Batteries and Fuel Cells
5. Energy Storage Technologies
Learning outcomes
- I can identify different methods of energy storage.
- I can compare batteries, fuel cells, and other storage technologies.
- I can evaluate the efficiency of energy-storage systems.
- I can explain the importance of energy storage in renewable energy systems.
- I can assess future developments in energy-storage technology.
What Is Energy Storage?
Energy storage is the process of capturing energy so that it can be used:
at a later time.
Energy cannot simply disappear into a storage device. Instead, it is converted from one form into another.
For example, a rechargeable battery converts:
electrical energy → chemical energy
during charging.
When the battery is used:
chemical energy → electrical energy
Different storage technologies store energy in different forms.
These include:
- chemical energy
- gravitational potential energy
- kinetic energy
- elastic potential energy
- thermal energy
- electrical energy
Why Do We Need Energy Storage?
Electricity production and electricity demand do not always occur at the same time.
For example, solar panels may generate large amounts of electricity:
during the day.
However, people may require large amounts of electricity:
after sunset.
Energy storage allows some of the daytime energy to be saved and used later.
The Basic Storage Cycle
Most energy-storage systems follow the same general process:
Energy input
↓
Energy converted into a storable form
↓
Energy stored
↓
Stored energy converted back
↓
Useful energy output
Every conversion involves some energy loss.
Therefore no real storage system is:
100% efficient.
Forms of Energy Storage
Energy can be stored in many different forms.
Chemical
- batteries
- hydrogen
Gravitational
- pumped hydroelectric storage
- gravity-storage systems
Kinetic
- flywheels
Thermal
- hot water
- molten salts
- other thermal-storage materials
Electrical
- capacitors
- supercapacitors
Mechanical
- compressed-air systems
Different technologies are suitable for different:
applications and timescales.
Rechargeable Batteries
Rechargeable batteries are one of the most familiar energy-storage technologies.
During charging:
electrical energy → chemical energy
During discharge:
chemical energy → electrical energy
Examples include:
- lithium-ion batteries
- lead-acid batteries
- nickel-metal hydride batteries
- sodium-ion batteries
Lithium-Ion Battery Storage
Lithium-ion batteries are widely used because they offer:
- high energy density
- high efficiency
- relatively fast response
- rechargeable operation
- useful cycle life
They are used in:
- smartphones
- laptops
- electric vehicles
- homes
- businesses
- large electricity grids
Large battery installations are often called:
battery energy storage systems, or BESS.
How Grid Batteries Work
Imagine a solar farm producing more electricity than the grid currently needs.
The excess electricity can charge batteries:
electrical → chemical
Later, when electricity demand increases:
chemical → electrical
The stored electricity can then be supplied back to the:
grid.
Advantages of Battery Storage
Rechargeable batteries can offer:
- rapid response
- high round-trip efficiency
- modular construction
- installation at many different scales
- no moving mechanical parts in the cells themselves
- useful storage from seconds to several hours or longer depending on design
Battery systems can respond quickly to changes in electricity:
supply and demand.
Limitations of Battery Storage
Battery systems also have limitations.
These can include:
- cost
- gradual degradation
- limited cycle life
- raw-material requirements
- fire and safety considerations
- environmental impacts from manufacturing
- eventual recycling requirements
Different battery chemistries have different:
advantages and disadvantages.
Fuel Cells and Hydrogen
Hydrogen provides another method of storing energy.
Strictly speaking, the fuel cell itself is an energy-conversion device, rather than the main storage device.
The energy is stored in:
hydrogen fuel.
A fuel cell converts the chemical energy of hydrogen into:
electrical energy.
Hydrogen Energy Storage
Electricity can first be used to produce hydrogen through:
The sequence can be represented as:
electricity
↓
↓
hydrogen
↓
storage
↓
fuel cell
↓
electricity
Hydrogen therefore acts as an:
energy carrier.
Advantages of Hydrogen Storage
Hydrogen may be useful for:
- long-duration storage
- storing large quantities of energy
- transportation
- industrial applications
- storing excess renewable energy
- situations where batteries may be less practical
Hydrogen can potentially be stored for:
long periods.
This makes it interesting for seasonal energy storage.
Limitations of Hydrogen Storage
The electricity-to-hydrogen-to-electricity pathway involves several conversions.
Energy can be lost during:
- electrolysis
- compression
- storage
- transportation
- fuel-cell operation
Therefore its overall round-trip efficiency is generally lower than that of:
battery storage.
Hydrogen storage also requires specialized infrastructure.
Pumped Hydroelectric Storage
One of the most established forms of large-scale energy storage is:
pumped hydroelectric storage.
It uses two reservoirs at different elevations.
When excess electricity is available, water is pumped from the lower reservoir to the:
upper reservoir.
This stores energy as:
gravitational potential energy.
How Pumped Hydro Works
During charging:
electrical energy → kinetic energy of pumps → gravitational potential energy
Water is moved uphill.
During discharge:
Water flows downhill through turbines.
Energy conversion:
gravitational potential energy → kinetic energy → electrical energy
The same water can be cycled between the reservoirs many times.
Advantages of Pumped Hydro
Pumped hydro can provide:
- very large storage capacity
- long operating life
- large power output
- storage for many hours
- well-established technology
It is especially useful for:
large electricity grids.
Limitations of Pumped Hydro
Pumped hydro requires suitable:
geography.
A site generally needs:
- significant elevation difference
- appropriate reservoirs
- access to water
- suitable geology
- substantial construction
Projects can also affect:
- ecosystems
- land use
- waterways
- local communities
Therefore pumped hydro cannot simply be built:
anywhere.
Gravitational Potential Energy
The gravitational energy stored can be estimated using:
E = mgh
where:
E = gravitational potential energy in joules
m = mass in kilograms
g = gravitational field strength
h = height difference in metres
Increasing either the mass of water or the height difference increases the amount of:
stored energy.
Worked Example: Pumped Hydro
Suppose:
m = 10,000 kg
g = 9.8 N/kg
h = 50 m
Then:
E = mgh
E = 10,000 × 9.8 × 50
E = 4,900,000 J
Therefore:
E = 4.9 MJ
This assumes an ideal system. The useful energy recovered would be less because of:
energy losses.
Flywheel Energy Storage
A flywheel stores energy in a rapidly rotating object.
During charging, an electric motor accelerates the flywheel.
Energy conversion:
electrical energy → rotational kinetic energy
When energy is required, the rotating flywheel drives a generator.
Energy conversion:
rotational kinetic energy → electrical energy.
Advantages of Flywheels
Flywheels can:
- respond extremely quickly
- provide high power
- undergo many cycles
- operate without the same electrochemical degradation mechanisms as batteries
They are particularly useful for:
short-duration energy storage and power stabilization.
Limitations of Flywheels
Flywheels generally store energy for shorter periods than some other technologies.
Energy is gradually lost through:
- friction
- bearing losses
- air resistance
- electrical losses
Advanced flywheels may operate in vacuum chambers and use specialized bearings to reduce these:
losses.
Compressed-Air Energy Storage
Another method is:
compressed-air energy storage, or CAES.
Excess electricity powers compressors.
Air is compressed and stored under pressure.
Energy is therefore stored partly through the:
compressed state of the gas.
Later, the compressed air can be expanded through machinery to help generate electricity.
Compressed-Air Storage
The basic process is:
electrical energy
↓
compressor
↓
compressed air
↓
storage
↓
expansion
↓
generator
↓
electrical energy
Large systems may use underground:
caverns.
Advantages of Compressed-Air Storage
Potential advantages include:
- large-scale energy storage
- relatively long storage duration
- potentially long equipment lifetime
However, performance depends strongly on how the system manages:
heat.
Heat and Compression
When a gas is compressed, its temperature tends to:
increase.
When compressed gas expands, its temperature tends to:
decrease.
Managing this thermal energy is an important part of designing efficient compressed-air storage systems.
Thermal Energy Storage
Energy can also be stored as:
thermal energy.
Examples include:
- hot water tanks
- molten salt
- heated rocks
- chilled water
- ice storage
Thermal storage is particularly useful when the desired final form of energy is:
heating or cooling.
Molten Salt Storage
Some solar thermal power plants use:
molten salt.
Solar energy heats the salt.
The thermal energy can be stored and later used to:
- heat water
- produce steam
- drive a turbine
- generate electricity
This allows solar energy collected during daylight hours to be used:
after sunset.
Advantages of Thermal Storage
Thermal energy storage can be:
- relatively simple
- inexpensive in some applications
- useful over several hours
- valuable for heating and cooling
- compatible with solar thermal systems
It can reduce the need to convert energy repeatedly when the final requirement is:
heat.
Limitations of Thermal Storage
Thermal energy gradually moves from warmer objects to cooler surroundings.
This causes:
heat loss.
Good insulation reduces heat transfer but cannot eliminate it completely.
If stored heat must later be converted back into electricity, additional energy losses occur.
Capacitors
A capacitor stores energy using separated electrical charges.
Unlike a battery, it does not primarily store energy through bulk chemical reactions.
Capacitors can:
- charge rapidly
- discharge rapidly
- provide high power
However, conventional capacitors store relatively:
small amounts of energy.
Supercapacitors
Supercapacitors are designed to store much more energy than conventional capacitors while retaining rapid charging and discharging.
They are useful where energy must be:
absorbed or released very quickly.
Advantages of Supercapacitors
Supercapacitors can provide:
- extremely fast charging
- extremely fast discharging
- high power
- very large numbers of cycles
They can be useful in:
- regenerative braking
- short-term backup
- power stabilization
- rapid energy recovery
Limitations of Supercapacitors
Compared with batteries, supercapacitors generally have much lower:
energy density.
This means a large supercapacitor system may be required to store the same amount of energy as a much smaller battery.
Therefore they are better suited to:
short bursts of power
than long-duration energy storage.
Gravity Storage
Pumped hydro is not the only way to store gravitational potential energy.
Other proposed and developing systems lift:
- heavy blocks
- large masses
- weights in shafts
When excess electricity is available:
electrical energy → gravitational potential energy
When electricity is required:
gravitational potential energy → electrical energy
Energy Storage Is About Trade-Offs
No storage technology is ideal for every purpose.
A system that is excellent for:
one second
may be poor for:
one month.
A system suitable for a:
smartphone
may be unsuitable for an:
electricity grid.
Energy storage must therefore be selected according to the:
application.
Storage Timescales
Different technologies are useful over different timescales.
Very short duration:
- capacitors
- supercapacitors
- flywheels
Short to medium duration:
- batteries
Long duration:
- pumped hydro
- compressed air
- some battery systems
Very long or potentially seasonal storage:
- hydrogen
- some thermal-storage approaches
These categories overlap depending on system design.
Power vs Energy
An important distinction is between:
power
and:
energy.
Energy describes:
how much can be stored.
Power describes:
how quickly that energy can be delivered.
A storage system may have high power but relatively little total energy.
For example, a supercapacitor can release energy:
very quickly
but may not store enough energy to operate a home overnight.
Energy Capacity
Energy-storage capacity is commonly measured in:
- joules (J)
- kilojoules (kJ)
- megajoules (MJ)
- watt-hours (Wh)
- kilowatt-hours (kWh)
- megawatt-hours (MWh)
Remember:
1 Wh = 3600 J
Therefore:
1 kWh = 3.6 MJ
Worked Example: Energy Conversion
A storage system contains:
20 kWh
Convert this to megajoules.
Since:
1 kWh = 3.6 MJ
Then:
20 × 3.6 = 72 MJ
Therefore:
20 kWh = 72 MJ.
Round-Trip Efficiency
An important way to evaluate storage is:
round-trip efficiency.
This compares the useful energy recovered with the energy originally supplied.
The equation is:
Efficiency (%) = useful energy output ÷ energy input × 100
A storage system can never return more energy than was originally supplied.
Worked Example: Storage Efficiency
A battery receives:
100 kWh
during charging.
Later it returns:
90 kWh.
Efficiency:
90 ÷ 100 × 100 = 90%
Therefore the round-trip efficiency is:
90%.
The remaining energy has been converted into less useful forms, mainly:
thermal energy.
Worked Example: Comparing Systems
System A receives 500 kWh and returns 450 kWh.
Efficiency = 450 ÷ 500 × 100
= 90%
System B receives 500 kWh and returns 300 kWh.
Efficiency = 300 ÷ 500 × 100
= 60%
System A has the higher:
round-trip efficiency.
However, efficiency alone does not determine which system is better for a particular application.
Where Does the Lost Energy Go?
Energy is not destroyed.
Instead, some energy is transformed into forms that are less useful for the intended purpose.
Losses can occur through:
- electrical resistance
- friction
- heating
- chemical side reactions
- pumps
- compressors
- generators
- power electronics
Eventually much of the lost energy becomes:
thermal energy.
Efficiency Is Not the Only Factor
Imagine two storage systems.
System A has very high efficiency but can store energy for only:
a few minutes.
System B has lower efficiency but can store enormous quantities of energy for:
several months.
The best system depends on what the energy storage is required to:
do.
Other Important Factors
When comparing storage technologies, consider:
- round-trip efficiency
- storage capacity
- power output
- storage duration
- response time
- cycle life
- cost
- size
- mass
- location requirements
- safety
- environmental impact
- material availability
A complete evaluation considers several factors rather than:
only efficiency.
Renewable Energy and Storage
Solar and wind power are:
variable energy sources.
Their output depends on environmental conditions.
Solar generation changes with:
- time of day
- season
- cloud cover
Wind generation changes with:
wind conditions.
Electricity demand does not automatically change in the same way.
The Solar Energy Problem
Solar panels may produce their greatest power around:
the middle of the day.
However, electricity demand may remain high during the:
evening.
Without storage, excess midday electricity might not be available after:
sunset.
Storage allows:
daytime generation → stored energy → evening electricity.
The Wind Energy Problem
Wind turbines produce electricity when:
wind is available.
Wind speed can change from:
- hour to hour
- day to day
- season to season
Energy storage can absorb electricity during periods of:
high wind generation
and return electricity during periods of:
lower generation.
Balancing the Electricity Grid
Electricity supply and demand must be balanced continuously.
If generation suddenly exceeds demand, storage systems can:
absorb energy.
If demand suddenly exceeds generation, storage systems can:
release energy.
Fast-response technologies such as batteries, flywheels, and supercapacitors can help stabilize:
electricity grids.
Peak Demand
Electricity demand is not constant.
There are periods of:
peak demand
when many consumers require electricity simultaneously.
Storage systems can charge during periods of lower demand and discharge during:
peak demand.
This process can reduce stress on electricity generation and transmission infrastructure.
Renewable Energy Curtailment
Sometimes renewable generators could produce electricity, but the grid cannot use or transport all of it.
The generator may therefore have to reduce its output.
This is called:
curtailment.
Energy storage can sometimes reduce curtailment by absorbing electricity that would otherwise be:
unused.
Short-Term vs Long-Term Storage
Different renewable-energy challenges require different storage durations.
For fluctuations lasting seconds:
flywheels or batteries may be useful.
For shifting solar energy from afternoon to evening:
batteries or pumped hydro may be suitable.
For periods lasting several days:
long-duration storage becomes more important.
For seasonal differences:
technologies such as hydrogen may potentially play a role.
Comparing Major Storage Technologies
| Technology | Stored Energy Form | Major Strength | Major Limitation |
|---|---|---|---|
| Batteries | Chemical | Fast and efficient | Degradation and materials |
| Hydrogen | Chemical | Long-duration potential | Lower round-trip efficiency |
| Pumped hydro | Gravitational | Large capacity and long life | Geography required |
| Flywheel | Kinetic | Very fast response | Short storage duration |
| Compressed air | Mechanical/thermal | Large-scale potential | Site and efficiency challenges |
| Thermal storage | Thermal | Useful and potentially inexpensive | Heat loss |
| Supercapacitor | Electrical | Very high power and cycle life | Low energy density |
Batteries vs Hydrogen
Batteries generally offer:
- higher round-trip efficiency
- rapid response
- compact systems
- direct electrical charging
Hydrogen can offer:
- potentially very long storage duration
- large-scale storage
- transportable fuel
- use outside the electricity sector
Therefore batteries and hydrogen may serve:
different roles.
Batteries vs Pumped Hydro
Batteries can be installed in many locations and expanded in:
modular units.
Pumped hydro can store very large quantities of energy but requires:
appropriate geography.
Pumped hydro facilities may operate for many decades.
Batteries are generally easier to deploy in smaller:
distributed systems.
Batteries vs Supercapacitors
Batteries have much greater:
energy density.
Supercapacitors have much greater ability to:
charge and discharge rapidly.
Therefore:
battery → better for storing more energy
supercapacitor → better for delivering short bursts of power
Hybrid systems can use:
both.
Hybrid Energy-Storage Systems
Sometimes the best solution is to combine technologies.
For example:
Battery + supercapacitor
The battery stores substantial energy.
The supercapacitor handles sudden power demands.
Another example:
Solar + battery + hydrogen
The battery can manage daily variations.
Hydrogen could potentially store surplus energy for:
longer periods.
Future Battery Technologies
Researchers are developing new battery technologies to improve:
- energy density
- safety
- cycle life
- charging speed
- cost
- material availability
- recycling
Examples under development or expanding commercialization include:
- solid-state batteries
- sodium-ion batteries
- flow batteries
- improved lithium-ion chemistries
Solid-State Batteries
Many conventional lithium-ion batteries use a liquid or gel electrolyte.
A solid-state battery uses a:
solid electrolyte.
Potential advantages may include improvements in:
- safety
- energy density
- battery design
However, challenges remain in:
- manufacturing
- interfaces between materials
- durability
- cost
Sodium-Ion Batteries
Sodium-ion batteries use:
Na⁺ ions
instead of lithium ions as the main charge carrier.
Potential advantages include the widespread availability of:
sodium-containing resources.
They may become particularly useful where:
- low cost
- material availability
- stationary storage
are more important than maximum energy density.
Flow Batteries
A flow battery stores chemical energy in liquid electrolytes held in external tanks.
The liquids are pumped through an electrochemical cell during operation.
A major advantage is that storage capacity can be increased by using:
larger electrolyte tanks.
Why Flow Batteries Are Interesting
Flow batteries can be attractive for stationary storage because:
- energy capacity can be scaled using tank size
- they can be designed for many cycles
- long-duration storage may be possible
However, they generally have lower energy density than lithium-ion batteries.
This matters less when the battery does not need to:
move.
Future Hydrogen Technology
Future developments may improve:
- electrolyzer efficiency
- fuel-cell efficiency
- hydrogen storage
- hydrogen transport
- catalyst cost
- system durability
Hydrogen may become particularly useful in sectors where direct battery electrification is:
difficult.
Future Thermal Storage
New thermal-storage systems are exploring materials such as:
- molten salts
- rocks
- ceramics
- phase-change materials
These systems may allow inexpensive materials to store large quantities of:
thermal energy.
Phase-Change Materials
A phase-change material stores or releases energy when it changes state.
For example:
solid → liquid
can absorb thermal energy.
Later:
liquid → solid
can release thermal energy.
These materials can be useful for:
- buildings
- heating systems
- cooling systems
- industrial thermal storage
Gravity-Based Storage
Some emerging systems use heavy masses rather than water.
When excess electricity is available:
motors lift the mass.
This stores:
gravitational potential energy.
Later, the mass is lowered while driving a generator.
The principle is the same as:
pumped hydro.
Improving Recycling
Future energy storage also depends on better:
recycling.
Battery materials can be valuable.
Recycling can potentially recover materials such as:
- lithium
- nickel
- cobalt
- copper
- lead
Improved recycling could:
- reduce waste
- reduce demand for new mining
- recover valuable materials
- improve resource security
Second-Life Batteries
A battery that is no longer suitable for an electric vehicle may still have useful:
storage capacity.
It could potentially be used in less demanding stationary applications before final recycling.
This is called:
second-life use.
Evaluating Future Technologies
New technology should not be evaluated only by asking:
"Does it store more energy?"
We should also ask:
- How efficient is it?
- How expensive is it?
- How long does it last?
- Is it safe?
- Can it be manufactured at large scale?
- Are its materials abundant?
- Can it be recycled?
- What is its environmental impact?
- How quickly can it respond?
- How long can it store energy?
The best technology depends on the:
problem being solved.
Worked Example 1
A storage system receives:
800 kWh
and later returns:
680 kWh.
Calculate its round-trip efficiency.
Efficiency = output ÷ input × 100
= 680 ÷ 800 × 100
= 85%
Worked Example 2
A system is 75% efficient and receives:
200 kWh.
Useful output:
0.75 × 200 = 150 kWh
Therefore:
150 kWh
can be recovered as useful energy.
Worked Example 3
A solar farm produces excess electricity during the afternoon but the community needs more electricity after sunset.
A suitable storage system could:
store afternoon energy and release it in the evening.
A battery or pumped-hydro system might be suitable depending on the:
scale and location.
Worked Example 4
A system must provide a very large burst of power for only a few seconds.
A:
supercapacitor or flywheel
may be more appropriate than a storage technology designed mainly for long-duration energy supply.
Worked Example 5
A system needs to store a very large amount of energy for several months.
A conventional short-duration battery system may not be the most practical choice.
A technology such as:
hydrogen or another long-duration storage system
may be considered.
The final choice would depend on cost, efficiency, infrastructure, and location.
Worked Example 6
Why isn't the storage technology with the highest efficiency automatically the best?
Because other factors matter, including:
- cost
- storage duration
- capacity
- power
- location
- lifetime
- materials
- safety
A lower-efficiency system may be more useful if it can meet a requirement that the higher-efficiency system:
cannot.
Worked Example 7
A pumped-hydro system stores 1000 MWh and returns 800 MWh.
Its efficiency is:
800 ÷ 1000 × 100 = 80%
Energy not recovered:
1000 − 800 = 200 MWh
The energy was transformed mainly into less useful:
thermal energy.
Worked Example 8
Why might a grid use both batteries and pumped hydro?
Batteries can provide:
fast response and flexible installation.
Pumped hydro can provide:
large capacity and long operating life.
Combining technologies allows each to perform tasks suited to its:
strengths.
Common Mistake: Energy Storage Creates Energy
Storage systems do not create energy.
They:
store and transform energy.
Because of energy losses, they always return less useful energy than was originally supplied.
Common Mistake: Electricity Is Always Stored as Electricity
Most storage technologies convert electrical energy into another form.
For example:
Battery:
electrical → chemical
Pumped hydro:
electrical → gravitational
Flywheel:
electrical → kinetic
Hydrogen:
electrical → chemical
Common Mistake: A Fuel Cell Stores Hydrogen
A fuel cell converts chemical energy into electrical energy.
The hydrogen itself must be stored in a separate:
storage system.
The fuel cell is mainly an:
energy-conversion device.
Common Mistake: Efficiency Is the Only Important Factor
A storage system must also be evaluated by:
- cost
- capacity
- power
- duration
- lifespan
- location
- safety
- environmental impact
A highly efficient system may still be unsuitable for a particular application.
Common Mistake: Batteries Are Always the Best Storage Technology
Batteries are extremely useful, but they are not ideal for every situation.
Large-scale or long-duration storage may sometimes be better served by:
- pumped hydro
- hydrogen
- compressed air
- thermal storage
- other technologies
Common Mistake: Renewable Energy Cannot Work Without Constant Sun or Wind
Renewable generation is variable, but electricity systems can use combinations of:
- energy storage
- transmission
- different generation sources
- demand management
- geographic diversity
Storage is one important tool for matching:
energy supply with demand.
Check Your Understanding
- Define energy storage.
- Why is energy storage useful?
- Name six forms in which energy can be stored.
- What energy conversion occurs when a battery charges?
- What energy conversion occurs when a battery discharges?
- Give two advantages of lithium-ion battery storage.
- Give two limitations of battery storage.
- Explain how hydrogen can be used to store energy.
- Why is a fuel cell not technically the main energy-storage component of a hydrogen system?
- What is pumped hydroelectric storage?
- What form of energy is stored in pumped hydro?
- Write the equation for gravitational potential energy.
- Explain how a flywheel stores energy.
- Give one advantage and one limitation of flywheels.
- Describe compressed-air energy storage.
- Give three examples of thermal energy storage.
- How does molten salt store energy?
- What is a supercapacitor?
- Why are supercapacitors useful for short bursts of power?
- Why are supercapacitors less suitable for long-duration storage?
- Distinguish between energy and power.
- Convert 5 kWh into MJ.
- Define round-trip efficiency.
- A storage system receives 500 kWh and returns 425 kWh. Calculate its efficiency.
- Why can no storage system be 100% efficient?
- Identify four sources of energy loss in storage systems.
- Why is efficiency not the only factor used to compare storage technologies?
- Why is energy storage important for solar power?
- Why is energy storage important for wind power?
- What is peak demand?
- What is renewable-energy curtailment?
- Explain how storage can reduce curtailment.
- Compare batteries and hydrogen storage.
- Compare batteries and pumped hydro.
- Compare batteries and supercapacitors.
- Why might an electricity grid use several different storage technologies?
- What is a hybrid energy-storage system?
- What is a solid-state battery?
- What is a sodium-ion battery?
- Describe how a flow battery works.
- Why can flow batteries be useful for stationary storage?
- What is a phase-change material?
- Explain the basic principle behind gravity-storage systems.
- What is meant by second-life battery use?
- Why is battery recycling important?
- A storage system is 70% efficient and receives 600 kWh. Calculate the useful energy output.
- Explain why long-duration storage may become increasingly important as renewable electricity generation increases.
- Compare at least four energy-storage technologies in terms of energy form, efficiency, duration, cost, and limitations.
- Evaluate which storage technology might be suitable for storing solar energy from afternoon until evening.
- Explain why the future electricity grid may use a mixture of different energy-storage technologies rather than relying on only one.
Key Terms
Energy storage: Capturing energy for use at a later time.
Battery energy storage system (BESS): System using rechargeable batteries to store electrical energy as chemical energy.
Hydrogen storage: Storage of energy in the chemical form of hydrogen.
Pumped hydroelectric storage: Storage of energy by pumping water to a higher elevation.
Gravitational potential energy: Energy stored because of an object's position in a gravitational field.
Flywheel: Device that stores energy as rotational kinetic energy.
Compressed-air energy storage: Storage of energy using compressed gas.
Thermal energy storage: Storage of energy as heat or cold.
Supercapacitor: Device capable of rapidly storing and releasing electrical energy.
Round-trip efficiency: Percentage of input energy recovered as useful output after storage.
Energy capacity: Total amount of energy a system can store.
Power: Rate at which energy is transferred.
Curtailment: Reduction of electricity generation because the available electricity cannot currently be used or transported.
Long-duration energy storage: Storage designed to supply energy over extended periods.
Flow battery: Rechargeable electrochemical system using liquid electrolytes stored in external tanks.
Solid-state battery: Battery using a solid electrolyte rather than a conventional liquid or gel electrolyte.
Sodium-ion battery: Rechargeable battery using sodium ions as charge carriers.
Phase-change material: Material that stores or releases thermal energy during a change of state.
Second-life battery: Used battery repurposed for a less demanding application before recycling.
Key Takeaways
- Energy storage captures energy so that it can be used later.
- Storage systems usually convert energy into another form rather than simply storing electricity directly.
- Batteries store energy as chemical energy.
- Hydrogen stores chemical energy and can later supply a fuel cell.
- Pumped hydro stores gravitational potential energy.
- Flywheels store rotational kinetic energy.
- Compressed-air systems store energy using compressed gas.
- Thermal systems store energy as heat or cold.
- Supercapacitors can absorb and release energy extremely quickly.
- Different technologies are suitable for different timescales and applications.
- Round-trip efficiency compares useful energy recovered with energy originally supplied.
- No real storage system is 100% efficient.
- Efficiency is important, but so are cost, capacity, power, duration, lifetime, safety, location, and environmental impact.
- Energy storage is particularly important for variable renewable sources such as solar and wind.
- Storage can shift renewable electricity from times of high generation to times of high demand.
- Storage can help stabilize electricity grids and reduce renewable-energy curtailment.
- Batteries are useful for fast response and short-to-medium-duration storage.
- Pumped hydro can provide very large-scale storage where suitable geography exists.
- Hydrogen may have advantages for some forms of long-duration or seasonal storage, although its round-trip efficiency is generally lower.
- Future technologies include solid-state batteries, sodium-ion batteries, flow batteries, advanced thermal storage, hydrogen systems, and gravity storage.
- Future energy systems will likely use multiple storage technologies, because no single technology is ideal for every energy-storage problem.