Towards development of a prototype high-temperature latent heat storage unit as an element of a RES-based energy system (part 2)

Transcription

1 BULLETIN OF THE POLISH CDEMY OF SCIENCES TECHNICL SCIENCES, Vol. 64, No., 6 DOI:.55/bpasts-6-45 Towards development of a prototype high-temperature latent heat storage unit as an element of a RES-based energy system (part ) J. KRWCKI*, K. BOGUCK-BYKUĆ, W. WŁOSIŃSKI, and S. BYKUĆ 3 Department of Heat Transfer, The Szewalski Institute of Fluid-Flow Mahinery Polish ademy of Sienes (IMP PN), Fiszera 4 St. Gdańsk 8-3, Poland Polish ademy of Sienes, Defilad Sq., -9 Warszawa, Poland 3 Department of Distributed Energy, The Szewalski Institute of Fluid-Flow Mahinery Polish ademy of Sienes (IMP PN), Fiszera 4 st. Gdańsk 8-3, Poland bstrat. This paper presents an experimental study performed with the general aim of defining proedures for alulation and optimization of shell-and-tube latent thermal energy storage unit with metals or metal alloys as PCMs. The experimental study is foused on reeiving the exat information about heat transfer between heat transfer fluid (HTF) and phase hange material (PCM) during energy aumulation proess. Therefore, simple geometry of heat transfer area was seleted. Two onfigurations of shell-and-tube thermal energy storage (TES) units were investigated. The paper also highlights the emerging trend (refleted in the literature) with respet to the investigation of metal PCM-based heat storage units in reent years and shortly presents unique properties and appliation features of this relatively new lass of PCMs. Key words: latent heat storage (LHS), phase hange material (PCM), metal alloy, middle melting point metal PCM. Nomenlature speifi heat [J/kgK] h latent heat [J/kg] k overall heat transfer oeffiient [W/m K] m average mass flow rate [kg/s] q heat flux [W/m ] Q heat flow rate [W] Q heat flow rate lost to environment [W] t temperature [ C] average inlet temperature gradient [K/s] t nominal temperature range of phase transition, Eq. () [K] α heat transfer oeffiient [W/m K] δ material thikness [m] λ thermal ondutivity [W/m K] τ time [s] CFD omputational fluid dynamis DSG-CSP DSC HTF diret steam generation onentrated solar power differential sanning alorimetry heat transfer fluid LHS latent heat storage PCM PI TES phase hange material proportional-integral ontroller thermal energy storage * jkarwaki@imp.gda.pl Subsripts: ambient air e outlet value H heat transfer fluid (HTF) i inlet value P phase hange material (PCM) R inner tube at phase hange outer tube. Introdution Phase hange material (PCM) investigation is one of the two elementary stages of the development of a latent heat storage (LHS) unit, next to designing the heat exhanger. Moreover, thermo-physial properties of a PCM seleted for a given working temperature of a system determine the design of the heat exhanger, and thus the whole onept of the LHS unit. Sine the majority of PCMs exhibit a low thermal ondutivity, the means for improving overall heat transfer in LHS systems are key researh topis in this field sine deades [ 9], both from material side and from heat exhanger design perspetive. mong different approahes aimed at ondutivity enhanement in storage systems with a phase hange, appliation of metals or metal alloys diretly as PCMs appears an intriguing onept. That is due to their muh higher thermal ondutivity when ompared to onventional PCMs. This observation an be exemplified by a omparison of average thermal ondutivity data for organi materials (from.5 up to.3 W/mK) or inorgani salt hydrates (from.4 up to.7 W/mK) with ther- Bull. Pol..: Teh. 64() 6 4 Unauthentiated

2 J. Karwaki, K. Boguka-Bykuć, W. Włosiński, and S. Bykuć mal ondutivity of seleted low melting point metals or metal alloys (eg. from 8. up to even 86.9 W/mK) []. However, sine no PCM ould meet not only all, but even most of the expetations, the advantages and disadvantages of using metals or its alloys as PCMs need to be thoroughly analysed and weighed not to be overrated. Metalli PCMs are rather rarely addressed in literature when ompared to organi and non-metalli inorgani phase hanging materials. However, the idea of applying them as suh is not a novelty, sine probably the first ones to investigate the onept of LHS using metal PCM were Birhenall and Telkes already in 976 []. Nevertheless, reently published artiles [, 4] indiate an emerging trend in the field of metal PCM-based heat storage in reent years. The thorough investigation of unique properties and appliation features of a new lass of PCMs, being the low melting point liquid metals or their alloys, oupled with their omparison with onventional PCMs, was reently published in []. Still, literature dediated to middle melting point PCMs () (4 C, mostly Bi-based metal alloys), being of interest for authors of this paper, as well as low melting point metalli PCMs ( 3 C, mostly Ga-based metal alloys) is very limited when ompared to high melting point metalli PCMs (above C) [, 5 9]. The general advantages of metalli PCMs, ompared to non-metalli, onventional ones, apart from already mentioned (i) high thermal ondutivity, are: (ii) high heat of fusion per unit volume, (iii) relatively low vapour pressure, (iv) moderate superooling effet, (v) no phase separation, (v) non-flammability, (vi) long-term stability after millions of solidifying and melting proesses. On the other hand, onsidering their disadvantages, mention may be made of: (i) signifiant weight, (ii) low heat of fusion per unit weight, (iv) low speifi heat, (v) orrosion/stability/ompatibility effet between metal and ontainer as well as (vi) high ost of material [6, ]. Blano-Rodríguez et al. [] analysed a performane of a magnesium and zin euteti alloy Mg5%-n (T m = 34 C, ΔH m = 55 J/g [3]) applied as a PCM and loated in between two onentri tubes in a laboratory sale LHS unit. The results of systemati sets of experiments (heating and ooling tests) were used to validate a D model developed within nsys Fluent CFD software. ppliation of the metal PCMs for TES system dediated for diret steam generation onentrated solar power (DSG-CSP) was demonstrated and proved benefiial due to the quasi-onstant melting and solidifiation temperatures and to its high heat transfer apaity. Nevertheless, Blano-Rodríguez et al. [] highlighted that even when high thermal ondutivity PCMs was applied, the overall heat transfer was influened by both ondution and onvetion phenomena and thus the latter also needed to be modelled. Kotze et al. [4] investigated appliation of a metalli PCM in the form of euteti aluminium silion lsi (T m = 577 C, ΔH m =46 J/g) for LHS in CSP both experimentally and mathematially. The test rig was onstruted as a m long ylinder, where lsi was plaed, with a heat transfer pipe running entrally through the () Division into low, middle and high melting point metal PCM after [] ontainer. Due to safety restritions in the laboratory, it was not possible to apply liquid metals as metalli heat transfer fluids to heat the lsi through the heat transfer pipe (this innovative approah was earlier suggested in []). Instead, the metal PCM was heated from the outside of a ylinder via eletri heaters. Therefore only the disharge proess was examined. The mathematial model was ompared with the results of experiments during disharge proess and onfirmed the rationality of the onept. Still, the need for improving data auray with respet to material properties was noted.. Experimental set-up and proedure The main objetive of presented investigation is to reate proedures for alulation and optimisation of shell-and-tube latent thermal energy storage unit with metals or metal alloys as PCMs. Therefore, the experimental study is foused on reeiving the exat information about heat transfer between HTF and PCM during energy aumulation proess. That is why simple geometry of heat transfer area was seleted. Two onfigurations of shell-and-tube TES unit were investigated. Fig.. shows the sheme of the first shell-and-tube storage tank in whih the length of heat transfer area is mm. The PCM was plaed inside the inner tube and heat transfer fluid (HTF) flowed in a slot between the tubes. The PCM stored the thermal energy in both sensible and latent forms. Fig.. Shemati view of shell-and-tube thermal energy storage (TES): inner tube, phase hange material (PCM), 3 outer tube, 4 heat transfer fluid (HTF), 5 leading ring, 6 seal assembly, 7 thermoouple mounting port In the first LHS unit the inner diameter of outer tube is 6 mm and its wall thikness is mm. The tube is overed throughout its length with 5 mm of mineral wool insulation and an aluminium ladding. There are two leading rings plaed at the opposite tube sides to entre the inner tube. The slot between the inner and outer tubes is mm. There is a seal as- 4 Bull. Pol..: Teh. 64() 6 Unauthentiated

3 Towards development of a prototype high-temperature latent heat storage unit as an element of a RES-based energy system (part ) sembly in the upper part of the tubes to allow a hange of the internal tube. The inner diameter of internal tube is mm and its wall thikness is mm. Both tubes are made of opper. Constrution of the seond shell-and-tube storage tank is similar but the inner diameter of the tubes is mm and 6 mm, respetively. Detailed information about both LHS units is listed in Table. Information in Table is only relevant to the heat transfer part of LHS units ( mm). Table Primary information about the geometry of the two investigated LHS units. loops, separated by the 3-way ontrol mixing valve (3), an be identified. The flow of the HTF was driven by the irulating pump (4). In the first loop an eletri irulation heater (6) supplies HTF at a desired temperature. The power of eletri heater (6) as well as the settings of irulation pump (4) and 3-way valves (3,8) are ontrolled by PI software regulators based on NI PXI platform and LabVIEW. The HTF flow rate in the seond loop depends on position of the 3-way valve (3). In this manner suitable HTF temperature differene between the inlet and outlet of the LHS unit is ahieved. The dry ooler (7) is used to ool HTF during the disharging proess. inner tube outer tube LHS LHS inner diameter [mm] wall thikness [mm] weight [kg] volume of PCM [dm 3 ] inner diameter [mm] 6 6 wall thikness [mm] weight [kg] Desription of metal alloy PCM Sn6Bi4 was seleted as a PCM for LHS unit under onsideration in IMP PN in Gdańsk []. With its melting temperature equal to 4.7 C, it falls into the ategory of middle melting point metal PCMs (4 C). The table below presents basi properties of the Sn6Bi4 and HTF fluid. Table Basi properties of the PCM and HTF used in experiments. HTF weight [kg] PCM HTF PCM weight [kg].96 kg kg The LHS units were investigated on a test stand that was built in the Heat Transfer Department laboratory in IMP PN. Fig.. shows the shemati view of the experimental set-up. The experimental system was omposed of vertial shell-andtube LHS unit, two Coriolis flow meters, two mixing valves, a pump, an eletri irulation heater and a dry ooler. n ethylene glyol-water mixture was used as HTF. Two different material Sn6Bi4 ethylene glyol-water mixture density [kg/m 3 ] liquid phase speifi heat [J/kgK] solid phase speifi heat [J/kgK] 8 enthalpy of fusion [J/kg] 55 melting/solidifiation temperature [ C] 4.7 thermal ondutivity [W/mK] 3 Fig.. Shemati diagram of the experimental set-up: LHS unit, Coriolis flow meter, 3 ontrol mixing valve, 4 irulating pump, 5 Coriolis flow meter, 6 eletri irulation heater, 7 dry ooler, 8 ontrol mixing valve, T inlet HTF temperature measurement, T outlet HTF temperature measurement.. Test proedure The results are evaluated on the basis of omparing inlet and outlet temperature, harging time and thermal energy stored by the LHS unit. The analysis involves onstant temperature rise during harging test. This type of experimental proedure is similar to most ommon operating modes for differential sanning alorimetry (DSC) a ramp temperature profile with onstant heating rate. The harging experiments were initiated when the entire LHS unit with PCM and HTF was in steady state at about C. In this state first referene value of heat losses was measured. Next, the harging proess started. In this operating mode onstant rate of inlet HTF temperature was hold at. K/s,. K/s,.3 K/s or.4 K/s, aordingly. The harging proess was stopped when the inlet temperature reahed about 5 C. Then, after reahing the steady state again, the seond referene value of heat losses was measured. s shown in Fig.., the temperature evolution of HTF at the inlet and outlet of the test unit was monitored with two thermoouples T and T (of K-type). In order to ahieve suitable Bull. Pol..: Teh. 64() 6 43 Unauthentiated

4 J. Karwaki, K. Boguka-Bykuć, W. Włosiński, and S. Bykuć dynami temperature response, mm diameter thermoouples were used. The measurements of the PCM-filled storage units are ompared to the referene results obtained with empty and water-filled inner tube. 3. Modelling of LHS unit thermal dynamis Time variations of average temperatures in the investigated LHS unit were modelled with D model desribing heat transfer between the HTF, PCM, the tubes material and ambient air. The model equations are derived from the heat balanes for eah pair of the mentioned materials in ontat, see Fig. 3. s the heat flow rate Q aumulated in the material of mass M and speifi heat depends on the differene between the inflow Q in and outflow Q out rates, these balanes an be written in the following general form dt Q M Q in ( ) Q out( ), () d where t denotes temperature and τ is time. dopting the designations given on the sheme in the Fig. 3, aording to () balane equations for eah of the heat aumulating material in the storage unit take forms presented below. PCM: HTF: k M HR M H P H HR P dt d P t k t t ) PR PR( R P dth m HH t Hi the d ( tr th ) kh H ( t th, (), (3) ) internal pipe R: dtr M RR d external pipe : M k PR dt d PR k ( tp tr ) khrhr( th tr ), (4) H H ( t t ) Q. (5) In above equations, heat transfer from the liquid zones and surroundings was treated as purely onvetive. In the remaining zones, only ondutive heat transfer was assumed. For the PCM zone, ondution ours in the layer with the thikness of half of the internal radius of the pipe, r Ri /, as shown in Fig. 3. The balane for the HTF (3) was written in referene to average temperature t H that is equal to arithmeti mean value of inlet and outlet temperatures, t H H thi the. (6) The heat apaity of the insulation U was omitted beause of its low mass, therefore there is no balane equation for it. However, its influene on the heat transfer is inluded in the balane Eq. (5), in the heat flow rate Q lost to the ambient air. The Q value was determined experimentally as a linear funtion of temperature differene t H t. The overall heat transfer oeffiient between the PCM and the internal pipe R is equal to k HR R R,, (7) R Fig. 3. Simplified temperature profile along the ross-setion of the modelled heat storage unit assumed in derivation of D model equations 44 Bull. Pol..: Teh. 64() 6 Unauthentiated

5 Towards development of a prototype high-temperature latent heat storage unit as an element of a RES-based energy system (part ) between the internal pipe R and the HTF: k HR R and between the HTF and the external pipe k H R, (8) R. (9) The HTF flow in the investigated storage unit was partially turbulent with the Reynolds number of about 55 (for m H = kg/h and t H = 4 C). Therefore, the heat transfer oeffiients α and α R in (8), (9) were determined from Hausen orrelation orreted for the annular shape of the flow hannel []. Their typial value was about 3 W/m K. The values of overall oeffiients k H and k HR were pratially the same as α and α R, while the value of k PR was onsiderably larger: 8 W/m K. PCM speifi heat in the model is temperature dependent. It was assumed after [3] that its value is equal to the sum of single phase speifi heat mp and two-phase speifi heat tp, aording to the following relationships: mp tp P t t t mp, () S L S L t tanht t t h tp tanhbt t tanhbt t t t, (), () where t denotes the phase hange temperature. ording to [3], when the onstant B in () is equal to 5, 9% of the phase hange ours in the temperature range of t ± Δt /. Exemplary speifi heat distribution defined by equations () () is presented in Fig. 4. Surfae area under the urve P, in the spe. heat [kj/kgk] t temperature [ C] Fig. 4. Speifi heat distribution of the PCM alulated from the equation () for Sn6Bi4 (omp. Table ) phase hange temperature range, is equal to the latent heat h. Integration of equation () with respet to temperature results in enthalpy distribution (assuming P is the onstant pressure speifi heat). Exemplary result of suh integration is depited in Fig. 5, whih shows that enthalpy is inreased by the latent heat h in the neighborhood of the phase hange temperature t. enthalpy [kj/kg] t temperature [ C] Fig. 5. PCM speifi enthalpy distribution resulting from integration of the speifi heat presented in Fig. 4. In the neighborhood of the phase hange temperature t, an inrease of enthalpy by the latent heat ours, marked with horizontal dashed lines Solution of model equations () (5) for a predetermined inlet profile of HTF temperature t Hi (τ) gives time distributions of average temperatures t P (PCM), t H (HTF), t R (inner tube) and t (outer tube). Next, from Eq. (6), outlet HTF temperature distribution an be alulated. Fig. 6 shows a differene between inlet (t Hi, thermoouple T) and outlet (t He or T) temperature alulated in this way with a onstant heating rate dynamis. t H [K] air water PCM t Hi h τ [s] Fig. 6. Calulated differene between HTF inlet and outlet temperature Δt H during a onstant heating rate dynamis measurement as a funtion of harging proess time: blak line empty tube, red line waterfilled tube, green line tube with PCM. The profile of inlet HTF temperature thi is also shown (blue line) t i Bull. Pol..: Teh. 64() 6 45 Unauthentiated

6 J. Karwaki, K. Boguka-Bykuć, W. Włosiński, and S. Bykuć Table 3. Results of the measurements performed for LHS and LHS units and used to determine heat losses Q. run No. LHS unit m H [kg/s] [K/s] t Hi [ C] t Hi [ C] t [ C] Q [W] Q [W] LHS LHS LHS LHS LHS LHS LHS Results and disussion Experimental studies were onduted in parallel with modelling aimed at elaboration of proedures for design and optimization of shell and tube storages with aumulation of heat in the latent form. Therefore, the results of experiments onstitute an important element in verifiation of the proposed mathematial model as well as in determination of the neessary empirial oeffiients. Heat storage units under investigation operate in dynami systems and thus, the developed model inludes time-dependent outlet temperature distributions depending on the inlet parameters. Due to the fat that the lumped parameters model does not take into aount the geometry of the storage unit, validation based on the experimental results will enable verifiation of its suitability and, if neessary, introdution of orretion fators. First, referene tests were performed for empty and water-filled tanks and the heat aumulation in this ase was of sensible type only. During these tests additional measurements were done to determine heat losses Q to the environment. For this purpose, the LHS units operated at onstant inlet onditions with low (t Hi = C) and high (t Hi = 5 C) HTF temperature and the obtained values of the heat flow rates Q are presented in Table 3. Fig. 7 and 8 exhibit the example results of experiments with the orresponding model predition. Both figures show onsiderable flutuations of the heat flux assoiated with the operation of the HTF heater. Divergene of experimental data and the values alulated theoretially originates from the transition period during the stabilization of the inlet temperature gradient. The obtained results indiate good agreement between the model and the experimental data. q [W/m ] q [W/m ] t H Fig. 7. The heat flux aumulated in the empty heat storage unit as a funtion of the average HTF temperature for the following experiment onditions: LHS, m H = kg/h, =.3 K/s t H Fig. 8. The heat flux aumulated in the heat storage unit filled with water as a funtion of the average HTF temperature for the following experiment onditions: LHS, m H = kg/h, =.3 K/s The measurements onduted were dynami in nature. They were done with a onstant inrease of temperature at the inlet of the heat storage unit (T Hi ). Exeution of this kind of measurements requires a ontinuous hange of the eletri power of the liquid heater. In the ase of small temperature differene between the inlet and outlet of the LHS unit, the implementation of suh measurement with adequate preision is diffiult. For the presented measurement results (Fig. 7 and 8) it is assoiated with signifiant flutuations in the measured heat flux values. 46 Bull. Pol..: Teh. 64() 6 Unauthentiated

7 Towards development of a prototype high-temperature latent heat storage unit as an element of a RES-based energy system (part ) q [W/m ] =.4 K/s =.3 K/s =. K/s =. K/s t H q [W/m ] =.4 K/s =.3 K/s 45 4 =. K/s =. K/s τ [s] Fig. 9. The heat flux aumulated in the heat storage unit filled with Sn6Bi4 as a funtion of the average HTF temperature for the following experiment onditions: LHS, m H = kg/h Fig.. Instantaneous values of heat flux aumulated in the storage unit as a funtion of time for seleted gradients of average inlet temperature and the following experiment onditions: LES, m H = kg/h 3 5 =. K/s =. K/s the presented mathematial model. full analysis of the measurement results will be ompleted in the ourse of further work and will be published in a subsequent paper. q [W/m ] t H Fig.. Distribution of the heat flux aumulated in the latent heat storage unit as a funtion of the average HTF temperature for LHS (after dedution of losses to the ambient environment). Next, the measurements with PCM-filled storage units were performed and their results are presented in Fig. 9 and Fig.. signifiant inrease of heat flux aumulated in the storage unit within a temperature range orresponding to the phase transition in the euteti alloy Sn6/Bi4 an be notied. The shift of reorded temperature peaks towards higher temperatures is due to the dependene of temperature equalization in heat storage unit on the heating rate. For different heating rates, aumulated latent heat should be the same. In the presented measurements, a measure of this heat is the area under the urves presented in Fig., whih shows instantaneous values of aumulated heat flux during the experiment. Due to dynami nature of the measurements, detailed information on the onditions of heat transfer during its aumulation in the PCM requires a onfrontation with the preditions of 5. Conlusions The main problem assoiated with the use of heat aumulators with phase hanging materials is related to very low heat transfer on the PCM side, due to poor thermal ondutivity. However, this is not the ase for metalli materials and metal alloys. Nevertheless, proper design in the latter ase also requires knowledge of the dependenies desribing heat transfer proess, partiularly on the side of the PCM. The results of the harging proess of a heat storage unit with euteti SnBi alloy as PCM are presented in this paper. test rig with a pipe in pipe LHS unit was desribed as well. The results of onduted experiments will be used for modifiation and verifiation of the developed mathematial model. Ultimately, the model will be used to design and optimise heat storage units with phase hanging materials. Finally, the results show that the suggested metalli material may serve as a filling for a heat storage with short harging time and high heat apaity. knowledgements. This projet was supported by a grant from Polish National Centre for Researh and Development. Referenes [] J. Chen, D. Yang, J. Jiang,. Ma, and D. Song, Researh Progress of Phase Change Materials (PCMs) Embedded with Metal Foam (a Review), Proedia Materials Siene 4, , (4). []. hang, N. hang, J. Peng, X. Fang, X. Gao, and Y. Fang, Preparation and thermal energy storage properties of paraffin/ expanded graphite omposite phase hange material, pplied Energy 9, (). Bull. Pol..: Teh. 64() 6 47 Unauthentiated

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