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Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs
Accepted Manuscript Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs Xiaoqin Sun, Youhong Chu, Mario A. Medina, Yajing Mo, Siyuan Fan, Shuguang Liao PII: DOI: Reference:
S1359-4311(18)36212-4 https://doi.org/10.1016/j.applthermaleng.2018.12.032 ATE 13039
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 October 2018 3 December 2018 5 December 2018
Please cite this article as: X. Sun, Y. Chu, M.A. Medina, Y. Mo, S. Fan, S. Liao, Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.032
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Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs Xiaoqin Sun1,2,*, Youhong Chu1, Mario A. Medina2,†, Yajing Mo1, Siyuan Fan1, Shuguang Liao3 1
School of Energy and Power Engineering, Changsha University of Science & Technology, Changsha,
Hunan 410114, PR China 2
Civil, Environmental & Architectural Engineering Department, The University of Kansas, Lawrence,
KS 66045, USA 3
Changsha Maxxom High-tech Co., Ltd., Changsha 410015, PR China
Abstract Experiments were conducted using a small wind tunnel to identify the thermal behavior during energy charging processes of phase change material (PCM) in rectangular slabs. A commercially available paraffin-based PCM with a nominal melting temperature range of 26-28 °C was encapsulated in high-density polyethylene (HDPE) slabs for thermal storage applications in buildings. The influence of air temperature and velocity as well as the slab inclination angle was evaluated using melting time and energy charging speed. The melting time was 18.8%-50.8% shorter at top zone than it at bottom zone of the slabs because of the natural convection. The average energy charging speed increased from 36.3 W to 109.4 W by 201.7% when the inlet air temperature increased from 35 °C to 55 °C. However, it was slightly enhanced by 8.7% when the air velocity increased from 4 m /s to 5 m/s. The aspect ratio should be considered when studying the impact of inclination angle on energy charging process. The maximum energy storage capacity was 97.2 W·hr with a fraction of 20.2% sensible heat with an energy charging speed of 109.4 W when the inlet air temperature was 55 °C, the air velocity was 3 m/s, and the inclination angle was 90°.
Keywords: phase change material (PCM); latent energy storage; energy storage capacity; energy charging speed; ventilated slabs
*
Corresponding authors. Address: School of Energy and Power Engineering, Changsha University of Science & Technology, Changsha, Hunan410004, PR China E-mail address: [email protected] (Xiaoqin Sun) † Civil, Environmental & Architectural Engineering Department, The University of Kansas, Lawrence, KS 66045, USA E-mail address:[email protected] (Mario A. Medina) 1
1 Introduction According to the most recent data from Energy Information Administration’s (EIA’s) Residential Energy Consumption Survey (RECS), home electricity consumption peaks in July and August when temperatures and cooling demand are the highest. It is estimated that 18% of annual household electricity use is for air conditioning in US. Therefore, energy efficient solutions for buildings, especially for space cooling are in urgent demand. The combination of energy efficient building design and renewable energies is attractive for reducing cooling energy consumption in buildings. For example, night ventilation is a well-known technology to reduce or eliminate the required active cooling during day time. The night coolness is stored when the outdoor air temperature is low. The stored coolness is utilized during the following day, so the cooling energy consumption is reduced. However, the night coolness is intermittent, thermal energy storage systems are required to store the coolness as much as possible [1, 2]. Among the thermal energy storage methods (i.e., sensible heat, latent heat, and thermos-chemical heat storage), storage through latent heat offered by phase change material (PCM) during their phase transitions, which far exceeds those associated with sensible heat of traditional materials is considered to be a very promising option [3]. However, the latent heat storage systems suffer from a low heat storage and release rates. The reason for this is that most PCM have a low thermal conductivity (k≤0.2 W/(m·°C)), which in many cases leads to incomplete melting or solidification processes. Also, a significant temperature difference within the PCM can lead to material failure and system overheating [4]. It is crucial to achieve efficient heat exchange between the heat transfer fluid and the PCM, which is strongly affected by the thermal boundary conditions and specific geometric configurations. PCMs are contained in various shapes for actual applications, including rectangular [5-7], spherical [8], and cylindrical or annular containers [9-11]. Melting of PCM in rectangular enclosures has received great attention due to its wide-ranging engineering applications in such fields as casting, metallurgy and thermal energy storage, especially the PCM to air heat exchangers for building applications [1, 12]. To study the thermal performance of a PCM-air rectangular heat exchanger under different thermal boundary conditions, Dolado et al. [13] carried out a real-scale test with different inlet air temperatures and air flow rates. The PCM used was paraffin based organic material (RT27 from Rubitherm). An average thermal power up to 4.5 kW for one hour was obtained for melting. An empirical relationship between melting time and temperature difference between inlet air and the PCM 2
was developed. Ji et al. [14] studied the transient energy storage process within an aluminum rectangular heat exchanger using a three-dimensional model for waste heat recovery from a low temperature gas flow (<230 °C) under different air flow rates and heater power inputs. The heat transfer mechanism of melting processes of PCM was described using 3D temperature contours in the presence of natural convection. The PCM at top side melted faster than that in the bottom side due to the natural convection. As the air flow rate increasing, the PCM temperature rose more slowly, resulting in a longer melting time. However, increasing heater power input fastened the melting process. Arzamendia Lopez et al. [5] developed a two-dimensional model to predict the thermal performance of a PCM-air heat exchanger for building application. The heat transfer process was supposed to be conduction only in an equivalent homogeneous material. The model was validated using a 3-cm PCM slab in thickness from ENERGAIN boards. Natural convection within liquid PCM was observed during melting, which should be considered when optimizing the geometric configurations [15, 16]. The energy charging and discharging rates of PCM could be enhanced readily through natural convection by optimizing the slab arrangements. Zhao et al. [17] experimentally analyzed the influence of the inclination angle on the natural convection during the melting process using image digitalization. The variation of melting time depended on the aspect ratio. When the aspect ratio was bigger than 1, with the melting process going on, the natural convection heat transfer was intensified; otherwise, it weakened. It was shown that with an inclination angle of 60° the melting process was achieved in the shortest time. Kamkari et al. [18, 19] observed the natural convection flow structures of a lauric acid within an enclosure that was heated isothermally from one side. The results showed that the convection in the enclosure increased and a disorganized flow structures appeared as the inclination angle of the enclosure was decreased from 90° to 0°. That is, the melting time was the shortest when the rectangular heat exchanged was placed horizontally. Prieto and González [20] compared the flow behavior using velocity plots and volumetric liquid fraction contours within PCM slabs arranged vertically and horizontally during melting and solidification. Vertical arrangement produced higher flow intensity than the horizontal arrangement for intermediate values of the PCM liquid fraction. However, the mean heat fluxes increased if the slabs were horizontally positioned. In summary, the energy charging processes of PCM-air heat exchangers are significantly influenced by the boundary conditions and configuration arrangements. However, there is no 3
“one-size-fits-all” version of a PCM-air heat exchanger. Each study has its own conclusions that make it more or less appropriate for a specific application. A challenge is in identifying these factors and subsequently matching the beneficial storage systems. This paper proposed an experimental study of melting PCM in rectangular slabs under different boundary conditions and slab inclinations. A commercially available paraffin-based PCM with a nominal melting temperature range of 26-28 °C was used. Thermal behavior was evaluated using energy charging speed and melting time. The impact on these metrics of the air temperature and velocity as well as the slab inclination angle was analyzed. The best thermal performance using the largest energy charging speed and the least melting time will be obtained.
2 Experimental setup 2.1 Experimental apparatus The schematic drawing of the experimental apparatus is shown in Figure 1. It consists of a climate chamber, a wind tunnel with a cross area of 330 mm × 330 mm, four PCM slabs, monitoring devices, and a data acquisition system. To bring the air to the designed temperatures in the climate chamber, two chillers (cooling capacities: 7.5 kW and 12.0 kW) and a heater (heating capacity: 18 kW) were used to bring air temperatures ranging from -10 °C to 60 °C with an accuracy of ± 0.5 °C. The chillers and heater adjusted their power automatically according to the set temperature. Air from the climate chamber was supplied to the wind tunnel. A variable speed fan was installed in the wind tunnel to control the air velocity. One flow straightener was set upstream of the slabs to stabilize the air flow.
Figure 1. Schematic drawing of the experimental apparatus
Type T thermocouples (Omega TT-T-30-SLE) were used to measure the air temperatures before and after passing through the PCM slabs and PCM temperature variation during melting. Air velocity was measured using a hot wire anemometer (KIMO-VT110). All temperatures were collected and transferred to a computer via a data acquisition system at the rate of one reading per second. The accuracy of sensors is shown in Table 1. Table 1 Sensors and their accuracy
4
2.2 Phase change materials Solgi et al. [21] recommended the PCM with a melting temperature of 27 °C for night ventilation to improve indoor thermal comfort. Therefore, the PCM used in this study was a paraffin-based material, which was sold under the name of ZDJN-28 with a melting temperature of 28 °C. Figure 2 shows the DSC analysis of this PCM under a heating rate of 5 °C/min. Two peaks were observed in the heat flux curve, which represented a solid-solid phase transition (crystal packing changes without entering an isotropic liquid phase) and a solid-liquid phase transition, respectively. The solid-solid phase transition occurred at 17.5 °C and the corresponding enthalpy was 53.2kJ/kg. The melting process started at a temperature of 22.1oC. The enthalpy at this point was 81.4kJ/kg. The second heat flux peak occurred at 26.4 °C. The melting process finished at a temperature of 32.5 °C and the corresponding enthalpy of 312.6 kJ/kg. The latent heat of fusion caused by the solid-liquid phase transition was 231.2kJ/kg [(312.6-81.4) kJ/kg]. The thermal parameters of the PCM are shown in Table 2.
Figure 2. DSC analysis for the paraffin-based PCM
Table 2 Thermal parameters of the PCM
PCM was encapsulated in high-density polyethylene (HDPE) slabs with a thermal conductivity of 0.49 W/(m°C). The thickness of the slab wall was 1 mm. The conductive heat transfer coefficient of the slab wall was 490 W/(m2°C), much higher than the conductive heat transfer of PCM. Therefore, the thermal resistance of the slabs was ignored during the analysis. Each slab was a rectangular cavity with outside dimension of 28 mm in width, 315 mm in length and 200 mm in height. The quantity of PCM in each slab is shown in Table 3. The average energy storage capacity of one slab was 72.73
Whr. A void space of 8.3% was left for PCM expansion.
Table 3 PCM mass and energy storage capacity within each slab
Figure 3 illustrates the arrangement of PCM slabs in wind tunnel and the measuring points. The 5
PCM slabs were placed at the bottom of the wind tunnel. Therefore, all surfaces except for the slab bottom were exposed to the air flowing through the wind tunnel. The spatial distance between two neighboring slabs was 44 mm. Four zones were selected within the slabs to study the temperature response of the PCM. These are labeled from A through H. The selected zones were windward-top (locations A and E), windward-bottom (locations B and F), leeward-top (locations C and G) and leeward-bottom (locations D and H). Four thermocouples were installed inside the slabs (locations A through D) using two wood stands and the others (locations E through H) were on the surface of the slabs. The coordinates of each location are in Figure 3a with an origin at the center of the slab.
Figure 3. PCM slab arrangement and measuring points
2.3 Experimental conditions Before the tests, a relationship between the motor frequency of the variable speed fan and the air velocity within the wind tunnel without slabs was figured out. It was assumed the air passed through the air channels with same velocity. The air velocity passing through the PCM slabs was calculated using the total air channel cross area based on the readings of a hot wire anemometer. Five different air temperatures were tested, which increased from 35 to 55 °C by 5 °C. Air velocity between the slabs increased from 1 m/s to 5 m/s. The inclination angle () of the slabs varied between 30° to 90°, as illustrated in Figure 4. Two slabs were used for the inclination angle test. Therefore, the motor frequency was different with the value in Cases 1-5. Before the start of melting process, PCM was cooled to solid state. The cooling process was stopped when the PCM temperature was lower than its melting temperature. The series of tests are summarized in Table 4.
Table 4 Experimental conditions
Figure 4. PCM slab inclination angle
3 Results and analysis PCM absorbed energy in the form of latent as well as sensible heat during the melting process. As hot air passed through the slabs, heat was transferred via convection from the air, conduction through 6
the slab wall, and potentially through both conduction and convection within the PCM. During melting, liquid PCM moved upward driven by the density differences. Moreover, the air passing the top surfaces of the slabs accelerated the melting at top zone, which in turn contributed to natural convection in the melted PCM inside the slab. Therefore, the melting time at top zone of the slab was shorter than it at bottom zone. The melting time difference between the top and bottom zones was used to evaluate the heat transfer enhancement caused by natural convection, as shown in Equation 1.
f where
BD- AC 100% BD and
(1)
were the PCM melting time at the top (locations A and C) and bottom (locations B
and D) zones, min, respectively. The melting time was defined as the time duration when the PCM temperature was between 22.1 °C and 32.5 °C, as shown in Equations 2 and 3.
(2)
(3)
AC
1 4 32.5 22.1 AC ,n AC ,n 4 n 1
BD
1 4 32.5 22.1 BD ,n BD ,n 4 n 1
where the subscript n was the slab number; AC meant the locations A and C; and superscripts 22.1 and 32.5 were the PCM melting temperature range. For example,
32.5 AC , n was
the time when the mean
temperature at locations A and C was 32.5 °C for slab #n. The melting time reduction thus obtained was used to evaluate the heat transfer enhancement caused by natural convection. 3.1 Reliability and repeatability Repeatability study was conducted to examine the consistency of experimental results. Three energy charging experiments were performed for Case 8 and 10, respectively. Transient temperature profiles were plotted for thermocouples installed at location C for Case 8 and location D for Case 10, as shown in Figure 5(a) and 5(b), respectively. It was observed that phase transition time for all experiments were clearly identical. The statistical standard deviations for phase transition time were calculated to be 0.902 and 1.036, respectively. Thus, it assured the reliability the repeatability of experimental results showing thermal performance.
(a) Case 8: Air temperature: 50 °C; Air velocity: 3 m/s; Inclination angle: 90° 7
(b) Case 10: Air temperature: 50 °C; Air velocity: 5 m/s; Inclination angle: 90° Figure 5. Illustration of repeatability of transient temperature profiles 3.2 Temperature and the amount of stored heat 3.2.1 Temperature variation Taking Case 5 as example, the temperature variation is illustrated in Figure 6. PCM temperature increased from 15 °C to around 55 °C. Three regions were separated by two ends of the temperature plateau. The first region was where the PCM temperature increased almost linearly from 15 °C until it reached to 22.1 °C. Sensible heat was stored in the solid PCM at this region. The second region was where the PCM started to melt and to store latent heat. The PCM temperature maintained almost constant for all zones at this stage. The last region was where the PCM temperature increased again when the PCM was completely melted. During the melting process, heat transfer occurred between the hot wall of the slab and the solid PCM via conduction, which dominated the melting process at the early stage and caused a very thin layer of liquid to surround the wall of the slab; while the rest of the PCM remained solid without any phase change. This was the close-contact melting process [22]. As the melting layer grew, convection heat transfer gradually prevailed until it was the only heat transfer mode. The melted PCM gained heat from the slab surface, and it moved upward because of the density differences until it reached near the top of the slab. The liquid PCM at top zone was thus at a higher temperature. This caused more solid to melt at this location. The energy charging speed was higher in top zone of the slabs because of the upward motion of the melted PCM. Therefore, the melting process finished earlier in locations A and C. Similarly, the slab surface temperature was higher at the top zone because of the convection of liquid PCM, as shown in Figure 6(b). The temperature at location G decreased at 16 minutes. The temperature at this location increased first when the PCM absorbed sensible heat. When the PCM was melting, natural convection occurred. The circulation of liquid PCM affected the temperature distribution by mixing the hot liquid with warm liquid, preventing the slab from overheating. As a result, the slab surface temperature reduced. Moreover, the temperature at location G was higher than it at location E during the first and second regions. When the hot air passed location E, the PCM at this point started to melt and natural convection formed around this location. More liquid PCM with lower 8
temperature was induced to this zone and formed a large circulation cell. Natural convection at this location was stronger with more liquid PCM. Therefore, the slab surface temperature was lower. This phenomenon was evidenced by the temperature distribution in Figure 7. When the PCM completed its melting at the third stage, temperature at location E was higher than in at location G.
(a) PCM temperature profiles during the melting process
(b) PCM slab surface temperature profiles during the melting process Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55oC; Air velocity: 3m/s; Inclination angle: 90o)
Figure 7 shows the sequential infrared photos of the temperature distribution. The temperatures below each photo were for the windward-bottom zone (location B). When the PCM temperature at location B was 20 °C (Fig. 7a), all the PCM within the slab was solid. The heat transfer was controlled by thermal conduction. When the PCM temperature at location B was 25 °C (Fig. 7b), the solid PCM at windward-top (location A) started melting and the PCM temperature at this location was the highest. Natural convection was found around this location. When the PCM temperature at location B was 30 °C and 35 °C (Figs. 7c and 7d), most of the PCM was melting. Because of the natural convection more melting occurred in top zone of the slabs whereas in bottom zone the melting proceeded at an insignificant rate. The PCM temperature at bottom zone was much lower than it at top zone. When the PCM temperature at location B was 40 °C (Fig. 7e), most of the PCM had finished the melting process and the natural convection was reduced. It was interesting to note that in photos (c)-(e) the temperature seemed higher than 31 °C. That is, all PCM was in liquid state. However, PCM with temperature lower than 28 °C was observed in the following photos. For example, when the PCM temperature at location B was 45 °C (Fig. 7f), PCM with dark purple color was observed at the bottom of the slab. This was tricky by the natural convection when it was strong in photos (c)-(e), which disturbed the temperature distribution. It cannot be concluded that the melting was finished under these conditions. Slight convection was observed at the windward-bottom (location B) in photo (f). When the PCM temperature at location B was 50 °C, the melting process was finished except for bottom zone of the slabs. There was no convection according to the thermovision picture. The melt progresses reached towards the 9
thermally stratified condition [23]. PCM temperature increased layer by layer from bottom to top zones. It was interesting to note that even for this high bulk temperature, the PCM in bottom zone of the slab was still in a mushy state.
Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°)
3.2.2 Energy charging speed and amount of stored heat The variation of energy storage for Case 5 is shown in Figure 8. Two inflexion points separated the sensible and latent heat storage at each measuring point, as points a and b on the curve for location A. The slope of the accumulated energy storage illustrated the energy charging speed, which was calculated using Equation 4 for latent energy charging speed and Equation 5 for sensible energy charging speed. The energy charging speed was the average value of four slabs at each location. At the first region where energy was stored as sensible heat, the energy charging speed was 40.7 W, which was same for various zones. During the latent heat energy storage region, the energy charging speed was 176.0 W at location C, 170.0 W at location A, 89.6 W at location B and 86.5 W at location D, respectively. The energy charging speed increased during the latent heat energy storage region because of the large latent heat of fusion when PCM melted. The 1 kg PCM was able to absorb 231.2 kJ thermal energy when it melted. To absorb the same amount of energy, its temperature had to increase by 100.5 °C for liquid PCM and 107.5 °C for solid PCM, respectively. Moreover, the energy charging speed was higher for top zone because of natural convection. However, it was lower for top zone at the last sensible heat storage region, since the temperature difference between the PCM and hot air was smaller.
qil
1 4 mi ,n h 1000 22.1 60 4 n1 i32.5 ,n i ,n
qis, j where
i A, B, C , D
1 4 mi ,n c p Ti , j ,n Ti , j 1,n 1000 4 n 1 60
(4)
i A, B, C , D
(5)
was the latent energy charging speed at location i, W; mi,n was the PCM mass at location i
within slab #n, kg; h was the latent heat of fusion, kJ/kg; i , n and 32.5
10
i22.1 ,n
were the time when the
PCM temperature at location i within slab #n reached to 32.5oC and 22.1oC, min, respectively; was the sensible energy charging speed at location i and time j, W; Ti,j,n and Ti,j-1,nwere the PCM temperature at location i and time j and j-1 within slab #n, °C; was the time step, 1min; and cp was the thermal capacity of pure liquid or solid PCM, kJ/(kg·°C). Each slab was divided into four equal volumes along the y and z axis. The quantity of PCM at bottom zone was more than it at top zone because of the void space. To be specific, the average PCM mass was 0.308 kg for zones B and D and 0.257 kg for zones A and C, respectively. It should be noted that the PCM mass was calculated based on its liquid density. When the slab changed with different inclinations, the PCM mass at each zone did not change because the void space never merged into the bottom zone. The average amount of stored heat in Whr including both latent and sensible heat of each slab was calculated using Equation 6. D 1 1 4 Qtotal mi ,n h qis, j d i A 60 j 0 4 n 1
i A, B, C , D
(6)
The relative uncertainty of the results caused by indirect measurements was obtained using Equation 7. The error involved in the measured and derived parameters were calculated based on the accuracy of the measuring instruments.
y
1 n y xi y i 1 xi
(7)
The amount of stored heat in Whr as a function of time is shown in the right Y-axis in Figure 8. It increased with time to 97.1 Whr with a relative error of ±2.56%. It was higher than the values in Table 2 because of the sensible heat. That is, with an average latent energy charging speed of 130.5 W, this slab would recover thermal energy for 45 minutes. Sensible heat made up 20–28% of the total thermal energy storage. The highest ratio was 27.6% for location A, followed by 27.5% for location C, 22.5% for location B, and 19.6% for location D.
Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 °C; Air velocity: 3 m/s; Inclination angle: 90o)
3.3 Effects of inlet air temperature (Cases 1-5) 11
3.3.1 Temperature variation as a function of inlet air temperature Figure 9 shows the PCM temperature variation at windward-top zone (location A) as a function of inlet air temperature. PCM started to melt when its temperature was around 23.5 °C for various inlet air temperatures; though its initial temperature was different. That is, the temperature when PCM started to melt was not affected by inlet air temperatures or PCM initial temperatures. However, the temperature when the melting process completed increased with increasing inlet air temperature. Under the testing conditions, the PCM melting temperature range was 23.5 °C - 28.5 °C at location A. It was noteworthy that the melting temperature of bulk PCM was different with the DSC test results.
Figure 9. Temperature variation at windward-top zone (location A) during melting
3.3.2 Melting time as a function of inlet air temperature Figure 10 shows the PCM melting time as a function of inlet air temperature. It decreased with increasing inlet air temperature. The rise of the air inlet temperature increased the temperature difference between the air and PCM, Ta−TPCM, and therefore enhanced the heat exchange between PCM and hot air. Consequently, the time required to complete the melting process was reduced. It was observed that the melting time was shorter at top zone than it at bottom zone for all inlet air temperatures. Table 5 shows the melting time reduction, calculated using Equation 1. The melting time reduction was maintained between 43%-47% for various inlet air temperatures.
Figure 10. Melting time at different locations with inlet air temperature
Table 5 Melting time reduction (f) as a function of air temperature 3.3.3 Latent energy charging speed as a function of inlet air temperature Figure 11 shows the latent energy charging speed as a function of inlet air temperature, which was calculated using Equation 4. It was 71.7% higher for top zone than it for bottom zone, indicating that natural convection improved the heat transfer. The average latent energy charging speed for four zones increased from 36.3 W to 109.4 W by 201.7% when the inlet air temperature increased from 35 °C to 55 °C.
12
Figure 11. Energy charging speed at different locations with inlet air temperature
The average amount of stored heat is shown in Table 6, which increased with increasing inlet air temperature. The variation of the stored heat was caused by the stored sensible heat. More sensible heat was absorbed and stored when the inlet air temperature was higher.
Table 6 Amount of stored heat as a function of air temperature
3.4 Effect of air velocity (Cases 6-10) 3.4.1 Melting time as a function of air velocity Figure 12 shows the melting time for various air velocities and Table 7 presents the corresponding melting time reduction. As the air velocity increased, the melting time reduced. However, this was not true when the air velocity increased from 4 m/s to 5 m/s at location A. When the air velocity increased from 1 m/s to 2 m/s, the melting time at each location was reduced by average 6.3%. That is, the increase in air velocity was not enough to accelerate the melting process in a significant manner. The reason is that the thermal resistance of PCM is the main thermal resistance under these conditions. The natural convection within liquid PCM was not strong, which was evidenced by the melting time reduction in Table 7. Therefore, increasing air velocity did not enhance the melting process obviously. When the air velocity increased from 2 m/s to 3 m/s, the corresponding melting time was reduced by 15.6%. When the air velocity increased from 3 m/s to 4 m/s, the melting time was reduced by 17.1%. Under these two conditions (3 m/s and 4 m/s), the natural convection was strong with an average melting time reduction of 48.5%. The decrease in melting time was the result of stronger convection effect in both air and PCM. When the air velocity increased from 4 m/s to 5 m/s, the melting time decreased by 0.6%. Under this condition, increasing air velocity did not enhance the melting process effectively. The melting time was even longer at location A when the air velocity was 5 m/s than it when the air velocity was 4 m/s. This result coincided with the conclusion in Ref. [14]. Under the studied conditions, it was effective to improve the melting by increasing air velocity when the air velocity was lower than 4 m/s. The reason behind this phenomenon was the bottleneck in the convective heat transfer process on the air side [13]. That is, increasing air velocity will result in a reduction in temperature rise inside PCM [14]. 13
From Table 7 it was observed that the melting time was much shorter at top zone when the air velocity was 3 m/s and 4 m/s. That is, natural convection brought high temperature melted PCM upward to top zone, and lead to stronger heat transfer. The larger melting time reduction indicated a stronger convection.
Figure 12. Melting time at different locations with air velocity
Table 7 The melting time reduction (f) as a function of air velocity
3.4.2 Latent energy charging speed as a function of air velocity Figure 13 shows the latent energy charging speed as a function of air velocity. The average latent energy charging speed increased from 66.3 W to 108.7 W by 63.9% with increasing air velocity. It was not obvious when the air velocity increased from 1 m/s to 2 m/s and from 4 m/s to 5 m/s. As explained in Section 3.3.1, the energy charging process under these conditions was impacted by the PCM thermal resistance and the bottleneck in the convective heat transfer process on the air side. The energy charging speed at top zone was 106.7% higher than it at bottom zone because of the natural convection.
Figure 13. Latent energy charging speed at different locations with air velocity
Table 8 shows the average amount of stored heat as a function of air velocity. The inlet air temperature was maintained at 50 °C when the air velocity increased. The PCM mass was same for various air velocities. Therefore, the amount of stored heat did not change significantly for different cases. The variation of the stored heat was caused by the varied stored sensible heat because of different PCM initial and final temperatures.
Table 8 Amount of stored heat as a function of air velocity
3.5 Effect of slab inclination angle (Cases 11-15) It is evident that the natural convection affected the melting process significantly, which depends 14
on the spatial extent of the gravitational field and the established temperature difference. Grashof number, the ratio of the buoyancy to viscous force acting on a fluid, was used to evaluate the effects of natural convection within the liquid PCM. The mathematical definition is shown in Equation 8. Gr
g vl 3 TAC TBD
(8)
2
where g was the gravity constant, m/s2; v was the PCM volume expansion coefficient, 0.0008K-1; l was the characteristic length, 0.1m; v was the PCM kinematic viscosity at characteristic (melting) temperature [24], m2/s; and
and
were the PCM temperatures at top and bottom zones, °C.
Based on the temperature at the windward-top zone (location A), Grashof number within the slabs with various inclination angles is shown in Figure 14. Grashof number increased with the rise of inclination angle. That is, natural convection increased. This result was because of the bigger vertical gap between the top and bottom zones when the inclination angle increased. The upward flow of the melted PCM was suppressed by the smaller vertical gap between the top and bottom zones with lower inclination angles. When the temperature at location A was 45 °C and 50 °C, Grashof number increased significantly when the inclination angle increased from 30o to 45o. For other conditions, Grashof number increased slightly. When the PCM temperature at location A was lower than 45oC, it was the beginning of the melting process and the temperature of the PCM domain was maintained at the melting temperature range. The temperature difference of PCM at the top and bottom zones was small; therefore, the variation of Grashof number was moderate. When the temperature at location A was 45 °C and 50 °C, PCM at top zone had completed melting. Liquid PCM accumulated at the top zone; while the solid PCM gathered at the bottom zone. The temperature difference between the top and bottom zones increased, so did the Grashof number.
Figure 14. Grashof number as a function of slab inclination angle
3.5.1 Melting time as a function of slab inclination angle Figure 15 shows the melting time as a function of inclination angle and Table 9 presents the corresponding melting time reduction. It was observed that with increasing inclination angle the melting time decreased. When the inclination angle was 30°, Grashof number was the lowest. That is, the natural convection was the weakest. The PCM melting time at top zone was reduced by 18.8% 15
compared to it at bottom zone. When the inclination angle increased from 30° to 45°, the melting time at top zone was reduced significantly. Under this condition, the effective aspect ratio increased, so did the natural convection. It was proved by the melting time reduction, which increased from 18.8% to 36.2%. When the inclination increased from 45° to 75°, the melting time was reduced in a linear pattern. The melting time reduction was between 36.2% and 41%. When the inclination increased from 75° to 90°, the melting time reduction was 50.8% and the natural convection was the strongest. Based on the melting time variation, it was recommended to install the PCM slabs vertically, which was different with the results in Ref [18-20]. The reasons for this phenomenon were: 1) the aspect ratio was smaller than 1 when the slabs were arranged horizontally, so the natural convection was weak; 2) The void space at the top of the slabs affected the heat transfer.
Figure 15. Melting time as a function of slab inclination angle
Table 9 The melting time reduction (f) as a function of slab inclination angle
3.5.2 Latent energy charging speed as a function of slab inclination angle The latent energy charging speed as a function of slab inclination angle is shown in Figure 16. The average latent energy charging speed increased from 46.1 W to 77.3 W by 67.8% when the slab inclination angle increased from 30° to 90°. The energy charging speed at top zone was 43.9% higher than it at bottom zone. Table 10 shows the amount of stored heat with various inclination angle. The different stored heat was the result of different sensible heat.
Figure 16. Latent energy charging speed as a function of slab inclination angle
Table 10 Amount of stored heat as a function of slab inclination angle
4 Conclusions The influence of air temperature and velocity as well as slab inclination angle on the melting processes of PCM was investigated. The energy charging speed as well as the melting time were calculated to evaluate the melting process. Based on experimental results, the maximum storage 16
capacity was 97.2 Whr with a fraction of 20.2% sensible heat. The highest energy charging speed was 109.4 W when the inlet air temperature was 55 oC, the air velocity was 3 m/s, and the inclination angle was 90°. 1. PCM at top zone of the slabs melted faster than it at bottom zone of the slabs because of the natural convection in liquid PCM. The increase in air inlet temperature enhanced the energy charging speed and improved the amount of stored heat. PCM initial temperature and inlet air temperature had no impact on the temperature when the melting started. However, the temperature when the melting finished increased with the increase of inlet air temperature. 2. Increasing air velocity enhanced the energy charging speed and reduced the melting time. However, it was effective when the air velocity was lower than 4 m/s, which depended on the PCM thermal resistance. For example, when the air velocity increased from 4 m/s to 5 m/s, the melting time was only reduced by 0.6%. 3. Increasing inclination angle of slab enhanced the natural convection and thus the energy charging speed was higher. The proposed slabs were recommended to install vertically. Acknowledgments This work was supported by the National Natural and Science Funding (51308051), the Science and Technology Department of Hunan (2017RS3036), Hunan Association for Science and Technology (2017TJ-Q05), Changsha City Fund for Distinguished and Innovative Young Scholars (kq1707013) and Hunan Province 2011 Collaborative Innovation Center of Clean Energy and Smart Grid. References [1] N.S. Dhaidan, J.M. Khodadadi, Melting and convection of phase change materials in different shape containers: A review, Renewable and Sustainable Energy Reviews, 43 (2015) 449-477. [2] E. Solgia, Z. Hamedani, R. Fernando, B.M. Kari, H. Skates, A parametric study of phase change material behaviour when used with night ventilation in different climatic zones, Building and Environment, 147 (2019) 327-336. [3] A.F. Regin, S.C. Solanki, J.S. Saini, Heat transfer characteristics of thermal energy storage system using PCM capsules: A review, Renewable and Sustainable Energy Reviews, 12 (2008) 2438-2458. [4] A.M. Abdulateef, J. Abdulateef, S. Mat, K. Sopian, B. Elhub, M.A. Mussa, Experimental and numerical study of solidifying phase-change material in a triplex-tube heat exchanger with longitudinal/triangular fins, International Communications in Heat and Mass Transfer, 90 (2018) 73-84. [5] J.P. Arzamendia Lopez, F. Kuznik, D. Baillis, J. Virgone, Numerical modeling and experimental validation of a PCM to air heat exchanger, Energy and Buildings, 64 (2013) 415-422. [6] N. Stathopoulos, M. El Mankibi, R. Issoglio, P. Michel, F. Haghighat, Air–PCM heat exchanger for peak load management: Experimental and simulation, Solar Energy, 132 (2016) 453-466. 17
[7] U. Stritih, An experimental study of enhanced heat transfer in rectangular PCM thermal storage, International Journal of Heat and Mass Transfer, 47 (2004) 2841-2847. [8] H. Cui, W. Tang, Q. Qin, F. Xing, W. Liao, H. Wen, Development of structural-functional integrated energy storage concrete with innovative macro-encapsulated PCM by hollow steel ball, Applied Energy, 185 (2017) 107-118. [9] S. Seddegh, X. Wang, A.D. Henderson, Numerical investigation of heat transfer mechanism in a vertical shell and tube latent heat energy storage system, Applied Thermal Engineering, 87 (2015) 698-706. [10] S. Seddegh, X. Wang, A.D. Henderson, A comparative study of thermal behaviour of a horizontal and vertical shell-and-tube energy storage using phase change materials, Applied Thermal Engineering, 93 (2016) 348-358. [11] S. Seddegh, X. Wang, M.M. Joybari, F. Haghighat, Investigation of the effect of geometric and operating parameters on thermal behavior of vertical shell-and-tube latent heat energy storage systems, Energy, 137 (2017) 69-82. [12] V.V. Tyagi, D. Buddhi, PCM thermal storage in buildings: A state of art, Renewable and Sustainable Energy Reviews, 11 (2007) 1146-1166. [13] P. Dolado, A. Lazaro, J.M. Marin, B. Zalba, Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model, Renewable Energy, 36 (2011) 2906-2917. [14] C. Ji, Z. Qin, S. Dubey, F.H. Choo, F. Duan, Three-dimensional transient numerical study on latent heat thermal storage for waste heat recovery from a low temperature gas flow, Applied Energy, 205 (2017) 1-12. [15] R.E. Murray, D. Groulx, Experimental study of the phase change and energy characteristics inside a cylindrical latent heat energy storage system: Part 1 consecutive charging and discharging, Renewable Energy, 62 (2014) 571-581. [16] A. Pizzolato, A. Sharma, K. Maute, A. Sciacovelli, V. Verda, Design of effective fins for fast PCM melting and solidification in shell-and-tube latent heat thermal energy storage through topology optimization, Applied Energy, 208 (2017) 210-227. [17] J. Zhao, J. Zhai, Y. Lu, N. Liu, Theory and experiment of contact melting of phase change materials in a rectangular cavity at different tilt angles, International Journal of Heat and Mass Transfer, 120 (2018) 241-249. [18] B. Kamkari, H. Shokouhmand, F. Bruno, Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure, International Journal of Heat and Mass Transfer, 72 (2014) 186-200. [19] B. Kamkari, H.J. Amlashi, Numerical simulation and experimental verification of constrained melting of phase change material in inclined rectangular enclosures, International Communications in Heat and Mass Transfer, 88 (2017) 211-219. [20] M.M. Prieto, B. González, Fluid flow and heat transfer in PCM panels arranged vertically and horizontally for application in heating systems, Renewable Energy, 97 (2016) 331-343. [21] E. Solgi, B.M. Kari, R. Fayaz, H. Taheri, The impact of phase change materials assisted night purge ventilation on the indoor thermal conditions of office buildings in hot-arid climates, Energy and Buildings, 150 (2017) 488-497. [22] M.K. Moallemi, Analysis of close-contact melting heat transfer, International Journal of Heat and Mass Transfer, 29 (1986) 855-867. 18
[23] V. Soni, A. Kumar, V.K. Jain, Modeling of PCM melting: Analysis of discrepancy between numerical and experimental results and energy storage performance, Energy, 150 (2018) 190-204. [24] C.J. Ho, Solid-liquid phase change heat transfer in enclosures, Purdue University, (1982).
19
Figure 1. Schematic drawing of the experimental apparatus Figure 2. DSC analysis for the paraffin-based PCM Figure 3. PCM slab arrangement and measuring points Figure 4. PCM slab inclination angle Figure 5. Illustration of repeatability of transient temperature profiles Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o) Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°) Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o) Figure 9. Temperature variation at windward-top zone (location A) during melting Figure 10. Melting time at different locations with inlet air temperature Figure 11. Energy charging speed at different locations with inlet air temperature Figure 12. Melting time at different locations with air velocity Figure 13. Latent energy charging speed at different locations with air velocity Figure 14. Grashof number as a function of slab inclination angle Figure 15. Melting time as a function of slab inclination angle Figure 16. Latent energy charging speed as a function of slab inclination angle
Figure 1. Schematic drawing of the experimental apparatus
400 Enthalpy Heat flux 26.4 oC 22.1 oC 81.4 kJ/kg
Enthalpy (kJ/kg)
300 250
3.0 312.6 kJ/kg
2.5 2.0
200 1.5 150
50 0 -50
1.0
17.5 oC 53.2 kJ/kg
100
32.5 oC Mixture
Solid 0
10
20
30
Liquid
0.5 0.0
40
o
Temperature ( C)
Figure 2. DSC analysis for the paraffin-based PCM
50
Heat flux (kW/kg)
350
315 mm
50 mm
100 mm Flow direction
Slab #1
Air flow
Z axis
C/G
Slab #3 Y axis
X axis
D/H
Slab #2
A/E O
50 mm
200 mm
100 mm
B/F
Slab #4
(b) Photo of HDPE slabs
Coordinates A (0, 57.5, 50)
B (0, 57.5, -50)
C (0, -57.5, 50)
D (0, -57.5, -50)
E (14, 57.5, 50)
F (14, 57.5, -50)
G (14, -57.5, 50)
H (14, -57.5, -50)
(a) Thermocouple locations
(c) Top view of HDPE slabs
Figure 3. PCM slab arrangement and measuring points
void space Air flow HDPE slab
Wind tunnel bottom
(a) front view
(b) side view Figure 4. PCM slab inclination angle
55 Leeward-Top (C) Experiment 01 Experiment 02 Experiment 03
50
Temperature (oC)
45 40 35 30 25 20 15
0
20
40
60
80
100
Time (min)
(a) Case 8: Air temperature: 50 °C; Air velocity: 3 m/s; Inclination angle: 90° 55 Leeward-Bottom (D) Experiment 01 Experiment 02 Experiment 03
50
Temperature (oC)
45 40 35 30 25 20 15
0
20
40
60
80
100
Time (min)
(b) Case 10: Air temperature: 50 °C; Air velocity: 5 m/s; Inclination angle: 90° Figure 5. Illustration of repeatability of transient temperature profiles
70
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
PCM temperature (oC)
60 50 40 30 20 10
0
20
40
60
80
100
120
Time (min)
(a) PCM temperature profiles during the melting process
HDPE panel surface temperature (oC)
60
50 50.8 50.4 50.0 49.6 49.2 48.8 8 12 16 20 24 28
40
30 Windward-Top (E) Windward-Bottom (F) Leeward-Top (G) Leeward-Bottom (H)
20
10
0
20
40
60
80
100
120
Time (min)
(b) PCM slab surface temperature profiles during the melting process Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o)
Air flow
(a) 20 o C/5 min
(b) 25 o C/27 min
(c) 30 o C/58 min
(d) 35 o C/70 min
(e) 40 o C/77 min
(f) 45 o C/85 min
(g) 50 o C/96 min
(h) 53 o C/107 min
Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°)
120
300
100
b 250
80 Amount of stored heat
200
60 150 100 50 0
a 0
20
40
60
40 Windward-Top (A) Windward-Bottom (B) 20 Leeward-Top (C) Leeward-Bottom (D) 0 80 100 120
Amount of stored heat (W·hr)
Accumulated energy storage (kJ/kg)
350
Time (min)
Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o)
80 70
Temperature (oC)
60 50
24 22 20 18 16 14 14:02
Boundary temperature 50oC 55oC 45oC 40oC o 35 C 14:07
14:12
40 30 20 10 14:02
28.5 oC
o o 27.7 oC 26.8 C 25.9 C
14:25
14:48
15:11
15:34
15:57
Time of day
Figure 9. Temperature variation at windward-top zone (location A) during melting
180 Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
160
Melting time (min)
140 120 100 80 60 40 20 30
35
40
45
50
55
60
o
Inlet air temperature ( C)
Figure 10. Melting time at different locations with inlet air temperature
180 Latent energy charing speed (W)
160 140 120
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
100 80 60 40 20 30
35
40
45
50
55
60
o
Inlet air temperature ( C)
Figure 11. Energy charging speed at different locations with inlet air temperature
110
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
100 Melting time (min)
90 80 70 60 50 40 30 20
0
1
2
3
4
5
6
Air velocity (m/s)
Figure 12. Melting time at different locations with air velocity
220 Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
Latent energy charing speed (W)
200 180 160 140 120 100 80 60 40 20
0
1
2
3
4
5
6
Air velocity (m/s)
Figure 13. Latent energy charging speed at different locations with air velocity
（×106）
14
Temperature at Windward-Top (A) 30℃ 35℃ 40℃ 45℃ 50℃
12
Grashof number
10 8 6 4 2 0
15
30
45
60
75
90
105
Inclination angle (°)
Figure 14. Grashof number as a function of slab inclination angle
150
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
Melting time (min)
120
90
60
30
0 15
30
45
60
75
90
105
Inclination angle (°)
Figure 15. Melting time as a function of slab inclination angle
Latent energy charing speed (W)
120
100
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
80
60
40
20 15
30
45
60
75
90
105
o
Inclination angle ( )
Figure 16. Latent energy charging speed as a function of slab inclination angle
Table 1 Sensors and their accuracy Table 2 Thermal parameters of the PCM Table 3 PCM mass and energy storage capacity within each slab Table 4 Experimental conditions Table 5 Melting time reduction (f) as a function of air temperature Table 6 Amount of stored heat as a function of air temperature Table 7 The melting time reduction (f) as a function of air velocity Table 8 Amount of stored heat as a function of air velocity Table 9 The melting time reduction (f) as a function of slab inclination angle Table 10 Amount of stored heat as a function of slab inclination angle
20
Table 1 Sensors and their accuracy Sensor
Range
Hot wire anemometer
0.15-30 m/s
Thermocouples
Accuracy
o
0-100 C
21
± 0.05 m/s ± 3% of reading for 0.15-3 m/s ± 0.2 m/s ± 3% of reading for 3.1-30 m/s ±0.5 oC
Table 2 Thermal parameters of the PCM
Melting
Heat of
Thermal conductivity
Density (kg/m3)
o
(W/(m· C))
Thermal capacity (kJ/(kg·oC))
temperature
fusion
(oC)
(kJ/kg)
Solid
Liquid
Solid
Liquid
Solid
Liquid
22.1-32.5
231.2
0.36
0.16
850
765
2.15
2.30
22
Table 3 PCM mass and energy storage capacity within each slab Slab #1
Slab #2
Slab #3
Slab #4
Average
PCM mass (kg)
1.13
1.09
1.16
1.15
1.13
Energy storage capacity (Whr)
72.57
70.00
74.50
73.86
72.73
23
Table 4 Experimental conditions Set of experiments
Set air temperature (oC)
Motor frequency
Air velocity
Inclination angle
(Hz)
(m/s)
(°)
Cases 1-5
35, 40, 45, 50, 55
27.6
3
90
Cases 6-10
50
12.8, 18.4, 27.6, 36.8, 46.0
1, 2, 3, 4, 5
90
Cases 11-15
50
43.5
3
30, 45, 60, 75, 90
24
Table 5 Melting time reduction (f) as a function of air temperature Set air temperature (oC) Melting time reduction (f)
35
40
43.1%
44.0%
25
45
50
55
43.0%
43.8%
47.4%
Table 6 Amount of stored heat as a function of air temperature
temperature (oC)
Air temperature (oC)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
stored heat
35 40 45 50 55
37.3 39.9 44.3 49.9 54.3
20.4 15.3 15.7 16.7 14.1
22.7 17.6 17.4 17.5 15.3
35.8 39.3 42.0 49.6 53.4
77.0 83.1 85.9 93.6 97.2
Set air
26
Amount of
Qs/(Qs+Ql) (%)
(Whr)
3.2 9.6 11.7 16.9 20.2
Table 7 The melting time reduction (f) as a function of air velocity Air velocity (m/s)
1
2
3
4
5
Melting time reduction (f)
36.6%
39.5%
48.1%
49.2%
46.7%
27
Table 8 Amount of stored heat as a function of air velocity
Air velocity (m/s)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
stored heat
1 2 3 4 5
20.8 20.5 18.3 16.7 18.3
21.9 21.7 19.7 18.3 19.7
49.5 47.5 48.4 50.3 50.4
85.59 84.67 86.82 90.41 93.16
28
Amount of
Qs/(Qs+Ql) (%)
(Whr) 13.7 13.2 15.1 17.2 16.1
Table 9 The melting time reduction (f) as a function of slab inclination angle Inclination angle (°)
30
45
60
75
90
Melting time reduction (f)
18.8%
36.2%
40.7%
41%
50.8%
29
Table 10 Amount of stored heat as a function of slab inclination angle
Inclination angle (°)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
Amount of
30 45 60 75 90
20.7 21.0 20.6 19.1 19.4
22.8 21.8 22.0 20.3 22.7
47.5 58.1 58.1 47.6 54.1
77.16 93.55 93.66 86.68 87.06
30
stored heat
Qs/(Qs+Ql) (%)
(Whr) 13.2 19.9 19.8 13.7 17.6
Highlight Energy charging processes of phase change materials (PCMs) in rectangular slabs were analyzed. Thermal response of the ventilated slabs to various boundary conditions was tested. An energy storage capacity of 97.2 W·hr with a charging rate of 109.4 W was obtained.
31
S1359-4311(18)36212-4 https://doi.org/10.1016/j.applthermaleng.2018.12.032 ATE 13039
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 October 2018 3 December 2018 5 December 2018
Please cite this article as: X. Sun, Y. Chu, M.A. Medina, Y. Mo, S. Fan, S. Liao, Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigations on the thermal behavior of phase change material (PCM) in ventilated slabs Xiaoqin Sun1,2,*, Youhong Chu1, Mario A. Medina2,†, Yajing Mo1, Siyuan Fan1, Shuguang Liao3 1
School of Energy and Power Engineering, Changsha University of Science & Technology, Changsha,
Hunan 410114, PR China 2
Civil, Environmental & Architectural Engineering Department, The University of Kansas, Lawrence,
KS 66045, USA 3
Changsha Maxxom High-tech Co., Ltd., Changsha 410015, PR China
Abstract Experiments were conducted using a small wind tunnel to identify the thermal behavior during energy charging processes of phase change material (PCM) in rectangular slabs. A commercially available paraffin-based PCM with a nominal melting temperature range of 26-28 °C was encapsulated in high-density polyethylene (HDPE) slabs for thermal storage applications in buildings. The influence of air temperature and velocity as well as the slab inclination angle was evaluated using melting time and energy charging speed. The melting time was 18.8%-50.8% shorter at top zone than it at bottom zone of the slabs because of the natural convection. The average energy charging speed increased from 36.3 W to 109.4 W by 201.7% when the inlet air temperature increased from 35 °C to 55 °C. However, it was slightly enhanced by 8.7% when the air velocity increased from 4 m /s to 5 m/s. The aspect ratio should be considered when studying the impact of inclination angle on energy charging process. The maximum energy storage capacity was 97.2 W·hr with a fraction of 20.2% sensible heat with an energy charging speed of 109.4 W when the inlet air temperature was 55 °C, the air velocity was 3 m/s, and the inclination angle was 90°.
Keywords: phase change material (PCM); latent energy storage; energy storage capacity; energy charging speed; ventilated slabs
*
Corresponding authors. Address: School of Energy and Power Engineering, Changsha University of Science & Technology, Changsha, Hunan410004, PR China E-mail address: [email protected] (Xiaoqin Sun) † Civil, Environmental & Architectural Engineering Department, The University of Kansas, Lawrence, KS 66045, USA E-mail address:[email protected] (Mario A. Medina) 1
1 Introduction According to the most recent data from Energy Information Administration’s (EIA’s) Residential Energy Consumption Survey (RECS), home electricity consumption peaks in July and August when temperatures and cooling demand are the highest. It is estimated that 18% of annual household electricity use is for air conditioning in US. Therefore, energy efficient solutions for buildings, especially for space cooling are in urgent demand. The combination of energy efficient building design and renewable energies is attractive for reducing cooling energy consumption in buildings. For example, night ventilation is a well-known technology to reduce or eliminate the required active cooling during day time. The night coolness is stored when the outdoor air temperature is low. The stored coolness is utilized during the following day, so the cooling energy consumption is reduced. However, the night coolness is intermittent, thermal energy storage systems are required to store the coolness as much as possible [1, 2]. Among the thermal energy storage methods (i.e., sensible heat, latent heat, and thermos-chemical heat storage), storage through latent heat offered by phase change material (PCM) during their phase transitions, which far exceeds those associated with sensible heat of traditional materials is considered to be a very promising option [3]. However, the latent heat storage systems suffer from a low heat storage and release rates. The reason for this is that most PCM have a low thermal conductivity (k≤0.2 W/(m·°C)), which in many cases leads to incomplete melting or solidification processes. Also, a significant temperature difference within the PCM can lead to material failure and system overheating [4]. It is crucial to achieve efficient heat exchange between the heat transfer fluid and the PCM, which is strongly affected by the thermal boundary conditions and specific geometric configurations. PCMs are contained in various shapes for actual applications, including rectangular [5-7], spherical [8], and cylindrical or annular containers [9-11]. Melting of PCM in rectangular enclosures has received great attention due to its wide-ranging engineering applications in such fields as casting, metallurgy and thermal energy storage, especially the PCM to air heat exchangers for building applications [1, 12]. To study the thermal performance of a PCM-air rectangular heat exchanger under different thermal boundary conditions, Dolado et al. [13] carried out a real-scale test with different inlet air temperatures and air flow rates. The PCM used was paraffin based organic material (RT27 from Rubitherm). An average thermal power up to 4.5 kW for one hour was obtained for melting. An empirical relationship between melting time and temperature difference between inlet air and the PCM 2
was developed. Ji et al. [14] studied the transient energy storage process within an aluminum rectangular heat exchanger using a three-dimensional model for waste heat recovery from a low temperature gas flow (<230 °C) under different air flow rates and heater power inputs. The heat transfer mechanism of melting processes of PCM was described using 3D temperature contours in the presence of natural convection. The PCM at top side melted faster than that in the bottom side due to the natural convection. As the air flow rate increasing, the PCM temperature rose more slowly, resulting in a longer melting time. However, increasing heater power input fastened the melting process. Arzamendia Lopez et al. [5] developed a two-dimensional model to predict the thermal performance of a PCM-air heat exchanger for building application. The heat transfer process was supposed to be conduction only in an equivalent homogeneous material. The model was validated using a 3-cm PCM slab in thickness from ENERGAIN boards. Natural convection within liquid PCM was observed during melting, which should be considered when optimizing the geometric configurations [15, 16]. The energy charging and discharging rates of PCM could be enhanced readily through natural convection by optimizing the slab arrangements. Zhao et al. [17] experimentally analyzed the influence of the inclination angle on the natural convection during the melting process using image digitalization. The variation of melting time depended on the aspect ratio. When the aspect ratio was bigger than 1, with the melting process going on, the natural convection heat transfer was intensified; otherwise, it weakened. It was shown that with an inclination angle of 60° the melting process was achieved in the shortest time. Kamkari et al. [18, 19] observed the natural convection flow structures of a lauric acid within an enclosure that was heated isothermally from one side. The results showed that the convection in the enclosure increased and a disorganized flow structures appeared as the inclination angle of the enclosure was decreased from 90° to 0°. That is, the melting time was the shortest when the rectangular heat exchanged was placed horizontally. Prieto and González [20] compared the flow behavior using velocity plots and volumetric liquid fraction contours within PCM slabs arranged vertically and horizontally during melting and solidification. Vertical arrangement produced higher flow intensity than the horizontal arrangement for intermediate values of the PCM liquid fraction. However, the mean heat fluxes increased if the slabs were horizontally positioned. In summary, the energy charging processes of PCM-air heat exchangers are significantly influenced by the boundary conditions and configuration arrangements. However, there is no 3
“one-size-fits-all” version of a PCM-air heat exchanger. Each study has its own conclusions that make it more or less appropriate for a specific application. A challenge is in identifying these factors and subsequently matching the beneficial storage systems. This paper proposed an experimental study of melting PCM in rectangular slabs under different boundary conditions and slab inclinations. A commercially available paraffin-based PCM with a nominal melting temperature range of 26-28 °C was used. Thermal behavior was evaluated using energy charging speed and melting time. The impact on these metrics of the air temperature and velocity as well as the slab inclination angle was analyzed. The best thermal performance using the largest energy charging speed and the least melting time will be obtained.
2 Experimental setup 2.1 Experimental apparatus The schematic drawing of the experimental apparatus is shown in Figure 1. It consists of a climate chamber, a wind tunnel with a cross area of 330 mm × 330 mm, four PCM slabs, monitoring devices, and a data acquisition system. To bring the air to the designed temperatures in the climate chamber, two chillers (cooling capacities: 7.5 kW and 12.0 kW) and a heater (heating capacity: 18 kW) were used to bring air temperatures ranging from -10 °C to 60 °C with an accuracy of ± 0.5 °C. The chillers and heater adjusted their power automatically according to the set temperature. Air from the climate chamber was supplied to the wind tunnel. A variable speed fan was installed in the wind tunnel to control the air velocity. One flow straightener was set upstream of the slabs to stabilize the air flow.
Figure 1. Schematic drawing of the experimental apparatus
Type T thermocouples (Omega TT-T-30-SLE) were used to measure the air temperatures before and after passing through the PCM slabs and PCM temperature variation during melting. Air velocity was measured using a hot wire anemometer (KIMO-VT110). All temperatures were collected and transferred to a computer via a data acquisition system at the rate of one reading per second. The accuracy of sensors is shown in Table 1. Table 1 Sensors and their accuracy
4
2.2 Phase change materials Solgi et al. [21] recommended the PCM with a melting temperature of 27 °C for night ventilation to improve indoor thermal comfort. Therefore, the PCM used in this study was a paraffin-based material, which was sold under the name of ZDJN-28 with a melting temperature of 28 °C. Figure 2 shows the DSC analysis of this PCM under a heating rate of 5 °C/min. Two peaks were observed in the heat flux curve, which represented a solid-solid phase transition (crystal packing changes without entering an isotropic liquid phase) and a solid-liquid phase transition, respectively. The solid-solid phase transition occurred at 17.5 °C and the corresponding enthalpy was 53.2kJ/kg. The melting process started at a temperature of 22.1oC. The enthalpy at this point was 81.4kJ/kg. The second heat flux peak occurred at 26.4 °C. The melting process finished at a temperature of 32.5 °C and the corresponding enthalpy of 312.6 kJ/kg. The latent heat of fusion caused by the solid-liquid phase transition was 231.2kJ/kg [(312.6-81.4) kJ/kg]. The thermal parameters of the PCM are shown in Table 2.
Figure 2. DSC analysis for the paraffin-based PCM
Table 2 Thermal parameters of the PCM
PCM was encapsulated in high-density polyethylene (HDPE) slabs with a thermal conductivity of 0.49 W/(m°C). The thickness of the slab wall was 1 mm. The conductive heat transfer coefficient of the slab wall was 490 W/(m2°C), much higher than the conductive heat transfer of PCM. Therefore, the thermal resistance of the slabs was ignored during the analysis. Each slab was a rectangular cavity with outside dimension of 28 mm in width, 315 mm in length and 200 mm in height. The quantity of PCM in each slab is shown in Table 3. The average energy storage capacity of one slab was 72.73
Whr. A void space of 8.3% was left for PCM expansion.
Table 3 PCM mass and energy storage capacity within each slab
Figure 3 illustrates the arrangement of PCM slabs in wind tunnel and the measuring points. The 5
PCM slabs were placed at the bottom of the wind tunnel. Therefore, all surfaces except for the slab bottom were exposed to the air flowing through the wind tunnel. The spatial distance between two neighboring slabs was 44 mm. Four zones were selected within the slabs to study the temperature response of the PCM. These are labeled from A through H. The selected zones were windward-top (locations A and E), windward-bottom (locations B and F), leeward-top (locations C and G) and leeward-bottom (locations D and H). Four thermocouples were installed inside the slabs (locations A through D) using two wood stands and the others (locations E through H) were on the surface of the slabs. The coordinates of each location are in Figure 3a with an origin at the center of the slab.
Figure 3. PCM slab arrangement and measuring points
2.3 Experimental conditions Before the tests, a relationship between the motor frequency of the variable speed fan and the air velocity within the wind tunnel without slabs was figured out. It was assumed the air passed through the air channels with same velocity. The air velocity passing through the PCM slabs was calculated using the total air channel cross area based on the readings of a hot wire anemometer. Five different air temperatures were tested, which increased from 35 to 55 °C by 5 °C. Air velocity between the slabs increased from 1 m/s to 5 m/s. The inclination angle () of the slabs varied between 30° to 90°, as illustrated in Figure 4. Two slabs were used for the inclination angle test. Therefore, the motor frequency was different with the value in Cases 1-5. Before the start of melting process, PCM was cooled to solid state. The cooling process was stopped when the PCM temperature was lower than its melting temperature. The series of tests are summarized in Table 4.
Table 4 Experimental conditions
Figure 4. PCM slab inclination angle
3 Results and analysis PCM absorbed energy in the form of latent as well as sensible heat during the melting process. As hot air passed through the slabs, heat was transferred via convection from the air, conduction through 6
the slab wall, and potentially through both conduction and convection within the PCM. During melting, liquid PCM moved upward driven by the density differences. Moreover, the air passing the top surfaces of the slabs accelerated the melting at top zone, which in turn contributed to natural convection in the melted PCM inside the slab. Therefore, the melting time at top zone of the slab was shorter than it at bottom zone. The melting time difference between the top and bottom zones was used to evaluate the heat transfer enhancement caused by natural convection, as shown in Equation 1.
f where
BD- AC 100% BD and
(1)
were the PCM melting time at the top (locations A and C) and bottom (locations B
and D) zones, min, respectively. The melting time was defined as the time duration when the PCM temperature was between 22.1 °C and 32.5 °C, as shown in Equations 2 and 3.
(2)
(3)
AC
1 4 32.5 22.1 AC ,n AC ,n 4 n 1
BD
1 4 32.5 22.1 BD ,n BD ,n 4 n 1
where the subscript n was the slab number; AC meant the locations A and C; and superscripts 22.1 and 32.5 were the PCM melting temperature range. For example,
32.5 AC , n was
the time when the mean
temperature at locations A and C was 32.5 °C for slab #n. The melting time reduction thus obtained was used to evaluate the heat transfer enhancement caused by natural convection. 3.1 Reliability and repeatability Repeatability study was conducted to examine the consistency of experimental results. Three energy charging experiments were performed for Case 8 and 10, respectively. Transient temperature profiles were plotted for thermocouples installed at location C for Case 8 and location D for Case 10, as shown in Figure 5(a) and 5(b), respectively. It was observed that phase transition time for all experiments were clearly identical. The statistical standard deviations for phase transition time were calculated to be 0.902 and 1.036, respectively. Thus, it assured the reliability the repeatability of experimental results showing thermal performance.
(a) Case 8: Air temperature: 50 °C; Air velocity: 3 m/s; Inclination angle: 90° 7
(b) Case 10: Air temperature: 50 °C; Air velocity: 5 m/s; Inclination angle: 90° Figure 5. Illustration of repeatability of transient temperature profiles 3.2 Temperature and the amount of stored heat 3.2.1 Temperature variation Taking Case 5 as example, the temperature variation is illustrated in Figure 6. PCM temperature increased from 15 °C to around 55 °C. Three regions were separated by two ends of the temperature plateau. The first region was where the PCM temperature increased almost linearly from 15 °C until it reached to 22.1 °C. Sensible heat was stored in the solid PCM at this region. The second region was where the PCM started to melt and to store latent heat. The PCM temperature maintained almost constant for all zones at this stage. The last region was where the PCM temperature increased again when the PCM was completely melted. During the melting process, heat transfer occurred between the hot wall of the slab and the solid PCM via conduction, which dominated the melting process at the early stage and caused a very thin layer of liquid to surround the wall of the slab; while the rest of the PCM remained solid without any phase change. This was the close-contact melting process [22]. As the melting layer grew, convection heat transfer gradually prevailed until it was the only heat transfer mode. The melted PCM gained heat from the slab surface, and it moved upward because of the density differences until it reached near the top of the slab. The liquid PCM at top zone was thus at a higher temperature. This caused more solid to melt at this location. The energy charging speed was higher in top zone of the slabs because of the upward motion of the melted PCM. Therefore, the melting process finished earlier in locations A and C. Similarly, the slab surface temperature was higher at the top zone because of the convection of liquid PCM, as shown in Figure 6(b). The temperature at location G decreased at 16 minutes. The temperature at this location increased first when the PCM absorbed sensible heat. When the PCM was melting, natural convection occurred. The circulation of liquid PCM affected the temperature distribution by mixing the hot liquid with warm liquid, preventing the slab from overheating. As a result, the slab surface temperature reduced. Moreover, the temperature at location G was higher than it at location E during the first and second regions. When the hot air passed location E, the PCM at this point started to melt and natural convection formed around this location. More liquid PCM with lower 8
temperature was induced to this zone and formed a large circulation cell. Natural convection at this location was stronger with more liquid PCM. Therefore, the slab surface temperature was lower. This phenomenon was evidenced by the temperature distribution in Figure 7. When the PCM completed its melting at the third stage, temperature at location E was higher than in at location G.
(a) PCM temperature profiles during the melting process
(b) PCM slab surface temperature profiles during the melting process Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55oC; Air velocity: 3m/s; Inclination angle: 90o)
Figure 7 shows the sequential infrared photos of the temperature distribution. The temperatures below each photo were for the windward-bottom zone (location B). When the PCM temperature at location B was 20 °C (Fig. 7a), all the PCM within the slab was solid. The heat transfer was controlled by thermal conduction. When the PCM temperature at location B was 25 °C (Fig. 7b), the solid PCM at windward-top (location A) started melting and the PCM temperature at this location was the highest. Natural convection was found around this location. When the PCM temperature at location B was 30 °C and 35 °C (Figs. 7c and 7d), most of the PCM was melting. Because of the natural convection more melting occurred in top zone of the slabs whereas in bottom zone the melting proceeded at an insignificant rate. The PCM temperature at bottom zone was much lower than it at top zone. When the PCM temperature at location B was 40 °C (Fig. 7e), most of the PCM had finished the melting process and the natural convection was reduced. It was interesting to note that in photos (c)-(e) the temperature seemed higher than 31 °C. That is, all PCM was in liquid state. However, PCM with temperature lower than 28 °C was observed in the following photos. For example, when the PCM temperature at location B was 45 °C (Fig. 7f), PCM with dark purple color was observed at the bottom of the slab. This was tricky by the natural convection when it was strong in photos (c)-(e), which disturbed the temperature distribution. It cannot be concluded that the melting was finished under these conditions. Slight convection was observed at the windward-bottom (location B) in photo (f). When the PCM temperature at location B was 50 °C, the melting process was finished except for bottom zone of the slabs. There was no convection according to the thermovision picture. The melt progresses reached towards the 9
thermally stratified condition [23]. PCM temperature increased layer by layer from bottom to top zones. It was interesting to note that even for this high bulk temperature, the PCM in bottom zone of the slab was still in a mushy state.
Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°)
3.2.2 Energy charging speed and amount of stored heat The variation of energy storage for Case 5 is shown in Figure 8. Two inflexion points separated the sensible and latent heat storage at each measuring point, as points a and b on the curve for location A. The slope of the accumulated energy storage illustrated the energy charging speed, which was calculated using Equation 4 for latent energy charging speed and Equation 5 for sensible energy charging speed. The energy charging speed was the average value of four slabs at each location. At the first region where energy was stored as sensible heat, the energy charging speed was 40.7 W, which was same for various zones. During the latent heat energy storage region, the energy charging speed was 176.0 W at location C, 170.0 W at location A, 89.6 W at location B and 86.5 W at location D, respectively. The energy charging speed increased during the latent heat energy storage region because of the large latent heat of fusion when PCM melted. The 1 kg PCM was able to absorb 231.2 kJ thermal energy when it melted. To absorb the same amount of energy, its temperature had to increase by 100.5 °C for liquid PCM and 107.5 °C for solid PCM, respectively. Moreover, the energy charging speed was higher for top zone because of natural convection. However, it was lower for top zone at the last sensible heat storage region, since the temperature difference between the PCM and hot air was smaller.
qil
1 4 mi ,n h 1000 22.1 60 4 n1 i32.5 ,n i ,n
qis, j where
i A, B, C , D
1 4 mi ,n c p Ti , j ,n Ti , j 1,n 1000 4 n 1 60
(4)
i A, B, C , D
(5)
was the latent energy charging speed at location i, W; mi,n was the PCM mass at location i
within slab #n, kg; h was the latent heat of fusion, kJ/kg; i , n and 32.5
10
i22.1 ,n
were the time when the
PCM temperature at location i within slab #n reached to 32.5oC and 22.1oC, min, respectively; was the sensible energy charging speed at location i and time j, W; Ti,j,n and Ti,j-1,nwere the PCM temperature at location i and time j and j-1 within slab #n, °C; was the time step, 1min; and cp was the thermal capacity of pure liquid or solid PCM, kJ/(kg·°C). Each slab was divided into four equal volumes along the y and z axis. The quantity of PCM at bottom zone was more than it at top zone because of the void space. To be specific, the average PCM mass was 0.308 kg for zones B and D and 0.257 kg for zones A and C, respectively. It should be noted that the PCM mass was calculated based on its liquid density. When the slab changed with different inclinations, the PCM mass at each zone did not change because the void space never merged into the bottom zone. The average amount of stored heat in Whr including both latent and sensible heat of each slab was calculated using Equation 6. D 1 1 4 Qtotal mi ,n h qis, j d i A 60 j 0 4 n 1
i A, B, C , D
(6)
The relative uncertainty of the results caused by indirect measurements was obtained using Equation 7. The error involved in the measured and derived parameters were calculated based on the accuracy of the measuring instruments.
y
1 n y xi y i 1 xi
(7)
The amount of stored heat in Whr as a function of time is shown in the right Y-axis in Figure 8. It increased with time to 97.1 Whr with a relative error of ±2.56%. It was higher than the values in Table 2 because of the sensible heat. That is, with an average latent energy charging speed of 130.5 W, this slab would recover thermal energy for 45 minutes. Sensible heat made up 20–28% of the total thermal energy storage. The highest ratio was 27.6% for location A, followed by 27.5% for location C, 22.5% for location B, and 19.6% for location D.
Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 °C; Air velocity: 3 m/s; Inclination angle: 90o)
3.3 Effects of inlet air temperature (Cases 1-5) 11
3.3.1 Temperature variation as a function of inlet air temperature Figure 9 shows the PCM temperature variation at windward-top zone (location A) as a function of inlet air temperature. PCM started to melt when its temperature was around 23.5 °C for various inlet air temperatures; though its initial temperature was different. That is, the temperature when PCM started to melt was not affected by inlet air temperatures or PCM initial temperatures. However, the temperature when the melting process completed increased with increasing inlet air temperature. Under the testing conditions, the PCM melting temperature range was 23.5 °C - 28.5 °C at location A. It was noteworthy that the melting temperature of bulk PCM was different with the DSC test results.
Figure 9. Temperature variation at windward-top zone (location A) during melting
3.3.2 Melting time as a function of inlet air temperature Figure 10 shows the PCM melting time as a function of inlet air temperature. It decreased with increasing inlet air temperature. The rise of the air inlet temperature increased the temperature difference between the air and PCM, Ta−TPCM, and therefore enhanced the heat exchange between PCM and hot air. Consequently, the time required to complete the melting process was reduced. It was observed that the melting time was shorter at top zone than it at bottom zone for all inlet air temperatures. Table 5 shows the melting time reduction, calculated using Equation 1. The melting time reduction was maintained between 43%-47% for various inlet air temperatures.
Figure 10. Melting time at different locations with inlet air temperature
Table 5 Melting time reduction (f) as a function of air temperature 3.3.3 Latent energy charging speed as a function of inlet air temperature Figure 11 shows the latent energy charging speed as a function of inlet air temperature, which was calculated using Equation 4. It was 71.7% higher for top zone than it for bottom zone, indicating that natural convection improved the heat transfer. The average latent energy charging speed for four zones increased from 36.3 W to 109.4 W by 201.7% when the inlet air temperature increased from 35 °C to 55 °C.
12
Figure 11. Energy charging speed at different locations with inlet air temperature
The average amount of stored heat is shown in Table 6, which increased with increasing inlet air temperature. The variation of the stored heat was caused by the stored sensible heat. More sensible heat was absorbed and stored when the inlet air temperature was higher.
Table 6 Amount of stored heat as a function of air temperature
3.4 Effect of air velocity (Cases 6-10) 3.4.1 Melting time as a function of air velocity Figure 12 shows the melting time for various air velocities and Table 7 presents the corresponding melting time reduction. As the air velocity increased, the melting time reduced. However, this was not true when the air velocity increased from 4 m/s to 5 m/s at location A. When the air velocity increased from 1 m/s to 2 m/s, the melting time at each location was reduced by average 6.3%. That is, the increase in air velocity was not enough to accelerate the melting process in a significant manner. The reason is that the thermal resistance of PCM is the main thermal resistance under these conditions. The natural convection within liquid PCM was not strong, which was evidenced by the melting time reduction in Table 7. Therefore, increasing air velocity did not enhance the melting process obviously. When the air velocity increased from 2 m/s to 3 m/s, the corresponding melting time was reduced by 15.6%. When the air velocity increased from 3 m/s to 4 m/s, the melting time was reduced by 17.1%. Under these two conditions (3 m/s and 4 m/s), the natural convection was strong with an average melting time reduction of 48.5%. The decrease in melting time was the result of stronger convection effect in both air and PCM. When the air velocity increased from 4 m/s to 5 m/s, the melting time decreased by 0.6%. Under this condition, increasing air velocity did not enhance the melting process effectively. The melting time was even longer at location A when the air velocity was 5 m/s than it when the air velocity was 4 m/s. This result coincided with the conclusion in Ref. [14]. Under the studied conditions, it was effective to improve the melting by increasing air velocity when the air velocity was lower than 4 m/s. The reason behind this phenomenon was the bottleneck in the convective heat transfer process on the air side [13]. That is, increasing air velocity will result in a reduction in temperature rise inside PCM [14]. 13
From Table 7 it was observed that the melting time was much shorter at top zone when the air velocity was 3 m/s and 4 m/s. That is, natural convection brought high temperature melted PCM upward to top zone, and lead to stronger heat transfer. The larger melting time reduction indicated a stronger convection.
Figure 12. Melting time at different locations with air velocity
Table 7 The melting time reduction (f) as a function of air velocity
3.4.2 Latent energy charging speed as a function of air velocity Figure 13 shows the latent energy charging speed as a function of air velocity. The average latent energy charging speed increased from 66.3 W to 108.7 W by 63.9% with increasing air velocity. It was not obvious when the air velocity increased from 1 m/s to 2 m/s and from 4 m/s to 5 m/s. As explained in Section 3.3.1, the energy charging process under these conditions was impacted by the PCM thermal resistance and the bottleneck in the convective heat transfer process on the air side. The energy charging speed at top zone was 106.7% higher than it at bottom zone because of the natural convection.
Figure 13. Latent energy charging speed at different locations with air velocity
Table 8 shows the average amount of stored heat as a function of air velocity. The inlet air temperature was maintained at 50 °C when the air velocity increased. The PCM mass was same for various air velocities. Therefore, the amount of stored heat did not change significantly for different cases. The variation of the stored heat was caused by the varied stored sensible heat because of different PCM initial and final temperatures.
Table 8 Amount of stored heat as a function of air velocity
3.5 Effect of slab inclination angle (Cases 11-15) It is evident that the natural convection affected the melting process significantly, which depends 14
on the spatial extent of the gravitational field and the established temperature difference. Grashof number, the ratio of the buoyancy to viscous force acting on a fluid, was used to evaluate the effects of natural convection within the liquid PCM. The mathematical definition is shown in Equation 8. Gr
g vl 3 TAC TBD
(8)
2
where g was the gravity constant, m/s2; v was the PCM volume expansion coefficient, 0.0008K-1; l was the characteristic length, 0.1m; v was the PCM kinematic viscosity at characteristic (melting) temperature [24], m2/s; and
and
were the PCM temperatures at top and bottom zones, °C.
Based on the temperature at the windward-top zone (location A), Grashof number within the slabs with various inclination angles is shown in Figure 14. Grashof number increased with the rise of inclination angle. That is, natural convection increased. This result was because of the bigger vertical gap between the top and bottom zones when the inclination angle increased. The upward flow of the melted PCM was suppressed by the smaller vertical gap between the top and bottom zones with lower inclination angles. When the temperature at location A was 45 °C and 50 °C, Grashof number increased significantly when the inclination angle increased from 30o to 45o. For other conditions, Grashof number increased slightly. When the PCM temperature at location A was lower than 45oC, it was the beginning of the melting process and the temperature of the PCM domain was maintained at the melting temperature range. The temperature difference of PCM at the top and bottom zones was small; therefore, the variation of Grashof number was moderate. When the temperature at location A was 45 °C and 50 °C, PCM at top zone had completed melting. Liquid PCM accumulated at the top zone; while the solid PCM gathered at the bottom zone. The temperature difference between the top and bottom zones increased, so did the Grashof number.
Figure 14. Grashof number as a function of slab inclination angle
3.5.1 Melting time as a function of slab inclination angle Figure 15 shows the melting time as a function of inclination angle and Table 9 presents the corresponding melting time reduction. It was observed that with increasing inclination angle the melting time decreased. When the inclination angle was 30°, Grashof number was the lowest. That is, the natural convection was the weakest. The PCM melting time at top zone was reduced by 18.8% 15
compared to it at bottom zone. When the inclination angle increased from 30° to 45°, the melting time at top zone was reduced significantly. Under this condition, the effective aspect ratio increased, so did the natural convection. It was proved by the melting time reduction, which increased from 18.8% to 36.2%. When the inclination increased from 45° to 75°, the melting time was reduced in a linear pattern. The melting time reduction was between 36.2% and 41%. When the inclination increased from 75° to 90°, the melting time reduction was 50.8% and the natural convection was the strongest. Based on the melting time variation, it was recommended to install the PCM slabs vertically, which was different with the results in Ref [18-20]. The reasons for this phenomenon were: 1) the aspect ratio was smaller than 1 when the slabs were arranged horizontally, so the natural convection was weak; 2) The void space at the top of the slabs affected the heat transfer.
Figure 15. Melting time as a function of slab inclination angle
Table 9 The melting time reduction (f) as a function of slab inclination angle
3.5.2 Latent energy charging speed as a function of slab inclination angle The latent energy charging speed as a function of slab inclination angle is shown in Figure 16. The average latent energy charging speed increased from 46.1 W to 77.3 W by 67.8% when the slab inclination angle increased from 30° to 90°. The energy charging speed at top zone was 43.9% higher than it at bottom zone. Table 10 shows the amount of stored heat with various inclination angle. The different stored heat was the result of different sensible heat.
Figure 16. Latent energy charging speed as a function of slab inclination angle
Table 10 Amount of stored heat as a function of slab inclination angle
4 Conclusions The influence of air temperature and velocity as well as slab inclination angle on the melting processes of PCM was investigated. The energy charging speed as well as the melting time were calculated to evaluate the melting process. Based on experimental results, the maximum storage 16
capacity was 97.2 Whr with a fraction of 20.2% sensible heat. The highest energy charging speed was 109.4 W when the inlet air temperature was 55 oC, the air velocity was 3 m/s, and the inclination angle was 90°. 1. PCM at top zone of the slabs melted faster than it at bottom zone of the slabs because of the natural convection in liquid PCM. The increase in air inlet temperature enhanced the energy charging speed and improved the amount of stored heat. PCM initial temperature and inlet air temperature had no impact on the temperature when the melting started. However, the temperature when the melting finished increased with the increase of inlet air temperature. 2. Increasing air velocity enhanced the energy charging speed and reduced the melting time. However, it was effective when the air velocity was lower than 4 m/s, which depended on the PCM thermal resistance. For example, when the air velocity increased from 4 m/s to 5 m/s, the melting time was only reduced by 0.6%. 3. Increasing inclination angle of slab enhanced the natural convection and thus the energy charging speed was higher. The proposed slabs were recommended to install vertically. Acknowledgments This work was supported by the National Natural and Science Funding (51308051), the Science and Technology Department of Hunan (2017RS3036), Hunan Association for Science and Technology (2017TJ-Q05), Changsha City Fund for Distinguished and Innovative Young Scholars (kq1707013) and Hunan Province 2011 Collaborative Innovation Center of Clean Energy and Smart Grid. References [1] N.S. Dhaidan, J.M. Khodadadi, Melting and convection of phase change materials in different shape containers: A review, Renewable and Sustainable Energy Reviews, 43 (2015) 449-477. [2] E. Solgia, Z. Hamedani, R. Fernando, B.M. Kari, H. Skates, A parametric study of phase change material behaviour when used with night ventilation in different climatic zones, Building and Environment, 147 (2019) 327-336. [3] A.F. Regin, S.C. Solanki, J.S. Saini, Heat transfer characteristics of thermal energy storage system using PCM capsules: A review, Renewable and Sustainable Energy Reviews, 12 (2008) 2438-2458. [4] A.M. Abdulateef, J. Abdulateef, S. Mat, K. Sopian, B. Elhub, M.A. Mussa, Experimental and numerical study of solidifying phase-change material in a triplex-tube heat exchanger with longitudinal/triangular fins, International Communications in Heat and Mass Transfer, 90 (2018) 73-84. [5] J.P. Arzamendia Lopez, F. Kuznik, D. Baillis, J. Virgone, Numerical modeling and experimental validation of a PCM to air heat exchanger, Energy and Buildings, 64 (2013) 415-422. [6] N. Stathopoulos, M. El Mankibi, R. Issoglio, P. Michel, F. Haghighat, Air–PCM heat exchanger for peak load management: Experimental and simulation, Solar Energy, 132 (2016) 453-466. 17
[7] U. Stritih, An experimental study of enhanced heat transfer in rectangular PCM thermal storage, International Journal of Heat and Mass Transfer, 47 (2004) 2841-2847. [8] H. Cui, W. Tang, Q. Qin, F. Xing, W. Liao, H. Wen, Development of structural-functional integrated energy storage concrete with innovative macro-encapsulated PCM by hollow steel ball, Applied Energy, 185 (2017) 107-118. [9] S. Seddegh, X. Wang, A.D. Henderson, Numerical investigation of heat transfer mechanism in a vertical shell and tube latent heat energy storage system, Applied Thermal Engineering, 87 (2015) 698-706. [10] S. Seddegh, X. Wang, A.D. Henderson, A comparative study of thermal behaviour of a horizontal and vertical shell-and-tube energy storage using phase change materials, Applied Thermal Engineering, 93 (2016) 348-358. [11] S. Seddegh, X. Wang, M.M. Joybari, F. Haghighat, Investigation of the effect of geometric and operating parameters on thermal behavior of vertical shell-and-tube latent heat energy storage systems, Energy, 137 (2017) 69-82. [12] V.V. Tyagi, D. Buddhi, PCM thermal storage in buildings: A state of art, Renewable and Sustainable Energy Reviews, 11 (2007) 1146-1166. [13] P. Dolado, A. Lazaro, J.M. Marin, B. Zalba, Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model, Renewable Energy, 36 (2011) 2906-2917. [14] C. Ji, Z. Qin, S. Dubey, F.H. Choo, F. Duan, Three-dimensional transient numerical study on latent heat thermal storage for waste heat recovery from a low temperature gas flow, Applied Energy, 205 (2017) 1-12. [15] R.E. Murray, D. Groulx, Experimental study of the phase change and energy characteristics inside a cylindrical latent heat energy storage system: Part 1 consecutive charging and discharging, Renewable Energy, 62 (2014) 571-581. [16] A. Pizzolato, A. Sharma, K. Maute, A. Sciacovelli, V. Verda, Design of effective fins for fast PCM melting and solidification in shell-and-tube latent heat thermal energy storage through topology optimization, Applied Energy, 208 (2017) 210-227. [17] J. Zhao, J. Zhai, Y. Lu, N. Liu, Theory and experiment of contact melting of phase change materials in a rectangular cavity at different tilt angles, International Journal of Heat and Mass Transfer, 120 (2018) 241-249. [18] B. Kamkari, H. Shokouhmand, F. Bruno, Experimental investigation of the effect of inclination angle on convection-driven melting of phase change material in a rectangular enclosure, International Journal of Heat and Mass Transfer, 72 (2014) 186-200. [19] B. Kamkari, H.J. Amlashi, Numerical simulation and experimental verification of constrained melting of phase change material in inclined rectangular enclosures, International Communications in Heat and Mass Transfer, 88 (2017) 211-219. [20] M.M. Prieto, B. González, Fluid flow and heat transfer in PCM panels arranged vertically and horizontally for application in heating systems, Renewable Energy, 97 (2016) 331-343. [21] E. Solgi, B.M. Kari, R. Fayaz, H. Taheri, The impact of phase change materials assisted night purge ventilation on the indoor thermal conditions of office buildings in hot-arid climates, Energy and Buildings, 150 (2017) 488-497. [22] M.K. Moallemi, Analysis of close-contact melting heat transfer, International Journal of Heat and Mass Transfer, 29 (1986) 855-867. 18
[23] V. Soni, A. Kumar, V.K. Jain, Modeling of PCM melting: Analysis of discrepancy between numerical and experimental results and energy storage performance, Energy, 150 (2018) 190-204. [24] C.J. Ho, Solid-liquid phase change heat transfer in enclosures, Purdue University, (1982).
19
Figure 1. Schematic drawing of the experimental apparatus Figure 2. DSC analysis for the paraffin-based PCM Figure 3. PCM slab arrangement and measuring points Figure 4. PCM slab inclination angle Figure 5. Illustration of repeatability of transient temperature profiles Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o) Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°) Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o) Figure 9. Temperature variation at windward-top zone (location A) during melting Figure 10. Melting time at different locations with inlet air temperature Figure 11. Energy charging speed at different locations with inlet air temperature Figure 12. Melting time at different locations with air velocity Figure 13. Latent energy charging speed at different locations with air velocity Figure 14. Grashof number as a function of slab inclination angle Figure 15. Melting time as a function of slab inclination angle Figure 16. Latent energy charging speed as a function of slab inclination angle
Figure 1. Schematic drawing of the experimental apparatus
400 Enthalpy Heat flux 26.4 oC 22.1 oC 81.4 kJ/kg
Enthalpy (kJ/kg)
300 250
3.0 312.6 kJ/kg
2.5 2.0
200 1.5 150
50 0 -50
1.0
17.5 oC 53.2 kJ/kg
100
32.5 oC Mixture
Solid 0
10
20
30
Liquid
0.5 0.0
40
o
Temperature ( C)
Figure 2. DSC analysis for the paraffin-based PCM
50
Heat flux (kW/kg)
350
315 mm
50 mm
100 mm Flow direction
Slab #1
Air flow
Z axis
C/G
Slab #3 Y axis
X axis
D/H
Slab #2
A/E O
50 mm
200 mm
100 mm
B/F
Slab #4
(b) Photo of HDPE slabs
Coordinates A (0, 57.5, 50)
B (0, 57.5, -50)
C (0, -57.5, 50)
D (0, -57.5, -50)
E (14, 57.5, 50)
F (14, 57.5, -50)
G (14, -57.5, 50)
H (14, -57.5, -50)
(a) Thermocouple locations
(c) Top view of HDPE slabs
Figure 3. PCM slab arrangement and measuring points
void space Air flow HDPE slab
Wind tunnel bottom
(a) front view
(b) side view Figure 4. PCM slab inclination angle
55 Leeward-Top (C) Experiment 01 Experiment 02 Experiment 03
50
Temperature (oC)
45 40 35 30 25 20 15
0
20
40
60
80
100
Time (min)
(a) Case 8: Air temperature: 50 °C; Air velocity: 3 m/s; Inclination angle: 90° 55 Leeward-Bottom (D) Experiment 01 Experiment 02 Experiment 03
50
Temperature (oC)
45 40 35 30 25 20 15
0
20
40
60
80
100
Time (min)
(b) Case 10: Air temperature: 50 °C; Air velocity: 5 m/s; Inclination angle: 90° Figure 5. Illustration of repeatability of transient temperature profiles
70
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
PCM temperature (oC)
60 50 40 30 20 10
0
20
40
60
80
100
120
Time (min)
(a) PCM temperature profiles during the melting process
HDPE panel surface temperature (oC)
60
50 50.8 50.4 50.0 49.6 49.2 48.8 8 12 16 20 24 28
40
30 Windward-Top (E) Windward-Bottom (F) Leeward-Top (G) Leeward-Bottom (H)
20
10
0
20
40
60
80
100
120
Time (min)
(b) PCM slab surface temperature profiles during the melting process Figure 6. Temperature variation during the melting process (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o)
Air flow
(a) 20 o C/5 min
(b) 25 o C/27 min
(c) 30 o C/58 min
(d) 35 o C/70 min
(e) 40 o C/77 min
(f) 45 o C/85 min
(g) 50 o C/96 min
(h) 53 o C/107 min
Figure 7. Infrared images of the PCM during the melting process (Air temperature: 55 °C; Air velocity: 5 m/s; Inclination angle: 90°)
120
300
100
b 250
80 Amount of stored heat
200
60 150 100 50 0
a 0
20
40
60
40 Windward-Top (A) Windward-Bottom (B) 20 Leeward-Top (C) Leeward-Bottom (D) 0 80 100 120
Amount of stored heat (W·hr)
Accumulated energy storage (kJ/kg)
350
Time (min)
Figure 8. Energy storage as a function of time (Case 5: Air temperature: 55 oC; Air velocity: 3 m/s; Inclination angle: 90o)
80 70
Temperature (oC)
60 50
24 22 20 18 16 14 14:02
Boundary temperature 50oC 55oC 45oC 40oC o 35 C 14:07
14:12
40 30 20 10 14:02
28.5 oC
o o 27.7 oC 26.8 C 25.9 C
14:25
14:48
15:11
15:34
15:57
Time of day
Figure 9. Temperature variation at windward-top zone (location A) during melting
180 Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
160
Melting time (min)
140 120 100 80 60 40 20 30
35
40
45
50
55
60
o
Inlet air temperature ( C)
Figure 10. Melting time at different locations with inlet air temperature
180 Latent energy charing speed (W)
160 140 120
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
100 80 60 40 20 30
35
40
45
50
55
60
o
Inlet air temperature ( C)
Figure 11. Energy charging speed at different locations with inlet air temperature
110
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
100 Melting time (min)
90 80 70 60 50 40 30 20
0
1
2
3
4
5
6
Air velocity (m/s)
Figure 12. Melting time at different locations with air velocity
220 Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
Latent energy charing speed (W)
200 180 160 140 120 100 80 60 40 20
0
1
2
3
4
5
6
Air velocity (m/s)
Figure 13. Latent energy charging speed at different locations with air velocity
（×106）
14
Temperature at Windward-Top (A) 30℃ 35℃ 40℃ 45℃ 50℃
12
Grashof number
10 8 6 4 2 0
15
30
45
60
75
90
105
Inclination angle (°)
Figure 14. Grashof number as a function of slab inclination angle
150
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D)
Melting time (min)
120
90
60
30
0 15
30
45
60
75
90
105
Inclination angle (°)
Figure 15. Melting time as a function of slab inclination angle
Latent energy charing speed (W)
120
100
Windward-Top (A) Windward-Bottom (B) Leeward-Top (C) Leeward-Bottom (D) Average
80
60
40
20 15
30
45
60
75
90
105
o
Inclination angle ( )
Figure 16. Latent energy charging speed as a function of slab inclination angle
Table 1 Sensors and their accuracy Table 2 Thermal parameters of the PCM Table 3 PCM mass and energy storage capacity within each slab Table 4 Experimental conditions Table 5 Melting time reduction (f) as a function of air temperature Table 6 Amount of stored heat as a function of air temperature Table 7 The melting time reduction (f) as a function of air velocity Table 8 Amount of stored heat as a function of air velocity Table 9 The melting time reduction (f) as a function of slab inclination angle Table 10 Amount of stored heat as a function of slab inclination angle
20
Table 1 Sensors and their accuracy Sensor
Range
Hot wire anemometer
0.15-30 m/s
Thermocouples
Accuracy
o
0-100 C
21
± 0.05 m/s ± 3% of reading for 0.15-3 m/s ± 0.2 m/s ± 3% of reading for 3.1-30 m/s ±0.5 oC
Table 2 Thermal parameters of the PCM
Melting
Heat of
Thermal conductivity
Density (kg/m3)
o
(W/(m· C))
Thermal capacity (kJ/(kg·oC))
temperature
fusion
(oC)
(kJ/kg)
Solid
Liquid
Solid
Liquid
Solid
Liquid
22.1-32.5
231.2
0.36
0.16
850
765
2.15
2.30
22
Table 3 PCM mass and energy storage capacity within each slab Slab #1
Slab #2
Slab #3
Slab #4
Average
PCM mass (kg)
1.13
1.09
1.16
1.15
1.13
Energy storage capacity (Whr)
72.57
70.00
74.50
73.86
72.73
23
Table 4 Experimental conditions Set of experiments
Set air temperature (oC)
Motor frequency
Air velocity
Inclination angle
(Hz)
(m/s)
(°)
Cases 1-5
35, 40, 45, 50, 55
27.6
3
90
Cases 6-10
50
12.8, 18.4, 27.6, 36.8, 46.0
1, 2, 3, 4, 5
90
Cases 11-15
50
43.5
3
30, 45, 60, 75, 90
24
Table 5 Melting time reduction (f) as a function of air temperature Set air temperature (oC) Melting time reduction (f)
35
40
43.1%
44.0%
25
45
50
55
43.0%
43.8%
47.4%
Table 6 Amount of stored heat as a function of air temperature
temperature (oC)
Air temperature (oC)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
stored heat
35 40 45 50 55
37.3 39.9 44.3 49.9 54.3
20.4 15.3 15.7 16.7 14.1
22.7 17.6 17.4 17.5 15.3
35.8 39.3 42.0 49.6 53.4
77.0 83.1 85.9 93.6 97.2
Set air
26
Amount of
Qs/(Qs+Ql) (%)
(Whr)
3.2 9.6 11.7 16.9 20.2
Table 7 The melting time reduction (f) as a function of air velocity Air velocity (m/s)
1
2
3
4
5
Melting time reduction (f)
36.6%
39.5%
48.1%
49.2%
46.7%
27
Table 8 Amount of stored heat as a function of air velocity
Air velocity (m/s)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
stored heat
1 2 3 4 5
20.8 20.5 18.3 16.7 18.3
21.9 21.7 19.7 18.3 19.7
49.5 47.5 48.4 50.3 50.4
85.59 84.67 86.82 90.41 93.16
28
Amount of
Qs/(Qs+Ql) (%)
(Whr) 13.7 13.2 15.1 17.2 16.1
Table 9 The melting time reduction (f) as a function of slab inclination angle Inclination angle (°)
30
45
60
75
90
Melting time reduction (f)
18.8%
36.2%
40.7%
41%
50.8%
29
Table 10 Amount of stored heat as a function of slab inclination angle
Inclination angle (°)
PCM slab surface initial temperature (oC)
PCM initial temperature (oC)
PCM final temperature (oC)
Amount of
30 45 60 75 90
20.7 21.0 20.6 19.1 19.4
22.8 21.8 22.0 20.3 22.7
47.5 58.1 58.1 47.6 54.1
77.16 93.55 93.66 86.68 87.06
30
stored heat
Qs/(Qs+Ql) (%)
(Whr) 13.2 19.9 19.8 13.7 17.6
Highlight Energy charging processes of phase change materials (PCMs) in rectangular slabs were analyzed. Thermal response of the ventilated slabs to various boundary conditions was tested. An energy storage capacity of 97.2 W·hr with a charging rate of 109.4 W was obtained.
31