Energy Resources and Sustainability
3. Energy Storage Systems
Learning outcomes
- I can explain why energy storage is important in modern energy systems.
- I can compare batteries, pumped-storage hydroelectric systems, flywheels, and hydrogen storage technologies.
- I can describe how energy is stored and later released for use.
- I can evaluate the advantages and limitations of different energy storage methods.
- I can analyze the role of energy storage in supporting renewable energy generation.
Energy Storage Systems
Energy storage allows energy to be captured at one time and used later.
This is increasingly important because electricity supply and electricity demand do not always occur at the same time.
For example:
- solar panels may generate excess electricity during sunny periods
- wind turbines may generate strongly when electricity demand is low
- electricity demand may suddenly increase
- renewable generation may suddenly decrease
- emergency backup power may be required
Energy-storage systems help balance these differences.
A storage system does not create energy. It transfers energy into a store and later transfers some of that energy back into a useful form.
Why Store Energy?
Traditional electricity grids were designed so that power stations adjusted their output to approximately match demand.
Modern grids increasingly include variable renewable resources such as:
- solar
- wind
Their electricity production depends partly on environmental conditions.
For example, a solar farm might produce more electricity than needed at midday but none at night.
Without storage:
generation must approximately match demand at that moment
With storage:
excess generation → storage → later electricity supply
This provides much greater flexibility.
Energy Storage and Conservation of Energy
Energy is conserved during storage.
However, real storage systems are not 100% efficient.
During charging and discharging, some energy is transferred into less useful forms, often thermal energy.
Therefore:
energy returned < energy originally supplied
Efficiency can be calculated using:
efficiency = useful energy output / energy input
or:
efficiency (%) = useful energy output / energy input × 100
Round-Trip Efficiency
For energy storage, we often use round-trip efficiency.
This compares the energy recovered with the energy originally supplied.
Suppose a battery receives:
100 kWh
and later returns:
90 kWh
Round-trip efficiency:
efficiency = 90/100 × 100
efficiency = 90%
Energy dissipated:
100 − 90 = 10 kWh
Four Important Storage Technologies
There are many ways to store energy.
Four important examples are:
- batteries
- pumped-storage hydroelectricity
- flywheels
- hydrogen
These systems store energy in very different ways.
| Technology | Main Energy Store |
|---|---|
| Battery | Chemical |
| Pumped hydro | Gravitational potential |
| Flywheel | Kinetic |
| Hydrogen | Chemical |
Understanding the energy store helps explain how each system works.
Battery Storage
A battery stores energy chemically.
During charging:
electrical energy → chemical energy store
During discharge:
chemical energy store → electrical energy
Rechargeable batteries can repeat this process many times.
Battery Cells
A battery contains one or more electrochemical cells.
Chemical reactions within the cell allow charge to move through an external circuit.
Rechargeable batteries use chemical reactions that can be reversed to a useful extent.
When charging, an external electrical supply drives the system toward a higher-energy chemical state.
When discharging, chemical processes allow electrical energy to be supplied to a circuit.
Types of Rechargeable Batteries
Different battery chemistries are used for different purposes.
Examples include:
- lithium-ion
- lead-acid
- nickel-metal hydride
- sodium-ion
- flow batteries
Each has different characteristics involving:
- energy density
- power output
- cost
- cycle life
- safety
- materials
- operating temperature
There is therefore no single battery technology that is best for every application.
Battery Energy Density
Energy density describes how much energy can be stored per unit mass or volume.
High energy density is particularly important when storage must be moved.
Examples include:
- electric cars
- mobile phones
- laptops
- aircraft
For stationary grid storage, mass may be less important because the batteries do not need to move.
Other factors such as cost and lifetime may become more important.
Battery Power
Energy capacity and power are different.
A battery may store a large amount of energy but be unable to release it very quickly.
Recall:
P = E/t
A high-power battery can transfer energy rapidly.
A high-energy battery can provide a large total quantity of energy.
Good storage design therefore considers both:
energy capacity
and:
power capability
Worked Example: Battery Storage
A battery stores:
60 kWh
of usable energy.
A device draws:
5 kW
of power.
Ignoring additional losses:
t = E/P
t = 60/5
t = 12 h
The battery could theoretically supply the device for 12 hours.
Advantages of Batteries
Battery systems can:
- respond extremely quickly
- be installed in many locations
- range from tiny to grid-scale systems
- store electricity locally
- support renewable energy
- provide emergency backup
- stabilize electrical grids
Their rapid response makes them particularly useful for managing sudden changes in supply or demand.
Limitations of Batteries
Possible limitations include:
- limited lifetime
- gradual degradation
- material requirements
- mining impacts
- cost
- fire risks for some chemistries
- recycling challenges
- finite storage capacity
Large batteries can also become expensive when enormous quantities of energy must be stored for long periods.
Battery Degradation
Rechargeable batteries do not normally maintain their original capacity forever.
Repeated:
charging → discharging → charging
can gradually change battery materials.
Over time:
- maximum capacity may decrease
- internal resistance may increase
- performance may decline
The number of useful charging and discharging cycles is called the battery's cycle life.
Pumped-Storage Hydroelectricity
Pumped-storage hydroelectricity stores energy by moving water to a higher elevation.
It normally uses:
- a lower reservoir
- an upper reservoir
- pumps
- turbines
- generators
During periods of excess electricity:
electrical energy → mechanical energy → gravitational potential energy
Water is pumped uphill.
Releasing Energy from Pumped Storage
When electricity is needed, water flows downhill.
Energy pathway:
gravitational potential energy → kinetic energy → mechanical energy → electrical energy
The moving water turns turbines.
The turbines drive generators.
This is essentially a giant gravitational energy-storage system.
Gravitational Potential Energy
Recall:
ΔEₚ = mgΔh
where:
- m = mass
- g = gravitational field strength
- Δh = change in height
Pumped storage therefore benefits from:
- large quantities of water
- large height differences
Worked Example: Pumped Storage
Suppose:
1,000,000 kg
of water is pumped:
100 m
upward.
Using:
g = 9.8 N/kg
Stored gravitational potential energy:
ΔEₚ = mgΔh
ΔEₚ = 1,000,000 × 9.8 × 100
ΔEₚ = 980,000,000 J
or:
980 MJ
This is the ideal gravitational potential energy increase.
The actual electrical energy recovered later would be lower because the system is not 100% efficient.
Advantages of Pumped Storage
Pumped-storage systems can provide:
- very large energy capacity
- long operating lifetimes
- rapid response
- grid-scale energy storage
- repeated charging and discharging
- storage lasting many hours
They are particularly useful for storing large amounts of energy.
Limitations of Pumped Storage
Pumped storage requires suitable geography.
Ideally, there must be:
- significant elevation difference
- space for reservoirs
- appropriate geology
- access to water
Construction can also:
- flood land
- disturb habitats
- alter landscapes
- require large infrastructure
Therefore, pumped storage cannot simply be installed anywhere.
Pumped Storage Is Not an Energy Source
This is an important distinction.
A pumped-storage station does not produce a net supply of new energy.
It stores energy supplied from somewhere else.
Because of efficiency losses:
electrical energy recovered < electrical energy used for pumping
Its value comes from moving energy from one time to another.
Flywheel Energy Storage
A flywheel stores energy in a rapidly rotating object.
Energy pathway during charging:
electrical energy → mechanical energy → rotational kinetic energy
During discharge:
rotational kinetic energy → mechanical energy → electrical energy
A motor accelerates the flywheel.
Later, the rotating flywheel drives a generator.
Energy in a Flywheel
A rotating flywheel contains rotational kinetic energy.
The amount of stored energy depends strongly on:
- rotational speed
- mass
- distribution of mass
High-performance flywheels can rotate extremely rapidly.
Engineers therefore use strong materials and carefully designed containment systems.
Reducing Flywheel Energy Losses
A rotating flywheel loses energy through friction and air resistance.
Engineers can reduce these losses using:
- low-friction bearings
- magnetic bearings
- vacuum chambers
- carefully balanced rotors
Reducing resistance allows the flywheel to continue rotating for longer.
Advantages of Flywheels
Flywheels can:
- charge rapidly
- discharge rapidly
- provide high power
- survive many cycles
- respond extremely quickly
- require relatively little chemical processing during operation
They are particularly useful when energy must be supplied or absorbed for short periods.
Limitations of Flywheels
Flywheels generally store less energy for long-duration applications than very large battery or pumped-hydro systems.
They may also:
- gradually lose energy while spinning
- require specialized bearings
- require strong containment
- involve high rotational speeds
They are therefore often better suited to short-duration, high-power applications.
Flywheels in Transportation
Flywheel systems have been investigated or used in:
- buses
- racing vehicles
- trains
- regenerative braking systems
During braking:
vehicle kinetic energy → rotational energy of flywheel
During acceleration:
flywheel rotational energy → vehicle kinetic energy
This allows some braking energy to be recovered rather than dissipated entirely as heat.
Hydrogen Energy Storage
Hydrogen can also be used as an energy-storage medium.
Electricity can be used to split water by electrolysis.
A simplified equation is:
2H₂O → 2H₂ + O₂
This requires an energy input.
The hydrogen can then be stored.
Green Hydrogen
When hydrogen is produced using electricity from renewable sources, it is commonly called green hydrogen.
For example:
wind electricity → electrolysis → hydrogen
The hydrogen stores energy chemically.
Later, it can be used in:
- fuel cells
- industrial processes
- some transportation systems
- electricity generation
Hydrogen and Fuel Cells
A fuel cell can convert the chemical energy associated with hydrogen into electrical energy.
A simplified overall reaction is:
2H₂ + O₂ → 2H₂O
Energy is transferred to an electrical circuit and the surroundings.
The overall storage pathway can therefore be:
electrical energy → hydrogen production → hydrogen storage → fuel cell → electrical energy
Several conversions occur, so significant energy losses can occur.
Hydrogen Storage
Hydrogen can be stored in several ways, including:
- compressed gas
- liquid hydrogen
- chemical carriers
- underground geological storage in suitable locations
Each method presents engineering challenges.
Hydrogen gas has very low density under ordinary conditions, so storing large quantities can require substantial volume unless it is compressed or liquefied.
Advantages of Hydrogen Storage
Hydrogen may offer:
- long-duration energy storage
- potentially large storage capacity
- connection between electricity and industrial energy use
- storage of excess renewable electricity
- potential seasonal storage
It may be particularly useful when energy must be stored for much longer than a few hours.
Limitations of Hydrogen Storage
Challenges include:
- energy losses during electrolysis
- energy required for compression or liquefaction
- storage difficulties
- transportation infrastructure
- fuel-cell or turbine conversion losses
- cost
- leakage management
Therefore, hydrogen usually has a lower round-trip electrical efficiency than directly storing electricity in a battery.
Why Use Hydrogen If Batteries Are More Efficient?
Efficiency is only one engineering consideration.
Imagine storing energy from summer for use several months later.
A battery large enough to store enormous amounts of energy for months may be impractical or expensive.
Hydrogen can potentially be produced when renewable electricity is abundant and stored for much longer periods.
This illustrates an important principle:
The most efficient technology is not automatically the best technology for every application.
Comparing Storage Technologies
| Technology | Energy Store | Response | Typical Strength |
|---|---|---|---|
| Battery | Chemical | Very fast | Flexible electrical storage |
| Pumped hydro | Gravitational potential | Fast | Large-scale, long-duration storage |
| Flywheel | Rotational kinetic | Extremely fast | Short-duration high power |
| Hydrogen | Chemical | Depends on system | Long-duration/large-scale storage |
Storage Duration
Different storage technologies are useful over different timescales.
Seconds to Minutes
Flywheels and batteries can respond rapidly.
Useful for:
- grid stabilization
- sudden power changes
- frequency control
Minutes to Hours
Batteries are particularly useful.
Applications include:
- shifting solar electricity into the evening
- managing peak demand
- backup power
Hours to Days
Large batteries and pumped-storage hydroelectricity can be useful.
Days to Months
Hydrogen and other long-duration storage technologies may become more attractive.
The exact choice depends on system size, cost, geography, and technology.
Power Versus Energy Capacity
Storage systems must be compared using both power and energy.
Suppose two batteries each have:
100 kWh
of energy capacity.
Battery A can deliver:
100 kW
Battery B can deliver:
20 kW
They store the same energy but have very different power capabilities.
Battery A could theoretically discharge its energy much more rapidly.
Worked Example: Power and Storage Time
A storage system contains:
500 kWh
and supplies:
100 kW
Ignoring losses:
t = E/P
t = 500/100
t = 5 h
It could theoretically provide 100 kW for 5 hours.
If demand increased to:
250 kW
then:
t = 500/250
t = 2 h
Higher power demand empties the same energy store more quickly.
Renewable Energy and Storage
Solar and wind energy are variable.
Consider a solar farm.
During midday:
solar generation > electricity demand
Without storage, some excess electricity might not be used.
With storage:
solar electricity → storage
Later in the evening:
storage → electrical grid
Storage therefore allows renewable energy to be shifted from periods of high generation to periods of high demand.
Example: Solar and Battery Storage
A solar farm produces:
120 MWh
during the day.
Consumers immediately use:
80 MWh
Excess energy:
120 − 80 = 40 MWh
Suppose all 40 MWh enters a battery system with a round-trip efficiency of:
85%
Energy later returned:
40 × 0.85
= 34 MWh
The remaining:
6 MWh
has been transferred into less useful forms during the storage cycle.
Wind Energy and Storage
Wind generation can vary significantly.
Sometimes:
wind generation > demand
At other times:
wind generation < demand
Storage can absorb excess wind-generated electricity and return energy when wind speeds decrease.
This can make variable renewable generation easier to integrate into the electricity grid.
Grid Stabilization
Electricity grids must maintain a balance between:
generation and demand
If this balance changes suddenly, grid frequency and voltage can be affected.
Fast-response storage systems can rapidly:
- absorb excess power
- supply additional power
Batteries and flywheels are particularly useful for these rapid adjustments.
Peak Demand
Electricity demand is not constant throughout the day.
Demand may increase:
- in the morning
- during very hot weather
- during very cold weather
- in the evening
Storage can charge when demand is lower and discharge during peak demand.
This can reduce the need to operate additional power stations only for short periods.
Energy Arbitrage
Electricity can have different values at different times.
Storage systems can:
charge when electricity is abundant
and:
discharge when electricity is scarce
This practice is sometimes called energy arbitrage.
From an energy-system perspective, it shifts electricity from periods of surplus to periods of greater need.
Backup Power
Storage can provide electricity when the main supply fails.
Applications include:
- hospitals
- data centres
- homes
- telecommunications
- factories
- emergency services
The required storage technology depends on:
- required power
- required duration
- response time
- reliability
Microgrids
A microgrid is a smaller local electrical system that can include:
- solar panels
- wind turbines
- batteries
- generators
- local consumers
Storage can help a microgrid continue operating when renewable generation changes or when connection to the main grid is interrupted.
This can be especially useful in remote communities.
Electric Vehicles as Energy Storage
Electric vehicles contain large rechargeable batteries.
Normally:
grid → vehicle battery
Some systems can potentially operate in the opposite direction:
vehicle battery → building or electrical grid
This concept is called vehicle-to-grid, or V2G.
A large number of connected vehicles could potentially provide significant distributed storage capacity.
However, practical systems must consider:
- battery degradation
- vehicle availability
- charging needs
- grid infrastructure
Storage and Energy Security
Energy storage can improve energy security.
A system with stored energy is less dependent on electricity being generated at exactly the moment it is needed.
Storage can provide protection against:
- short-term supply interruptions
- sudden demand increases
- renewable-generation decreases
- some grid failures
However, storage capacity is finite.
Environmental Impacts of Batteries
Battery production requires materials that may include:
- lithium
- nickel
- cobalt
- graphite
- copper
Mining and processing these materials can cause environmental and social impacts.
Battery evaluation should therefore consider:
mining → manufacturing → operation → recycling/disposal
Recycling can recover some valuable materials and reduce demand for newly mined resources.
Environmental Impacts of Pumped Storage
Pumped-storage systems may require large reservoirs.
Potential impacts include:
- habitat alteration
- landscape changes
- water-system changes
- construction impacts
However, installations can operate for many decades.
Some newer designs use existing reservoirs, mines, or other altered environments to reduce impacts.
Environmental Impacts of Flywheels
Flywheels require materials and manufacturing, but they do not depend on large quantities of electrochemical materials for repeated cycling.
Their long cycle life can be an advantage.
However, they still require:
- specialized materials
- manufacturing energy
- containment systems
- eventual recycling or disposal
Environmental Impacts of Hydrogen
The environmental impact of hydrogen depends strongly on how the hydrogen is produced.
Hydrogen is an energy carrier, not a primary energy source.
If renewable electricity powers electrolysis, emissions can be relatively low.
If hydrogen is produced from fossil fuels, greenhouse-gas emissions can be significant unless additional technologies reduce them.
Therefore:
hydrogen itself does not tell us the environmental impact
We must ask:
How was the hydrogen produced?
Overall Storage Efficiency
Suppose electricity passes through several stages.
Electrolyzer:
75% efficient
Hydrogen storage and handling:
90% efficient
Fuel cell:
60% efficient
Overall efficiency:
0.75 × 0.90 × 0.60
= 0.405
Overall efficiency:
≈ 41%
If the system began with:
100 kWh
approximately:
41 kWh
would be returned under these simplified assumptions.
This demonstrates why multi-stage energy conversions can significantly reduce overall efficiency.
Comparing Two Storage Systems
System A:
90% round-trip efficiency
System B:
70% round-trip efficiency
Both receive:
1000 kWh
System A returns:
900 kWh
System B returns:
700 kWh
However, we cannot automatically conclude that System A is always better.
System B might offer:
- lower cost
- much larger capacity
- longer storage duration
- better geographical suitability
Engineering decisions require multiple criteria.
Choosing Storage for a Solar Farm
Suppose a solar farm regularly produces excess electricity during the afternoon that is needed four hours later.
A battery system may be suitable because it offers:
- rapid response
- high efficiency
- several hours of storage
- flexible installation
Using hydrogen might involve unnecessary conversion losses for such short-duration storage.
Choosing Storage for Seasonal Energy
Suppose a region generates enormous amounts of renewable electricity during one season but needs the energy several months later.
A battery system large enough for months of storage could be extremely expensive.
Long-duration storage such as hydrogen may become more attractive despite its lower round-trip efficiency.
Choosing Storage in Mountainous Regions
A mountainous region may have:
- existing reservoirs
- significant elevation differences
- suitable water resources
Pumped-storage hydroelectricity may be particularly attractive.
A flat region without suitable reservoirs would require a different technology.
Choosing Storage for Rapid Grid Response
Suppose an electrical grid needs a system that can supply large amounts of power almost instantly for only a short period.
Good candidates may include:
- batteries
- flywheels
A flywheel may be particularly suitable where extremely rapid response and very high cycle life are important.
Engineering Trade-Offs
When selecting energy storage, engineers consider:
- energy capacity
- power
- response time
- storage duration
- efficiency
- energy density
- cost
- lifetime
- cycle life
- geography
- safety
- environmental impact
- materials
- maintenance
- self-discharge
- infrastructure
No storage technology is best in every category.
Common Mistakes
Saying Energy Storage Creates Energy
Storage systems do not create energy.
They move energy from one time to another.
Assuming All Stored Energy Is Recovered
Real storage systems are not 100% efficient.
Some energy is dissipated during charging, storage, and discharge.
Confusing Power and Energy Capacity
Energy capacity tells us how much energy is stored.
Power tells us how quickly that energy can be transferred.
Calling Pumped Storage an Energy Source
Pumped-storage hydroelectricity is primarily an energy-storage technology.
It requires energy to pump water uphill.
Thinking Batteries Store Electricity Directly
Rechargeable batteries store energy primarily in chemical form.
Thinking Flywheels Store Electrical Energy
Flywheels store rotational kinetic energy.
Thinking Pumped Hydro Stores Electrical Energy
Pumped hydro stores gravitational potential energy.
Calling Hydrogen a Primary Energy Source
Hydrogen is generally an energy carrier.
Energy must first be used to produce it.
Assuming Hydrogen Is Automatically Environmentally Friendly
Its environmental impact depends heavily on how it is produced.
Assuming the Most Efficient Storage Is Always Best
Efficiency is only one factor.
Engineers must also consider capacity, duration, power, cost, geography, lifetime, and environmental impact.
Check Your Understanding
- Explain why energy storage is important in modern electricity systems.
- Name four major energy-storage technologies.
- What type of energy is stored in a battery?
- Describe the energy transformations when charging and discharging a battery.
- Define round-trip efficiency.
- A battery receives 200 kWh and later returns 180 kWh. Calculate its round-trip efficiency.
- Calculate the energy dissipated in Question 6.
- Explain the difference between energy capacity and power.
- A 100 kWh battery supplies 20 kW. For how long could it theoretically operate?
- Explain how pumped-storage hydroelectricity stores energy.
- Describe the energy transformations when pumped-storage energy is released.
- Calculate the gravitational potential energy stored when 500,000 kg of water is raised 80 m. Use g = 9.8 N/kg.
- Explain why pumped-storage hydroelectricity requires suitable geography.
- Why is pumped storage not considered an original energy source?
- What type of energy does a flywheel store?
- Describe how a flywheel is charged and discharged.
- Give two advantages of flywheel storage.
- Why are flywheels particularly useful for short-duration, high-power applications?
- Explain how hydrogen can be produced using electrolysis.
- What is meant by green hydrogen?
- Explain how stored hydrogen can be used to generate electricity.
- Why does hydrogen storage generally have lower round-trip electrical efficiency than battery storage?
- Explain why hydrogen might still be useful for long-duration storage.
- A solar farm produces 60 MWh of excess electricity. A storage system has an 80% round-trip efficiency. How much energy can later be returned?
- Explain how storage can support solar electricity generation.
- Explain how storage can support wind electricity generation.
- Why are batteries and flywheels useful for grid stabilization?
- Compare batteries and pumped-storage hydroelectricity.
- Compare flywheel and hydrogen storage in terms of likely storage duration.
- A region has abundant wind generation in one season but high electricity demand several months later. Evaluate which storage technology might be suitable and explain your reasoning.
Key Terms
Energy storage — Capturing energy so it can be used at a later time.
Energy-storage system — Technology that transfers energy into a store and later releases it for useful purposes.
Energy capacity — Total amount of energy a storage system can hold.
Power — Rate at which energy is transferred.
Round-trip efficiency — Percentage of input energy recovered after a complete storage and release cycle.
Battery — Electrochemical device that stores energy chemically and can supply electrical energy.
Electrochemical cell — Device in which chemical processes are associated with electrical energy transfer.
Cycle life — Number of charging and discharging cycles a storage system can complete while maintaining acceptable performance.
Battery degradation — Gradual decline in battery performance or capacity.
Pumped-storage hydroelectricity — Storage technology that uses electricity to pump water to a higher elevation.
Gravitational potential energy — Energy stored because of position in a gravitational field.
Flywheel — Rotating device used to store kinetic energy.
Rotational kinetic energy — Energy associated with rotational motion.
Electrolysis — Process that uses electrical energy to drive a chemical reaction, such as splitting water to produce hydrogen and oxygen.
Hydrogen storage — Storage of energy chemically using hydrogen as an energy carrier.
Green hydrogen — Hydrogen produced using renewable electricity, commonly through electrolysis.
Fuel cell — Device that converts chemical energy from continuously supplied reactants into electrical energy.
Energy carrier — Substance or system used to transport or store energy obtained from another source.
Grid stabilization — Maintaining stable operation of an electrical grid by balancing supply and demand.
Peak demand — Period when electricity demand is particularly high.
Self-discharge — Loss of stored energy over time without useful energy being withdrawn.
Vehicle-to-grid (V2G) — System allowing suitable electric vehicles to return stored electrical energy to the grid.
Key Takeaways
- Energy storage allows energy generated at one time to be used later.
- Storage is increasingly important as solar and wind generation increase.
- Storage systems do not create energy.
- Every real storage system loses some useful energy during a complete storage cycle.
- Round-trip efficiency compares recovered energy with the original energy input.
- Batteries store energy chemically.
- Pumped-storage hydroelectricity stores gravitational potential energy.
- Flywheels store rotational kinetic energy.
- Hydrogen stores energy chemically as an energy carrier.
- Batteries provide rapid response and can be installed at many different scales.
- Pumped storage can provide enormous energy capacity but requires suitable geography.
- Flywheels provide extremely rapid power transfer and can survive many cycles.
- Hydrogen can potentially provide very long-duration or seasonal energy storage.
- Batteries generally have higher round-trip efficiency than hydrogen-based electrical storage.
- Energy capacity and power are different quantities.
- P = E/t connects storage capacity, power, and operating time.
- Pumped storage uses ΔEₚ = mgΔh.
- Storage can absorb excess renewable electricity when generation exceeds demand.
- Stored energy can later be released when demand exceeds generation.
- Batteries and flywheels can respond rapidly to sudden changes in grid conditions.
- Pumped hydro can provide large-scale storage over many hours.
- Hydrogen may be useful when enormous quantities of energy must be stored for long periods.
- The environmental impact of hydrogen depends strongly on how it is produced.
- Battery production and disposal have material and environmental impacts that must also be considered.
- Storage technologies should be evaluated using their complete life cycles.
- No single storage technology is ideal for every situation.
- Engineers must compare capacity, power, duration, efficiency, cost, lifetime, geography, safety, and environmental impact.
A useful way to remember the four systems is:
Battery → chemical energy
Pumped hydro → gravitational potential energy
Flywheel → kinetic energy
Hydrogen → chemical energy
The central role of energy storage is to solve a timing problem:
Energy is not always generated when we need it, and we do not always need it when it is generated.