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Experimental research on thermal characteristics of PCM thermal energy storage units
Journal of the Energy Institute xxx (xxxx) xxx
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Experimental research on thermal characteristics of PCM thermal energy storage units Tianshi Zhang a, b, Yubin Liu b, Qing Gao a, b, *, Guohua Wang a, b, Zhenmin Yan b, Ming Shen b a b
State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025, Changchun, China College of Automotive Engineering, Jilin University, 130025, Changchun, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2017 Received in revised form 22 March 2019 Accepted 1 April 2019 Available online xxx
To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage unit Phase change material Multiple melting points Thermal characteristics
1. Introduction Phase-change heat storage technologies have received considerable attention in the field of vehicle-mounted waste heat utilization. For the traditional internal combustion engine car, Schatz [1] proposed the concept of a heat battery, which adopts a phase change material (PCM) to store waste heat from engine cooling water. The heat could be used to preheat the cabin and improve engine cold start performance in winter. Korin [2] used latent heat energy to preheat a catalytic converter until it reached the optimum working temperature. Dinker [3] showed that PCM stores much heat more than sensible heat storage materials at constant temperature and many researchers have been studying to use PCMs in battery thermal management. Aldoss [4] built a mathematical model to investigate a lighting thermal management system with a PCM. Adamczyk [5] proposed a vacuum insulation method, and experimental results showed that this method could efficiently realize heat preservation for a long time. Compared with traditional vehicles, extending the operational range and reducing energy consumption are more critical problems for EVs. Because pure electric vehicles (PEVs) do not have engine waste heat, winter heating and battery cold starts have become difficult technical problems and large obstacles for all climate applications. Al-Hallaj [6] considered using heat stored in PCMs to improve the cold start performances of battery packs in cold regions. Park [7] proposed charging the heat and electricity at the same time for EVs using a heat storage device, and discussed coordinated control and application methods. Gao [8] et al. indicated that PCMs can absorb considerable heat from batteries, PCUs, and motors to make their working temperatures stable. In addition, waste heat can be used to defrost and preheat the cabin facilitating heat recovery and energy savings. Therefore, a thermal energy storage setup is important for vehicle-mounted heat energy reutilization and complementation, and it will promote electric vehicle thermal management (VTM) integration involving the battery packs, motors, power control units (PCUs), heat pumps (HPs), etc. At the same time, realizing rapid heat storage and release is a key problem in vehicle applications. Therefore, heat transfer
* Corresponding author. State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025, Changchun, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (Q. Gao). https://doi.org/10.1016/j.joei.2019.04.007 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Nomenclature
T2
hs PCM PCUs VTM Tol Qhs Qhr teht qhp Tao
r c
t Q Qhsc Qrec
Q qv T1
phase change material power control units vehicle thermal management outlet liquid temperature ( C) heat storage capacity (kJ) heat release capacity (kJ) essential heat transfer time (s) heat power (kW) average outlet temperature ( C) density of liquid (kg/m3) specific heat capacity of liquid (kJkg1K1) heat transfer time (s) heat storage/release capacity (kJ) heat storage capacity of PCM units,(kJ) cooling capacity of refrigeration unit in combined units, (kJ) heat storage/release power of PCM (kW) volume flow rate of liquid (m3/s) inlet liquid temperature ( C)
Qe
hr
Qre
hsc
Qec
hrc
Qhrc
outlet liquid temperature ( C) thermal efficiency of the unit in heat storage process electricity consumption of unit (kJ) thermal efficiency of the unit in heat release process cooling capacity of refrigeration unit (kJ) thermal efficiency of the combined units in heat storage process electricity consumption of combined units, (kJ) thermal efficiency of the combined units in heat release process heat release capacity of PCM units in heat release process, (kJ)
Subscripts ol outlet liquid hs heat storage hr heat release eht essential heat transfer hp heat power ao average outlet
enhancement technologies are necessary and important. At present, technological development mainly focuses on two aspects: the improvement of thermal conductivity and the optimization of the structure for heat exchange. Khateeb et al. [9] find it can enchance heat transfer and temperature uniformity to fill the PCM with aluminum foam. The results showed that the comprehensive thermal conductivity of the PCM increased. Mills [10] filled a PCM with a graphite matrix and experimentally investigated the effects of the filling proportion on the thermal conductivity and latent heat. Fukai [11,12] proposed the use of carbon fibers as heat transfer ducts inside PCMs. Banaszek [13] designed a spiral thermal energy storage unit and conducted theoretical and experimental research on this type of unit. E. Assis [14] adopted a small spherical shell to encapsulate PCM and investigated the effects of different diameters on the heat storage rate. Among other researchers' valuable findings, the use of multiple PCMs with different melting temperatures inside a latent heat thermal energy storage unit has been recommended as more efficient than single-PCM systems. Farid and Kanzawa [15] were the first to suggest the use of multiple PCMs arranged in a cascade, and showed the advantage of such an arrangement. Then multiple PCMs in various geometries has been developed experimentally and numerically, such as recently, Yang et al. [16] investigated a heat storage based on three types of PCMs in spherical capsules, it's indicate that there is an advantage of using multiple materials in the energy aspect, but the exergy efficiency et al. [17] explored a two-PCM unit is lower during the melting process using a finite difference method for the numerical solution. Peiro experimentally and demonstrated an effectiveness enhancement of above 19% as compared with a single-PCM configuration. Ezra et al. [18] present a numerically solved mathematical model to explore the effects of different parameters, including the inlet velocity and temperature of the HTF, the number of rows, the number of materials, and the PCMs' melting temperature span, for a latent heat thermal energy storage unit where an arbitrary number of PCMs, which can attain the shortest melting (charging) time for an entire multiple-PCM unit under given conditions. And the generalized results of the model are applicable in this more realistic case as well, comparing favorably with experimental results from the other literature. To realize heat energy recycling for electric vehicle and improve cold start performances of battery pack, the present work demonstrates a PCM thermal energy storage unit using flat tubes and corrugated fins. And the multiple PCMs with different melting temperatures (30#, 40#, and 50#)were cascaded to help extend the range of applicable temperature and satisfy multiple heat sources (battery, motor, PCUs, etc) with different temperature classes. Based on the experimental, the method of fluid active commutation is adopted to deal with different conditions of heat storage and heat release, i.e. during the heat storage process, adopting descending order by melting point to increase the phase change uniformity of the units and the heat storage rate, and ascending order by melting point is adopting during the heat release process. On this basis, we explore the factor of the heat storage and release rates of PCM units, such as the different inlet temperature, liquid flow rate, melting point of the PCM and the combination order of the units, and the thermal characteristics of the PCM units were investigated, such as the temperature variation, heat capacity, and heat transfer time. 2. Design of PCM unit and experimental system 2.1. Thermal energy storage unit The structure and geometric elements of the thermal energy storage unit is shown in Fig. 1. The core of unit consisted of flat tubes and corrugated fins with louvers. Liquid flowed inside the flat tubes. A PCM was added between the flat tubes, and the thickness of flat tube was 2 mm. The PCM was paraffin and the corrugated fins with louvers were used to increase surface area for heat transfer. The thickness of corrugated fin was 0.1 mm, and the width of it was 4 cm. The mass of PCM is 1.15 kg in the thermal energy storage unit. Thermo-physical properties of PCMs are shown in Table 1. Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Fig. 1. The structure and geometric elements of the thermal energy storage unit.
To concurrently recover waste heat from heat sources at different temperatures, a cascading heat storage combination was built, which adopted multiple melting point PCM units throughout the experimental process. The melting point of the PCMs included 30 C, 40 C, and 50 C, and the corresponding heat storage units were called 30#, 40#, and 50#, respectively. The cascading combinations have two arrangements that make use of a change in the direction of the flow of the liquid. These orders include the melting point ascending and descending orders, as show in Fig. 2. The solid arrow represents the forward flow direction, and the hollow arrow represents the backward flow direction. 2.2. System and apparatus The experimental system consisted of the thermal energy storage unit, a refrigerating system, a heating tank, a cooling tank, an expansion tank, an air duct, an axial fan, a radiator, pumps, flow distributors, regulating valves, an agitator, filters, etc. The experimental system loop and apparatus are presented in Fig. 3 (a) and (b), respectively. The liquid temperature was regulated using the heating tank, cooling tank, refrigerating system, and radiator. The expansion tank was used to maintain the system pressure and to add liquid. The liquid was a 50% aqueous ethylene glycol solution by capacity. During the experimental process, insulation measures were adopted for the PCM unit, heating tank, cooling tank, and pipes to reduce the environment impact. In addition, the test system mainly included a signal-acquiring device (National Instrument), a temperature control unit, a turbine flow meter, thermocouples (its measurement error is ±1.5 C), electrical energy meter, etc. 2.3. Working conditions and calculation method To systematically investigate the heat transfer characteristics and effects of the inlet temperature, liquid flow rate, PCM melting point, and order of the PCM units, a series of experimental conditions were defined. It's defined the liquid flow rate as 1 L/min with an initial PCM temperature of 0 C and an inlet liquid temperature of 60 C as the basic conditions during the heat storage process. Other conditions are shown in Table 2. Furthermore, the authors defined the liquid flow rate as 1 L/min, the initial PCM temperature as 60 C, and the inlet liquid flow temperature as 0 C as the basic conditions of the heat release process. The other conditions are shown in Table 3. In fact, the PCM heat storage includes the latent heat and the sensible heat; the former is more dominant in the early and middle periods than in the latter periods. Therefore, the essential heat transfer period was defined as the storage or release periods of the 70% heat capacity, and the heat storage or release capacity in the corresponding period was defined as the essential heat storage or release capacity. Some of the investigations and analyses were performed during the essential heat storage and release periods. In addition, the heat storage or release power is calculated as follows [19]:
Q ¼ qv r cðΤ 2 Τ 1 Þ Where:
Q is heat storage/release power of PCM, kW. qv is volume flow rate of liquid, m3/s. ris density of liquid, kg/m3. cis specific heat capacity of liquid, kJkg1K1. Τ 1 is inlet liquid temperature, C. Τ 2 is outlet liquid temperature, C. Therefore, the heat storage or release capacity can be calculated as follow: Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Table 1 Thermo-physical properties of PCM. Melting point of PCM ( C)
Density (kg/m3)
Specific heat capacity (kJ/kg K)
Heat of fusion (kJ/kg)
30 40 50
840 852 845
2.5 2.5 2.5
207 220 210
Fig. 2. Cascading combination of thermal energy storage units.
Fig. 3. Experimental system composition.
Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Table 2 The working conditions during the heat storage process. liquid flow rate (L/min)
initial PCM temperature ( C)
inlet liquid flow temperature ( C)
melting point of PCM ( C)
0.5 1 2
0
50 60 70
30 40 50
Table 3 The working conditions during the heat release process. liquid flow rate (L/min)
initial PCM temperature ( C)
inlet liquid flow temperature ( C)
melting point of PCM( C)
0.5 1 2
60
10 0 10
30 40 50
Zt Q¼
Q ðtÞdt
0
Where: Q is heat storage/release capacity, kJ.
t is heat transfer time, s. It is simplified into measured time in the experiment and calculation. 3. Results and analysis The performance of the thermal energy storage unit was evaluated in the basic condition. In addition, the authors investigated the effects of the inlet liquid temperature, the liquid flow rate, and the PCM melting point, calculated the thermal efficiency in some typical cases and comparatively analyzed the performances of the different cascading combination orders. 3.1. Effect of inlet temperature The effects of the inlet liquid temperature were investigated based on the 40# PCM unit. In the heat storage process, the inlet temperature of the unit was 50 C, 60 C, or 70 C. In the heat release process, the inlet liquid temperature of the unit was 10 C, 0 C, or 10 C. Other parameters were consistent with the basic heat storage and release conditions. Fig. 4 (a) demonstrates the variations in the outlet liquid temperature (Tol) of the thermal energy storage unit and the heat storage capacity (Qhs) under different inlet liquid temperature conditions ranging from 50 C to 60 C during the heat storage process. Because the latent heat exceeded the sensible heat for the PCM, the outlet temperature curves had smaller slopes at the beginning of the phase change heat storage. Furthermore, the slope of Qhs was low for the 50 C condition, and the Tol curve even presented a temperature slowly rise period during the phase change heat storage period. The phase change heat storage rate was lowest for the 50 C condition. Meanwhile, the slopes of the Tol and Qhs curves were greatest and the heat storage rates were the highest for the 70 C condition in the same period. When the inlet temperature was 60 C (the basic heat storage condition), the essential heat storage time was 165 s, and the essential heat storage capacity was 309.1 kJ. Fig. 4(b)demonstrates the variations in the outlet temperature (Tol) and the heat release capacity (Qhr) under different inlet temperature conditions ranging from 10 C to 10 C in the heat release process. The slope of the Tol curve and the heat release rate were at their lowest values for the 10 C condition during the phase change heat release period. Meanwhile, the slope of the Tol curve and the heat release rate were at their highest values for the 10 C condition in the same period. The essential heat release time was 180 s, and the essential heat release capacity was 317.9 kJ for the 0 C condition (the basic heat release condition). It could be seen that the designed thermal energy storage unit exhibited well heat storage and release rates, and it satisfies the practical application for the battery preheating. Under the condition of heat storage, the total heat storage capacity is 309.1 kJ with a 1.15 kg 40# PMC, and we can calculate the latent heat is 253 kJ (1.15 kg*220 kJ/kg ¼ 253 kJ). And the condition of heat release is in like manner, which the total heat release capacity is 317.9 kJ with a 1.15 kg 40# PMC. We can see the latent heat accounts for a large proportion of the total storage or release of heat. 3.2. Effect of melting point The effects of the melting point were investigated in the basic heat storage and release conditions, and the PCM melting point was set to 30 C, 40 C, or 50 C in the experimental process. Fig. 5 (a) demonstrates the variations of the Tol and Qhs under different PCM melting points ranging from 30# to 50# during the heat storage process. It could be seen that the outlet water temperature rose the most, but the heat storage capacity increased most slowly for 50# PCM in defined the essential heat transfer period and 40# PCM secondly. As for 30# PCM, the outlet water temperature rose most slowly, but the heat storage capacity increased fastest. Analysis shows that this is due to the same inlet temperature, flow rate and other conditions, the PCM unit with high melting point temperature cannot rapidly carry out latent heat storage because it goes into the phase change melting stage slowly. Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
T. Zhang et al. / Journal of the Energy Institute xxx (xxxx) xxx
Tol /ć
ć7RO ć7RO ć7RO ć4KV ć4KV ć4KV
ć7RO ć7RO ć7RO ć4KU ć4KU ć4KU
Tol/ć
Qhs/kJ
Qhr/kJ
6
Time/s
Time/s
˄E˅Heat release process
˄D˅Heat storage process
Fig. 4. The variations in the outlet temperature and heat capacity for several inlet liquid temperatures.
7RO
7RO
7RO
4KU
7RO
4KV 4KV
4KV
Time/s
˄D˅Heat storage process
7RO
4KU
4KUN-
7RO
7 ROć
4 N-
T /ć
4KU
Time/s
˄E˅Heat release process
Fig. 5. The variations in the outlet liquid temperature and heat capacity for several PCM melting points.
Fig. 5 (b) demonstrates the variations of the Tol and Qhr under different PCM melting points from 30# to 50# during the heat release process. It could be seen that the outlet water temperature drops the slowest, but the heat release capacity increases fastest for 50# PCM in defined the essential heat transfer period and 40# PCM secondly. As for 30# PCM, the outlet water temperature drops fastest, but the heat release capacity increased most slowly. Analysis shows that this is due to the same inlet temperature, flow rate and other conditions, the PCM unit with high melting point temperature enter the phase change melting exothermic stage quickly, and the latent heat can be released rapidly. Fig. 6 illustrates the comparisons and analyses of PCMs with different melting points in the essential heat transfer (storage or release) periods. In the figure, as the PCM melting point increased, the essential heat transfer time (teht) was extended, the heat power (qhp) decreased, and the average outlet temperature (Tao) rose gradually in the basic heat storage condition. In the basic heat release condition, as the PCM melting point increased, the qhp and Tao also rose, but teht gradually decreased, and the higher melting point PCMs maintained higher outlet liquid temperatures.
3.3. Effect of liquid flow rate The effects of the liquid flow rate were investigated based on the 40# PCM unit, and the liquid flow rate was 0.5 L/min, 1 L/min, or 2 L/min in the experimental process. Other parameters were consistent with the basic heat storage and release conditions. Fig. 7(a) demonstrates the variations of the Tol and Qhs under different liquid flow rates ranging from 0.5 L/min to 2 L/min during the heat storage process. As demonstrated, the slope of the Tol curve was at a maximum and the rising speed of Qhs was fastest under the 2 L/min condition during the phase change heat storage period, which means that the phase change heat storage rate was the fastest. At the same time, the rising speed of Qhs was slowest and the heat storage rate was lowest under the 0.5 L/min condition in the same period. When the liquid flow rate was 1 L/min, the phase change heat storage rate and rising speed of Qhs were between 0.5 L/min and 2 L/min. Fig. 7(b) illustrates variations of the Tol and Qhr under different liquid flow rates ranging from 0.5 L/min to 2 L/min during the heat release process. It could be seen that the slope of the Tol curve was at its maximum value, and the rising speed of Qhr was fastest under the 2 L/min condition during the phase change heat release period, which means that the phase change heat release rate was fastest. At the same time, Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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30# 40# 50#
t eht /s
heat storage
heat release
˄a˅Essential heat transfer time
30# 40# 50#
Tao /ć
qhp/kW
30# 40# 50#
heat storage
heat storage
heat release
˄b˅Average heat power
heat release
˄c˅Average outlet temperature
Fig. 6. Comparisons and analyses of PCMs with different melting points.
/PLQ7RO /PLQ7RO /PLQ7RO /PLQ4KU /PLQ4KU /PLQ4KU
/PLQ7RO /PLQ7RO /PLQ7RO
/PLQ4KV /PLQ4KV
/PLQ4KV
T /ć
Q /kJ
T /ć
Q /kJ
Time/s
˄D˅Heat storage process
Time/s
˄E˅Heat release process
Fig. 7. Variations in the outlet temperature and heat capacity for several liquid flow rates.
the rising speed of Qhr was slowest and the heat release rate was lowest under the 0.5 L/min condition in the same period. When the liquid flow rate was 1 L/min, the heat release rate and rising speed of Qhr were between 0.5 L/min and 2 L/min. Fig. 8 demonstrates the comparisons and analyses of different liquid flow rates in the range of essential heat transfer (storage and release) periods. With increasing liquid flow rate, teht gradually decreased, and qhp increased. Meanwhile, Tao increased in the heat storage process but decreased gradually in the heat release process. Furthermore, in the middle and later periods, the role of the liquid flow rate continued to decrease compared with the early period. Therefore, heat transfer enhancement could not be realized through a continuous increase in the liquid flow rate. 3.4. Effect of combination order of the units The cascaded combination of thermal energy storage units using multiple melting point PCM units is employed to help extend the applicable temperature range, and it is also benefited waste heat recycling from multiple heat sources with different temperature classes such as the battery pack, power control unit (PCU), and motor for electric vehicle. In addition, the authors hope that the heat of the cascading thermal energy storage combination can release rapidly in practical application such as battery preheating in cold weather, and find out the effects of the different cascade order through experimental investigation. The thermal energy storage units include 30# PCM, 40# PCM, and 50# PCM, and the cascading combination has two orders including ascending melting point and descending melting point, as show in Fig. 2. The inlet and outlet liquid temperatures of each unit were Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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0.5L/min 1L/min 2L/min
t eht /s
heat storage
heat release
˄a˅Essential heat transfer time
0.5L/min 1L/min 2L/min
1L/min
2L/min
T /ć
qhp/kW
0.5L/min
heat storage
heat storage
heat release
˄b˅Average heat power
heat release
˄c˅Average outlet temperature
Fig. 8. Comparisons and analyses of different liquid flow rates.
70
60
50
40
Tol /ć
Tol /ć
measured. The inlet temperature of the first unit was also the inlet temperature of the cascading combination, and the outlet temperature of the last unit was also the outlet temperature of the cascading combination according to the two orders. During the heat storage process, the initial temperature of the cascading combination was 0 C, the inlet liquid temperature was 60 C, and the liquid flow rate was 2 L/min. Meanwhile, the initial temperature of the cascading combination was 60 C, the inlet liquid temperature was 0 C, and the liquid flow rate was still 2 L/min during the heat release process. Fig. 9 (a) demonstrates the variations in the outlet liquid temperature under the melting point ascending order condition during the heat storage process. With the exception of 40# PCM, it could be seen that the outlet temperature curves of the 30# PCM and 50# PCM did not present an obvious temperature slowly rise period, and this phenomenon means that the phase changes of 30# PCM and 50# PCM did not occur rapidly and completely in the early and middle periods. By analyzing the results, the authors found that the melting point ascending order was not good for the phase change uniformity during heat storage process. Fig. 9(b) illustrates the variations in the outlet liquid temperature under the melting point descending order condition during the heat storage process. Notably, all outlet temperature curves exhibited obvious temperature slowly rise period in the same time, and the phase change was very uniform. Fig. 10(a) demonstrates the variations in the outlet liquid temperature under the melting point ascending order condition during the heat release process. As shown, the phase change uniformity was better than the descending order in the same period. Fig. 10 (b) demonstrates the variations in the outlet liquid temperature under the melting point descending order condition during the heat release process. As shown, the phase change was not very uniform in the early and middle periods.
30# unit
30
40# unit 50# unit
20
30# unit
40# unit 50# unit
10
0
Time/s
Time/s
˄D˅Melting point ascending order˄E˅Melting point descending order Fig. 9. The variations of the outlet liquid temperature of each unit during the heat storage process.
Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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60
50
40
30# unit
Tol /ć
Tol /ć
9
40# unit
30
50# unit
30# unit 40# unit
20
10
50# unit
0
Time/s
Time/s
˄D˅Melting point ascending order˄E˅Melting point descending order Fig. 10. Variations in the outlet liquid temperatures of each unit during the heat release process.
In addition, after the heat storage and release times reaching 240 s, we can see the variations of the outlet liquid temperature of each unit during the heat storage and release process were not that sharp, the PCM phase change process nearly finished. Thus, analysis was performed during this heat storage and release periods, the Qhs of the descending order condition was 951.7 kJ, it was more than the Qhs of the ascending order condition by approximately 89 kJ in the same heat storage period. At the same time, the Qhr of the ascending order condition was 921.8 kJ, it was more than the Qhr of the descending order condition by approximately 88 kJ in the same heat release period. 3.5. Analysis of thermal efficiency Experiments using electric heaters and water tanks to simulate the heat release source, so the heat storage efficiency of the PCM heat storage unit is defined as the ratio of storage heat of the PCM heat storage unit to the release heat of the simulated heat source in a heat storage process. The electrical power is completely converted to heat, ignoring the heater electro-thermal conversion losses. Taking the Qhs of Fig. 4 (a) as an example, at the inlet temperature of 70 C at 360s, the stored heat of the heat storage unit is 481.8 kJ and the electrical energy is 0.2609 kWh measured by electrical energy meter. Then the heat storage efficiency of the heat storage unit in heat storage process is:
hs ¼
Q hs Qe
Where:
hs is the thermal efficiency of the unit in heat storage process. Qhs is heat storage capacity of PCM unit in heat storage process, kJ. Qe is electricity consumption of the unit, kJ. Similarly, define the heat release efficiency of the PCMs as the ratio of the refrigeration capacity of the refrigeration unit to the heat released by the heat storage unit in a heat release process. Taking the Qhr of Fig. 4(b) as an example, at the inlet temperature of 10 C at 360s, the released heat of the heat storage unit is 368.6 kJ, Refrigeration unit cooling power is 0.6 kW, so the thermal efficiency of heat storage unit in heat release process is:
hr ¼
Q re Q hr
Where:
hr is the thermal efficiency of the unit in heat release process. Qhr is heat release capacity of PCM unit in heat release process, kJ Qre is cooling capacity of refrigeration unit, kJ From the equations above, the thermal efficiency of the heat storage unit in heat storage process is 51.3%, the efficiency in heat release process is 58.6%. As for the combined units, the calculation process is similar with each unit. Taking the combinations of Figs. 9(b) and 10(A) at 240s as examples, the stored heat of the heat storage unit is 951.7 kJ and the electrical energy is 0.4665 kWh measured by electrical energy meter. The thermal efficiency of the heat storage units in descending order during heat storage process is:
hsc ¼
Q hsc Q ec
Where: Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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hsc is the thermal efficiency of the combined units in heat storage process. Qhsc is heat storage capacity of PCM units, in heat storage process kJ. Q ec is electricity consumption of combined units, kJ. The thermal efficiency of the heat storage units in ascending order during heat release process is:
hrc ¼
Q rec Q hrc
Where:
hrc is the thermal efficiency of the combined units in heat release process. Q hrc is heat release capacity of PCM units in heat release process, kJ Qrec is cooling capacity of refrigeration unit in combined units, kJ. From the equations above, the thermal efficiency of the heat storage units in descending order during heat storage process is 56.7%, the efficiency in ascending order during heat release process is 57.3%.Because of the much heat loss of the experimental system, such as the large heat capacity of the water in the heating tank, long duct, multiple components, the calculated heat storage efficiency form the experiment is much lower than real vehicle heat storage system in further research. Therefore, the main purpose of the system constructed in this experiment is to study the heat storage and release characteristics of the PCM heat storage unit. The rest cases will not be described in this paper, because of the same calculation process of each case. 4. Conclusions Within the experimental system and operating range, the designed PCM unit reaches the defined 70% heat capacity at 165s during the heat storage process and 180s during the heat release process, respectively. Moreover, under given condition, the heat storage efficiency and heat release efficiency of unit are 51.3% and 58.6%, and the heat storage efficiency and heat release efficiency of combined units are 56.7% and 57.3%. From the analysis indicates that the designed PCM thermal energy storage unit exhibits well heat storage and release performance. Furthermore, the phase change heat storage and release rates increase with increasing temperature differences between the PCM melting point and the inlet liquid temperature of the unit, and this characteristic affects the melting and solidification of the PCM. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Meanwhile, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. Meanwhile, the cascading combination of the PCM units is also help to recycle multiple heat energy with different temperature classes. During the heat storage process, adopting descending order by melting point can increase the phase change uniformity of the units and the heat storage rate. Also, the unit in ascending order by melting point is better than descending order during the heat release process. These findings are conducive to the selection of the optimal condition, and establishment of the control strategy for the PCM utilization in further study, especially for heat recovery system of EV. Acknowledgements The authors gratefully acknowledge the financial fund of the National Natural Science Foundation of China (No. U1864213) and “Double Ten” Science & Technology Innovation Project (No. 17SS022) of Jilin Province of China. Thanks also go to the continued support of Specific programs for industrial innovation of Jilin Development and Reform Committee (No.2016C022) and Science Foundation for The Excellent Youth Scholars of Jilin Province of China (No. 20180520066JH). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
O. Schatz, Cold Start Improvements with a Heat Store, SAE International, 1991. E. Korin, R. Reshef, D. Tshernichovesky, E. Sher, Improving Cold-Start Functioning of Catalytic Converters by Using Phase-Change Materials, 1998. A. Dinker, M. Agarwal, G.D. Agarwal, Heat storage materials, geometry and applications: a review, J. Energy Inst. 90 (2015) 1e11. T. Aldoss, Z.D.J.E. Yan, L. Ma, Numerical investigation of phase change materials for thermal management systems, SAE Int. J. Mater. Manuf. 2 (2009) 85e91. A.A. Adamczyk, C.P. Hubbard, F. Ament, S.H. Oh, M.J. Brady, M.C. Yee, Experimental and modeling evaluations of a vacuum-insulated catalytic converter, in: International Fuels & Lubricants Meeting & Exposition, 1999. S.A. Hallaj, J.R. Selman, ChemInform abstract: a novel thermal management system for electric vehicle batteries using phase-change material, ChemInform 32 (2001) nono. J.K. Park, H.S. Park, H.S. Kim, J.H. Yun, H. Kim, H.J. Jeong, Apparatus and Method for Air Conditioning Vehicle Interior Using Battery Charge Control of Electric Vehicle, in, US, 2011. T. Zhang, C. Gao, Q. Gao, G. Wang, M.H. Liu, Y. Guo, C. Xiao, Y.Y. Yan, Status and development of electric vehicle integrated thermal management from BTM to HVAC, Appl. Therm. Eng. 88 (2015) 398e409. S.A. Khateeb, S. Amiruddin, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal management of Li-ion battery with phase change material for electric scooters: experimental validation, J. Power Sources 142 (2015) 345e353. A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal conductivity enhancement of phase change materials using a graphite matrix, Appl. Therm. Eng. 26 (2006) 1652e1661. J. Fukai, M. Kanou, Y. Kodama, O. Miyatake, Thermal conductivity enhancement of energy storage media using carbon fibers, Energy Convers. Manag. 41 (2000) 1543e1556. J. Fukai, Y. Hamada, Y. Morozumi, O. Miyatake, Effect of carbon-fiber brushes on conductive heat transfer in phase change materials, Int. J. Heat Mass Transf. 45 (2002) 4781e4792. N. Vol, Experimental study of solid-liquid phase change in a spiral thermal energy storage, Appl. Therm. Eng. 19 (1999) 1253e1277. E. Assis, L. Katsman, G. Ziskind, R. Letan, Numerical and experimental study of melting in a spherical shell, Int. J. Heat Mass Transf. 50 (2007) 1790e1804.
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[15] M.M. Farid, A. Kanzawa, Thermal performance of a heat storage module using PCM's with different melting temperatures: mathematical modeling, J. Sol. Energy Eng. 111 (2) (1989) 152e157. [16] L. Yang, X. Zhang, G. Xu, Thermal performance of a solar storage packed bed using spherical capsules filled with PCM having different melting points, Energy Build. 68 (2014) 639e646. , J. Gasia, L. Miro , L.F. Cabeza, Experimental evaluation at pilot plant scale of multiple PCMs (cascaded) vs. single PCM configuration for thermal energy storage, [17] G. Peiro Renew. Energy 83 (2015) 729e736. [18] M. Ezra, Y. Kozak, V. Dubovsky, G. Ziskind, Analysis and optimization of melting temperature span for a multiple-PCM latent heat thermal energy storage unit, Appl. Therm. Eng. 93 (2016) 315e329. [19] A. Bejan, A.D. Kraus, Heat Transfer Handbook, John Wiley & Sons, 2003.
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Experimental research on thermal characteristics of PCM thermal energy storage units Tianshi Zhang a, b, Yubin Liu b, Qing Gao a, b, *, Guohua Wang a, b, Zhenmin Yan b, Ming Shen b a b
State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025, Changchun, China College of Automotive Engineering, Jilin University, 130025, Changchun, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2017 Received in revised form 22 March 2019 Accepted 1 April 2019 Available online xxx
To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage unit Phase change material Multiple melting points Thermal characteristics
1. Introduction Phase-change heat storage technologies have received considerable attention in the field of vehicle-mounted waste heat utilization. For the traditional internal combustion engine car, Schatz [1] proposed the concept of a heat battery, which adopts a phase change material (PCM) to store waste heat from engine cooling water. The heat could be used to preheat the cabin and improve engine cold start performance in winter. Korin [2] used latent heat energy to preheat a catalytic converter until it reached the optimum working temperature. Dinker [3] showed that PCM stores much heat more than sensible heat storage materials at constant temperature and many researchers have been studying to use PCMs in battery thermal management. Aldoss [4] built a mathematical model to investigate a lighting thermal management system with a PCM. Adamczyk [5] proposed a vacuum insulation method, and experimental results showed that this method could efficiently realize heat preservation for a long time. Compared with traditional vehicles, extending the operational range and reducing energy consumption are more critical problems for EVs. Because pure electric vehicles (PEVs) do not have engine waste heat, winter heating and battery cold starts have become difficult technical problems and large obstacles for all climate applications. Al-Hallaj [6] considered using heat stored in PCMs to improve the cold start performances of battery packs in cold regions. Park [7] proposed charging the heat and electricity at the same time for EVs using a heat storage device, and discussed coordinated control and application methods. Gao [8] et al. indicated that PCMs can absorb considerable heat from batteries, PCUs, and motors to make their working temperatures stable. In addition, waste heat can be used to defrost and preheat the cabin facilitating heat recovery and energy savings. Therefore, a thermal energy storage setup is important for vehicle-mounted heat energy reutilization and complementation, and it will promote electric vehicle thermal management (VTM) integration involving the battery packs, motors, power control units (PCUs), heat pumps (HPs), etc. At the same time, realizing rapid heat storage and release is a key problem in vehicle applications. Therefore, heat transfer
* Corresponding author. State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025, Changchun, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (Q. Gao). https://doi.org/10.1016/j.joei.2019.04.007 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Nomenclature
T2
hs PCM PCUs VTM Tol Qhs Qhr teht qhp Tao
r c
t Q Qhsc Qrec
Q qv T1
phase change material power control units vehicle thermal management outlet liquid temperature ( C) heat storage capacity (kJ) heat release capacity (kJ) essential heat transfer time (s) heat power (kW) average outlet temperature ( C) density of liquid (kg/m3) specific heat capacity of liquid (kJkg1K1) heat transfer time (s) heat storage/release capacity (kJ) heat storage capacity of PCM units,(kJ) cooling capacity of refrigeration unit in combined units, (kJ) heat storage/release power of PCM (kW) volume flow rate of liquid (m3/s) inlet liquid temperature ( C)
Qe
hr
Qre
hsc
Qec
hrc
Qhrc
outlet liquid temperature ( C) thermal efficiency of the unit in heat storage process electricity consumption of unit (kJ) thermal efficiency of the unit in heat release process cooling capacity of refrigeration unit (kJ) thermal efficiency of the combined units in heat storage process electricity consumption of combined units, (kJ) thermal efficiency of the combined units in heat release process heat release capacity of PCM units in heat release process, (kJ)
Subscripts ol outlet liquid hs heat storage hr heat release eht essential heat transfer hp heat power ao average outlet
enhancement technologies are necessary and important. At present, technological development mainly focuses on two aspects: the improvement of thermal conductivity and the optimization of the structure for heat exchange. Khateeb et al. [9] find it can enchance heat transfer and temperature uniformity to fill the PCM with aluminum foam. The results showed that the comprehensive thermal conductivity of the PCM increased. Mills [10] filled a PCM with a graphite matrix and experimentally investigated the effects of the filling proportion on the thermal conductivity and latent heat. Fukai [11,12] proposed the use of carbon fibers as heat transfer ducts inside PCMs. Banaszek [13] designed a spiral thermal energy storage unit and conducted theoretical and experimental research on this type of unit. E. Assis [14] adopted a small spherical shell to encapsulate PCM and investigated the effects of different diameters on the heat storage rate. Among other researchers' valuable findings, the use of multiple PCMs with different melting temperatures inside a latent heat thermal energy storage unit has been recommended as more efficient than single-PCM systems. Farid and Kanzawa [15] were the first to suggest the use of multiple PCMs arranged in a cascade, and showed the advantage of such an arrangement. Then multiple PCMs in various geometries has been developed experimentally and numerically, such as recently, Yang et al. [16] investigated a heat storage based on three types of PCMs in spherical capsules, it's indicate that there is an advantage of using multiple materials in the energy aspect, but the exergy efficiency et al. [17] explored a two-PCM unit is lower during the melting process using a finite difference method for the numerical solution. Peiro experimentally and demonstrated an effectiveness enhancement of above 19% as compared with a single-PCM configuration. Ezra et al. [18] present a numerically solved mathematical model to explore the effects of different parameters, including the inlet velocity and temperature of the HTF, the number of rows, the number of materials, and the PCMs' melting temperature span, for a latent heat thermal energy storage unit where an arbitrary number of PCMs, which can attain the shortest melting (charging) time for an entire multiple-PCM unit under given conditions. And the generalized results of the model are applicable in this more realistic case as well, comparing favorably with experimental results from the other literature. To realize heat energy recycling for electric vehicle and improve cold start performances of battery pack, the present work demonstrates a PCM thermal energy storage unit using flat tubes and corrugated fins. And the multiple PCMs with different melting temperatures (30#, 40#, and 50#)were cascaded to help extend the range of applicable temperature and satisfy multiple heat sources (battery, motor, PCUs, etc) with different temperature classes. Based on the experimental, the method of fluid active commutation is adopted to deal with different conditions of heat storage and heat release, i.e. during the heat storage process, adopting descending order by melting point to increase the phase change uniformity of the units and the heat storage rate, and ascending order by melting point is adopting during the heat release process. On this basis, we explore the factor of the heat storage and release rates of PCM units, such as the different inlet temperature, liquid flow rate, melting point of the PCM and the combination order of the units, and the thermal characteristics of the PCM units were investigated, such as the temperature variation, heat capacity, and heat transfer time. 2. Design of PCM unit and experimental system 2.1. Thermal energy storage unit The structure and geometric elements of the thermal energy storage unit is shown in Fig. 1. The core of unit consisted of flat tubes and corrugated fins with louvers. Liquid flowed inside the flat tubes. A PCM was added between the flat tubes, and the thickness of flat tube was 2 mm. The PCM was paraffin and the corrugated fins with louvers were used to increase surface area for heat transfer. The thickness of corrugated fin was 0.1 mm, and the width of it was 4 cm. The mass of PCM is 1.15 kg in the thermal energy storage unit. Thermo-physical properties of PCMs are shown in Table 1. Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Fig. 1. The structure and geometric elements of the thermal energy storage unit.
To concurrently recover waste heat from heat sources at different temperatures, a cascading heat storage combination was built, which adopted multiple melting point PCM units throughout the experimental process. The melting point of the PCMs included 30 C, 40 C, and 50 C, and the corresponding heat storage units were called 30#, 40#, and 50#, respectively. The cascading combinations have two arrangements that make use of a change in the direction of the flow of the liquid. These orders include the melting point ascending and descending orders, as show in Fig. 2. The solid arrow represents the forward flow direction, and the hollow arrow represents the backward flow direction. 2.2. System and apparatus The experimental system consisted of the thermal energy storage unit, a refrigerating system, a heating tank, a cooling tank, an expansion tank, an air duct, an axial fan, a radiator, pumps, flow distributors, regulating valves, an agitator, filters, etc. The experimental system loop and apparatus are presented in Fig. 3 (a) and (b), respectively. The liquid temperature was regulated using the heating tank, cooling tank, refrigerating system, and radiator. The expansion tank was used to maintain the system pressure and to add liquid. The liquid was a 50% aqueous ethylene glycol solution by capacity. During the experimental process, insulation measures were adopted for the PCM unit, heating tank, cooling tank, and pipes to reduce the environment impact. In addition, the test system mainly included a signal-acquiring device (National Instrument), a temperature control unit, a turbine flow meter, thermocouples (its measurement error is ±1.5 C), electrical energy meter, etc. 2.3. Working conditions and calculation method To systematically investigate the heat transfer characteristics and effects of the inlet temperature, liquid flow rate, PCM melting point, and order of the PCM units, a series of experimental conditions were defined. It's defined the liquid flow rate as 1 L/min with an initial PCM temperature of 0 C and an inlet liquid temperature of 60 C as the basic conditions during the heat storage process. Other conditions are shown in Table 2. Furthermore, the authors defined the liquid flow rate as 1 L/min, the initial PCM temperature as 60 C, and the inlet liquid flow temperature as 0 C as the basic conditions of the heat release process. The other conditions are shown in Table 3. In fact, the PCM heat storage includes the latent heat and the sensible heat; the former is more dominant in the early and middle periods than in the latter periods. Therefore, the essential heat transfer period was defined as the storage or release periods of the 70% heat capacity, and the heat storage or release capacity in the corresponding period was defined as the essential heat storage or release capacity. Some of the investigations and analyses were performed during the essential heat storage and release periods. In addition, the heat storage or release power is calculated as follows [19]:
Q ¼ qv r cðΤ 2 Τ 1 Þ Where:
Q is heat storage/release power of PCM, kW. qv is volume flow rate of liquid, m3/s. ris density of liquid, kg/m3. cis specific heat capacity of liquid, kJkg1K1. Τ 1 is inlet liquid temperature, C. Τ 2 is outlet liquid temperature, C. Therefore, the heat storage or release capacity can be calculated as follow: Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Table 1 Thermo-physical properties of PCM. Melting point of PCM ( C)
Density (kg/m3)
Specific heat capacity (kJ/kg K)
Heat of fusion (kJ/kg)
30 40 50
840 852 845
2.5 2.5 2.5
207 220 210
Fig. 2. Cascading combination of thermal energy storage units.
Fig. 3. Experimental system composition.
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Table 2 The working conditions during the heat storage process. liquid flow rate (L/min)
initial PCM temperature ( C)
inlet liquid flow temperature ( C)
melting point of PCM ( C)
0.5 1 2
0
50 60 70
30 40 50
Table 3 The working conditions during the heat release process. liquid flow rate (L/min)
initial PCM temperature ( C)
inlet liquid flow temperature ( C)
melting point of PCM( C)
0.5 1 2
60
10 0 10
30 40 50
Zt Q¼
Q ðtÞdt
0
Where: Q is heat storage/release capacity, kJ.
t is heat transfer time, s. It is simplified into measured time in the experiment and calculation. 3. Results and analysis The performance of the thermal energy storage unit was evaluated in the basic condition. In addition, the authors investigated the effects of the inlet liquid temperature, the liquid flow rate, and the PCM melting point, calculated the thermal efficiency in some typical cases and comparatively analyzed the performances of the different cascading combination orders. 3.1. Effect of inlet temperature The effects of the inlet liquid temperature were investigated based on the 40# PCM unit. In the heat storage process, the inlet temperature of the unit was 50 C, 60 C, or 70 C. In the heat release process, the inlet liquid temperature of the unit was 10 C, 0 C, or 10 C. Other parameters were consistent with the basic heat storage and release conditions. Fig. 4 (a) demonstrates the variations in the outlet liquid temperature (Tol) of the thermal energy storage unit and the heat storage capacity (Qhs) under different inlet liquid temperature conditions ranging from 50 C to 60 C during the heat storage process. Because the latent heat exceeded the sensible heat for the PCM, the outlet temperature curves had smaller slopes at the beginning of the phase change heat storage. Furthermore, the slope of Qhs was low for the 50 C condition, and the Tol curve even presented a temperature slowly rise period during the phase change heat storage period. The phase change heat storage rate was lowest for the 50 C condition. Meanwhile, the slopes of the Tol and Qhs curves were greatest and the heat storage rates were the highest for the 70 C condition in the same period. When the inlet temperature was 60 C (the basic heat storage condition), the essential heat storage time was 165 s, and the essential heat storage capacity was 309.1 kJ. Fig. 4(b)demonstrates the variations in the outlet temperature (Tol) and the heat release capacity (Qhr) under different inlet temperature conditions ranging from 10 C to 10 C in the heat release process. The slope of the Tol curve and the heat release rate were at their lowest values for the 10 C condition during the phase change heat release period. Meanwhile, the slope of the Tol curve and the heat release rate were at their highest values for the 10 C condition in the same period. The essential heat release time was 180 s, and the essential heat release capacity was 317.9 kJ for the 0 C condition (the basic heat release condition). It could be seen that the designed thermal energy storage unit exhibited well heat storage and release rates, and it satisfies the practical application for the battery preheating. Under the condition of heat storage, the total heat storage capacity is 309.1 kJ with a 1.15 kg 40# PMC, and we can calculate the latent heat is 253 kJ (1.15 kg*220 kJ/kg ¼ 253 kJ). And the condition of heat release is in like manner, which the total heat release capacity is 317.9 kJ with a 1.15 kg 40# PMC. We can see the latent heat accounts for a large proportion of the total storage or release of heat. 3.2. Effect of melting point The effects of the melting point were investigated in the basic heat storage and release conditions, and the PCM melting point was set to 30 C, 40 C, or 50 C in the experimental process. Fig. 5 (a) demonstrates the variations of the Tol and Qhs under different PCM melting points ranging from 30# to 50# during the heat storage process. It could be seen that the outlet water temperature rose the most, but the heat storage capacity increased most slowly for 50# PCM in defined the essential heat transfer period and 40# PCM secondly. As for 30# PCM, the outlet water temperature rose most slowly, but the heat storage capacity increased fastest. Analysis shows that this is due to the same inlet temperature, flow rate and other conditions, the PCM unit with high melting point temperature cannot rapidly carry out latent heat storage because it goes into the phase change melting stage slowly. Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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Tol /ć
ć7RO ć7RO ć7RO ć4KV ć4KV ć4KV
ć7RO ć7RO ć7RO ć4KU ć4KU ć4KU
Tol/ć
Qhs/kJ
Qhr/kJ
6
Time/s
Time/s
˄E˅Heat release process
˄D˅Heat storage process
Fig. 4. The variations in the outlet temperature and heat capacity for several inlet liquid temperatures.
7RO
7RO
7RO
4KU
7RO
4KV 4KV
4KV
Time/s
˄D˅Heat storage process
7RO
4KU
4KUN-
7RO
7 ROć
4 N-
T /ć
4KU
Time/s
˄E˅Heat release process
Fig. 5. The variations in the outlet liquid temperature and heat capacity for several PCM melting points.
Fig. 5 (b) demonstrates the variations of the Tol and Qhr under different PCM melting points from 30# to 50# during the heat release process. It could be seen that the outlet water temperature drops the slowest, but the heat release capacity increases fastest for 50# PCM in defined the essential heat transfer period and 40# PCM secondly. As for 30# PCM, the outlet water temperature drops fastest, but the heat release capacity increased most slowly. Analysis shows that this is due to the same inlet temperature, flow rate and other conditions, the PCM unit with high melting point temperature enter the phase change melting exothermic stage quickly, and the latent heat can be released rapidly. Fig. 6 illustrates the comparisons and analyses of PCMs with different melting points in the essential heat transfer (storage or release) periods. In the figure, as the PCM melting point increased, the essential heat transfer time (teht) was extended, the heat power (qhp) decreased, and the average outlet temperature (Tao) rose gradually in the basic heat storage condition. In the basic heat release condition, as the PCM melting point increased, the qhp and Tao also rose, but teht gradually decreased, and the higher melting point PCMs maintained higher outlet liquid temperatures.
3.3. Effect of liquid flow rate The effects of the liquid flow rate were investigated based on the 40# PCM unit, and the liquid flow rate was 0.5 L/min, 1 L/min, or 2 L/min in the experimental process. Other parameters were consistent with the basic heat storage and release conditions. Fig. 7(a) demonstrates the variations of the Tol and Qhs under different liquid flow rates ranging from 0.5 L/min to 2 L/min during the heat storage process. As demonstrated, the slope of the Tol curve was at a maximum and the rising speed of Qhs was fastest under the 2 L/min condition during the phase change heat storage period, which means that the phase change heat storage rate was the fastest. At the same time, the rising speed of Qhs was slowest and the heat storage rate was lowest under the 0.5 L/min condition in the same period. When the liquid flow rate was 1 L/min, the phase change heat storage rate and rising speed of Qhs were between 0.5 L/min and 2 L/min. Fig. 7(b) illustrates variations of the Tol and Qhr under different liquid flow rates ranging from 0.5 L/min to 2 L/min during the heat release process. It could be seen that the slope of the Tol curve was at its maximum value, and the rising speed of Qhr was fastest under the 2 L/min condition during the phase change heat release period, which means that the phase change heat release rate was fastest. At the same time, Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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30# 40# 50#
t eht /s
heat storage
heat release
˄a˅Essential heat transfer time
30# 40# 50#
Tao /ć
qhp/kW
30# 40# 50#
heat storage
heat storage
heat release
˄b˅Average heat power
heat release
˄c˅Average outlet temperature
Fig. 6. Comparisons and analyses of PCMs with different melting points.
/PLQ7RO /PLQ7RO /PLQ7RO /PLQ4KU /PLQ4KU /PLQ4KU
/PLQ7RO /PLQ7RO /PLQ7RO
/PLQ4KV /PLQ4KV
/PLQ4KV
T /ć
Q /kJ
T /ć
Q /kJ
Time/s
˄D˅Heat storage process
Time/s
˄E˅Heat release process
Fig. 7. Variations in the outlet temperature and heat capacity for several liquid flow rates.
the rising speed of Qhr was slowest and the heat release rate was lowest under the 0.5 L/min condition in the same period. When the liquid flow rate was 1 L/min, the heat release rate and rising speed of Qhr were between 0.5 L/min and 2 L/min. Fig. 8 demonstrates the comparisons and analyses of different liquid flow rates in the range of essential heat transfer (storage and release) periods. With increasing liquid flow rate, teht gradually decreased, and qhp increased. Meanwhile, Tao increased in the heat storage process but decreased gradually in the heat release process. Furthermore, in the middle and later periods, the role of the liquid flow rate continued to decrease compared with the early period. Therefore, heat transfer enhancement could not be realized through a continuous increase in the liquid flow rate. 3.4. Effect of combination order of the units The cascaded combination of thermal energy storage units using multiple melting point PCM units is employed to help extend the applicable temperature range, and it is also benefited waste heat recycling from multiple heat sources with different temperature classes such as the battery pack, power control unit (PCU), and motor for electric vehicle. In addition, the authors hope that the heat of the cascading thermal energy storage combination can release rapidly in practical application such as battery preheating in cold weather, and find out the effects of the different cascade order through experimental investigation. The thermal energy storage units include 30# PCM, 40# PCM, and 50# PCM, and the cascading combination has two orders including ascending melting point and descending melting point, as show in Fig. 2. The inlet and outlet liquid temperatures of each unit were Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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0.5L/min 1L/min 2L/min
t eht /s
heat storage
heat release
˄a˅Essential heat transfer time
0.5L/min 1L/min 2L/min
1L/min
2L/min
T /ć
qhp/kW
0.5L/min
heat storage
heat storage
heat release
˄b˅Average heat power
heat release
˄c˅Average outlet temperature
Fig. 8. Comparisons and analyses of different liquid flow rates.
70
60
50
40
Tol /ć
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measured. The inlet temperature of the first unit was also the inlet temperature of the cascading combination, and the outlet temperature of the last unit was also the outlet temperature of the cascading combination according to the two orders. During the heat storage process, the initial temperature of the cascading combination was 0 C, the inlet liquid temperature was 60 C, and the liquid flow rate was 2 L/min. Meanwhile, the initial temperature of the cascading combination was 60 C, the inlet liquid temperature was 0 C, and the liquid flow rate was still 2 L/min during the heat release process. Fig. 9 (a) demonstrates the variations in the outlet liquid temperature under the melting point ascending order condition during the heat storage process. With the exception of 40# PCM, it could be seen that the outlet temperature curves of the 30# PCM and 50# PCM did not present an obvious temperature slowly rise period, and this phenomenon means that the phase changes of 30# PCM and 50# PCM did not occur rapidly and completely in the early and middle periods. By analyzing the results, the authors found that the melting point ascending order was not good for the phase change uniformity during heat storage process. Fig. 9(b) illustrates the variations in the outlet liquid temperature under the melting point descending order condition during the heat storage process. Notably, all outlet temperature curves exhibited obvious temperature slowly rise period in the same time, and the phase change was very uniform. Fig. 10(a) demonstrates the variations in the outlet liquid temperature under the melting point ascending order condition during the heat release process. As shown, the phase change uniformity was better than the descending order in the same period. Fig. 10 (b) demonstrates the variations in the outlet liquid temperature under the melting point descending order condition during the heat release process. As shown, the phase change was not very uniform in the early and middle periods.
30# unit
30
40# unit 50# unit
20
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40# unit 50# unit
10
0
Time/s
Time/s
˄D˅Melting point ascending order˄E˅Melting point descending order Fig. 9. The variations of the outlet liquid temperature of each unit during the heat storage process.
Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007
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60
50
40
30# unit
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9
40# unit
30
50# unit
30# unit 40# unit
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50# unit
0
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˄D˅Melting point ascending order˄E˅Melting point descending order Fig. 10. Variations in the outlet liquid temperatures of each unit during the heat release process.
In addition, after the heat storage and release times reaching 240 s, we can see the variations of the outlet liquid temperature of each unit during the heat storage and release process were not that sharp, the PCM phase change process nearly finished. Thus, analysis was performed during this heat storage and release periods, the Qhs of the descending order condition was 951.7 kJ, it was more than the Qhs of the ascending order condition by approximately 89 kJ in the same heat storage period. At the same time, the Qhr of the ascending order condition was 921.8 kJ, it was more than the Qhr of the descending order condition by approximately 88 kJ in the same heat release period. 3.5. Analysis of thermal efficiency Experiments using electric heaters and water tanks to simulate the heat release source, so the heat storage efficiency of the PCM heat storage unit is defined as the ratio of storage heat of the PCM heat storage unit to the release heat of the simulated heat source in a heat storage process. The electrical power is completely converted to heat, ignoring the heater electro-thermal conversion losses. Taking the Qhs of Fig. 4 (a) as an example, at the inlet temperature of 70 C at 360s, the stored heat of the heat storage unit is 481.8 kJ and the electrical energy is 0.2609 kWh measured by electrical energy meter. Then the heat storage efficiency of the heat storage unit in heat storage process is:
hs ¼
Q hs Qe
Where:
hs is the thermal efficiency of the unit in heat storage process. Qhs is heat storage capacity of PCM unit in heat storage process, kJ. Qe is electricity consumption of the unit, kJ. Similarly, define the heat release efficiency of the PCMs as the ratio of the refrigeration capacity of the refrigeration unit to the heat released by the heat storage unit in a heat release process. Taking the Qhr of Fig. 4(b) as an example, at the inlet temperature of 10 C at 360s, the released heat of the heat storage unit is 368.6 kJ, Refrigeration unit cooling power is 0.6 kW, so the thermal efficiency of heat storage unit in heat release process is:
hr ¼
Q re Q hr
Where:
hr is the thermal efficiency of the unit in heat release process. Qhr is heat release capacity of PCM unit in heat release process, kJ Qre is cooling capacity of refrigeration unit, kJ From the equations above, the thermal efficiency of the heat storage unit in heat storage process is 51.3%, the efficiency in heat release process is 58.6%. As for the combined units, the calculation process is similar with each unit. Taking the combinations of Figs. 9(b) and 10(A) at 240s as examples, the stored heat of the heat storage unit is 951.7 kJ and the electrical energy is 0.4665 kWh measured by electrical energy meter. The thermal efficiency of the heat storage units in descending order during heat storage process is:
hsc ¼
Q hsc Q ec
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hsc is the thermal efficiency of the combined units in heat storage process. Qhsc is heat storage capacity of PCM units, in heat storage process kJ. Q ec is electricity consumption of combined units, kJ. The thermal efficiency of the heat storage units in ascending order during heat release process is:
hrc ¼
Q rec Q hrc
Where:
hrc is the thermal efficiency of the combined units in heat release process. Q hrc is heat release capacity of PCM units in heat release process, kJ Qrec is cooling capacity of refrigeration unit in combined units, kJ. From the equations above, the thermal efficiency of the heat storage units in descending order during heat storage process is 56.7%, the efficiency in ascending order during heat release process is 57.3%.Because of the much heat loss of the experimental system, such as the large heat capacity of the water in the heating tank, long duct, multiple components, the calculated heat storage efficiency form the experiment is much lower than real vehicle heat storage system in further research. Therefore, the main purpose of the system constructed in this experiment is to study the heat storage and release characteristics of the PCM heat storage unit. The rest cases will not be described in this paper, because of the same calculation process of each case. 4. Conclusions Within the experimental system and operating range, the designed PCM unit reaches the defined 70% heat capacity at 165s during the heat storage process and 180s during the heat release process, respectively. Moreover, under given condition, the heat storage efficiency and heat release efficiency of unit are 51.3% and 58.6%, and the heat storage efficiency and heat release efficiency of combined units are 56.7% and 57.3%. From the analysis indicates that the designed PCM thermal energy storage unit exhibits well heat storage and release performance. Furthermore, the phase change heat storage and release rates increase with increasing temperature differences between the PCM melting point and the inlet liquid temperature of the unit, and this characteristic affects the melting and solidification of the PCM. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Meanwhile, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. Meanwhile, the cascading combination of the PCM units is also help to recycle multiple heat energy with different temperature classes. During the heat storage process, adopting descending order by melting point can increase the phase change uniformity of the units and the heat storage rate. Also, the unit in ascending order by melting point is better than descending order during the heat release process. These findings are conducive to the selection of the optimal condition, and establishment of the control strategy for the PCM utilization in further study, especially for heat recovery system of EV. Acknowledgements The authors gratefully acknowledge the financial fund of the National Natural Science Foundation of China (No. U1864213) and “Double Ten” Science & Technology Innovation Project (No. 17SS022) of Jilin Province of China. Thanks also go to the continued support of Specific programs for industrial innovation of Jilin Development and Reform Committee (No.2016C022) and Science Foundation for The Excellent Youth Scholars of Jilin Province of China (No. 20180520066JH). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Please cite this article as: T. Zhang et al., Experimental research on thermal characteristics of PCM thermal energy storage units, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.04.007