There are several key methods used for storing energy in modern power systems. The choice of energy storage technology depends on the specific application, with factors like response time, storage capacity, capital cost, lifespan, and efficiency playing important roles. Common energy storage methods include pumped hydropower, compressed air energy storage (CAES), flywheels, electrochemical batteries, hydrogen storage, thermal energy storage, and supercapacitors. This article will provide an overview of these key energy storage technologies, their strengths and limitations, and example use cases.
Pumped Hydropower Storage
Pumped hydropower is currently the largest-capacity and lowest-cost form of grid energy storage available. With this method, water is pumped from a lower reservoir to an upper reservoir when electricity demand is low. Then when demand is high, the stored water is released from the upper reservoir through a turbine to generate electricity. Pumped hydro facilities can respond very quickly to changes in electricity demand and offer large storage capacities. However, geographical constraints limit widespread deployment.
How It Works
Pumped hydro storage works by using excess electricity to pump water uphill into a reservoir. When electricity demand is high, the water can be released back down through a turbine to regenerate electricity. The upper and lower reservoirs allow the storage of large amounts of potential energy.
Efficiency
Pumped hydro storage facilities typically have round-trip efficiencies of 70-80%. This means about 20-30% of the electricity is lost in pumping the water uphill and generating electricity. The cycle of pumping and generating leads to some energy losses.
Capacity and Power Rating
Currently, pumped hydro storage has a total worldwide capacity of around 127 gigawatts (GW), providing around 97% of all utility-scale energy storage. Individual facilities can have capacities from 1-3 GW. Power ratings range from 1-1000 megawatts (MW). Larger capacity and power outputs require bigger reservoirs and turbine generators.
Response Time
Pumped hydro storage can respond very quickly to changes in demand, making it well-suited for grid stability. Turbines can ramp from zero to full power in just seconds to minutes. Quick response times help match supply with unpredictable renewables.
Storage Duration
The amount of energy able to be stored depends on the size of the upper reservoir. Larger reservoirs allow more storage capacity. Typical storage durations range from 8-16 hours, but some facilities can store up to an entire season’s worth of energy.
Geography and Topography Needs
Suitable geography and topography are required for pumped hydro sites. An upper and lower reservoir at different elevations are needed, connected by pipework. Hilly or mountainous regions near loads offer the best sites. Suitable sites are geographically limited.
Other Considerations
Advantages of pumped hydro include low maintenance costs and long lifetimes of around 50 years. Main limitations are geographic constraints, high capital costs, and potential environmental impacts. Overall, pumped hydro offers reliable, large-scale energy storage, but suitable sites are limited.
Compressed Air Energy Storage (CAES)
Compressed Air Energy Storage (CAES) stores energy by compressing and storing air in underground reservoirs. When electricity is needed, the pressurized air is heated and expanded through a turbine to generate power. CAES provides large-scale storage capabilities.
How It Works
During low electricity demand, a motor compresses air which is stored under pressure in an underground reservoir. To regenerate electricity, the air flows back up, is heated, and expands through a turbine, spinning a generator. The compression and expansion stages use natural gas.
Efficiency
CAES has an overall efficiency of around 50-70%. About two-thirds of energy is lost in the compression, storage, and expansion process. The use of natural gas for heating improves efficiency but results in some emissions.
Capacity and Power Rating
There are only two large-scale CAES plants worldwide, with capacities of 110-290 megawatts (MW) and storage capacities of up to 3000 megawatt-hours. Individual CAES plants can have power ratings from 10-400+ MW. Larger underground reservoirs allow greater storage capacity.
Response Time
CAES systems can ramp up to full power generation within minutes, providing quick response times. Quick start-up and shutdown times help balance electricity supply and demand efficiently.
Storage Duration
CAES reservoirs can store large amounts of high-pressure air. This allows long storage times from hours up to even months. Long storage duration makes CAES suitable for both daily peak shaving and seasonal storage needs.
Geography Needs
Suitable geology is needed to develop underground air storage reservoirs. Most sites use salt caverns, aquifers, or depleted natural gas fields for the air storage. This limits potential CAES sites.
Other Considerations
CAES provides large-scale, long-duration energy storage. Key advantages include quick response times and low capital costs compared to alternatives. However, opportunities are geographically limited and use some natural gas.
Flywheel Energy Storage
Flywheel energy storage works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When electricity is needed, the flywheel’s kinetic energy is converted back into electricity.
How It Works
A motor spins up a heavy rotor to very high speeds of 20,000-50,000 rpm. This stores rotational kinetic energy. When electricity is needed, the flywheel’s momentum drives a generator to produce electricity.
Efficiency
Flywheel storage can achieve efficiencies over 90% as there are minimal energy conversion losses. The enclosing vacuum reduces friction allowing sustained low-loss operation.
Capacity and Power Rating
Individual flywheels typically have storage capacities from 3-25 kilowatt-hours. Outputs range from 100 kW to 1 MW. Flywheels are often grouped together for higher capacities. Storing more energy requires bigger and heavier rotors.
Response Time
Flywheels offer very rapid response times. Full discharge can occur within seconds, while full charge takes minutes. This makes them ideal for smoothing short-term fluctuations in electricity.
Storage Duration
Discharge durations are short, ranging from seconds up to around 15 minutes. Other technologies are better suited for longer term storage needs.
Geography Needs
Flywheels have relatively few geographical restrictions. They can be sited close to where the electricity is needed. Small modular systems make distribution easier.
Other Considerations
Flywheel advantages include high efficiency, fast response, and long lifetime. Limitations include short discharge times and high self-discharge. Overall, flywheels excel at power quality applications.
Electrochemical Batteries
Batteries store energy in electrochemical form as chemical potential between two electrodes. Many types of batteries are used for grid energy storage, including lithium-ion, sodium-sulfur, lead-acid, nickel-based, and flow batteries.
How It Works
During charging, energy drives a reaction forcing electrons to the battery’s positive electrode. Discharging reverses the reaction and electrons flow from the negative to positive electrode producing electricity.
Efficiency
Battery round-trip efficiencies range from around 65-90%, depending on battery chemistry and operating conditions. Lead-acid batteries have lower efficiencies of about 65-75%, while lithium-ion can reach 85-95%.
Capacity and Power Rating
Battery capacities vary widely from kilowatt-hours to megawatt-hours depending on battery size and chemistry. Similarly, power ratings range from kilowatts to megawatts. Larger battery banks allow greater capacity and power.
Response Time
Batteries have fast response times, often able to fully charge or discharge within milliseconds to seconds. Fast ramping helps balance grid fluctuations.
Storage Duration
Discharge durations vary from minutes to hours. Flow batteries with actively supplied electrolytes can sustain longer discharge times. Faster responding batteries handle short durations well.
Geography Needs
Battery storage has minimal geographic limitations. Batteries can locate close to generation sources or end-users. Smaller modular systems enable distributed siting.
Other Considerations
Key battery advantages include fast response, flexibility, and low maintenance. Limitations include shorter lifetimes and decreasing capacity over time. Overall, batteries serve short and medium duration storage needs.
Hydrogen Energy Storage
Hydrogen can provide very large-scale, long-term energy storage. Excess electricity is used to electrolyze water into hydrogen and oxygen. The hydrogen can then be stored and later used in fuel cells or combusted to generate electricity.
How It Works
During low demand, electricity powers electrolysis splitting water into hydrogen and oxygen. The hydrogen is compressed and stored. Later, fuel cells convert the hydrogen back into electricity, or it is burned in generators.
Efficiency
Overall system efficiency is low, around 30-45%. Losses occur during electrolysis, compression, storage, and reconversion to electricity. Technical improvements to electrolyzers and fuel cells will enhance efficiency.
Capacity and Power Rating
Hydrogen storage capacities are only limited by the size of the storage infrastructure. Underground storage caverns or tanks can store massive amounts of energy. Similarly, power ratings depend on the scale of generation equipment and can be very large (e.g. 100+ MW).
Response Time
Hydrogen systems response times are relatively slow overall. However, hydrogen turbines and fuel cells can provide electricity within minutes once the hydrogen fuel is supplied.
Storage Duration
A key advantage of hydrogen storage is extremely long storage duration – potentially months or even years. The hydrogen remains stored until needed.
Geography Needs
Suitable geology such as salt caverns are needed for large scale underground hydrogen storage. Otherwise, above ground tanks can store hydrogen almost anywhere.
Other Considerations
Hydrogen allows very large energy storage capacity and long storage times. Disadvantages are low efficiency and high costs currently. Hydrogen storage remains an active research area.
Thermal Energy Storage
Thermal energy storage (TES) involves storing energy as heat or cold to be used for heating/cooling at a later time. Common techniques include sensible heat storage, phase change materials, and thermochemical reactions.
How It Works
Sensible heat TES stores thermal energy by heating or cooling a storage material. Phase change materials melt or solidify to store latent heat. Thermochemical reactions use reversible chemical reactions to absorb and release heat when needed.
Efficiency
TES systems can achieve reasonably high efficiencies of around 60-90%. Phase change materials and thermochemical reactions often have higher efficiencies than sensible TES. Heat loss from the thermal reservoir is the main limiting factor.
Capacity and Power Rating
TES capacities range from kilowatt-hours to megawatt-hours. Large tanks of phase change materials or mineral reactions can store hundreds of megawatt-hours of energy. Power ratings span kilowatts to tens of megawatts.
Response Time
Discharge times depend on the temperature differential and system design, but are typically hours to days. This makes TES more suitable for longer term storage rather than short bursts. Response time is slower than batteries.
Storage Duration
The stored thermal energy remains until needed, allowing storage durations ranging from daily cycles to even seasonal storage. Long storage duration is a major benefit for TES systems.
Geography Needs
Thermal reservoirs require insulation but otherwise have few geographical limitations. Storage near end-users helps reduce heat losses in transport. Underground pits are an option for larger reservoirs.
Other Considerations
Advantages of TES include low costs and long storage times. Heat losses can be a disadvantage for longer term storage. Overall, TES provides an efficient option for daily to seasonal thermal energy storage needs.
Supercapacitors
Supercapacitors (SCs), also called ultracapacitors, store energy in electric fields between two electrodes. They have properties between traditional capacitors and batteries, offering high power density but lower overall storage capacity compared to most batteries.
How It Works
Charging builds up positive and negative charges on each electrode separated by an ion-permeable membrane. The electric field between the electrodes stores energy. Discharging connects the electrodes and releases the energy.
Efficiency
Supercapacitor efficiency can reach as high as 95-98%. Very high power density enables efficient storage and release of bursts of energy.
Capacity and Power Rating
SCs have high power density up to 10 kW/kg but low energy density around 5 Wh/kg. Capacities are typically a few kilowatt-hours while power ratings are a few hundred kilowatts.
Response Time
Extremely fast response time reaching full power draw in milliseconds is a major advantage of SCs. This makes SCs ideal for short, high power events.
Storage Duration
Discharge durations are very short, from seconds up to about a minute. Other technologies are better suited for longer term storage needs.
Geography Needs
Minimal geographic limitations. Small size allows distribution near end users. Can locate anywhere near generation or demand.
Other Considerations
Key pros of SCs are high power density, fast response, and long lifetime. Downsides are low energy density and high costs. SCs fill a niche role for short bursts of power.
Comparison of Technologies
| Technology | Efficiency | Discharge Time | Capital Cost | Lifetime |
|---|---|---|---|---|
| Pumped hydro | 70-80% | Hours-Months | High | 50+ years |
| CAES | 50-70% | Hours-Months | Low | 20-40 years |
| Flywheels | 90%+ | Secs-Mins | High | 20+ years |
| Batteries | 65-95% | Mins-Hours | Moderate | 5-15 years |
| Hydrogen | 30-45% | Hours-Months | High | Long with maintenance |
| TES | 60-90% | Hours-Months | Low | 10-20 years |
| Supercapacitors | 95-98% | Secs-Mins | Very High | 10-20 years |
Conclusion
In summary, pumped hydropower and compressed air offer large-scale, long-duration storage, while flywheels and supercapacitors are well-suited for short duration smoothing. Batteries balance duration, efficiency, and modularity. Hydrogen and thermal energy storage can provide very long but inefficient storage. Overall, each technology has strengths and weaknesses that determine suitable applications depending on the specific needs of the grid. Ongoing research aims to improve efficiency, capacity, and costs to support grid modernization through energy storage.