The impact of velocity on thermal energy storage performance of tube type thermocline tank

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1 Available online Jounal of Chemical and Phamaceutical Reseach, 2014, 6(6): Reseach Aticle ISSN : CODEN(USA) : JCPRC5 The impact of velocity on themal enegy stoage pefomance of tube type themocline tank Yongping Yang, Jingxiao Han and Hongjuan Hou School of Enegy Powe and Mechanical Engineeing, Noth China Electic Powe Univesity, Beijing, China ABSTRACT Established an indiect contact themocline single tank heat stoage two-dimensional mathematical model, its themal enegy stoage popeties has been studied. Using Fluent softwae to pefom CFD simulations on tubula accumulato, the themal stoage medium used fo the model was sand, heat tansfe fluid was ai, two types of media wee sepaated by a stainless steel tube in the middle. The simulation esults ageed well with the existing small-scale test bed egeneative esults. The esults showed that fo a 6m long heat stoage tank, when the flow ate changes in the ange of 4,6,8 m/s, the heat stoage efficiency was 0.708, 0.534, 0.425,espectively. High flow ates, tempeatue distibution inside the tank tends to unifom, which is not conducive to the fomation of themocline.. Keywods: sola enegy, themocline, themal enegy stoage, efficiency, CFD, ai, sand. INTRODUCTION Sola themal powe[1] plants using themal enegy stoage(tes) Technology, can help achieve continuous plant opeation even at a lowe intensity of sunshine o night, without elying on a natual gas o diesel back-up geneato. Howeve, the high cost of existing stoage technology has esticted the development of lage-scale sola themal powe. Theefoe, to find out suitable heat stoage mateials and egeneative mode ae impotant issues in sola themal utilization field. The Califonia SEGS tough powe plants use themal oil[2] as the heat tansfe fluid, the Hitec molten salt as the stoage media, heat exchange is accomplished though a heat exchange, thus the heat can be emained in the molten salt. To futhe educe the cost of the heat accumulato, the use of poous ock o simila heat stoage medium as the fille, so that heat tansfe fluid flows though the themal stoage medium in diect contact with the heat stoage mode attacted attention in ecent yeas [3].Compaed with othe heat-accumulating mateial, such as the molten salt and phase change mateial (PCM), the biggest advantage of the ock is cheap and easy to obtain. Despite its egeneative popeties less than the fome one, when inceased in volume, you can still get good esults. Sandia National Laboatoies ceated a single themocline tank using molten salt fo themal stoage[4], the tank body using quatz and silica sand as the themal stoage medium to tansfe heat with molten salt[5]. Flueckige et al[6] established a themocline heat stoage sola towe powe plant level model, the pebble bed in the molten salt contact heat tansfe model. Within a themocline tank, duing the heat stoage pocess, the a statified tempeatue laye inside the tank body will be fomed, high tempeatue laye at the top of the tank, and the low tempeatue egion at the bottom end, between which exists a middle tempeatue gadient egion which is called themocline. Xu et al[7] investigated the packed bed heat stoage system using molten salt as the heat tansfe fluid, the two-dimensional simulation esults showed that, with the exothemic pocess, while the themocline egion moving upwad, a slight expansion of the themocline egion phenomenon will occu. To impove heat tansfe and heat stoage effect of the molten salt in the solid paticles, smalle solid paticles should be used[8]. 1620

2 Since the heat-tansfe fluid needs to flow all though the heat stoage aea, the pessue loss is elatively lage; in addition, fo lage sola powe egeneative demanding, a need fo the constuction of a huge volume of the heat stoage tank aises, theeby inceasing the cost of. Laing et al[9] consideed using concete as the stoage mateial, a heat-tansfe fluid flowing though the pipe embedded in the concete, as the heat tansfe fluid passing though the tunnels, tank body investment can be educed. Howeve, the effect of the poo themal conductivity of concete [10], make it necessay fo heat tansfe enhancement, and afte epeated use, the concete becomes easy to cack, esulting in poo heat tansfe. In this pape, gavel was used as a sensible heat stoage mateial because it s simila themal conductivity to ocks. And the ai was used as heat tansfe fluid, heat tansfe is achieved though the heat conduction to the stainless steel pipe in the heat stoage tank. The aim of this pape was to study the effect of the flow ate on the heat stoage popeties. If lage-scale seasonal heat stoage can be ealized, which can be achieved by diectly embedding the egeneative modula in the deset, the constuction costs will see a significant eduction. EXPERIMENTAL SECTION Mathematical Model Refeence [11] gives the model desciption. Using ANSYS Fluent@ 6.3softwae,the model assumed that the heat tansfe and flow inside the heat stoage tank is tansient, with incompessible fluid, constant popeties within the flow ate anges fom 4-8m/s, the Reynolds numbe in the ange of , the flow is lamina. The involved govening equations, momentum, continuous and enegy equations ae: V = 0 V ( V ) V = p + ν V t ρ T + 2 ( V ) T = k T (3) t Since the themocline effects (the hot fluid floats on top ofthe cold fluid), geneally natual convection is not consideed. Single pipe calculation aea bounday conditions ae as follows: (1) (2) R z R f H a b Fig.1 Tansfomation of hexagon into 2D contol volume In the pipeline centeline, u =0, =0, Along the diection, outside bounday,u =0, u =0, At the solid wall whee z=0and z=h, u =0,u =0, At the outlet bounday,u =0, =0, Expeimental Validation To validate the simulated esults, one simulation was compaed to pevious expeimental esults. The expeimental setup is constucted as follows: Heat stoage tank is cylindical, pocessed with low cabon steel. Its diamete D = 203mm, height H = mm, the tunnels ae fomed with stainless steel tube (1010 low cabon steel, density 7.86 g/cm3, the themal conductivity of 1621

3 51.9 W/(m K), specific heat capacity 448J/(kg C), gavel heat 733 J/(kg K), themal conductivity 0.15W/(m K). Wheein the pipe out diamete dout =12.7mm, inside diamete din=9.4mm, its height is the same to the tank s height, namely mm. Inside the tank, cabon steel tubes ae aanged in hexagonal honeycomb aangement. Fom the cente to the outside of the tank along hoizontal diection, the tubes ae aanged in thee layes. Fo any two adjacent tubes, the distance between two tubes cente spacing is 40.6mm. So fo the inne ing of tube, the heat tansfe can be seen as well themal insulated. Using Omega company poduced K-type themocouples, whose themocouple diamete is 3mm, and length is 152mm, the tempeatue does not exceed 0.2 of the absolute measuement eo. Fom Fig.2 we can easily come to the conclusion that the esults conveges well. So in the futue we will use the model to simulate lage TES units T1 exp T1 sim Tout exp T3 exp T3 sim Tout sim T6 exp T6 sim Tin Time /s Fig.2 Compaison of sand expeiment and 2D simulation RESULTS AND DISCUSSION Accoding to the popeties mentioned in the pevious section, a lage TES tank was simulated. The length of the tank is zoomed out to 6m, but each tube diamete and its elative elation emains the same. Hee we come to investigate egeneative popeties of diffeent ai flow ates. Ai flow ate was set to be 4, 6 and 8m/s, espectively. The initial tempeatue of the tank was set to be k, and ai inlet tempeatue was k. (a) 4m/s (b) 6m/s 1622

4 (c) 8m/s Fig.3 Tempeatue pofiles of TES tank unde vaious inlet velocity Figue 3 shows the tempeatue distibution of thetank at diffeent locations. The impact of flow ate on heat stoage tank s themal enegy stoage popeties is pesented. The simulation esults ae indicated by the tempeatue of themocouples embedded in the tank. Fom the bottom of the tank to the top, altogethe 7 themocouples was placed, and thee ae called T1-7. Thei distance to the bottom of the tank ae 0.72, 1.48, 2.24, 3, 3.76, 4.52, 5.28 m. Easy to see that, unde diffeent flow ates, the tempeatue inside the tank tends to be unifom, indicating that the tempeatue inside the tank unde high velocity distibution, is not conducive to geneating themocline. To evaluate the pefomance of diffeent types of gavel, fom the pespective of the enegy input and output, we get thethemal enegy stoage efficiency η in out Q m& f C TES p, f (Tf -T f )dt η= = in t0 QIdeal m& C (T -T s )dt f p, f f (4) This equation epesent fo a cetain peiod of themal stoage pocess, if the heat tansfe fluid can actually pass all of its enegy to the themal stoage medium and its tempeatue is loweed to the initial tempeatue of the tank, we get the theoetical maximum heat tansfe. Unde this assumption, the themal stoage medium in the pocess of themal stoage will not ise its tempeatue, hee we can getq Ideal. Then the actual heat tansfe is Q TES. Afte calculation, themal enegy stoage efficiency fo diffeent flow ates, ae η708, 0.534, We may each the conclusion that themal enegy stoage efficiency at low flow ates can be geatly inceased. CONCLUSION Efficiency of themal stoage tank s unde diffeent ai flow ates showed that: at low flow ates, the heat tansfe between the heat tansfe fluid and the themal enegy stoage media is good. With high flow ates, the tempeatue inside the tank tend to convege, which is not conducive to the fomation of themocline. If we want to obtain good themal enegy stoage, the themocline should be maintained at low ai flow ate. Acknowledgements All of the authos ae gateful to the financial suppot bythe National Natual Science Foundation of China (Gant No , ), National High-tech R&D Pogam of China (863 Pogam) (Gant No.2012AA050604) and State Key Laboatoy of Altenate Electical Powe System with Renewable Enegy Souces(LAPS12). REFERENCES [1]D Balev; R Vidu; P Stoeve. Sola Enegy Mateials and Sola Cells, 2011,95(10), [2]TE Boukelia; M Mecibah. Renewable and Sustainable Enegy Reviews, 2013, 21, [3]DLZhao; Y Li; YJ Dai; RZ Wang.Enegy Convesion and Management, 2011,52(6), [4]JE Pacheco; SK Showalte; WJ Kolb.ASME Jounal of Sola Enegy Engineeing, 2002,124, [5]L Helle; PGauché. Sola Enegy, 2013,93, [6]SMFlueckige; BD Iveson; SV Gaimella; JE Pacheco.Applied Enegy, 2014,113, [7]C Xu;ZFWang;YLHe;X Li;FWBai. Applied Enegy, 2012,92, [8]C Xu;X Li;ZF Wang;YL He;FWBai.Applied Themal Engineeing, 2013,57(1-2), [9]Laing D, Steinmann W, Tamme R, Richte C, Solid media themal stoage fo paabolic tough powe plants[j], 1623

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