Heat Transfer Analysis of Stratified Chilled Water Storage Tank for District Cooling Systems

In district cooling and heating systems, the colony of buildings are air-conditioned as against central air conditioning where only one building is maintained at comfort conditions. The chilled water is produced with a large chiller and supplied to different buildings through the pipe network. DCS systems avoid the initial cost of installing chillers and cooling towers. Thus, the chilled water is supplied as a utility like electricity, water, and gas.

District cooling systems are gaining momentum in India considering the large demand for comfort and commercial applications with environmental solutions. A stratified chilled water storage system eliminates shortcomings of conventional ice bank storage system which is high power consumption due to lower evaporating temperature. District cooling plants are used due to less fixed cost and decarbonization. Thermal Energy Storage (TES) tanks work as a reservoir that can sustain peak load easily [1]. The TES tank reduces the need for using chillers at its peak capacity, fulfilling the disparity between demand and supply of energy. The DCS systems can offer large prospects for energy-saving, energy efficiency, and societal benefits in terms of climate change. In India, at Gujarat International Finance Tec-City (GIFT), the DCS system used is 180,000 TR against the capacity of 270,000 TR. The chilled storage tank is designed in such a way that they are charged during an off-peak period whereas discharged in peak time minimizing electrical demand from 240 MW to 135 MW as per the district cooling guidelines, India. Thus, there is a tremendous saving in energy and environment. The substantial advantage of the DCS is to reduce the required capacity of the chilling plant as systems are designed based on average demand rather than peak demand. TES with a district cooling system provides many advantages over fluctuating refrigeration needs.

It is found that TES reduces around one-third of peak load along with peak electricity demand by 10% to 20% [2]. The thermal energy in the TES is stored in either latent heat or sensible heat. The warm water and cold water coexist in the stratification tank due to the proper design of diffusers. The stratification is observed when the mixing of water, thermal losses, and thermocline thickness is at a minimum. The stratified tank consists of three zones at the bottommost of the tank as chilled water zone, the top, the warm water zone, and the intermediate zone as thermocline region separating chilled and warm water.

The researchers showed that for proper diffusers and height of the tank, stratification efficiency is over 90%. They used cylindrical and radial diffusers in the study of stratification to find out thermocline thickness and found that radial diffusers are better over cylindrical diffusers for Froude number (Fr) of two. They compared the performance of small and large diffusers with octagonal diffusers and found that octagonal diffusers produced minimum mixing for Fr=1. The performance of stratified cooled water storage systems serving in hot, humid regions presented quantitative information about performance [3-5]. It is observed that due to higher daytime ambient temperature than night, 65% more demand for electricity in the daytime [6-8]. They found that TES could save energy, operation, maintenance, and initial costs. In the DCS system, smaller air handling units and ducting are needed due to more temperature difference. The lower the thermocline thickness, the better the stratification and FOM [9-11]. It is observed that the stratification phenomenon is dependent on the MIX Number, stratification coefficient, and exergy loss. They studied that exergy efficiency is an important frame of reference than energy efficiency [12]. They proved that with DCS, waste heat application has improved from 77% to 96% with carbon dioxide reduction by 8% considering environmental conditions [13]. Energy consumption has significantly increased due to population, modernization, and economic development in different countries. The solar and wind energy resources are synchronous [14, 15]. District heating systems are useful for thermal energy consumption in residential and commercial buildings. They developed the Modelica library to study DCS with charging and discharging operations. Thus, they saved the time needed for district heating/cooling system testing and analysis [16, 17].

The CFD simulations are performed for thermocline thickness in a storage tank with different cylindrical openings and diameters. They studied flow patterns with the simulation during charging and discharging operations [18]. They studied thermal energy systems and the parameters affecting the stratification with CFD. They considered five tank dimensions with varying diameters to study the outcome of velocity at the inlet and aspect ratio of the tank for thermocline thickness calculation [19]. They studied the stratification phenomenon and validated it with CFD [20]. Thus, different researchers studied the stratification in district cooling for residential and commercial applications. In the existing research, the experimental setup is designed, fabricated, and developed to conduct trials at different flow rates for charging and discharging operations.

3.1 Thermocline formation in stratification tank

In a stratified storage tank, warm and chilled water coexist at the same time due to the proper design of upper and lower diffusers. Heat transfer by conduction is observed between warm and chilled water through the thermocline section. Thermocline thickness formation is a significant contribution to the stratified tank. The highly stratified system is inversely proportional to the thickness of the thermocline. Thus, a high stratification system has a narrow thermocline and minimum temperature difference. Thermocline thickness (T_H) measures mixing intensity in a stratification tank as shown in Figure 4.

$T_H=\frac{T-T_C}{T_h-T_C}$           (1)

where, T_c: inlet water temperature, T_h: initial temperature of water in TES tank.

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Figure 4. Thermocline in storage tank

It is observed that the dimensionless temperature ratio varies from zero to one. Thus, thermoclines give a clear indication of mixing intensity. The small stable thermocline is a better sign of stratification. It is observed that thermocline thickness depends on natural convection between the outside wall and ambient temperature, mixing of water due to high velocity through the diffusers, diffusion and conduction properties of fluids, and aspect ratio of the stratified tank. Thermocline thickness in the experimental trial was found to be 0.175 m for 100 LPH water flow rate. Figure 5 shows the actual diffusers designed and developed for the storage system at the bottom and top of the stratification tank with 10 holes inside and outside the ring. Thus, thermocline gives a clear indication of mixing between warm and chilled water as shown in Figure 6. Figure 7 represents the temperature of water is different from inlet to outlet.

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Figure 5. Diffusers used in stratification tank

Thermoclines are plotted for different flow rates such as 100 LPH, 150 LPH, and 200 LPH. It is observed that mixing intensity is proportional to incoming flow. Thermocline thickness increases with an increase in flow rate and hence badly affects the stratification. Thermocline thickness increases with time while charging the tank due to disturbance in the water. Figures 6, Figure 7, and Figure 8 show thermocline formation at 100 ltr/hr, 150 ltr/hr and 200 ltr/hr respectively.

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Figure 6. Thermocline for a flow rate of 100 ltr/hr

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Figure 7. Thermocline for a flow rate of 150 ltr/hr

ANSYS Fluent 2022 is used to simulate a stratified tank with 2D surface model. All quad mesh with a total of 15421 cells, 31107 faces, and 15687 nodes with unsteady state, first order implicit laminar model is used. The boundary conditions given are inlet, outlet, wall, hot water, and cold water. The energy equation is used to assign temperature conditions. The material used is water as liquid with different boundary conditions using standard initialization and assigned different temperature water conditions to hot and cold-water zones. The laminar model in Fluent 2022 is used for simulation for 10000 iterations with an inlet velocity of 0.097 m/s. Figure 9 shows the temperature profile developed during the charging operation.

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Figure 8. Thermocline for a flow rate of 200 ltr/hr

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Figure 9. Temperature profile in tank

3.2 Aspect ratio

The aspect ratio is an indicator of tank height and diameter. A higher aspect ratio is better for stratification. Thermocline thickness is inversely proportional to the aspect ratio of the tank. In the experimentation, the aspect ratio of the tank used is 2.75. It is observed that the optimum design of a chilled water tank could help in better stratification along with control of temperature difference, fluid velocity, and thermocline region.

3.3 Flow rate of water

The flow rate of water that passes through a given surface per unit of time is important for mixing. The efficiency of stratification decreased for a large water flow rate. In the experimentation, trials are performed for volume flow rates varying from 50 LPH to 350 LPH.

3.4 Mixing coefficient

Richardson (Ri) and Reynolds number (Re) indicate the significance of stratification. Richardson number provides information on natural and forced convection while the Reynolds number is a measure of laminar or turbulent flow. The mixing coefficient is the ratio of Reynolds number to Richardson number. The mixing coefficient of unity represents perfect stratification. It is observed that the Mixing coefficient is a direct measure of thermocline thickness. Reynolds number and Richardson number are expressed as:

$R e=\frac{\rho v d}{\mu}=\frac{1000 \times 0.0010078 \times 1}{0.001427}=706.23$         (2)

$\begin{aligned} R i=\frac{g \beta \Delta T h}{v^2}= & \frac{9.81 \times 0.07333 \times 10^{-3} \times 8 \times 5}{(0.0010078)^2} =28331\end{aligned}$      (3)

The mixing coefficient Z can be expressed as:

$\begin{aligned} Z= & 1.688 \times 10^4\left(\frac{R e}{R i}\right)^{0.67} =1.688 \times 10^4\left(\frac{706.23}{28331}\right)^{0.67} =1422.82\end{aligned}$       (4)

3.5 Figure of merit (FOM1/2)

The Figure of Merit for Half Cycle (FOM_1/2) is a performance parameter during the charging and discharging cycle. It is the relative measure of discharge capacity to ideal capacity. It is an energy-based performance to calculate cooling capacity lost due to the conduction of heat in the thermocline. The lost capacity is determined when for exit temperature is at the highest temperature (Tlimit) [28-38]. For a charging cycle:

$\operatorname{Cmax}=\rho A H c(\mathrm{Th}-\mathrm{Tin})$          (5)

Cint $=\rho A H c($ Tini $-T f)$           (6)

Clost $=\rho A H c(T f-$ Tin $)$           (7)

Cmax $=($ Cint + Clost $)$           (8)

FOM $_{1 / 2}=\frac{T_h-T_f}{T_h-T_{i n}}$            (9)

3.6 Equivalent lost tank height (ELH)

Equivalent lost tank height is the ratio of loss capacity to the extreme capacity of the tank. It is a measure of loss of energy in a storage tank due to a decrease in water height due to mixing and heat loss by conduction and convection.

$\mathrm{ELH}=\frac{\mathrm{C}_{\mathrm{LOST}}}{\rho \mathrm{cA}\left(\mathrm{T}_{\mathrm{h}}-\mathrm{T}_{\mathrm{in}}\right)}$            (10)

where, A=tank projected area, ρ=water density, c=specific heat.

The experimentation is performed on fabricated DCS for different flow rates varying from 50 LPH to 350 LPH. The sample reading is shown below for reference. Water flow rate: 100 LPH, Chiller temperature: 7℃, Hot water temperature: 30℃, Suction pressure: 40 PSIG, Discharge pressure: 130 PSIG. Table 4 shows the instruments used in the experimental setup with the accuracy.

Table 4. Instruments used in the setup

The system parameters are studied by Hasan and Theeb [1], and the results are compared for different water flow rates. It is found that the results are in good agreement for performance parameters such as thermocline, figure of merit, and tank loss height.

Experiments are performed on fabricated test rigs for different water flow rates from 50 LPH to 350 LPH with linear diffusers in stratified thermal energy cylindrical tanks for charging and discharging operation and validated results with CFD simulation. The following conclusions are made from the analysis.

• TES tank is a thermal reservoir. Energy is stored in chilled water during off-peak time which can be used in peak time
• District cooling system reduces plant capacity due to energy stored in chilled water during off-peak time. The plant can run on 70% of rated capacity.
• In the case of central air conditioning, more units will be needed as standby for different buildings. In a District cooling system, one stand-by unit is sufficient which reduces operational and initial cost.
• Smaller refrigeration equipment sizes and costs apply to DCS as they are designed for average loads rather than peak loads.
• For a water flow rate of 100 LPH, the performance parameters such as thermocline thickness of 175 mm, half Figure of Merit as 0.9 with equivalent loss tank height as 0.07 are observed.
• District cooling systems are superior to other air conditioning systems considering operating costs. The district cooling systems can be used as district heating systems in colder countries. Various chemicals can be used with the water to increase the specific heat of the water to increase the cooling energy storage capacity.
• The future research includes computational fluid dynamics study of thermal energy storage for stratification in district cooling systems with different simulation models.