Cost-Effectiveness Opportunities for Thermal Energy Storage Systems: A Case Study of School Building in Saudi Arabia

Transcription

1 International Journal of Sustainable and Green Energy 2016; 5(4): doi: /j.ijrse Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia Badr Habeebullah 1, Rahim Jassim 2, Nedim Turkmen 1, Ahmad Bokhary 1, Majed Alhazmy 1 1 Mehanial Engineering, King Abdulaziz University, Jeddah, Saudi Arabia 2 Saudi Eletri Servies Polytehni (SESP), Baish, Jazan Provine, Kingdom of Saudi Arabia address: bhabeeb@kau.edu.sa (B. Habeebullah), mhazmy@kau.edu.sa (M. Alhazmy), r_jassim@sesp.edu.sa (R. Jassim) To ite this artile: Badr Habeebullah, Rahim Jassim, Nedim Turkmen, Ahmad Bokhary, Majed Alhazmy. Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia. International Journal of Sustainable and Green Energy. Vol. 5, No. 4, 2016, pp doi: /j.ijrse Reeived: June 11, 2016; Aepted: June 20, 2016; Published: July 6, 2016 Abstrat: Air onditioning in houses, offie buildings and shools onsume high portion of the generated eletriity in Saudi Arabia. This paper presents a study of the eonomi opportunities afforded by installing an ie storage system to existing air onditioning plants of a shool in Jeddah, Saudi Arabia. In this paper, the assumptions are i) fixed interest rate of 10%, ii) a tenure of 10 years and iii) estimated operational tariff struture depending on both the number of operating hours and the ambient temperature. The study examines both full and partial load storage senarios then alulates the effet of various priing tariffs on ost optimization. The results show that the urrent fixed eletriity tariff rate of $0.0267/kWh whih is not eonomially feasible. Combining both the energy storage and an inentive time strutured rate shows reasonable daily bill savings. For a base tariff of $0.07/kWh during daytime operation and $0.0267/kWh for the off-peak period, savings of $33/d and $73.36/d is ahievable for full load storage and partial load senarios, respetively. These savings will inrease to $159/d for full load storage and $124.06/d for partial load storage after 10 years. Keywords: Ie Storage, Cooling Load, Eonomi Analysis 1. Introdution The air onditioning (A/C) systems in Saudi Arabia onsume more than 60% of the total eletri energy available for buildings. The high onsumption rates enourage the authorities to work on both inreasing the energy generating rates and reduing the demand. Therefore, shifting the energy onsumption from peak to off-peak hours and improving A/C systems performane are neessary for fulfilling the eletriity demand redution. The main purpose of using a thermal storage system (TES) is to shift the eletriity peak load assoiated with buildings' ooling from peak time to off-peak periods. When applying variable tariff poliy, TES beomes a good andidate for utilities to infore demand management for many users. Cold TES tehnology provides a feasible solution for solving peak load problems (Yau and Rismanhi [1]; Parameshwaran et al [2]; Habeebullah [3]). The TES is suitable for buildings having disontinuous working hours suh as offies, shools (Mihael [4]), ourt-halls, ampus buildings (Yau and Rismanhi [1]), subway stations (Keisuke [5]) and many others (Habeebullah [3]). Applying different tariffs based on the day time of high and low onsumption levels reates opportunities for the use of energy storage systems. Ihm et al [6] presented a TES simulation model within the EnergyPlus (EnergyPlus [7]) buildings analysis pakage. Henz et al [8] arried out an investigation of the possible savings in the eletriity bill for various storage strategies, different ombinations of hiller types, building type and weather onditions. Sanaye and Shirazi [9] performed a study on the thermo-eonomi modeling and optimum design of an ie TES for A/C appliations. The same study took into onsideration the penalty for CO 2 emission. Sebzali et al [10] have studied the implementation of a hilled water thermal system in Kuwait ity and have demonstrated that this redues the peak power demand of A/C systems.

2 International Journal of Sustainable and Green Energy 2016; 5(4): Cost Funtion The total annual ost of the ombined hillers and a storage system onsists of the annual apital ost of both the hillers C h and the energy storage tanks C st in addition to the annual operational expenses. The annual operational ost whih is a funtion of the operation period t op, the onsumed eletrial energy E el, and energy rate C el ($/ kwh) is given as 3. Operation Strategies Cold storage systems usually operate on either full storage or partial storage modes. The storage supplies the total energy needed during peak hours while the hillers operate only at nighttime. Figure 1 shows an illustration for this senario for a daily ooling load with a peak period between 7:00 AM and 4:00 PM. C total = a t op [ Ch + Cst ] + CelEeldt 0 ($/y) (1) where, ny ( ) ( ) a = i 1+ i 1+ i 1 r r r is the apitalreovery fator after a period of ny years. The apital expenses omprise the purhase of the equipment, equipment installation, and maintenane. Aording to the mehanial equipment index, the hillers' apital ost depends on the ooling apaity C A. Therefore, the hillers the ost is C = α C h h A, where α h is a multipliation ost index in $/kw or $/ton refrigeration (TR) (Habeebullah [3]). This analysis estimates the maintenane ost as a perentage of the apital ost α m ; hene the hillers ost is C h The apital ost of the ie storage tank depends on the required mass of ie during the ie build-up period. The speified ooling load variation for the orresponding ie buildup proess affets size seletion as well as the tank internal oils and auxiliaries. If the thermal storage apaity is S st (kwh), the apital ost of ie storage inluding installation, piping, aessories and ontrol units is expressed as where α st is in $/kwh, whih is furnished by manufatures. Substituting Eqs. 2 and 3 into Eq. 1 gives C total = a (2) (3) ($/y) (4) In Eq. 4, the eletrial energy onsumption E el (kw) is funtion of time. It inludes the energy onsumed by the ooling hillers and/or ie making units during the period t op. The hourly ooling load determines the needed eletri power to drive the air onditioning system. The ooling load varies with the building struture, ativities of the oupants in addition to the ambient onditions. Eletrial energy ost is also another fator. Eletriity ost might follow from one of many aounting method and ould have a fixed flat rate or variable tariff rate (e.g. funtion of time of use). ny ( m ) αhca =1+α C = α st st [( 1+ αm ) αhca + αstsst ] + CelEeldt S st t op 0 Figure 1. Shemati of full load storage senario. For partial storage systems, the peak load overs a small segment of the load while the hillers meet the rest as shown in Fig. 2. The part load senario is suitable for load leveling or demand limiting. In this senario, the hillers operate ontinuously for 24 hours mostly at the rated apaity. During the periods of low demand, the exess energy is stored, whih is used later to over the peak load. In our ase study, the hillers operate between 6:00 PM to next 6:00 AM depending on the time required to harge the storage beause there is no ooling load during this time. 4. Appliation to the Shool Air Conditioning Plant New shool buildings in Jeddah are onstruted to a standard plan alled Mubassat Modon. Shools are normally built as a omplex of three idential buildings. Eah shool ontains twenty-five lassrooms, two offies, two labs, an open area, and a library distributed on three floors. A typial shool plan is shown in Figs. 3 and 4. The shool is subdivided into a total of five main zones (Table 1) and the running time for the purpose of ooling load is established as between 6:00 AM to 4:00 PM. The ooling loads have been determined from shool building simulations using EnergyPlus software (Energyplus 7 ). The total area of m 2 exhibits an approximately 1608 GJ peak eletrial demand for non-thermal loads. Details of maximum loads are presented in Table 1. The ooling load is estimated around 1273 TR h using EnergyPlus. Fig. 5 learly shows how the shool ooling load hanges from 6:00 AM to 5:00 PM, as a result of the outside temperature variation.

3 61 Badr Habeebullah et al.: Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia Figure 2. Shemati of partial storage or load leveling senario. Figure 3. Typial shool omplex in Jeddah.

4 International Journal of Sustainable and Green Energy 2016; 5(4): Figure 4. Floor design of a shool building in Jeddah.

5 63 Badr Habeebullah et al.: Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia 5.2. Cooling Load Calulation Proedures Zone Figure 5. Shool ooling load urve. Table 1. Summary of EnergyPlus results. Max. Design Load [W] Cooling Interior lighting Interior equipment Fans Pumps 78 Total (W) Total (TR) Total End Uses (GJ) 2314 Eletriity [GJ] The data presented in Fig. 5 indiates that two hillers in whih eah apaity is of 100 TR are suffiient for the ooling load between 6:00 AM and 5:00 PM. One hiller is operating at full load and the other one is partially in operation to over the peak load. During off-peak operation, the units operate at part load and onsequently at a low oeffiient of performane (COP) that diretly inreases the power onsumption per kw. For the evaporation temperature of -1 C, the COP varies daily in terms of the ondensing temperature as alulated from the log data sheet and provided by the maintenane department of the shool. It an be seen that during the peak load hours and beause of the high ambient temperature, the COP drops from 2.46 at dawn to 2.2 at midday. This simply means a derease in the units' performane and an inrease in the energy onsumption. 5. Shool Cooling Load Calulations The analysis in this study uses Energyplus to perform the ooling load alulations for the shool s three floors Comfort Standards The onditions that affet the omfort levels of both students and shool staff have been onsidered. Using the reommended indoor air onditions for omfort as published in ASHRAE Standard 55 [11], Thermal Environmental Conditions for Human Oupany, the ranges of air temperature and relative humidity (RH) are aeptable to at least 80%. The following step-by-step instrutions summarize how to alulate shool ooling loads: 1. Selet indoor and outdoor design onditions. 2. Use arhitetural plans (Fig. 4) to measure dimensions of all surfaes through whih there will be external heat gains, for eah zone. 3. Calulate areas of all these zones. 4. Selet heat transfer oeffiient U-values for eah element from appropriate tables, or alulate them from individual R-values. 5. Determine time of day and month of peak load for eah zone by alulating external heat gains at times for whih they are expeted to be a maximum. Hourly shool ooling loads for 24 hours were onverted to ooling load temperature differene (CLTD) values by dividing the roof or wall area and the overall heat transfer oeffiient so that the shool ooling load an be alulated for any wall or roof by the following relation: where Q ɺ w Q ɺ w = A U CLTD : the rate of heat transfer in Watts (W). A: the outside surfae area of the wall (m 2 ). U: the overall oeffiient of heat transmission in W/m 2.K. CLTD : Correted Cooling Load Temperature Differene CLTD = CLTD + LM + R t DR: Daily Range LM: CLTD orretion for Latitude and Month t a : indoor temperature ( C) t o : outdoor temperature ( C) 5.3. Class Room Peak Cooling Load a = t o ( 25.5 t ) + ( ta 29.5) ( DR/ 2) Air onditioning system must be sized to handle peak loads that should be determined based on the estimated ooling load. The external heat gain omponents vary in intensity with time of day and time of year following the hanging in solar radiation aused by the hanges of the sun orientation and hanges in the outdoor temperature. This results in a hange in the total room ooling load. Oasionally, it is immediately apparent by inspeting the tables at what time the peak load ours, but often alulations are required at a few different times. Some general guidelines an be offered to simplify this task. From the CLTD, SHGF, and CLF tables (ASHRAEStandard 55 [11]) we an note the following: 1. For west-faing glass, the maximum load is in midsummer in the afternoon. 2. For east-faing glass, maximum solar load is in early or (5) (6) (7)

6 International Journal of Sustainable and Green Energy 2016; 5(4): mid-summer in the morning. 3. For south-faing glass, maximum solar load is in the fall or winter in early afternoon. 4. For southwest-faing glass, maximum solar load is in the fall in the afternoon. 5. For roofs, maximum load is in the summer in the afternoon or evening. 6. For walls, maximum load is in the summer in the afternoon or evening. These generalizations an be used to loalize approximate times of zone peak loads. For instane, we might expet a south-faing zone with a very large window area to have a peak load in early afternoon in the fall, not in the summer. If the zone had a small glass area, however, the wall and glass heat ondution might dominate and the peak load time would be a summer afternoon. One the appropriate day and time are established, a few alulations will determine the exat time and value of the peak load. 7. Calulate eah zone peak load, using the values for the external heat gains determined above and by alulating and adding the internal heat gains from student/staff, lights, and equipment. If there is infiltration, this must be added to the zone ooling load. 8. Find the time of shool peak ooling load - see suggestions below Shool Peak Cooling Load The shool ooling load is the rate at whih heat is removed from all air-onditioned zones in the shool when the shool ooling load is at its peak value. If peak ooling loads for eah zone were added, the total would be greater than the peak ooling load required for the whole shool due to the fat that these peaks do not our at the same time. Therefore, the designer must also determine the time of year and time of day at whih the shool ooling load is at a peak, and then alulate it. A reasoning and investigation similar to that arried out in finding zone peak loads is used. From our experiene, the following guidelines emerge: 1. The shool buildings have approximately squared shapes in plan with similar onstrution on all four walls. The peak load is usually in late afternoon in summer. This is beause the outside temperature is highest then, and there is no differential influene of solar radiation on one side of a building. 2. For shool buildings with a long south or southwest exposure having large glass areas, the peak load may our in the fall, around mid-day, beause radiation is highest then. This ase requires areful analysis. 3. For one-storey shool buildings with very large roof areas, the peak load usually ours in the afternoon in summer. These suggestions must be verified in eah ase beause there are so many variations in the onstrution and orientation of shool buildings. One the peak load time is determined, the total building heat gains an be alulated. The searh for the time and value of peak zone and building ooling loads is greatly simplified by using Energyplus software. After the neessary data are entered, a omplete time profile of loads for many hours an be developed in a few minutes. 4. Calulate the shool building load at peak time, adding all external and internal gains and infiltration, if any. Add supply dut heat gain, dut heat leakage, and drawthrough supply fan heat gain if signifiant 5. Find the ooling oil and refrigeration load by adding the ventilation load to the shool building heat gains; add blow-through fan, return air fan, and pump heat gains, if signifiant Cooling Coil Load After the shool building ooling load is determined, the ooling oil load is found. The ooling oil load is the rate at whih heat must he removed by the air onditioning ooling oil(s). The ooling oil load will be greater than the building load beause there are heat gains to the air onditioning system itself. These gains may inlude: 1. Ventilation (outside air) 2. Heat gains to duts 3. Heat produed by the air onditioning system fans and pumps 4. Air leakage from duts 5.6. Refrigeration Load The refrigeration load is the load on the refrigeration equipment. For entral systems with remote hilled water ooling oils, the pump heat is a load on the refrigeration hiller, but not the ooling oil. For a diret expansion system, the refrigeration load and ooling oil load are equal. For hilled water system, the refrigeration load is the ooling oil(s) load plus the hilled water pump heat Shool Cooling Load Calulations Summary The HVAC system with storage for the example Building will have the following harateristis: The primary hillers and the partial modular or enapsulated ie storage system meet design day ooling load. Thermal storage plaement is downstream of hillers. The atual apaity of the primary hillers at design ambient ondition of 46 C is 200 tons to over the required shool apaity of 1273 TR h. Ethylene glyol brine (25%) is the heat transfer fluid in the Primary distribution, hillers and storage loops. Chilled water is the heat transfer in the Seondary networks. On-peak period loads are ooled by the ombination of primary hillers and storage system. A seondary hiller meets oinident off-peak ooling. Variable flow pumps are used for storage, Primary and Seondary loops. Constant volume pumps are used for hiller loop. Deoupler line is used for flow balanes. No blending

7 65 Badr Habeebullah et al.: Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia or bypass valves are required. In-building heat exhangers to separate the Primary brine and the Seondary hilled water networks. Twoway ontrol valves loated at outlets of exhangers and AHU oils ontrol both sides of the heat exhangers. Coinidental ooling loads have the brine pipe onneted diretly to the AHU oils. Charging of the storage system is ontrolled by Time of Day Programming. Two indiators ontrol storage ie level: tank water level measurement and brine leaving temperature. Disharge strategies are Time of Day Programming and BMS seletable disharge. Charging shedule is between 5 pm to 6 am the day after. System an operate in six operating modes: hillers only, storage disharge, hiller and storage disharge, storage harge, Charge with oinidental ooling load, and off. Operating temperatures of storage; During harging: at inlet T is = -5 C (23 F) at outlet T os = - 1 C (30 F) During disharge: at inlet T isd = 15.5 C (60 F) at outlet T osd = 5.5 C (42 F) Operating temperatures of hillers; During harging: at inlet T i = -1 C (30 F) at outlet T o = -5 C (23 F) During disharge: at inlet T id = 15.5 C (60 F) at outlet T od = 5.5 C (42 F) Set point Control: Chiller Only = 5.5 C (42 F) Storage Only = 17 C (62 F) Charge = -7.7 C (18 F) Operating temperatures of building brine-side heat exhangers; at inlet T ie = 5.5 C (42 F) at outlet T oe = 15.5 C (60 F) Total additional ost of introduing the ie storage system is SR 850,000 for full load and SR 350,000 for partial load. Based on the shool s available data it is of interest to look Senario 1 Full load storage Time Hours Required load, TR for an energy storage option and operation shedule that provide the most eonomi ost funtion, C total. 6. Costing Senarios For the present ase, an ie storing tehnology is adapted and two senarios are investigated in the following setion. Senario Full Load Storing Sheme For full load storing, it is suggested that a substantial amount of ie be produed overnight to handle the entire peak load between 6:00 and 16:00 o'lok. In this proposed operation senario and beause of the ooling load, two hillers with a apaity of 100 TR operate at full load between 7 pm to 5 am as shown in Table 2. No hiller is in operation as water hiller to provide the shool with the neessary air onditioning load, where the other hillers harge the storage tank (produe the required amount of ie) this is beause there is no load at shool between 6 am to 4 pm. The present hiller ontrol is set to provide hilled water at 5 C at evaporator temperature of -1 C. Using the same hillers to produe ie requires a redution of the evaporator temperature to -10 C therefore, the ie making hillers' ooling apaity is redued to less than 100 TR. The refrigeration yle is solved for evaporator temperature of - 10 C with R404A and the drop in apaity was found to be 4.95%. Therefore, a orretion fator of for the evaporating temperature hange is used. In addition, storing energy in the form of ie passes through a freezing proess where the rate of heat transfer is affeted by the ie buildup thikness. A fator of 0.75 is assumed for the ie formation proess (Anonymous [12]). This makes an overall onversion fator of , whih means that the hiller apaity when ontrolled to make ie redues from 100 TR to 71 TR. Table 2. Senario 1 Operation sheme for full load storage (based on Carrier HXC30-100TR). Water Chiller Ie Chiller Storage No. of Chillers in operation TR No. of Chillers in operation TR disharge TR 0 0 off off * off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off off Total No. of hillers in operation

8 International Journal of Sustainable and Green Energy 2016; 5(4): Senario 1 Full load storage Time Hours Required load, TR Water Chiller Ie Chiller Storage No. of Chillers in operation TR No. of Chillers in operation TR disharge TR off off off off off off off off off off off off off off off off off off off off off off off off off off off off Total, TR-h * The hiller apaity is multiplied by orretion fator (0.7088) to onvert the water hiller into ie hiller Total No. of hillers in operation The total ooling load, whih is the integrated area under the ooling urve, amounts to 1273 TR-h as shown in Fig. 6, of whih 510 TR-h is the off-peak ooling load and the rest is stored in the ie tanks. Therefore, the plant is operated during nighttime and early morning, only making use of the relatively low ambient temperature and high COP. The operation time shedule is given in Table 2, whih indiates that only 1 hiller operating lose to full load for only 9 hours is required to handle the entire total daily ooling needs. The main advantage of the suggested senario is the improved operation of the hillers the entire time (average COP a = 2.2). Figure 6. Full load storing senario (based on Carrier HXC30-100TR). Let us investigate the eonomis of the proposed senario where the hiller operates only for 11 hours at their rated apaity. The eletriity tariff as used in Saudi Arabia is shown in Table 3. The daily operational ost (the last term in Eq. 4) for normal operation without ie storage using the tariff rate that shown in Table 3. Table 3. Saudi Arabia eletriity tariff am-8 am am-12 pm 0.15 Saturday to Thursday 12 pm-5 pm pm-12 am C el E el dt = =670 SR/d (177 $/d) 24 ( Qɺ evap COPa ) Celdt = The daily operation ost of making ie during the 9 hours

9 67 Badr Habeebullah et al.: Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia nighttime and morning period is alulated in two parts using the data of Tables 1 and 2 to give ( ) 3.51 ( ) =314 SR/d (84 $/d) The apital investment of the ie storage tank is determined by alulating the mass of ie formed during the 9 hours operation period that is neessary to provide 1273 TR-h (4468 kw h). Following the urrent priing data for loal market ost of 668 SR/ TR h. The apital investment for the storage tanks C st is then 850,000 SR. Assuming an interest rate of 10% and 10 years paybak period the apital reovery fator a is The apital investment annuity, A, is a Cst A = = x 850,000 =138,550 SR (36,947 $) Assuming 300 working days per year, the storage apital ontributes 126 $/d (462 SR/d) for the daily total investment repayment. The total daily effetive expenses, inluding the ost of the water hillers in Eq. 1, is C [ C + C ] d, n = a h st / n day topqɺ evapc + COP =126+84=$210 (788 SR) (8) The daily ost feasibility, or in other words the daily benefit when introduing ie storage tanks, is the differene between the expenses of the plant without and with the ie storage system as = -33 $/d (-124 SR/d). The negative sign here indiates that there is no saving and the shool AC plant is performing well with the urrent tariff. a el However, after 10 years the apital will be paid off and the saving will be =93 $/d (349 SR/d). Additionally, assuming the base tariff struture where the urrent low rate of 0.1 SR/kWh is fixed as nighttime rate and the daytime rate is maintained at its level of 0.26 SR/kWh, the daily ost of operation will be inreased from 177 $/d to 243$/d. Thus, the daily saving will be =33 $/d (124 SR/d) and after 10 years it will be =159 $/d (596 SR/d). Senario Partial Load Storing Sheme Inspetion of the ooling load shows that the peak load falls between 6 AM to 4 PM, where the load reahes 134 TR. For partial storing the peak load is leveled at 94.5 TR for 8 hour only so that the energy above this load and from 4 pm to 6 pm is supplied from storage tanks (Fig. 7). This arrangement will redued the apital invested in ie storage at the same time redued the transformer and ables ost. The operation senario is one hiller overs the energy required below the seleted level of 94.5 TR running at 100% apaity the extra loads are overed from the ie storage tank whih harge by running 2 hillers between 1 am-5 am. The advantage of this arrangement is the operation of all hillers with maximum COP and at lowest tariff. The energy stored during this period is 567 TR h while the required energy during the disharge period is 517 TR h. In this ase adjusting the ontrol of the hillers to have a shorter ie-harging period is required to orret the differene. In this senario only 350,000 SR will be invested in ie storage system. Table 4 shows the detailed operation sheme for partial storage senario. Figure 7. Partial load storing senario (based on Carrier HXC30-100TR.

10 International Journal of Sustainable and Green Energy 2016; 5(4): Table 4. Senario 2 of ooling load profile. Senario 2 Part load storage Time Hours Water Chiller Ie Chiller Required No. of Chillers in No. of Chillers Total No. of hillers load, TR TR Exess Storage disharge TR TR operation in operation in operation * off off off off off off off off off off off off off off off off off off - - Total, TR-h * The hiller apaity is multiplied by orretion fator (0.7088) to onvert the water hiller into ie hiller Let us investigate the ost for the proposed partial load-storing senario, where the ie hillers operate only for 4 hours, 1-5 AM, at their rated apaity and average COP a. Noting that the average atual COP a is 2.46 for the time between 1 am -5 am and 2.2 between 10 am and 1 pm. Following the same alulation proedure and using the data of Table 4 the ost items are summarized as, a Operation ost for the total ooling load, 1273 TR-h 177 $/d Operation ost for the water hillers $/d produing 756 TR-h= b Operation ost during ie making period ( ) =30.44 $/d 2.46 d The i Ie storage apital to form 517 TR-h equivalene of ie in 4 h = 93,333 $ e Fixed harges rate for the ie storage apital (300 working days/y and fixed harges rate = 50.7 $/d C d, n f Total daily ost with ie storage = b+ + e $/d g Net daily benefit (a f) 39.7 $/d Here, it is seen that the positive sign means that there is a little saving with the partial storage senario. The reason is the eletriity tariff rate as harged by the authorities is relatively affordable. However, the annual saving is 11,910 $ (44,663 SR) and after 10 years it will be ( = 27,120$) (101,700 SR). The net daily savings of 39.7 $/d an be inreased to $/d if a base tariff struture is assumed where the urrent low rate of 0.1 SR/kWh is fixed as nighttime rate and while daytime rate is maintained at its level of 0.26 SR/kWh. Table 5 shows the savings using this senario where the annual saving inreases from 11,910 $ to 22,008 $. Table 5. Daily saving of partial load storing senario. Funtion Current Tariff Fig. 2 $/d a b e f= b+ + e g=a-f Assumed Tariff SR/kWh nighttime 0.26 SR/kWh daytime $/d

11 69 Badr Habeebullah et al.: Cost-Effetiveness Opportunities for Thermal Energy Storage Systems: A Case Study of Shool Building in Saudi Arabia 7. Conlusions This study investigated the potential savings that result from the installation of a thermal energy storage system to the air onditioning plant of a shool building in Saudi Arabia. In the analysis eonomi outomes are onsidered in terms of what effet time-strutured tariffs an have, oupled with the role ative energy storage systems an play, on the daily eletriity utility bill. The results based on measured ooling load and ambient temperature showed that with the urrent subsidized eletriity rates as shown in Table 3 there is no gain in Table 6. Daily saving for different senario and tariff. introduing ie storage systems for a full load storage sheme. However, there is a daily saving of $39.7 for partial load storage shemes as shown in Table 6. Combining energy storage and an inentive time strutured rate showed reasonable daily bill savings. A base tariff of $0.07 /kwh during daytime operation and $0.0267/kWh for the off-peak period, make a total savings of $33/d and $73.36/d for full load storage and partial load senarios, respetively. These savings inreases to $159/d for full load storage and $124.06/d for partial load storage after 10 years of operation as summarized in Table 6. Full load Saving $/d Partial load Saving $/d Current base tariff Inentive base tariff Current base tariff Inentive base tariff Present time After 10 years Present time After 10 years Present time After 10 years Present time After 10 years Full load Saving $/d Table 7. Daily saving for all Jeddah shools. Partial load Saving $/d Current base tariff Inentive base tariff Current base tariff Inentive base tariff Present time After 10 years Present time After 10 years Present time After 10 years Present time After 10 years 0 17,786 6,311 30,409 7,593 17,289 14,076 23,726 Aknowledgement This projet was funded by the National Plan for Siene, Tehnology and Innovation (MAARIFAH) King Abdulaziz City for Siene and Tehnology - the Kingdom of Saudi Arabia award number (08-ENE 194-3). The authors also, aknowledge with thanks Siene and Tehnology Unit, King Abdulaziz University for tehnial support Notation apital reovery fator C unit ost ($/kwh) C el unit ost of eletrial energy ($/kwh) SHGF solar heat gain fator CLF ooling load fator COP oeffiient of performane d day E el onsumption of eletrial energy (kw) = a i r interest rate n number of operation days per year ny number of years of repayment Q ɺ evaporator apaity Ton-Refrigeration evap t op period of operation per year (h) Subsripts a average h hiller el eletriity ie inlet heat exhanger oe outlet heat exhanger i inlet hiller during harging id inlet hiller during disharging o outlet hiller during harging Q ɺ evap COP a is inlet storage during harging isd inlet storage during disharging od outlet hiller during disharging os outlet storage during harging osd outlet storage during disharging s, st storage GENERAL COMMENTS Referenes [1] Yau Y H, Rismanhi B (2012). A review on ool thermal storage tehnologies and operating stratigies. Renewable and sustainable Energy Reviews, 16: [2] Parameshwaran R, Kalaiselvam S, Harikrishnan S, Elayaperumal A (2012). Sustainable thermal energy storage tehnologies for buildings: A review. Renewable and sustainable Energy Reviews, 16: [3] Habeebullah, B (2007). Eonomi feasibility of thermal energy storage systems. Energy and Buildings, 39: [4] Mihael H (2003). Ie thermal storage for Colorado Shool. ASHRAE Journal, 45: [5] Keisuke O, (2002). Thermal storage air onditioning system in subway station building. Japanese Railway Engineering, 148: [6] Ihm P, Krarti M, Henze G (2004). Development of a thermal energy storage model for EnergyPlus. Energy and Buildings, 36: [7] EnergyPlus (2014). EnergyPlus Manual, Doumentation V , program DOE 2014, USA [8] Henze G, Krarti M, Brandemuehl M (2003). Guidelines for improved performane of ie storage systems. Energy and Buildings, 35: p

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