Optimisation of A Phase Change Thermal Storage System

Transcription

1 World Acadey of Science, Engineering and Technology Optiisation of A Phase Change Theral Storage Syste Nasrul Ari Mohd Ain, Martin Belusko, Frank Bruno Abstract PCMs have always been viewed as a suitable candidate for off peak theral storage, particularly for refrigeration systes, due to the high latent energy densities of these aterials However, due to the need to have the encapsulated within a container this density is reduced Furtherore, PCMs have a low theral conductivity which reduces the useful aount of energy which can be stored To consider these factors, the true energy storage density of a PCM syste was proposed and optiised for PCMs encapsulated in slabs Using a validated nuerical odel of the syste, a paraetric study was undertaken to investigate the ipact of the slab thickness, gap between slabs and the ass flow rate The study showed that, when optiised, a PCM syste can deliver a true energy storage density between 53% and 83% of the latent energy density of the PCM Keywords Phase change aterial, refrigeration, sustainability, theral energy storage I INTRODUCTION LECTRICITY deand during the suer season [6] E represents a significant ipact to the cost of electricity [7] One of the contributors to this deand during peak suer days is refrigeration Therefore, there is a need for off peak storage for refrigeration However, as with all coercial off peak storage systes, there is a need to prevent an increase in energy consuption [8] Storing the coolness fro the off peak period using phase change aterial (PCM) is a potential option for refrigeration systes [9] PCMs are a substance in which the heat at the solidliquid phase transition point is used in applications for storing large aounts of theral energy [0,] at a certain teperature A principle advantage of PCMs is the large energy storage density relative to sensible energy storage with rock or water (liquid) [2] PCMs also have soe disadvantages The ain drawbacks with PCMs are the low theral conductivity and the encapsulation design In a latent heat theral energy storage syste, the heat transfer fluid (HTF) passes through an area of encapsulated PCM In a PCM, during elting or freezing, the theral resistance increases as the solid/liquid boundary NAMA was with School of Mechanical Engineering at the University of Adelaide He is now with the Institute for Sustainable Systes and Technologies at the University of South Australia, Mawson Lakes, South Australia 5095, Australia (phone: ; fax: ; eail: nasrulariohdain@yahoocoau) Both of second and third author are with the Institute for Sustainable Systes and Technologies at the University of South Australia, Mawson Lakes, South Australia 5095, Australia (phone: ; fax: ; eail: artinbelusko@unisaeduau) oves away fro the heat exchange surface [0] This resistance is significant and reduces the theral capacity of the syste [3] This resistance changes with the design of the encapsulation In a PCM theral storage unit, the percentage of the PCM volue per total syste volue is defined as the copactness ratio The challenge in PCM encapsulation design is to have a large copactness ratio so that the axiu aount of latent energy can be used The copactness ratio can be as low as 50%, as in a sphere type PCM heat exchanger [4] In this project, a plate type theral storage syste is used because of the high storage density of PCM which can be up to 95% of the total volue [5] II SYSTEM DESIGN Optiisation involves varying the PCM slab thickness, gap width between slabs and the HTF ass flow rate A PCM with a elting point of 27 C is used for this analysis because it is suitable for refrigeration applications [9] along with Dynalene HC40 as the HTF [4] The aount of PCM used in this odel is fixed at 3 Therefore, when the slab thickness is varied, the nuber of slabs is changed so that the total PCM volue is constant The actual perforance of a PCM energy storage syste depends on the effectiveness of charging and discharging, and the copactness ratio These paraeters can be incorporated into the true energy storage density factor, α The value of α can be applied to the latent heat of fusion, H L The resulting product represents the true energy density of the PCM theral storage syste III THEORETICAL ANALYSIS The storage syste consists of several flat PCM slabs No heat transfer to the surrounding is assued The HTF is flowing between these slabs as in Fig heat transfer fluid adiabatic wall x Fig Scheatic diagra of the PCM odel [9] PCM A latent heat theral energy storage syste can be described as a heat exchanger Effectiveness is an iportant 765

2 World Acadey of Science, Engineering and Technology paraeter in heat exchanger design and can be applied to PCM storage systes as well The effectiveness of a phase change storage syste is equal to the ratio of the actual heat transfer to the axiu possible heat transfer [6] In this work, the effectiveness of elting, ε and freezing, ε 2 was calculated by using equations () and (2) over the phase change process These equations ignore the sensible energy storage Cp ( Tin Tout ) / Cp ( Tin T Cp ( Tout Tin ) / Cp ( T pc ε = ) () pc ε = T ) (2) 2 in For ideal latent heat theral energy storage, both values of ε and ε 2 are Theoretically, this eans the exit teperature of the HTF ust be the sae as the PCM phase change teperature The ost critical objective in theral energy design is to get the highest α as possible So the values of elting and freezing effectiveness ust be high enough together with the proportion of PCM in the syste, the copactness ratio, γ Values of α and γ was calculated using equations (3) and (4) α = ε ε 2γ (3) γ = t/(w + t) (4) To consider puping losses a odified effectiveness value was deterined using equations (5) and (6), total losses of the syste are then calculated using equations (7) ε * = ε ( Losses / Q th ) (5) ε = ε ( Losses / Q ) (6) 2* 2 th Losses Δ P V η p p s = / η (7) The Losses define the priary energy of the puping losses Pup efficiency, η p and power station efficiency, η ps were fixed at 50% and 35% respectively To consider these losses, α value becae α* as defined in (8) * = ε ε γ (8) α * 2* IV SIMULATION SOFTWARE In this work, the siulations were done using TRNSYS TRNSYS is a transient systes siulation progra with a odular structure [7] The software has been used for analysing the tie dependent nuerical odel for any heat exchanger types [] in refrigeration systes [8] In order to calculate the true energy storage density factor, α, the value of ε and ε 2 values need to be known TRNSYS was used to calculate these values flat PCM slabs which are then siulated in the coputer using the finitedifference calculation ethod The ideal perforance occurs when the inlet teperature is close to the elting teperature Therefore a C teperature different was applied between the inlet teperature and the PCM elting point This teperature difference also iniised the sensible energy of the PCM Siulations on the odel are based on the following assuptions: No super cooling occurs, 2 Siulation stops when T in T out (elting) or T out T in T < 00 Results have shown that the effectiveness values for elting and freezing have inial differences (coparing T < 00 and T < 000) and can be neglected, 3 Conduction in the PCM is ignored as it has been shown that the resistance to heat flow is sall due to the thin slabs that are used [5,9,20], 4 Natural convection is ignored, 5 Sensible energy is ignored as it is only 25% of the latent energy Table show the paraeters which had to be set on the coputer odel These values are fixed through siulations Paraeter HTF inlet teperature Solid theral diffusivity Liquid theral diffusivity Solid specific heat Liquid specific heat Solid theral conductivity Liquid theral conductivity Solid density Liquid density PCM elting teperature PCM freezing teperature Initial teperature Latent heat of fusion Wall theral conductivity Length of slabs Width of slabs TABLE I PRESET PARAMETERS Quantity 257 (for elting) 277 (for cooling) (for elting) 265 (for cooling) Unit 2 /s 2 /s J/kgK J/kgK W/K W/K kg/ 3 kg/ 3 The three ain paraeters investigated in this project are the gap between slabs, w, HTF ass flow rate,, and PCM slab thickness, t Table 2 shows the range investigated J/kg W/K V PCM MODEL An existing one diensional coputer odel of latent heat theral energy [9] in TRNSYS was used in this research This experientally validated odel [9] consists of several 766

3 World Acadey of Science, Engineering and Technology TABLE II CHANGING PARAMETERS Paraeter No of slabs Thickness of slabs Gap between slabs Mass flow rate Quantity 5 to 05 Default is to Default is to 075 Default is to 7500 Default is 00 Unit VI SIMULATION FINDINGS A Gap width effects The first siulation investigated the effect of the gap width between PCM slabs, w on the true energy storage density factor, α The w value is increased fro 0005 to 075 through repeated siulations The PCM slab thickness and ass flow rate of the HTF are fixed at 00 and 00 respectively The result is shown in Fig 2 Fig 3 Melting effectiveness, ε and ε * versus gap width, w () B Mass flow rate effects The ipact of HTF ass flow rate was investigated The result is shown in Fig 4 Fig 2 True energy storage density factor, α and α * versus gap width, w () There are two lines in the graph As the values for both theoretical, α and the one including losses, α * are alost identical, it can be concluded that the puping losses are negligible The α values declined sharply for w < 0 due to a decreasing copactness ratio, γ As shown in Fig 3, an increasing w had a sall effect on the effectiveness The sall reduction was due to the decrease in overall heat transfer coefficient between the wall and the HTF that a larger gap would cause The range of w < 0 is typical for any flat slabs type of PCM storage because of their high value of α The syste becae inefficient for larger gap widths Fig 4 True energy storage density factor, α and α * versus ass flow rate, () The heat transfer fro the HTF to PCM occurs best for low ass flow rates This can be explained by referring to Fig 5 The elting effectiveness, ε and ε * decreased with increasing flow rate as expected in heat exchangers The ipact of losses is sall, increasing at higher flow rate The sae phenoenon applies to the freezing process 767

4 World Acadey of Science, Engineering and Technology Fig 5 Melting effectiveness, ε and ε * versus ass flow rate, () C Slab thickness effects The ipact of the PCM slab thickness, t is shown in Fig 6 The nuber of slabs was increased fro 5 to 05, as the thickness decreased fro to The siulation is run repeatedly with 50, 00, 500, and 000 ass flow rate of the HTF (fro top to botto in the graph) Fig 7 Melting effectiveness, ε and ε * versus slab thickness, t () This optial thickness increases with decreasing flow rates At the lowest flow rate, α reaches a constant axiu and this is due to the effectiveness also being relatively constant (Fig 7) This occurs as with a decrease in area there is also an increase in the overall heat transfer coefficient between the wall and the HTF as the ass flow rate is fixed Fig 6 True energy storage density factor, α and α * versus slab thickness, t () The graph shows an increasing α at sall slab thicknesses Given that the gap width, w is fixed, this increase is due to an increasing copactness ratio Fig 7 shows the elting effectiveness decreasing at larger slab thickness which is due to the decreasing heat exchanger area as the nuber of slabs decreased Therefore an optial slab thickness exists VII OPTIMAL DESIGN The ost critical finding is depicted fro Fig 6 It can be seen that the slab thickness plays an iportant role in theral energy storage design It is clearly shown that the optiu value for α is different depending on the ass flow rate supplied into the syste As the ass flow rate increased, the axiu α dropped off In Fig 6, the top line represents α for 50 flow rate Slab thickness greater than gives an alost constant α It was concluded that for low flow rate, the optiu size of thickness is with 5 slabs in this theral storage unit (with 00 gap between slabs) Another conclusion is that, for low flow rates, the α values are independent of losses Overall an optial α of 083 was obtained 000k/hr represents a typical high flow rate for the syste The optiu slab thickness is with 45 slabs It can be seen that, the losses fro puping have a sall effect At this flow rate, the optiu α was 053 Overall, it has been shown that with a plate design, the true energy storage density of a PCM theral storage facility is heavily dependent on the design and the flow rates used Optiisation has shown that the true energy storage density of a PCM syste can be increased to within the range of 53% to 83% of the latent energy storage density of the PCM With PCMs being any ties ore energy dense than sensible storage systes, it can be concluded that optiized PCM systes with plates can deliver true storage densities considerably higher than sensible systes 768

5 World Acadey of Science, Engineering and Technology APPENDIX TABLE III NOMENCLATURE Sybol Quantity Unit ε elting effectiveness ε 2 ε * ε 2* freezing effectiveness odified elting effectiveness odified freezing effectiveness ass flow rate of HTF between slabs Cp specific heat J/kgK T in inlet teperature outlet teperature T out T pc Q th Q act teperature of PCM theoretical stored energy J/s actual stored energy J/s P pressure drop kg/ 2 η p puping efficiency η ps power station efficiency f relative roughness l length of PCM slab ρ density of the HTF kg/ 3 v velocity of the HTF /s D h Re w l t μ γ α α * Q sensible Q latent H L hydraulic diaeter Reynolds nuber width of gap length of gap / PCM slab thickness of PCM slab dynaic viscosity of the HTF proportion of PCM true energy storage density factor odified true energy storage density factor sensible heat latent heat latent heat of fusion ACKNOWLEDGMENT kg/s J J J/kg The authors are very grateful to the Institute for Sustainable Systes and Technologies at the University of South Australia for providing technical supports NAMA also be thankful for the financing grant by the Universiti Malaysia Perlis which governed by the Ministry of Higher Education, Malaysia [6] L Jianyou, "Nuerical and experiental investigation for heat transfer in triplex concentric tube with phase change aterial for theral energy storage", Solar Energy, vol 82, no, pp , Nov 2008 [7] W Muriel, O Hugh, S Ted, M Iain, O Monica, and P Scott, "Photovoltaics and peak electricity loads, Suer ", in Report for BCSE, May 2005 [8] V Butala and U Stritih, "Experiental investigation of PCM cold storage", Energy and Buildings, vol 4, no 3, pp , Mar 2009 [9] M Liu, F Bruno and W Saan, Theral perforance of a PCM theral storage unit, in Proc of Int Solar Energy Society for Solar World Congress, Beijing, China, 82Sept 2007 [0] N A M Ain, F Bruno, and M Belusko, Maxiizing the energy storage perforance of phase change theral storage systes, in Proc of the IASTED Int Conf, Solar Energy (SOE 2009), Phuket, Thailand, 68 Mar 2009, pp 5560 [] H Mehling and LF Cabeza, Heat and Cold Storage with PCM: An up to date introduction into basics and applications Berlin Heidelberg: SpringersVerlag, 2008 [2] F Bruno, Centralised PCM systes for shifting cooling loads during peak deands in buildings, in Suppleentary Technical Research Paper, City of Melbourne, May 2006 [3] H Ettouney, H ElDessouky, and A AlAli, Heat transfer during phase change of paraffin wax stored in spherical shells, Journal of Solar Energy Engineering, vol 27, no 3, pp , Aug 2005 [4] J P Bedecarrats, F Strub, B Falcon and J P Duas, Phasechange theral energy storage using spherical capsules: perforance of a test plant, International Journal of Refrigeration, vol 9, no 3, pp 87 96, 996 [5] M Belusko and F Bruno, 40 Design ethodology of PCM theral storage systes with parallel plates, in Report for st Int Congress on Heating, Cooling, and Buildings, EUROSUN 2008, Lisbon, Portugal, 7 0 Oct 2008 [6] The Board of Regents of the University of Wisconsin Syste (2006, Noveber, 6) [Online] Available: [7] S V Dosky, D Heinze, and J Wolf, Nuerical siulation of a refrigeration cycle for scaling towards sall geoetries, International Journal of Refrigeration, vol 3, no 8, pp , Dec 2008 [8] E Halawa, F Bruno, and W Saan, Nuerical analysis of a PCM theral storage syste with varying wall teperature, Energy Conversion and Manageent, vol 46, no 56, pp , Sept 2005 [9] E Halawa, Theral perforance analysis of a roof integrated solar heating syste incorporating phase change theral storage, PhD thesis, School of AME, University of South Australia, 2005 [20] D J Morrison and S I AbdelKhalik, Effects of phasechange energy storage on the perforance of airbased and liquidbased solar heating systes, Solar Energy, vol 20, no, pp 5767, 978 REFERENCES [] F Wang, G Maident, J Missenden, and R Tozer, "A review of research concerning the use of PCMS in air conditioning and refrigeration engineering", in Proc of the Int Conf on Advances Building Technology, Hong Kong, China, 4 6 Dec 2002, pp [2] A Pasupathy, L Athanasius, R Velraj, and R V Seeniraj, "Experiental investigation and nuerical siulation analysis on the theral perforance of a building roof incorporating phase change aterial (PCM) for theral anageent", Applied Theral Engineering, vol 28, no 56, pp , Apr 2008 [3] M M Farid, A M Khudhair, S A K Razack, and S AlHallaj, "A review on phase change energy storage: aterials and applications", Energy Conversion and Manageent, vol 45, no 90, pp 59765, June 2004 [4] S M Vakilaltojjar, Phase change theral storage syste for space heating and cooling, PhD thesis, School of AME, University of South Australia, 2000 [5] A M Papadopoulos, S Oxizidis, and N Kyriakis, "Perspectives of solar cooling in view of the developents in the airconditioning sector", Renewable and Sustainable Energy Reviews, vol 7, no 5, pp 49438, Oct