Performance Estimation of Vertical Inflow Diffuser for Temperature-stratified Type Thermal Storage Tank by CFD Analysis

- Effect of Tank Water Level and Step Change of Flow Rate on Temprature Distribution -

Student Member †Taichi HINOTSU 1 SHASE Technical Fellow Kazunobu SAGARA 1

Member Tomohiro KOBAYASHI 2 SHASE Technical Fellow Toshio YAMANAKA 1

SHASE Technical Fellow Hisashi KOTANI 1 Member Yoshihisa MOMOI 1

Member Osamu KOGA 3 Member Kyouhei ICHITANI 3

SHASE Technical Fellow Mitsuru NISHIYAMA 4

1 Osaka University 2 Osaka City University 3 The Kansai Electric Power Co., Inc. 4 Taikisha Ltd.

Thermal energy storage air-conditioning system has been introduced in many buildings because it will contribute to electric-load leveling and business continuity plan, which are especially focused on after the Great East Japan Earthquake. In this paper, the effect of water level in storage tank and step change in flow rate into tank on the storage performance of temperature-stratified type thermal energy storage water tank installed in an actual high-rise building was estimated by CFD.

Introduction Thermal energy storage (TES) air conditioning system has been introduced in many buildings in Japan. In TES air conditioning system, thermal energy required in the next daytime is generated and stored during nighttime using cheaper nighttime electric power and is supplied for cooling and heating load in daytime, which can contribute to electric-load leveling as well as business continuity plan, and these advantage are especially focused on after the Great East Japan Earthquake. Water is often used as the thermal storage media because of its large heat capacity and low cost. The thermal storage tank of water TES can be classified two types; a labyrinth type and a thermal-stratified type, according to condition of water mixing. This study focuses on the temperature-stratified water TES tank where water of different temperature is stored by density difference without mixing. The temperature-stratified water TES shown in Figure-1 schematically was installed in the building (the F building) located in the center of Osaka to contribute to electric-load levelling. There are two things to be considered. One is the effect of water level in tank on the heat storage performance when water level rises due to any malfunctions. The other is the effect of step change in flow rate on the heat storage performance when

3600 3600 3600 3600 3600 3600 3600 3600 3600 3600 3600 3600 3600 3600 3600

Tank3-5 Tank

3-3 Tank3-1

Tank2-5

Tank2-1

Tank1-5

Tank1-3

Tank1-1

[the TES tank (No.3)] [the TES tank (No.2)] [the TES tank (No.1)]

Tank2-3

the input flow rate increases under peak cut operation of the tank. Yu et al.1) studied the accuracy of the computational fluid dynamics (CFD) by comparing the vertical temperature distribution in a tank between CFD and experimental results for temperature-stratified water TES tank. Yaici et al.2) used CFD analysis to investigate the influence of several design and operating parameters of a hot water storage tank installed in solar thermal energy system and compared CFD results with experimental measurements and observations. The CFD technique is efficient diagnosis tool for the performance evaluation of water TES tank in which buoyancy effect is strong,

Figure-2 Schematic plan of the TES tank installed in the F building

Figure-3 A-A' cross-section of the TES tank installed in the F building

5℃

15℃

=10KΔθ

chilled water charging

Chiller

Temperature-stratified water TES tank

5℃

15℃

=10KΔθ

AHU

Temperature-stratified water TES tank

chilled water discharging

Figure-1 Schematic diagram of charging and discharging operation

A A’Tank2-1

36003600

36003600

Tank3-5

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[the TES tank (No.3)] [the TES tank (No.2)] [the TES tank (No.1)]

Tank3-6

Tank3-3

Tank3-4

Tank3-1

Tank3-2

Tank2-5

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Tank2-6

Tank2-4

Tank2-2

Tank1-5

Tank1-3

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Tank1-4

Tank1-2

and CFD analysis was also used in this study to evaluate the effects of water level and step flow rate change on the storage performance of temperature-stratified water TES tank.

1. CFD analysis outline 1.1 Analysis domain Figure-2 and Figure-3 show the schematic plan and cros-section of TES tank in the F building. In this study, the analyzed tank is the Tank 2-3. Figure-4 shows the CFD geometric model of the Tank 2-3. Circular tubes are used at inlet and outlet vertical pipe in the actual tank. In this model, however, it is replaced by square pipes for easy handling of CFD as shown in Figure-5. The distance between water surface and the punched-metal shown in Figure-6 is here called “installation depth” and it is varied to examine the effect on thermal storage performance. In order to reduce the calculation load, computation domain was made to be a quarter of tank assuming space symmetry as shown in Figure-4.1.2 Analysis condition and case Table-1 shows the summary of CFD analysis. Since the density difference of water is very important for thermal stratification, the water density is not treated as constant value, but given by polynomial function3) as :

To investigate the effect of water level on the heat storage performance, three cases of diffuser installation depth (100mm, 200mm, and 400mm) are analyzed as Case A, which are shown in Figure-6 and Table-2. In Case B, investigating the effect of step change in flow rate on the heat storage performance, five cases shown in Table-2 are studied. In these five cases, their installation depth is fixed at 100mm. Nakahara et al.4) introduced a turbulent model for analysis of

thermal-stratified type water tank. However, this model had not become popular for its complexity. The RNG k-ε model is a popular turbulent model in recent CFD tools and has a good performance to simulate strong thermal stratification, and was used as a turbulent model in this study.1.3 Modeling of Punched metal In this study, punched metal was attached to diffuser top of vertical inflow type in order to obtain uniform inflow velocity distribution. In using CFD analysis, it is required to concern how to simplify the modeling of punched metal. In the previous studies1), punched metal has been modeled by the 1-dimensional pressure drop model. However, Iwata et al.5) found that flow behavior of this model has not agreed with that of the flow visualization experiments. Higuchi et al.6) found that 3-pressure drop model showed better agreement with the experiment than 1-pressure drop model in thermal storage performance as well as vertical flow pattern observed in the experiment. In 3-pressure drop model, pressure drops occur in horizontal direction in addition to vertical direction. By setting the three dimensional flow resistances, the outflow direction at the diffuser can be controlled appropriately by this model. Accordingly, 3-pressure drop model is used as the modeling method of punched metal in this paper. 1.4 Evaluation method of thermal storage performance The degree of stratification is one of the most common performance indexes of temperature-stratified water TES tank. In this study, temperature gradient of thermocline region obtained from temperature profile is used as the performance index. Nakahara and Sagara7) developed an index of stored chilled water in TES tank to evaluate TES tank efficiency. The definitional equation of the index can be written follows:

Figure-4 Analysis domain

Table-2 Case of analysys

Table-1 CFD analysis condition

Figure-5 Punched-metal and diffuserCaseCase A-1Case A-2Case A-3Case B-1Case B-2Case B-3Case B-4Case B-5

Initial flow rate22.74 m3/h22.74 m3/h22.74 m3/h28.43 m3/h28.43 m3/h56.85 m3/h28.43 m3/h227.44 m3/h

Frow rate afterstep changefixedfixedfixed56.86 m3/h after 1 hour56.86 m3/h after 4 hourfixed227.44 m3/h after 1 hour fixed

Installation depth100 mm200 mm400 mm100 mm100 mm100 mm100 mm100 mmFigure-6 Caces of diffuser installation depth

CFD codeTurbulence modelDensityDiscretization schemeAlgorithmNumber of meshTime step

Initial tempratureInlet tempratureInlet flow late

Fluent14.4.7RNG k-ε modelPolynomial (quartic function)Second order upwindTransient state (SIMPLE)650000(Case A) 1.0s(Case B-1,2,3) 0.5s(Case B-4,5) 0.125s5℃15℃(Case A) 22.74 m3/h(Case B) Table 2Define flow rateDefine flow rateWall:Generalized log lowSymmetry:free slip

Boundary condition InletOutletTank wallWater surface

100mm 200mm400mm

(1) Case A-1 (2) Case A-2 (3) Case A-3diffuser

installation depth

(example)

water surface

500 1800

1600

33002340

3650

XY

Z

(b) 1/4 of Tank 2-3

Vertical pipe:200mm×200mmLength 1000mm

Box diffuser：4500mm×400mm×200mm

Punched-metal:bore 1.5mmpich2.0mm60°staggered arreangementaperture ratio 22.7%thickness 2mm

ρ = 999.869 + 6.72307 × 10-2θ - 8.87995×10-3θ2

+ 8.0389× 10-3θ3- 4.78507×10-7θ4

Thermal storage tank efficiency (η) =Storage of heat

Standard storage of heat

500 3600500

1600

16007300

33002340

(a) Tank 2-3（Capacity:227m3 ）

Inlet

Outlet

Figure-9 Comparison of the transition of vertical temperature profiles and the thermal storage tank efficiency in Case A-1, Case A-2 and Case A-3

Figure-8 Comparison of the transition of vertical temperature profiles in Case A-1, Case A-2 and Case A-3

According to its definition, as the temperature gradient of thermocline region becomes larger, water TES tank efficiency increases, i.e. thermal storage performance gets better when the temperature gradient is larger.

2. Result and discussion of CFD analysis 2.1 Effect of water level in tank on the heat storage performance Figure-7 shows the time progress of the temperature contour in Case A-2. T* is the dimensionless time as the ratio of charged water from diffuser into tank until a given time towater capacity of the tank. Figure-8 shows the time progress of the temperature profiles in Case A during discharging operation every 0.2 of dimensionless time. It is observed that there is visible difference in temperature profiles of different installation depth. The thermal storage performance of tank decreases as installation depth becomes larger. The rationale of this tendency is that the mixing of hot and cold water occur in early stage of discharging in case of large installation depth. To evaluate the result quantitatively, furthermore, the author uses the thermal storage tank efficiency. To caluculate this, the condition when discharging operation came to an end was applied as the initial condition of charging operation. Then, the boundary conditions of inlet and outlet were exchanged to start the calculation of charging operation. The calculation stopped when the process of charging came to an end. The upper temprature profiles in Figure-9 were obtained at the time when

charging operation came to an end and lower temprature profiles were obtained when discharging came to an end. It is assumed that the standard storage of heat in Case A-1 is the same as that in Case A-2 and Case A-3, to compare other cases with Case A-1. The storage of heat in each case is calculated from the area enclosed by the temperature profiles at the end of discharging and charging. The thermal storage tank efficiency is also shown in Figure-9. Based on this result, it was found that it is important to keep the installation depth small to improve the thermal storage performance.2.2 Effect of step change in flow rate on the heat storage performance Figure-10 shows the time progress of the temperature contour in Case B-4. After the flow rate was changed to 8 times as large as that at the beginning, the thermal layer is shaken temporarily, then turned back to the condition when it was generated before. While Figure-11 shows the comparison of the three cases in which the flow rates are abruptly changed during discharging operation, Figure-12 shows the comparison of Case B-1 with two other cases in which the flow rates are different and fixed from the beginning. In Case B-5 (flow rate is 8 times from the beginning), a relatively weak temperature-stratified layer is created; i.e. the heat storage performance becomes worse. In Case B-1, Case B-2, and Case B-4, no difference could be seen in the transient variation of vertical temperature distribution. According to this result, it is believed that the effect of step change in flow rate on the heat storage performance is insignificant if the strong temperature-stratified layer has been

Figure-7 Transition of temperature contours in Case A-2T* = 0.1 T* = 0.3 T* = 0.5 T* = 0.7 T* = 0.9

15.0

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[℃]

( )5,540mm( )5,640mm( )5,840mm

Diffuser installation depth

100mm

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400mm

0 0.2 0.4 0.8 10.6

Hei

ght [

mm

]

0

T*=0.2

T*=0.4

T*=0.6

T*=0.8

T*=1.0

(Case A-1)

(Case A-2)

(Case A-3)

Dimensionless temperature (θ-θini)/(θin-θini) [-]

6,000

5,000

3,000

2,000

1,000

4,000

0

1

2

3

4

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6

0 0.5 10 0.2 0.4 0.8 10.6

0

6,000

(Case A-1) η =100

(Case A-2) η =200

(Case A-3) η =400

= 93.28%

= 92.88%

= 91.63%

Dimensionless temperature (θ-θini)/(θin-θini) [-]

Hei

ght [

mm

]

5,000

3,000

2,000

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(Case A-1)

(Case A-2)

(Case A-3)

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100mm

200mm

400mm

This research was supported by MEXT Grant-in-Aid for Scientific Research (B), Sagara. K., 2013

Acknowledgement

References1)

2)

already generated in the early stage of discharging process.

3. Conclusion In this study, the authors discussed the effect of water level in tank and step change in flow rate for the heat storage performance. It was clarified that the heat storage performance got worse to some degree when the water level rose, and the effect of step change in flow rate on the heat storage performance was not significant if there exists strong temperature-stratified layer in the early stage of discharging operation.

Yu, Y., Sagara, K., Yamanaka, T., Kotani, H., Momoi, Kobayashi, T., Iwata, T., Kitora, H., and Taguchi, Y.: CFD Analysis on Thermal Storage Performance of Temperature-stratified Water TES Tank with New Type Diffuser, Proc. of SET2010 - 9th International Conference on Sustainable Energy Technologies, Shanghai, China, In CD-ROM, No.SE-049, 2010Yaici, W., Ghorab, M., Entchev, E. and Hayden, S.: Three-

Figure-11 Comparison of the transition of the vertical temperature profiles in Case B-1, Case B-2 and Case B-4

T* = 0.1 T* = 0.3 T* = 0.5 T* = 0.7 T* = 0.9Figure-10 Transition of temperature contours in the Case B-4, in which flow rate was changed to 8 times at T*=0.13

Figure-12 Comparison of the transition of the vertical temperature profiles in Case B-1, Case B-3 and Case B-5

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dimensional Unsteady CFD Simulations of a Thermal Storage Tank Performance for Optimum Design, Applied Thermal Engineering, No. 60, pp.152-163, 2013Niwa, H., Sagara, K., Sekimomo, Y., Inooka, T.: Study on Optimization of Thermal Storage Performance for a Temperature Stratified Thermal Storage Tank, Transactions of SHASE Japan, Vol.56, 57-68, 1994 (In Japanese)Nakahara, N., Sagara, K. and Tsujimoto, M.: Water Thermal Storage Tank Part 2 Mixing Model and Storage Estimation for Temperature-stratified Tanks, ASHRAE Transactions, Vol. 94, Part 2, pp.371-394, 1988 Iwata, T., Sagara, K. et al.: An Experimental Study on Inflow Behavior of Vertical Inlet for Temperature-stratified Type of Thermal Storage Tank, Proc. Annual meeting of AIJ, 1033-1034, 2012 (In Japanese)Higuchi, A., Kobayashi, T., Iwata, T., Sagara, K., Yamanaka, T., Kotani, H., Momoi, Y., Koga, O., Ichitani, K., and Nishiyama, M.: CFD Analysis of Temperature-stratified Water Thermal Energy Storage Tank - Modeling of Punched Metal Attached to Top Surface of Diffuser -, Proc. of Annual Meeting SHASE Kinki Chapter, Special Issue of International Exchange Workshop, Osaka, Japan, pp.39-42, 2013Nakahara, N., Sagara, K.: Water Thermal Storage Tank Part 1 Basic Design Concept and Storage Estimation for Multi-connected Complete Mixing Tanks, ASHRAE Transactions, Vol.94, part 2, 346-370, 1988

3)

4)

5)

6)

7)

0 0.2 0.4 0.8 10.6

T*=0.1

T*=0.3

T*=0.5

T*=0.7

T*=0.9

Case B-1 (changed from 28.43 m3/h to 56.86 m3/h after 1 hour)Case B-2 (changed from 28.43 m3/h to 56.86 m3/h after 4 hour)Case B-4 (changed from 28.43 m3/h to 227.44 m3/h after 1 hour)

6,000

Dimensionless temperature (θ-θini)/(θin-θini) [-]

Hei

ght [

mm

]

5,000

3,000

2,000

1,000

4,000

00 0.2 0.4 0.8 10.6

0

T*=0.1

T*=0.3

T*=0.5

T*=0.7

T*=0.9

Case B-1 (changed from 28.43 m3/h to 56.86 m3/h after 1 hour)Case B-3 (fixed flow late, 56.86 m3/h)Case B-5 (fixed flow late, 227.44 m3/h)

6,000

Dimensionless temperature (θ-θini)/(θin-θini) [-]

Hei

ght [

mm

]

5,000

3,000

2,000

1,000

4,000

ρ Water density [kg/m3]θ Water temperature [℃ ] θin Temperature of the water supplied from the diffuser [℃ ]θini Initial water temperature [℃ ]

Nomenclature