Hollow block s structural impact on the thermal behaviour of PCM wall

Transcription

1 th Internatinal Cnference On Building Energy, Envirnment Hllw blck s structural impact n the thermal behaviur f PCM wall Y. Zhang, Q. Wang Schl f Energy Pwer Engineering Jiangsu University, Zhenjiang, Jiangsu, China SUMMARY he impact laws f five structural cnfiguratin factrs n the thermal perfrmance f cmpsite PCM hllw blck wall (CPCMHBW) have been analyzed, using tw-dimensinal enthalpy mdel. hen a series f imprtant rules have been btained. Firstly, fr the wall equipped with PCM, that the PCM is nt brken dwn is the guarantee fr high unsteady-state thermal perfrmance (UP). Secndly, the mre the PCM is clse t the interir surface f wall, the wider the utside temperature fluctuatin range which the wall can bear becmes, vice versa. Finally, it is summarized that, the factr f the lcatin f PCM in thickness directin, shuld be taken int special cnsideratin when designing the structure f hllw blck equipped with PCM, the PCM shuld be intrduced int the lcatin which is clse t the interir surface f wall; meanwhile, the staggered arrangement f PCM hles in hllw blck is advisable. INRODUCION Intrducing phase change material (PCM) int building envelpe is ne f the ptential methds fr imprving its thermal capacity. By prperly using the phase change latent (PCLH) f PCM, the thermal capacity f building envelpe can be increased substantially. Alqallaf analyzed the thermal perfrmance f a PCM cncrete rf. He fund the gain reduced with the increased amunt f PCM, the phase change temperature affected the gain (Alqallaf Alawadhi ). Bntemps used PCM int the partitin wall f an experimental bx with twin-bx structure, the temperature fluctuatin in the experimental bx was significantly lwer than the case withut PCM (Bntemps et al. ). Cabeza built a cncrete rm under natural cnditin, the walls with suth west rientatins rf cntained PCM. With the experiments, the peak temperature flux had been significantly decayed delayed (Cabeza et al. ). It can be seen that the building envelpe equipped with PCM shws better thermal perfrmance than the traditinal building envelpe. But, whether the thermal characteristics f the PCM in building envelpe can be prperly used directly influences the thermal perfrmance f the building envelpe equipped with PCM. In this type f building envelpe, the energy strage functin f PCM depends n the ccurrence f its phase change effect. Hwever, the thermal perfrmance f the building envelpe equipped with PCM nt nly depends n the therm-physical prperties f PCM, but als lies n the structural cnfiguratin f the wall. On the ne h, the therm-physical prperties f PCM determine hw strng the energy strage functin is; n the ther h, the structural cnfiguratin f PCM wall determines whether the functin f PCM can be efficiently used t a large extent. Vicente tested analyzed the thermal perfrmance f a perfrated brick wall utfitted with PCM thermal insulatin material. He fund that this type f wall regulated the indr thermal envirnment well (Vicente Silva ). abares-velasc analyzed the thermal perfrmance f an paque PCM wall using mathematical mdel, the preliminary cnclusin was that the thermal perfrmance f PCM wall depended n the factrs including the thermal prperties the psitin f PCM (abares-velasc et al. ). With the experiments calculatins, Silva realized the significance f the structural cnfiguratin n the thermal perfrmance f PCM wall (Silva et al. ). At present, as a gd frm f energy-saving wall, hllw blck wall has been widely used in sme cuntries arund the wrld. If the PCM is intrduced int the hles f hllw blcks frms the cmpsite PCM hllw blck wall (CPCMHBW), the new type f wall will be easy cnvenient fr cnstructin with gd applicatin effect, in which the PCM will nt ccupy the thickness f wall, the wall will be mre safe with the well sealed PCM. he authrs f this paper have analyzed the influence laws f the thickness, number, width rib width f hles, the number f hle rws, the staggered r unstaggered arrangement f hles, n the thermal perfrmance f the hllw blck wall withut PCM in the past (Zhang et al. ). Whereas, after intrducing PCM int hllw blck, the functining mde fr the energy strage f PCM is quite different frm the air layers in hles. herefre, the influence law n the thermal perfrmance f the hllw blck wall equipped with PCM shuld als be entirely different frm that f the hllw blck wall withut PCM. Currently, it is insufficient fr the systematic cmplete analysis f the influence laws n the thermal perfrmance f CPCMHBW by hllw blck s structural cnfiguratin. On accunt f this, in this paper, in rder t supply the research gap f the influence laws f hllw blck s structural cnfiguratin n the thermal perfrmance f PCM hllw blck wall, the influence laws f the structural cnfiguratin f hllw blck n the thermal perfrmance f CPCMHBW have been analyzed using tw-dimensinal enthalpy mdel. he structural cnfiguratin f PCM hllw blck mainly cvers five factrs: the lcatin f PCM in thickness directin, the thickness f PCM, PCM s hle number, hle width, rib width f each rw, the number f PCM rws, the staggered r unstaggered arrangement f PCM hles. MEHODS Assumptins simplify the transfer mdel, the fllwing assumptins were made: () all the materials were cnsidered t be thermally hmgeneus istrpic; () because the transfer prcess was mainly cnductin, the thermal cnductivity f the liquid PCM was cnsidered cnstant; () because the vlume f the liquid ISBN: ---- COBEE-Paper page

2 th Internatinal Cnference On Building Energy, Envirnment state t that f the slid state fr the same mass was less than., the vlume change f the PCM during the phase change prcess was ignred; () the super-cling effect during the freezing prcess the natural cnvectin effect were ignred; () the equivalent thermal cnductivity was applied fr the air layers inside the blcks (Ministry f Husing Urban-Rural Develpment f the Peple s Republic f China ); () because the net flw was zer in the vertical directin f wall, the transfer in the cmputatinal dmain was simplified t be tw-dimensinal transfer, the ttal flw was in the directin f thickness. Gverning equatin In this research, tw-dimensinal enthalpy mdel was adpted t slve the transfer prblem f the PCM walls. ρ H = λ( + ) τ x y where ρ, λ, H are the density, thermal cnductivity, enthalpy f PCM, respectively. he thickness width directins are represented by x y, the temperature at any pint in time τ is. he enthalpy f PCM is calculated by cps, < m ε L () H = cps, m+ [ ( m ε) ] m ε m+ ε ε cps, m + L+ cpl, [ ( m + ε) ] > m + ε where c ps, c pl, are the specific f the PCM in its slid liquid states, respectively, m is the mean temperature f the phase change temperature range (PCR). he phase change radius ε represents the half range f the PCR L is the latent during phase change. he temperature range f phase change is frm m ε t m + ε. When < m ε r when > m + ε, the value f c ps, is equal t the value f c pl, (=.kj kg K ); when m ε m + ε, the value f c ps, is equal t cp,equ = L, where c p,equ is the equivalent specific f the PCM during phase change. Initial bundary cnditins Because the experiments are the testing thermal perfrmance f wall under peridical thermal bundary cnditins, fr eliminating the influence f initial cnditin n the results, fur perids are perfrmed in each experiment. he initial temperature is shwn as fllws: () x, y, τ = τ = init () where xy,, τ is the temperature at pint ( xy, ) at time τ. init is the initial temperature. he third type thermal bundary cnditin is applied fr the exterir interir surface in thickness directin. he bundaries in the directin f width directin are adiabatic. Numerical methd he pint iteratin methd is used fr the tw-dimensinal numerical calculatin in this wrk. he gverning equatins bundary cnditins are discretized by Finite Difference Methd (FDM). Central differences are applied in space step while a fully implicit finite difference scheme frward differences are applied in time step. he Gauss-Seidel scheme is emplyed in iteratin. he calculatin prgram is develped in MALAB envirnment. Mdel validatin he enthalpy mdel used in this research had been validated in the reference (Zhang et al. ). It was demnstrated that the errrs between the calculatin results f this mathematical mdel the crrespnding experimental data were less than %, which can satisfy the requirement f the analysis n the prblems f unsteady-state phase change transfer. Evaluatin Indexes he authrs f this paper have prpsed fllwing indexes fr evaluating the UP (Zhang et al. ). () Abslute damping f the temperature fluctuatins θdec where θ = θ θ () dec ut, air in, wall θdec is abslute damping f the temperature fluctuatins, θ ut, air is the amplitude f the external temperature disturbance, θ in, wall is the temperature amplitude n the interir surface f building envelpe cnsidering the time lag. () PCM utilizatin rati a) PCM utilizatin rati f ing prcess he represents the functinal level f PCM during the ing prcess. t ( t ε ) % max m = () ε where t max is the maximum temperature f PCM, t m is the central temperature f the ing temperature range, ε is the half width f the ing temperature range. b) PCM utilizatin rati f freezing prcess he represents the functinal level f the PCM during the freezing prcess. ( t + ε ) t m min = % () ε where t m is the central temperature f the freezing temperature range, ε is the half width f the freezing ISBN: ---- COBEE-Paper page

3 th Internatinal Cnference On Building Energy, Envirnment (c) PCM in the middle hles (Blck) (d) PCM in the inside hles (Blck) Figure. PCM s lcatins in the blck able. herm-physical prperties f materials Sensible hermal Density Parameters cnductivity - (kg m ) (J kg K ) (W m K ) Blck material. Air layer (mm.. thickness) (h) (a) time lags Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface Utilizatin rates f ing prcess (%) Density - (kg m ) able. herm-physical prperties f PCM Sensible Latent hermal emperature cnductivity range (J kg K ) (J kg ) (W m K ) ( C). ~ get rid f the disturbance f ther factrs, the blck shape shwn in Figure is selected as the physical mdel. here are tw rws f hles in the hllw blck, in which the PCM is intrduced. he therm-physical parameters f the blck material PCM are shwn in able. he PCM intrduced int the three types f hles in the blck has the same ttal latent ; that is t say, the thicker the PCM is, the lwer the latent per unit mass is. Figure shws the UP values f each blck wall, when the thicknesses f the PCM hles are mm (Case-Case), mm (CaseCase) mm (Case-Case). Figure., time lags, the It can be seen frm Figure that, alng with the increase f the RR f left bundary air, althugh the Influence f the thickness f PCM (the ttal latent is cnstant) (a) PCM mm thick PCM mm thick n the PCM in the utside hles (Blck) (a) Blck structure (mm) get rid f the disturbance f ther factrs, the blck shape shwn in Figure a is selected as the physical mdel. he therm-physical parameters f the materials in hllw blck are shwn in able. Figure shws the UP values f each blck wall, when the central temperature f the left bundary air the temperature f the right bundary air are bth C, the PCM is in different lcatins f blck in thickness directin, the wall is under different temperature range radiuses (RRs) f the left bundary air ( C C). Influence f the lcatin f PCM in thickness directin RESULS AND DISCUSSION left right surfaces f PCM hles rise gradually. Fr the Case - Case (Blck ), the phase change effect has nt cmpletely taken place in all f the PCM under the bundary cnditins f Case - Case, the blck walls maintain high UP; hwever, the temperature f PCM has exceeded its PCR under the bundary cnditins f Case - Case, namely the PCM has been brken dwn, which significantly decreases its applicatin effect; als, the higher the degree f the break dwn is, the lwer the thermal perfrmance f wall becmes. Fr the Case - Case (Blck ), the phase change effect has nt cmpletely taken place under the cnditins f Case - Case, then the walls maintain high UP; while the PCM is brken dwn fr the Case Case fr the same reasn, cnsequently the applicatin effect f wall is weaken. he phase change effect has nt cmpletely taken place fr all the cases frm Case t Case (Blck ), which maintains steady temperatures n the interir surfaces f the blcks, then reaches high UP. Besides, under the same temperature range f left bundary, the grwth f the PCM s utilizatin rati f the Blck is the fastest, that f the Blck is the slwest (Figure b); this demnstrates that the thermal perfrmance f the Blck wall is the easiest t be influenced by external temperature variatins, n the cntrary, that f the Blck wall is the mst difficult t be influenced. It can als be seen that, the lwer the PCM utilizatin rati is, the higher the thermal perfrmance f wall reaches in mst cases, the pssibility f the PCM being brken dwn becmes lwer the UP f wall is mre difficult t be influenced by external temperature variatins. In additin, the UP f the Blck wall (PCM lcates in the utside hles) is the lwest (the decrement factr time lag are bth lw); the decrement factr f the Blck wall (PCM lcates in the middle hles) is slightly higher than the Blck wall (PCM lcates in the inside hles), while the time lag f the Blck wall is a bit lwer than the Blck wall, therefre, it is nt much difference between the thermal perfrmance f the Blck Blck. PCM. perfrm n a declining curve; the is the minimum temperature f the tmin temperature range, (c) PCM mm thick Figure. Blck with different PCM hle thicknesses (mm) θ dec able. herm-physical prperties f materials cntinuusly rises, the decrement factr time lag ISBN: ---- COBEE-Paper page

4 th Internatinal Cnference On Building Energy, Envirnment Parameters Density (kg m - ) Sensible (J kg K ) Blck material PCM tal latent (J) hermal cnductivity (W m K ) Melting freezing temperature range ( C) ~ (c) (a) & time lags Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface (h) Utilizatin rates f ing prcess (%) f utside PCM hles Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface Utilizatin rates f ing prcess (%) f inside PCM hles Figure., time lags, the As is shwn in Figure, with the rise f the RR f the left bundary air (the bundary temperature range becmes wider), the decrement factr time lag bth decrease, the PCM s utilizatin ratis increase cntinuusly. In these cases, nt all the PCM s utilizatin ratis exceed % (Case - Case, Case - Case, Case - Case) with the RR f the left bundary air f C, C C, while the utilizatin ratis f ther are larger than %, the UP f the blck walls dramatically reduces. With the same temperature range f the left bundary air different thicknesses f PCM, the UP f each blck wall des nt vary clearly: as the thickness f PCM gradually increases, the decrement factr under same bundary cnditins slightly declines, but the unidirectinal persistent variatin f the time lag values cannt be seen. herefre, fr the hllw blcks with equal ttal latent same phase change temperature f PCM, the variatin f PCM s thickness (mm, mm mm) des nt significantly influence the UP f the blck wall. Althugh the thickness f PCM in blck rises, the ttal latent des nt change; at this pint, the value f the ttal latent is the key factr determining the UP f wall. herefre, when designing the cmpsite PCM hllw blck with the cnstant ttal latent f PCM, the variatin f PCM s thickness cannt significantly imprve the UP f wall. Influence f PCM s hle number, hle width rib width f each rw Figure shws the variatin f the decrement factrs, time lags, the n the left right surfaces f PCM f cmpsite PCM hllw blck wall with the basic structure in Figure. guarantee the cnstant ttal latent f the cmpsite PCM, the latent per mass has been adjusted accrding t the amunt f the intrduced PCM in the calculatin. Based n the blck structure, the PCM s hle number, hle width rib width f each rw changes fllwing the able. Als, the blck thickness f.m the hle thickness f.m are invariable, the RR f the left bundary air is C, respectively. he calculatin results are in Figure. Figure. Basic blck structure (the unit is mm) able. Variatin f PCM s hle number, hle width rib width f each rw Hle number Hle width when the rib width is mm (mm) Hle width when the rib width is mm (mm) Hle width when the rib width is mm (mm) (Case) (Case) (Case) (Case) (Case) (Case) (Case) (Case) (Case) (a) & time lags Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface (h) Utilizatin rates f ing prcess (%) f utside PCM hles (Case) (Case) (Case) ISBN: ---- COBEE-Paper page

5 th Internatinal Cnference On Building Energy, Envirnment (c) Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface Utilizatin rates f ing prcess (%) f inside PCM hles Figure., time lags, the (the ttal latent f PCM is cnstant) In additin, the influence f anther grup f PCM s hle number, hle width rib width f each rw n the UP f wall is here fr analyzing. he cnstant latent per mass is perfrmed n the cmpsite PCM, which is different frm the abve analysis; therefre, the ttal latent f PCM will vary with the intrduced amunt. he calculatin results are shwn in Figure. (c) (a) & time lags Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface n the left & right surfaces f utside PCM hles Utilizatin rates f ing prcess n the left surface Utilizatin rates f ing prcess n the right surface n the left & right surfaces f inside PCM hles Figure., time lags, the (the latent per mass f PCM is cnstant) It can be seen frm the Figure Figure that, when the ttal latent r the latent per mass f PCM is cnstant, the variatin f PCM s hle number, hle width rib width f each rw in the blck has very slight impact n the UP f wall, which can be ignred. his demnstrates that the variatin f PCM s hle number, hle width rib width f each rw is nt beneficial t the imprvement f the UP f CPCMHBW; meanwhile, because the thermal cnductivity f PCM is nt necessarily lwer than the blck material, the variatin f hle number, hle width rib width f each rw has slight impact n the steady-state thermal perfrmance f the hllw blck equipped with PCM t. herefre, fr the hllw blck equipped with PCM, it is (h) Utilizatin rates f ing prcess (%) Utilizatin rates f ing prcess (%) n need t reduce the rib width vary the PCM s hle number f each rw intentinally. his cnclusin is much different frm the previus cnclusin fr the hllw blck wall withut PCM. he reasn fr this is: in the hllw blck wall withut PCM, the functin f hles is resisting the flux; therefre, the key factr influencing the thermal perfrmance f the hllw blck withut PCM is the thermal resistance f the air layers in hles the width f the thermal bridge at the hle s lcatin (namely the rib width) (Zhang et al. ). While in the hllw blck equipped with PCM, the main functin f the PCM hles is regulating the exterir temperature wave flw wave making them mre smth steady by prperly using the latent thermal strage functin f PCM; therefre, if the phase change temperature latent f PCM have n bvius change, the UP f the hllw blck equipped with PCM will nt significantly vary with the PCM s hle number, hle width rib width f each rw. Influence f the number f PCM rws analyze the influence f the relatinship between the number f PCM rws the bundary temperature range n the thermal perfrmance f wall, the thermal prcess f the blck walls with single rw, duble rws triplex rws shwn in Figure has been calculated under the cases f different RR f left side air. he amunt f the PCM used in different blcks is equal. (a) Single rw Duble rws (c) riplex rws Figure. Blck structure (the unit is mm) It can be seen frm the Figure that, the UP f the blcks with different number f PCM rws has nt varied significantly, with the same amunt f PCM bundary temperature range. herefre, the number f PCM rws has slight influence n the UP f blck wall with the cnstant ttal latent f PCM, which can be ignred. Fr the hllw blck equipped with PCM, there is n need t add r reduce the rws f PCM intentinally. Figure. & time lags with different rws Influence f the staggered r unstaggered arrangement f PCM hles analyze the influence f the staggered r unstaggered arrangement f PCM hles n the thermal perfrmance f wall, the blck structure shwn in Figure is taken as the basic structure, the UP f the CPCMHBW (with different staggered r unstaggered arrangement f PCM hles shwn in able ) has been calculated under the same bundary cnditin. Figure is the decrement factrs time lags f the CPCMHBWs. (h) ISBN: ---- COBEE-Paper page

6 th Internatinal Cnference On Building Energy, Envirnment able. Variatin f the hle widths f each rw Case Case Case Case Hle widths f each rw / / / / (mm) Staggered r nt N N Yes Yes Case Case Case Case Hle widths f each rw (mm) / / / / Staggered r nt Yes Yes Yes Yes Figure. & time lags with different staggered r unstaggered arrangement f hles In Figure, the UP f Case Case is slightly lwer than the UP frm Case t Case; but verall, the UP f different cases des nt vary bviusly. hat is t say, in spite f the staggered r unstaggered arrangement f PCM hles (frm unstaggered t staggered with increasingly deeper extent), this factr has slight influence n the UP f blck wall, the UP with staggered arrangement f PCM hles is a bit better than that with unstaggered arrangement f PCM hles; meanwhile, the steady-state thermal perfrmance with staggered arrangement f PCM hles is better than that with unstaggered arrangement f PCM hles t (Zhang et al. ). herefre, under the premise f nt adding prductin difficulty cst, it is advisable t make the rws f PCM hles be staggerly arranged in blcks. CONCLUSIONS In this paper, the influence laws f the structural cnfiguratin n the thermal perfrmance f cmpsite PCM hllw blck wall (CPCMHBW) has been analyzed, using tw-dimensinal enthalpy mdel. With the analysis f this paper, fllwing cnclusins has been drawn: () Fr the wall equipped with PCM, that the PCM is nt brken dwn (the PCM is brken dwn means that the temperature f PCM exceeds the phase change temperature range f PCM) is the guarantee fr high unsteady-state thermal perfrmance (UP). () On the basis f cmplete phase change cycle that PCM experiences, the lwer the PCM utilizatin rati is, the higher the UP f wall reaches in mst cases. () With the same wall structure cmpsitin, the mre the PCM is clse t the exterir surface f wall, the easier the UP f wall is influenced by external temperature variatins, the narrwer the utside temperature fluctuatin range which the wall can bear becmes, the lwer the UP becmes, vice versa. () btain high stable UP, the PCM shuld be intrduced int the lcatin which is clse t the interir surface f wall. () On the basis f cnstant ttal latent, the variatins f the thickness f PCM, PCM s hle number, hle width, rib width f each rw, the number f PCM rws, the staggered r unstaggered arrangement f PCM hles, cannt significantly influence the thermal (h) perfrmance f CPCMHBW, the impact f them can be ignred. It is advisable t make the rws f PCM hles be staggerly arranged (the thermal perfrmance f the blck wall with staggered arrangement f PCM hles is a bit better than that with unstaggered arrangement f PCM hles). ACKNOWLEDGEMEN his wrk was financially supprted by the Natinal Natural Science Fundatin f China (N.), the Natural Science Fundatin f Jiangsu Prvince (N.BK), China Pstdctral Science Fundatin (N.M), Jiangsu Prvince Pstdctral Science Fundatin (N.B), Jiangsu Gvernment Schlarship fr Overseas Studies Scientific Research, "Yuth alent Cultivatin Plan" Excellent Yung Key eachers raining Prject f Jiangsu University, the Scientific Research Starting Fundatin fr Advanced alents f Jiangsu University (N.JDG). REFERENCES Alqallaf H. Alawadhi E.. Cncrete rf with cylindrical hles cntaining PCM t reduce the gain. Energy Buildings, Vl., pp. - Bntemps A., Ahmad M., Jhannes K., et al.. Experimental mdelling study f twin cells with latent strage walls. Energy Buildings, Vl. (), pp. Cabeza L., Castellón C., Ngués M., et al.. Use f micrencapsulated PCM in cncrete walls fr energy savings. Energy Buildings, Vl. (), pp. Vicente R. Silva.. Brick masnry walls with PCM macrcapsules: An experimental apprach. Applied hermal Engineering, Vl. (-), pp. - abares-velasc P., Christensen C. Bianchi M.. Verificatin validatin f EnergyPlus phase change material mdel fr paque wall assemblies. Building Envirnment, Vl., pp. Silva., Vicente R., Sares N., et al.. Experimental testing numerical mdelling f masnry wall slutin with PCM incrpratin: A passive cnstructin slutin. Energy Buildings, Vl., pp. - Zhang Y., Du K., He J., et al.. Impact factrs analysis n the thermal perfrmance f hllw blck wall. Energy Buildings, Vl., pp. Ministry f Husing Urban-Rural Develpment f the Peple s Republic f China.. hermal design cde fr civil building (GB-). Beijing: China Planning Press Zhang Y., Du K., He J., et al.. Impact Factrs Analysis f the Enthalpy Methd the Effective Heat Capacity Methd n the ransient Nnlinear Heat ransfer in Phase Change Materials (PCMs). Numerical Heat ransfer, Part A: Applicatins, Vl. (), pp. - Zhang Y. He J.. Impact f the relatinship between phase change temperature bundary temperature n the thermal perfrmance f PCM wall the presentatin f PCM thermal perfrmance indexes. Heat ransfer - Asian Research, Vl. (), pp. - ISBN: ---- COBEE-Paper page