THERMAL ENERGY STORAGE USING DESERT SAND: A NUMERICAL STUDY OF THE THERMOFLUIDIC PERFORMANCES.

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1 12th International Conerence on Heat Traner, Fluid Mechanic and Thermodynamic THERMAL ENERGY STORAGE USING DESERT SAND: A NUMERICAL STUDY OF THE THERMOFLUIDIC PERFORMANCES Mahoudi N.*, Kheiri A., El Ganaoui M., Moummi A. *Author or correpondence Laboratoire de Génie Civil, Hydraulique, Développement Durable et Environnement, LAR-GHYDE, Univerité Mohamed Khider, BP 145, RP, 07000, Bikra, Algérie Univerité de Lorraine, CNRS, LEMTA UMR 7536, 2 Rue Jean Lamour, Vandoeuvre-lè-Nancy, F-54500, France. Univerité de Lorraine, Laboratoire d Energétique de Longwy/LERMAB, IUT Henri Poincaré de Longwy, 186 rue de Lorraine Cone Et Romain Laboratoire de Génie Civil, Hydraulique, Développement Durable et Environnement, LAR-GHYDE, Univerité Mohamed Khider, BP 145, RP, 07000, Bikra, Algérie nadjiba213@yahoo.r ABSTRACT The Thermal Energy Storage (TES) enhance the availability o renewable energy plant. It reduce the mimatch between the production and the demand o the electric energy. However, the high cot o the TES lead to a high overall cot o the produced electric kwh. In the cae o olar power plant built in Saharan region, the ue o and a the toring medium in the TES i a priori a uitable technique that can olve thi problem. In act, the and preent good phyical and chemical propertie and it i locally available with a very low cot. In thi paper, a numerical tudy ha been conducted to ae the thermal and luidic perormance o a ixed bed and o a luidized bed by uing and a toring medium. The heat i tranerred to, and rom, the and by air. 2D and 3D imulation are conducted. The temperature proile o the bed are examined a well a the toring rate. Parametric tudie with the air peed and the height o the bed were conidered. The reult gained in thi paper indicate that it i very viable and promiing to integrate and a toring media in the olar thermal application epecially where thi material i plentiul. INTRODUCTION Energy torage technologie are a neceary component or any eicient ue o renewable energy ource. Among them, TES ha attracted increaing interet or both thermal application and power plant. O the many material available or enible toring, the mot commonly ued are Water, Sand, Concrete and ground [1-4]. A promiing enible heat toring material mut alo be ecologic, inexpenive, and mut have a good thermal diuivity [5]. The advantage o TES ytem uing and a a torage media, include very low cot o thermal energy torage media, high and timely table heat traner rate into (and out o) and, eay handling operation. Recent tudie in USA and Spain howed the interet o uing and a toring media [6,7]. S. Haan et al. [8] hown that and ha uitable thermal propertie and tand a a candidate or toring heat. El Sebaii et al. [9] invetigated the eectivene to ue and a a torage material in an active ingle bain olar till. They have ound NOMENCLATURE C d [-] Drag coeicient C p [J/kg.K] Speciic heat d [m] Diameter o olid phae g [m/ 2 ] Acceleration due to gravity h [J/.m.K] Heat traner coeicient between c and d phae H [kj./kg] Enthalpy H 0 [m] Initial height k [W/mK] Thermal conductivity L [m] Lenght o the luidized bed p [Pa] Preure t [] Time T [K] Temperature u [m/] Velocity Q V [J] [m 3 ] Heat torage capacity Sand Volume Special character β [-] Inter-phae momentum exchange coeicient μ [Pa ] Vicoity [kg/m 3 ] Denity ϴ [m 2 /. 2 ] Granular temperature Ф [-] Volume raction [Pa] Stre tenor Subcript c ch d ini k Continiou (air) Charging Dipered (olid) Fluid Initial Phae k, continiou or dipered Solid that the annual average o daily productivity with torage i 23.8% higher than that when i ued without torage [9]. Thank to it good thermal inertia, and it availability in a plentiul way in everal region o the world (North o Arica, Gul tate, America, Middle eat ), it will be intereting to ue and a a torage medium [10-12]. Hence, in the preent work, the thermal behavior and the torage perormance o enible TES uing and a the toring media i invetigated uing two technologie: ixed and luidized bed. 603

2 12th International Conerence on Heat Traner, Fluid Mechanic and Thermodynamic THERMAL MODELING OF SENSIBLE HEAT STORAGE Fixed bed Figure 1 how the phyical model o the TES unit or numerical calculation. The torage unit i a regenerative type heat exchanger which aborb/releae heat energy by paing the hot/cold Heat Traner Fluid (HTF) repectively through the charging tube. It conit o cubic bed with charging tube embedded into it. The outer urace o the unit i well inulated to avoid heat loe to ambient environment. Furthermore, the tube thickne i neglected and a a boundary layer condition, the equality o the urace heat rate i conidered between the HTF and the and. The heat torage capacity i calculated uing: Q VC T (5) p ch Where ΔT ch i the and temperature change ince the toring tart. At the initial condition (t = 0), there i no low o HTF through the charging tube and all the domain i at uniorm temperature T ini. A illutrated in Figure 2, at the inlet o the tube, there i a contant temperature T inlet and a contant luid velocity, at any time (t > 0). All the outer urace o the TES are well inulated. Figure 1. Phyical model o and heat torage ixed bed While developing the thermal model or heat traner the ollowing aumption were made: The HTF entering inide wa laminar and ully developed. The enible heat torage material i iotropic. The rictional preure drop o HTF i neglected through the tube. Laminar low and adiabatic wall were aumed. Thermal proprietie, including denity, o the luid and o the and were aumed contant Axial (z direction) conduction in HTF i neglected. The HTF low inide the charging tube i imulated by olving the continuity and the Navier-Stock equation imultaneouly. t.( v) 0 D 2 P u (2) Dt The heat traner rom the HTF to the wall take place by convection; the energy equation decribing heat traner proce i given by (radiative traner are neglected): DT 2 C p T (3) Dt In the toring media, i.e. and, the heat conduction equation to be olved tand a: T t 2 Cp KS T (4) (1) Figure 2 Initial and boundary condition o the model Fluidized bed The computational domain or imulation i hown in Figure 3. It i repreented by a 2D planar with a width o L and a height o H. The initial height o packed bed o the dipered phae (Sand) i H 0 when there i no luid low. Figure 3 Study domain o and luidized bed In the luidized bed, the governing equation o the multiphae low are: Ma conervation or phae k (k = c or air and d or and) with: (6) (7) 604

3 12th International Conerence on Heat Traner, Fluid Mechanic and Thermodynamic Where ϕ and are repectively the volume raction and the denity o the phae k. Momentum conervation or continuou phae: Momentum conervation or dipered phae: (8) (9) Where u k i the velocity o the phae k, P i the preure, i the tre tenor o the phae k, and i the inter-phae momentum exchange coeicient. Thermal energy conervation continuou phae: RESULTS AND DISCUSSION The charging time or ixed bed i hown in Figure 4. It i een that the and bed take about 10 day to be completely charged. The variation o the thermal energy torage rate or the elected material i hown in Figure 5. The amount o the thermal energy tored at the charging time i calculated uing Eq. 5. The energy tored in the and ixed bed i MJ. The energy torage rate o the bed i initially zero when there i no heat input and it rie with time till the torage bed i ully charged. Since the energy torage rate i unction o volume average temperature o the torage bed, it ha the ame proile. Thermal energy conervation dipered phae: (10) (11) Where H k i the enthalpy o the phae k, k k i the thermal conductivity o the phae k, i the heat traner coeicient between c and d phae, and T k i the temperature o the phae k. The algebraic granular temperature wa adopted [13]: (12) Figure 4 Charging time o and ixed bed i the granular temperature. Material property and imulation parameter ued or the imulation are lited in Table 1. Table 1 Material propertie o ga and olid phae and Simulation parameter Flow type Ga-olid model Time tep ued Laminar Euler-Euler, with kinetic theory Convergence criteria 10-3 Supericial ga velocity 0.25 m/ Max. olid packing volume raction 0.60 Initial air temperature 473 K Initial and temperature 300 K Initial height o and 40 cm Sand heat capacity 830 J.kg -1.K -1 Sand denity 2650 kg.m -3 Sand particle diameter 162 μm Figure 5 Rate o energy tored in and ixed bed In order to how the HTF temperature variation along the torage bed, HTF temperature i depicted along the central charging tube at a contant radial poition at dierent interval o charging time, 1 hour, 2 hour, 5 hour, and 24 hour (Figure 6). During the irt hour o charging the temperature 605

4 12th International Conerence on Heat Traner, Fluid Mechanic and Thermodynamic drop o HTF i 12 C. However, ater 24 hour the temperature drop i about 0.1 C. Initially, the TES i at a temperature lower than T inlet the torage i initiated by upplying HTF at temperature T inlet. A a reult, the HTF temperature drop along the charging tube and exit at a lower temperature, T outlet. During the initial period o charging time, the temperature drop o HTF i dominant, ater certain duration, it value i almot contant and no urther drop in temperature i regitered a torage temperature itel rie with time and the driving potential or conduction i reduced. rate, the heat input to torage bed per unit time i higher o it take le time to complete the charging cycle (ΔT ch ). The decreae in charging time o torage bed with HTF velocity o 2 m/ a compared to 10 m/ i higher than the decreae in charging time o torage bed with HTF velocity o 10 m/ a compared to 20 m/. Hence, the increae in HTF velocity beyond 10 m/ will have le impact on the charging time o torage bed. Moreover increaing the HTF velocity caue more rictional preure drop in the HTF and thu lead to more pumping power a the rictional preure drop i proportional to the quare o velocity o HTF luid. Concerning the thermal behaviour o the luidized bed, Figure 8 how the time variation o the mean dipered phae temperature at eight o H = 15 cm in the and luidized bed. Note that, the average olid temperature hown i the mean o the particle temperature averaged acro the ection o the column at a given height. It i een that or each time duration o about 1 minute, the and temperature increae with gain o 6 C. The luctuation oberved are due to the paage o the air bubble through the bed allow both the cooling and the heating o the and particle. It i hown that the hydrodynamic o the low aect the thermal behaviour o the luidized bed. Figure 6 : Axial variation o HTF temperature in and ixed bed The eect o low rate o HTF on charging time or and i preented in Figure 7. Figure 8 Variation o the temperature o the and at height o H = 15 cm in the and luidized bed. Figure 7 : Eect o HTF velocity on charging time o and ixed bed In order, to preent the eect o the air velocity on the luidized bed hydrodynamic, Figure 9 illutrate the expanion and the bubble ormation in the bed or dierent air velocitie (0.25, 0.5 and 1 m/). It ha been een that with the increae o the air upericial velocity, the bubble ize in the bed increae. At high velocitie, a group o a mall bubble are generated rom the air ditributor. Subequently, the bubble coalece and produce then bigger tructure a they move upward and become tretched becaue o the interaction with other bubble and becaue the bed wall eect. It i een that the coniderable reduction in charging time occur with increaing the ma low rate o HTF (or velocity a cro ection o tube i contant). Since, the increae in velocity o HTF lead to the improvement in convective heat traner 606

5 12th International Conerence on Heat Traner, Fluid Mechanic and Thermodynamic 12,96 MJ. The eect o HTF velocity on charging time o torage media ha been analyzed. For the luidized bed, the reult how that the hydrodynamic ha an important eect on the thermal behaviour o the TES and thu on the torage capability. The chaotic ormation o the bubble allow a good heat traner between the continuou phae (air) and dipered phae (and). Thi work i in progre to undertand and compare the reult with the irt data obtained rom an experimental TES with and bed that i under contruction in our laboratory. REFERENCES Figure 9 Eect o the air velocity on the hydrodynamic o the luidized bed. Figure 10 how the variation o the olid phae temperature a unction o the width o the luidized bed at t = 45 and or dierent height o the bed. It i een that the temperature i uniorm in all the bed. At thi moment the chaotic regime i reached, promoting thu the heat exchanger in the luidized bed. Figure 10 Variation o the temperature o the and a unction a the width o the luidized bed. CONCLUSION In thi paper, 2D and 3D imulation o enible TES uing deert and, in ixed and luidized bed were conducted to invetigate it perormance. For the ixed bed, a acility employing a coniguration o nine charging tube ha been preented. It i ound that the and ixed bed take about 10 day to be ully charged. In thi cae the capacity o torage i [1] A.I. Fernandez, M.Martınez, M.Segarra, I.Martorell, L.F.Cabeza, Selection o material with potential in enible thermal energy torage, Solar Energy Material & Solar Cell, vol. 94, pp , [2] I. Dinçer, M. A. Roen, Thermal Energy Storage Sytem and Application, 2nd ed., Willey, 2011, pp [3] F. Bai, C. Xu, Perormance analyi o a two-tage thermal energy torage ytem uing concrete tream accumulator, Applied Thermal Engineering, vol. 31, pp , [4] M.E. Navarro, M.Martı nez, A.Gil, A.I.Ferna ndez, L.F.Cabeza, R.Olive, X.Py, Selection and characterization o recycled material or enible thermal energy torage, Solar Energy Material & Solar Cell, vol. 107,pp , [5] M. Medrano, A. Gil, I. Martorell, X. Potau, L. Cabeza, State o the art on high-temperature thermal energy torage or power generation. Part 2 Cae tudie, Renewable and Sutainable Energy Review, vol. 14, pp [6] A. Gil, M. Medrano, I. Martorell, X. Potau, L. Cabeza, State o the art on high-temperature thermal energy torage or power generation. Part 1 Concept, material, and modelization, Renewable and Sutainable Energy Review, vol. 14, pp , [7] D. Fernande, F. Pitié, G. Cácere, J. Baeyen, Thermal energy torage: How previou inding determine current reearch prioritie, Energy, vol. 39, , [8] S. Haan, R. S. Hernandez, K. Vonzabern, J. Pierce, K. Kantearia, M. Rubin, Thermal Inertia o and at dierent Level o Water Saturation, Proceeding o the 44th Lunar and Planetary cience Conerence, Texa, [9] A.A. El-Sebaii, S.J. Yaghmour, F.S. Al-Hazmi, Adel S. Faidah, F.M. Al-Marzouki, A.A. Al-Ghamdi, Active ingle bain olar till with a enible torage medium, Dealination, vol. 249, pp , [10] N. Mahoudi, A. Moummi, M. El. Ganaoui, Sand a a Heat Storage Media or a Solar Application: Simulation Reult, Applied Mechanic and Material, 621(214), [11] N. Mahoudi, A. Khachkouch, A Moummi B. Benhaoua, M. El Ganaoui, Deign and characterization o a portable heat torage acility, Mechanic & Indutry, 16 (4), pp , [12] N. Mahoudi, A. Moummi, M. El. Ganaoui, F. Mnari, K. M. Aboudou, Simulation du comportement thermique d un milieu poreux detinée au tockage enible : Application à l énergie olair, Proceeding o the 3 th Colloque internationale Francophone d énergétique et mécanique, Comore, PP Mai, [13] M. Lungu, J. Wang, Y. Yang, Numerical imulation o low tructure and heat traner in a central jet bubbling luidized bed, Powder technology, PP , 269,