Thermal Analysis of Molten Salt Thermocline Thermal Storage System with Packed Phase Change Bed

Thermal Analysis of Molten Salt Thermocline Thermal Storage System with Packed Phase Change Bed

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 61 (2014) 2038 – 2041 The 6th International Conference on Applied Energy – I...

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Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 61 (2014) 2038 – 2041

The 6th International Conference on Applied Energy – ICAE2014

Thermal analysis of molten salt thermocline thermal storage system with packed phase change bed Jianfeng Lu*, Tao Yu, Jing Ding, Yibo Yuan School of Engineering, Sun Yat-Sen University, Guangzhou, 510006, China

Abstract Energy storage performances of molten salt thermocline thermal storage system with packed phase change bed are numerically studied by using transport model of porous media and phase change model of thermocline bed. The results show that the packed phase change bed can remarkably increase the energy storage density and discharging efficiency. The thermocline with phase change material can be divided into three stages, or high temperature thermocline, low temperature thermocline and phase change layer. As the melting point of phase change material approaches to the outlet temperature, the effective discharging energy remarkably increases, so the optimal melting point should be equal to the outlet temperature for good heat storage performance. As the phase change material content increases, the phase change layer thickness and discharging time increases. Keywords: energy storage; thermocline system; molten salt; phase change materials; solar thermal power

1. Introduction The molten salt thermal energy storage system [1, 2] is widely used in concentrating solar power (CPS), and the thermocline system [3] is a very promising technology for high heat capacity and low cost. Pacheco et al. [4] first demonstrated packed-bed molten salt thermocline system with 2.3 MWht in Sandia National Laboratory. Brosseau et al. [5] suggested that quartzite rocks and sands were the low-cost and efficient solid fillers for packed bed. Zuo and Li [6] proposed a molten-salt hybrid thermocline thermal storage system with two storage subsystems. Yang et al. [7] and Xu et al. [8] numerically studied the thermal performance of a packed-bed molten salt thermocline thermal storage system. Since phase change materials have high energy storage density, the thermal storage system with packed phase change bed is expected to have good heat storage performance. In this paper, the thermocline storage system with packed phase change bed is proposed, and the numerical model is developed by using the transport model of porous media and phase change model of thermocline bed. Based on the simulation results, the thermocline layer structure, discharging efficiency and optimal melting point of the thermocline system with phase change bed are further described.

* Corresponding author. Tel.: +8620-3933-2320; fax: +8620-3933-2319. E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.070

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2. Numerical model The molten salt thermocline thermal storage system with packed phase change bed is a cylindrical tank that contains storage material. The tank has an inlet and an outlet on the bottom and top for the hot and cold molten salt. The storage material includes molten salt and packed bed. In order to increase the energy storage density, the packed bed is made of ceramic sphere that containing phase change material, and the porosity and particle diameter of the bed are ε and d. According to the thermocline thermal storage system, the numerical model is established by using transport model of porous media and phase change model of thermocline bed. The flow and heat transfer is axisymmetrical, and a uniform flow is imposed at the inlet. Packed phase change bed is a continuous, homogeneous and isotropic porous media, and molten salt through the bed is laminar and incompressible. The transport model of porous media and its validation can refer to available literature [7-9]. In the present article, phase change occurs in the bed, so the energy equation has the source q=(1-ε)ρpcphsl∂fp/∂t at the melting point Tm, where hsl denotes the latent heat of phase change material, and fp means the phase change material content (volume) in the bed. The radius and length of tank are 1.5 m and 5.9 m, and ε=0.22, d=0.019 m. The properties of molten salt (60wt%NaNO3-40wt%KNO3) are [2]: ρl=2090-0.636t kgm-3, cl=1443-0.172t Jkg-1K-1, λl=0.4430.00019t Wm-1K-1, μl=22.714-0.12t+0.0002281t2-0.0000001474t3 gm-1s-1, where t means degree Celsius. The properties of ceramic material are: ρc=2500 kgm-3, cc=830 Jkg-1K-1, λc=5.69 Wm-1K-1. The effective conductivities of fluid and bed [10] are keff,f=0.5PrRekl, keff,s=kl(ks/kl)m, m=0.28-0.757logε-0.057log(ks/kl). The heat transfer coefficient between solid and fluid is h=6(1-ε)kf(1+1.1Re0.6Pr1/3)/d2. The heat transfer coefficient of heat loss through insulation is 0.5 W/K. The inlet, outlet and surrounding temperatures are respectively 290oC, 390oC and 0oC. The properties of phase change material are presented in Table 1. Table 1. Properties of phase change materials Number

Phase change material

ρp (kgm-3)

cp (Jkg-1K-1)

λp (Wm-1K-1)

Tm (oC)

hls (kJ/kg)

1

NaF-BeF2

2010

2176

0.87

340

376

2-4

Material 2-4

2010

2176

0.87

310, 360, 390

376

5

MgCl2-NaCl-KCl

2250

960

0.95

385

461

3. Results and Discussions 3.1. Basic heat transfer and storage performances

o

Tf ( C)

o

320

1h

3h

6h

10 h

2

340 Y2

320

1 300

300

280 0

1

2

3 x (m)

4

5

Y3

o

340

3

Ts-Tf ( C)

360

360

tf ( C)

Tf Ts-Tf

380

380

280

4

400

400

Y1 0

1

2

3 x (m)

4

5

0

Fig. 1 Temperature profiles of molten salt at the axis Fig. 2 Temperature profile and temperature difference between solid and fluid (NaF-BeF2, fp=0.5) (NaF-BeF2, fp=0.5)

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Figs. 1-4 present the basic heat transfer and storage performances of molten salt thermocline system, where the phase change material (NaF-BeF2) content in the bed is 0.5. Because of phase change material, the thermocline can be divided into three stages: the high temperature thermocline, low temperature thermocline and phase change layer. In Fig. 2, the temperature difference between solid and fluid has two peaks in the high temperature thermocline and low temperature thermocline regions, and its maximum is less than 2.3oC. Compared with the system without phase change material in Fig. 3, the discharging time with high outlet temperature can be significantly increased. In addition, the thicknesses of the high temperature thermocline, low temperature thermocline and phase change layer can be further defined as Y3(T Tin+A
4 with PCM 1 without PCM

380

Y (m)

360

Tf,out

Y1 Y2 Y3

3

340

2

320 1

300 280

0

2

4

6 t (h)

8

10

12

0

0

2

4

6 8 t (h)

10

12

Fig. 3 The outlet temperature variation during the discharging process Fig. 4 Thermocline thickness during the discharging process (NaF-BeF2, fp=0.5) (NaF-BeF2, fp=0.5)

3.2. Heat storage performances under different melting points t 7

25 ttot teff

6

Qefff (MWh)

20

t (h)

15 10

5 4 3 2

5 0

1 o

310 C

o

340 C

o

360 C

o

390 C

Melting temperature

Fig. 5 The whole discharging time and effective discharging time (NaF-BeF2 and material 2-4, fp=0.5)

0

Ref

o

o

o

o

310 C 340 C 360 C 390 C Melting temperature

Fig. 6 The effective discharging energy (NaF-BeF2 and material 2-4, fp=0.5)

Figs. 5-6 presents the heat storage performances of molten salt thermocline under different melting points, where fp=0.5, and materials 2-4 in Table 1 are used to investigate the effects of melting point. As the melting point increases, the whole discharging time ttot (T Tout>T Tin) remarkably drops, while the effective f charging time teff (T Tout>T Tout-B, B=20oC in present article) increases. Compared with the system without phase change material (Ref in Fig. 6), the packed phase change bed can increase the effective discharging energy. When the melting point approaches to the outlet temperature, the effective discharging energy is

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remarkably increased, so the melting point of phase change material should be a little below the outlet temperature for good energy storage performance. 3.3. Heat storage performances under different phase change material content Table 2 presents the heat storage performances of molten salt thermocline under different phase change material contents, where fp (MgCl2-NaCl-KCl)=0, 0.5, 0.7. As the phase change material content increases, the phase change layer thickness and effective discharging energy increase, and the effective discharging efficiency also rises. When the phase change material content rises from 0 to 0.7, the effective discharging energy rises from 2.21 MWh to 8.30 MWh. Table 2. Heat storage performances under different phase change material content (MgCl2-NaCl-KCl, fp=0, 0.5, 0.7)

fp

teff (h)

Qtot (MWh)

Qeff (MWh)

η (-)

0

3.18

2.52

2.21

87.6%

0.5

9.13

7.25

6.57

90.6%

0.7

11.51

9.14

8.30

90.8%

4. Conclusions Molten salt thermocline thermal storage system with packed phase change bed has high heat storage density for latent heat. The thermocline with phase change can be divided into three stages: the high temperature thermocline, low temperature thermocline and phase change layer. As the melting point below outlet temperature and phase change material content increases, the effective discharging energy and efficiency both increase. 5. Acknowledgements This paper is supported by National Natural Science Foundation of China (51176206) and National High Technology Research and Development Program of China (2013AA050503, 2012AA050604) 6. References [1] Kalogirou S.A. Solar thermal collectors and applications. Progress in Energy and Combustion Science, 2004, 30, 231-295. [2] Zavoico A.B. Solar power tower design basis document. Sandia National Laboratories. Report no. SAND2001-2100, 2001. [3] Panthalookaran V., Heidemann W., Muller-Steinhagen H. A new method of characterization for stratified thermal energy stores. Solar Energy, 2007, 81: 1043-1054. [4] Pacheco J.E., Showalter S.K., Kolb W.J. Development of a molten-salt thermocline thermal storage system for parabolic trough plants. J. Sol. Energy, ASME, 2002, 124: 153-159. [5] Brosseau D., Kelton J.W., Ray D., Edgar M. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems in parabolic trough power plants. J. Sol. Energy, ASME, 2005, 127: 109-116. [6] Zuo Y., Li X. Scheme and experiments of a molten-salt hybrid thermocline thermal storage system. Chem. Indust. Eng. Progr. 2007, 26: 1018-1022. [7]Yang Z., Garimella S.V. Molten-salt thermal energy storage in thermocline under different environmental boundary conditions. Appl. Energy, 2010, 87: 3322-3329. [8] Xu C., Wang Z., He Y., Li X., Bai F. Sensitivity analysis of the numerical study on the thermal performance of a packed-bed molten salt thermocline thermal storage system. Appl. Energy 2012, 92: 65-75. [9] FLUENT 6.1 Documentation. . [10] Ismail K.A.R, Stuginsky J.R. A parametric study on possible fixed bed models for PCM and sensible heat storage. Appl. Therm. Eng. 1999, 19: 757-788.