National Power & Energy Conference (PECon) 2004 Proceedings, Kuaia Lumpur, Malaysia 705

Modeling and Simulation of Sodium Sufhr Battery for Battery Energy Storage System and

Custom 1 'Power Devices Lee Wei Chung, Mohd Fadzil Mohd Siam. Amir Basha Ismail and Zahrul Faizi Hussien, Member, IEEE

Abstract-This paper presents an overview of Sodium- Sulfur NAS battery used for Battery Energy Storage System and custom power devices for power quality applications. Several electrical battery models are reviewed and discussed, Using one of the most accurate electrical battery modets, the voltage- current behavior and characteristics of NAS battery are modeled and simulated using EMTDCIPSCAD software tool. Comparison between simulation and experimental results are presented for a 50kW battery model to validate the simulation model and f o analyze NAS battery output performance with respect to battery's depth of discharge and temperature change.

hdex Ternis-Electrical Battery Model, NAS Battery. Battery Energy Storage, Peak Shaving, Power Quality

I. INTRODLCTION HE Sodium Sulfur (NAS) Battery is an advanced T secondary battery that is being developed by Tokyo

Electric Power Company (TEPCO) and NGK Insulators, Ltd. since 1983 [ 11. Feasibility studies of various demonstration projects [1]-[4] showed that the NAS battery technology is attractive for use in relatively large-scale Battery Energy- Storage System (BESS) applications due to its outstanding energy density, efficiency, zero maintenance and long life cycle (up to 15 years).

NAS battery is a relatively new development in BESS and hence requires more research on its operational characteristics and performance. There are several published works investigating on the applications and developments of NAS battery [ I]-[3]. Recent advances include the development of NAS battery for multiple hnction storags system that performs a variety of energy management function and custom power devices for power quality applications. However as far as the authors are aware, little work has been published in the modeling and simulation of the NAS battery. Since the performance of NAS battery is the main factor in determining its utilization in various applications, the need for an accurate model is therefore indispensable.

T h i s work was jointly supported by TNB Research Sdn. Bhd., Tokyo Electric Power Company (TEPCO) and University Tenaga Nasional

Lee Wei Chung, Amir Basha Ismail and Zahrul Faizi Hussien are w l i h Department of Electrical & Electronic, University Tenaya Nasional. ,Malaysia (e-mail: leeweichung@tnrd.com.my).

Mohd Fadzil Mohd Siam is with Power System Group, TNB Research Sdn. Bhd., Malaysia.

Important factors such as energy, power, voltage, internal resistance, service life, depth of discharge and temperature are taken into account for optimal design of the battery model. These factors are considered because they influence the capacity, operational characteristics and performance of NAS battery.

This paper initially gives a quick glance of NAS battery features in section IT. Factors considered in NAS battery modeling is discussed in section 111. Section IV describes the modeling of the NAS battery. Finally, Section V presents the simulation results and compares them with the experimental data for validation.

11. FEATURES OF NAS BATTERY

A simplified schematic diagram of a NAS cell is shown in Fig. I . As described in [2] and [ 7 ] , the cylindrical structure NAS cell uses sodium as the negative active material, sulfur as the positive active materia1 and a solid beta alumina ceramics act as electrolyte by conducting sodium ions (ionic charge transfer) between the positive and negative electrodes: The metal container, which is housed inside the beta alumina tube, acting as a safety tube to prevent an over current and an internal temperature rise in case of a beta alumina tube failure. The fearures and advantages of using NAS battery is discussed in detail in [ 11, [Z], [7] and [8].

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Fig I : Schematic diagram of NAS cell snucture and its specifications

NAS battery operates at nearly 300 degree C, it is used as a battery module (BM) in which cells are collected and housed in a thermal enclosure, The BM is equipped with an electrical heater to raise and maintain cell temperature. Cells inside the

0-7803-8724-4/04/$20.00 a2004 IEEE.

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module are anchored in place by filling and solidifying them with sand, which is used to prevent fires as shown in Fig. 2. Since this BM is a basic unit to consist a system, the electrical NAS battery model will be first modeled and discuss as per cell basis in section I11 and IV. Later in section V, the electrical circuit model will be compiled to form a 50kW battery module for simulation studies.

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Fig. 2: (a) 50kW NAS-BM Structure, and (b) 50kW NAS-BM specification

111. IMPORTANT FACTORS 1N NAS BATTERY MODELING

In order to model the NAS battery accurately, the following factors are vital to determine battery capacity and voltage- current behavior. The cell's parameters and factors being discussed in this paper are based on the TEPCO's type "T5" . NAS cell.

A. lnternal Resistance Charge and Discharge resistance:

There are resistance associated with electrolyte resistance, plate resistance and fluid resistance. All these resistance are different during charging and discharging operation. Due to non-equality of composition [7], the resistance in NAS battery changes with temperature and depth of discharge (DOD) as shown in Fig. 3.

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Fig. 3: DOD V.S. intemal cell resistance at different temperature levels (Experimental)

Charge-Discharge life cycle resistance: Through charge and discharge cycles, NAS battery deteriorates and internal resistance increases. This resistance represents deterioration-resistance that arises based on the increase of charge-discharge cycle as shown in Fig. 4. This factor is important because it determines the remaining. available maximum pulse power and voltage output of a NAS battery.

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Fig. 4: Deterioration-cell resistance V.S. number of charge-discharge cycle (Manufacturer Data)

B. Temperature Effect NAS battery operates within 300-360°C and its intemal

resistance fluctuates with temperature as described in Fig. 3, where the higher module temperature is, the smaller intemal resistance becomes. This factor is very important because it determines the limitation of its peak power output with respect to DOD. NAS battery's temperature varies differently between charging, standby and discharging state of operation.

During discharging state, NAS battery generates resistance heat and entropic heat causing the battery to store heat and raises its temperature. While at charging state, the amount of resistance heat generation is nearly equal to that of entropic heat absorption. Thus, the battery temperature is gradually lowered. Heat stored in the battery only dissipated during standby state. As a result, the battery temperature drops gradually. When the temperature exceeds the lower limit (300"C), the heater installed inside the BM will be tumcd on to raise the temperature and maintain it within nominal temperature range. Fig. 5 shows the battery temperature variations at different state of operation.

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Fig. 5: Battery Temperature variations at different stale of openlion (Expcrimcnlal)

207

ia some Power Quality (PQ) application as discussed in [I], [?I and [ 7 ] , NXS batteries is subjected to pulse output ol'up to 4-5 times rated power output. Pulse power output witli largei- citrretit generates more j ou le heat by internal resistance. .As illustrated in [ 7 ] , tlie 50kW NAS battery module with 5 times rated power output for 30 seconds will make temperature rise

t I I by around 3 degree as we can observe in Fig. 6. During rhis . Y ; ? h , \ j ! S 4 , , ) S .

operation, the temperature m u s t be kept within nornial I1 !U ill 0" x o 100 ill> llil

operating condition in ordcr to maintain high efficiency Stair u l d i r r h r r u l : 1 % 16 11. YI.S,I

without reaching unacceptably high temperahire or creating undesirable temperahire differentials within the battery. Pig. 7 : N A S ccll clcctromotive Forcc (EMF)

D. Depth of Discharge (DOD) As discussed previously i n Section 111 A-C, this factor is

important to represcnt the chargeidischarge status of the battery. It is used to dctermine the capacity left in the battcry. internal resistance change, temperature and battery'l EMF

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260 1 In this section, several battery models are rcviewcd and

c i I0.L 1 I.? 2 I i J J 5 .I I 5 j described Lbr modeling the NAS battery characteristic. Subscquently, one of thcse is selected to be niost suitable to Inode{ represent luost of tile itllportallt beilavior of tile NAS battery.

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Fig. 6: Rclariorisliip bc!\\.c.cn pulse poWrfiiid tcmpcralurc risc (cxpcriiuenral)

C. Battery Electronloti\-e Force (EMF) The EIMF of NXS battery mainly depends on tlic DOD.

Due to the composition reaction, the EMF of NAS hatter!. is relatively constam and drops linearly aftel. (30-7556 depth of discharge, depending on tlie tevniination point of the particular celI as shown in Fig. 7. In practice, NAS battei-y is limited to discharge less than 100% of its tlicoretical capacity (NaS;) because of the more corrosivc properties ofNa2S3 [Sj, and due to the remaining \.olunie of sodium. Before all the material changes to NarS3, the sodiuin in the cell moves to active electrode and the room for sodium becomes empty. In such a ' case, there is no path for electron in the negative electrode, causing poor performance at discharging. Hence, the battery is designed to stop discharging before all tlie sodium goes to active electrode. To provide an operational safety margin, rhe remaining volume of sodium per cell has to be considcred and is described as follows:

- Volume ofsodium is 7XOg. - -

Volume ofsodium usable is 675% Remaining volume ofsodium = 13.5%

As a result, NAS cell typically deliver 8540% of their theoretical capacity, which means that the approximate sodium polysulfide composition corresponding to I .S2V per cell is a mixture ofNa2S4 and Na2S2 at the end of discharge. As explained above, this factor is important atid needs to bc considcrcd in the simulation in order to observe the remaining voltage left at the end of discharge and to predict the possible maximum power output of the NAS battery at specific DOD.

A. Sinipic Model The most commonly used battery model [ 5 ] is shorrn it1

Fig. 8, which consists of a constant internal resistance (Ro) and open circuit voltage (Eu). Yo is thc tcrmiiial voltage of thc battery.

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Fig. 8: Siinpte Battery Modcl

Since NAS battery's internal resistance is sensitivc to and varies with temperature and DOD, this model is not suitablc in modeling the battery because it does not take into account the varying characteristic of the internal resistance of the battery with respect to DOD and temperature changes. Such modcl only applies in some circuit calculation or simulation where tlie energy from Bo is assumed to be imiimited.

R . Thevenin Battery Model The second most coninioii used model is the Thevenin

battery model [5]-[h] , which iiicludcs an ideal no load battery voltage (Eo) , internal resistance (X), capacitance (CO) and over-voltage resistance (Xo) as shown in Fig. 9. The component Cu represents the battcry capacity wlicre the battery mostly delivers or stores energy and behaves as a large capacitor.

208

. Fig. 9: Thevenin Baacry Model

This model has a disadvantage in modeling the NAS battery because the elements are all assumed to be constant, yet in fact all the values are function of battery conditions [5]. In addition, the internal open circuit voltage drops (or refers as EMF drops) in NAS battery is not taken into account.

C. Modified Battery Model

configuration shown in Fig. 8. Fig. 10 shows a niodified battery niodel based on the basic

Fig. IO: Improved Onticry Modcl

This battery model is relatively simple but yct meets most requirements for modeling of the NAS battery. It takes into account the non-linear battery element cliaracteristic during charging and discharging as well as the internal resistance which depends on the temperature changes and DOD of the battcry as discussed previously in section 111. Therefore, it is sclccted as the most appropriate in modeling the NAS battery using EMTDUPSCAD simulation tool. Bascd on the NAS characteristic, the above compotients are described as follows:

Charge and Discharge resistance (Rc & Rd): RC and Rd internal resistance are a function of tlic temperature and depth of discharge. The purpose of wing the diode shown in Fig.10 is to make NAS battery possessing different internal resistancc value during charge and discharge operation, Charge-Discharge life cycle resistance (Rlc): Rlc represents deterioration-resistance that is based on the number of charge-discharge life cycle resistance as describe in Fig.4.

E represents the cell's EMF as a function of DOD. Based on the characteristic shown in Fig. 7, it is expressed as follows: E = E o ; at DOD ,560 % E = Eo - k.f ; at DOD > 60 % where: k constants obtained by experiment

.

1 Battery open-circuit voltage, E:

f depth of discharge (DODO/) Eo EMF at fully charged

The battcry modcl prcscntcd hi Fig. 10 reprcscnts thc pcrformancc of only oiic NAS ccll. In fact, a BESS uscs BM which includes several cells connected in sesies and/or parallel. Hence, we are more interested in modeling thc BM than just a cell. In this paper a 50kW NAS-BM (Type G50), which consists of 320 cells connected in series and demanded, for high capacity storage device is sclccted. The 5OkW-BM voltage-current behavior can be estimated and simulated by multiplying the internal cell resistance and cell's EMF by 320 based 011 the given internal cell resistance data and performance information from battery manufacturer.

EMTDCiPSCAD is an industry standard simulation tool for studying the transient behavior and custom power application of electrical networks. Using one of the built-in components available in EMTDCIPSCAD, all thc important manufacturer data and parameters are tabulated and compiled into a data- reference look-up table. Consequently, the feature of components (Rc, Rd, E and Rlc) as described in Fig 10 are plotted and dcrived from the data-reference look-up table by using linear interpolation or extrapolation calculation with respect to the input value of DOD% and temperature-change. This is prescnted in Fig. 1 I . Fig. 1 I(a) shows the block diagram of XYZ component which outputs the valuc of x (Rd curve) based on the input values x (temperature-cliange) and y (DODX) using interpolation calculation by referring into tlic data references files bascd on the experimcntal data. Delail of this XYZ relation is illustratcd i n Fig. I I (b).

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Fig. I I : (a) X Y Z Companeiil which i s provided in EMTDCiPSCAD (b) Rclarionship bclwecn data rerercnccs look-up curve and simulated Rd-curvc as a fiinction oltcmperaturc aid DOD%,.

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V. SIMULATION RESULTS A N D VALIDAT~ON

In this section, the siriiulation is divided iiito hvo parts. Firstly, the electrical NAS battery modcl i s tested and simulated as per cell basis to examine its voltage-current behavior at different powcr output discharge Icvel. Thcti, the simulation is carried out for modeling the pulse power capability of SOkW-BM (320 cells i n serics). Both the simulations results are compared with esprriinental rosults for validation purposes.

A. NAS cell Simulation and Results In order to validate the NAS cell electrical battery model,

simulation is caii+d out for Peak Shaving (PS) application and its result is compared with the experimcntal result having the same operations as foollows:

(a) 4 hours ofconstant power discharging at X4W, (b) 3 hours of constant power discharging at 136W, (c) 4 hours of constant power discharging at 84W, (d) 2 hours standby, and ( e ) 9 hours of constant power charging at 135W.

Fig. 12 and I 3 show the results obtained ftom simulation and experiment respectively. The characteristic and behavior of the NAS cell terminal voltage and current obtained from experitnenral measurement appear to be similar to the simulation. When observed closely, the simulated terminal voltage waveform shown in Fig. 12 is actually slightIy different fi-om that in Fig. 13 at several instances duc to the following reasons.

The simulation does not take into account the short duration alteration of polarization effect at the edge of switching operation, which slightly affects NAS internal resistance in practice. However, this polarization effect is known to be of little concern in PS application [2]; hence, it is not taken into consideration in the siniulatioti studies.

Besides that, the charging pattern at the end of charging is different. In practice, the internal resistance at the end of charging operation is high (as shown in Fig 3) due to the insulating nature of pure sulfur [8]. When "AS battery continues to be charged at constant power, its internal resistance increases significantly causing larger energy loss at higher terminal voltage. To avoid this, wlien terinind voltage reaches a set point, charging power is decreased to reduce excessive energy toss. This is calIed "supplement charge" as is evident in Fig. 13. In the simulation, the "supplement charge" is not simulated for simplicity, The ceIl is only subjected to a canstant charging power, which raises the terminal voltage as it comes to thc end of charging.

It can be seen that the behavior of voltage and current demonstrated a somewhat concave pattern at, the shoulder discharging hours. The reason for this is that during discharging, the EMF of NAS battery decreases when DOD exceeds around 60%. Hence the cell is pushed for higher output current with terminal voltage drops to produce a constant power output.

Fis. 12 Voltclgz-currcnt Beliavror of a NAS cell (Simulation)

B. 50kW NAS-BM Siniulation and Results Fig. 14 shows the simulation result of combined peak

shaving with appl'0Xi"eiy 5 times the rated power for 30 scconds pulse power test having I hour time iiirerval between each pulse. The simulation performs peak shavitig at 50kW for 5 hours and 5 pulse power events during PQ application. The initial temperature is set at 295 degree C.

The sampling time interval of data shown in Fig. 14 is 10 seconds. The top waveform shows the variation of temperature and the bottom waveform shows the DC power output capability of the 50kW battery module. During each pulse power event, the BM temperature increase around 3 degree C. Temperature rise is one of the cause for concern becausc,it limits the PQ and PS application. When pulse power outpiit interval is shortened or pulse power events are more frequent,'' non-equality of temperature is not dissolved and temperature imbalance is enlarged. Therefore, pulse power output will be stopped due to significant temperature rise until it reaches the limitation of 360 degree C.

As seen in Fig. 14, the first pulse power output is slightly lower than the second one. This is because the internal resistance is high at the beginning initial temperature. Referring to the internal resistance chart shown in Fig. 3, the internal resistance rises at deep discharge region, which means that the pulse power capability at end of discharge will bc reduced. For example shown in Fig. 14, the fifth pulse power has a small decrement compared to thc previous pulse powcr Ou(p1It.

By comparing the data-result given by manufacturer as shown in Fig. 6 with that in Fig. 14, the results are found to be in agreement. This shows that the battery model dcvcloped and simulated is accurately representing the NAS battery.

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[7] Ilyogo Takaini, Ainir l3:islia Isin;iil and Molid Ftidzil Moiril Siriii l. 'NAS Ihltcry Eiicrgy Storiigc Syslctii for I'owcr Qu;ility Suppart iii M;ilaysi;i', presented nt IPEC2003 Conr. Novcntbcr 28, 2003. David Liiidcri and T. Rcddy. ' I Iiindbook of Uattcrics', Sccoiid Edi~ioii~ library of Congrcss Catalo~iiig-iir-Ptiblicatioii Dalii, Soduiiii tJCIil

Bntlcrics, Chapter 40: pg40.1-40.23,

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Fig. 14: Relationship between Pulse Powcr Output aiid tempcralurc change o f a SOkW NAS-BM (Simulation).

VI. CONCLUSlON

NAS battery is relatively new in the market, therefore essential work is required to model the performance of NAS battery to observe its capability and potential in making high

IX. BIOGRAPHIES

Lcc Wci Chuiig was born iii Malaysia, on April 2, 1980. t lc rcccivcd his BSc . dcgrcc i i i

Elcdricat and Eleciroiiic iroiii Uiiivcrsily o f Adelaide, Australia i n 2002. I-le is prcscnlly studying for a Mastcr of Enginccring dcgrcc ;it

Uiiivcrsity Tciiaga Niisioiial. Malaysia. while working as a Rcsearcli Assislaill 011 tlic NAS bnttcry projccl i n Maliiysia at TNB tlcscnrch Siiii. Blid. since 2003.

sinlulating NAS batteryicell, The simo\a t jon results showed that the NAS battery provides cxcellent pulsc power

turbinc cogcnclntioii systcms. I lis ctirrciit resciircii iiitcrcst iiicliidcs p w c r qu:ilily, powcr syslcnis, p o w r clcctronics niid iiu drives.

capabilities arid should perform well in PQ and PS applications.

VI1. ACKNOWLEDGMENT The authors would like to thank Hyogo Takami-san from

Tokyo Electric Power Company, TEPCO for providing the information and experimental data on NAS battcry tcclmology.

V111. REFERENCES

[ I ] Makoto Kanlibayashi and Kouji Tanaka. 'Recent Sodium Sulfiir Baitcry Applications,' in Proc. 2OOl IEEE Power Eugi,iewitrg Society Tianstriirsiort ~ n d Di.r!ribulion CO$, vo1.2, pp, I 169-1 173. Tamyurek, B.; Nichols, D.K.; Dcmirci, O., 'The NAS Rattcry: A Mulli- Function Energy Storage System', Powcr Bngit;eering Society Gcncral Meeting, 2003, IEEE, Volume: 4 , 13-17 July 2003 Pages: 1996 Vol. 4 F. M. Stackpool, W Auxer, M McNarnee and M F Mangan, 'Sodiimi Sulphur Ratlory Developmcnl', CH2761-3/89/0U00-2765, I989 IEE Takaslisma, K.; Ishiinaru, F.; Kuniniato, A.; Kagawa, M.; Matsui, IC.; Nomura, E.; Matsumaru, Y.; Kita, A.; Iijima. S.; Kaio, T.; Mnrsuo, Y . ; Nakayama, T.; Sern. Y., 'A Plan for a IMW/BMWH Sodium-Sulfur Battery Energy Storage Plant', Energy Conversion Enginccriiig Conference, 1990. IECEC-90. Proceedings of the 25th lntcrsocicty , Volume: 3 ,August 12-17, 1990 Pagcs:%7 - 371 H.L. Cban and D. Sutanto. 'A Ncw Battery Model for usc with Battery Energy Storage System and Electrical Vehiclcs Power Systems.' 2UU0 IEEE. Yoon-Ho Kim. Member, IEEE, and Hoi-Doo Ha, 'Design of Intcrfacc Circuits With Electrical Batter Modcls,' IEEE Transuc!ious OII

induslriai Ekcrronics, ~01.44, No. 1, February 1997.

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