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Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry
Accepted Manuscript Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry Yubo Miao, Hongtao Xu, Qiang Gao PII: DOI: Article Number: Reference:
S2451-9049(18)30234-8 https://doi.org/10.1016/j.tsep.2019.100362 100362 TSEP 100362
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
31 March 2018 22 November 2018 28 May 2019
Please cite this article as: Y. Miao, H. Xu, Q. Gao, Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry, Thermal Science and Engineering Progress (2019), doi: https://doi.org/10.1016/j.tsep.2019.100362
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Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry Yubo Miao, Hongtao Xu*, Qiang Gao aSchool
of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, PR China *Corresponding Author. Tel: 021–55272015; E-mail: [email protected]
Abstract Microencapsulated phase change material (MPCM) slurry has received attention due to their higher thermal storage density, was introduced as a storage medium in latent thermal energy storage (LTES) system. Based on this background, this article presented the experimental results of the thermal storage and heat transfer characteristics of MPCM slurry during cooling discharging process, a series of experiments were carried out by controlling variable. Hexadecane (C16H34) and urea-formaldehyde was used as the core and shell of PCM to form MPCM with core–shell ratio of 7:3 by weight. The temperature evolutions of the MPCM slurry were obtained during the experiment, and the volumetric thermal storage capacity, cooling discharging rate were calculated under different stirring rates and heat transfer fluid (HTF) initial inlet temperatures. From the experimental results, it is estimated that the thermal storage and heat transfer performance of MPCM slurry both were enhanced remarkably by phase change. In addition, accelerating the stirring rate of stirrer can enhance the thermal storage capacity. At 300 rpm, the volumetric thermal storage capacity and mean discharging rate are about 1.28 times higher than that of MPCM slurry without stirring. At the same stirring rate, the mean discharging rate of 20 ºC is 1.24 times higher than that of 10 ºC. Keywords: MPCM slurry; LTES; Cooling discharging rate; Volumetric thermal storage capacity; Nomenclature A cp E EV ΔE M m P Rt T ΔT ΔU
heat transfer area, m2 specific heat, kJ/( kg·K) energy, J volumetric thermal energy storage capacity, J/m3 cooling energy storage, J mass flow rate, kg/s mass, kg mean charging/discharging rate, W product of heat transfer coefficient and heat transfer area, W/◦C temperature, ◦C temperature difference, ◦C sensible heat storage
Greek symbols τ
time, s
Subscripts c ch d e h i lab loss m t w
charging chiller discharging end heating device initial laboratory heat loss mean storage tank, storage medium water
1 Introduction Energy consumption has grown rapidly in recent decades due to the growth of economy, business and population. Therefore, new technologies of conserving available energy and improving its utilization are the focuses of the current and future researches [1]. LTES has been proven to be an effective technology to improve the energy efficiency because of large latent storage capacity and small temperature variation from storage to extraction. Due to the above outstanding performance, LTES can be applied in many occasions, such as building energy conservation systems, air-conditioning and refrigeration systems, electronic thermal management and solar thermal systems [2][4]. Apparently, the performance of a LTES unit is influenced by the thermophysical properties of the thermal storage medium. Starting from early 1980s, using phase change material (PCM) for thermal storage has gained considerable interest because of its desirable thermophysical properties, such as large latent heat, good thermal and chemical stabilities, and environment friendly. However, a barrier to the wide use of paraffin is the low conductivity of most PCM, which lead to low rate of heat transfer during charging and discharging of LTES, and as a result practical LTES units have been difficult to develop. Several approaches have been developed for the purpose of heat transfer enhancement, including the use of fins in the thermal energy storage unit [6], addition of high conductive fibers [10], and immersion of porous metal foam [11]. In recent years, a new technique is proposed, in which the phase change material is microencapsulated and suspended in a conventional single-phase heat transfer fluid to form microencapsulated phase change material slurry. Such slurry has a large surface area to volume ratio, so the heat can be quickly absorbed and released from the core to the carrier fluid, which means a higher transfer rate compared with traditional TES using PCM without microencapsulating. In addition, the core material is separated from the carrier fluid by microencapsulating the PCM particles, and therefore the deposition and
agglomeration of particles in the slurry are avoided [12]. Several studies on the flow and heat transfer characteristics of MPCM slurry have been investigated in the past by a number of researchers. Liu et al.[13] developed a numerical model of laminar forced convection heat transfer for MPCM slirry with constant heat flux, they found mass concentration and Stefan number are the most influential parameters on the improvement of heat transfer in MPCM slurries. Sabbah et al. [14] developed a three-dimensional, one-phase, laminar flow model of a rectangular channel using MPCM slurry with temperature dependent physical properties, they found that the heat transfer coefficient is mainly dependent on the channel inlet and outlet temperatures and the selected MPCM melting temperature. Ma et al. [15] investigated the flow and heat transfer characteristics of MPCM slurry in a horizontal pipe under constant heat, they found that small MPCM particle and high inlet velocity results in high heat transfer coefficient. Languri et al. [16] performed the numerical and analytical modeling to study the flow of MPCM slurry through helically coiled heat exchangers, the result indicated that the radius of helically coiled influence the heat transfer and MPCM slurry at low mass fraction can be treated as homogenous heat transfer fluids. Inaba et al. [17] proposed a computation model to analyze natural convection in a rectangular enclosure with non-Newtonian MPCM slurry. It was observed that the heat transfer coefficients are higher when compared to slurries without phase change and when the PCM concentration increased within the range 20–40%, the convection coefficient decreased, while for the range 10–20%, the convective coefficient increased. Several experiments were also conducted to deal with the flow and heat transfer characteristics of MPCM slurry. Goel et al. [18] experimentally investigated the laminar, hydrodynamically fully developed flows in a circular duct with a constant wall heat flux. They found the most dominant parameter influencing the heat transfer is the bulk Stefan number and the effect of concentration is insignificant. Wang et al. [19] measured the pressure drop and local heat transfer coefficients in order to investigate the flow and convective heat transfer behaviors of MPCM slurry in a horizontal circular tube, and they found heat transfer coefficients of MPCM slurry are significantly higher than single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Li et al. [20] investigated the flow characteristic of the slurries flowing in a horizontal circular tube flow, they found that compared to water the heat density of MPCM slurry is higher and it can save pumping consumption accordingly. Ma et al. [21] investigated the thermofluidic performances of the MPCM slurry in laminar flow in the mini-tubes, results showed that the heat transfer performance of the MPCM slurry is significantly better than water at similar Reynolds number and the heat transfer coefficient increase at higher mass fraction. Kong et al. [22] experimentally investigated the flow and heat transfer characteristics of MPCM slurry as a HTF in a heated helically coiled tube. Results showed that Nusselt number of MPCM slurry significantly increases during the phase change process, except transporting more energy, MPCM slurry has no advantage over that of water in the aspect of heat transfer performance. Alvarado et al. [23] conducted turbulent flow heat transfer experiments to determine the convective heat transfer coefficient of MPCM slurry. Results showed MPCM slurry convective heat transfer
coefficients are typically lower than that of water under the constant heat flux, and the heat transfer coefficient increases considerably during the phase change process. The primary characteristics of above MPCM slurries studied by researchers are summarized in Table 1. Table 1 Characteristics of different TES systems containing MPCM slurry. Thermal Particle Fusion Phase conductivity PCM diameter MPCM heat of change of of Core material mass slurry PCM temperature PCM(solid) MPCM fraction (J/g) (°C) (W/m•K) (μm) Liu et al. [13]
n-Octadecane
263
23–29
0.25
5–20%
-
Sabbah et al. [14]
-
110
-
0.2
5-25%
10
-
167
27
0.31
1225%
-
-
-
-
-
5.910.9%
-
-
-
30
-
1040%
-
n-Eicosane
-
36
0.15
5-20%
-
160
14.3–17.2
0.14
5– 27.6%
10.1
196.5
78–85
-
15–25 %
10
Ma et al. [15] Languri et al. [16] Inaba et al. [17] Goel et al. [18]
Wang et al. Error! Reference Bromohexadecae source not found. Li et al. Error! Reference source not found.
Paraffin
Ma et al.
n-Hexadecane
226.8
13.3715.92
-
1020%
-
Kong et al. Error! Reference source not found.
Butyl stearate
88
17.4–22.2
0.23
2.1– 10.9%
5–10
Alvarado et al.[23]
Tetradecane
215
5.5
-
-
2-10
[21]
The effective and appropriate applications of MPCM slurries can reduce the buildings energy consumption and associated carbon emission in buildings [24]. The cooled ceiling (CC) system employed the cooled ceiling to treat a sensible cooling load and used an independent HVAC system for the ventilation and humidity control. As the energy storage medium, MPCM slurry can be used in a cooled ceiling (CC) system. However, more experiments are needed to evaluate whether a competitive thermal storage and heat transfer performance can be achieved. To the authors' knowledge, the literature available on the natural convection process outside a helical coil is limited. Heinz and Streicher [25] indicated that the natural convective heat transfer coefficient of 20% mass concentration is approximately 30% lower than that of water because the viscosity of MPCM slurry is high. Similar conclusions were drawn by Huang et al. [26], they found that the MPCM slurry with 50% mass concentration is not applicable, because the low thermal conductivity and high viscosity reduced the heat transfer from the heat exchanger to the storage media. Diaconu et al. [27] found that the natural convective coefficient for MPCM slurry during the phase change process could be up to five times compare to water. Zhang and Niu [28] found that In the CC system, MPCM slurry capacity of volumetric thermal energy storage is found higher, whereas the discharging rate is still considerably lower than that of water. This article aims to obtain reliable experimental data of the thermal storage and heat transfer behaviors of MPCM slurry at different parameters by building a coil-in-tank LTES unit. Experimental setup was validated with experimental data of a coil-in-tank water system. In order to introduce forced convection, an axial stirrer with variable rating speeds was added into the LTES system. The temperature evolutions of MPCM slurry, volumetric thermal storage capacity and cooling discharging rate are reported under different initial inlet temperatures of the HTF and stirring rate of stirrer. These test results will be used to assess the suitability of MPCM slurry for integration into building cooling applications. 2 Experimental approach 2.1 Materials
The PCM and micro-encapsulated PCM were supplied by Aerospace Institute of Advanced Material & Processing Technology. The MPCM slurry was synthesized by in situ polymerization [29], urea-formaldehyde was used as the shell, and hexadecane (C16H34) was used as the core PCM material, with a melting temperature of 18 ºC and latent heat of 224 kJ/kg [30]. The core-shell ratio was 7:3 by weight, and the average diameter of capsules measured by a particle size analyzer is 12.5 μm. The flow diagram of in situ polymerization is in Fig. 1.Pure water was used as carrier fluid. The mass fraction of the MPCM slurry used in this study was 25%. The fluid could be analysed as Newtonian when the microcapsule volumetric concentration was less than 30% [31]. Fig. 2 shows the optical and SEM photographs of microcapsules used in the present study.
Fig. 1. Flow diagram of in situ polymerization
Fig. 2. (a) Optical micrograph and (b) Scanning electron microscopy photograph. The thermal storage and heat transfer of MPCM slurry in the storage tank are associated with the following properties: density, specific heat capacity, thermal conductivity and latent heat, which depend on the type of PCM, shell material, its carrier fluid and the particle concentrations. These properties given in Table 2 were calculated from the weighted fraction of the individual properties of PCM, coating material and water based on the thermal property models that have been validated and applied by various researchers, the details of which can be found in [19]. Table 2 Physical properties of MPCM slurry and its components Thermal Density Specific heat Latent heat conductivity kg/m3 J/(kg• ºC) kJ/kg W/(m• ºC)
Hexadecane(solid) Hexadecane(liquid) Urea-formaldehyde Water(at 20 ºC) MPCM particle(solid) MPCM particle(liquid) MPCM slurry (mass fraction)Φ=0.25
780 770 1490 998 960 950
1805 2221 1675 4183 1766 2057
0.4 0.21 0.433 0.599 0.382 0.203
224
990
3579
0.578
44
177
2.2 Experimental system An experimental system (Fig. 3) has been built to investigate the thermal storage and heat transfer performance of a coil in tank MPCM/water slurry cooling storage system. In order to avoid mechanical damage to the MPCM capsules by the pump, water was used as the HTF in both the chiller loop and heating device loop. Two helical coil heat exchangers immersed in a cylindrical tank, HE1 and HE2, were made of copper tube of 15 mm in diameter and 11m in length. The storage tank (Fig. 4) is a cylindrical stainless steel tank with an insulation layer of 40 mm polyethylene insulation layer. The diameter of the tank is 0.3 m, and the height is 0.5 m. The internal volume of the tank minus the volume occupied by the internal coils is 0.0278 m3. A variable speed stirrer driven by AC power was installed above the tank to keep the slurry mixed ideally and to generate forced convection during the charging and discharging processes.
Fig. 3. Schematic diagram of the experimental apparatus
Fig. 4. Detailed diagram of the storage tank (a) storage tank plan (b) site photograph of thermal storage tank For the sake of measuring the system operation parameter, we installed twenty Ttype thermocouples (±0.5 ºC, operating range of 0 to 50 ºC) and three rotameters (± 4%, operating range of 0 to 7 L/min).The temperatures (T2, T3, T6, T7) of water were measured by the thermocouples placed at the inlets and outlets of the two heat exchangers, HE1 and HE2, respectively. Five thermocouples were inserted from the top to the bottom of the thermal storage tank and the average of data recorded by the thermocouple was used as the temperature of MPCM slurry (Tt) to counteract the effect of the temperature gradient. All the data from the various instruments were recorded and transferred through real-time data acquisition system (DAS), and subsequently stored in a computer. 2.3 Experimental procedure During the experiment, the chiller (refrigeration capacity 8.4 kW) with an on/off mode first supplied the chilled water to a stratified water storage tank (SWST) tank by the pump, and then transported to the thermal storage tank with the temperature T2 of 4 ºC and a flow rate of 6 L/min. The MPCM/water slurry was cooled until the temperature reached 9 ºC through a heat exchanger HE1. For the discharging process, the cooling stored in the tank was released by using the other water circuits with a flow rate of 6 L/min through the heat exchanger HE2. In the test room, there was a heater of 1.5 kW to counteract the cooling released from the tank and the temperature of the MPCM/water slurry was raised from 9 ºC to 22 ºC. The discharging processes were tested at four stirring rates (0 rpm, 100 rpm, 200 rpm and 300 rpm) and three HTF initial inlet temperatures (10 ºC, 15 ºC and 20 ºC). 3 Data analysis method
In the process of charging, the thermal energy supplied by the chiller, Ec, equals the thermal energy stored in the tank, ΔE, plus the heat loss to surroundings, Eloss. For the discharging process, the thermal energy released to the heating device, Ed, equals the thermal energy reduction in the tank, ΔE, minus the heat loss to surroundings, Eloss. Therefore, in a specific time interval from τ1 to τ2, the heat balance of the charging and discharging processes can be defined as: Ec E Eloss
(2)
Ed E Eloss
(3)
where Ec and Ed can be obtained from: 2
Ec M ch c p.w (T3 T2 )d
(4)
1
2
Ed M h c p.w (T6 T7 )d
(5)
1
The heat loss to surroundings can be calculated by: 2
Eloss Rt (Tlab Tt )d
(6)
1
where Rt is the product of heat transfer coefficient and heat transfer area of the storage tank, calculated as 0.794 W/°C by the method mentioned in the literature [32] Before the start of the experimental measurement of MCPM slurry, we first used pure water to validate the experimental accuracy. Thermal energy is stored as sensible heat for water, and the total thermal storage capacity can be calculated as follow:
E U mwc p.w (Te Ti )
(7)
By combining of Eqs. (1) and (6), below heat balance equations can be obtained: 2
1
2
M ch c p.w (T3 T2 )d mwc p.w (Tt .i Tt .e ) Rt (Tlab Tt )d , 1
(8)
For the LTES tank, the cooling discharging rates qd can be obtained using the following equations:
qd M h c p.w (T6 T7 ) Rt Tlab Tt
(9)
Accordingly, the volumetric thermal storage capacities Ev from τ1 to τ2 can be obtained using the following equations: 2
Ev
E [ M c 1
h p.w
(T6 T7 ) Rt Tlab Tt ]dτ
Vt
The mean charging rate can be estimated by
Vt
(10)
2
Pd
E
1
t
M h c p.w (T6 T7 ) Rt Tlab Tt dτ 2 1
(11)
4 Results and Discussion 4.1 Accuracy and error analysis The accuracy of the data acquisition system (DAS) was first verified using pure water. The method to verify the accuracy of the thermal energy storage system by the heat balance analysis based on equation (1). The thermal energy supplied by the chiller, Ec, should equal to the cooling stored in the water as sensible heat, ΔE, plus the heat loss to surroundings, Eloss. The deviation between the two sides of the equation, ΔEerr, could be an good indicator of the measurement error of the system. 2
2
1
1
Eerr M ch c p.w (T3 T2 )d mwc p.w (Tt .i Tt .e ) Rt (Tlab Tt )d
(12)
ΔEerr at various stirring rates were calculated and shown in Table 3.
100rpm 200rpm 300rpm
Table 3 System heat balance analysis with water Ec ΔE Eloss ΔEerr kJ kJ kJ kJ 1894 1749 37 108 1770 1615 28 127 1645 1480 20 145
ΔEerr/ Ec % 5.7 7.2 8.8
From Table 3, we can find that ΔEerr is below 8.8% of the total input cooling energy. Maybe there are three reasons for the error. One is that the energy supplied by the chiller was stored not only in the water but also in the rest components of the tank. Another reason is that the stirrer inputs heat into the thermal storage tank. The last one is related to the accuracy of the thermocouples and rotameters used in the system. 4.2 Temperature evolutions of the cooling discharging processes Fig. 5 shows the temperature variations in HE2 inlet T6 and outlet T7 at a stirring rate of 100 rpm during cooling discharging. The results indicated that from the beginning of the experiment, the temperature increases gradually and the temperature difference ΔT = (T6-T7) is stable. Because the temperature difference between MPCM slurry and HTF is steady. Thereafter, the slope variation of temperature difference ΔT = (T6-T7) shows that phase change occurs from 605 s. The phase change significantly increased the temperature difference between the HE2 outlet and inlet, and heat transfer capacity between MPCM slurry and supply chilled water was enhanced accordingly. Overall, the temperature difference between the HE2 inlet and outlet decreased gradually because the temperature difference between MPCM slurry and HTF decreased. Therefore, the cooling-discharging capacity weakens as time progresses.
Fig. 5. Temperature variations of inlet and outlet with time during the cooling discharging at 100 rpm During cooling discharging, the temperature variation of MPCM slurry was recorded from 9 °C to 22 °C. Fig. 6 shows the average temperature variations of MPCM slurry at various parameters of stirring speed and initial HE2 inlet temperature. As shown in Fig. 6 the temperature of MPCM slurry increased rapidly at the initial stage because of the small thermal storage capacity of solid MPCM particles without phase changes. After the temperature reached approximately 18 °C, the phase change can be easily identified for the speed range of 100 rpm to 300 rpm. The MPCM slurry temperature was virtually constant owing to the involvement of the latent heat transfer. With further cooling discharge from the LTES tank, the melting process of MPCM was completed, and the temperature of MPCM slurry increased rapidly again because the heat transfer between HTF and MPCM slurry during sensible heat transfer process. At the stirring rate of 0 rpm, the temperature increase of MPCM slurry was generally linear, and the phase transition cannot be clearly identified because natural convective heat transfer was weak in the LTES tank due to high viscosity of MPCM slurry, and the MPCM particle phase change occurred in most of the temperature rise. Cooling discharging was apparently influenced by the initial inlet temperature of HE2. High HE2 initial inlet temperature accelerates the phase-transition speed owing to large temperature difference for heat transfer. Compared with that of the initial HE2 inlet temperature of 10 °C, the duration of the coolingdischarging completion was 327 s shorter for the initial HE2 inlet temperature of 20 °C.
28
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
26
Temperature (oC)
24 22 20 18 16 14 12 10 0
200
400
600
800 1000 1200 1400 1600 1800
Time (s) Fig. 6. Temperature variations of MPCM slurry in the LTES tank during cooling discharging at different stirring rates and HTF initial inlet temperature 4.3 Volumetric thermal storage capacity Fig. 7 presents the volumetric thermal storage capacity during the cooling discharging process of MPCM slurry. Within the range of stirring rates of 100–300 rpm, the volumetric thermal storage capacity increases linearly because the specific heat of MPCM slurry is almost constant during the sensible cooling discharging until the melting temperature of approximately 17 °C is achieved for the MPCM slurry. Above this temperature, the volumetric thermal storage capacity sharply increases owing to the release of latent heat from MPCM. After the completion of the phase change, the volumetric thermal storage capacity of MPCM slurry linearly increased again with a relatively low rate because of the constant specific heat. At the stirring rate of 0 rpm, the volumetric thermal storage capacity of MPCM slurry rises linearly, and the sharp increase trend cannot be distinguished. The decreased volumetric thermal storage capacity is due to partial MPCM particles floating on the top of the LTES tank without blade rotations, which leads to low utilization ratio of the latent heat of PCM capsules. At 300 rpm, the volumetric thermal storage capacity reaches approximately 88.24 MJ/m3, and it is 1.28 times higher than that of MPCM slurry at 0 rpm. The influence of HTF initial inlet temperature on the specific heat and utilization ratio of the latent heat of PCM capsules are insignificant, therefore the volumetric thermal storage capacity is almost same at a fixed stirring speed. Thus, the interior convective flow state of MPCM slurry itself in the LTES tank dominates volumetric thermal storage capacity in this experiment. The forced convection of MPCM slurry can be achieved by the rotary motion of the stirrer, or the enhanced thermal conductivity additives shall be integrated to increase the thermal conductivity of MPCM slurry. These subjects need further study.
Volumetric thermal storage capacity (MJ/m3)
90
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
80 70 60 50 40 30 20 10 0
10
12
14
16
18
20
22
Average MPCM slurry temperature (oC)
Fig. 7. The volumetric thermal storage capacity variation versus MPCM slurry temperature at different stirring rates and HTF initial inlet temperature 4.4 Discharging rate and mean discharging rate The instantaneous cooling discharging rate are presented in Fig. 8. With the stirrer operation and at the same HTF initial inlet temperature of 15 °C, MPCM slurry in the LTES tank was in a forced convection state, and the cooling discharging rate first decreases gradually because the temperature difference between circulating water and MPCM slurry decreases during the sensible heat transfer stage. The cooling discharging rate considerably increases because of the release of latent heat from MPCM particle during the melting MPCM stage of the temperature ranged from 16 °C to 18 °C. As shown in Table 4, high stirring rate enhances the mean cooling charging rate. After the phase-change completion of MPCM, the cooling discharging rate decreases again. Different from the forced convection driven by the stirrer in the LTES tank, the cooling discharging rate of MPCM slurry with the stirring rate of 0 rpm is extremely low at the beginning of the experiment. With the high viscosity and temperature gradient in the MPCM slurry, the heat transfer driven by natural convection is slow. At 300 rpm, the mean discharging rate is 1.525 kW, and it is 1.29 times higher than that of MPCM slurry at 0 rpm. It is also observed that the discharging rate is obviously more affected by HTF initial inlet temperature, high HTF initial inlet temperature accelerates the cooling discharging process because the temperature difference between MPCM slurry and HTF is high. At the same stirring rate, the mean discharging rate of 20 ºC is 1.24 times higher than that of 10 ºC. For the case with lower HTF initial inlet temperature, the discharging rate of MPCM slurry quickly rose at first because the temperature difference between HTF and MPCM slurry increased, it was contrary to the case with higher HTF initial inlet temperatures.
2.4 2.2
Discharging rate (kW)
2.0 1.8 1.6 1.4 1.2
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
1.0 0.8 0.6 0.4 0.2
10
12
14
16
18
20
22
Average MPCM slurry temperature (oC)
Fig. 8. Discharging rate of LTES system using MPCM slurry as storage medium at different stirring rates and HTF initial inlet temperatures Table 4 Mean discharging rate of the LTES unit during the discharging processes. Stirring rate rpm 0 100 200 300 100 100
HTF initial inlet temperature ºC 15 15 15 15 10 20
Pd kW 1.1803 1.4816 1.5211 1.5250 1.4108 1.7476
5. Conclusions In the present study, a LTES system is developed using 25 wt% MPCM slurry. A 1.5-kW heating wire was used to counteract the cooling load during the whole experimental process. A series of experiments were carried out to investigate the thermal storage and heat transfer performance of MPCM/water slurry at different stirring rates and HTF initial inlet temperatures during cooling discharging processes. The volumetric thermal storage capacity of MPCM slurry rapidly increased to a higher level in phase change process because of the release of latent heat. Accelerating the stirring rate of stirrer can enhance the thermal storage capacity of MPCM slurry because the utilization ratio of the latent heat of PCM capsules is increased. At 300 rpm, the volumetric thermal storage capacity reaches approximately 88.24 MJ/m3, and it is 1.28 times higher than that of MPCM slurry without stirring. The discharging rate was enhanced remarkably by the phase change in the temperature range from 16 to 18 ºC because the convection heat
transfer was strengthened by phase change. Larger stirring rates especially larger HTF initial inlet temperatures can obviously enhance the cooling discharging process.
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We used more thermocouples to measure the temperature gradient distribution in the LTES tank
MPCM slurry was used as thermal storage medium in forced convection introduced by stirrer.
The thermal storage and heat transfer performance of MPCM slurry during the cooling discharging processes were discussed under different stirring rates and heat transfer fluid (HTF) initial inlet temperatures.
Discharging and mean discharging rate were calculated to evaluate the heat transfer performance of MPCM slurry.
[33] No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
[34]
S2451-9049(18)30234-8 https://doi.org/10.1016/j.tsep.2019.100362 100362 TSEP 100362
To appear in:
Thermal Science and Engineering Progress
Received Date: Revised Date: Accepted Date:
31 March 2018 22 November 2018 28 May 2019
Please cite this article as: Y. Miao, H. Xu, Q. Gao, Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry, Thermal Science and Engineering Progress (2019), doi: https://doi.org/10.1016/j.tsep.2019.100362
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Experimental study on thermal storage and heat transfer performance of microencapsulated phase-change material slurry Yubo Miao, Hongtao Xu*, Qiang Gao aSchool
of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, PR China *Corresponding Author. Tel: 021–55272015; E-mail: [email protected]
Abstract Microencapsulated phase change material (MPCM) slurry has received attention due to their higher thermal storage density, was introduced as a storage medium in latent thermal energy storage (LTES) system. Based on this background, this article presented the experimental results of the thermal storage and heat transfer characteristics of MPCM slurry during cooling discharging process, a series of experiments were carried out by controlling variable. Hexadecane (C16H34) and urea-formaldehyde was used as the core and shell of PCM to form MPCM with core–shell ratio of 7:3 by weight. The temperature evolutions of the MPCM slurry were obtained during the experiment, and the volumetric thermal storage capacity, cooling discharging rate were calculated under different stirring rates and heat transfer fluid (HTF) initial inlet temperatures. From the experimental results, it is estimated that the thermal storage and heat transfer performance of MPCM slurry both were enhanced remarkably by phase change. In addition, accelerating the stirring rate of stirrer can enhance the thermal storage capacity. At 300 rpm, the volumetric thermal storage capacity and mean discharging rate are about 1.28 times higher than that of MPCM slurry without stirring. At the same stirring rate, the mean discharging rate of 20 ºC is 1.24 times higher than that of 10 ºC. Keywords: MPCM slurry; LTES; Cooling discharging rate; Volumetric thermal storage capacity; Nomenclature A cp E EV ΔE M m P Rt T ΔT ΔU
heat transfer area, m2 specific heat, kJ/( kg·K) energy, J volumetric thermal energy storage capacity, J/m3 cooling energy storage, J mass flow rate, kg/s mass, kg mean charging/discharging rate, W product of heat transfer coefficient and heat transfer area, W/◦C temperature, ◦C temperature difference, ◦C sensible heat storage
Greek symbols τ
time, s
Subscripts c ch d e h i lab loss m t w
charging chiller discharging end heating device initial laboratory heat loss mean storage tank, storage medium water
1 Introduction Energy consumption has grown rapidly in recent decades due to the growth of economy, business and population. Therefore, new technologies of conserving available energy and improving its utilization are the focuses of the current and future researches [1]. LTES has been proven to be an effective technology to improve the energy efficiency because of large latent storage capacity and small temperature variation from storage to extraction. Due to the above outstanding performance, LTES can be applied in many occasions, such as building energy conservation systems, air-conditioning and refrigeration systems, electronic thermal management and solar thermal systems [2][4]. Apparently, the performance of a LTES unit is influenced by the thermophysical properties of the thermal storage medium. Starting from early 1980s, using phase change material (PCM) for thermal storage has gained considerable interest because of its desirable thermophysical properties, such as large latent heat, good thermal and chemical stabilities, and environment friendly. However, a barrier to the wide use of paraffin is the low conductivity of most PCM, which lead to low rate of heat transfer during charging and discharging of LTES, and as a result practical LTES units have been difficult to develop. Several approaches have been developed for the purpose of heat transfer enhancement, including the use of fins in the thermal energy storage unit [6], addition of high conductive fibers [10], and immersion of porous metal foam [11]. In recent years, a new technique is proposed, in which the phase change material is microencapsulated and suspended in a conventional single-phase heat transfer fluid to form microencapsulated phase change material slurry. Such slurry has a large surface area to volume ratio, so the heat can be quickly absorbed and released from the core to the carrier fluid, which means a higher transfer rate compared with traditional TES using PCM without microencapsulating. In addition, the core material is separated from the carrier fluid by microencapsulating the PCM particles, and therefore the deposition and
agglomeration of particles in the slurry are avoided [12]. Several studies on the flow and heat transfer characteristics of MPCM slurry have been investigated in the past by a number of researchers. Liu et al.[13] developed a numerical model of laminar forced convection heat transfer for MPCM slirry with constant heat flux, they found mass concentration and Stefan number are the most influential parameters on the improvement of heat transfer in MPCM slurries. Sabbah et al. [14] developed a three-dimensional, one-phase, laminar flow model of a rectangular channel using MPCM slurry with temperature dependent physical properties, they found that the heat transfer coefficient is mainly dependent on the channel inlet and outlet temperatures and the selected MPCM melting temperature. Ma et al. [15] investigated the flow and heat transfer characteristics of MPCM slurry in a horizontal pipe under constant heat, they found that small MPCM particle and high inlet velocity results in high heat transfer coefficient. Languri et al. [16] performed the numerical and analytical modeling to study the flow of MPCM slurry through helically coiled heat exchangers, the result indicated that the radius of helically coiled influence the heat transfer and MPCM slurry at low mass fraction can be treated as homogenous heat transfer fluids. Inaba et al. [17] proposed a computation model to analyze natural convection in a rectangular enclosure with non-Newtonian MPCM slurry. It was observed that the heat transfer coefficients are higher when compared to slurries without phase change and when the PCM concentration increased within the range 20–40%, the convection coefficient decreased, while for the range 10–20%, the convective coefficient increased. Several experiments were also conducted to deal with the flow and heat transfer characteristics of MPCM slurry. Goel et al. [18] experimentally investigated the laminar, hydrodynamically fully developed flows in a circular duct with a constant wall heat flux. They found the most dominant parameter influencing the heat transfer is the bulk Stefan number and the effect of concentration is insignificant. Wang et al. [19] measured the pressure drop and local heat transfer coefficients in order to investigate the flow and convective heat transfer behaviors of MPCM slurry in a horizontal circular tube, and they found heat transfer coefficients of MPCM slurry are significantly higher than single-phase fluid flow in laminar flow conditions, but exhibit more complicated phenomena at low turbulent conditions. Li et al. [20] investigated the flow characteristic of the slurries flowing in a horizontal circular tube flow, they found that compared to water the heat density of MPCM slurry is higher and it can save pumping consumption accordingly. Ma et al. [21] investigated the thermofluidic performances of the MPCM slurry in laminar flow in the mini-tubes, results showed that the heat transfer performance of the MPCM slurry is significantly better than water at similar Reynolds number and the heat transfer coefficient increase at higher mass fraction. Kong et al. [22] experimentally investigated the flow and heat transfer characteristics of MPCM slurry as a HTF in a heated helically coiled tube. Results showed that Nusselt number of MPCM slurry significantly increases during the phase change process, except transporting more energy, MPCM slurry has no advantage over that of water in the aspect of heat transfer performance. Alvarado et al. [23] conducted turbulent flow heat transfer experiments to determine the convective heat transfer coefficient of MPCM slurry. Results showed MPCM slurry convective heat transfer
coefficients are typically lower than that of water under the constant heat flux, and the heat transfer coefficient increases considerably during the phase change process. The primary characteristics of above MPCM slurries studied by researchers are summarized in Table 1. Table 1 Characteristics of different TES systems containing MPCM slurry. Thermal Particle Fusion Phase conductivity PCM diameter MPCM heat of change of of Core material mass slurry PCM temperature PCM(solid) MPCM fraction (J/g) (°C) (W/m•K) (μm) Liu et al. [13]
n-Octadecane
263
23–29
0.25
5–20%
-
Sabbah et al. [14]
-
110
-
0.2
5-25%
10
-
167
27
0.31
1225%
-
-
-
-
-
5.910.9%
-
-
-
30
-
1040%
-
n-Eicosane
-
36
0.15
5-20%
-
160
14.3–17.2
0.14
5– 27.6%
10.1
196.5
78–85
-
15–25 %
10
Ma et al. [15] Languri et al. [16] Inaba et al. [17] Goel et al. [18]
Wang et al. Error! Reference Bromohexadecae source not found. Li et al. Error! Reference source not found.
Paraffin
Ma et al.
n-Hexadecane
226.8
13.3715.92
-
1020%
-
Kong et al. Error! Reference source not found.
Butyl stearate
88
17.4–22.2
0.23
2.1– 10.9%
5–10
Alvarado et al.[23]
Tetradecane
215
5.5
-
-
2-10
[21]
The effective and appropriate applications of MPCM slurries can reduce the buildings energy consumption and associated carbon emission in buildings [24]. The cooled ceiling (CC) system employed the cooled ceiling to treat a sensible cooling load and used an independent HVAC system for the ventilation and humidity control. As the energy storage medium, MPCM slurry can be used in a cooled ceiling (CC) system. However, more experiments are needed to evaluate whether a competitive thermal storage and heat transfer performance can be achieved. To the authors' knowledge, the literature available on the natural convection process outside a helical coil is limited. Heinz and Streicher [25] indicated that the natural convective heat transfer coefficient of 20% mass concentration is approximately 30% lower than that of water because the viscosity of MPCM slurry is high. Similar conclusions were drawn by Huang et al. [26], they found that the MPCM slurry with 50% mass concentration is not applicable, because the low thermal conductivity and high viscosity reduced the heat transfer from the heat exchanger to the storage media. Diaconu et al. [27] found that the natural convective coefficient for MPCM slurry during the phase change process could be up to five times compare to water. Zhang and Niu [28] found that In the CC system, MPCM slurry capacity of volumetric thermal energy storage is found higher, whereas the discharging rate is still considerably lower than that of water. This article aims to obtain reliable experimental data of the thermal storage and heat transfer behaviors of MPCM slurry at different parameters by building a coil-in-tank LTES unit. Experimental setup was validated with experimental data of a coil-in-tank water system. In order to introduce forced convection, an axial stirrer with variable rating speeds was added into the LTES system. The temperature evolutions of MPCM slurry, volumetric thermal storage capacity and cooling discharging rate are reported under different initial inlet temperatures of the HTF and stirring rate of stirrer. These test results will be used to assess the suitability of MPCM slurry for integration into building cooling applications. 2 Experimental approach 2.1 Materials
The PCM and micro-encapsulated PCM were supplied by Aerospace Institute of Advanced Material & Processing Technology. The MPCM slurry was synthesized by in situ polymerization [29], urea-formaldehyde was used as the shell, and hexadecane (C16H34) was used as the core PCM material, with a melting temperature of 18 ºC and latent heat of 224 kJ/kg [30]. The core-shell ratio was 7:3 by weight, and the average diameter of capsules measured by a particle size analyzer is 12.5 μm. The flow diagram of in situ polymerization is in Fig. 1.Pure water was used as carrier fluid. The mass fraction of the MPCM slurry used in this study was 25%. The fluid could be analysed as Newtonian when the microcapsule volumetric concentration was less than 30% [31]. Fig. 2 shows the optical and SEM photographs of microcapsules used in the present study.
Fig. 1. Flow diagram of in situ polymerization
Fig. 2. (a) Optical micrograph and (b) Scanning electron microscopy photograph. The thermal storage and heat transfer of MPCM slurry in the storage tank are associated with the following properties: density, specific heat capacity, thermal conductivity and latent heat, which depend on the type of PCM, shell material, its carrier fluid and the particle concentrations. These properties given in Table 2 were calculated from the weighted fraction of the individual properties of PCM, coating material and water based on the thermal property models that have been validated and applied by various researchers, the details of which can be found in [19]. Table 2 Physical properties of MPCM slurry and its components Thermal Density Specific heat Latent heat conductivity kg/m3 J/(kg• ºC) kJ/kg W/(m• ºC)
Hexadecane(solid) Hexadecane(liquid) Urea-formaldehyde Water(at 20 ºC) MPCM particle(solid) MPCM particle(liquid) MPCM slurry (mass fraction)Φ=0.25
780 770 1490 998 960 950
1805 2221 1675 4183 1766 2057
0.4 0.21 0.433 0.599 0.382 0.203
224
990
3579
0.578
44
177
2.2 Experimental system An experimental system (Fig. 3) has been built to investigate the thermal storage and heat transfer performance of a coil in tank MPCM/water slurry cooling storage system. In order to avoid mechanical damage to the MPCM capsules by the pump, water was used as the HTF in both the chiller loop and heating device loop. Two helical coil heat exchangers immersed in a cylindrical tank, HE1 and HE2, were made of copper tube of 15 mm in diameter and 11m in length. The storage tank (Fig. 4) is a cylindrical stainless steel tank with an insulation layer of 40 mm polyethylene insulation layer. The diameter of the tank is 0.3 m, and the height is 0.5 m. The internal volume of the tank minus the volume occupied by the internal coils is 0.0278 m3. A variable speed stirrer driven by AC power was installed above the tank to keep the slurry mixed ideally and to generate forced convection during the charging and discharging processes.
Fig. 3. Schematic diagram of the experimental apparatus
Fig. 4. Detailed diagram of the storage tank (a) storage tank plan (b) site photograph of thermal storage tank For the sake of measuring the system operation parameter, we installed twenty Ttype thermocouples (±0.5 ºC, operating range of 0 to 50 ºC) and three rotameters (± 4%, operating range of 0 to 7 L/min).The temperatures (T2, T3, T6, T7) of water were measured by the thermocouples placed at the inlets and outlets of the two heat exchangers, HE1 and HE2, respectively. Five thermocouples were inserted from the top to the bottom of the thermal storage tank and the average of data recorded by the thermocouple was used as the temperature of MPCM slurry (Tt) to counteract the effect of the temperature gradient. All the data from the various instruments were recorded and transferred through real-time data acquisition system (DAS), and subsequently stored in a computer. 2.3 Experimental procedure During the experiment, the chiller (refrigeration capacity 8.4 kW) with an on/off mode first supplied the chilled water to a stratified water storage tank (SWST) tank by the pump, and then transported to the thermal storage tank with the temperature T2 of 4 ºC and a flow rate of 6 L/min. The MPCM/water slurry was cooled until the temperature reached 9 ºC through a heat exchanger HE1. For the discharging process, the cooling stored in the tank was released by using the other water circuits with a flow rate of 6 L/min through the heat exchanger HE2. In the test room, there was a heater of 1.5 kW to counteract the cooling released from the tank and the temperature of the MPCM/water slurry was raised from 9 ºC to 22 ºC. The discharging processes were tested at four stirring rates (0 rpm, 100 rpm, 200 rpm and 300 rpm) and three HTF initial inlet temperatures (10 ºC, 15 ºC and 20 ºC). 3 Data analysis method
In the process of charging, the thermal energy supplied by the chiller, Ec, equals the thermal energy stored in the tank, ΔE, plus the heat loss to surroundings, Eloss. For the discharging process, the thermal energy released to the heating device, Ed, equals the thermal energy reduction in the tank, ΔE, minus the heat loss to surroundings, Eloss. Therefore, in a specific time interval from τ1 to τ2, the heat balance of the charging and discharging processes can be defined as: Ec E Eloss
(2)
Ed E Eloss
(3)
where Ec and Ed can be obtained from: 2
Ec M ch c p.w (T3 T2 )d
(4)
1
2
Ed M h c p.w (T6 T7 )d
(5)
1
The heat loss to surroundings can be calculated by: 2
Eloss Rt (Tlab Tt )d
(6)
1
where Rt is the product of heat transfer coefficient and heat transfer area of the storage tank, calculated as 0.794 W/°C by the method mentioned in the literature [32] Before the start of the experimental measurement of MCPM slurry, we first used pure water to validate the experimental accuracy. Thermal energy is stored as sensible heat for water, and the total thermal storage capacity can be calculated as follow:
E U mwc p.w (Te Ti )
(7)
By combining of Eqs. (1) and (6), below heat balance equations can be obtained: 2
1
2
M ch c p.w (T3 T2 )d mwc p.w (Tt .i Tt .e ) Rt (Tlab Tt )d , 1
(8)
For the LTES tank, the cooling discharging rates qd can be obtained using the following equations:
qd M h c p.w (T6 T7 ) Rt Tlab Tt
(9)
Accordingly, the volumetric thermal storage capacities Ev from τ1 to τ2 can be obtained using the following equations: 2
Ev
E [ M c 1
h p.w
(T6 T7 ) Rt Tlab Tt ]dτ
Vt
The mean charging rate can be estimated by
Vt
(10)
2
Pd
E
1
t
M h c p.w (T6 T7 ) Rt Tlab Tt dτ 2 1
(11)
4 Results and Discussion 4.1 Accuracy and error analysis The accuracy of the data acquisition system (DAS) was first verified using pure water. The method to verify the accuracy of the thermal energy storage system by the heat balance analysis based on equation (1). The thermal energy supplied by the chiller, Ec, should equal to the cooling stored in the water as sensible heat, ΔE, plus the heat loss to surroundings, Eloss. The deviation between the two sides of the equation, ΔEerr, could be an good indicator of the measurement error of the system. 2
2
1
1
Eerr M ch c p.w (T3 T2 )d mwc p.w (Tt .i Tt .e ) Rt (Tlab Tt )d
(12)
ΔEerr at various stirring rates were calculated and shown in Table 3.
100rpm 200rpm 300rpm
Table 3 System heat balance analysis with water Ec ΔE Eloss ΔEerr kJ kJ kJ kJ 1894 1749 37 108 1770 1615 28 127 1645 1480 20 145
ΔEerr/ Ec % 5.7 7.2 8.8
From Table 3, we can find that ΔEerr is below 8.8% of the total input cooling energy. Maybe there are three reasons for the error. One is that the energy supplied by the chiller was stored not only in the water but also in the rest components of the tank. Another reason is that the stirrer inputs heat into the thermal storage tank. The last one is related to the accuracy of the thermocouples and rotameters used in the system. 4.2 Temperature evolutions of the cooling discharging processes Fig. 5 shows the temperature variations in HE2 inlet T6 and outlet T7 at a stirring rate of 100 rpm during cooling discharging. The results indicated that from the beginning of the experiment, the temperature increases gradually and the temperature difference ΔT = (T6-T7) is stable. Because the temperature difference between MPCM slurry and HTF is steady. Thereafter, the slope variation of temperature difference ΔT = (T6-T7) shows that phase change occurs from 605 s. The phase change significantly increased the temperature difference between the HE2 outlet and inlet, and heat transfer capacity between MPCM slurry and supply chilled water was enhanced accordingly. Overall, the temperature difference between the HE2 inlet and outlet decreased gradually because the temperature difference between MPCM slurry and HTF decreased. Therefore, the cooling-discharging capacity weakens as time progresses.
Fig. 5. Temperature variations of inlet and outlet with time during the cooling discharging at 100 rpm During cooling discharging, the temperature variation of MPCM slurry was recorded from 9 °C to 22 °C. Fig. 6 shows the average temperature variations of MPCM slurry at various parameters of stirring speed and initial HE2 inlet temperature. As shown in Fig. 6 the temperature of MPCM slurry increased rapidly at the initial stage because of the small thermal storage capacity of solid MPCM particles without phase changes. After the temperature reached approximately 18 °C, the phase change can be easily identified for the speed range of 100 rpm to 300 rpm. The MPCM slurry temperature was virtually constant owing to the involvement of the latent heat transfer. With further cooling discharge from the LTES tank, the melting process of MPCM was completed, and the temperature of MPCM slurry increased rapidly again because the heat transfer between HTF and MPCM slurry during sensible heat transfer process. At the stirring rate of 0 rpm, the temperature increase of MPCM slurry was generally linear, and the phase transition cannot be clearly identified because natural convective heat transfer was weak in the LTES tank due to high viscosity of MPCM slurry, and the MPCM particle phase change occurred in most of the temperature rise. Cooling discharging was apparently influenced by the initial inlet temperature of HE2. High HE2 initial inlet temperature accelerates the phase-transition speed owing to large temperature difference for heat transfer. Compared with that of the initial HE2 inlet temperature of 10 °C, the duration of the coolingdischarging completion was 327 s shorter for the initial HE2 inlet temperature of 20 °C.
28
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
26
Temperature (oC)
24 22 20 18 16 14 12 10 0
200
400
600
800 1000 1200 1400 1600 1800
Time (s) Fig. 6. Temperature variations of MPCM slurry in the LTES tank during cooling discharging at different stirring rates and HTF initial inlet temperature 4.3 Volumetric thermal storage capacity Fig. 7 presents the volumetric thermal storage capacity during the cooling discharging process of MPCM slurry. Within the range of stirring rates of 100–300 rpm, the volumetric thermal storage capacity increases linearly because the specific heat of MPCM slurry is almost constant during the sensible cooling discharging until the melting temperature of approximately 17 °C is achieved for the MPCM slurry. Above this temperature, the volumetric thermal storage capacity sharply increases owing to the release of latent heat from MPCM. After the completion of the phase change, the volumetric thermal storage capacity of MPCM slurry linearly increased again with a relatively low rate because of the constant specific heat. At the stirring rate of 0 rpm, the volumetric thermal storage capacity of MPCM slurry rises linearly, and the sharp increase trend cannot be distinguished. The decreased volumetric thermal storage capacity is due to partial MPCM particles floating on the top of the LTES tank without blade rotations, which leads to low utilization ratio of the latent heat of PCM capsules. At 300 rpm, the volumetric thermal storage capacity reaches approximately 88.24 MJ/m3, and it is 1.28 times higher than that of MPCM slurry at 0 rpm. The influence of HTF initial inlet temperature on the specific heat and utilization ratio of the latent heat of PCM capsules are insignificant, therefore the volumetric thermal storage capacity is almost same at a fixed stirring speed. Thus, the interior convective flow state of MPCM slurry itself in the LTES tank dominates volumetric thermal storage capacity in this experiment. The forced convection of MPCM slurry can be achieved by the rotary motion of the stirrer, or the enhanced thermal conductivity additives shall be integrated to increase the thermal conductivity of MPCM slurry. These subjects need further study.
Volumetric thermal storage capacity (MJ/m3)
90
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
80 70 60 50 40 30 20 10 0
10
12
14
16
18
20
22
Average MPCM slurry temperature (oC)
Fig. 7. The volumetric thermal storage capacity variation versus MPCM slurry temperature at different stirring rates and HTF initial inlet temperature 4.4 Discharging rate and mean discharging rate The instantaneous cooling discharging rate are presented in Fig. 8. With the stirrer operation and at the same HTF initial inlet temperature of 15 °C, MPCM slurry in the LTES tank was in a forced convection state, and the cooling discharging rate first decreases gradually because the temperature difference between circulating water and MPCM slurry decreases during the sensible heat transfer stage. The cooling discharging rate considerably increases because of the release of latent heat from MPCM particle during the melting MPCM stage of the temperature ranged from 16 °C to 18 °C. As shown in Table 4, high stirring rate enhances the mean cooling charging rate. After the phase-change completion of MPCM, the cooling discharging rate decreases again. Different from the forced convection driven by the stirrer in the LTES tank, the cooling discharging rate of MPCM slurry with the stirring rate of 0 rpm is extremely low at the beginning of the experiment. With the high viscosity and temperature gradient in the MPCM slurry, the heat transfer driven by natural convection is slow. At 300 rpm, the mean discharging rate is 1.525 kW, and it is 1.29 times higher than that of MPCM slurry at 0 rpm. It is also observed that the discharging rate is obviously more affected by HTF initial inlet temperature, high HTF initial inlet temperature accelerates the cooling discharging process because the temperature difference between MPCM slurry and HTF is high. At the same stirring rate, the mean discharging rate of 20 ºC is 1.24 times higher than that of 10 ºC. For the case with lower HTF initial inlet temperature, the discharging rate of MPCM slurry quickly rose at first because the temperature difference between HTF and MPCM slurry increased, it was contrary to the case with higher HTF initial inlet temperatures.
2.4 2.2
Discharging rate (kW)
2.0 1.8 1.6 1.4 1.2
0 rpm, 15oC 100 rpm, 15oC 200 rpm, 15oC 300 rpm, 15oC 100 rpm, 10oC 100 rpm, 20oC
1.0 0.8 0.6 0.4 0.2
10
12
14
16
18
20
22
Average MPCM slurry temperature (oC)
Fig. 8. Discharging rate of LTES system using MPCM slurry as storage medium at different stirring rates and HTF initial inlet temperatures Table 4 Mean discharging rate of the LTES unit during the discharging processes. Stirring rate rpm 0 100 200 300 100 100
HTF initial inlet temperature ºC 15 15 15 15 10 20
Pd kW 1.1803 1.4816 1.5211 1.5250 1.4108 1.7476
5. Conclusions In the present study, a LTES system is developed using 25 wt% MPCM slurry. A 1.5-kW heating wire was used to counteract the cooling load during the whole experimental process. A series of experiments were carried out to investigate the thermal storage and heat transfer performance of MPCM/water slurry at different stirring rates and HTF initial inlet temperatures during cooling discharging processes. The volumetric thermal storage capacity of MPCM slurry rapidly increased to a higher level in phase change process because of the release of latent heat. Accelerating the stirring rate of stirrer can enhance the thermal storage capacity of MPCM slurry because the utilization ratio of the latent heat of PCM capsules is increased. At 300 rpm, the volumetric thermal storage capacity reaches approximately 88.24 MJ/m3, and it is 1.28 times higher than that of MPCM slurry without stirring. The discharging rate was enhanced remarkably by the phase change in the temperature range from 16 to 18 ºC because the convection heat
transfer was strengthened by phase change. Larger stirring rates especially larger HTF initial inlet temperatures can obviously enhance the cooling discharging process.
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We used more thermocouples to measure the temperature gradient distribution in the LTES tank
MPCM slurry was used as thermal storage medium in forced convection introduced by stirrer.
The thermal storage and heat transfer performance of MPCM slurry during the cooling discharging processes were discussed under different stirring rates and heat transfer fluid (HTF) initial inlet temperatures.
Discharging and mean discharging rate were calculated to evaluate the heat transfer performance of MPCM slurry.
[33] No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
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