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Abstract—While generation of energy is a very important task,

storing the energy is just as paramount. There have been numerous methods of storing energy throughout history, and it is a subject that is still heavily researched to this day. In order to understand the necessity of energy storage and why there is such a focus on advancement of its technology, various methods must be observed along with the benefits and issues with each.

I. HISTORY Energy storage is a vital technology that is vital to technological advancement both past and present. It allowed for portable electronics as early as the late 19th century, such as electric cars and flashlights. Grid balancing techniques rely on energy storage to be as efficient as possible. In the future, it could help stabilize intermittent power sources like wind and solar by providing a steady stored output.

Almost all energy storage is some form of potential energy. Storage devices, to give some examples, can be mechanical, gravitational, electrical, or thermal in nature. The only widely used kinetic energy storage is flywheels, which can convert electrical potential to rotational kinetic and back again.

Another factor to consider when looking at energy storage is the length that the energy is intended to be stored. Different systems produce different losses that make them suitable only for a certain timeframe of storage. Capacitors are intended to store electrical energy for a much shorter time than lithium-ion batteries. Each system is suited for a different purpose.

Early developments in energy storage were almost exclusively mechanical or gravitational potential energy storage. Reservoirs and dams, notably, were used to store and release water that could supply drinking water or be used to power water wheels [1].

The first practical advance in electrical energy storage was the Leyden jar. It was developed independently by two different European scientists in 1745-1746: Ewald Georg von Kleist and Pieter van Musschenbroek [3]. It stores static electricity between two electrodes on the inside and outside of a glass jar, which is filled with a liquid. It is considered to be the earliest useful form of capacitor and battery. Both stored voltage and capacitance could be changed by changing the properties of the jar. Leyden jars allowed scientists obtain enough charge to perform experiments, leading to other developments.

Pumped-storage hydroelectricity was used as early as the 1909 s in Switzerland [1]. It pumps water into a heightened reservoir to run through turbines later. Pumping losses actually make these facilities net consumers of power. These losses were lessened by the implementation of reversible turbine-generator assemblies that act as both a pump and turbine in the

1920s [2]. Because the pumps are only run during cheaper off-peak energy hours though, it remains profitable when more expensive power is sold during peak hours. It is also important as a grid-balancing measure so that power isn't wasted in large amounts.

Compressed air storage works on a similar concept as pumped-storage hydroelectricity. Cheap electricity is used to compress air, which is used later during peak hours. Paris had a system working as early as 1870, which generated 2.2 MW to be used with specific pneumatic machines. The first utility scale project was built in Germany in 1978, which was 290 MW scale [4]. The compressed air is often used to run turbines, which produce electricity. It is stored in tanks or sometimes in abandoned mines, which provide ample enclosed space. Ice storage air conditioning operates much like the other "off-peak" electricity technologies. It creates ice to be used in lieu of air conditioning later during peak hours. It is unusual in that it is a more widely available storage technique as it can be installed in businesses and homes. It isn't as generally useful though, because it can only be used as air conditioning.

Flywheels are rotors accelerated by usually electrical means that store energy as rotational kinetic energy. It slows down as it converts energy back to electricity. Modern flywheels date back to the 1950s when smaller systems were made bigger and more efficient for larger scale energy storage [5]. The rotor itself exists in a vacuum and has magnetic bearings to reduce friction losses as much as possible.

Batteries are important to several technologies, including portable electronics. Practical batteries were developed in 1836 when the Daniels cell introduced two (wet) electrolytes. The modern dry cell batteries are smaller, more efficient, and were developed in 1949 [1]. Rechargeable batteries were developed in the 1970s and have been improved until the present day.

II. STATE OF THE ART DESIGNS With continued research over the years, new, state of the art designs have been developed in order to improve upon older, historical methods of energy storage.

Research on Energy Storage Derek Lusby, Lu Wang, Tyler McGraw, Edward Palmer, Nathan Peck, and Jake Woods

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A. Lithium Poly-Sulfide Flow Battery The Lithium Poly-Sulfide flow battery differs from traditional redox flow batteries. Today’s redox based flow batteries have two streams of high cost liquids that are pumped through the system and pass through a high cost membrane where the streams undergo chemical reactions to wither produce or store energy. The Lithium Poly-Sulfide flow battery differs in two main ways. The Lithium Poly-Sulfide flow battery only uses one stream of relatively cheap Lithium Poly-Sulfides, which are dissolved in an organic solution to conserve the life of the battery. The Lithium Poly-Sulfide battery also does not use an expensive membrane. Instead the Lithium anode is coated with a barrier that protects the raw Lithium but still allows electrons to freely pass. During the discharge cycle, Lithium Ions are absorbed by the Lithium Poly-Sulfide molecules. During the charging cycle, the Lithium ions are lost back into the organic solution.

Fig. 1. Common Redox Flow battery design and state of the art Lithium Poly-Sulfide flow battery design

B. Hydrogen Storage The hydrogen energy storage uses a Hydrogen generator to

produce Hydrogen, which can be stored as a fuel for later use. The Hydrogen generator uses a battery-like design to produce Hydrogen from water through the process of electrolysis. An anode and cathode are submerged in water and when an electrical current is passed between the two, protons are able to pass through a Proton Exchange Membrane (PEM) where the protons combine with electrons to produce stable Hydrogen. This stable Hydrogen can then be stored in large tanks and are converted to energy when demand is high.

Fig. 2. Diagram of Hydrogen Generator

C. Pumped Water Storage During the off peak hours, water is pumped up a hill into a

reservoir for storage. These pumps are powered using excess off-peak energy, which is cheap and available. During times of high-energy demand, the water is released from the reservoir where gravity pulls it down through turbines generating electricity. This system as a whole has losses associated, but since the high-demand energy can be sold at a premium, a profit can be made.

D. Compressed Air Storage Compressed air storage uses the same principle as pumped

water storage. During low energy demand, excess energy is used to run pumps, which compress air into large underground reservoirs. When the energy is in high demand, a release valve is opened allowing the compressed air to shoot through air-turbines, which recovers the stored energy in the form of electrical energy, which can be sold at a premium price.

E. Flywheel Energy Storage Flywheels work in the same way as electrical motors and

generators work. The flywheel is supported by magnets and enclosed in a vacuum. During off-peak hours, excess energy is used to spin the flywheel to a very high speed just like an electrical motor. Then when the energy is in demand, the flywheel acts like an electrical generator and converts the kinetic energy of the flywheel into electrical energy, which slows the flywheel back down. This generated energy is then sold at a premium.

III. BENEFITS AND VALUES Each method of energy storage has certain benefits that make it a viable method.

A. Batteries Batteries have the most well known benefits compared to

other types of electrical energy storage. They are the most widely used storage due to their size and weight. From AAs to car batteries, individuals have the ability to manually change them and apply them to other applications. The capacity of different batteries changes, thus they can be designed to fit different products. Battery systems also are more reliable and

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run for a longer timespan compared to other methods such as flywheels and ultra-capacitors. Depending on the requirements of a particular system, batteries can be combined easily to apply for different uses.

B. Flow Batteries Flow Batteries are a special type of storage that has been

developed recently. The greatest benefit this type of battery provides is the increased utility of renewable energy storage. These batteries are efficient enough to be used during periods of peak demand for systems that are more dependent on renewable energy sources. Due to the lack of solid-solid phase transitions, flow batteries of long cycle lives. Flow batteries have the potential to be almost instantly recharged by replacing the electrolyte liquid while simultaneously recovering the spent material for re-energization. Some models of flow batteries offer very good tolerance to overcharging and over discharging. The power and energy are separated in the flow batteries with makes designing these for application very flexible. The design can be tailored specifically to the load or generating asset.

C. Hydrogen Storage Hydrogen energy storage is a method of converting

electricity to hydrogen using electrolysis. This method has much higher energy storage capacity compared to batteries or pumped hydro and CAES systems. Electrolytic hydrogen can be used for the production of synthetic liquid fuels from biomass; this significantly increases the efficiency of biomass utilization. Hydrogen has the ability to hold 120MJ/kg; this means that a small amount is needed to hold a significant amount of energy. Because hydrogen is a stable element, energy can be stored for a longer period of time compared to other mediums. Hydrogen fuel cells have a fast response time making them able to correct rapid fluctuations in electricity demand or supply, regulating the frequency.

Fig. 3. Discharge time versus storage capacity graph

D. Pumped Water Storage Pumped-storage hydroelectricity is mainly utilized to

balance out electricity during peak hours of use. This method

is also the most cost efficient after taking capital costs and geographic features into consideration. The graph below shows the amount of power consumed (green) compared to the amount of power generated (red).

Fig. 4. Pumped hydroelectric power graph

If a large body of water or a large variation in height is available, pumped storage has a relatively low energy density. Compared to thermal plants and their reactions to sudden changes in electrical demand, hydro storage can respond to load changes within seconds.

E. Compressed Air Storage Compressed Air Energy Storage is a method that reuses

underground tunnels and caverns as way to store electrical energy. If a salt cavern can be utilized, this method of storage obtains much higher flexibility due to the lack in pressure losses within the storage. CAES methods can generate three times the output of the same natural gas methods because CAES lacks the compression stage. The compression stage uses up 2/3 of the turbine capacity. This also reduces the gas consumption and reduces CO2 emissions by 40 to 60%.

F. Flywheel Energy Storage Flywheel energy storage is a method that uses kinetic energy

to store electricity. When compared to other methods of storing electricity, flywheels have very long lifetimes that required little to no maintenance. Full-cycle lifetimes for flywheels have been quoted at ranges from 105 up to 107 cycles of use. Flywheels also have high energy densities ranging from 360-500 kJ/kg and have large maximum power outputs. Flywheel energy efficiencies are also usually around 90%, which makes them one of the most efficient methods of storing energy. The capacity range of typical flywheels ranges from 3kWh to 133kWh.

G. Superconducting Magnetic Superconducting magnetic Energy Storage systems store

energy in the magnetic field created by the flow of direct current in a superconducting coil. These systems have a round –trip efficiency greater than 95%. The delay time during charge and discharge is very short. Power is available almost instantly and very high power output can be provided for a short duration. Not only do these systems provide stability to

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the grid, it has next to no environmental issues. SMES does not produce harmful chemicals, does not require radical changes to the landscape, and it is silent in operation.

H. Double-layered Capacitors Double Layer Capacitors are a new type of electrochemical

capacitors called super capacitors. These capacitors have the highest available capacitance values per unit volume and the greatest energy density of all capacitors. They have capacitance values of 10,000 times that of electrolytic capacitors. Super capacitors have power densities that are from 10-100x greater than conventional batteries. Because of this high density, the charge and discharge cycles are much faster than batteries as well. Superconductors have greater than 1000 cycles of charge and discharge, which means that they will last for the entire lifetime of most devices. This makes superconductors environmentally friendly. The internal resistance of the capacitors is low and consequently makes the cycle efficiency very high (95% or more). These capacitors use non-corrosive electrolytes and low material toxicity, which makes them, have improved safety.

Fig. 5. Energy density versus power density graph

IV. CONCERNS AND PROBLEMS Each method of storage also has a set of issues that keep it from performing ideally.

A. Pumped Water Storage Pumped hydroelectric storage a relatively high efficiency

method of energy storage, yet the maximum return is only in the range of 70 to 85% [23]. Aside from efficiency issues, this method requires a mountainous area, as it gravity to generate hydroelectric power from the reservoir water. This limits the use of the method to very specific geographic locations [23]. As this method requires mountains, electrical equipment must be installed in a mountainous region, such as transmission lines that must run from possibly isolated areas to the main grid. Not only does this make installation and sustentation difficult and inconvenient, it can also damage the environment [23].

B. Compressed Air Storage Compressed air energy storage has a comparable efficiency

to pumped hydroelectric, so the return is still less than unity.

This method also requires an underground cavern to store the air. While these are more common than the mountainous areas required by pumped hydroelectric, this may still limit its use in some areas. While the gas is in the storage process, it must be compressed. This compression causes the gas to heat. The amount of gas that can be stored is limited by this phenomenon, as temperatures could rise to dangerous levels if too much gas is stored [23]. What gas is stored underground is also subject to energy loss through heat transfer to the walls of the cavern, thus limiting its long-term storage capabilities [23].

C. Batteries Each different type of battery carries unique concerns and

issues. Lead acid batteries, commonly seen in automobiles, have a low energy density; thus they require large size to store large amounts of energy [23]. Building sizable batteries to store large amounts of energy is neither cost effective nor space effective. Sodium-sulfur (NaS) batteries require that the sodium and sulfur are kept molten and separated from one another [23]. This method of storage is sizeable, and not feasible in every situation. In addition, the battery may suffer critical damage if it is completely drained and left cold [23]. Lithium-ion batteries are very efficient, sometimes nearly 90% efficient, but also very expensive due to safety considerations. The lithium salt solution used in the batteries is flammable; thus extra measures must be taken to ensure that the batteries will not catch on fire [23].

D. Flywheel Energy Storage Flywheel storage is still a very developmental technology.

Key advances may eliminate issue that face flywheels, however this in not currently the case. Flywheels are expensive to build and require an incredible amount of precision in order to operate properly [24]. As energy is stored through the spinning of the wheel, they must be built strong enough to withstand heavy rotation while not breaking or seeing a drop-off in performance [23]. This necessary integrity limits the effectiveness of the flywheel as a long-term method of energy storage [23].

E. Ice Storage Ice storage is an effective way to reduce energy costs and

demand during times of stress on cooling systems [25]. However, this method still faces its share of issues. The major issue of ice storage arises from the nature of the method itself: it is only saving energy originally intended for cooling systems. This method is supplemental, and not viable for large-scale storage, as not all energy stored and supplied goes to the cooling system. Thus, while effective at reducing costs, ice storage is not the answer for the problem of effective, efficient energy storage [25].

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V. RESEARCH AND DEVELOPMENT There is great interest and focus on developing better, more efficient forms of energy storage.

A. Liquid Lead Batteries Batteries have always been a widely used storage technique

for the energy grid. Researchers at the Massachusetts Institute of Technology in Cambridge are developing a new battery, which is believed to be more competitive for large-scale use on the energy grid. Most batteries are made of solid electrodes and possibly a solid electrolyte, but this new development uses liquid electrodes and electrolytes. The latest model from the team at Cambridge uses a liquid lithium negative electrode, a liquid lithium salt electrolyte and a liquid lead-antimony positive electrode instead of the previously used magnesium-antimony electrode [26]. According to [26], “When the battery discharges, lithium atoms in the negative electrode give up an electron and travel through the electrolyte to the lead-antimony electrode. Charging pushes them back in the opposite direction, and the flow of current is enough to keep the metals liquefied.”

The latest version of the molten battery could run at 450°C over the previous 700°C which the magnesium-antimony version required to run. This not only lowers the energy needed to sustain the liquid battery but it will also help to decrease the rate of corrosion. After 1,800 hours of operation no sign of corrosion could be seen and the lead-antimony battery was able to maintain 94% of its capacity after 450 complete cycles. Estimations predict the battery will keep 85% of its charge capacity after a decade of complete cycles [26].

Cost effectiveness is the main concern for this new molten metal battery. Currently a large-scale lead-antimony battery is estimated to cost upwards of $500 per kilowatt-hour of electricity produced, but the Cambridge team’s goal is to lower the cost to around $100 per kilowatt-hour. This would allow the battery to become an alternative in energy grid storage. However, this is just the beginning of the Cambridge team’s research as they continue to refine the battery’s chemistry, exploring more ways to develop a longer lasting and more efficient battery [26].

B. Graphene Aerogel in Super-Capacitors Another energy storage development undergoing research is

a graphene aerogel and its ability to improve the power grid. Lawrence Livermore National Laboratory researchers believe that graphene aerogel could help improve energy storage by smoothing out power fluctuations in the grid. Graphene aerogel is currently one of the lightest materials in the world and may be especially useful in the super-capacitors. According to [27], “Graphene aerogel-based super-capacitor electrodes could be particularly useful… because they feature high surface area, good electrical conductivity, chemical inertness and long-term cycling stability.” Graphene aerogels also support enhanced pore size distribution and density control over currently used carbon-based super-capacitor electrodes. LLNL’s Patrick Campbell states in [27], “Our materials can potentially improve on the performance of these commercial super-capacitors by more than 100 percent.” This

new development is still being heavily tested but could lead to improved super-capacitors, in turn improved batteries, and an increase in the capability of energy storage [27].

C. Molten Salt Storage Molten salt storage is another form of energy storage that is

undergoing research and continual develop. Most current means of energy storage are not viable since the costs heavily outweigh the profit, so more unique ways of storage energy have been developed. Molten salt uses solar energy to heat sodium into a liquefied state that is then stored until necessary, such as peak or nighttime hours. Once more energy is needed the liquid sodium is pumped into a steam generator and the molten salt gives off enough heat to boil the water, creating steam. The steam is then directed through a path to a turbine, which it spins to generate electricity. This provides a new way to store thermal energy for a later time when electricity is needed. It is also an environmentally safe way to store great amounts of energy as well as a cheaper method than that of batteries while still keeping a reasonable efficiency [28].

The mixture of salt used varies, but the more common ones contain sodium nitrate, potassium nitrate, and calcium nitrate. The average temperature that the salt melts also varies by depending on the concentration of compounds, though the average temperature is around 131°C and is kept around 288°C in the storage tanks. The molten salt is kept here until it is finally reheated by a solar collector to around 566°C and finally sent to another storage tank where it is kept up to a week before the salt is used to generate energy though a steam generator and the cycle is continued [29].

VI. POWER ELECTRONICS FOR GRID-SCALE ENERGY STORAGE

Generally, power electronics is the process of using semiconductor switching devices to control and convert electrical power flow from one form to another to meet a specific need. They are widely used in the flexible alternating current transmission system (FACTS) and used as distributed energy interfaces. In 2005, approximately 30% of all electric power generated utilizes power electronics somewhere between the point of generation and its end use. By 2030, it is expected that perhaps as much as 80% of all electric power will use power electronics somewhere between generation and consumption [30].

A. Grid Interfacing Technology for Batteries Power electronics is expected to play an integral role in

interfacing battery storage to the grid. Bidirectional battery chargers and Solid State Transformers (SSTs) are two primary examples of making this role a reality. An advanced research project in US entitled “Gallium nitride switch technology for bi-directional battery to grid charger applications” (2010-2014) has achieved higher frequency (100kHz), 2 times faster charging (6.6kW), 2 times higher efficiency (>95%), and 10 times smaller (>120W/in3), Fig. 6 shows a demo of this project [31].

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Fig. 6 A demo of Bidirectional battery charger

The elements of an SST are a dual active bridge including a physically smaller high-frequency transformer and two converters (rectifier and inverter per given direction). An assembly diagram of the SST is found in Fig. 7 [32]. SST centers on FACTS features, namely reactive power compensation and harmonic filtering. Depending on the SST design used, advantages can include bidirectional power flow (four-quadrant operation), reactive power compensation, harmonic isolation, voltage sag compensation, fault isolation, a common dc link, or energy storage integration. Another benefit of the SST is communication capability between utilities, end users, various other switching devices in the network, and other SSTs.

Fig. 7 Diagram of Example SST configuration

SST development has been specifically targeted at the distribution level. They operate at 20 kVA, 20 kVA, 300 kVA, 500kVA and 1 MVA, respectively, and range between 7.2 and 15 kV on the distribution side, which are compared in [32] showing their unique benefits and limitations. The SST has great potential of becoming the standard for interfacing various power system technologies to the grid, including battery storage systems.

B. Utility Scale Power Electronics Utilizing Energy Source Reactive power compensator is very important for the

dynamic voltage support, dynamic reactive power compensation, power system stability, increased power transfer capability, renewable energy and energy storage integration, and the overall enhanced power quality. Static synchronous compensator (STATCOM) systems, which are designed with self-commutated voltage sourced converter (VSC) technology, have become a more widely accepted, advanced technology, solution for reactive power compensation applications because of faster response and improved operational characteristics. The widely used STACOM types are neutral point clamped (NPC) topology developed by Siemens and the modular multilevel converter (MMC) topology used by ABB. The design of the SATCOM

is flexible and has lower space requirements due to the reduced size and fewer passive components resulting in a smaller footprint. Standard sizes are available and include 25, 35, and 50 Mvar. The open rack modular system configuration enables a transformerless grid connection up to 36 kV and 100 Mvar [33].

The NPC topology for an n-level, multilevel converter is illustrated in Fig. 8. This topology creates the smallest converter ac voltage steps, has a small rate of rise in voltage, and generates lower harmonics and low switching losses. The total required capacitance is shared by the modules instead of being concentrated, as in other traditional power converter designs [34].

Fig. 8 NPC topology for STACOM

ABB has recently developed and tested an STATCOM solution referred to as DynaPeaQ [35], which is MMC with energy storage, as shown as Fig. 9. Currently, the amount of power that can be delivered by the energy storage system is about 20MWfor tens of minutes. But the technology permits up to 50 MW for periods of 60 min. There are four application areas where DynaPeaQ is expected to find widespread use, such as renewable generation grid connection, backup power, emergency and short-time power, intermittent loads of a railway [36].

Fig. 9 MMC topology for STATCOM

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C. Battery Storage Applications in Microgrids The fundamental microgrid requirements include the

capability of operating in islanding and/or on-grid modes with high stability, mode switching with minimum load disruption and shedding during transitions, and after a transition, stabilizing in a certain amount of time. The medium-voltage direct current (MVDC) architecture (Fig. 10) has often been referred to as a type of microgrid upon first view [36].

Fig. 10 MVDC Microgrid Example

Energy storage systems act as a buffer, either absorbing excess generation, or discharging energy to meet minimum load requirements. Energy storage can smooth renewable energy output and frequency deviations, thereby preventing voltage instability. Most importantly, energy storage can supply power during outages that last for extended periods of time.

Power electronic converters operate as three modes in Microgrid: grid feeding, grid forming, and grid supporting. Grid-feeding (Fig. 11b) power converters are mainly designed to deliver power to an energized grid. These units are modeled, simply, as a current source with high impedance in parallel with the source. Grid-forming power converters (Fig. 11a) are represented by a voltage-controlled source and low series impedance. Grid-supporting converters (Fig.12 13) are used to regulate their output current/voltage to keep the value of the grid frequency and voltage amplitude close to their rated values. Its main objective is to deliver proper values of active and reactive power to contribute to the regulation of the grid frequency and voltage.

Fig. 11. (a) Grid-forming converter and (b) grid feeding converter

Fig. 12. Current-sourced-based grid supporting

Fig. 13. Voltage-source-based grid supporting

D. Conclusions Power electronics do in fact have a key role to play in grid

scale energy storage applications. Any regulation of nonlinear output and any coupling of battery storage to the grid are performed by the use of power electronic systems. Bidirectional dc chargers, FACTS devices with integrated energy storage, battery storage within microgrids, and EVCSs are a number of examples that show power electronics being interfaced to battery storage. Successful realizations were expounded upon, exemplifying the utilization of power electronics in each. Within the transmission, microgrid, and distribution layers of the current and future grid, power electronics technologies are integral to the implementation of new equipment developments including battery storage systems.

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