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Design and experimental analysis of a helical coil phase change heat exchanger for thermal energy storage
Journal of Energy Storage 21 (2019) 9–17
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Design and experimental analysis of a helical coil phase change heat exchanger for thermal energy storage
T
Vahit Saydam, Mohammad Parsazadeh, Musaab Radeef, Xili Duan
⁎
Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John's, NL, A1B 3X5, Canada
ARTICLE INFO
ABSTRACT
Keywords: Thermal energy storage Helical coil heat exchanger Phase change material Paraffin wax
Latent heat energy storage systems have superior features over conventional sensible storage systems. With a large latent heat of fusion, a phase change material (PCM) can absorb and release a great amount of thermal energy at nearly a constant temperature. This improves the capacity and efficiency of the energy storage unit while extending the service time. In this study, a prototype PCM heat exchanger with a helical coil tube is designed, fabricated, and experimentally analyzed for its thermal storage performance under different operational conditions. Paraffin wax is used as PCM and Ethylene glycol (EG)-water mixture is used as heat transfer fluid (HTF). Different HTF inlet temperatures, flow directions, and flow rates were tested to find out the effects of these parameters on the performance, including charging and discharging time, of the thermal storage unit. It was found that the most significant factor during charging is the HTF inlet temperature. The experimental results showed that increasing the HTF inlet temperature from 70 °C to 75 °C shortened the charging time by 35% while charging time was reduced up to 21% with increasing flow rate from 0.5 to 4 L/min. The discharging time, however, did not change substantially with flow rates. It was also found that higher flow rate leads to higher recovery efficiency. The flow direction of the HTF was found to have an insignificant effect on the total charging and discharging time but showed effects on the temperature variations of PCM in the energy storage unit.
1. Introduction Energy from renewable sources, such as solar and wind, is abundant but intermittent in nature, and often with a time lag between energy supply and demand. To exploit these renewable energy resources, it is crucial to develop efficient energy storage systems that can store energy harvested in times of peak supply and provide it in times of high demand [1]. Conventional solar thermal to hot water systems are utilized for residential applications, with relatively low efficiency and limited capacity particularly in periods of scarce sunshine. One alternative is the implementation of Phase Change Materials (PCMs) to store thermal energy [2–4] both sensibly and latently. Latent heat thermal storage systems using PCMs have been proved to be more efficient than sensible heat storage systems thanks to the high energy density of the PCMs [5]. Heat is added to an energy storage unit when hot Heat Transfer Fluid (HTF), such as that from a solar panel, runs through the PCM tank. Water is commonly used as HTF in the latent thermal energy storage units. However, recent studies have proposed the use of water-based nanofluids, which have higher thermal conductivity and improved convective heat transfer than the base fluid [6,7]. Hot HTF transfers energy to the PCM and melts the latter thereby ⁎
storing thermal energy latently. PCMs can be broken down into three categories based on their chemical composition: organic, inorganic, and liquid metals [8]. Liquid metals are characterized by high melting points and hardness in their solid state. Inorganic PCMs are typically used in high temperature range as their melting points are also high. Prior investigations have proved critical disadvantages of inorganic PCMs, i.e., the sub-cooling issue, and corrosion of the containment material [8,9]. Organic PCMs are compounds that contain carbon, and usually hydrogen as well. This type of PCM is arguably the most popular, typically used in relatively low to medium temperature applications. It contains a wide range of compounds including alkane (paraffin) and fatty acids family. Among those, paraffin wax is favorable due to its abundance, high latent heat of fusion, chemically stability, and its compatibility with a wide range of materials with no corrosion in the containment material. Nevertheless, paraffin has low thermal conductivity [9], which makes the heat transfer rate low and leads to long charging and discharging times. This issue has been addressed by various heat transfer enhancement techniques [10–12]. Different designs of phase change energy storage systems have been studied [13–15]. A tube-and-shell unit is one of the simplest designs and among the most commonly used [16–18]. Numerous factors must
Corresponding author. E-mail address: [email protected] (X. Duan).
https://doi.org/10.1016/j.est.2018.11.006 Received 20 February 2018; Received in revised form 3 November 2018; Accepted 5 November 2018 2352-152X/ © 2018 Elsevier Ltd. All rights reserved.
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study aims to examine the detailed phase change behaviors of paraffin wax in a helical coil PCM heat exchanger, which were not well understood in previous studies. These data are useful in improving the design of helical coil PCM heat exchangers for thermal energy storage applications.
Table 1 Properties of PCM and HTF. Property
Values
mpcm
3.54 kg
Cp, pcm, l
2.981 J/g·K [28]
kpcm Cp, pcm, s
Hpcm Ti Tm TF mHTF Cp, HTF
0.34 W/m·K (solid), 0.28 W/m·K (liquid) 2.6 J/g·K [28]
2. PCM heat exchanger design and experimental setup
160 kJ/kg
2.1. Theoretical design of the PCM heat exchanger
18 °C 51 °C 70 °C 0.07-0.00875 kg/s 3.56 J/g·K [29]
Actual melting and solidification processes involve both latent heat and sensible heat due to the temperature change of the PCM. The amount of heat stored in the PCM can be theoretically calculated with Eq. (1):
be considered during the design process. In residential applications, the charging time plays a vital role in determining the functionality of the system, hence lengthy charging time is highly undesirable. Fabrication cost is another decisive factor, designing the system to be small can help control fabrication costs. Compactness and high efficiency of the design are achieved by a higher overall heat transfer rate between the HTF and the PCM. Another factor is the pressure drop developed from frictional losses through the heat exchanger. The optimal design will aim to limit the local pressure drop while not compromising the device’s performance. Among the several heat exchanger designs, the helical coil configuration stands out due to increased heat transfer surface area, which propelled several researchers to incorporate the helical coil design in their studies [19–26]; the prevailing helical coil configuration among those studies is the vertical configuration [19–24]. Nonetheless, horizontal configuration of the helical coil was explored as well [25,26]. Previous studies mostly focused on the effects of HTF temperature and flow rate on the charging and discharging processes. One of the major findings was the greater influence of the HTF inlet temperature on the charging and discharging performance compared to that of the flow rate [19–22]. More uniform charging was observed at lower HTF inlet temperature [19,25]. But these studies have not considered the effects of HTF flow direction for vertical coil configuration. The effect of HTF flow direction was found to be significant during melting in a simple shell and tube thermal storage unit [18,30]. Since natural convection of the melted PCM is significant, the flow direction of HTF might affect the heat transfer process in a coil configuration as well. This project aims to design, fabricate, and analyze a helical coil thermal energy storage unit with paraffin wax as the phase change material. The performance of the PCM thermal energy storage unit under different operational conditions is investigated. Melting and solidification characteristics of the PCM are examined with varying the HTF flow rate, inlet HTF temperature and the HTF flow direction. In addition, this
Qstored = mpcm [ Cp, pcm, s (Tm
Ti ) + Hpcm + Cp, pcm, l (Tf
Tm)]
(1)
where mpcm is the mass of the paraffin wax, Hpcm is the latent heat capacity of the paraffin wax, Ti is the initial temperature of the wax, Tm is the melting temperature of the wax, TF is the final temperature of the wax at the end of charging, Cp, pcm, s and Cp, pcm, l are the specific heat of the wax in solid and liquid phase respectively. The latent and sensible energy calculations assumed that the temperature of the wax is increased from 18 °C to 70 °C. The values of these parameters are summarized in Table 1. During the experiment, the heat supply or extraction rate during charging or discharging can be found from the following equation for the HTF [14]:
Qf = mHTF × Cp, HTF ×
Tf
(2)
Total energy supplied or recovered by the HTF during charging and discharging can be found by Eq. (3):
Qft =
t × Qf
(3)
where mHTF is the mass flow rate of the HTF, Cp, HTF is the specific heat of the HTF, Tf is the temperature difference between the HTF inlet and outlet, and t is the duration of each time step (20 s in the experiment). 2.2. Prototype design and fabrication An overall shell-and-tube heat exchanger lay out was selected for the prototype design. Different concepts for the tube configuration, HTF pattern, and insulation were considered and evaluated [27]. The final selection of design parameters is summarized here. Paraffin wax is selected as the PCM material with rationales discussed in the introduction. Paraffin has an excellent latent heat of fusion, with a desirable melting temperature (∼51 °C) for residential warm water applications. EG-water mixture was selected as the heat transfer fluid by the virtue of
Fig. 1. Fabrication of the PCM heat exchanger, (a) the top end plate, (b) the helical coil and shell, and (c) the whole thermal energy storage unit with insulation. 10
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its low cost, availability, and higher safety in handling. Polycarbonate was selected for the shell of this experimental prototype. Polycarbonate is durable, available, and easy to use with great heat/corrosion resistance. With its inherently low thermal conductivity, this material exhibits the behavior of a pseudo-insulator. In addition, polycarbonate has good transparency, which allows the monitoring of the PCM melting and solidification processes during lab testing. A helical tube design was selected for final fabrication. Preliminary CFD simulation and lab testing [27] proved that the spiral tube has better heat transfer performance than a straight one. The HTF tube was made from copper, which was chosen for its exceptionally high thermal conductivity, low cost, and availability. Figs. 1 and 2b show the key components and assembly dimensions of the PCM heat exchanger. The final design has the following dimensions: 29.53 cm (1 ft) in length and 16.64 cm (6.55 in. in outer diameter. The top and bottom ends of the container were attached to two square grooved acrylic plates having a thickness of 0.9525 cm (3/8 in. and 19.05 cm (7.5 in. in length (Fig. 1a). Four holes near each corner of the plates were drilled so stainless-steel rods can later be inserted through the holes on the upper and lower plates to hold the assembly together by fastening the nuts. Three additional holes were drilled to accommodate a thermocouple rod, an alignment rod which helps keep the helical coil in line, and the last hole was used for PCM filling and draining purposes. The shell is covered with insulation blanket made of cryogel (Scotia Insulations Co.), an advanced insulation material; the top and the bottom plates were also covered to prevent heat loss. The helical coil tubing was made from a 315 cm (124 in. long copper pipe with an inner and outer diameter of 0.635 cm (1/4 in. and 1.905 cm (3/ 8 in. respectively. The coil diameter was chosen to be 10.16 cm (4 in. with a pitch of 2.8575 cm (1.125 in. yielding 8.7 turns and a 7.62 cm (3 in. long straight tubing section extending from the center of the helical coil at each end (Fig. 1b). The fully prepared PCM thermal energy storage unit with insulation is shown in Fig. 1c. After placing the helical coil inside the storage tank, the heat exchanger prototype was sealed using a silicon caulking. Silicon sealed the gaps between the endcaps and the Lexan shell, and between the plastic tubing and end-cap penetrations. Paraffin wax in liquid phase was poured into the storage through the filling hole. After pouring the molten wax into the storage unit, the whole setup was left to cool down at room temperature (20 °C) to let the PCM completely solidify for lab testing. 2.3. Experimental setup A schematic of the experimental setup is shown in Fig. 2a. The main components include the cylindrical PCM heat exchanger with helical tubing, a thermal bath circulator, and a computer (PC) connected to a data acquisition system. The thermal bath circulator provides EG-water as HTF at desired temperatures for both charging and discharging experiments. The outlet of the thermal bath is connected to a three-way ball valve which allows for manual adjustments of the HTF flow rate.
Fig. 2. (a) Schematic of the experimental setup; (b) Drawing of the experimental PCM heat exchanger (dimensions in cm) with positions of the thermocouples, T1: Side bottom, T2: Center bottom, T3: Center middle, T4: Center Top, T5: HTF Inlet, T6: HTF Outlet. Table 2 Experiments under different operational conditions. Test #
Test Type
HTF Flow rate, LPM
HTF Inlet temperature, °C
HTF Flow direction
1 2 3 4 5 6 7 8 9 10 11
Charging Charging Charging Charging Charging Charging Discharging Discharging Discharging Discharging Discharging
4 2 1 0.5 4 4 2 1.5 1 0.5 1.5
75 75 75 75 70 75 20 20 20 20 20
Upward Upward Upward Upward Upward Downward Upward Upward Upward Upward Downward
Fig. 3. Comparison of charging and discharging times at different operational conditions. 11
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(6.875 in). The top thermocouple was placed just an inch (2.54 cm) above the middle thermocouple due to significant shrinkage upon solidification. Two thermocouples (T5, T6) are implemented to monitor the inlet and outlet temperatures of the HTF. Another thermocouple (T1) was inserted at the same height as the bottom center thermocouple but 6.35 cm apart from it to monitor the radial temperature difference. The bottom plate is the datum for the previous height measurements. The thermocouples were calibrated, and the uncertainty was found to be ± 1 °C, which is acceptable for this experimental study. 3. Results and discussions This section presents the results of the experiments under different operational conditions. Three main parameters, namely, HTF flow rate, HTF inlet temperature and HTF flow direction were investigated. The ranges of these parameters were chosen based on the capability of the thermal bath and the flowmeter used in the experiment. For all charging tests, the initial PCM temperature in the storage unit is 20 °C, a common room temperature in most residential warm water applications. For this type of applications, the charging HTF temperatures in previous studies, for example [16,19,24], are generally in the range of 70 °C–85 °C. In this study, the HTF inlet temperature is set to be in the range of 70–75 °C. In the discharging experiments, the initial PCM temperature is 69 °C and the HTF inlet temperature is set to be 20 °C. In addition, the lower limit of the flowmeter used in this study is 0.5 LPM (litre per minute, or L/min), while the maximum flow rate provided by the pump is 4 LPM. The operational conditions of 11 tests are summarized in Table 2. The objectives of these experiments are to determine the effects of each parameter on the performance of the
Fig. 4. Charging with a flow rate of 4 LPM at 75 °C inlet HTF temperature.
The flow rate of the HTF is monitored using a flow meter connected to the inlet line of the storage unit. The excess amount of HTF is directed back to the thermal bath through a bypass line. The outlet pipe connected to the top of the storage merges with the bypass line going back to the thermal bath. The thermocouples and flow meter are connected to the data acquisition (DAQ) system to record the measured data on the computer. Six T-type thermocouples are used to track the temperatures within the storage unit (see Fig. 2b). Three thermocouples (T2, T3, T4) are placed at the center but three different heights of the cylindrical tank. The bottom thermocouple was positioned 2.22 cm (0.875 in. above the bottom plate in the center. The middle thermocouple was attached to the same rod as the bottom center thermocouple at a height of 17.46 cm
Fig. 5. Pictures of charging with a flow rate of 4 LPM at 75 °C inlet temperature (a:30 min, b:60 min, c:90 min, d:120 min, e:145 min, f:170 min). 12
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clear in the literature and are often taken as the times when temperatures in the PCM become steady and close to the HTF temperature. In this study, the charging time is defined as the total time to attain a temperature of 69 °C from all thermocouples in the storage unit. Similarly, the discharging time for the storage unit is determined when all thermocouples in the storage unit read below 25 °C. Note that the total charging/discharging time includes both sensible and latent heat transfer processes. The subsequent sections present detailed discussions of the experimental data from these tests, including temperatures of the PCM, images of the melting or solidification process, HTF inlet and out temperature differences, energy addition/recovery rates, and energy recovery efficiency of the storage unit. 3.1. Charging Charging tests were carried out to investigate the melting behavior and how phase change evolves inside the storage unit. Fig. 4 illustrates the temperature profile of paraffin wax at different locations with HTF flow rate of 4 LPM at 75 °C inlet temperature. The melting behavior is observed by taking pictures of the system every half an hour; the pictures tracked the motion of the liquid/solid interface (Fig. 5). These results demonstrate that PCM melting initiated at the circumferential area of the helical coil; the closer the wax is to the coil, the earlier melting commences. This is also confirmed by thermocouple readings, since temperatures from the side bottom thermocouple are higher than those from the thermocouples placed in the center (Fig. 4). This also explains the phenomena observed in Fig. 5c where a big chunk of unmelted wax of a mountain shape exists at the center of the heat exchanger. As the melted portion grows, the liquid wax is pushed upward by buoyant force to trigger natural convection, which is the dominant heat transfer mechanism onwards (Fig. 5b). In later charging stages, the PCM temperature readings abruptly increase from the center top, and center middle thermocouples, due to the increased heat transfer through natural convection. Meanwhile, temperatures at the bottom increase linearly, since heat conduction dominate there. The rate of temperature increase at the top and middle thermocouple locations slows down after that sudden increase as more energy supplied from HTF is used to energize the phase change process rather than to increase sensible heat. It is evident that steep temperature increase at the top and middle thermocouples is present once the liquid/solid interface passes these probe locations and moves downwards over time (Fig. 5 d and e). This could be attributed to the significantly less energy required to increase the temperature of the PCM as opposed to causing phase change from solid to liquid. At the later stage of charging, the big conically-shaped chunk of PCM undergoes phase change and gradually disappears (Fig. 5d–f). Finally, the same sudden temperature increase is seen for the
Fig. 6. (a) HTF inlet and outlet temperature difference, (b) energy addition rate, and (c) cumulative energy supply during charging.
designed PCM storage unit, including the charging and discharging times, the energy storage and recovery rates, and energy recovery efficiency, etc. Fig. 3 shows the comparison of charging and discharging times for all the tests. This is only an overview of the results to show a general trend of the charging/discharging speed under different operational conditions. The definitions of charging and discharging times are not
Fig. 7. Discharging with a flow rate of 1 LPM at 20 °C inlet HTF temperature. 13
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Fig. 8. Pictures of the discharging process with a flow rate of 1 LPM at 75 °C inlet temperature (a:0 min, b:30 min, c:150 min).
thermocouples at the bottom with the one in the center being more distinct. It was found that the trend of the temperature increment during charging is very similar to [19]. However, the charging duration is longer as the current thermal storage unit is larger. The temperature differences at the inlet and outlet of the HTF, ΔTf, for four different flow rates during the charging tests are shown in Fig. 6a. The results show that lower flow rates lead to larger fluid temperature differences. This is expected since lower flow rates give the fluid more time to release heat to the PCM. The HTF inlet/outlet temperature difference is high at the beginning of the charging process for all flow rates, and then it decreases toward the end of the charging process. This is due to the decreasing heat transfer rate with decreasing temperature difference between the PCM and the HTF. When the flow rate is low, i.e., < 2 LPM, there is a sudden decrease of ΔTf before it levels out. This sudden decrease, shown approximately with the dashed line in Fig. 6a, may serve as an indicator of initial melting around the HTF tubing. The liquid PCM has lower thermal conductivity and causes a reduction in the heat transfer rate. However, such a behavior is not observable with the high flow rate of 4 LPM, since the ΔTf is very low during the whole charging process as the flow rate is very high. With these fluid temperature differences, the energy addition rates from the HTF are calculated and shown in Fig.6 b. At a given flow rate, the energy addition rate decreases with time with a similar trend of the ΔTf, which is expected from Eq. (2). This equation also predicts that the energy addition rate is determined by the product of the flow rate and the fluid temperature difference. Therefore, neither the case of highest flow rate (4 L/min), nor the one with highest fluid temperature difference (0.5 L/min) brings the highest energy addition rate. It’s the case of medium flow rate of 1 L/min that provides the highest energy addition rate from the HTF, as shown in Fig. 6b. This flow rate also brings the highest cumulative energy supply, as shown in Fig. 6c. In contrast, the lowest cumulative energy is supplied at the highest flow rate of 4 L/ min.
takes place at around 55 °C. The foreign structures within the storage unit, such as the metal rods, provide nucleation sites and therefore alleviate the supercooling effect. Due to its proximity to the helical coil, the side bottom thermocouple readings diverge from the ones in the center and decrease rapidly right after the phase change completes. After that, the heat extracted from the PCM decreases drastically, leading to insignificant temperature differences between the inlet and outlet HTF temperatures due to increased thermal resistance in the solid layer. The completion of phase change is followed by gradual cool down of the solid wax to the cold HTF temperature, with the center middle thermocouple being the slowest. Overall, the results confirm that the discharging process takes much longer time compared to the charging process (also in Fig. 3), due to poor conduction heat transfer within the solid paraffin wax. The discharging trend is very similar to the results in [19], although the current discharging duration is longer due to the larger capacity of the thermal storage unit. The temperature differences between the inlet and outlet of the HTF, ΔTf, are small for all four different flow rates in the discharging tests, as illuminated in Fig. 9a. These temperature differences are even smaller than those in the charging time depicted in Fig. 6a, due to the lower thermal conductivity of the liquid phase PCM compared with that of the solid PCM at early stage of charging. The ΔTf also decreases with time due to the increase of PCM temperature and the consequently decreasing heat transfer rate between the liquid and PCM. In discharging, the HTF flow rate shows less effects on the ΔTf. Therefore from Eq. (2) higher energy recovery rate is achieved at higher flow rates, which is shown in Fig. 9b. As a result, the cumulative energy recovery from the storage unit is higher at higher flow rates, as shown in Fig. 9c. 3.3. The effects of HTF flow rate on charing and discharging The effects of HTF flow rate on the charging and discharging processes were examined by varying the flow rates for the same inlet HTF temperature (75 °C). Results in Fig. 10 show that higher flow rates result in faster melting of the PCM. Faster charging is attributed to the increased forced convection between the HTF and inner surface of the helical coil, leading to increased overall heat transfer between the HTF and the PCM [22]. After warming up the solid PCM at the beginning of charging, melting starts, and natural convection takes control; PCM temperatures reveal higher heat transfer rate with higher flow rate (with more rapid temperature increases). The results indicate that the onset of natural convection is earlier at higher flow rate. The charging times with different HTF flow rates are shown in Fig. 3. These data show that increasing the flow rate from 0.5 L/min to 4 L/min decreases the charging time by 21%.
3.2. Discharging Fig. 7 shows the temperature variations in the storage unit during discharging with an HTF flow rate of 1 LPM at 20 °C inlet temperature. Significant drop in temperature for all the thermocouples is noted at the beginning of the discharging process due to the sensible energy loss. It results in considerable temperature difference between the inlet and outlet of the HTF. Solidification starts from the outer surface of the helical coil and inner wall of the shell (the latter is due to heat loss). This obstructs visual tracking of the solidification behavior of inner layers of the PCM (Fig. 8). As can be seen from Fig. 7, the liquid-solid transformation 14
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Fig. 10. Temperature profile of center top thermocouple at different flow rates during charging at 75 °C HTF inlet temperature.
Fig. 11. Temperature profile of the center bottom thermocouple at different flow rates during charging at 75 °C HFT inlet temperature.
Fig. 12. Temperature profile of the center top thermocouple at different flow rates during discharging at 20 °C HTF inlet temperature.
Fig. 9. (a) HTF inlet and outlet temperature difference, (b) energy extraction rate, and (c) cumulative energy recovery during discharging.
solid transition. After that, temperature remains at around 55 °C for a long time indicating the liquid-solid phase change transformation. Upon the completion of solidification around 50 °C, the temperature decreases almost linearly regardless of flow rate. The discharging process takes 5 h 32 min with an HTF flow rate of 0.5 L/min. Higher flow rates of the HTF do not reduce discharging time, as more clearly shown in Fig. 3. Although convective heat transfer rate increases with higher flow rate in the helical coil, the innate low thermal conductivity of the wax dominates the overall heat transfer process during discharge and prevents any reductions in the discharging time, which agrees with previous studies [16,24].
Temperature variations show different trends at other locations in the storage unit. It is seen from Fig. 11 that the for most of the time, the temperature increases linearly at the center bottom location before melting as it is the last part of the region to be melted. As discussed earlier, higher flow rate leads to earlier melting and onset of natural convection. After melting, the increased heat transfer at higher flow rates leads to rapid temperature increase. As compared to Fig. 11, results in Fig. 12 indicate that there is not much change in the readings of the center top thermocouple during discharging at different flow rates. At the beginning of discharge, a rapid drop in temperature is seen before the PCM goes through liquid15
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be more uniform at 70 °C inlet HTF temperature as opposed to a separation of top and middle thermocouples due to dramatic temperature rise at 75 °C inlet temperature. 3.5. Effects of HTF flow direction on charging and discharging Another factor investigated during the charging and discharging tests is the flow direction of HTF. A previous numerical study on a vertical tube-in-shell configuration showed that upward flow decreases the charging time due to enhanced natural convection effects [30]. In the upward flow, the hotter molten PCM at the bottom rises up due to its lower density. This effect has been shown to accelerate melting [31]. However, our experimental results in Fig.3 indicate that switching the HTF inlet from bottom to top does not lead to noticeable change in charging or discharging time. Results in Fig. 14 also show similar PCM temperature variations with different HTF flow directions during charging. One possible explanation is related to these high flow rates of HTF, which lead to very small difference between the inlet and outlet temperatures of the HTF. Hence the flow direction either being downward or upward does not cause any change in charging or discharging time. A very low HTF flow rate may give a different trend, but this study didn’t consider lower flow rates than 0.5 LPM due to the limitations of our flow meter. Furthermore, a very low flow rate would make the discharging or charging time too long for feasible lab test and not useful for engineering applications. For discharging, PCM temperature results in Fig. 15 indeed show some differences, although the total discharging time does not change much with switching the flow direction. Supplying cold HTF from the top of the storage unit results in longer phase transition (maintaining the temperature at around 55 °C). This might be due to the formation of weak natural convection with circulation of liquid wax when the cold HTF flow is downward, since cooled liquid wax on the top portion of the storage unit is replaced by the warmer liquid wax rising from the bottom. Therefore, more uniform temperature distribution is maintained. However, upon completion of the phase change, the PCM temperature decreased more quickly compared to the case where cold HTF is supplied from the bottom, eventually leading to similar overall discharging time.
Fig. 13. Temperature profile of the center top thermocouple at different inlet temperatures of HTF at 4 LPM during charging.
Fig. 14. Temperature profile of the center bottom thermocouple at different flow directions with 4 LPM during charging at 75 °C.
3.6. Energy recovery efficiency Experimental energy recovery efficiency can be determined from the ratio of the recovered energy by the HTF during discharging to the energy supplied by the HTF during charging, both calculated using Eqs. (2) and (3). The recovery efficiency of the storage unit can then be determined using Eq. (4).
=
Qf ,discharging Qrecovered = Qsupplied Qf ,charging
(4)
The recovery efficiency of the storage unit ranges between 35–62 % for the tested discharging flow rates. The highest efficiency was found at a HTF flow rate of 2 LPM while the lowest efficiency was at 0.5 LPM. It should be noted that these calculations did not consider the heat losses from the storage container to the surrounding environment.
Fig. 15. Temperature profile of the center bottom thermocouple at different flow directions with 1 LPM during discharging at 20 °C.
3.4. Effects of HTF inlet temperature on charging
4. Conclusions
The effects of inlet temperature of the HTF on charging time was also investigated. The charging tests were conducted at 70 and 75 °C for the same flow rate (4 L/min). Increasing the inlet temperature from 70 to 75 °C reduced the charging time by 35% (Fig. 3). It is also shown in Fig. 13 that the PCM temperature increase from the center top thermocouple was identical for both cases at the initial state. However, higher inlet temperature leads to faster melting and triggers the earlier onset of natural convection which dominates the melting process during charging [26,30]. The PCM temperature increase was found to
A PCM heat exchanger for latent heat storage of thermal energy was designed, fabricated, and analyzed experimentally. Performance of the thermal energy storage unit is investigated under different operational conditions. Charging and discharging processes were studied by varying the HTF flow rate, HTF inlet temperature and flow direction of the HTF. Temperature readings and images of the PCM revealed details of the phase change behaviors in the helical coil PCM heat exchanger. The experimental results show that the HTF inlet temperature has a greater 16
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impact on the charging time than the HTF flow rate. Charging time was reduced by 35% when inlet HTF temperature was increased from 70 to 75 °C. Increasing the flow rate from 0.5 to 4 L/min also reduced the charging time of the storage by 21% whereas the same effect did not result in any reduction in discharging time. Poor heat transfer rate stemming from low thermal conductivity of the paraffin wax was responsible for long charging/discharging times. Discharging processes were much longer than charging processes since only conduction exists as a main mode of heat transfer. It was also found that switching the inlet HTF position from the bottom to the top of the container did not lead to significant change in charging or discharging time, though different temperature variations were observed during discharging. Higher recovery efficiency was achieved at higher flow rates during discharging.
energy storage: a review, Renew. Sustain. Energy Rev. 24 (2013) 418–444. [12] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renew. Sustain. Energy Rev. 11 (9) (2007) 1913–1965. [13] S. Kumar, D. Nagarajan, L.A. Chidambaram, V. Kumaresan, Y. Ding, R. Velraj, Role of PCM addition on stratification behaviour in a thermal storage tank – an experimental study, Energy 115 (1) (2016) 1168–1178. [14] V. Pandiyarajan, M. Chinna Pandian, E. Malan, R. Velarj, R.V. Seeniraj, Experimental investigation on heat recovery from diesel engine exhaust using finned shell and tube heat exchanger and thermal storage system, Appl. Energy 88 (2011) 77–87. [15] Y. Dutil, D.R. Rousse, N. Ben Salah, S. Lassue, L. Zalewski, A review on phasechange materials: mathematical modeling and simulations, Renew. Sustain. Energy Rev. 15 (1) (2011) 112–130. [16] A. Sari, K. Kaygusuz, Thermal performance of palmitic acid as a phase change energy storage material, Energy Convers. Manage. 43 (6) (2002) 863–876. [17] A. Sarı, K. Kaygusuz, Thermal and heat transfer characteristics in a latent heat storage system using lauric acid, Energy Convers. Manage. 43 (18) (2002) 2493–2507. [18] M. Parsazadeh, X. Duan, Numerical and statistical study on melting ofr nanoparticle enhanced phase change material in a shell-and-tube thermal energy storage system, Appl. Therm. Eng. 111 (2017) 950–960. [19] A.I.N. Korti, F.Z. Tlemsani, Experimental investigation of latent heat storage in a coil in PCM storage unit, J. Energy Storage 5 (2016) 177–186. [20] M. Tayssir, S.M. Eldemerdash, R.Y. Sakr, A.R. Elshamy, O.E. Abdellatif, Experimental investigation of melting behavior of PCM by using coil heat source inside cylindrical container, J. Electr. Syst. Inf. Technol. 4 (2017) 18–33. [21] S. Zhang, L. Zhang, X. Yang, X. Yu, F. Duan, L. Jin, X. Meng, Experimental investigation of a spiral tube embedded latent thermal energy storage tank using paraffin as PCM, Energy Procedia 105 (2017) 4543–4548. [22] X. Yang, T. Xiong, J.L. Dong, W.X. Li, Y. Wang, Investigation of the dynamic melting process in a thermal energy storage unit using a helical coil heat exchanger, Energies 10 (8) (2017) 1129. [23] M.J. Huang, P.C. Eames, S. McCormack, P. Griffiths, N.J. Hewitt, Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system, Renew. Energy 36 (11) (2011) 2932–2939. [24] M. Kabbara, D. Groulx, A. Joseph, Experimental study of a latent heat storage unit with a helical coil heat exchanger, CSME Congress (2014). [25] P. Sundaram, R.K. Tiwari, S. Kumar, Experimental performance study of helical coil thermal storage unit filled with PCM, Int. J. Chem. Res. 9 (7) (2016) 2993–2997. [26] A. Dinker, M. Agarwal, G.D. Agarwal, Experimental assessment on thermal storage performance of beeswax in a helical tube embedded storage unit, Appl. Therm. Eng. 111 (2017) 358–368. [27] X. Duan, J. Roul, S. Ryan, S. Hodder, J. Stamp, Solar thermal energy storage with phase change material - heat exchanger design and heat transfer analysis, The 2016 International Conference on Energy Engineering and Environmental Protection (EEEP 2016), December,16-18 (2016). [28] N. Ukrainczyk, S. Kurajica, J. Šipušiæ, Thermophysical comparison of five commercial paraffin waxes as latent heat storage materials, Chem. Biochem. Eng. Q. 24 (2) (2010) 129–137. [29] Engineering ToolBox, https://www.engineeringtoolbox.com/ethylene-glycol-d_ 146.html, (Accessed 15 December 2017). [30] M. Parsazadeh, X. Duan, Numerical study on the effects of fins and nanoparticles in a shell and tube phase change thermal energy storage unit, Appl. Energy 216 (2018) 142–156. [31] H. Ettouney, H. El-Dessouky, E. Al-Kandari, Heat transfer characteristics during melting and solidification of phase change energy storage process, Ind. Eng. Chem. Res. 43 (17) (2004) 5350–5357.
Acknowledgments This study was financially supported by Natural Sciences and Engineering Research Council of Canada (NSERC) with grant # RGPIN 3940-2015 and NL Innovation Council with grant # 5404-1891-101. References [1] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294–303. [2] M. Akgün, O. Aydın, K. Kaygusuz, Thermal energy storage performance of paraffin in a novel tube-in-shell system, Appl. Therm. Eng. 28 (5) (2008) 405–413. [3] D. Zhou, C.Y. Zhao, Experimental investigations on heat transfer in phase change materials (PCMs) embedded in porous materials, Appl. Therm. Eng. 31 (5) (2011) 970–977. [4] A. Sari, A. Karaipekli, Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material, Appl. Therm. Eng. 27 (8) (2007) 1271–1277. [5] A.S. Fleischer, Thermal Energy Storage Using Phase Change Materials: Fundamentals and Applications, Springer, 2015. [6] S.U.S. Choi, J.A. Eastman, Enhancing thermal conductivity of fluids with nanoparticles, in: D.A. Siginer, H.P. Wang (Eds.), Developments and Applications of NonNewtonian Flows, ASME, FED-Vol. 231/MD 66, 1995, pp. 99–105. [7] L. Godson, B. Raja, D. Mohan Lal, S. Wongwises, Enhancement of heat transfer using nanofluids-an overview, Renew. Sustain. Energy Rev. 14 (2) (2010) 629–641. [8] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2) (2009) 318–345. [9] J.N. Chiu, Heat Transfer Aspects of Using Phase Change Material in Thermal Energy Storage Applications, PhD diss. (2011). [10] L. Fan, J.M. Khodadadi, Thermal conductivity enhancement of phase change materials for thermal energy storage: a review, Renew. Sustain. Energy Rev. 15 (1) (2011) 24–46. [11] J.M. Khodadadi, L. Fan, H. Babaei, Thermal conductivity enhancement of nanostructure-based colloidal suspensions utilized as phase change materials for thermal
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Contents lists available at ScienceDirect
Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Design and experimental analysis of a helical coil phase change heat exchanger for thermal energy storage
T
Vahit Saydam, Mohammad Parsazadeh, Musaab Radeef, Xili Duan
⁎
Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John's, NL, A1B 3X5, Canada
ARTICLE INFO
ABSTRACT
Keywords: Thermal energy storage Helical coil heat exchanger Phase change material Paraffin wax
Latent heat energy storage systems have superior features over conventional sensible storage systems. With a large latent heat of fusion, a phase change material (PCM) can absorb and release a great amount of thermal energy at nearly a constant temperature. This improves the capacity and efficiency of the energy storage unit while extending the service time. In this study, a prototype PCM heat exchanger with a helical coil tube is designed, fabricated, and experimentally analyzed for its thermal storage performance under different operational conditions. Paraffin wax is used as PCM and Ethylene glycol (EG)-water mixture is used as heat transfer fluid (HTF). Different HTF inlet temperatures, flow directions, and flow rates were tested to find out the effects of these parameters on the performance, including charging and discharging time, of the thermal storage unit. It was found that the most significant factor during charging is the HTF inlet temperature. The experimental results showed that increasing the HTF inlet temperature from 70 °C to 75 °C shortened the charging time by 35% while charging time was reduced up to 21% with increasing flow rate from 0.5 to 4 L/min. The discharging time, however, did not change substantially with flow rates. It was also found that higher flow rate leads to higher recovery efficiency. The flow direction of the HTF was found to have an insignificant effect on the total charging and discharging time but showed effects on the temperature variations of PCM in the energy storage unit.
1. Introduction Energy from renewable sources, such as solar and wind, is abundant but intermittent in nature, and often with a time lag between energy supply and demand. To exploit these renewable energy resources, it is crucial to develop efficient energy storage systems that can store energy harvested in times of peak supply and provide it in times of high demand [1]. Conventional solar thermal to hot water systems are utilized for residential applications, with relatively low efficiency and limited capacity particularly in periods of scarce sunshine. One alternative is the implementation of Phase Change Materials (PCMs) to store thermal energy [2–4] both sensibly and latently. Latent heat thermal storage systems using PCMs have been proved to be more efficient than sensible heat storage systems thanks to the high energy density of the PCMs [5]. Heat is added to an energy storage unit when hot Heat Transfer Fluid (HTF), such as that from a solar panel, runs through the PCM tank. Water is commonly used as HTF in the latent thermal energy storage units. However, recent studies have proposed the use of water-based nanofluids, which have higher thermal conductivity and improved convective heat transfer than the base fluid [6,7]. Hot HTF transfers energy to the PCM and melts the latter thereby ⁎
storing thermal energy latently. PCMs can be broken down into three categories based on their chemical composition: organic, inorganic, and liquid metals [8]. Liquid metals are characterized by high melting points and hardness in their solid state. Inorganic PCMs are typically used in high temperature range as their melting points are also high. Prior investigations have proved critical disadvantages of inorganic PCMs, i.e., the sub-cooling issue, and corrosion of the containment material [8,9]. Organic PCMs are compounds that contain carbon, and usually hydrogen as well. This type of PCM is arguably the most popular, typically used in relatively low to medium temperature applications. It contains a wide range of compounds including alkane (paraffin) and fatty acids family. Among those, paraffin wax is favorable due to its abundance, high latent heat of fusion, chemically stability, and its compatibility with a wide range of materials with no corrosion in the containment material. Nevertheless, paraffin has low thermal conductivity [9], which makes the heat transfer rate low and leads to long charging and discharging times. This issue has been addressed by various heat transfer enhancement techniques [10–12]. Different designs of phase change energy storage systems have been studied [13–15]. A tube-and-shell unit is one of the simplest designs and among the most commonly used [16–18]. Numerous factors must
Corresponding author. E-mail address: [email protected] (X. Duan).
https://doi.org/10.1016/j.est.2018.11.006 Received 20 February 2018; Received in revised form 3 November 2018; Accepted 5 November 2018 2352-152X/ © 2018 Elsevier Ltd. All rights reserved.
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study aims to examine the detailed phase change behaviors of paraffin wax in a helical coil PCM heat exchanger, which were not well understood in previous studies. These data are useful in improving the design of helical coil PCM heat exchangers for thermal energy storage applications.
Table 1 Properties of PCM and HTF. Property
Values
mpcm
3.54 kg
Cp, pcm, l
2.981 J/g·K [28]
kpcm Cp, pcm, s
Hpcm Ti Tm TF mHTF Cp, HTF
0.34 W/m·K (solid), 0.28 W/m·K (liquid) 2.6 J/g·K [28]
2. PCM heat exchanger design and experimental setup
160 kJ/kg
2.1. Theoretical design of the PCM heat exchanger
18 °C 51 °C 70 °C 0.07-0.00875 kg/s 3.56 J/g·K [29]
Actual melting and solidification processes involve both latent heat and sensible heat due to the temperature change of the PCM. The amount of heat stored in the PCM can be theoretically calculated with Eq. (1):
be considered during the design process. In residential applications, the charging time plays a vital role in determining the functionality of the system, hence lengthy charging time is highly undesirable. Fabrication cost is another decisive factor, designing the system to be small can help control fabrication costs. Compactness and high efficiency of the design are achieved by a higher overall heat transfer rate between the HTF and the PCM. Another factor is the pressure drop developed from frictional losses through the heat exchanger. The optimal design will aim to limit the local pressure drop while not compromising the device’s performance. Among the several heat exchanger designs, the helical coil configuration stands out due to increased heat transfer surface area, which propelled several researchers to incorporate the helical coil design in their studies [19–26]; the prevailing helical coil configuration among those studies is the vertical configuration [19–24]. Nonetheless, horizontal configuration of the helical coil was explored as well [25,26]. Previous studies mostly focused on the effects of HTF temperature and flow rate on the charging and discharging processes. One of the major findings was the greater influence of the HTF inlet temperature on the charging and discharging performance compared to that of the flow rate [19–22]. More uniform charging was observed at lower HTF inlet temperature [19,25]. But these studies have not considered the effects of HTF flow direction for vertical coil configuration. The effect of HTF flow direction was found to be significant during melting in a simple shell and tube thermal storage unit [18,30]. Since natural convection of the melted PCM is significant, the flow direction of HTF might affect the heat transfer process in a coil configuration as well. This project aims to design, fabricate, and analyze a helical coil thermal energy storage unit with paraffin wax as the phase change material. The performance of the PCM thermal energy storage unit under different operational conditions is investigated. Melting and solidification characteristics of the PCM are examined with varying the HTF flow rate, inlet HTF temperature and the HTF flow direction. In addition, this
Qstored = mpcm [ Cp, pcm, s (Tm
Ti ) + Hpcm + Cp, pcm, l (Tf
Tm)]
(1)
where mpcm is the mass of the paraffin wax, Hpcm is the latent heat capacity of the paraffin wax, Ti is the initial temperature of the wax, Tm is the melting temperature of the wax, TF is the final temperature of the wax at the end of charging, Cp, pcm, s and Cp, pcm, l are the specific heat of the wax in solid and liquid phase respectively. The latent and sensible energy calculations assumed that the temperature of the wax is increased from 18 °C to 70 °C. The values of these parameters are summarized in Table 1. During the experiment, the heat supply or extraction rate during charging or discharging can be found from the following equation for the HTF [14]:
Qf = mHTF × Cp, HTF ×
Tf
(2)
Total energy supplied or recovered by the HTF during charging and discharging can be found by Eq. (3):
Qft =
t × Qf
(3)
where mHTF is the mass flow rate of the HTF, Cp, HTF is the specific heat of the HTF, Tf is the temperature difference between the HTF inlet and outlet, and t is the duration of each time step (20 s in the experiment). 2.2. Prototype design and fabrication An overall shell-and-tube heat exchanger lay out was selected for the prototype design. Different concepts for the tube configuration, HTF pattern, and insulation were considered and evaluated [27]. The final selection of design parameters is summarized here. Paraffin wax is selected as the PCM material with rationales discussed in the introduction. Paraffin has an excellent latent heat of fusion, with a desirable melting temperature (∼51 °C) for residential warm water applications. EG-water mixture was selected as the heat transfer fluid by the virtue of
Fig. 1. Fabrication of the PCM heat exchanger, (a) the top end plate, (b) the helical coil and shell, and (c) the whole thermal energy storage unit with insulation. 10
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its low cost, availability, and higher safety in handling. Polycarbonate was selected for the shell of this experimental prototype. Polycarbonate is durable, available, and easy to use with great heat/corrosion resistance. With its inherently low thermal conductivity, this material exhibits the behavior of a pseudo-insulator. In addition, polycarbonate has good transparency, which allows the monitoring of the PCM melting and solidification processes during lab testing. A helical tube design was selected for final fabrication. Preliminary CFD simulation and lab testing [27] proved that the spiral tube has better heat transfer performance than a straight one. The HTF tube was made from copper, which was chosen for its exceptionally high thermal conductivity, low cost, and availability. Figs. 1 and 2b show the key components and assembly dimensions of the PCM heat exchanger. The final design has the following dimensions: 29.53 cm (1 ft) in length and 16.64 cm (6.55 in. in outer diameter. The top and bottom ends of the container were attached to two square grooved acrylic plates having a thickness of 0.9525 cm (3/8 in. and 19.05 cm (7.5 in. in length (Fig. 1a). Four holes near each corner of the plates were drilled so stainless-steel rods can later be inserted through the holes on the upper and lower plates to hold the assembly together by fastening the nuts. Three additional holes were drilled to accommodate a thermocouple rod, an alignment rod which helps keep the helical coil in line, and the last hole was used for PCM filling and draining purposes. The shell is covered with insulation blanket made of cryogel (Scotia Insulations Co.), an advanced insulation material; the top and the bottom plates were also covered to prevent heat loss. The helical coil tubing was made from a 315 cm (124 in. long copper pipe with an inner and outer diameter of 0.635 cm (1/4 in. and 1.905 cm (3/ 8 in. respectively. The coil diameter was chosen to be 10.16 cm (4 in. with a pitch of 2.8575 cm (1.125 in. yielding 8.7 turns and a 7.62 cm (3 in. long straight tubing section extending from the center of the helical coil at each end (Fig. 1b). The fully prepared PCM thermal energy storage unit with insulation is shown in Fig. 1c. After placing the helical coil inside the storage tank, the heat exchanger prototype was sealed using a silicon caulking. Silicon sealed the gaps between the endcaps and the Lexan shell, and between the plastic tubing and end-cap penetrations. Paraffin wax in liquid phase was poured into the storage through the filling hole. After pouring the molten wax into the storage unit, the whole setup was left to cool down at room temperature (20 °C) to let the PCM completely solidify for lab testing. 2.3. Experimental setup A schematic of the experimental setup is shown in Fig. 2a. The main components include the cylindrical PCM heat exchanger with helical tubing, a thermal bath circulator, and a computer (PC) connected to a data acquisition system. The thermal bath circulator provides EG-water as HTF at desired temperatures for both charging and discharging experiments. The outlet of the thermal bath is connected to a three-way ball valve which allows for manual adjustments of the HTF flow rate.
Fig. 2. (a) Schematic of the experimental setup; (b) Drawing of the experimental PCM heat exchanger (dimensions in cm) with positions of the thermocouples, T1: Side bottom, T2: Center bottom, T3: Center middle, T4: Center Top, T5: HTF Inlet, T6: HTF Outlet. Table 2 Experiments under different operational conditions. Test #
Test Type
HTF Flow rate, LPM
HTF Inlet temperature, °C
HTF Flow direction
1 2 3 4 5 6 7 8 9 10 11
Charging Charging Charging Charging Charging Charging Discharging Discharging Discharging Discharging Discharging
4 2 1 0.5 4 4 2 1.5 1 0.5 1.5
75 75 75 75 70 75 20 20 20 20 20
Upward Upward Upward Upward Upward Downward Upward Upward Upward Upward Downward
Fig. 3. Comparison of charging and discharging times at different operational conditions. 11
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(6.875 in). The top thermocouple was placed just an inch (2.54 cm) above the middle thermocouple due to significant shrinkage upon solidification. Two thermocouples (T5, T6) are implemented to monitor the inlet and outlet temperatures of the HTF. Another thermocouple (T1) was inserted at the same height as the bottom center thermocouple but 6.35 cm apart from it to monitor the radial temperature difference. The bottom plate is the datum for the previous height measurements. The thermocouples were calibrated, and the uncertainty was found to be ± 1 °C, which is acceptable for this experimental study. 3. Results and discussions This section presents the results of the experiments under different operational conditions. Three main parameters, namely, HTF flow rate, HTF inlet temperature and HTF flow direction were investigated. The ranges of these parameters were chosen based on the capability of the thermal bath and the flowmeter used in the experiment. For all charging tests, the initial PCM temperature in the storage unit is 20 °C, a common room temperature in most residential warm water applications. For this type of applications, the charging HTF temperatures in previous studies, for example [16,19,24], are generally in the range of 70 °C–85 °C. In this study, the HTF inlet temperature is set to be in the range of 70–75 °C. In the discharging experiments, the initial PCM temperature is 69 °C and the HTF inlet temperature is set to be 20 °C. In addition, the lower limit of the flowmeter used in this study is 0.5 LPM (litre per minute, or L/min), while the maximum flow rate provided by the pump is 4 LPM. The operational conditions of 11 tests are summarized in Table 2. The objectives of these experiments are to determine the effects of each parameter on the performance of the
Fig. 4. Charging with a flow rate of 4 LPM at 75 °C inlet HTF temperature.
The flow rate of the HTF is monitored using a flow meter connected to the inlet line of the storage unit. The excess amount of HTF is directed back to the thermal bath through a bypass line. The outlet pipe connected to the top of the storage merges with the bypass line going back to the thermal bath. The thermocouples and flow meter are connected to the data acquisition (DAQ) system to record the measured data on the computer. Six T-type thermocouples are used to track the temperatures within the storage unit (see Fig. 2b). Three thermocouples (T2, T3, T4) are placed at the center but three different heights of the cylindrical tank. The bottom thermocouple was positioned 2.22 cm (0.875 in. above the bottom plate in the center. The middle thermocouple was attached to the same rod as the bottom center thermocouple at a height of 17.46 cm
Fig. 5. Pictures of charging with a flow rate of 4 LPM at 75 °C inlet temperature (a:30 min, b:60 min, c:90 min, d:120 min, e:145 min, f:170 min). 12
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clear in the literature and are often taken as the times when temperatures in the PCM become steady and close to the HTF temperature. In this study, the charging time is defined as the total time to attain a temperature of 69 °C from all thermocouples in the storage unit. Similarly, the discharging time for the storage unit is determined when all thermocouples in the storage unit read below 25 °C. Note that the total charging/discharging time includes both sensible and latent heat transfer processes. The subsequent sections present detailed discussions of the experimental data from these tests, including temperatures of the PCM, images of the melting or solidification process, HTF inlet and out temperature differences, energy addition/recovery rates, and energy recovery efficiency of the storage unit. 3.1. Charging Charging tests were carried out to investigate the melting behavior and how phase change evolves inside the storage unit. Fig. 4 illustrates the temperature profile of paraffin wax at different locations with HTF flow rate of 4 LPM at 75 °C inlet temperature. The melting behavior is observed by taking pictures of the system every half an hour; the pictures tracked the motion of the liquid/solid interface (Fig. 5). These results demonstrate that PCM melting initiated at the circumferential area of the helical coil; the closer the wax is to the coil, the earlier melting commences. This is also confirmed by thermocouple readings, since temperatures from the side bottom thermocouple are higher than those from the thermocouples placed in the center (Fig. 4). This also explains the phenomena observed in Fig. 5c where a big chunk of unmelted wax of a mountain shape exists at the center of the heat exchanger. As the melted portion grows, the liquid wax is pushed upward by buoyant force to trigger natural convection, which is the dominant heat transfer mechanism onwards (Fig. 5b). In later charging stages, the PCM temperature readings abruptly increase from the center top, and center middle thermocouples, due to the increased heat transfer through natural convection. Meanwhile, temperatures at the bottom increase linearly, since heat conduction dominate there. The rate of temperature increase at the top and middle thermocouple locations slows down after that sudden increase as more energy supplied from HTF is used to energize the phase change process rather than to increase sensible heat. It is evident that steep temperature increase at the top and middle thermocouples is present once the liquid/solid interface passes these probe locations and moves downwards over time (Fig. 5 d and e). This could be attributed to the significantly less energy required to increase the temperature of the PCM as opposed to causing phase change from solid to liquid. At the later stage of charging, the big conically-shaped chunk of PCM undergoes phase change and gradually disappears (Fig. 5d–f). Finally, the same sudden temperature increase is seen for the
Fig. 6. (a) HTF inlet and outlet temperature difference, (b) energy addition rate, and (c) cumulative energy supply during charging.
designed PCM storage unit, including the charging and discharging times, the energy storage and recovery rates, and energy recovery efficiency, etc. Fig. 3 shows the comparison of charging and discharging times for all the tests. This is only an overview of the results to show a general trend of the charging/discharging speed under different operational conditions. The definitions of charging and discharging times are not
Fig. 7. Discharging with a flow rate of 1 LPM at 20 °C inlet HTF temperature. 13
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Fig. 8. Pictures of the discharging process with a flow rate of 1 LPM at 75 °C inlet temperature (a:0 min, b:30 min, c:150 min).
thermocouples at the bottom with the one in the center being more distinct. It was found that the trend of the temperature increment during charging is very similar to [19]. However, the charging duration is longer as the current thermal storage unit is larger. The temperature differences at the inlet and outlet of the HTF, ΔTf, for four different flow rates during the charging tests are shown in Fig. 6a. The results show that lower flow rates lead to larger fluid temperature differences. This is expected since lower flow rates give the fluid more time to release heat to the PCM. The HTF inlet/outlet temperature difference is high at the beginning of the charging process for all flow rates, and then it decreases toward the end of the charging process. This is due to the decreasing heat transfer rate with decreasing temperature difference between the PCM and the HTF. When the flow rate is low, i.e., < 2 LPM, there is a sudden decrease of ΔTf before it levels out. This sudden decrease, shown approximately with the dashed line in Fig. 6a, may serve as an indicator of initial melting around the HTF tubing. The liquid PCM has lower thermal conductivity and causes a reduction in the heat transfer rate. However, such a behavior is not observable with the high flow rate of 4 LPM, since the ΔTf is very low during the whole charging process as the flow rate is very high. With these fluid temperature differences, the energy addition rates from the HTF are calculated and shown in Fig.6 b. At a given flow rate, the energy addition rate decreases with time with a similar trend of the ΔTf, which is expected from Eq. (2). This equation also predicts that the energy addition rate is determined by the product of the flow rate and the fluid temperature difference. Therefore, neither the case of highest flow rate (4 L/min), nor the one with highest fluid temperature difference (0.5 L/min) brings the highest energy addition rate. It’s the case of medium flow rate of 1 L/min that provides the highest energy addition rate from the HTF, as shown in Fig. 6b. This flow rate also brings the highest cumulative energy supply, as shown in Fig. 6c. In contrast, the lowest cumulative energy is supplied at the highest flow rate of 4 L/ min.
takes place at around 55 °C. The foreign structures within the storage unit, such as the metal rods, provide nucleation sites and therefore alleviate the supercooling effect. Due to its proximity to the helical coil, the side bottom thermocouple readings diverge from the ones in the center and decrease rapidly right after the phase change completes. After that, the heat extracted from the PCM decreases drastically, leading to insignificant temperature differences between the inlet and outlet HTF temperatures due to increased thermal resistance in the solid layer. The completion of phase change is followed by gradual cool down of the solid wax to the cold HTF temperature, with the center middle thermocouple being the slowest. Overall, the results confirm that the discharging process takes much longer time compared to the charging process (also in Fig. 3), due to poor conduction heat transfer within the solid paraffin wax. The discharging trend is very similar to the results in [19], although the current discharging duration is longer due to the larger capacity of the thermal storage unit. The temperature differences between the inlet and outlet of the HTF, ΔTf, are small for all four different flow rates in the discharging tests, as illuminated in Fig. 9a. These temperature differences are even smaller than those in the charging time depicted in Fig. 6a, due to the lower thermal conductivity of the liquid phase PCM compared with that of the solid PCM at early stage of charging. The ΔTf also decreases with time due to the increase of PCM temperature and the consequently decreasing heat transfer rate between the liquid and PCM. In discharging, the HTF flow rate shows less effects on the ΔTf. Therefore from Eq. (2) higher energy recovery rate is achieved at higher flow rates, which is shown in Fig. 9b. As a result, the cumulative energy recovery from the storage unit is higher at higher flow rates, as shown in Fig. 9c. 3.3. The effects of HTF flow rate on charing and discharging The effects of HTF flow rate on the charging and discharging processes were examined by varying the flow rates for the same inlet HTF temperature (75 °C). Results in Fig. 10 show that higher flow rates result in faster melting of the PCM. Faster charging is attributed to the increased forced convection between the HTF and inner surface of the helical coil, leading to increased overall heat transfer between the HTF and the PCM [22]. After warming up the solid PCM at the beginning of charging, melting starts, and natural convection takes control; PCM temperatures reveal higher heat transfer rate with higher flow rate (with more rapid temperature increases). The results indicate that the onset of natural convection is earlier at higher flow rate. The charging times with different HTF flow rates are shown in Fig. 3. These data show that increasing the flow rate from 0.5 L/min to 4 L/min decreases the charging time by 21%.
3.2. Discharging Fig. 7 shows the temperature variations in the storage unit during discharging with an HTF flow rate of 1 LPM at 20 °C inlet temperature. Significant drop in temperature for all the thermocouples is noted at the beginning of the discharging process due to the sensible energy loss. It results in considerable temperature difference between the inlet and outlet of the HTF. Solidification starts from the outer surface of the helical coil and inner wall of the shell (the latter is due to heat loss). This obstructs visual tracking of the solidification behavior of inner layers of the PCM (Fig. 8). As can be seen from Fig. 7, the liquid-solid transformation 14
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Fig. 10. Temperature profile of center top thermocouple at different flow rates during charging at 75 °C HTF inlet temperature.
Fig. 11. Temperature profile of the center bottom thermocouple at different flow rates during charging at 75 °C HFT inlet temperature.
Fig. 12. Temperature profile of the center top thermocouple at different flow rates during discharging at 20 °C HTF inlet temperature.
Fig. 9. (a) HTF inlet and outlet temperature difference, (b) energy extraction rate, and (c) cumulative energy recovery during discharging.
solid transition. After that, temperature remains at around 55 °C for a long time indicating the liquid-solid phase change transformation. Upon the completion of solidification around 50 °C, the temperature decreases almost linearly regardless of flow rate. The discharging process takes 5 h 32 min with an HTF flow rate of 0.5 L/min. Higher flow rates of the HTF do not reduce discharging time, as more clearly shown in Fig. 3. Although convective heat transfer rate increases with higher flow rate in the helical coil, the innate low thermal conductivity of the wax dominates the overall heat transfer process during discharge and prevents any reductions in the discharging time, which agrees with previous studies [16,24].
Temperature variations show different trends at other locations in the storage unit. It is seen from Fig. 11 that the for most of the time, the temperature increases linearly at the center bottom location before melting as it is the last part of the region to be melted. As discussed earlier, higher flow rate leads to earlier melting and onset of natural convection. After melting, the increased heat transfer at higher flow rates leads to rapid temperature increase. As compared to Fig. 11, results in Fig. 12 indicate that there is not much change in the readings of the center top thermocouple during discharging at different flow rates. At the beginning of discharge, a rapid drop in temperature is seen before the PCM goes through liquid15
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be more uniform at 70 °C inlet HTF temperature as opposed to a separation of top and middle thermocouples due to dramatic temperature rise at 75 °C inlet temperature. 3.5. Effects of HTF flow direction on charging and discharging Another factor investigated during the charging and discharging tests is the flow direction of HTF. A previous numerical study on a vertical tube-in-shell configuration showed that upward flow decreases the charging time due to enhanced natural convection effects [30]. In the upward flow, the hotter molten PCM at the bottom rises up due to its lower density. This effect has been shown to accelerate melting [31]. However, our experimental results in Fig.3 indicate that switching the HTF inlet from bottom to top does not lead to noticeable change in charging or discharging time. Results in Fig. 14 also show similar PCM temperature variations with different HTF flow directions during charging. One possible explanation is related to these high flow rates of HTF, which lead to very small difference between the inlet and outlet temperatures of the HTF. Hence the flow direction either being downward or upward does not cause any change in charging or discharging time. A very low HTF flow rate may give a different trend, but this study didn’t consider lower flow rates than 0.5 LPM due to the limitations of our flow meter. Furthermore, a very low flow rate would make the discharging or charging time too long for feasible lab test and not useful for engineering applications. For discharging, PCM temperature results in Fig. 15 indeed show some differences, although the total discharging time does not change much with switching the flow direction. Supplying cold HTF from the top of the storage unit results in longer phase transition (maintaining the temperature at around 55 °C). This might be due to the formation of weak natural convection with circulation of liquid wax when the cold HTF flow is downward, since cooled liquid wax on the top portion of the storage unit is replaced by the warmer liquid wax rising from the bottom. Therefore, more uniform temperature distribution is maintained. However, upon completion of the phase change, the PCM temperature decreased more quickly compared to the case where cold HTF is supplied from the bottom, eventually leading to similar overall discharging time.
Fig. 13. Temperature profile of the center top thermocouple at different inlet temperatures of HTF at 4 LPM during charging.
Fig. 14. Temperature profile of the center bottom thermocouple at different flow directions with 4 LPM during charging at 75 °C.
3.6. Energy recovery efficiency Experimental energy recovery efficiency can be determined from the ratio of the recovered energy by the HTF during discharging to the energy supplied by the HTF during charging, both calculated using Eqs. (2) and (3). The recovery efficiency of the storage unit can then be determined using Eq. (4).
=
Qf ,discharging Qrecovered = Qsupplied Qf ,charging
(4)
The recovery efficiency of the storage unit ranges between 35–62 % for the tested discharging flow rates. The highest efficiency was found at a HTF flow rate of 2 LPM while the lowest efficiency was at 0.5 LPM. It should be noted that these calculations did not consider the heat losses from the storage container to the surrounding environment.
Fig. 15. Temperature profile of the center bottom thermocouple at different flow directions with 1 LPM during discharging at 20 °C.
3.4. Effects of HTF inlet temperature on charging
4. Conclusions
The effects of inlet temperature of the HTF on charging time was also investigated. The charging tests were conducted at 70 and 75 °C for the same flow rate (4 L/min). Increasing the inlet temperature from 70 to 75 °C reduced the charging time by 35% (Fig. 3). It is also shown in Fig. 13 that the PCM temperature increase from the center top thermocouple was identical for both cases at the initial state. However, higher inlet temperature leads to faster melting and triggers the earlier onset of natural convection which dominates the melting process during charging [26,30]. The PCM temperature increase was found to
A PCM heat exchanger for latent heat storage of thermal energy was designed, fabricated, and analyzed experimentally. Performance of the thermal energy storage unit is investigated under different operational conditions. Charging and discharging processes were studied by varying the HTF flow rate, HTF inlet temperature and flow direction of the HTF. Temperature readings and images of the PCM revealed details of the phase change behaviors in the helical coil PCM heat exchanger. The experimental results show that the HTF inlet temperature has a greater 16
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impact on the charging time than the HTF flow rate. Charging time was reduced by 35% when inlet HTF temperature was increased from 70 to 75 °C. Increasing the flow rate from 0.5 to 4 L/min also reduced the charging time of the storage by 21% whereas the same effect did not result in any reduction in discharging time. Poor heat transfer rate stemming from low thermal conductivity of the paraffin wax was responsible for long charging/discharging times. Discharging processes were much longer than charging processes since only conduction exists as a main mode of heat transfer. It was also found that switching the inlet HTF position from the bottom to the top of the container did not lead to significant change in charging or discharging time, though different temperature variations were observed during discharging. Higher recovery efficiency was achieved at higher flow rates during discharging.
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