: Wind turbine with integrated battery storage
Q.100 In a grid‑scale storage system, "power rating" (MW) and "energy rating" (MWh) together define:
The battery’s color and shape
How long the system can deliver its maximum power
The type of inverter required
The amount of water needed
Explanation - Duration = Energy (MWh) ÷ Power (MW).
Correct answer is: How long the system can deliver its maximum power
Q.101 Which storage technology can be installed underground without the need for large surface footprints?
Pumped hydro
Compressed air energy storage
Flywheel
Lithium‑ion battery
Explanation - CAES caverns are subterranean, minimizing surface impact.
Correct answer is: Compressed air energy storage
Q.102 The primary advantage of using a battery with a high "energy density" in electric vehicles is:
Longer driving range for a given weight
Faster charging times
Lower manufacturing cost
Higher thermal stability
Explanation - Higher energy per kilogram allows vehicles to travel farther without increasing mass.
Correct answer is: Longer driving range for a given weight
Q.103 Which storage method can directly provide "black start" capability to restart a power grid after a blackout?
Pumped hydro
Lithium‑ion battery
Supercapacitor
Flywheel
Explanation - Batteries can supply the initial power needed to energize control and startup equipment.
Correct answer is: Lithium‑ion battery
Q.104 When a battery system is said to have a "depth of discharge" of 80%, it means:
Only 20% of its capacity is used before recharging
It can be discharged to 80% of its full capacity
It loses 80% of its energy per cycle
It operates at 80 °C
Explanation - DoD of 80% means up to 80% of the total stored energy may be drawn before recharging.
Correct answer is: It can be discharged to 80% of its full capacity
Q.105 Which storage technology is most sensitive to the availability of suitable geological formations?
Lithium‑ion battery
Pumped hydro
Compressed air energy storage
Supercapacitor
Explanation - CAES requires stable underground caverns or porous rock to store compressed air safely.
Correct answer is: Compressed air energy storage
Q.106 The term "thermal runaway" in batteries refers to:
A rapid increase in temperature that can lead to fire or explosion
A slow loss of capacity over years
The battery’s inability to charge
A sudden voltage spike
Explanation - Thermal runaway is a dangerous condition where heat generation exceeds dissipation, causing catastrophic failure.
Correct answer is: A rapid increase in temperature that can lead to fire or explosion
Q.107 Which of the following is a key advantage of using a hybrid pumped‑hydro and battery system?
Eliminates the need for turbines
Combines long‑duration storage with fast response power
Reduces water consumption to zero
Allows operation without any control systems
Explanation - Pumped hydro provides bulk energy, while batteries handle rapid fluctuations.
Correct answer is: Combines long‑duration storage with fast response power
Q.108 In a flow battery, increasing the size of the electrolyte tanks will:
Increase the power rating
Increase the energy capacity
Decrease the round‑trip efficiency
Raise the self‑discharge rate
Explanation - More electrolyte volume stores more chemical energy, boosting total energy capacity while power depends on cell stack size.
Correct answer is: Increase the energy capacity
Q.109 Which storage technology typically uses a "separator" to prevent direct contact between the two electrodes?
Supercapacitor
Lithium‑ion battery
Flywheel
Pumped hydro
Explanation - Separators keep the anode and cathode apart while allowing ion flow.
Correct answer is: Lithium‑ion battery
Q.110 For a solar‑plus‑storage system intended to provide 8 hours of backup, which combination is most cost‑effective in most regions?
Lithium‑ion batteries + flywheels
Pumped hydro + compressed air
Lithium‑ion batteries alone
Thermal molten‑salt storage
Explanation - For 8‑hour durations, lithium‑ion offers a good balance of cost, efficiency, and footprint.
Correct answer is: Lithium‑ion batteries alone
Q.111 Which characteristic is most important for a storage system that must operate in a desert environment with high daytime temperatures?
Low self‑discharge
High temperature tolerance
Small physical size
High magnetic susceptibility
Explanation - Devices must withstand high ambient temperatures without performance loss or safety issues.
Correct answer is: High temperature tolerance
Q.112 The term "energy arbitrage" in electricity markets refers to:
Buying electricity when cheap, storing it, and selling when prices are high
Using renewable energy to replace fossil fuels
Balancing frequency in real time
Providing emergency backup power
Explanation - Arbitrage exploits price differences over time using storage.
Correct answer is: Buying electricity when cheap, storing it, and selling when prices are high
Q.113 Which storage technology can be considered "solid‑state" because it does not involve liquid electrolytes?
Lead‑acid battery
Solid‑state lithium‑ion battery
Flow battery
Compressed air
Explanation - Solid‑state batteries replace liquid electrolyte with a solid conductor, improving safety.
Correct answer is: Solid‑state lithium‑ion battery
Q.114 In a battery system, the term "state‑of‑health" (SoH) measures:
Current temperature of the battery
Remaining usable capacity compared to a fresh battery
Voltage at full charge
Number of cells in series
Explanation - SoH indicates how much of the original capacity is still available, reflecting degradation.
Correct answer is: Remaining usable capacity compared to a fresh battery
Q.115 Which storage technology is most suitable for providing high‑frequency reactive power support in a distribution network?
Lithium‑ion battery with advanced inverter
Pumped hydro
Flywheel
Compressed air
Explanation - Inverters can quickly inject or absorb reactive power to support voltage regulation.
Correct answer is: Lithium‑ion battery with advanced inverter
Q.116 When sizing a battery for a microgrid, the term "overnight autonomy" refers to:
The amount of energy stored to last through the night without generation
The time it takes to charge the battery fully
The maximum power the battery can deliver
The number of cycles per day
Explanation - Overnight autonomy is the storage capacity needed to cover load when solar is unavailable.
Correct answer is: The amount of energy stored to last through the night without generation
Q.117 Which storage technology can be directly coupled with a wind turbine without power conversion electronics?
Flywheel
Pumped hydro
Lithium‑ion battery
Supercapacitor
Explanation - Mechanical energy from the turbine can be stored directly in a rotating flywheel via a gearbox.
Correct answer is: Flywheel
Q.118 The primary factor limiting the deployment of pumped‑hydro storage in many regions is:
Low round‑trip efficiency
Requirement for suitable topography and water resources
High self‑discharge rate
Incompatibility with renewable generation
Explanation - Pumped hydro needs specific elevation differences and water availability, limiting site options.
Correct answer is: Requirement for suitable topography and water resources
Q.119 Which storage technology can be easily scaled by adding more tanks of electrolyte, making it attractive for large‑scale grid applications?
Lithium‑ion battery
Flow battery
Flywheel
Supercapacitor
Explanation - Increasing tank size directly raises energy capacity without changing the cell stack.
Correct answer is: Flow battery
Q.120 In the context of battery safety, a "vent" is primarily used to:
Increase the battery’s voltage
Release gases generated during overcharge or failure
Cool the battery with water
Inject electrolyte into the cell
Explanation - Vents prevent pressure buildup by allowing gases to escape safely.
Correct answer is: Release gases generated during overcharge or failure
Q.121 Which of the following storage technologies is typically the most environmentally friendly in terms of material toxicity?
Lead‑acid battery
Lithium‑ion battery
Sodium‑sulfur battery
Zinc‑air battery
Explanation - Zinc and air are abundant and non‑toxic, making zinc‑air batteries environmentally benign.
Correct answer is: Zinc‑air battery
Q.122 The term "grid‑forming" inverter is essential for which type of energy storage deployment?
Off‑grid microgrids
Large‑scale wind farms only
Solar panels without storage
Traditional coal power plants
Explanation - Off‑grid systems need an inverter that can set voltage and frequency without a utility grid.
Correct answer is: Off‑grid microgrids
Q.123 When a battery exhibits a rapid drop in capacity after a few hundred cycles, the most likely cause is:
Excessive depth of discharge
Low ambient temperature
High self‑discharge rate
Use of a DC‑DC converter
Explanation - Deep cycles accelerate degradation, reducing capacity quickly.
Correct answer is: Excessive depth of discharge
Q.124 Which storage technology can be used to store excess renewable electricity as "synthetic natural gas" through methanation?
Lithium‑ion battery
Compressed air energy storage
Power‑to‑gas (methanation)
Flywheel
Explanation - Electricity drives electrolysis to produce hydrogen, which is then combined with CO₂ to form synthetic methane.
Correct answer is: Power‑to‑gas (methanation)
Q.125 A battery with a high "C‑rate" is best suited for:
Long‑duration energy supply
Fast charging and discharging applications
Low‑temperature operation
Passive energy storage
Explanation - High C‑rate capability means the battery can handle rapid charge/discharge without damage.
Correct answer is: Fast charging and discharging applications
Q.126 Which energy storage technology can provide both high power and long duration by using multiple stacked stages?
Flywheel
Hybrid battery‑flow system
Supercapacitor
Pumped hydro
Explanation - Combining batteries (high power) with flow cells (high energy) yields both attributes.
Correct answer is: Hybrid battery‑flow system
Q.127 In a solar‑plus‑storage plant, the "capacity factor" of the storage system is defined as:
Actual energy output divided by the maximum possible output over a period
The ratio of storage voltage to load voltage
The speed at which a flywheel spins
The percentage of sunlight captured
Explanation - Capacity factor indicates how fully the storage is utilized relative to its rating.
Correct answer is: Actual energy output divided by the maximum possible output over a period
Q.128 Which storage technology can be directly coupled with a geothermal plant to store excess heat without converting to electricity?
Lithium‑ion battery
Thermal molten‑salt storage
Compressed air
Supercapacitor
Explanation - Molten‑salt can store high‑temperature heat from geothermal sources for later use.
Correct answer is: Thermal molten‑salt storage
Q.129 Which of the following best describes the term "inertia" in power systems?
The ability of a generator to store chemical energy
The kinetic energy stored in rotating masses that helps stabilize frequency
The amount of heat generated by a battery
The voltage level of an inverter
Explanation - Inertia comes from rotating generators or flywheels and dampens frequency deviations.
Correct answer is: The kinetic energy stored in rotating masses that helps stabilize frequency
Q.130 When integrating storage with a renewable energy system, "state‑of‑charge management" is important to:
Maximize self‑discharge
Prevent over‑charging and over‑discharging for longevity
Increase the system’s weight
Reduce the inverter’s size
Explanation - Proper SoC management protects battery health and extends lifespan.
Correct answer is: Prevent over‑charging and over‑discharging for longevity
Q.131 Which storage technology typically requires a "separator" made of a porous membrane?
Supercapacitor
Lithium‑ion battery
Pumped hydro
Flywheel
Explanation - Separators prevent electrical contact while allowing ion flow.
Correct answer is: Lithium‑ion battery
Q.132 A key advantage of using a modular battery system for a remote telecom tower is:
Ability to replace individual modules without shutting down the entire system
Zero need for temperature control
Infinite energy storage capacity
No need for any monitoring
Explanation - Modular design allows maintenance of one module while others continue operating.
Correct answer is: Ability to replace individual modules without shutting down the entire system
Q.133 Which storage technology is most commonly used for short‑duration, high‑frequency ancillary services such as "fast frequency response"?
Pumped hydro
Lithium‑ion battery
Thermal storage
Compressed air
Explanation - Lithium‑ion batteries can respond within milliseconds, ideal for fast frequency response.
Correct answer is: Lithium‑ion battery
Q.134 The term "grid‑forming" inverter is essential for which type of operation?
Islanding mode where the microgrid operates independently
Synchronizing with a large utility grid only
Providing only DC output
Running diesel generators
Explanation - Grid‑forming inverters can set voltage and frequency, enabling islanded operation.
Correct answer is: Islanding mode where the microgrid operates independently