application of combined ice external melt ...= u * a * cltd roof q wall = u * a * cltd wall q glass...

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, Quezon 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

Q sensible

= N (QS) (CLF)

Q latent

= N (QL)

Where

• N = number of people in space

• QS, Q

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

Q = 3.41 x W x FUT

x FSA

x (CLF)

Where

• W = Watts input from electrical lighting plan or lighting load data

• FUT

= Lighting use factor, as appropriate

• FSA

= 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

Q Sensible

= Qin

x Fu x F

r x (CLF)

Q Latent

= Qin

x Fu

Where

• Qin = rated energy input from appliances

• Fu = Usage factor

• Fr = Radiation factor

• CLF = Cooling Load Factor, by hour of occupancy

Heat Gain due to Infiltration of Air

Q sensible

= 1.08 x CFM x (To – Ti)

Q latent

= 4840 x CFM x (Wo – Wi)

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

Total Cooling Load for

Engineering Complex (TR-hr)Solar Time

Total Cooling Load

for Admin Building

Total Cooling Load for

Engineering Complex (Btu/hr)

Total Cooling Load Per

Engineering Building

Total Cooling Load for All

Engineering Buildings

Primary Heat Gain

Q Roof

= U * A * CLTD Roof

Q Wall

= U * A * CLTD Wall

Q Glass Conduction

= U * A * CLTD Glass

Q Glass Solar Radiation

= A * SC * SCL

Note: CLTD, CFM, SC, CLF are constants provided by ASHRAE for simpler computation.

Table 1: Cooling Load Calculations Summary

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 dry-

bulb 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

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

Ice

Th

ickn

ess

Time in Hours

Ice Thickness vs. Operating Time

Compressor 1

Compressor 2

Compressor 3

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.

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.

Table 2 Hourly ice build-up on coil

Time Accumulated ice thickness in

Remarks W50 168Y W40 168Y Z50 154Y

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








16:00 1.77 1.73 1.25 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 off-

peak 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)

ParticularsConventional 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

ParticularsConventional 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%

Percentage of reduction due to TOU power rate 86.24%

Basis of comparison

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

REFERENCES

Bongat, R., 2012, “Ice build-up rate on custom-designed external melt thermal energy storage (TES) – Mathematical model

and validation”, Ph.D. dissertation, University of the Philippines-Diliman, Quezon City

Bahtia A., Cooling Load Calculations and Principles, Continuing Education and Development, Inc. 9 Greyridge Farm Court

Stony Point, NY 10980

ASHRAE, ASHRAE Handbook 1997, Fundamentals, Atlanta, GA, 1997

National Power Corporation, 2009, “Time of Use Rates for Luzon Grid (ERC provisionally approved RORB-TOU rates)”.

http://www.napocor.gov.ph/Power%20Rates/eff_tou_rates_for_luzon_grid.html.

Stewart R., 1990, “Ice Formation Rate for a Thermal Storage”. American Society of Heating, Air-conditioning and

Refrigerating Engineers, ASHRAE Transactions, vol 96, pp. 400-405.

COOLTOOLS™ CHILLED WATER PLANT DESIGN GUIDE, Energy Design Resources, December 2009

Lopez, A. and Lacarra, G., 1999, “Mathematical Modeling of Thermal Storage Systems for the Food Industry”. Int.

Journal of Refrigeration, vol 22, pp. 650-658.

Fukusako, S. and Yamada, M., 1993, “ Recent Advances in Research on Water-Freezing and Ice-Melting Problems”.

Experimental Thermal and Fluid Science, volume 6, pp. 90-105.

American Society of Heating, Refrigerating and Airconditioning Engineers, Inc, 1997, “ASHRAE Handbook, Fundamentals

Volume”. McGraw-Hill Book Company, Atlanta.

Holman J.P., 1981, “Heat Transfer”. McGraw-Hill Inc., New York.

http://www.napocor.gov.ph/Power%20Rates/eff_tou_rates_for_luzon_grid.html

application of combined ice external melt ...= u * a * cltd roof q wall = u * a * cltd wall q glass...

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