Abstract-- The electrical energy storage systems serve many

applications to the power system like economically meeting peak loads, quickly providing spinning reserve, improving power quality and stability, and maintaining reliability and security. The rapidly increasing integration of renewable energy sources into the grid is driving greater attention towards electrical energy storage systems which are capable of stabilizing the output from renewable energy sources. The application of electrical energy storage systems will also defer the installation of new transmission lines. With the development of advanced energy storage technologies, the electrical energy storage systems are making the future power system more reliable, efficient and economical. This paper presents various energy storage technologies currently used, their applications and the role they are going to play in the future power system.

Index Terms Energy storage, renewable energy sources, smart grid.

I. INTRODUCTION lectrical energy storage systems (EESS) store the electrical energy in the kinetic, potential, electrochemical

or electromagnetic form which can be transferred back to the electrical energy when required. The conversion of electrical energy to different forms and back to electrical energy is done with power conversion systems. EESS can store the inexpensive energy during off-peak periods and be used to meet the loads during peak periods when the energy is expensive, which will improve the economic operation of the power system. Compared to conventional generators, the energy storage systems have a faster ramping rate which can quickly respond to the load fluctuations. Therefore, the energy storage systems can be a perfect spinning reserve source which provides a fast load following and reduces the need for spinning reserve sources from conventional generation plants.

Electrical energy storages were initially treated only for load leveling applications. Now, they are more seen as a tool to improve the power system quality and stability, to ensure a reliable and secure power supply to loads, and to black start the power system [1]. For example, due to increasing electronic loads, the power quality has been becoming a major concern to the utilities. Poor power quality exists with variations in voltage magnitude and frequency. Electrical energy storages can help in maintaining power quality by

The authors are with the Department of Electrical and Computer Engineering, Mississippi State University, Starkville, MS 39762 USA (e-mail: sy153@msstate.edu, fu@ece.msstate.edu )

providing necessary voltage and frequency support to the power grid. Energy storage systems can provide VARs to quickly increase and maintain voltage in periods when large amounts of inductive loads come online [2]. Drawing high power in a short interval directly from the power grid will exacerbate the power system stability. However, energy storage systems can be used to produce high power pulses with the trivial impact on the stability. Reliability is also a major issue which should be guaranteed by the utility, especially to the critical loads. In U.S. the electricity outages cost approximately $79 billion annually with two third of the cost due to interruptions under five minutes [3]. Energy storage devices can eliminate the short term interruptions until the backup generators are brought online. In addition, a proper location of EESS will provide the preventive actions to mitigate violations under any contingency. Consequently, the security of the power system will be maintained by the application of EESS.

In the United States it is expected that there will be a significant increase in amount of renewable generation. Renewable portfolio standards of 15 30 % are required in most states by 2020 [4]. Wind and solar, which are the major sources of renewable energy, are intermittent and volatile resulting in a varying power supply to the power grid. The combination of electrical energy storages with these energy sources can firm up the renewable power output by storing energy during high availability and using it while there is shortage. As a result, electrical energy storage systems can increase the penetration of renewable energy into the power grid and reduce the carbon emissions. In addition, when a critical line outage occurs, the wind energy has to be curtailed in order to remove the overloading on the transmission lines. Such problem can be mitigated by using large energy storage.

In general, upgrade of the transmission system is necessary to mitigate congestions in the power system with a large amount of increasing demand. However, such solution may not work because (1) obtaining rights and approvals to install new lines will take many years, (2) the construction of new lines in urban areas is almost impossible due to the space limitation, and (3) the cost of building a transmission line has been drastically increased. The energy storage at the load center could be a fast, flexible and economical solution to the operation of stressed power systems. The usage of energy storage systems can reduce the power transfer through the lines during the peak load periods and greatly decrease the congestion cost as the energy storage system can shift the load from the peak to off-peak load periods [5]. Thus, reducing the

Impacts of Energy Storage on the Future Power System

Sandeep Yeleti, Student Member, IEEE, and Yong Fu, Member, IEEE

E

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MR-TOWNHighlightThis can lead to a competitive market as the LMP as made to be the same since the congestion cost is reduced. Thus increasing the social welfare.

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loading on the transmission equipments can defer the upgrade of the transmission system. For example, a 1.0 MW, 7.2 MWh sodium-sulphur battery was employed successfully in 2006 to peak-shave the summer load on a 20 MVA transformer and delay the substation upgrade for 3-5 years until the firm load growth showed the need [6].

In addition, the energy storage can support critical load customers during fault situations. Energy storage devices also facilitate the black start of the entire power system. Overall, the integration of energy storages into the power system is making the usage of available generation, transmission and distribution infrastructures more reliable, efficient and economic.

The electrical energy storage system, as a market participant, can participate in electricity markets, such as NYISO and PJM. Depending on its storage and delivery characteristics, the energy storage system can enter into different markets, such as energy, ancillary service and installed capacity markets [12].

The rest of this paper is organized as follows. Section II describes various energy storage technologies. Section III presents the impact of energy storage on different sections of the power system. The importance of energy storage in future smart grid is discussed in Section IV. Section V gives a brief conclusion.

II. DIFFERENT ENERGY STORAGE TECHNOLOGIES There are different energy storage technologies of which some are already in use and some are yet to be implemented. Different energy storage technologies serve different applications depending on the amount of energy to be stored, the rate at which it is to be transferred, and the response time. The following energy storage technologies are with the high priority in the energy storage applications. The comparison of these technologies is summarized in TABLE I [3, 9, 12]. Besides, there are several other energy storage technologies, such as hydrogen energy storage, thermal storage, and fuel cells.

A. Pumped Hydro Storage The pumped hydro storage is the conventional and most widely used energy storage technology. In this storage, the electrical energy is stored as the potential energy by pumping water to a higher reservoir during off peak periods. This energy can be converted back to the electrical energy by allowing the water to flow from higher to lower reservoir and driving the hydro turbines. Of the grid energy storage techniques currently in use, the pumped hydro storage is the largest one in terms of capacity. The efficiency of the pumped hydro storage plant is about 70%80% which varies depending on the plant size, the type of turbine, the penstock diameter, the height between the reservoirs and the level of generation [5]. The usage of pumped storage plants is limited to rural areas because of the large area the system constitutes for setting up the reservoirs.

B. Battery Energy Storage The battery is charged to store the electrical energy in the form of a chemical reaction inside the battery. The reversal of this reaction will result in the discharge of the battery producing electrical energy from the chemical reaction. Because batteries store and produce DC power, the power converters are essential for this type of storage in order to interact with the AC power grid. Because they are non-polluting, easy to install and portable, the battery energy storage is the most convenient form of storage in urban areas. A battery energy storage system is made up of several low voltage battery modules connected in series and parallel to obtain the desired electrical characteristic [1]. There are many types of battery systems like lead-acid, Lithium-ion, Sodium sulfur and flow batteries. Among them, the Lead-acid battery is presently used in many applications due to its low cost. Although the batteries are the most convenient and practical method, their high cost and less cycle life are of great concern in their implementation. The implementation of batteries in plug-in hybrid electric vehicles is challenging the existing energy management system of power grids.

C. Compressed Air Energy Storage (CAES) The compressed air energy storage uses the excess power from the grid during off-peak load periods to compress air and stores it in an underground reservoir under the pressure. In case of the shortage of power, the compressed air is released and burnt with a fuel to drive the generator. Actually, this type of technology is known as hybrid energy storage as it uses fuel as well. However, for the same power output, the fuel consumption in this storage is only one third of the consumption for a regular combustion turbine. The compressed air energy storage is not widely being used presently due to the safety related issues of storing compressed air in the underground. Currently there is only one such kind of a plant in US, which is a 110 MW 26 hour compressed air energy storage plant in Alabama, built in 1991[5, 8].

D. Flywheel Storage The flywheel energy storage stores the electrical energy in the form of kinetic energy of a rotor or a disc spinning around its axis on a shaft. The charging or discharging of the flywheel storage system takes place by changing the amount of kinetic energy present in the accelerating or decelerating rotor, respectively. The flywheel is coupled with an electrical machine which acts as a motor to drive the flywheel while charging and acts as a generator to discharge the stored energy by decelerating the rotor to stationary position. The amount of energy stored depends on the moment of inertia and square of the rotational speed of the rotor. Therefore, composite materials are used for the rotor to reduce its weight allowing much high speeds. The flywheel has a high power density and high cycle life. A medium scale flywheel system is being incorporated in New York City with ten 100kW 30 seconds flywheels for regenerative braking and startup of subway transit cars [8].

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TABLE I COMPARISON OF DIFFERENT ENERGY STORAGE TECHNOLOGY

Storage Technology Energy Capacity

Discharge Duration at Max

Power Level Power Level Response Time

AC-AC Efficiency

Life Time Applications

Pumped Hydro

< 24000 MWh 12 hours < 2000 MW 30 ms 70 80 % 40 yrs

Energy arbitrage Frequency regulation Ancillary services

CAES 400 7200 MWh 4 24 hours 100300 MW 3-15 min 85 % 30 yrs Energy arbitrage Frequency regulation Ancillary services

Fly Wheel < 100 kWh Minutes to 1 hour < 100 kW 5 ms 80 85 % 20 yrs

Frequency regulation Power quality Emergency bridging power Fluctuation smoothing

Battery < 200 MWh 1 8 hours < 30 MW 30 ms 60 80 % 210 yrs

Peak shaving for T&D upgrade deferral

Backup power Small load leveling

applications

SMES 0.6 kWh 10 s 200 kW 5 ms 90 % 40 yrs Power quality Emergency bridging power

Capacitors 0.3 kWh 10 s 100 kW 5 ms 90 % 40 yrs Power quality Emergency bridging power Fluctuation smoothing

E. Superconducting Magnetic Energy Storage (SMES) The superconducting magnetic energy storage charges by storing the electrical energy in the form of magnetic field created by the flow of DC current through a coil made of superconducting material at very low temperatures. The DC power stored in the magnetic field can be discharged with high power output in a short interval time. The energy stored in the coil is proportional to the inductance of the coil and square of the dc current creating the magnetic field. The increase in the size of the coil can increase the storage capacity. Superconducting magnetic energy storage units up to 3 MW are in usage presently [1, 8]. Due to its high efficiency and fast response, the superconducting magnetic energy storage is driving greater attention. A small SMES unit of 1.0 MW, 0.75 MJ was installed and tested by Central Hudson Gas and Electric Company to improve local power quality and reliability [10].

F. Capacitors The capacitor consists of two parallel electrode plates which are separated by a dielectric. When the voltage is applied across the terminals the positive and negative charges get accumulated over the electrodes of opposite polarity. The amount of energy stored in the capacitor is proportional to the capacitance and the square of the voltage applied. So, the energy storage capacity can be increased with both capacitance and voltage. Although the voltage is restricted by the dielectric in the capacitors, the capacitance can be increased by increasing the permittivity and surface area of the electrodes or decreasing the distance between the electrode plates. The capacitor is generally used for the high power short term applications, such as a pulsed load. There has been much advancement in the capacitor storage like the super-capacitor which has much lesser volume for the same storage capability and with much longer cycle life. The super-capacitor is now available in the range of up to 100 kW with very a short discharge time of up to ten seconds [9].

III. ENERGY STORAGE APPLICATIONS IN DIFFERENT SECTIONS OF THE POWER SYSTEM

A. Energy Storage with Renewable Energy Sources Various renewable energy sources can be used for generating the power, such as wind, solar, geothermal, and tidal. Renewable energy sources are given the high priority in the power system because of their clean and economic nature. However, the volatile and intermittent nature of renewable energy affects the operation and planning of the power system. Energy storage connected with renewable energy sources can solve two major issues as follows,

Stabilize the intermittent and volatile power output of renewable energy sources

Benefit the power system by shifting the excessive renewable energy from the off-peak load period to the peak load period.

These impacts of energy storage connected with a wind farm can be explained with the following example shown in Fig .1. Since the wind farms generally request high capacity energy storage technologies, we use a pumped storage plant in this example. The hourly demand for the entire power system is shown in Fig. 2. The ten-minute forecasted wind power output from the wind farm P1 during a day is shown in Fig. 3. After connecting the pumped hydro storage with the wind farm, the coordinated wind power output P2 in Fig. 4 is obtained by executing the economic dispatch for the entire day. The power profile of the pumped storage plant P3 is shown in Fig .5. The generation that is not being supplied by wind farm is assumed to be supplied from conventional generation in the power system.

Fig. 1. Power system with a wind farm connected with pumped hydro storage

Fig.2. Hourly system demand

Fig.3. Wind power forecast without energy storage

Fig.4. Wind power output with energy storage

Fig.5. Energy storage changing and discharging

Considering the wind farm with the pumped hydro storage, the energy storage techniques can stabilize the wind power output such that a constant power shown in Fig.4 is generated in each hour to follow the system demand trend over 24 hours. Note that the forecasted wind power output shown in Fig. 2 is high during off-peak period (hours 1-9 and 19-24) and is very low during peak loads (hours 13-17). However, the coordinated wind power output shown in Fig. 4 is low during off-peak period (hours 1-9 and 19-24) and is very high during peak loads (hours 10-18). After comparing Fig. 2 and Fig. 4, we can find that a certain amount of inexpensive wind power during off-peak load hours is stored in the pumped hydro storage, and used during peak load hours. As a result, the total operation cost of the power system is reduced since it is not necessary to get the power from expensive conventional generators during peak load hours. Therefore, the integration of wind farm with the energy storage will firm up the wind energy output and make the system economic.

B. Grid Energy Storage

The grid energy storage means that the energy storage is connected at a certain bus. Grid energy storage can reduce the operating cost of the power system, mitigate the congestion, and improve the security of the power system. The following example shows such benefits from the grid energy storage. A simple 6-bus system is shown in Fig. 6 which has two thermal units at buses 1 and 3, two loads at buses 2 and 5, two battery storages at buses 4 and 6, and six lines with reactance 0.1 p.u. The characteristics of generators and lines are presented in Fig. 6, such as the operating capacity and the incremental cost. The batteries are operated with the capacity limit between 2 MWh and 8 MWh. The charging/discharging rate limit is assumed as 3 MW/hr and the initial stored energy in the battery is 2 MWh. This system is supplying the loads for 3 hours. The load L1 is 37 MW, 45 MW and 47 MW, and L2 is 65MW, 80 MW and 85 MW for 3 hours, respectively. Security constrained economic dispatch (SCED) with the DC network model is executed in the following four cases to show the economic and security benefits from the energy storage.

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Fig. 6. Six-bus power system with the battery energy storages Case 1: Normal condition without battery

In this normal case, the system is initially assumed without battery. After executing the economic dispatch considering the limits on generation and lines, the output of generating units is listed in TABLE II. The minimum generation dispatch cost of the system without the battery storage is $3733.324. At hour 1, the expensive unit G1 supplies its minimum and the remaining load is being supplied by the cheap unit G2. As there are high loads at hours 2 and 3, the cheap unit G2 has to supply more power, but no more than 77.7027 MW at hour 2 and 73.70571 MW at hour 3 due to the flow limit on the line 3-4. So, the remaining has to be supplemented with the expensive unit G1.

TABLE II OPTIMAL GENERATION DISPATCH WITHOUT THE BATTERY

Units Hour 1 Hour 2 Hour 3 P1 (MW) 30 47.2973 58.29429 P2 (MW) 72 77.7027 73.70571

Case 2: Normal condition with the battery

With the battery, the economic dispatch is solved and the generation dispatch is listed in TABLE III, where the negative sign indicates the power consumed by the battery during charging and the positive sign means the power delivered to the grid during discharging. At the first low-load hour, the batteries BAT1 and BAT2 are charged by 3 MW which is from the cheap unit G2. At hours 2 and 3, the output from BAT2 is mitigating the congestion on the line 3-4. Consequently, the cheap unit G2 can supply more power compared with Case 1. In addition, the outputs from batteries are replacing the generation from the expensive unit G1. Accordingly, the total generation dispatch cost is $ 3625.243 which is less than that in Case 1. The saving could be much higher in the large power system.

TABLE III OPTIMAL GENERATION DISPATCH WITH THE BATTERY

Units Hour 1 Hour 2 Hour 3 P1 (MW) 30 43.47 50.11258 P2 (MW) 78 80 77.41742

Pbat1 (MW) -3 0 +3 Pbat2 (MW) -3 +1.53 +1.47

The batteries can also decrease the power flow on the lines during the peak load. The power flow on line 1-6 at hour 3 is 35.53 MW in this case, but 40 MW in Case 1. So, such application could be helpful in deferring the transmission line upgrade due to the aging of lines or the increase in loads.

Case 3: Considering the contingency without battery

Consider the outage of line 1-2 at hour 2. If the system has no batteries, the load at the second hour could not be met because the outage of line 1-2 restricts the supply from G1 to 40MW through line 1-6. Thus, the total power that the generators could supply is 120 MW which can not meet the load of 125 MW at hour 2.

Case 4: Considering the contingency with battery

With the outage of line 1-2 at hour 2, the optimal generation dispatch of the system with batteries is listed in TABLE IV. In this case, the batteries have to discharge 5 MW at hour 2 in order to meet the system demand of 125 MW once the outage of line 1-2 occurs, and only 1 MW is left for the discharge from BAT 2 at the peak load hour 3. Accordingly, the minimum generation cost is $ 3645.135 which is higher than that in Case 2.

TABLE IV OPTIMAL GENERATION DISPATCH WITH THE BATTERY

CONSIDERING LINE 1-2 OUTAGE Units Hour 1 Hour 2 Hour 3

P1 (MW) 30 40 55.79279 P2 (MW) 78 80 75.20721

Pbat1 (MW) -3 +3 0 Pbat2 (MW) -3 +2 +1

According to the above discussions, the grid energy storage can be helpful in reduction of generation cost, decrease in line loading and enhancement of the power system against contingencies. In addition, by comparing the installation and maintenance costs of energy storage devices with the benefits from them over the studied period, the system planner can make a decision on whether, when and where to build a grid energy storage.

C. Energy Storage with Loads Energy storage systems connected to the load can meet the peak loads, provide uninterrupted supply to critical loads, eliminate spikes in the load profile, and supply pulsed loads. The power supply to critical loads should be ensured all the time. In a fault situation when the supply to critical loads gets interrupted, the energy storage on the load side can supply the power until the system is restored. For example, a distribution system shown in Fig. 7 has three loads and one energy storage at bus 4. The Energy storage is connected with a critical load at bus 4. When power system is running in the normal situation, the energy storage gets charged. Assume that a fault occurs on the line 1-2. The supply to loads L1 and L2 is interrupted because of opening of switches on the line 1-2. In this case, the energy storage will discharge to supply the critical load L4 till the system is restored.

Fig .7. One line diagram of a distributed system with energy storage

The energy storage has a higher ramping rate so it can

mitigate spikes present in the load profile. The pulsed load draws a large power in a very short interval time, which can be supplied by an energy storage system which charges during no-load periods and discharges instantaneously to supply the pulsed load.

IV. ENERGY STORAGE IN FUTURE SMART GRID Due to the aging infrastructure, environmental concerns, security issues, to avoid blackouts and to ensure high efficiencies, there has been much interest laid by the United States government in transforming the conventional electric grid into a future smart grid. With the large scale integration of renewable energy sources and distributed generation into the power grid, energy storage devices will play a key role in the future smart grid. The smart grid without energy storage devices is said to be like a computer without hard drive [13]. Presently only 2.5 % of the total electric power delivered in the United States passes through energy storage, almost all of which is pumped hydro storage [12].

The distributed energy storage devices connected to the grid on a large scale can be helpful in meeting peak loads, maintaining power quality, and reducing the reserve capacity. With the less response time, the energy storage can reduce spikes in the load and ensure the power supply to the loads during the faults until the system is restored. The distributed energy storage can help in reducing losses on the distribution lines. The energy storage can also play a major role in the micro grid operation by helping in islanding and reconnection process, and increasing the range of load that could be supplied by the micro grid. In addition, the plug-in hybrid electrical vehicle is driving greater attention in the smart grid, which charge when the vehicle is running and can supply the power to the grid when the vehicle is parked. The plug-in hybrid electrical vehicle can facilitate the micro grid to meet load demand transients or grid power quality events [18].

With many applications in the power grid, the large scale integration of energy storage is still lacking because of the following challenges,

A economic energy storage device is to be developed since the existing devices are requiring the high capital investment

The optimal location, type, and size of the energy storage system is still a major concern to the utilities.

Both optimal software and hardware models for the energy storage applications are to be developed.

The high-efficient power electronic conversion system is to be developed to increase the efficiency of the energy storage system.

The proper coordination of the energy storage with the renewable energy sources should be determined.

The long-life and clean battery technology should be developed for minimizing its negative impact on the environment.

V. CONCLUSION This paper presents various energy storage techniques and their applications in the power system. The benefits from the energy storage with the renewable energy sources, at the grid, and with the loads are discussed based on simple examples. It has been shown that energy storage devices not only enables the large scale integration of renewable energy sources into the grid, but also ensures the economic generation by the application of grid energy storages. With the energy storage serving different sections in the power system as discussed in the paper, they will be installed widely in the grid in the future, which requires much more knowledge in determining the optimal design, location, type and size of the energy storage devices.

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Sandeep Yeleti received his BS degree in Electrical and Electronics Engineering from Osmania University, India, in 2009. Currently, he is pursuing MS degree in Electrical Engineering at Mississippi State University. Yong Fu (M05) received his BS and MS in E.E. from Shanghai Jiaotong University, China, in 1997 and 2002, respectively and Ph.D degree in E.E. from Illinois Institute of Technology, USA, in 2006. From 2006 to 2009, he was a senior research associate in the Electric power and Power Electronics center at Illinois Institute of Technology. Presently, he is an assistant professor in the department of Electrical and Computer engineering at Mississippi State University.

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