APPLICATION OF COMBINED ICE EXTERNAL MELT PARTIAL THERMAL ENERGY STORAGE AND DISTRICT COOLING SYSTEM FOR OPTIMAL ENERGY EFFICIENCY FOR ACADEMIC, REEARCH AND DEVELOPMENT SETTING Binoe E. ABUAN, Menandro S. BERANA, Ruben A. BONGAT University of the Philippines Diliman, uezon City, Philippines Abstract: This paper primarily discusses the combination of ice-external-melt partial thermal-energy-storage (TES) system and district cooling system (DCS) on the design of air conditioning system for complexes that are used for academic and research and development operations. An institution of such natureand operating in a complex of buildings can take advantage of the TES system and the DCS.Giventhe option for commercial and industrial applications to avail of the time-of-use (TOU) electricity rate, where the electricity rate at night is cheaper than that at day, the cost of operation can be reduced when ice is made at night and melted at the following day for the complex.the efficiency of the chiller system used in making ice can even be improved because the ambient temperature at night is relatively lower than that at day. The efficiency of distribution of cooling medium in a complex, which is normally chilled water, is enhanced when DCS is used.the TES design was based on the computed cooling load of the complex. Cooling loads wereobtained using the Cooling Load Temperature Difference (CLTD) Method on per room or cooling space basis. The designed TES storage tanks are proposed to be installed underground together with the piping network for the DCS distribution. The compressor rack system wherein compressors are placed in racks in the central cooling plant is employed in the design for versatility and economy of operations of the compressors and overall chiller systems. Air handling units (AHU s) were designed based on the area to be conditioned and the cooled air distribution system throughout the buildings is chosen to befabric air ducts which will reduce maintenance costs and noise caused by flow and vibrations.economic comparison of the proposed system with a conventional one wherein each building will be using separated chillers in the basement was performed. The investment cost is outweighed by the lower operational cost and the resulting payback period is attractively short. The overall coefficient of performance (COP) of the complex using combined TES and DC systems is higher and thus the operational cost and payback period are lower compared to the conventional system. Aside from functioning as the cooling system of the complex, the proposed system can also become a facility for further research and development in ice TES, DCS and cooling systems in general. Keywords: Thermal Energy Storage (TES), District Cooling System (DCS), Time of Use (TOU), Cooling Load Temperature Difference (CLTD) Method, Coefficient of Performance (COP) 1. INTRODUCTION The latest update from the Energy Information Administration (EIA) shows that there is a constant increase of 6.7% on carbon emissions for the whole world (from vehicles and buildings) starting 2010.This is due to the fact that there is also an increase on commercial buildings, vehicles and machineries across the world. Increasing carbon emissions contributes largely to the environmental problems the world is facing nowadays, that s why energy efficient systems are needed to help reduce energy consumption without sacrificing the environment. One of the energy efficient systems emerging today is the Thermal Energy Storage (TES) whereas energy is stored when it is not needed and is used at the most optimized time. This system can be used for air conditioning by making ice on coils at night (using external melt partial system) when air conditioning at schools is not needed, when compressor temperature is low, and when electricity is cheaper.it is melted on day time and chilled water system distribution is enhanced using the district cooling method.
2. METHODOLOGY 2.1 Design Intent The Engineering Complex is composed of nine 2-floor Engineering buildings (one per department and one graduate studies building) and a 3-floor administrative building. The 8 departments are Mechanical, Civil, Electrical and Electronics, Industrial, Chemical, Geodetic, and Materials and Metallurgical Engineering. The Engineering Complex should provide the needed thermal comfort for students and teachers based on standards using the combination of external-melt partial thermal energy storage system and district cooling system (DCS) distribution. 2.2 Cooling Load Calculation The Cooling Load Temperature Difference (CLTD) Method was used to approximate heat gains from different heat generating components inside the buildings. These sources of heat include solar transmission and absorption through the roof, walls and windows, heat generated by building occupants, heat generated by lights and appliances, and finally the infiltration air from the ambient. The CLTD method includes the following formula for computing the cooling load: Heat Gain from Occupants sensible = N ( S ) (CLF) latent = N ( L ) Where N = number of people in space S, L = Sensible and Latent heat gain from occupancy CLF = Cooling Load Factor, by hour of occupancy. (This is a constant provided by ASHRAE for the CLTD method) Heat Gain from Lighting = 3.41 x W x F UT x F SA x (CLF) Where W = Watts input from electrical lighting plan or lighting load data F UT = Lighting use factor, as appropriate F SA = special ballast allowance factor, as appropriate CLF = Cooling Load Factor, by hour of occupancy. (This is a constant provided by ASHRAE for the CLTD method) Heat Gain from Appliances Sensible = in x F u x F r x (CLF) Latent = in x F u Where in = rated energy input from appliances F u = Usage factor F r = Radiation factor CLF = Cooling Load Factor, by hour of occupancy Heat Gain due to Infiltration of Air sensible = 1.08 x CFM x (To Ti) latent = 4840 x CFM x (Wo Wi)
Primary Heat Gain Roof = U * A * CLTD Roof Wall = U * A * CLTD Wall = U * A * CLTD Glass Conduction Glass = A * SC * SCL Glass Solar Radiation Note: CLTD, CFM, SC, CLF are constants provided by ASHRAE for simpler computation. Table 1: Cooling Load Calculations Summary Solar Time Total Cooling Load Per Total Cooling Load for All Total Cooling Load Total Cooling Load for Total Cooling Load for Engineering Building Engineering Buildings for Admin Building Engineering Complex (Btu/hr) Engineering Complex (TR-hr) 7am 310850.582 2486804.656 358568.984 2845373.64 237.11447 8am 376362.9852 3010903.882 431312.8512 3442216.733 286.8513944 9am 384791.03 3078328.24 440379.392 3518707.632 293.225636 10am 396061.9844 3168495.875 452597.1264 3621093.002 301.7577501 11am 409887.5082 3279100.066 468495.8312 3747595.897 312.2996581 12nn 423191.2138 3385529.71 483661.1288 3869190.839 322.4325699 1pm 436567.4224 3492539.379 499156.7944 3991696.174 332.6413478 2pm 448522.591 3588180.728 513309.596 4101490.324 341.7908603 3pm 459864.6696 3678917.357 527022.0336 4205939.39 350.4949492 4pm 467100.8992 3736807.194 536118.7032 4272925.897 356.0771581 5pm 474202.3358 3793618.686 545063.8408 4338682.527 361.5568773 6pm 475255.8414 3802046.731 546197.1584 4348243.89 362.3536575 7pm 476309.347 3810474.776 547330.476 4357805.252 363.1504377 The cooling loads for the nine engineering buildings are assumed to be the same since they are all composed of the same number of classrooms, laboratories, etc. There is a separate computation for the administrative building s second and third floor cooling loads. The first floor of the admin building (the cafeteria) will not be included in the centralized air conditioning system. Table 1 shows the summary of the total hourly cooling load calculations that will be the basis for designing the size and capacity to be used for the engineering complex. 2.3 Thermal Energy Storage System Design Thermal Energy Storage (TES) systems, in general, refer to a system that stores energy for later use. This scheme can be applied for district cooling of the engineering complex. Figure 1 and 2 shows the schematic of a conventional water chiller system and an external melt ice on coil TES system respectively. Utilizing TES can lower operating costs and reduce maintenance expenses of the university. The external melt ice on coil system builds and stores ice on the external surface of a heat exchanger coils submerged in a non-pressurized water tank. This system operates at night when electricity costs are lower and it will then melt the stored ice to meet the cooling demand the next day when the power costs are higher. Capitalizing on this price difference between on-peak and off-peak electricity cost makes TES an attractive alternative to conventional water chiller systems. The external melt ice on coil system requires less electricity than conventional water chiller systems since the ambient drybulb conditions are lower at night than day time. These result in lower system discharge pressure, higher coefficient of performance and lower compressor drawn current. Wet-bulb temperature during night is also lower than it is at day time thus; cooling tower operation is more efficient, requiring less water flow rate and lower pumping power requirement. Lastly, TES has a lower chilled water discharge temperature which results in higher available water temperature difference. This lowers chilled water flow rate requirement of building cooling systems requiring smaller pumping power. Partial ice storage was selected over the full ice storage for the air conditioning system of the engineering complex. In a partial ice storage system, the compressors still operate at part-load and aid the ice thermal energy storage during the day while in a full TES the compressors are totally shut down during the day. Partial ice storage is practical for comfort cooling applications compared to full ice storage which is best suited for process cooling applications. Full ice storage has advantages such as a higher COP and lower operations cost but partial ice storage has a significantly lower capital installation. Full ice storage system requires more compressors, pumps and cooling towers since the amount of ice made should be greater to meet the
building cooling load. Partial ice storage also has a simpler control system due to fewer components. The operating cost savings of full ice storage are not sufficient to justify its high installation cost,while advantages of the partial ice storage system justify its high operating cost in the setting of this research. One constraint in the use of TES for building air conditioning is the usual space constraint but in the case of our engineering complex, the ice storage tanks would be placed underground so that the space above it can be utilized for other facilities such as a sports complex or a park. This scheme also lessens the solar heat absorbed by the ice storage tanks which would otherwise hasten the melting of the ice and have a negative impact on the system. Figure 1 Schematic Diagram of water chiller with shell and tube condenser, cooling tower, semi-hermetic compressor and centrifugal pumps Our engineering complex will utilize an external melt ice-on-coil system which uses submerged evaporator coils where refrigerant or secondary coolant is circulated, resulting in ice accumulating on the external surface of the evaporator coils during the night. Storage is discharged by circulating warm return over the evaporator coils during the day, melting ice from the outside. Figure 2 shows the schematic diagram of an external melt ice-on-coil TES system which includes a heat exchanger (aimed to isolate the open storage tank from the building distribution system) and a chiller barrel to supplement stored cooling during discharging periods. Pre-cooling of return chilled water was done on the water chiller, reducing its temperature from 16⁰C to 8⁰C. Melting the ice from the TES storage tank further reduces its temperature to 2⁰C before supplying to the engineering complex buildings. Outlet and inlet temperatures at TES storage tank are 1⁰C and 6⁰, respectively. Figure 2: Schematic diagram of external melt ice on coil system
Ice Thickness Table 2 shows the results for the ice thickness computations for three types of compressors. The calculation procedure used is based on mathematical modeling made by Bongat, 2012 [1]. The results are also presented graphically on Figure 3. The ice build-up time is from 8 pm to 7 am and during this time, the engineering complex is no longer under operation while the system is in full blast. From 8 am to 3 pm the compressors will just aid in the cooling done primarily by melting the ice made overnight. From 4 pm to 8 pm, the compressors will be totally shut down emptying the ice formed on the evaporator coils for another cycle of the ice thermal energy storage system to initialize again. 3.5 3 2.5 2 1.5 1 0.5 0 Ice Thickness vs. Operating Time 0 5 10 15 20 25 30 35 Time in Hours Compres sor 1 Compres sor 2 Compres sor 3 Figure 3 The corresponding plot of ice thickness on coils versus time of day for the 3 compressor models Four (4) Compressors each with a capacity of 182 tons of refrigeration will be used for the ice build-up of the thermal energy storage system of the engineering complex. The system will have an evaporator temperature of 20.13 F (-6.54 C). Computations for the compressor capacity and evaporator temperature can be found in Appendix B. Table 3 shows the comparison of power consumption for two centralized air conditioning systems with the same total compressor capacity (tons of refrigeration). One system utilizes ice TES partial storage and the other system uses conventional water chillers only. The values for the annual projected energy consumption are from the electric motors of the two systems that could be found in the compressors, cooling tower fans, chilled water pumps and cooling tower pumps. Time Table 2 Hourly ice build-up on coil Accumulated ice thickness in W50 168Y W40 168Y Z50 154Y Remarks 01:00 1.92 1.72 1.38 ice build-up 02:00 2.19 1.95 1.61 ice build-up 03:00 2.44 2.15 1.83 ice build-up 04:00 2.67 2.34 2.02 ice build-up 05:00 2.88 2.52 2.20 ice build-up 06:00 2.88 2.69 2.37 ice build-up 07:00 2.88 2.85 2.53 ice build-up 08:00 2.83 2.76 2.42 melting while comp in operation
09:00 2.76 2.67 2.30 melting while comp in operation 10:00 2.69 2.58 2.18 melting while comp in operation 11:00 2.62 2.49 2.04 melting while comp in operation 12:00 2.55 2.39 1.91 melting while comp in operation 13:00 2.48 2.28 1.76 melting while comp in operation 14:00 2.40 2.18 1.61 melting while comp in operation 15:00 2.10 2.07 1.44 melting while comp in operation 16:00 1.77 1.73 1.25 forced compressor shut-down 17:00 1.38 1.33 0.71 forced compressor shut-down 18:00 1.00 0.94 0.00 forced compressor shut-down 19:00 0.50 0.41 0.00 forced compressor shut-down 20:00 0.00 0.00 0.00 pre-cooling and ice build-up 21:00 0.23 0.37 0.00 ice build-up 22:00 0.80 0.84 0.34 ice build-up 23:00 1.26 1.18 0.78 ice build-up 24:00 1.61 1.47 1.11 ice build-up Annual electricity consumption for the water chiller option was computed at 1,419,632 kw-hrs, valued at PhP 7,898,321 while for the TES option: 1,661,587 kw-hrs/year at PhP 6,986,787.68. Note that the annual projected power cost of TES system is 11.54% lower than that of conventional water chiller with cost difference amounting to PhP 911,533. Operating the system at night when the temperature is lower will result in an increase of COP contributing 13.76% to the total electricity cost reduction. The bulk of the electricity cost savings (86.24%) are due to the time of use (TOU) ratesimplemented in the country wherein electricity rates for utilities using more than 500 kwh per month are lower during off-peak hours. As can be seen from Table 4, electricity costs are significantly lower from 10 pm to 8 am compared to the normal electricity rates shown on Table 5. The ice thermal energy storage system utilized by our engineering complex predominantly runs at full capacity during these offpeak hours to build ice thus, greatly reducing the monthly electricity bills of the university. Table 3 Comparison of operating parameters (conventional chiller and TES partial storage) Particulars Conventional Water Chiller (1) TES partial storage (2) Annual operating time, hours 3,696 5,016 Total annual power consumption, kw-hrs 1,419,632 1,661,587 Average hourly power consumption, kw 384 331 Total compressor capacity, TR 497 497 Average annual kw/tr 0.773 0.667 kw/tr reduction Basis of comparison 13.76% Computation of total savings due to COP improvement and TOU power rate Particulars Conventional Water Chiller (1) TES partial storage (2) Power cost, PhP/yr 7,898,321 6,986,788 Annual power savings, PhP Basis of comparison 911,533 Annual power cost reduction, % Basis of comparison 11.54% Percentage of reduction due to COP improvement 13.76% Basis of comparison Percentage of reduction due to TOU power rate 86.24%
The cooling towers that will cool the chiller condensers will also have a better operation during the night since the wet bulb temperature of the ambient air that the water being cooled should approach is lower as compared to its wet bulb temperature during the day. Table 5 Time of Use (TOU) Rates in Luzon Table 6: Normal Electricity Rates in Luzon
Figure 4 The air conditioning ducting and floor plans for the mechanical engineering building. Note: Other engineering buildings just follow the same layout as the ME building. Figure 5 The air conditioning ducting and floor plans for the administrative building There will be one air handling unit (AHU) per building that will blow air (a mix of warm return air and fresh air from the outside) towards the coils containing the supply chilled water from the ice thermal energy storage system. The ducting used will be rectangular galvanized iron sheet covered with polyurethane insulation. Vibration isolators are present at the entrance and exit of the AHU to dampen noise and vibration. Volume control dampers are also present inside the ducts to control the volume of air flow to various parts of the system. Each duct is subdivided into a supply air duct which supplies cold air to the
building load and a return air duct which delivers warm air back to the AHU for cooling. A fresh air duct is connected to the AHU to replenish the oxygen content of air inside the building which is used up by the building occupants. 3 CONCLUSIONS The layout of the proposed engineering institution is shown in Figure 8. The engineering complex would utilize external melt partial energy storage with district cooling system distribution. There would be four chillers as computed connected to the ice storage tanks. The ice storage tanks will be located underground and so with the pipes connecting it to the distribution system for the solar heat effect on the tanks to be minimized and to provide more space for the complex. Four cooling towers are located beside the mechanical engineering department to serve cooling water for the condensers. Power cost savings of the proposed engineering complex for utilizing thermal energy storage over the conventional water chiller district air conditioning system amounts to Php 911,533.00. This is due to the utilization of Time of Use (TOU) and the ambient temperature at night time in the ice thermal system. Figure 8: Proposed Engineering University Complex Layout 4 RECOMMENDATIONS Partial external melt ice thermal energy storage, a compressor aided system, was used in the design and computation in this paper. In this scheme, the compressors still run during some of the system cooling time. Full storage system, another type of thermal energy storage wherein compressors are fully shut down during cooling time of the system, can be modeled in replacement for the partial energy storage. This system will benefit on low cost operation but the initial capital cost will be very high compared to the compressor aided system.
APPENDICES Appendix A. Hourly cooling load profile
Appendix B. Design Evaporator Temperature in F
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