Experimental study of short term thermal energy storage unit based on enclosed phase change material in polyethylene film bag

rm m Zu al U form ne nit pap sto e P � 2006 Elsevier Ltd. All rights reserved. Keywords: Thermal energy storage (TES); Phase change material (PCM); Paraffin wax demand of low energy consumption houses results from device has to be attached to the heating system. This unit accumulates energy at night. During the day, when the cost of electricity is higher, the unit can be discharged. second law analysis by Watanabe and Kanzawa [4]. They is no (i) heat exchange between capsules filled with PCM, (ii) heat conduction in a radial direction and (iii) heat con- duction in the working fluid. The authors observed a strong influence of a cyclic variation of the working fluid inlet tem- perature on the results of the calculations from all models. Farid and Kong tested experimentally an underfloor * Tel.: +48 85 7469680. E-mail addresses: [email protected], [email protected]. Energy Conversion and Managem heating of ventilation air. The author proposes a new con- struction of the air based heating system. In this applica- tion, warm air flows through ventilation ducts of an underfloor heat exchanger and is blown into a room near the external wall. More details about the underfloor air dis- tribution system are presented in Refs. [1–3]. The ventila- tion air circulating in a house can be heated in an electric furnace. This solution is economically efficient under con- dition that low over night electricity tariff is available to customers. Furthermore, a short term energy storage described a heat storage unit that consisted of encapsulated PCM in horizontal cylindrical tubes [5]. The capsules were fixed in an in line arrangement. Ismail and Stuginsky inves- tigated four basic groups of models suitable for sensible and latent heat thermal storage systems [6]. They tested the fixed bed storage system in cylindrical geometry. The authors found that a thermal model with an internal thermal gradi- ent inside the solid particles seems to consume more than twenty times the computing required by Schumann’s model [7]. This one-dimensional approximation assumes that there 1. Introduction Warm air heating systems are frequently designed for new residential buildings. A high thermal resistance of walls and a new generation of windows technologies are the main causes of a great reduction of heat losses through building envelopes and leakage. The main part of heat The PCM used in the thermal storage units can be encapsulated in another material or a porous material can be impregnated directly by the PCM. In this paper, only the first case is considered. Encapsulation of PCMs in polymeric materials is the subject matter of significant works. The optimum melting point distribution of PCMs was optimized on the basis of Experimental study of short te based on enclosed phase change Miroslaw Department of Heat Engineering, Bialystok Technic Received 12 April 2005; received in revised Available onli Abstract This paper describes a short term thermal energy storage (TES) u film bag. As a storage medium, paraffin wax (RII-56) was used. The pressure drop characteristics of the tested unit. The total enthalpy ranges from 240 to 262 kJ/kg. The investigations data show that th 0196-8904/$ - see front matter � 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.04.020 thermal energy storage unit aterial in polyethylene film bag kowski * niversity, Wiejska 45 A, 15-351 Bialystok, Poland 28 October 2005; accepted 28 April 2006 19 June 2006 based on an enclosed phase change material (PCM) in polyethylene er presents experiment results, which include charge, discharge and rage in the module depends on the PCM temperature change and CM RII-56 can be successfully used in heat storage applications. www.elsevier.com/locate/enconman ent 48 (2007) 166–173 and heating system with encapsulated CaCl2 Æ 6H2O placed in a bed of concrete [8]. They applied a one-dimensional heat conduction model and simulated heat transfer assuming that the specific heat capacity of the PCM is in proportion to temperature during the phase change process. The geo- metrical and operational parameters of a storage unit com- posed of spherical capsules filled with PCM and placed inside a cylindrical tank were investigated both numerically and experimentally by Ismail and Henriquez [9]. A numer- ical model to simulate the system was based on dividing the tank into a number of axial layers. The solidification pro- cess was treated by using only one-dimensional heat con- duction. They provided that the use of polyethylene or PVC for the spherical capsule facilitates the construction of the storage and reduces its costs. A short term storage unit based on polyethylene spheres with encapsulated PCM was designed and investigated by Arkar and Medved Nomenclature A surface area of casing walls (m2), c air specific heat at constant pressure (kJ/kg K) h convective heat transfer coefficient inside unit (W/m2 K) H enthalpy (kJ/kg) k thermal conductivity (W/m K) Q heat transfer rate (W) t time (s) T temperature (K) V volumetric flow rate (m3/s) Greek symbols DP pressure drop (Pa) DT temperature difference (K) M. Zukowski / Energy Conversion [10]. A one-dimensional numerical model described heat transfer inside the duct with porous media. The model assumed that the melting and solidification processes occur at the same and constant temperature, and in the liquid phase, convection is not considered. Measured and calcu- lated results showed that it is possible to prevent or to reduce building overheating by using a short term heat stor- age unit tested by the authors. Several thermal andmechanical properties of compounds made of polyethylene, paraffin wax, ethylene vinyl-acetate and exfoliated graphite powder were investigated by Bader [11]. Takeda et al. constructed a computer simulation pro- gram for the floor supply air conditioning system with a PCM packed bed [12,13]. The enthalpy method was applied to simulate a one-dimensional phase change process. The results of calculations for the office building showed that the heat storage amount in the night time can be increased by 27.9% by utilizing the PCM packed bed. Khudhair and Farid summarized the investigations related to energy con- servation in building applications. The problems of encapsu- lating suitable phase change materials in concrete, gypsum wallboard, floor and ceiling were widely presented and dis- cussed in Ref. [14]. A new structure of underfloor electric heating system with shape stabilized PCM plates was investigated experi- mentally and numerically by Lin and co-authors [15,16]. They used the enthalpy method to develop a model to study the influences of various factors on the thermal per- formance of the radiant panel heating unit. They neglected natural convection inside the plates arising during melting. The simulated results indicated that this heating system can perform well in ordinary buildings. An underfloor heating system with high storage capacity was developed by RUBI- THERM GmbH [17]. Granulated PCM (RUBITHERM GR 41) used in this construction affords possibilities for reduction (up to 50%) of the total floor thickness. TEAP developed encapsulation methods in spherical, cylindrical and microencapsulation shapes. TEAP’s TH29 capsules q density (kg/m3) Subscripts A air IN inlet MID middle OUT outlet PCM phase change material SF surface W wall 0 starting value 1 ending value Management 48 (2007) 166–173 167 have an application in underfloor heating. Water piping or electric cables can be positioned between the capsules. This system greatly increases the density of heat energy that can be accumulated within a building structure. CRISTOPIA energy systems developed storage technology based on encapsulation of phase change materials. This solution, called STL, is composed of a tank filled with spherical nodules (approximately 60% of the volume), which are blow molded from a proprietary blend of poly- olefin and filled with PCM. The remaining 40% is occupied with a heat transfer fluid. A range of PCMs allow storing energy at temperatures between �33 �C and +27 �C [18,19]. The main goals of the project presented in this paper are the following: • Design a short term storage module that can cooperate with the UFAD (underfloor air distribution) system briefly described above. • Experimentally determine the performance characteris- tic of the prototype TES device. and utilizing streamlined shapes of the inlet and outlet air passages. The dimensions of the unit were the following: length – 0.32 m, width – 0.25 m and height – 0.065 m. The casing is shown in Fig. 2. Fig. 2. The construction of the TES unit casing. Table 1 Thermophysical properties of the paraffin wax RII-56 qPCM cPCM kPCM HPCM 842 2174 0.267 156 and Management 48 (2007) 166–173 • Creation of a mathematical model and its numerical solution for simulating the phase change process and air forced convection at the same time. • Build a computer code that can be a useful tool for design- ing thermal storage systems for heating applications. • Validation of the computational model based on data from earlier experiments. The two first stages were realized, and the results are described below. The basic features of the designed module are a rela- tively low cost of storage material and the construction. Fig. 1 shows an idea of the prototype TES device. The PCM is encapsulated in elastic polyethylene film bags. This solution ideally reduces the thermal stress to a minimum, which is a result of the volume change during the melting process. Moreover, a very thin wall of polyeth- ylene bag is characterized by a minimal thermal resistance. The proposed device with a minimal pressure drop can operate as a part of a ventilation ducting. An additional fan is not required because in this case the charging and Fig. 1. Schematic of the TES device under study. 168 M. Zukowski / Energy Conversion discharging cycles are realized by a variable speed circulat- ing fan, which is located inside a furnace. The available literature and internet resources review indicates that this type of short term TES unit has not been tested yet. Moreover, according to the author’s knowledge of the subject matter, there are no reported applications of the paraffin wax (RT-56) as a heat storage medium. 2. Description of the tested heat storage unit The main aspect of the experimental investigations con- cerned the charge, discharge and pressure drop characteris- tics of the short term TES unit for design requirements. The storage module consisted of the following parts: a Plexiglas casing, light steel nets and PCM enclosed in poly- ethylene film bags. The special attention was focused on the problem of uniform flow distribution inside the module. This goal was achieved by installing two relatively thick matrix layers Fig. 3. Paraffin wax RII-56 enclosed in polyethylene film bags and the arrangement of layers. M. Zukowski / Energy Conversion and Management 48 (2007) 166–173 169 The light steel nets were used to keep a uniform distance between the PCM layers and to protect the capsules against shape changing. This solution improved the experiment repeatability and accuracy. For commercial applications, the support construction will be removed. The paraffin wax RII-56 was selected as a storage material. This sub- stance is produced in large quantities in Poland, and the price is kept at a relative low level. The thermophysical properties of the tested material were prepared on the basis of a manufacturer’s data and are presented in Table 1. The PCM was enclosed in polyethylene film bags and Fig. 4. Polyethylene film bags filled with PCM. arranged in three layers (see Fig. 3). The bag was com- pletely filled with paraffin wax and extended simulta- neously in accordance with the PCM volume change (see Fig. 4). The total mass of the material was 1.92 kg. The bedplate of the system was insulated with a 0.1 m polysty- rene layer and the remaining walls with 0.04 m thick poly- ethylene foam sheet. 3. Experimental arrangement 3.1. Experimental apparatus Fig. 5 presents the photograph of the experimental apparatus and instrumentation. charge period ou la Fig. 6. The air circulation The tested module (1) was connected with the axial fan (2) via the plastic rectangular ducts. They were joined in a closed loop circuit during the charging process, whereas the arrangement of the ducts was open during the discharging period, and the fan drove the air from the laboratory as shown in Fig. 6. The air volumetric flow rate was changed by using the electronic frequency controller (3) operated with the fan. The vane anemometer (4), mounted inside the outlet duct, was used to measure the volumetric flow rate of air. The circulating air was heated inside with the electrical resis- tance heater (5). The temperature of the inlet fluid was con- trolled by the digital PID thermo-regulator (6) with accuracy ±0.2 K. The regulator’s temperature sensor was mounted inside the inlet duct. The digital multimeter (7) was used to confirm the energy consumption of the air hea- ter. Temperatures were measured using T-type thermocou- ples (wire diameter 1/0.315 mm) installed inside the inlet and outlet part of the unit and at the top and bottom sur- face of the TES module. The thermocouples were placed inside and at the surface of PCM capsules in the first and Fig. 5. The experimental apparatus and instrumentation. last row. One thermocouple measured the wall temperature inside the module. The location of the thermocouples inside the polyethylene bags is shown in Fig. 7. A temperature field at the capsules surfaces of the top layer was additionally scanned by the infrared thermometer after removing the top insulation sheet. The data logger (8) (16 channel, inner solid state memory, real time clock) was used to collect the experimental data. The information was stored locally and transferred to a PC (9) by the RS485/ air from the laboratory t of the boratory discharge period during the experiment. ation. The inflow TA,IN and outflow TA,OUT air tempera- tures and internal surface temperature of the casing TW,SF were taken from the experiment.� � 20 30 40 50 0 20 40 60 80 100 120 140 160 180 T AIR(IN) TAIR (OUT) TS,PCM(IN) TS,PCM(OUT) TIN,PCM(IN) TIN,PCM(OUT) TA,IN TA,OUT TPCM,IN(SF) TPCM,OUT(SF) TPCM,IN(MID) TPCM,OUT(MID) t [min] T [° C ] T [° C ] 20 30 40 50 60 70 0 20 40 60 80 100 T AIR(IN) TAIR (OUT) TS,PCM(IN) TS,PCM(OUT) TIN,PCM(IN) TIN,PCM(OUT) TA,IN TA,OUT TPCM,IN(SF) TPCM,OUT(SF) TPCM,IN(MID) TPCM,OUT(MID) t [min] (a) (b) Fig. 8. The air and the PCM temperature variation during charging (a) and discharging (b) period for case-5. R n o and RS232 interface after each experimental case. All electric equipment was protected by an uninterruptible power sup- ply system (10). The electronic micro-manometer was used to measure the differential pressure. The static pressure taps were located in the inlet and outlet TES unit ducts. The errors in the measurement process were the follow- ing: temperature – ±0.2 K, pressure – ±1 Pa and volume flow rate – ±0.000055 m3/s (0.2 m3/h). 3.2. Experimental methods Experimental cases were realized for the following three air flow rates: 20, 30 and 40 m3/h. Inlet air temperature was changed between 60 �C and 70 �C by steps of 5 �C. The intervals between the data logger port scanning were set at 2 min. Recording of the data was stopped when the inlet air temperature did not change during 15 min. An air tem- perature inside the laboratory was stabilized with an elec- tric convector heater with ±0.3 �C accuracy. 4. Results and discussion The experimental investigations included the following cases: Case-1 (TA,IN = 60 �C, VA = 20 m3/h), case-2 (TA,IN = 60 �C, VA = 30 m3/h), case-3 (TA,IN = 60 �C, VA = 40 m3/ h), case-4 (TA,IN = 65 �C, VA = 20 m3/h), case-5 (TA,IN = 65 �C, VA = 30 m3/h), case-6 (TA,IN = 65 �C, VA = 40 m3/ h), case-7 (TA,IN = 70 �C, VA = 20 m3/h), case-8 (TA,IN = 70 �C, VA = 30 m3/h) and case-9 (TA,IN = 70 �C, VA = 40 m3/h). After the charging process, the storage unit was cooled and the initial air temperature was stabilized at 21 �C for all the cases. Fig. 8 shows the exemplary transient temperature response for case-5. Analyzing the graphs in Fig. 8 (particularly the inte- rior temperatures of the PCM – TPCM,IN(MID) and TPCM,OUT(MID)), it can be concluded that the region of high TPCM,IN(SF) AI PCM TPCM,IN(MID) TA,IN Fig. 7. The locatio 170 M. Zukowski / Energy Conversion thermal capacity begins at approximately 49 �C and ends at 57 �C. It can also be noticed by observing the discharge curves that the outflow air temperature TA,OUT rapidly decreases from 65 �C to 45 �C during the first few minutes. Subsequently, the air temperature changes more slowly and approximately linearly within the next 1 h. The heat flux QPCM absorbed by the PCM during the charging period and emitted during the cooling process for case-4–case-6 is shown in Fig. 9. The value of QPCM was calculated from Eq. (1), in which the heat losses from the TES unit were taken into consider- 60 70 TPCM,OUT(SF) TPCM,OUT(MID) TA,OUT f thermocouples. Management 48 (2007) 166–173 QPCM ¼ V AcA qA;IN þ qA;OUT 2 TA;IN � TA;OUTj j � hLAW TA;IN þ TA;OUT 2 � TW;SF ���� ���� ð1Þ The dissimilarity of the TES unit thermal characteristics for the charge and discharge periods is illustrated in Fig. 10. The cooling process shows more intense heat transfer by forced convection at the PCM surface than that during the charging period. The heat flux for the heating process 0 20 40 60 80 100 120 0 50 100 150 200 250 300 0 20 40 60 80 100 120 140 160 180 200 Tin=65C, V=20m3/h Tin=65C, V=30m3/h Tin=65C, V=40m3/h TA,IN=65 C,VA =20m3/h TA,IN=65 C,VA =30m3/h TA,IN=65° ° ° C,VA=40m3/h t [m Q P CM [W ] he M. Zukowski / Energy Conversion and Management 48 (2007) 166–173 171 200 250 300 Tin=65C, V=30m3/h-ch dzenie Tin=65C, V=30m3/h - adowanie TA,IN=65 °C,VA=30m3/h (discharge) TA,IN=65 °C,VA=30m3/h (charge) [W ] Fig. 9. The heat flux absorbed (on the left) and emitted (on t rapidly decreases during the first 20 min of the experiment, and then, QPCM decreases more slowly. This effect influ- ences the final charging and discharging time as shown in Fig. 11. It can be concluded that the difference between the PCM temperatures at the start TPCM,0 and the end TPCM,1 of the experiments influences only the final charg- ing time. Sizing the heat storage system requires information about the enthalpy of the PCM contained in the TES unit. The change of the total PCM enthalpy as a function of 0 50 100 150 0 20 40 60 80 100 120 140 160 Q P CM t [min] Fig. 10. The comparison of heat transfer characteristics for charge and discharge periods in case-5. 0 50 100 150 200 250 20 25 30 35 4 DT=39 DT=44 DT=49ΔT=39 °C ΔT=44 °C ΔT=49 °C VA [m3/h t[ mi n ] Fig. 11. The final charging (on the left) and discharging (on the right) time v in] right) from PCM depends on the volumetric flow rate of air. 240 245 250 255 260 265 39 41 43 45 47 49 ΔTPCM [°C] H P CM [kJ /k g] Fig. 12. The change of the total PCM enthalpy versus PCM temperature PCM temperature difference between the TPCM,0 and TPCM,1 values during the charging process is shown in Fig. 12. This dependence, as a linear function of DTPCM, can be approximated by the following equation: DEPCM ¼ 2:17DT PCM þ 156 ð2Þ where DT PCM ¼ T PCM;0 � T PCM;1j j ð3Þ For visualization and a closer examination of the phase change process, the bags with the PCM were placed in the air free stream (TA = 70 �C for melting, TA = 21 �C for solidification). The characteristic stages of this physical effect were photographed and are presented in Fig. 13. The infrared thermometer was used to scan the top surface tem- perature field. From the photos, it can be observed that the 20 25 30 35 400 ] ersus air volume flow rate for three values of PCM temperature changes. change. Fig. 13. The particular stage es o 172 M. Zukowski / Energy Conversion and melting process starts in the corners of the capsule where the PCM layer is thick. Then, the liquid phase increases along the outer edges. At the end of the phase change pro- cess, the PCM melts in the centre of the capsule. As we can see in Fig. 14, the solidification process starts Fig. 14. The particular stag quickly on almost the whole surface (except the corners) of the capsule. Then, the solid phase grows larger slowly and uniformly in the capsule interior direction. The range of the experiment was included to determine the pressure drop characteristics of the storage unit. The static pressure difference between the inlet and outlet of the module as a function of the volume rate of air flow is presented in Fig. 15. The pressure losses are negligible because of the small length of the tested unit, but significant decreases of the dis- tance between capsules can lead to a great increase of the pressure drop. On the other hand, this change can result in an increase of the thermal energy storage per unit volume. 0 2 4 6 8 10 0 10 20 30 40 VA [m3/h] Δp [P a] Fig. 15. The pressure drop characteristic of TES module. 5. Concluding remarks The short term TES unit was tested experimentally, and the following conclusions can be drawn: s of the melting process. f the solidification process. Management 48 (2007) 166–173 • The total enthalpy storage in the module depends on the PCM temperature change and ranges from 240 to 262 kJ/kg. The value of the energy storage can be deter- mined from Eq. (2). • The region of high thermal capacity of the investigated paraffin wax RII-56 is comprised between 49 �C and 57 �C. • The charging time of the tested unit, which depends on air flow rate and the PCM temperature change is rela- tively long and ranges from 80 to 240 min. • The cooling process occurs faster and ranges from 50 to 90 min for the tested unit. The results of the investigations showed that paraffin wax RII-56 is a material that can be successfully used in TES applications for processes and buildings that require energy to be stored in the short term. Future work on the current project will include creation and validation of a numerical model and a computer program for simulation of the heat transfer performance of the storage unit based on enclosed PCM in polyethylene film bags. Acknowledgements This investigation was conducted under grants of the Pol- ish StateCommittee for ScientificResearchNo. 4T10B01324 and Bialystok Technical University W/IIS/36/03. References [1] Zukowski M. Heat transfer and pressure drop characteristics of the underfloor air distribution system. Energ Buildings 2005;37(8):890–6. [2] Zukowski M. A method of designing thermal comfort conditions in the room with UFAD system. In: Proceedings of the 9th international conference on air distribution in rooms – ROOMVENT 2004, Coimbra; 2004. [3] Zukowski M. A numerical analysis of heat and mass transfer in a room with the air-underfloor heating. 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[19] http:// [accessed 14.10.05]. M. Zukowski / Energy Conversion and Management 48 (2007) 166–173 173 Experimental study of short term thermal energy storage unit based on enclosed phase change material in polyethylene film bag Introduction Description of the tested heat storage unit Experimental arrangement Experimental apparatus Experimental methods Results and discussion Concluding remarks Acknowledgements References