1504 IEEE Transactions on Power Systems, Vol. 8, No. 4, November 1993

COST BENEFIT ANALYSIS OF A COOLING ENERGY STORAGE SYSTEM

C.S. CHEN, MemberJEEE J.N. SHEEN Department of Electrical Engineering

National Sun Yat-Sen University Kaohsiung, Taiwan, R.O.C.

-Since the air conditioner(A/C) load contributes 30% of the peak demand of Taiwan Power Company(Taip0wer) system during the summer season, load management by clipping A/C load has become one of the most important topics in Taipower system. Since the eutectic salt is a complete inorganic compound with freezing point at 470F and latent heat 41 Btu/lb, it is a good medium for the energy storage system. In this paper, an A/C system with 1800 RT-HR eutectic salt energy storage tank was built on the campus of National Sun Yat-Sen University (NSYSU) for the demonstration of the cooling energy storage(CES) system. Six operation modes of the CES-system are designed to meet different cooling load requirements. By computer simulation, it is found that 41.1% of the electric peak demand has been reduced and 56% of the energy consumption has been shifted from peak hours to off-peak hours by the CES-system.

In this paper, the mathematical model of an eutectic salt CES- system has been developed for the computer simulation of the energy storage system. It is found that the optimal capacity of the storage tank is determined by the off-peak time period and the largest annual electricity charge saving could be obtained by applying the smallest nominal chiller size under the optimal operating conditions. According to the results of the computer simulation and the field test, the payback period of the test system is estimated to be 3.6 years by considering the additional investment cost and the electricity charge saving of the CES-system.

Kev Words: energy storage system, eutectic salt, peak demand reducnon, avoided cost, payback period, tank-priority .

INTRODUCTION

Since the oil embargo in 1973, various strategies of load management have been implemented by utilities to reduce the system peak demand and increase the system efficiency. Fig.1 shows the peak demand, average load and installation capacity of Taipower system. It is found that the system peak demand has increased dramatically and the load factor becomes worse as more and more A/C loads are used in the commercial and residential area. For instance, the annual average load is increased by 849 MW while the peak demand is increased by 2180 MW during the past three years, and system available spinning reserve has been reduced to be only 5% in 1991. Fig.2 shows the A/C load in Taipower system. It is increased by more than 10 % annually and contributes more than 30% of the system peak demand. Fig.3 shows the temperature sensitivity of peak demand for Taipower system. It is found that the system peak demand is increased by 326 MW for each 1°C when the ambient temperature is above 28oC in 1990. Therefore, load management by controlling the A/C loading is a very effective strategy to reduce the peak demand for the Taipower system[ 1,2].

Direct A/C load control and A/C system with cooling energy storage are the most effective load management strategies to clip the

93 WM 196-6 PWRS A paper recommended and approved by the IEEE Power System Engineering Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1993 Winter Meeting, Columbus, OH, January 31 - February 5, 1993. Manuscript submitted April 13, 1992; made available for printing November 23, 1992.

system peak demand. Direct A/C load control strategy requires high operation cost for utilities because of the hardware and software facilities involved and only the avoided operation cost can be achieved. On the other hand, A/C system with energy storage, such as hot water storage and cooling energy storage have been implemented by utilities to provide both energy saving and demand reduction[3,4,5,6,7,8,9]. The CES-system is a technique that produces and stores cooling energy during off-peak period and uses the energy stored during peak period. By this method, both the avoided operation cost and avoided cupacity cosr of the power system can be obtained by improving the system load factor effectively without any investment. For example, if 100 refrigerant tons (RT, 1RT=12 OOO BTU/Hour) cooling load for 8 hours is required for a space during the daytime, the total cooling energy is 800 RT-HR. By operating the chiller of the A/C system 16 hours a day with a 400 RT-HR energy storage system, only 50 RT cooling capacity is required for the same cooling load. The system peak demand is reduced by 50% because the A/C load has been shifted from peak period to off-peak period.

In this paper, a mathematical model of the energy storage system has been developed to determine the proper component size of an eutectic salt CES-system. The computer simulation of the CES- system is executed to find the energy consumption and the cost/benefit analysis is performed to estimate the payback years of the system. Further, the best operation strategies of an eutectic salts CES- system is determined according to the weather conditions and the electricity rate in Taiwan.

COOLING E- SYSTEM

Fig.4 shows the block diagram of the CES-system[ lo]. Three different coolant paths are illustrated in the figure. Path A shows the charging process. Path B indicates the conventional air conditioner operation mode in which the coolant bypasses through the storage tank. Path C shows the discharging process in which the low temperature coolant flows through the cooling space to reach a comfortable room temperature. For a CES-system, various combinations of the coolant paths can be achieved by changing the control mode according to the desired operation criterion.

FigS(a) shows the electric demand of an office building with a conventional A/C system during the summer season. It is found that a very high percentage of the total demand is consumed by the A/C during the system peak period. Fig.S(b) shows the electric demand of the building with full energy storage air conditioner system. All the compression load of A/C is shifted from peak to off-peak period and the efficiency of the chiller is very high because it is operated during the night time while the ambient temperature is low. Besides, the electricity charge is reduced significantly because most of the load is shifted from peak to off-peak. However, a large storage tank and- a large chiller are required, which means that the initial investment will be larger than the other systems. It is more suitable for the concentrated cooling load such as churches. FigS(c) shows the electric demand of the building with partial energy storage air conditioner system. The key advantage of the system is the less investment required because the capacity of the storage tank and the chiller are smaller. Besides, during the light load seasons, the storage tank may supply the cooling load completely as does a full storage system.

Fig.6 shows the feasibility study procedure of a CES-system. The proper type and component size of the CES-system are determined according to the electricity charge by TOU rate structure and the cooling load profile of the study building. After deciding on the operation mode, the electricity load pattern and the electricity charge saving are then solved and the operation and maintenance cost saving can be obtained. The Payback Years is applied to evaluate the

0885-8950/93/$03.00 0 1993 IEEE

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. 1505

MW/'C

15000:

1oooO:

5000:

1978 1980 1982 1984 1986 1988 1990 Year

1980 1982 1984 1986 1988 1990 Year

Fig.3 Temperature sensitivity of Taipower peak demand

1980 1982 1984 1986 1988 1990 VI%U

Fig.1 Peak demand, ave&geload and installation Fig2 Air conditioning load of Taipower capacity of Tapower

kW

1000

500

0 0 7:30 15 2230 (b). Full energy storage A/C system

Fig3 Daily elecmc load demand of a typical building

(a) Conventional A/C system (C) partial energy Storage A/c System

Coo 1 in g n Space STH NCS =

rli*H, +rld*Hd Tower

,where NCS is nominal chiller size; Q i and 'qd are the operation efficiency of the chiller during charging and discharging period respectively. The tank size(X*STH, X is the storage capacity ratio to STH) is determined by the product of the cooling capacity and operation efficiency of the chiller during the charging hours, i.e.

A .B Storage tank

I -

I C h i l l e d w a t e r pump A d 1 I + B,C

Fig.4 Block diagram of the CES-system X*STH = NCS*qi*Hi

effectiveness of the CES-system in this paper.

By Eq.(l) and (2), the charging hours of the storage tank is solved as a function of X, i.e.

For a CES-system, the storage tank and the chiller must be properly designed to meet the system peak cooling load and the maximum daily cooling energy requirement. The chiller must be large enough to charge the storage tank completely during the off-peak period and total output of the chiller must be equal to the summation of the system cooling load and losses [11,12]. According to these criteria, the mathematical model of the CES-system is developed to determine the nominal chiller size and storage tank size as u function of the off-peak time period.

For a CES-system, the maximum charging hours Hi should be equal to the off-peak time period H, therefore,

(3) IHo Hi IH,

A).Charging hours, Hi

The maximum daily cooling energy requirement STH of a building is equal to the cooling energy produced by the chiller during the charging hour Hi and discharging hours €Id, i.e.

B).Optimal storage tank capacity ratio, Xo

For the CES-system operated at the optimal condition, the charging hours Hi is equal to the off-peak period &. Therefore, the o~timal storage tank caDacitv ratio M is solved bv Ea.(3):

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. 1506

Basic design conditions of air condotioning of the selected building

Climatic information (Average Weather Yearly of the study area)

+ +

Cooling load profile simulation

I ApplyingTOU-rate 1 .c I DecidetypeofCES-system I I Decide the components size of

CES-system

Operation mode decided

Investment 9 I Electricity ioad pattern I simulation

1 Electricity charge

Fig.6 Feasibility study procedure of a CES-system

1 xo=--- HO - C , + H o l + C 2 (4)

qd*Hd - ‘1 w h e r e C2=-- - rli*Hi Ho . .

Fig.7 shows the optimal storage tank capacity ratio to the maximum daily cooling energy requirement of the system as a function of the off-peak time period €&,.

C).Nominal chiller size, NCS

Substituting Eq.(4) into Eq.(2), the optimal nominal chiller size NCSO is,solved as a function of XO:

NCS, = C3*X,

STH w h e r e C =- 3 “*H0

The relationship of the nominal chiller size NCS and the storage tank capacity ratio X is illustrated in Fig.8. The straight line with

positive slope indicates the over optimal situation(X>Xo) and the straight line with negative slope indicates the under optimal situation(XcX0). The comer point in the figure is the optimal nominal chiller size NCSO and the corresponding storage tank capacity ratio xo.

D).Annual electricity charge saving, ES

The electric peak demand reduction PDR by the CES-system is calculated as the difference between the electric demand of the conventional A/C system and the CES-system at system peak cooling load, i.e.

PDRZEPL-ECSd

,where EPL and ECSd are the electric peak demand of the conventional A/C and the CES-system respectively. The annual elecmcity charge ECC of a conventional A/C system is calculated as the summation of electricity demand charge and energy charge:

ECC=(EPL*Dp* 12)+(EPL*Ep*Hd*L*OD) (7)

,where Dp and Ep are the demand and peak energy charge rate of the electricity tariff, and IC is the annual load factor of the conventional A/C, OD is the annual operation day of this system.

The annual electricity charge ECS of a CES-system is solved as:

E a =(ECSd*Dp*12)+(ECSd*Ep*Hd*lld+ECSi*Eo*Ho*hi)*OD = c3*c4*xo ( 8 )

,where C4=RESd*Dp* 12+(RESd*Hd*hd*Ep+RESi*Ho*hi*Eo), ECSi is the elecmc demand of the CES-system during discharging period and Eo is the off-peak energy charge rate of the elecmcity taxiff. hd and hi are the annual load factor of the CES-system during charging and discharging period respectively, E S d and RESi are the electric energy consumption rate of the CES-system during discharging and charging period respectively.

Therefore, the annual electricity charge saving of the CES- system is represented as a function of the optimal storage tank capacity ratio Xo:

(9)

Fig9(a) illustrates the relationship of annual electricity charge saving ES and storage tank capacity ratio X at various HO with Taipower energy charge ratio R=&/Ep=O.4681. It is found that the elecmcity charge saving is increased with the off-peak time period. Fig9(b) illustrates the relationship of ES and X for off-peak period HO equals to nine hours. The electricity charge saving is increased as the difference of the energy charge between off-peak and peak time period becomes larger.

E).Additional investment cost, AIC

For the CES-system, additional investment such as the storage tank, control system and tank circulating pump must be made. The additional investment cost is very country-dependent and diffcult to be represented by a gereral model.

F).Payback years of a CES-system

The annual total cost saving TS of a CES-system is solved as the summation of the electricity charge saving and the maintenance

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Hoc12

0- 0.0 0.2 0.4 0.6 0.8 1.0

Off-peak period hours, HO Tank storage capacity ratio, X

Fig.8 Relationship of NCS and X

cost saving Ms.

TS = ES + Ms

If the interest rate is r% and the inflation rate is IC%; then

1 0 (1 + r)

Present value factor PVF(n,r) = +

Futurevalue factor FVF(n,x) = Q +

The summation of the net present value over N years is solved as:

1 N PVF(n,r) * FVF(n,n) * TS - AIC

The payback period is determined as the years N when NPV is equal to zero, i.e.

EUTECTIC SALT CES-SYSTEM IN NsYSY

To solve the dynamic cooling load profile of the building so that proper energy storage system can be designed, an accurate yearly cooling load simulation program HASP (Heating, Air-condition and Sanitary Engineering Program) is executed with the Average Weather Yearly(AWY) infomation in Kaohsiung, Taiwan. Fig.10 shows the typical daily cooling load of the study building during June. According to the cooling load profile and the off-peak time period of Taipower, the tank storage capacity ratio and the nominal chiller size are determined from Eq.(4) and (5). The maximum daily cooling energy requirement STH of the study building is 3915 RT-HR. Therefore, a storage tank with 1800 RT-HR stoage capacity and two chiller with total cooling capacity 24ORT are installed. Fig.11 shows the diagram of the CES-system in NSYSU campus. Six operation modes have been designed for the CES-system as shown in Table 1 according to the cooling load requirement.

A digital control system is installed to perform functions of auto-operation, data acquisition and analysis as shown by Fig.12.

Many sensors are installed in the pipes and tank to detect the temperature, humidity and flow rate of the coolant so that the

2- 0.0 0.2 0.4 0.6 0.8

~ R-0 .1

4 R-0.4681

-- 0.0 0.2 0.4 0.6 0.8 I

1507

0 ...

Tank storage capacity ratio, X Tank storage capacity ratio, X (b) H o S hours

(The base value is $29055, it is determined at Ho=9 and R4.4681) (a) R=0.468 1

Fig9 Electricity charge saving of CES-system

performance of the CES-system can be evaluated. A kwh recorder is also installed in the power panel to record the electricity consumption of this system. The software program for the automatic-operation of the CES-system is setup in a personnel computer to execute system model control. The remote processor unit(RPU) is a microprocessor based device which accepts commands from personnal computer to perform the control of the chiller, pump and valves so that the optimal operation of the CES-system can be achieved.

RT (refrigerant tons) I I

Daily time Fig. 10 Typical daily cooling load of the study building in June

r - - - - - - - - computer * R P U I (AT)

(Remote ,r) I I processor I 'I I

Equipments *chiller

*cooling

0 Printer

Power panel *chiller kwh-Recorder

*cooling

Fig. 12 Auto-control and data-monitoring system

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1508 - From cooLin4 space

Fig.11 Configuration diagram of the CES-system

Table 1 Operation modes of the CES-system

Mod$ Process I Function I Time to apply 1 I charging I storage tank charging I off-peak hours

discharging (ch i l le r - p r ior i ty) discharging (ch i l le r - bypass) discharging (tank- parallel) discharging ( tank- pr ior i ty) conventiona AfC

tank discharging in series after chiller to supply load simultaneously tank discharging to supply load

tank discharging parallel with chiller to supply load simultaneously tank discharging in series before chiller to supply load simultaneously chiller operating to supply load

peak hours during summer

peak hours during fall

peak hours during summer

peak hours during summer

conventional A/C system operation

Abbreviation: CXchiller CHP:chilled water pump mcooling tower CV:conml valve CWP:cooling water pump m f l o w meter FS:flow switch LS:level switch PS:pressure switch PSV:pressure safety valve STFWorage tank pump Ttemperature sensor zP:mne pump

A monthly computer simulation of the cooling load of the study building is performed for June, 1991. Fig.13 shows the cooling load solved by the typical Average Weather Yearly (AWY-HASP) and the actual daily temperature cwe(0A-HASP). It is found that the average mismatch of cooling load by AWY-HASP and OA-HASP is about 3.7%, which implies that the typical weather pattern can be used to simulate the cooling load of the building for the design and analysis of the CES-system.

To perform the cost/benefit analysis of the CES-system, the monthly cooling load from April to November are calculated by HASP program with typical weather data in Taiwan. Four cases of computer simulation, which represent different operation modes of the A/C system are described as follows:

Case 1: Conventional A/C system, i.e. mode 6 only. Case 2: CES-system with tank-priority, i.e.

combination of mods 1 and mode 5. Case 3: CES-system with chiller-priority. i.e. combination of

mode 1 and mode 2. Case 4: CES-system with chiller-parallel, i.e. combination of

mode 1 and mode 4.

The parameters of the mathematical model in the previous section for the CES-system in NSYSU campus are listed as follow: - PL=420 RT STHz3915 RT-HR

OD=200 days/year D p = $ 6 . 4 3 6/k W Ep=$0.06836/kWh Eo=$O.O32/kWh H o = 9 hours r= 10% h c=O. 8 3 Xi=0.95 h 1 ~ 0 . 6 5 RESd=l.35 kW/RT RESi=1.07 kW/RT

In order to execute the energy consumption analysis of the CES-system, the energy consumption pattern of the test system should be derived from the performance equation of each component.

acquired by the monitoring system[ 131.

H d = 9 hours R= 4% The least square regression method is applied to solve the coefficients

of the performance equation of the components with system data

OA-HASP

4000

3000

1000

A

" 1' 3 4 5 6 7 8' 10 1 1 12 13 14 IS' 17 18 19 20 21 2 2 ' 2 4 25 26 2 7 28 29' June, 1991 (date with * are weekend)

Fig. 13 Monthly cooling load solved by AWY-HASP and OA-HASP

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1509

Fig. 14 and Fig. 15 are the typical daily load pattern in June and load duration curve for both the conventional A/C and the CES- system during the cooling months in 1991. The electric peak demand of the conventional A/C and the CES-system are calculated as 504kW and 297kW respectively while the system peak cooling demand is 420RT and the storage capacity of the tank is 1800 RT-HR. It is found that 41.1% of the electric peak demand has been reduced. Besides, 56% of the electricity energy consumption has been shifted from peak to off-peak period and the annual electricity charge saving is then solved by Eq.(7) as $29055. For this study case, the annual maintenance cost saving of the CES-system is estimated to be $4OOO and the additional investment cost is $101200. Therefore, the required payback period of this system is solved to be 3.6 by Eq.( 10).

A computer simulation is performed from the system data acquired by the monitoring system to verify the mathematical model. Fig.16 and Fig.17 show the electricity energy consumption and electricity energy charge for each operation case. It is found that all three operation cases of the CES-system introduce the electricity charge saving by shifting the cooling load and energy usage from peak to off-peak period. The electricity charge is the smallest if the CES-system is operated with the tank-priority method. By this method, 54% or 385.2 MWh of the energy consumption has been shifte+.from peak to off-peak period, and 35% or $30450 of the elecmcity charge can be saved annually by the CES-system

The CES-system has been proved to be one of the most effective strategies of load management to reduce the system peak demand for the utilities with high percentage of air conditioner loads during summer season. For the eutectic salt CES-system installed in the NSYSU campus. it is found that 41.1% of the electric peak

500

5 400

3 300 g 200

100

0 0 7:30 15 2230

Fig.14 Typical daily load pattern of conventional A/C and CES-system of the study building in June.

Daily time

kW

0 20 40 60 80 100 Time percentage (during cooling months, 100%-4800 hours)

demand has beenreduced and 56% of the energy consumption has been shifted from peak to off-peak hours by the tank-priority operation method of the CES-system

Fig.15 Load duration curve of the conventional A/C and CES-system

k case 4, mode 1

800

case 2 . mode 1 ea case 2 . mode 5 case 3 , mode 2

case 4. mode 4 600 a case 3 , mode 1

400

200

APR. MAY JUN. JUL. AUG. 1991, cooling month

SEP. OCT. NOV. ANNUAL

Fig.16 annual electricity energy consumption of conventional A/C and CES-system 50000

w g 40000 c 0 8000 E 30000

3 6000 20000 x

I .r(

10000 $ 4000 A w 2000

APR. MAY JUN. JUL. AUG. SEP. OCT. NOV. ANNUAL 1991, cooling month

Fig.17 Annual electricity energy charge of conventional A/C and CES-system

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1510

In this paper, the mathematical model of the eutectic salt CES- system has been derived. The optimal ratio of the storage tank capacity to the maximum daily cooling energy requirement of the building is solved as a function of the off-peak hours. According to the typical daily temperature profile in Taiwan, an optimal storage tank with the capacity to store 47.36% of the maximum daily cooling energy requirement should be designed if the off-peak time period is 9 hours. Different operation modes of the CES-system have been designed and implemented according to the variations of the cooling load conditions. Jt is found that the largest electricity charge saving can be obtained by operating the CES-system with the combination of different operation mode such as tank-priority. The monthly cooling load, energy consumption and the electricity charge of the study building have been solved by computer simulation. The payback period is then determined according to the annual electricity charge saving and the actual additional investment of the CES-system. It is recommended that the electricity rate structure of Taipower company should be revised by increasing the energy charge difference between peak period and off-peak period so that more incentive could be provided to all the customers with cooling energy storage air conditioner system.

ACKNOWLEDGMENT The authors gratefully acknowledge the help from Mr. H. S.

Chen at the Energy Committee of MOEA, ROC in the success of this work. Financial supports from the Energy Committee are also appreciated.

[ 11 "The impact of load management to Taipower system" Report from MOEA,ROC. pp.28-92, June 1989.

[2] "A case study of the CES-system in Taiwan", Energy Committee of MOEA, ROC pp.3, September 1986.

[3] C.W. Gellings and J.R. Redmon, "Electric system impacts of storage heating and storage water heating, Part I and U", IEEE

[4] C.W. Gellings and J.P. Stovall, "Generation system impacts of storage heating and storage water heating", IEEE Trans. PAS, Vo1.103, pp.1439-1446, June 1984.

[5] M.D. Adams,"American electric power system electric thermal storage program: A evaluation of the impact of the generation system", IEEE Trans. PAS, Vol.101, pp.886-894, April 1982.

[6] D.R. Laybourn and V.A. Baclawski, "The benefits of thermal energy storage for cooling commercial building", IEEE Trans. PAS. Vo1.104, pp.2356-2360, September 1985.

[7] W.G. Bentley and John C. Evelyn, "Customer thermal energy storage: A marketing opportunity for cooling off electric peak demand", IEEE Trans. on power svstemg, Vol.1, N0.4,pp.57- 61, November 1986.

[8] Y.Y. Hsu, C.J. Wu, K.L. Liou and P.S. Sung,"Design and implementation of an air-conditioning system with storage tank for load shifting", M E Trans. on Power svstem , V01.2, N0.4, pp.973-979, November 1987.

[9] G.A. Comnes, Edward Kahn, Chris Pignone and Mashuri Warren, "An integrated economic analysis of commercial thermal energy storage", IEEE Trans. on Power Svstems, Vo1.3, N0.4, pp.1717-1722, November 1988.

[lo] E.I. Mackie,"Influence of discharge characteristics on design of chilled water storage", Symposium on stratified chilled water storage: Design and Operating experience, pp.708-721, NY 87- 02.

[l 1) J.M. Ayres, H. Lau and J.R. Scott, "Sizing of thermal storage systems for cooling building with time-of-use electric rates", ASHRAE Tra n L Part.lb, Vol.%, 1984.

TIWIS. PAS, V01.101, pp.2068-2085, July 1982.

[ 121 L.K. Rawlings, "Ice storage system optimization and control strategies", ASHRAE Technical Data Bulletin: Thermal Storage, pp.7-17, January 1985.

[13] W.F. Stoecker, "Procedures for simulating the performance of components and system for energy calculations", 3rd. Edition, pp.30-65, ASHRAE, Atlanta,GA. 1975.

BIOGRAPHIES Ches was born in Pint-Tung, Taiwan on 1954. He

received the B.S. degree from National Taiwan University in 1976 and received the M.S. and Ph.D degree in electrical engineering from the University of Texas at Arlington in 1981 and 1984 respectively. Since 1984, he has been at National Sun Yat-Sen University, where he is a professor of electrical engineering. From 1989 to 1990, he is on sabbatical at Empros System International, where he works as a consultant. His research interests are in the field of power system analysis, distribution automation and load management. Dr. Chen is a member of IEEE.

Jen-Nan was born in Hu-Wei, Taiwan, on January 4,1956. He received the B.S. and M.S. degree in electrical engineering from National Taipei Inst. of Tech. and National Sun Yat-Sen University in 1976 and 1989 respectively. He worked as a director of electrical and instrumental depamnent of Taiwan Synthetic Rubber Corporation from 1980 to 1988. He is working for his Ph.D degree in National Sun Yat-Sen University.

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