A review of current energy storage technologies
Characteristics and potential applications
Although large scale energy storage already existed in the beginning of last century in the form of PHS*, the energy storage sector expanded and gained more importance only in the recent years. The main reasons are the rise of renewable energies, the rising interest for electric cars, the need for backup electricity in case of natural disaster and more generally, the need to update the energy sector. As such, a broad variety of storage solutions have emerged as well as new promising applications for energy storage. Today, these solutions rely on very different mechanisms and consequently show potential for certain applications while not being suited for others. Despite such variety, storage solutions can be grouped in 5 different families :
- Mechanical: converts electricity into potential or kinetic energy to be stored.
- Electrochemical: stores energy in batteries, the incoming current is used to trigger chemical reactions which can be reverted when energy is needed.
- Electrical: stores energy in an electric field or a magnetic field powered via incoming current.
- Chemical: incoming energy is used to synthesize a chemical which can later be used a fuel to produce energy.
- Thermal: incoming electricity is used to heat a material/liquid in an isolated reservoir.
Figure 1. Main energy storage technologies grouped in their family
Information source: Five Steps to Energy Storage Insight Brief – World Energy Council
Used by permission of the World Energy Council
Among these technologies, many are still in development and more research and experimentation is needed to make them profitable for market-driven diffusion. Because it is the most mature technology, PHS represents the major part of energy storage and proves to be very useful to store large quantities of energy over long periods. Nevertheless, PHS can’t solve all problems and has its drawbacks too: because of slow discharge reactivity, it can’t be used to quickly restore the frequency in the grid or to provide emergency backup. Even though storing energy this way today is simple, efficient and economical, the infrastructure needs a lot of investment and about 3 to 5 years to be installed . Construction is a challenge as two reservoirs are needed and suitable locations are limited. Because of this, only 300 projects have been commissioned since 1950 approximately . In comparison, while the capacity of storing electricity is relatively smaller, other technologies like Li-ion batteries are capable of delivering more energy at a quicker pace. Because of its high efficiency, high energy density, little needs for maintenance and low weight. Li-ion batteries have received a lot of attention and also represent an important part of the storage market today. They also progressively replaced the lead acid/Pb-A batteries which were not as efficient, durable and dense in energy . They are very useful to supply energy on a short period of time. Li-ion batteries are also used to assist the implementation of renewables into the grid despite some security concerns due to accidents like the fire and explosion of Arizona Public Service’s Surprise Battery Storage in April 2012 and the similar incidents which happened later in Europe and in South Korea in 2019 . . Unfortunately, the lifetime of Li-ion batteries is quite short, and there are still no proper recycling methods because they are complicated and dangerous to dismantle. There are also costly to use today compared to other batteries like VRFB or Na-S batteries . This is why many countries are investing in new types of storage such as flywheels, CAES* or flow batteries to find alternatives.
Applications for energy storage vary from very small scale to large scale, and most storage technologies tend to be suited for large scale or small scale exclusively but not both of them. The main reasons for that are the differences in characteristics such as the size of the storage capacity and the response time to discharge energy into the grid. Some solutions have high maintenance costs or progressively lose energy via self-discharge when storing energy for a long time, consequently they are more profitable to use on a shorter term. There are many possible applications as mentioned before but depending on the discharge reactivity and time, it is possible to distinguish 3 main types of applications:
- Fixing power quality: use the most reactive technologies with response duration of 1 second and below to maintain the power frequency flowing in the grid. Applications include frequency stabilization or acting as a spinning reserve to feed energy into the grid very quickly when needed. The most suited solutions for this are flywheels, SMES*, Supercapacitors or batteries with rapid discharge rates like Na-NiCl2 . The power cost ($/kW) of these solutions is more important than the energy cost ($/kWh) as the main focus is to improve the power quality, not to deliver large amount of energy into the grid. The above solutions are therefore interesting because their power cost is much lower than their energy cost.
- Bridging power: delivering power during transition between two sources of power. This concerns reactive technologies with response duration of a few seconds maximum but with discharge duration higher than the previous solutions to make sure that power can be discharged into the grid during the transition time. Applications include emergency back-up or voltage regulation. The most suited technologies for this are usually batteries like Na-NiCl2, Na-S, Ni-Cd, Ni-MH or lead acid batteries .
- Energy management: the remaining solutions with slower response duration, the ability to store energy during longer times or to discharge their energy into the grid at a slower pace will be used to provide energy to the grid when it is needed or to store it when they are excesses. Because these solutions focus on providing energy rather than power, the energy cost ($/kWh) is more important than the power cost ($/kW). Applications include energy arbitrage, emergency back-up, black start, peak shaving, load levelling, seasonal storage… The most suited technologies for this are PHS, CAES, Li-ion batteries, VRFB*, MSES*, Hydrogen… . These solutions are also the most suited to help for the integration of renewables into the grid or to anticipate/partly replace network expansion. Today, the cheapest solutions are PHS and CAES, but the energy costs of newer technologies (especially batteries) are decreasing quickly, so they are also very competitive.
Even if in one storage category, there are many different solutions (flywheels and PHS both rely on mechanical energy to operate but they have very different characteristics and applications for example), there are still some trends related to them. Electrochemical storage technologies tend to have bigger energy densities than the others for example, but they also usually have more negative impacts on the environment . Among electrochemical storage technologies, solid state and flow batteries try to improve the risks caused by classic batteries which often operate at high temperatures and their complicated maintenance/recycling process because of corrosion . Classic batteries have a lifetime of 7 to 15 years approximately. Solid state batteries and flow batteries have a better life expectancy but they still need more development to be profitable . In general batteries seem to be more adapted for occasional discharge rather than frequent discharge because it increases their life expectancy . Electrical storage technologies have the shortest response delays and discharge rates, they are all very efficient technologies and have a long lifetime but they remain very expensive today. They are more suitable to frequent discharge utilizations because of their longer life expectancy than batteries . Chemical storage solutions like hydrogen or ammonia are very promising solutions which would be very practical to use as they would easily fit in our current system. However, they remain too expensive and not so effective today . Moreover, in theory they don’t have an impact on the environment but in reality they heavily rely on fossil fuels to work properly today . Mechanical storage technologies are usually costly to manufacture and set up. They need a lot of investment, but they also provide the cheapest energy during operation (except flywheels) . They are more suitable to large scale storage and energy management applications because of their longer discharge period, but they can also handle frequent discharge/charge cycles thanks to their longer life expectancy . Thermal storage is both suited for large and small scale applications. It has potential applications related to heating and cooling. Consequently, it could be installed in small quantities in small communities to provide heat as well as a being associated with solar power plants to store the electricity produced .
As a lot of improvements are currently made via research and development, improved versions of current technologies could be developed. If some issues related to the previous versions are solved, then technologies will be eligible to a larger range of applications.” For example, batteries with a short life expectancy and high self discharge rates could also be used for energy management over longer period if these issues are solved.” New more efficient technologies already partly replace the old ones: lead-acid batteries are considered to be outdated compared to Li-ion batteries, and solid state batteries, VRFB or other flow batteries could also replace Li-ion batteries in the future if they were to become more affordable. Similarly, Ni-MH batteries have more advantages than the current Ni-Cd batteries and could potentially replace them in the future. There is also potential for hybrid supercapacitors to replace supercapacitors and liquid air storage to replace CAES in the future, but these technologies need a lot more research to be improved first. For each application, the perfect storage solution doesn’t exist yet, but as people today come up with a lot of new ideas and new methods to improve them, there is a lot to expect from storage technologies in the future.
Possible applications for storage:
Power quality: Fixing power supply as a secondary power source when needed
Energy arbitrage: On the market, buy energy at a particular time/place for a low price, then go somewhere else and/or wait and sell it for a higher price to make benefit.
RES Integration: Associate with renewable energy to facilitate their integration in the grid: stabilize their energy output when it is intermittent.
Emergency back-up: Provide energy supply in case of emergencies.
Peak shaving: Store energy during peak production times to avoid wasting unused energy.
Time shifting: Re-emit energy stored later in time depending on prices to gain money
Load levelling: Store energy when not so much is needed, emit energy when during peak energy demand times.
Black start: Restart energy supply in particular places after energy supply is suddenly stopped, without the help of an outside source like the power grid. Useful in case of emergencies when some places are temporarily cut off from the grid or the grid can’t supply energy.
Seasonal storage: Store energy over seasons.
Spinning reserve: Feed energy very quickly in the grid when needed. The storage solution is linked to the network but doesn’t operate unless asked to.
Network expansion: Act as a part of the network in itself in order to replace/delay network expansion which can be costly and time-consuming.
Network stabilization: Help balancing network instabilities like frequency resonance or unstable voltage.
Voltage regulation: Make sure the energy and power flows regularly to match demand accurately.
End-user services: Offer customers the possibility to reduce their electricity bills by doing energy arbitrage themselves for example.
Reference for this part: World Energy Council (2020) -Five Steps to Energy Storage .
PHS: Pumped Hydro Storage
CAES: Compressed Air Energy Storage
SMES: Superconducting Magnetic Energy Storage
MSES: Molten Salt Energy Storage
VRFB: Vanadium RedOx Flow Batteries
-  World Energy Council (2020). Five Steps to Energy Storage. https://www.worldenergy.org/assets/downloads/Five_steps_to_energy_storage_v301.pdf
-  EESI (2019, February). Fact Sheet | Energy Storage. https://www.eesi.org/papers/view/energy-storage-2019
-  Pumped Storage Tracking Tool. Retrieved May 10th, 2021 from https://www.hydropower.org/hydropower-pumped-storage-tool
-  Energy Storage. Retrieved April 19, 2021 from https://www.iea.org/reports/energy-storage
-  Henok A.B., Maarten M., Thierry C., Maitane B., Kinde A.F., Abraham A., Joeri V.M. (2020, December). A Review of Energy Storage Technologies’ Application Potentials in Renewable Energy Sources Grid Integration. Research Gate. https://www.researchgate.net/publication/347270076_A_Review_of_Energy_Storage_Technologies%27_Application_Potentials_in_Renewable_Energy_Sources_Grid_Integration
-  Md Mustafizur R., Abayomi O.O., Eskinder G., Amit K. (2020, November 1st). Assessment of energy storage technologies: A review. Science Direct. https://www.sciencedirect.com/science/article/abs/pii/S0196890420308347
-  IRENA (2017, October). Electricity Storage and Renewables: costs and market to 2030. https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2017/Oct/IRENA_Electricity_Storage_Costs_2017_Summary.pdf?la=en%26hash=2FDC44939920F8D2BA29CB762C607BC9E882D4E9
-  Susan M. (2011, January). Research Gate. Energy Storage Systems Cost Update: A Study for the DOE Energy Storage Systems Program. https://www.researchgate.net/publication/255240406_Energy_Storage_Systems_Cost_Update_A_Study_for_the_DOE_Energy_Storage_Systems_Program
-  Amela A., Albert H., Reinhard H. (2020, June 1st). On the role of storage for electricity in smart energy systems. Science Direct. https://www.sciencedirect.com/science/article/abs/pii/S0360544220305806
-  Future Bridge (2020, September 9th). Green Ammonia for Energy Storage. https://www.futurebridge.com/industry/perspectives-energy/green-ammonia-for-energy-storage/