Energy Consumption Based on Actual Building Load Profile

Energy Consumption Based on Actual Building Load Profile
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  Energy Consumption Based on Actual Building Load Profile  H. Hazran,W.M.W.A. Najmiand M. Suhairil  Department of Thermo-FluidsFaculty of Mechanical EngineeringUniversiti Teknologi MARA40450 Shah Alam Keywords : Thermal energy storage, load profile, energy consumption. 1 Abstract Thermal energy storage (TES) has gained popularity among  building owners in Malaysia. An attractive tariff package is offered by the utility power provider to promote more  participation from customers. However, poor operation and maintenance of the system may lead to disappointing savings and even higher operational costs than conventional system. This paper describes the results of an investigation on actual operating condition of a centralized air-conditioning plant integrated with ice thermal energy storage. The actual operation conditions and building load profile are observed and simulations are carried out to measure its energy consumption. The results are then compared with the designed operation conditions and load profile. The results show the actual operation is under cooling load demand capacity and the ice storage is not fully utilized. The ice charging process only stored 6,000 RTh compare to the nominal tank capacity of 10,800 RTh. Energy consumption is higher since the chiller has to top up the remaining 4,800 RTh during peak period. The results are then used to develop control modes to meet daily fluctuating cooling demand. Based on the designed modes and the chiller efficiency, the  bill is RM 199,216.34 compare to the current average monthly electricity bill of RM 250,000. In conclusion, there are rooms for improvement and after the fine-tuning process is fully successful, the energy consumption is expected to be reduced 2 Introduction The rapid pace of industrial development and the increasingly higher standard of living in Malaysia has led to continual rise in industrial and residential power consumption. Demand for energy has increase with increasing of population and industries. The increase in electricity surcharge recently indicates that the supply of energy has been critical and will grow up in the future. Cooling of buildings in Malaysia is a major contributor to the peak electrical load. By some estimates it contributes up to 45 percent of the total electrical demand [1].There is a significant difference in electrical power imbalance between daytime need and nighttime. During the night, utilities have electricity to spare, and this off-peak electricity is much cheaper. Thermal energy storage (TES) system has been identified as a proven technology in electrical load management, shifting the energy usage to a later period to take advantage of cheaper rates and toreduce overall energy demand [2]. The contribution of TES system could reduce theelectrical peak demand toas much as 20% to 30 percent [1]. Tenaga Nasional Berhad (TNB), the national electricity  provider is taking steps in sustaining the economic growth with energy supply. One major strategy to efficiently manage the imbalance energy utilization is to promote better customer participation through a program called Demand Side Management (DSM). TES system has been identified as one of the solutions and has been gaining popularity among  building owners in Malaysia. In order to further encourage the implementation of TES in the country, TNB has introduced the C2 TES electricity tariff specially developed for TES application [1].  Table 1: Electricity rate structure for C2 TESMany education institutions have participated in DSM  program through application of TES technology. By some estimates, energy consumption in educational building contributes up to 65 percent of the total electrical demand of the building [3]. They are motivated to incorporate TES in a cooling system to reduce operating costs. However, as the technology is new to many people in Malaysia, operators are rarely knowledgeable about the energy consumption rates of using TES. It usually consume more energy than conventional cooling systems due to the inefficiencyof making ice at subfreezing evaporator temperatures [4].The objective of this paper is to investigate the actual operating condition of a centralized air-conditioning plant integrated with ice thermal energy storage. The actual operation conditions and building load profile are observed and simulations are carried out to measure its energy consumption. 3 Case Study The air-conditioning system integrated with TES in Kompleks Sains & Teknologi, Universiti Teknologi MARA was completed and commissioned in December 2002. The  plant consists of 3 closed pumping loops namely primary, secondary and tertiary loops with the TES system in the upper stream. The liquid flowing in the primary loop is ethylene glycol and chilled water flows in the secondary and tertiary loops. This design of multiple loops allowed the system to operate in all 5 operating modes; ice build, ice  build with cooling, cooling with ice, cooling with chiller, and cooling with ice and chiller.Figure 1: Operation schematic diagram of the plantThe plant utilizes partial storage operating strategy in which during discharge mode, the ice storage and the ice chiller running day mode work hand in hand to cover the building load. This strategy is quite critical, where controlling the contribution of chiller and ice storage are critical to the system economy and comfort. Modulating valves are used to manage the relative contributions of storage and chiller.As the load and chiller contribution varies, the modulating valves will automatically allow sufficient flow through the storage system to maintain 42ºF of ethylene glycol to be supplied to the primary HX. Heat transfer process take place  between ethylene glycol and chilled water, resulting to 44ºF chilled water temperaturewhich will be supplied to the Air Handling Units (AHU) and Fan Coil Units (FCU) for the entire building [5].The storage tank in the plant consists of 45 number of modular ice thermal storage tank with storage capacity of 240 ton-hour each. The tank is filled with water, in which is submerged a polyethylene tube heat exchanger. The heat exchanger consists of horizontal rows of serpentine coils held in a rigid bundle by radial plastic spacer bars. A glycol solution at about 26ºF (-3.3ºC) flowing inside thetubes causes the surrounding water to freeze. This water that is frozen never leaves the storage tank.The chilled glycol solution is typically provided by a chiller having about 900 tons of cooling capacity. When fully frozen, the ice cell stores 10,800tons-hours of cooling capacity. Then to serve a cooling load, the ice can be melted at a rate dictated by load. The ice cell could serve a steady load of 900 tons for 12 hours or 1800 tons for 6 hours. In  providing cooling, the glycol solution flows from the ice cell Tariff  Description C2 TESFor each kilowatt of maximumdemand per month during the RM 25.70 per kWPeakPeriod For all units during the Peak RM 0.208 per kWhPeriod For all units during the Off-Peak RM 0.118 per kWhPeriod Chiller CTIce cellAHU & FCUBlock 1,2,3Tower 1,2AHU & FCUBlock 4,5 PHXSHX  to cool the chilled water in the secondary loop through heat transfer process. 4 Analysis The tonnage for the ice cell, chiller and building loads for the current operation are calculated using the formula below. The calculations are performed using a spreadsheet software i.e. Microsoft Excel for its ability to calculate formula in a very user friendly environment. 24* T Q RT    (1)The flow rate, Q (USgpm) and temperature different, ΔT ( ºF) for ice cell, chiller and building are obtained from the temperature and flow meter sensors located at critical points in the system. Q  ΔT Ice cellFlow rate of ethylene glycol through the ice cellT leaving ice cell  –  T entering ice cell Chiller Flow rate of ethylene glycol through the chiller T leaving chiller   –  T entering chiller  BuildingFlow rate of chilled water supply to the  buildingT return header  –  T supply header  Table 2: Descriptionon the flow rate and temperature differentCooling requirementis the main factor in outlining the design of a cooling system. The fact is that the air-conditioning process is meant to meet the demand of the cooling requirement, which varies from time totime in a  particular day. A chart that represents the cooling requirement in a day is called building load profile. The cooling requirement of the building is being observed through the building automation system from time to time to obtain the pattern ofload profile.Figure 2:Building load profile for peak cooling requirementFigure 3: Load profile graph for chiller-priority control modeChiller efficiency is measured by power consumption per unit ton. Manufacturer data of chiller capacity and power consumption are those under standard operating conditions and full-load state, called nominal cooling capacity and nominal power consumption respectively. Actual process loads are significantly less than full loaddesign conditions, therefore chillers operate at full load for only a fraction of the operating time. Therefore, the Integrated Part-Load Value (IPLV) is calculated to estimate the average chiller efficiency. Building Load Profile 05001000150020002500300035000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Hour     R   T Building RT vs Time 05001000150020002500300035000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Hour     R   T Ice cell Chiller Building   DC  B A  IPLV 1  (2)A, B, C and D are the chiller efficiency (kW/ton) at 100%, 75%, 50% and 25% respectively. Table 3: IPLV value for chiller efficiencyFigure 4: Chiller power consumption OperationkW/RT chiller only (charging at 26ºF)0.9chiller only (discharging at 44ºF)0.7chiller and ice storage 0.5ice storage only0.2 TimeIce cell RTChiller RTBuilding RTkW/RTkWTariffRMMD 21:00:01 981.8 981.8 0 0.9 883.62 0.208 183.7922:00:01 981.8 981.8 0 0.9 883.62 0.118 104.2723:00:01 981.8 981.8 0 0.9 883.62 0.118 104.270:00:01 981.8 981.8 0 0.9 883.62 0.118 104.271:00:01 981.8 981.8 0 0.9 883.62 0.118 104.272:00:01 981.8 981.8 0 0.9 883.62 0.118 104.273:00:01 981.8 981.8 0 0.9 883.62 0.118 104.274:00:01 981.8 981.8 0 0.9 883.62 0.118 104.275:00:01 981.8 981.8 0 0.9 883.62 0.118 104.276:00:01 981.8 981.8 0 0.9 883.62 0.118 104.277:00:01 981.8 981.8 0 0.9 883.62 0.118 104.278:00:01  119.2   1580.8 1700 0.6 1020 0.118 120.369:00:01  819.2   1580.8 2400 0.6 1440 0.208 299.5210:00:01  769.2   1580.8 2350 0.6 1410 0.208 293.2811:00:01  569.2   1580.8 2150 0.6 1290 0.208 268.3212:00:01  619.2   1580.8 2200 0.6 1320 0.208 274.5613:00:01  1419.2   1580.8 3000 0.6 1800 0.208 374.414:00:01  1519.2   1580.8 3100 0.6 1860 0.208 386.88 4780215:00:01  1319.2   1580.8 2900 0.6 1740 0.208 361.9216:00:01  1319.2   1580.8 2900 0.6 1740 0.208 361.9217:00:01  969.2   1580.8 2550 0.6 1530 0.208 318.2418:00:01  419.2   1580.8 2000 0.6 1200 0.208 249.619:00:01  519.2   1580.8 2100 0.6 1260 0.208 262.0820:00:01  419.2   1580.8 2000 0.6 1200 0.208 249.6 *bold and italic print indicate ice dischargingElectricity consumptionelectricity cost per day:RM 5047.14456electricity cost per month (*30 days) :RM 151414.3368monthly maximum demand:RM 47802Total Cost:RM199216.34   Total Kilowatt-Hour  050010001500200025000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Hour     E   l  e  c   t  r   i  c   i   t  y   C  o  n  s  u  m  p   t   i  o  n   (   k   W   )  5 Discussion and Conclusion A good plant management is the key element to a successful ice storage system implementation. Experience in operation of ice storage system demonstrates that poor design and operation due to the control strategy can lead to disappointing savings performance. However, with the development computer software there is a tremendous  potential to reduce operating cost and increase energy efficiency with improved control.From the designed mode of operation, it is expected that the monthly electricity consumption to be RM 199,216.34 compare to the latest average monthly electricity bill of RM 250,000.00. This result can be achieved by fully utilized the ice charging process from 6,000 RTh to its nominal tank capacity of 10,800 RTh and the chiller will run at lower load. However, there are rooms of improvement that can be carried out to enjoy more saving especially by using other control modes. References [1]Salim Sairan & Rosli Mohamed, Electric Thermal Energy Storage –Mutual Benefit,  Jurutera -Journal  Institution of Engineers Malaysia , 1999.[2]W.F. Stoecker and J.W. Jones,  Refrigeration and  Air Conditioning  , McGraw-Hill, Singapore, 1982, ch. 11, 20.[3]E.M. Thomas, HVAC Principles and Applications Manual, McGraw-Hill, New York, 1997, ch. 2-5.[4]G.P. Henze, M. Krarti and M.J. Brandemuehl, Guidelines for improved performance of ice storage system,  Journal of Energy and Building Publication , 2001.[5]R. Zainuddin  , Review on Operating Improvement of the Air Conditioning System at Kompleks Sains & Teknologi, UiTM Shah Alam , Universiti Teknologi MARA, Shah Alam, Malaysia,2005.
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