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Analysis of temperature control effect of composite phase change structure used in thermoelectric conversion system
Journal Pre-proofs Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System Jia Yu, Haoqing Wang, Li Kong, Hongji Zhu, Qingshan Zhu, Jiawen Su, Jialin Guan PII: DOI: Reference:
S1359-4311(18)37811-6 https://doi.org/10.1016/j.applthermaleng.2019.114760 ATE 114760
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
20 December 2018 28 November 2019 2 December 2019
Please cite this article as: J. Yu, H. Wang, L. Kong, H. Zhu, Q. Zhu, J. Su, J. Guan, Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114760
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© 2019 Published by Elsevier Ltd.
Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System
Jia Yu1, Haoqing Wang1,*, Li Kong1, , Hongji Zhu1, Qingshan Zhu1, Jiawen Su1, Jialin Guan1
1
College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, P. R. China
* Corresponding authors: [email protected] (Haoqing Wang)
1
Abstract In this study, phase change materials made using graphite foam (GF) are placed on the hot side of a thermoelectric generator for thermal control and additional energy output. The experiment results prove the feasibility of this method, and the simulation results indicate a balance between the thermal conductivity and the amount of heat absorption. Compared with GF of 50 Wm-1K-1, GF of 100 Wm-1K-1 can produce 47% more electricity, whereas GF of 200 Wm-1K-1 can produce 69% more electricity, but overheats within 54 s. Overheating will cause various problems and should therefore be avoid. A porosity of 0.9 can produce 71.81% more electricity than a porosity of 0.95, whereas a porosity of 0.8 can produce 114% more electricity, but the material overheats in 79 s. It can thus be predicted that the materials with a change in their composite phase should have a high thermal conductivity and sufficient volume for a temperature control of a thermoelectric generator.
Keywords: Phase change materials; Graphite foam; Porosity; Thermal conductivity; Temperature control
2
1. Introduction It is well known that an optimised energy use is extremely important. Phase change materials (PCMs) can absorb excess energy during manufacturing abrupt temperature changes in the environment usually have a low thermal conductivity
[4]
[3]
[1][2]
, and significantly reduce any
. However, a problem exists in that PCMs
. A thermoelectric generator (TEG) can then
convert energy from waste heat into electrical energy
[5]-[7]
, although its thermoelectric
conversion efficiency is based on the temperature of the thermal environment. Therefore, in recent years, researchers have begun to use PCMs to construct the thermal environment required for a TEG [8]-[15].
In this regard, Kiziroglou et al. produced a thermoelectric absorption conversion device using water as a phase change material
[16]
. The results show that the use of PCMs can increase
approximately 30% of the total amount of thermoelectric energy conversion as compared to the sole use of thermoelectric materials. Tu et al. improved the design of Kiziroglou’s by applying composite PCMs made from expanded graphite and paraffin
[17]
study
. The composite
PCMs (paraffin, 5 wt% expanded graphite) further increased the total output energy by 32.32%, as compared with the results obtained using pure PCMs. This finding was also confirmed from the results by Carneiro et al., who used a certain amount of PCMs for practical feasibility and thermoelectric materials to generate 6 kJ of electrical energy
[18]
,
which was sufficient to satisfy the use of a submersible in a single cycle. The advantages of PCMs and a TEG can be effectively combined to save energy and reduce emissions.
Rather than using PCMs solely as a heat sink at the cold side, such as in the above studies, this research aims to use PCMs at the hot side of the TEG for thermal control. An illustration 3
of this study is shown in Fig. 1. When the temperature of the heat source quickly changes by a °C, the temperature at both sides of the TEG will then change by b °C and c °C, making the voltage output completely unstable. At this stage, a PCM with a melting point of approximately 60 °C is added between the heat source and TEG. Theoretically, this addition will prevent the temperature at both sides of the TEG from changing significantly within a short time, and the temperature will thus be maintained, which will also stabilise the efficiency of the TEG. However, the thickness of the phase change material affects the heat transfer process, and thus the thermal conductivity of the phase change material must be increased.
Figure 1. Schematic diagram of temperature control effect of PCM
2. Experiment Section 2.1. Design of experiment and simulation models Based on the above-stated ideas, a waste heat recovery power generation structure was designed in this study, as shown in Fig. 2. In this structure, graphite foam composite phase change materials (G-PCMs) were placed on the hot side of the TEG, which was packaged in aluminium foil to prevent leakage. In the simulation calculation, the presence of aluminium foil can be ignored owing to its high thermal conductivity and thinness. The other surface of 4
the G-PCM was in direct contact with the working environment temperature (T0) as a heat flow inlet, and the cold side of the TEG was used as a water-cooling temperature control system to keep the temperature T2 constant, serving as a heat flow outlet. Insulation materials were used to reduce heat leakage from the remaining surfaces. When the entire structure was placed in a high-temperature environment, the temperature difference of the TEG was directly determined using temperature T1. It can be inferred that when the temperature changes, the G-PCMs will maintain temperature T1 for a certain amount of time, and the stored energy can also allow the TEG to operate for an additional period of time.
Figure 2. Schematic diagram of (a) experiment and (b) simulation models. 2.2. Actual temperature control effect of composite phase change materials In general, pure PCMs have a low thermal conductivity
[19]
. For example, the thermal
conductivity of paraffin wax is approximately 0.15 Wm-1K-1, which limits its capacity for temperature control. A number of methods have been mentioned in previous studies for improving the thermal conductivity of PCMs [22]
[20]
, namely the use of fins
[21]
, nanomaterials
, expanded graphite [23], and porous materials [24]-[27], among others. G-PCMS composed of
graphite foam (GF) demonstrates a higher thermal conductivity and lower mass. Such composite materials are becoming widely popular, particularly in studies in the automotive and aerospace sectors. 5
Figure 3. (a) GF, (b) paraffin, and (c)–(f) production process of G-PCMS. The G-PCMS used in this experiment was constructed from purchased GF and paraffin. The total size of the structure was 46.2 × 44.8 × 12.5 mm3, its thermal conductivity was approximately 75 Wm-1K-1, the porosity was 0.9, and the hole diameter was approximately 1.2–1.5 mm. The thermal conductivity of the paraffin wax was approximately 0.15 Wm-1K-1 and its melting point ranged from 53°C to 60 °C. Moreover, the entire equivalent thermal conductivity was approximately 2–3 Wm-1K-1, which is 20-times that of pure PCM. The model specifications of the TEG are TEP1-126T200. Based on the provided instructions, the maximum continuous working temperature was 120 °C, and the structure will become completely damaged at 138 °C. The size of the TEG was 40 × 40 × 3.4 mm3, the main material is Bi2Te3, and its thermal conductivity is 1.2–1.5 Wm-1K-1. The main instruments used, as shown in Fig. 4, included an electric heating platform, a water-cooling temperature control platform, a voltage tester, a temperature tester, a weight for pressurisation, and some foam for simulating an adiabatic environment.
6
Figure 4. (a) G-PCMS and TEG and (b) experiment instruments. To simulate the temperature change of the electronic chip, the temperature of the heat source was increased from 50 Ԩ to 65 Ԩ during the first step. Once the temperature stabilised, it was
further increased to 75 Ԩ during the second step. Owing to a heating of the instrument, the heating temperature slightly exceeded the set temperature, and then decreased, and thus the experiment results are likely to exceed the value for the expected temperature. Each test instrument was tested before commencing the formal experiment. The heat source temperature was set to a constant 45 Ԩ, and the cold side temperature was set to a room temperature of 22 Ԩ. The output voltage of the TEG at 0.01 V was considered stable.
Subsequently, the values were recorded as follows: T0 = 45.0 Ԩ, T1 = 27.3 Ԩ, T2 = 25.3 Ԩ, and a voltage of 0.1238 V. Based on the instructions of TEG, each 1 Ԩ temperature difference will produce 65 mV of voltage. Therefore, when the temperature difference was
20 Ԩ, the voltage was 0.13 V. The error between the actual measured value and the calculated value was found to be 4.77%. Such an error is small, implying a normal working capacity of the experimental instrument. The simulation was calculated using the solidification and melting model found in the commercial software, Fluent. As shown in Fig. 5, the calculation model size was drawn according to the physical size, with the size of each mesh cell being 1 mm. The GF had a 7
porosity of 0.9, and a porous model was used to simulate the G-PCMs. The hot and cold sides were set to the instantaneous temperature boundary conditions. The remaining boundary conditions were adiabatic, and the property parameters for the material were set as shown in Table 1. In addition, the time step was 0.5–1 s. Factors such as the interfacial thermal resistance and thermal radiation were ignored, and the material parameters were assumed to be constants.
Figure 5. Data of the simulation model. Table 1. Material property parameters Material
Density (kgm-3)
Specific heat (kJkg-1K-1)
Thermal conductivity (Wm-1K-1)
Latent heat (kJkg-1)
Melting point (K)
Paraffin wax
800
2850
0.15
200
330
Graphite foam
2000
710
75
/
/
TEG
7740
152
1.5
/
/
Copper
8978
381
387.6
/
/
2.3 Analysis and results The experiment time was approximately 660 s. The simulated heat source was obtained using a curve fitting method in MATLAB. To approximate the experiment data, fourth- and
8
fifth-order polynomials were used for fitting. In addition, some data points were hidden for easier viewing. It is clear from Fig. 6 that the results of the first set of numerical simulations, T1_N1, are quite different from the experiment results, T1_E, causing the PCM to melt completely in advance. The reason for this is the incredibly high thermal conductivity in the simulation calculation at approximately 7.5 Wm-1K-1. Based on the experience of other researchers
[28][29][30]
, necessary corrections were made to the overall calculation of the
thermal conductivity of the G-PCMS using Fluent. The actual thermal conductivity is lower than the value obtained using the built-in formula of the Fluent porous media model. A calculation formula for the macroscopic thermal conductivity of the GF was proposed by Klett
[31]
; based on this formula, the thermal conductivity ultimately changed to
approximately 2.06 Wm-1K-1, and the second group of simulation calculations was made. It is clear from the figure that the corrected result, T1_N2, is closer to the actual test result, T1_E. Therefore, the same correction method was used in the simulation calculations for this paper. The average difference between T1_N2 and T1_E was observed to be 12.5%, because the boundary conditions of the external environment were simplified in the simulation calculation, and heat loss to the surrounding air was not considered. Since the value set in the simulation calculation cannot be the same as the values for actual situation, therefore, the difference between the simulation calculation results and actual experiment is considered to be normal.
9
Temperature (Ԩ )
80 70
T0_N T1_N1 T1_N2
60 50
T0_E T1_E
40 30 -100
0
100 200 300 400 500 600 700 Time (s)
Figure 6. Comparison of experiment and simulation results
Figure 7. Melting process of G-PCMS at different times in the second set of simulation calculations. During the experiment, the heat sources T0_E and T0_N were subjected to two large mutations within the comparative simulation, increasing by 15 °C and 10 °C, respectively. It is clear from Fig. 6 that the temperature, T1, at the other end of the composite phase-change material also changed, although its magnitude of change was relatively small. The maximum temperature change of the heat source in a working cycle was 79–50 = 29 °C. In addition, the temperature change for T1_E was 33.2–27.9 = 5.3 °C, which only accounts for 18.3% of the original change. In the simulation, the value of T1_N2 was 37.8–30.2 = 7.6 °C, which is 26.2%
10
of the original change. All values are less than half of the values for the original change, thus proving that the use of G-PCMS in maintaining a stable temperature is completely feasible. At the same time, the analysis of the two sets of simulation results in the above figure show that the overall thermal conductivity of the composite has a certain level of influence on the results for the temperature control. The two sets of simulation calculations can be considered as a comparison of two different types of GF. When the thermal conductivity of the GF is high, the instantaneous temperature, T1, is also obviously high with a low thermal conductivity. The above experiment and calculation results show that the use of G-PCMS can effectively reduce the temperature fluctuations. In addition, the high thermal conductivity of G-PCMS can increase the temperature of the hot side of the TEG to obtain a higher electrical output as compared to the use of a material with a low thermal conductivity. 3. Simulations and Discussion 3.1. Evaluation method of thermal control capability of G-PCMS The feasibility of the numerical simulation method and the practicality of G-PCMS for thermoelectric generation was verified in the previous section. However, it is clear from Fig. 6 that the low thermal conductivity leads to a decrease in the output temperature range. To promote the optimal working efficiency of the thermoelectric generator, the temperature range is a significant factor. In the first aspect, based on the maximum temperature of the TEG hot end, the thermoelectric generator has a range of operating temperatures, and if the maximum temperature exceeds this range, the device will be damaged. However, when conducting a thermoelectric conversion, focus is on the conversion efficiency, and the more 11
energy available under the same heat source conditions, the better the design is. In this study, the total energy output and the maximum temperature for different materials are considered. Referring to the heater used in the experiment and the heat source boundary conditions found in the literature, in the new simulation calculation, the heat source was defined as indicated in Fig. 7. It was heated twice with a heating power of 200 W and studied using different types of GF. To reduce the temperature fluctuation and achieve a more complete and gradual melting, the simulation experiments were carried out in an environment with a temperature exceeding 120 °C. An Erythritol-based composite material was used owing to its melting point of close to 120 °C. Based on the total heating energy and heating time, we can estimate that the volume required for the phase change material was approximately 21.6 cm3. The thermal conductivity of the GF was changed from 50 to 200 Wm-1K-1, and the porosity was changed from 0.8 to 0.95.
SFPBKRWIDFH WHJBKRWIDFH KHDWHUBSRZHU
3RZHU :
7HPSHUDWXUH .
7LPH V
Figure 8. Schematic diagram of heating progress and temperature of heat source Table 3. Material parameters of erythritol
Material
density (kgm-3)
specific heat (kJkg-1K-1) 12
thermal latent conductivity heat (Wm-1K-1) (kJkg-1)
melting point (K)
1300(liquid)
2760(liquid)
1451(solid)
1380(solid˅
Erythritol
0.326
316.8
392.94
3.2. Effects of the thermal conductivity of GF The thermal conductivity of the GF itself directly affected the effective thermal conductivity of the composite, and GF with different thermal conductivities was then calculated under the same porosity and other conditions. Taking 200 Wm-1K-1 as an example, as indicated in Fig. 8, a high thermal conductivity can quickly increase the temperature of the TEG to above 100 °C. However, owing to a high heat source power, the temperature of the TEG increases beyond the normal temperature range and leads to an overheating. Therefore, the thermal conductivity should not be too high. A TEG ratio of 0.131 mVK-1
[17]
is used to calculate the open-circuit voltage at the
instantaneous temperature, the externally output electrical power is then determined, and finally, the time is integrated to obtain the total power output. In the calculation, a voltage of 5 V or more is used as an effective output, and a voltage of less than 5 V is ignored. Considering an overheating, all temperatures above 120 °C are changed to 120 °C for the purposes of the calculation. The results are shown in Table 4. We can see that in the current calculation the electric energy of the TEG can increase by 69.27%. However, at 150 Wm-1K-1 and 200 Wm-1K-1, overheating occurs for 44 and 54 s, respectively, which is almost the same as half the length of the heating time. Oppositely, at 100 Wm-1K-1, not only is the TEG well protected during the working cycle, sufficient electrical energy is also generated. The electric energy is increased by 47.19% from 50 to 100 Wm-1K-1, and is only increased by 14.59% from 100 to 200 Wm-1K-1. It can be seen that continuing to increase the thermal
13
conductivity will cause the PCMs to melt faster and cause an overheating, whereas the increments of the output energy of the TEG are reduced.
Figure 9. (a) Liquid fraction and (b) temperature comparison charts. Table 4. Total value of electrical energy generated Thermal
50 /Wm-1K-1
75/Wm-1K-1 100/Wm-1K-1 150/Wm-1K-1 200/Wm-1K-1
conductivity Electric energy
12,772.16J
16,646.18J
18,866.96J
20,667.41J
21,619.36J
Increments
0
30.33%
47.19%
61.82%
69.27%
3.3. Effect of GF porosity Graphite foam is a porous material with a high thermal conductivity. In addition to its thermal conductivity, the porosity of GF is often used to calculate the equivalent thermal conductivity and total energy storage. The following is a simulated calculation for multiple levels of porosity of between 0.8 and 0.95. According to the melting rate shown in Fig. 9, the melting rate is found to be close to 100% when the porosity is 0.9, and the melting condition can continue to operate for another cycle when the porosity is above 0.9. It is clear that when the total volume is the same, increasing 14
the porosity can ensure additional working time. From the perspective of a thermal response, as the porosity decreases, the amount of GF and the overall thermal conductivity of the G-PCMS both increase. As shown in Table 5, the effective thermal conductivity still affects the thermoelectric output. The electrical energy output increases by 114% when the porosity changes from 0.95 to 0.8. However, the overheating time is extended owing to a reduction of the phase-change material; for example, the overheating time is 79 s at a porosity of 0.8, which is 65.83% of the total heating time. Thus, a porosity of 0.9 is the best design offering full protection and energy output.
Figure 10. (a) Liquid fraction and (b) temperature comparison charts. Table 5. Total value of electrical energy generated Porosity
0.95
0.92
0.9
0.85
0.8
Electric energy
9,734.26J
14,215.33J
16,724.63J
19,716.96J
20,831.61J
Increments
0
46.03%
71.81%
102.55%
114.00%
4. Conclusions 15
It is feasible to place a phase-change material having an improved thermal conductivity between the heat source and the thermoelectric generator. In the first aspect, G-PCMS can not only provide a stable temperature output in an unstable environment, it can also lower the temperature to below its phase change temperature. In the second aspect, the composite PCMs can provide the TEG with a greater temperature difference than pure PCMs. In an environment heated twice at 200 W, G-PCMs made from GF with a thermal conductivity of 100 Wm-1K-1 and a porosity of 0.9 can protect the TEG from damage while generating a large amount of electricity. However, appropriate G-PCMs should be chosen for different working conditions. The results of the simulations show that a rate of 100 Wm-1K-1 can produce 47% more electrical energy than a rate of 50 Wm-1K-1, although 200 Wm-1K-1 can produce 69% more energy while overheating within 54 s. Thus, a porosity of 0.9 can produce 71.81% more electrical energy than a porosity of 0.95, whereas a porosity of 0.8 can produce 114% more energy while overheating within 79 s. Based on these conclusions, we can see that the G-PCMs have the potential to control the temperature of the TEG. However, the G-PCMs should be improved to obtain a balance between the thermal conductivity and the heat absorption, rather than simply enhancing the thermal conductivity.
Notes The authors declare no competing financial interest.
Acknowledgments
16
We gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (Project No. 51672054).
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Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System
Jia Yu1, Haoqing Wang1,*, Li Kong1, , Hongji Zhu1, Qingshan Zhu1, Jiawen Su1
1
College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, P. R. China
* Corresponding authors: [email protected] (Haoqing Wang)
22
Highlights l
Experiments verify the feasibility of simulation calculation.
l
CPCM's thermal conductivity is better than PCM.
l
Heat storage and thermal conductivity must be considered simultaneously during temperature control.
l
The design of each set of CPCM is related to the location, and the C value is also the same.
23
S1359-4311(18)37811-6 https://doi.org/10.1016/j.applthermaleng.2019.114760 ATE 114760
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
20 December 2018 28 November 2019 2 December 2019
Please cite this article as: J. Yu, H. Wang, L. Kong, H. Zhu, Q. Zhu, J. Su, J. Guan, Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114760
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Ltd.
Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System
Jia Yu1, Haoqing Wang1,*, Li Kong1, , Hongji Zhu1, Qingshan Zhu1, Jiawen Su1, Jialin Guan1
1
College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, P. R. China
* Corresponding authors: [email protected] (Haoqing Wang)
1
Abstract In this study, phase change materials made using graphite foam (GF) are placed on the hot side of a thermoelectric generator for thermal control and additional energy output. The experiment results prove the feasibility of this method, and the simulation results indicate a balance between the thermal conductivity and the amount of heat absorption. Compared with GF of 50 Wm-1K-1, GF of 100 Wm-1K-1 can produce 47% more electricity, whereas GF of 200 Wm-1K-1 can produce 69% more electricity, but overheats within 54 s. Overheating will cause various problems and should therefore be avoid. A porosity of 0.9 can produce 71.81% more electricity than a porosity of 0.95, whereas a porosity of 0.8 can produce 114% more electricity, but the material overheats in 79 s. It can thus be predicted that the materials with a change in their composite phase should have a high thermal conductivity and sufficient volume for a temperature control of a thermoelectric generator.
Keywords: Phase change materials; Graphite foam; Porosity; Thermal conductivity; Temperature control
2
1. Introduction It is well known that an optimised energy use is extremely important. Phase change materials (PCMs) can absorb excess energy during manufacturing abrupt temperature changes in the environment usually have a low thermal conductivity
[4]
[3]
[1][2]
, and significantly reduce any
. However, a problem exists in that PCMs
. A thermoelectric generator (TEG) can then
convert energy from waste heat into electrical energy
[5]-[7]
, although its thermoelectric
conversion efficiency is based on the temperature of the thermal environment. Therefore, in recent years, researchers have begun to use PCMs to construct the thermal environment required for a TEG [8]-[15].
In this regard, Kiziroglou et al. produced a thermoelectric absorption conversion device using water as a phase change material
[16]
. The results show that the use of PCMs can increase
approximately 30% of the total amount of thermoelectric energy conversion as compared to the sole use of thermoelectric materials. Tu et al. improved the design of Kiziroglou’s by applying composite PCMs made from expanded graphite and paraffin
[17]
study
. The composite
PCMs (paraffin, 5 wt% expanded graphite) further increased the total output energy by 32.32%, as compared with the results obtained using pure PCMs. This finding was also confirmed from the results by Carneiro et al., who used a certain amount of PCMs for practical feasibility and thermoelectric materials to generate 6 kJ of electrical energy
[18]
,
which was sufficient to satisfy the use of a submersible in a single cycle. The advantages of PCMs and a TEG can be effectively combined to save energy and reduce emissions.
Rather than using PCMs solely as a heat sink at the cold side, such as in the above studies, this research aims to use PCMs at the hot side of the TEG for thermal control. An illustration 3
of this study is shown in Fig. 1. When the temperature of the heat source quickly changes by a °C, the temperature at both sides of the TEG will then change by b °C and c °C, making the voltage output completely unstable. At this stage, a PCM with a melting point of approximately 60 °C is added between the heat source and TEG. Theoretically, this addition will prevent the temperature at both sides of the TEG from changing significantly within a short time, and the temperature will thus be maintained, which will also stabilise the efficiency of the TEG. However, the thickness of the phase change material affects the heat transfer process, and thus the thermal conductivity of the phase change material must be increased.
Figure 1. Schematic diagram of temperature control effect of PCM
2. Experiment Section 2.1. Design of experiment and simulation models Based on the above-stated ideas, a waste heat recovery power generation structure was designed in this study, as shown in Fig. 2. In this structure, graphite foam composite phase change materials (G-PCMs) were placed on the hot side of the TEG, which was packaged in aluminium foil to prevent leakage. In the simulation calculation, the presence of aluminium foil can be ignored owing to its high thermal conductivity and thinness. The other surface of 4
the G-PCM was in direct contact with the working environment temperature (T0) as a heat flow inlet, and the cold side of the TEG was used as a water-cooling temperature control system to keep the temperature T2 constant, serving as a heat flow outlet. Insulation materials were used to reduce heat leakage from the remaining surfaces. When the entire structure was placed in a high-temperature environment, the temperature difference of the TEG was directly determined using temperature T1. It can be inferred that when the temperature changes, the G-PCMs will maintain temperature T1 for a certain amount of time, and the stored energy can also allow the TEG to operate for an additional period of time.
Figure 2. Schematic diagram of (a) experiment and (b) simulation models. 2.2. Actual temperature control effect of composite phase change materials In general, pure PCMs have a low thermal conductivity
[19]
. For example, the thermal
conductivity of paraffin wax is approximately 0.15 Wm-1K-1, which limits its capacity for temperature control. A number of methods have been mentioned in previous studies for improving the thermal conductivity of PCMs [22]
[20]
, namely the use of fins
[21]
, nanomaterials
, expanded graphite [23], and porous materials [24]-[27], among others. G-PCMS composed of
graphite foam (GF) demonstrates a higher thermal conductivity and lower mass. Such composite materials are becoming widely popular, particularly in studies in the automotive and aerospace sectors. 5
Figure 3. (a) GF, (b) paraffin, and (c)–(f) production process of G-PCMS. The G-PCMS used in this experiment was constructed from purchased GF and paraffin. The total size of the structure was 46.2 × 44.8 × 12.5 mm3, its thermal conductivity was approximately 75 Wm-1K-1, the porosity was 0.9, and the hole diameter was approximately 1.2–1.5 mm. The thermal conductivity of the paraffin wax was approximately 0.15 Wm-1K-1 and its melting point ranged from 53°C to 60 °C. Moreover, the entire equivalent thermal conductivity was approximately 2–3 Wm-1K-1, which is 20-times that of pure PCM. The model specifications of the TEG are TEP1-126T200. Based on the provided instructions, the maximum continuous working temperature was 120 °C, and the structure will become completely damaged at 138 °C. The size of the TEG was 40 × 40 × 3.4 mm3, the main material is Bi2Te3, and its thermal conductivity is 1.2–1.5 Wm-1K-1. The main instruments used, as shown in Fig. 4, included an electric heating platform, a water-cooling temperature control platform, a voltage tester, a temperature tester, a weight for pressurisation, and some foam for simulating an adiabatic environment.
6
Figure 4. (a) G-PCMS and TEG and (b) experiment instruments. To simulate the temperature change of the electronic chip, the temperature of the heat source was increased from 50 Ԩ to 65 Ԩ during the first step. Once the temperature stabilised, it was
further increased to 75 Ԩ during the second step. Owing to a heating of the instrument, the heating temperature slightly exceeded the set temperature, and then decreased, and thus the experiment results are likely to exceed the value for the expected temperature. Each test instrument was tested before commencing the formal experiment. The heat source temperature was set to a constant 45 Ԩ, and the cold side temperature was set to a room temperature of 22 Ԩ. The output voltage of the TEG at 0.01 V was considered stable.
Subsequently, the values were recorded as follows: T0 = 45.0 Ԩ, T1 = 27.3 Ԩ, T2 = 25.3 Ԩ, and a voltage of 0.1238 V. Based on the instructions of TEG, each 1 Ԩ temperature difference will produce 65 mV of voltage. Therefore, when the temperature difference was
20 Ԩ, the voltage was 0.13 V. The error between the actual measured value and the calculated value was found to be 4.77%. Such an error is small, implying a normal working capacity of the experimental instrument. The simulation was calculated using the solidification and melting model found in the commercial software, Fluent. As shown in Fig. 5, the calculation model size was drawn according to the physical size, with the size of each mesh cell being 1 mm. The GF had a 7
porosity of 0.9, and a porous model was used to simulate the G-PCMs. The hot and cold sides were set to the instantaneous temperature boundary conditions. The remaining boundary conditions were adiabatic, and the property parameters for the material were set as shown in Table 1. In addition, the time step was 0.5–1 s. Factors such as the interfacial thermal resistance and thermal radiation were ignored, and the material parameters were assumed to be constants.
Figure 5. Data of the simulation model. Table 1. Material property parameters Material
Density (kgm-3)
Specific heat (kJkg-1K-1)
Thermal conductivity (Wm-1K-1)
Latent heat (kJkg-1)
Melting point (K)
Paraffin wax
800
2850
0.15
200
330
Graphite foam
2000
710
75
/
/
TEG
7740
152
1.5
/
/
Copper
8978
381
387.6
/
/
2.3 Analysis and results The experiment time was approximately 660 s. The simulated heat source was obtained using a curve fitting method in MATLAB. To approximate the experiment data, fourth- and
8
fifth-order polynomials were used for fitting. In addition, some data points were hidden for easier viewing. It is clear from Fig. 6 that the results of the first set of numerical simulations, T1_N1, are quite different from the experiment results, T1_E, causing the PCM to melt completely in advance. The reason for this is the incredibly high thermal conductivity in the simulation calculation at approximately 7.5 Wm-1K-1. Based on the experience of other researchers
[28][29][30]
, necessary corrections were made to the overall calculation of the
thermal conductivity of the G-PCMS using Fluent. The actual thermal conductivity is lower than the value obtained using the built-in formula of the Fluent porous media model. A calculation formula for the macroscopic thermal conductivity of the GF was proposed by Klett
[31]
; based on this formula, the thermal conductivity ultimately changed to
approximately 2.06 Wm-1K-1, and the second group of simulation calculations was made. It is clear from the figure that the corrected result, T1_N2, is closer to the actual test result, T1_E. Therefore, the same correction method was used in the simulation calculations for this paper. The average difference between T1_N2 and T1_E was observed to be 12.5%, because the boundary conditions of the external environment were simplified in the simulation calculation, and heat loss to the surrounding air was not considered. Since the value set in the simulation calculation cannot be the same as the values for actual situation, therefore, the difference between the simulation calculation results and actual experiment is considered to be normal.
9
Temperature (Ԩ )
80 70
T0_N T1_N1 T1_N2
60 50
T0_E T1_E
40 30 -100
0
100 200 300 400 500 600 700 Time (s)
Figure 6. Comparison of experiment and simulation results
Figure 7. Melting process of G-PCMS at different times in the second set of simulation calculations. During the experiment, the heat sources T0_E and T0_N were subjected to two large mutations within the comparative simulation, increasing by 15 °C and 10 °C, respectively. It is clear from Fig. 6 that the temperature, T1, at the other end of the composite phase-change material also changed, although its magnitude of change was relatively small. The maximum temperature change of the heat source in a working cycle was 79–50 = 29 °C. In addition, the temperature change for T1_E was 33.2–27.9 = 5.3 °C, which only accounts for 18.3% of the original change. In the simulation, the value of T1_N2 was 37.8–30.2 = 7.6 °C, which is 26.2%
10
of the original change. All values are less than half of the values for the original change, thus proving that the use of G-PCMS in maintaining a stable temperature is completely feasible. At the same time, the analysis of the two sets of simulation results in the above figure show that the overall thermal conductivity of the composite has a certain level of influence on the results for the temperature control. The two sets of simulation calculations can be considered as a comparison of two different types of GF. When the thermal conductivity of the GF is high, the instantaneous temperature, T1, is also obviously high with a low thermal conductivity. The above experiment and calculation results show that the use of G-PCMS can effectively reduce the temperature fluctuations. In addition, the high thermal conductivity of G-PCMS can increase the temperature of the hot side of the TEG to obtain a higher electrical output as compared to the use of a material with a low thermal conductivity. 3. Simulations and Discussion 3.1. Evaluation method of thermal control capability of G-PCMS The feasibility of the numerical simulation method and the practicality of G-PCMS for thermoelectric generation was verified in the previous section. However, it is clear from Fig. 6 that the low thermal conductivity leads to a decrease in the output temperature range. To promote the optimal working efficiency of the thermoelectric generator, the temperature range is a significant factor. In the first aspect, based on the maximum temperature of the TEG hot end, the thermoelectric generator has a range of operating temperatures, and if the maximum temperature exceeds this range, the device will be damaged. However, when conducting a thermoelectric conversion, focus is on the conversion efficiency, and the more 11
energy available under the same heat source conditions, the better the design is. In this study, the total energy output and the maximum temperature for different materials are considered. Referring to the heater used in the experiment and the heat source boundary conditions found in the literature, in the new simulation calculation, the heat source was defined as indicated in Fig. 7. It was heated twice with a heating power of 200 W and studied using different types of GF. To reduce the temperature fluctuation and achieve a more complete and gradual melting, the simulation experiments were carried out in an environment with a temperature exceeding 120 °C. An Erythritol-based composite material was used owing to its melting point of close to 120 °C. Based on the total heating energy and heating time, we can estimate that the volume required for the phase change material was approximately 21.6 cm3. The thermal conductivity of the GF was changed from 50 to 200 Wm-1K-1, and the porosity was changed from 0.8 to 0.95.
SFPBKRWIDFH WHJBKRWIDFH KHDWHUBSRZHU
3RZHU :
7HPSHUDWXUH .
7LPH V
Figure 8. Schematic diagram of heating progress and temperature of heat source Table 3. Material parameters of erythritol
Material
density (kgm-3)
specific heat (kJkg-1K-1) 12
thermal latent conductivity heat (Wm-1K-1) (kJkg-1)
melting point (K)
1300(liquid)
2760(liquid)
1451(solid)
1380(solid˅
Erythritol
0.326
316.8
392.94
3.2. Effects of the thermal conductivity of GF The thermal conductivity of the GF itself directly affected the effective thermal conductivity of the composite, and GF with different thermal conductivities was then calculated under the same porosity and other conditions. Taking 200 Wm-1K-1 as an example, as indicated in Fig. 8, a high thermal conductivity can quickly increase the temperature of the TEG to above 100 °C. However, owing to a high heat source power, the temperature of the TEG increases beyond the normal temperature range and leads to an overheating. Therefore, the thermal conductivity should not be too high. A TEG ratio of 0.131 mVK-1
[17]
is used to calculate the open-circuit voltage at the
instantaneous temperature, the externally output electrical power is then determined, and finally, the time is integrated to obtain the total power output. In the calculation, a voltage of 5 V or more is used as an effective output, and a voltage of less than 5 V is ignored. Considering an overheating, all temperatures above 120 °C are changed to 120 °C for the purposes of the calculation. The results are shown in Table 4. We can see that in the current calculation the electric energy of the TEG can increase by 69.27%. However, at 150 Wm-1K-1 and 200 Wm-1K-1, overheating occurs for 44 and 54 s, respectively, which is almost the same as half the length of the heating time. Oppositely, at 100 Wm-1K-1, not only is the TEG well protected during the working cycle, sufficient electrical energy is also generated. The electric energy is increased by 47.19% from 50 to 100 Wm-1K-1, and is only increased by 14.59% from 100 to 200 Wm-1K-1. It can be seen that continuing to increase the thermal
13
conductivity will cause the PCMs to melt faster and cause an overheating, whereas the increments of the output energy of the TEG are reduced.
Figure 9. (a) Liquid fraction and (b) temperature comparison charts. Table 4. Total value of electrical energy generated Thermal
50 /Wm-1K-1
75/Wm-1K-1 100/Wm-1K-1 150/Wm-1K-1 200/Wm-1K-1
conductivity Electric energy
12,772.16J
16,646.18J
18,866.96J
20,667.41J
21,619.36J
Increments
0
30.33%
47.19%
61.82%
69.27%
3.3. Effect of GF porosity Graphite foam is a porous material with a high thermal conductivity. In addition to its thermal conductivity, the porosity of GF is often used to calculate the equivalent thermal conductivity and total energy storage. The following is a simulated calculation for multiple levels of porosity of between 0.8 and 0.95. According to the melting rate shown in Fig. 9, the melting rate is found to be close to 100% when the porosity is 0.9, and the melting condition can continue to operate for another cycle when the porosity is above 0.9. It is clear that when the total volume is the same, increasing 14
the porosity can ensure additional working time. From the perspective of a thermal response, as the porosity decreases, the amount of GF and the overall thermal conductivity of the G-PCMS both increase. As shown in Table 5, the effective thermal conductivity still affects the thermoelectric output. The electrical energy output increases by 114% when the porosity changes from 0.95 to 0.8. However, the overheating time is extended owing to a reduction of the phase-change material; for example, the overheating time is 79 s at a porosity of 0.8, which is 65.83% of the total heating time. Thus, a porosity of 0.9 is the best design offering full protection and energy output.
Figure 10. (a) Liquid fraction and (b) temperature comparison charts. Table 5. Total value of electrical energy generated Porosity
0.95
0.92
0.9
0.85
0.8
Electric energy
9,734.26J
14,215.33J
16,724.63J
19,716.96J
20,831.61J
Increments
0
46.03%
71.81%
102.55%
114.00%
4. Conclusions 15
It is feasible to place a phase-change material having an improved thermal conductivity between the heat source and the thermoelectric generator. In the first aspect, G-PCMS can not only provide a stable temperature output in an unstable environment, it can also lower the temperature to below its phase change temperature. In the second aspect, the composite PCMs can provide the TEG with a greater temperature difference than pure PCMs. In an environment heated twice at 200 W, G-PCMs made from GF with a thermal conductivity of 100 Wm-1K-1 and a porosity of 0.9 can protect the TEG from damage while generating a large amount of electricity. However, appropriate G-PCMs should be chosen for different working conditions. The results of the simulations show that a rate of 100 Wm-1K-1 can produce 47% more electrical energy than a rate of 50 Wm-1K-1, although 200 Wm-1K-1 can produce 69% more energy while overheating within 54 s. Thus, a porosity of 0.9 can produce 71.81% more electrical energy than a porosity of 0.95, whereas a porosity of 0.8 can produce 114% more energy while overheating within 79 s. Based on these conclusions, we can see that the G-PCMs have the potential to control the temperature of the TEG. However, the G-PCMs should be improved to obtain a balance between the thermal conductivity and the heat absorption, rather than simply enhancing the thermal conductivity.
Notes The authors declare no competing financial interest.
Acknowledgments
16
We gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (Project No. 51672054).
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21
Analysis of Temperature Control Effect of Composite Phase Change Structure Used in Thermoelectric Conversion System
Jia Yu1, Haoqing Wang1,*, Li Kong1, , Hongji Zhu1, Qingshan Zhu1, Jiawen Su1
1
College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, P. R. China
* Corresponding authors: [email protected] (Haoqing Wang)
22
Highlights l
Experiments verify the feasibility of simulation calculation.
l
CPCM's thermal conductivity is better than PCM.
l
Heat storage and thermal conductivity must be considered simultaneously during temperature control.
l
The design of each set of CPCM is related to the location, and the C value is also the same.
23