Phase change material Chinese Kang: Design and experimental performance study

Phase change material Chinese Kang: Design and experimental performance study

Journal Pre-proof Phase Change Material Chinese Kang: Design and Experimental Performance Study Gang Li, Xiaoxuan Bi, Guohui Feng, Lan Chi, Xianfang ...

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Journal Pre-proof Phase Change Material Chinese Kang: Design and Experimental Performance Study

Gang Li, Xiaoxuan Bi, Guohui Feng, Lan Chi, Xianfang Zheng, Xueting Liu PII:

S0960-1481(20)30005-7

DOI:

https://doi.org/10.1016/j.renene.2020.01.004

Reference:

RENE 12866

To appear in:

Renewable Energy

Received Date:

19 June 2019

Accepted Date:

02 January 2020

Please cite this article as: Gang Li, Xiaoxuan Bi, Guohui Feng, Lan Chi, Xianfang Zheng, Xueting Liu, Phase Change Material Chinese Kang: Design and Experimental Performance Study, Renewable Energy (2020), https://doi.org/10.1016/j.renene.2020.01.004

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.

Journal Pre-proof

Phase Change Material Chinese Kang: Design and Experimental Performance Study Gang Li1, Xiaoxuan Bi1, Guohui Feng1*, Lan Chi2, Xianfang Zheng1, Xueting Liu1 (1.Municipal and Environmental Engineering Faculty, Shenyang Jianzhu University, Shenyang, Liaoning, China, 110168; 2. Law Faculty, Fujian Engineering College, Fuzhou, Fujian, China, 350118) Abstract The Chinese Kang is indispensable for domestic heating during the winter in the rural residence of northern China. However, it still has certain drawbacks such as the huge surface temperature difference and limited effective heating time. A novel phase change material Chinese Kang (PCMCK) is developed by combining paraffin and the traditional Kang to overcome such disadvantages. A comparison test was conducted to analyze the thermal performance of PCMCK and traditional

Kang. According to the

data, the coefficient of variation of the PCMCK is significantly lower than that of the traditional

Kang during each time period. Thus, the PCMCK surface temperature

distribution is much more uniform. Additionally, after the burning ceased, the indoor temperature of a PCMCK-heated room still kept rising until midnight due to the excessive heat absorbed by #48 paraffin. The mean indoor temperature is 2.94◦C higher than that of the traditional

Kang-heated room. As a result, the thermal character of phase change

* Corresponding author: Guohui Feng(1964—), male, doctor, professor, [email protected], 13940515506

E-mail addresses: [email protected](Gang Li), [email protected](Xiaoxuan Bi), [email protected](Lan Chi), [email protected](Xianfang Zheng), [email protected](Xueting Liu) 1

Journal Pre-proof material is fully utilized. The PCMCK can effectively improve the indoor thermal comfort, the uniformity of Kang surface temperature and extend the heating period significantly as well. Keywords: Phase Change Material, Heat Storage, Paraffin, Chinese Kang 1. Introduction In 2015, the total annual household energy consumption in rural China reached the equivalent of 317 million tons of standard coal, which is a large quantity (tce)(9.3*109 GJ)[1], out of which more than 80% is used for heating[2]. The Chinese Kang has been used in China since the Zhou dynasty, 2500 years ago [3] and it has similar functions of heating systems such as the ondol (heated floor) in Korea [4] and hypocaust in ancient Rome [5]. Nearly 85% of rural residences use Chinese Kang for domestic heating in northern China during the cold and dry winter [6]. The Chinese Kang consists of a stove, a Kang body and a chimney. The traditional grounded Chinese Kang(Figure 1.A) and the new elevated Chinese Kang(Figure 1.B) [6] are mostly used. However, there are still some drawbacks such as the weak ability to maintain indoor temperature, the air pollution from the cooking smoke, and the nonuniform temperature distribution. Some researches have been reported on the study of the performance of the Chinese Kang. Chen et al. [7] took field surveys to investigate the rural resident thermal environment with the coupled Chinese Elevated Kang and passive solar power collecting wall heating system. Zhuang et al. investigated the model of smoke flow [8] and thermal storage performance [9] of the Chinese Kang and presented an experimental assessment on the smoke flow characteristics and heat transfer of a typical 2

Journal Pre-proof elevated Chinese Kang [10]. A set of models were established to simulate the Kang heating system’s energy performance by Cao et al. [11]. He et al. [12] presented a mathematical model of the solar Chinese Kang to study the energy consumption and thermal performance. Yang et al. [13] developed a heat transfer model to simulate the dynamic characteristics of a new Kang system that integrates a solar air collector into the convectional Chinese Kang system. A series of numerical parametric analysis were conducted based on the previous heat transfer model [14]. Some new kinds of Chinese Kang were proposed, such as a Chinese Kang with forced convection (CKFC) by Wang et al. [15] and a novel combined Kang system that integrates the solar Kang and solar air collector by Wei et al. [16]. Zhai et al. developed the model [17] and analyzed the result [18] to improve the design and to optimize the domestic Kang heating system. Field measurements and assessments of the Kang system were conducted to improve it by Li et al. [19]. Niu et al. [20] modeled the thermal effectiveness of two types of Kang to improve indoor thermal comfort. Even though various kinds of research have been developed, the disadvantages, such as the non-uniform temperature distribution and the overheating of the Kang inlet, have not been effectively solved yet. Various technologies of the Phase Change Materials (PCM) that are integrated into buildings have been proposed to improve the energy utility [21]. Paraffin waxes with a suitable latent heat are excellent materials for energy storage [22], which are widely used in various applications such as building walls, roofs, ceilings, or floors [23]. Kuznik and Virgone [24] investigated a PCM copolymer composite wallboard experimentally in a full scale test room. Microencapsulated paraffin constitutes 60% of the PCM wallboard. A 3

Journal Pre-proof honeycomb wallboard made of an aluminium honeycomb panel filled with paraffin was presented by Hasse et al. [25] to enhance the thermal conductivity and to avoid leaks. Eddhahak-Ouni et al. [26] developed a Portland cement concrete modified with organic microencapsulated PCMs containing paraffin. The mechanical properties and thermal properties were investigated. The paraffin was encapsulated into the prototype roof structure to manage the thermal energy efficiently by Akeiber et al. [27]. A model which can describe the transient behavior of a phase change energy storage unit is developed by Esen and Ayhan [28], the performance of a solar assisted cylindrical energy storage tank is investigated theoretically. The performance of a cylindrical latent heat storage tank is optimized by Esen [29], the method is using two different models describing the diurnal transient behavior of the PCM, optimal geometric design of the store depending on these parameters and PCMs is presented. A cylindrical phase change storage tank linked to a solar powered heat pump system is investigated experimentally and theoretically by Esen [30], a simulation model was used to define the transient behaviour of the phase change unit, both the mean temperature of water within the tank and the inlet and outlet water temperature of the tank are measured by the experiments. Bastani et al. [31] developed a tool to characterize the required thickness of a PCM wallboard which needs to be charged during the grid off-peak. A study which focuses on the characterizing heat transfer of PCM wallboards, and to identify the influential parameters on the charging procedure of a PCM wallboard is done by Bastani and Haghighat [32], the correlations between the dimensionless parameters and the performance of the PCM wallboard were determined and a procedure was developed to expand Heisler chart 4

Journal Pre-proof application to study thermal behavior of PCM wallboards. PCM were also used for temperature uniform improvement by many researchers. A new concentrator photovoltaic system integrated with phase change material heat sink was developed by Rabie et al. [33]. Low and uniform temperature distribution along the solar cell were introduced and discussed. Sabbah et al. [34] studied the cooling and temperature uniform effectiveness of passive cooling by PCM on high power lithium-ion packs. Li et al. [35] designed a sandwiched cooling structure using copper metal foam saturated with PCM which can effectively reduce the battery’s surface temperature and improve the uniformity of the temperature distribution. The Chinese Kang is usually heated by the residual heat during the cooking period. The stove operates intermittently, but the Kang body needs to heat the indoor room continuously. This is definitely a difficult problem to resolve. However, the phase change material can store and release the energy by the process of phase changing. Two outstanding merits are expected by fully utilizing the heat storage performance of the phase change material in the Kang plate. On the one hand, the excess heat can be converted into energy and stored, which will be released when the temperature of the Kang plate is lower than the room temperature, thus significantly extend the heating period. On the other hand, the non-uniformity of the Kang Body’s temperature will be greatly improved. In order to overcome the existing disadvantages of the traditional Kang, a new phase change material Chinese Kang (PCMCK) is developed in this paper. Relevant temperatures are measured and analyzed by the experiment to guide future design of Chinese Kang and rural residence. 5

Journal Pre-proof 2. System Description

2.1 System Principle

The phase change material Chinese Kang (PCMCK) combines the traditional Chinese grounded Kang with PCM. Chinese Kang utilizes the residual heat of smoke from an adjacent cooking stove which burns biomaterials or coals. The operation principle is that the high temperature smoke from the stove flows through the flue to heat the Kang Body. The Kang Plate is made of materials with large thermal capacity. So a great deal of heat is absorbed and stored during the firing period and then gradually released to ensure the Kang plate to maintain a certain temperature level. Therefore the indoor air is also heated, maintaining a heating period of several hours or more. The heat transfer mode includes convection and radiation. The #48 paraffin is designed to be encapsulated into the Kang plate to improve the thermal performance. The phase change materials can absorb or release a large number of latent heat when changing from one phase to another, but the temperature remains almost constant. It actually can be treated as a constant temperature energy storage. The paraffin can store a great deal of excess heat and release it when the temperature of the Kang plate drops below the indoor air temperature. So the heating time can be effectively extended. In addition, the non-uniformity of Kang surface temperature distribution can be improved, energy will be saved, and the indoor thermal comfort will also be increased.

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Figure 1. System Description of the traditional

Kang (A, B) and PCMCK (C)

2.2 System Component

The traditional Chinese grounded Kang consists of three parts, which are the stove, the Kang body (Kang plate and flue), and the chimney. Because the corrosion resistance and the heat transfer performance of the galvanized steel is good, three boxes are made with 1.5mm thick galvanized steel to hold the paraffin with the size of 1800mm× 900mm × 40mm. The paraffin is infused after melting and covered with a layer of plastic film after solidification. Then a plasterboard is placed on t h e top to act as the Kang surface. The system description is shown in Figure 1 (C). Figure 2 and Figure 3 present the photographs of the traditional Kang and the galvanized steel boxes with paraffin we used in the experiment respectively.

Figure 2. Photograph of the traditional Kang

Figure 3. Galvanized steel boxes with paraffin

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Journal Pre-proof 2.3 Phase Change Character of Paraffin

Phase change temperature and the latent heat of #48 paraffin were tested by Perkin Elmer type DSC4000 differential scanning calorimeter, as can be seen in Figure 4, which was made in USA. Liquid nitrogen was used for cooling and high purity nitrogen was adopted as purge gas with a flow rate of 20ml/min. In the process, the experimental temperature was controlled at range of 0 ~ 90◦C. #48 paraffin was first cooled down to a lower temperature by liquid nitrogen, and then start to be heated up, and the heating rate was set as 10◦C/min. By analyzing the melting process of it, The phase change character curve of #48 paraffin was presented in Figure 5. The phase change initial temperature is 47.69◦C and the peak temperature is 56.15◦C. The phase change latent heat is 170.24kJ/kg.

(A) Differential scanning calorimeter test system

(B) Perkin Elmer type DSC4000

Figure 4. Photographs of DSC4000 and the test system

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Figure5. DSC curve of #48 paraffin

2.4. Quantity of Paraffin

The heat absorbed by Chinese Kang comes from the fuel combustion in the stove, which should be considered to calculate the quantity of PCM. If the stove provides the quality of heating effects will be determined by the thermal efficiency of

constant heat, the Chinese

Kang. Guo et al. [36] found that the heat in the flue smoke (Heat Source of the Chinese Kang)is 41.1% of the total calorific value provided by burning fuel. The comprehensive thermal efficiency of the grounded Kang is relatively low, which is 40% - 50%. Thus, the quantity of PCM can be calculated by the following equations: 𝑊

𝑀𝑃𝐶𝑀 = 𝐻

(1)

𝑊 = 𝜂𝑧 ∙ 𝜂𝑘 ∙ 𝑀𝐹𝑢𝑒𝑙 ∙ 𝑄

(2)

Where MP CM (kg) is the quantity of the PCM. W (kJ) is the heat released per operation. H(kJ/kg) is the latent heat of the PCM. MFuel(kg) is the fuel consumption during one operation. Q(kJ/kg) is the application base lower heating value of the fuel. ηz is the ratio of the 9

Journal Pre-proof flue smoke heat gained from the burning fuel by the Chinese Kang (which is set as 40% in this experiment). ηk is the comprehensive thermal efficiency of the Chinese Kang (which is set as 40% in this experiment). According to the rural residents’ experience, dry straw is chosen as fuel in this experiment. During every operation, about 6kg of straw are needed, which will last about 90minutes. The application-based lower heating value of the straw can be considered as 15000kJ/kg [37]. By calculation, the quantity of the #48 paraffin is 84.6kg. 3. Experimental Set-up

3.1. Test Environment Information

The experimental site is a detached house built in Shenyang Jianzhu University, Shenyang, Liaoning, China. Figure 6 shows the outlook of the experiment house. Shenyang is located in northeastern China, 41.48◦N and 123.40◦E and belongs to the severe cold zone of the country. The heating period in winter is 152 days, the heating outdoor calculated temperature is -19◦C, the average temperature is -5.7◦C and the average wind speed is 3.2m/s.

Figure 6. Photograph of the detached house 10

Journal Pre-proof Two testing rooms were selected for the experiment in the detached house. The size, structure, and layout were almost the same. The east room is used to establish the test platform of the PCMCK. The original setup was maintained with a traditional Kang for the contrast test. There are no insulations on the exterior wall, roof, or floor. Figure 7 shows the layout of the two experimental rooms. Other detailed information is listed in Table 1.

Figure 7. Layout of test rooms Table 1. Detailed information of the detached house and the Chinese Kang Component Detail Room Size (L × W × H) 5000mm × 4000mm × 3200mm Chinese Kang (L × W × H) 2800mm × 1800mm × 600mm. Precast Concrete Slab and Brick External Walls (Outside to Inside) 370mm brick. Granitic Plaster + Cement Mortar + Brick + Cement Mortar + Calcimine Roof(Ceiling to Roof Surface) 250mm. Wooden Beam + Reed Mat + Alkaline Soil + Stove Ash + Cement Basic Floor(Ground to Floor Surface) 100mm. Pebble + Stove Ash + Cement Window 2000mm × 1500mm. Single Frame, Double Glazing, Plastic Steel Door Single Wooden 11

Journal Pre-proof 3.2. Test Methodology

In order to analyze the surface temperature distribution of the two kinds of different Kang and the indoor thermal comfort of the two testing rooms, the PCMCK and the traditional

Kang

were operating at the same time. The relevant temperatures were recorded by the XMZ*-J Series multi-input temperature detector every 20 minutes. The thermal probes are E type nickel chrome-constantan thermocouple that have been calibrated to ensure precision. Based on the test method for measuring the performance of a Chinese Kang, (NY/T 58-2009) [38], 9 testing points were arranged on each Kang as shown in Figure 8. The probes were fixed on the Kang surface using high temperature resistance adhesive tape. At the same time, a total of 11 thermocouples were set indoors and outdoors to measure the air temperature, with a, b, c, d, e as indoor points and o as outdoor point, they were all hung 1.5m high ,and the outdoor thermocouple were covered in a ventilated box coated with aluminium foil, as shown in Figure 8. Figure 9 shows the actual testing point scenario of the Kang plates.

Figure8. Test points arrangement

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Figure 9. Photograph of the test points The comparative analysis method was adopted in the experiment. The heating time for the two testing rooms was the same. Both used 6kg of straw for fuel. There were three operating periods per day, each lasting 90 minutes, namely 7:30-9:00 in the morning, 11:30-13:00 at noon, 17:00-18:30 in the evening (based on local experience). The experiment lasted from 7:30 a.m. on March 15th to 17:00 p.m. on March 18th, 2016.We chose the data of steady operation period (from8:00 a.m. on March 17th to 8:00 a.m. on March 18th)to carry out comparative analysis of the relevant temperatures of the two testing rooms. The Coefficient of Variation and the one-way analysis of variance (abbreviated one-way ANOVA) were utilized to analyze the tested values, which can evaluate the measures of dispersion. 4. Results and Discussion

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Journal Pre-proof 4.1 Surface Temperature

4.1.1 Temperature Distribution

The temperature variations of PCMCK and traditional Kang are shown in Figure 10 and Figure 11 according to the data.

Figure 10. Surface temperature variation of PCMCK

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Figure 11. Surface temperature variation of the traditional Kang Comparing the two figures, it can be observed that the surface temperature of the traditional Kang increased dramatically as soon as the fuel started burning. However, due to the huge thermal inertia, the temperature rising rate of the PCMCK was greatly weakened. In the traditional Kang, the highest surface temperature reached 117.39◦C at 13:20, the minimum was 19.83◦C. But for the PCMCK, the peak temperature was 53.73◦C at 21:20 and the minimum was 29.1◦C. The surface temperature difference of the traditional Kang was huge, which was more than 70◦C, while that of the PCMCK was less than 15◦C. The mean temperature difference between the inlet and middle parts was 25.12◦C, and that between the middle and outlet parts was 4.26◦C. But in the PCMCK, the temperature difference were 3.33◦C and 1.84◦C respectively, which demonstrated better uniformity. The main reason of the improvement of the temperature uniformity on the Kang plate is that phase change materials can absorb or release a large number of latent heat when changing from one phase to another, but the temperature remains almost constant. So the 15

Journal Pre-proof huge temperature difference of the traditional Kang is significantly narrowed. To further analyze the improvement of temperature uniformity caused by the PCMCK, the statistical software Minitab is used to calculate the coefficient of variation. Table 2. Coefficient of variation analysis

Time

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00

Surface temperature of traditional Chinese Kang Standard Coefficient Mean deviation of variation (℃) (%) (℃) 18.65 4.07 21.8 40.29 15.56 38.61 61.86 24.73 39.98 59.32 27.09 45.67 50.43 26.51 52.56 57.78 28.12 48.68 58.21 28.06 48.2 51.69 22 42.56 46.1 18.61 40.37 42.28 16.78 39.68 43.82 15.2 34.69 53.62 17.84 33.28 58.67 22.53 38.39 53 19.1 36.04 48.75 16.27 33.37 44.87 12.92 28.8 40.75 8.71 21.38 37.95 6.6 17.38 35.56 5.15 14.47 33.92 4.48 13.22 31.97 3.73 11.66 30.18 3.64 12.05 28.05 3.77 13.43 26.04 3.63 13.94 22.77 3.08 13.51

Surface temperature of PCMCK Mean (℃) 31.097 31.389 31.733 32.311 31.878 33.733 34.622 35.744 36.944 38.344 40.11 42.18 44.34 46.28 46.51 45.34 43.98 42.522 41.144 40.1 38.467 36.844 35.122 33.211 31.311

Standard deviation (℃) 1.154 1.149 1.177 1.393 2.99 1.616 1.822 2.122 2.502 2.879 3.36 3.85 4.01 4.15 3.85 3.51 3.23 2.96 2.749 3.32 2.991 2.915 2.637 2.318 2.132

Coefficient of variation (%) 3.71 3.66 3.71 4.31 9.38 4.79 5.26 5.94 6.77 7.51 8.38 9.12 9.05 8.96 8.29 7.73 7.34 6.96 6.68 8.29 7.78 7.91 7.51 6.98 6.81

According to the data shown in Table 2, the coefficient of variation of the PCMCK is significantly lower than that of the traditional Kang during each time period. Thus, the PCMCK surface temperature distribution is much more uniform. 16

Journal Pre-proof 4.1.2. Highest, Lowest and Mean temperature

To analyze the significance of the influence on the surface temperature under different working scenario, the one-way ANOVA regarding the surface average temperature, maximum temperature, and minimum temperature is studied. Table 3 shows that the F value is greater than the critical F value. The P value (Statistically Significant Value) is far less than0.01. Different types of the Chinese Kang have extremely significant effects on the surface temperature distribution. (The significance level, α = 0.05) Table 3. One-way ANOVA between the traditional Kang and PCMCK Analysis Object

F

P-value

F crit

Mean Temperature

28.822

3.110E-07

3.907

Highest temperature

74.723

9.540E-15

3.907

Lowest temperature

8.273

4.637E-3

3.907

The comparison of the highest, the lowest and mean temperature distribution of the two types of Kang is shown in Figure 12.

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Figure 12. Surface temperature distribution contrast

Frequency distribution (%)

Traditional kang

PCMCK

40.00% 35.00% 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% 0-25

25-30

30-35

35-40

40-45

45-50

50-

Mean temperature of Kang plate surface(℃)

Figure 13. Contrast of mean temperature frequency distribution As can be seen from Figure 12, the fuel started burning at 7:30, in the traditional

Kang,

the mean temperature before burning was 18.64◦C. It rose quickly and fluctuated dramatically. About 90 minutes after the first firing period, the temperature reached the first peak and then declined rapidly as well, the maximum temperature of the Kang surface was 117.39◦C at 13:20. 18

Journal Pre-proof The difference of maximum temperature was as high as 90◦C, and that of the minimum temperature was up to 30◦C as well. Whereas the trend in case of the PCMCK was different. The maximum surface temperature of the PCMCK was 53.73◦C at 21:20, which was much lower than the traditional

Kang; and the fluctuation was much smaller, the temperature distribution

was also more uniform. The difference of the maximum temperature was about 20◦C and that of the minimum temperature was only 11◦C. From 18:00 till 3:00 next day, the surface mean temperature remained stably between 40◦C and 45◦C. The thermal storage effect was remarkable, which can meet the heating demands of people at night more effectively. Considering the frequency distribution of mean temperature of Kang plate surface, we can discover the strength of PCMCK from another perspective, as illustrated in Figure 13 , the most significant difference between the two types of Kang was the surface temperature distribution during a whole day, temperatures stayed below 25 ℃ and above 50℃ make up more than half of the day which is not suitable for people to stay on for traditional

Kang, on the contrary, all

temperatures of the PCMCK range from 30℃ to 50℃. The percentage of temperatures staying from 30℃ to35 ℃ and 35℃ to 40 ℃ for PCMCK are both more than 5 times as large as that of the traditional Kang, with the total percentage of temperatures ranging from 30℃ to 40 ℃ accounts for 63% of the whole testing time. In contrast, the surface temperature difference of the traditional

Kang is great, most of the time, it is far more from comfort temperature

needed for residents. Obviously, the thermal comfort performance of the PCMCK is much better than the traditional Kang.

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Journal Pre-proof 4.2. Indoor Temperature

The outdoor and indoor temperature variations of the two rooms are shown in Figure 14.

24

Outdoor Temperature PCMCK Traditional Kang

22 20

Temperature(℃)

18 16 14 12 10 8 6 4 2 0 -2

9:

00

11

:0

0 13

:0

0 15

:0

0 17

:0

0 19

:0

0 21

:0

0 23

:0

0

1:

00

3:

00

5:

00

7:

00

Time (h)

Frequency distribution (%)

Figure 14. Indoor air temperature variation of the two test rooms

Traditional Kang

60.00%

PCMCK

50.00% 40.00% 30.00% 20.00% 10.00% 0.00% 0-10

10-12

12-14

14-16

16-18

Indoor air temperature (℃)

Figure 15. Contrast of indoor air temperature frequency distribution As can be seen from the chart, the outdoor mean temperature was 2.1◦C during the testing period, and the indoor mean temperature was 12.91◦C in the room equipped with the traditional

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Journal Pre-proof Kang, while that in the room severed by PCMCK was 15.85◦C, which was about 3◦C higher. The temperature fluctuation in the room using traditional Kang was more dramatic; the maximum temperature was 16.6◦C and the minimum was 9.4◦C. It rose rapidly at the time of burning, but dropped markedly after the firing period as well. By contrast, in the room using the PCMCK, the indoor temperature rose gradually. At about 00:00, it reached the maximum temperature of 17.8◦C, then declined steadily. The air temperature in the room using the traditional

Kang was higher than that in the room using the PCMCK from 9:30 to12:30.

But after that the opposite scenario occurred, as the temperature in the room using the PCMCK was higher than that in the room using the traditional Kang for rest of the time, In addition, after 19:20, the temperature of the room using traditional Kang started to decrease, whereas the PCMCK-heated room’s temperature kept rising, which corroborated the effective improvement by the PCMCK. Moreover, the temperature of the PCMCK- heated room kept rising until about 00:00. Obviously, the PCMCK can effectively extend the heating time for about 5 hours. As far as the frequency distribution of indoor air temperature is concerned (Figure 15), it is clear that the indoor air temperatures in PCMCK never stayed below 12◦C, and the proportion of temperature higher than 16◦C accounts for 49.32% of all, which is 12 times as large as that of the traditional Kang (4.11%).On the contrary, temperatures under 14◦C of the traditional Kang take up approximately 67% of the whole day. Over all, PCMCK both extend the heating period and improve the indoor thermal comfort effectively. 5. Conclusions In this paper, the Kang surface temperature and indoor temperature of the two testing 21

Journal Pre-proof rooms were analyzed. Several useful conclusions that were obtained are as follows: The Phase Change Material-Chinese Kang (PCMCK) with #48 paraffin effectively overcomes the drawback of the surface temperature non-uniformity inherited in traditional Kang. According to the data, the coefficient of variation of the PCMCK is significantly lower than that of the traditional Kang during each test period. Thus, the PCMCK surface temperature distribution is much more uniform. In addition, during the stage when the temperature decreased at night, the temperature of the PCMCK dropped at a slower rate, which was apparently higher than that of the traditional Kang between 0:00-8:00. The indoor temperature of PCMCK-heated room kept rising until midnight. The heating time was extended effectively. During the experiment period, the outdoor mean temperature was 2.1◦C, the indoor mean temperature of the PCMCK-heated room was 15.84◦C, which was about 3◦C higher than that of the traditional Kang- heated room, which was 12.91 ◦C. In conclusion, compared to the traditional Kang, the phase change material Chinese Kang (PCMCK) can effectively improve the surface temperature uniformity, extend the heating time and raise the indoor thermal comfort.

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CRediT author statement Gang Li: Conceptualization, Methodology. Xiaoxuan Bi: Validation, WritingOriginal draft preparation. Guohui Feng: Conceptualization, Supervision. Lan Chi: Validation, Writing-Original draft preparation. Xianfang Zheng: WritingReviewing and Editing. Xueting Liu: Writing-Reviewing and Editing.

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Highlights

The PCMCK surface temperature distribution is much more uniform comparatively. The indoor mean temperature of the PCMCK room is raised about 3℃ comparatively. The indoor temperature of PCMCK-heated room kept rising until midnight. The temperature of the PCMCK dropped more slowly when it decreased after midnight.