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Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in an intermittently heated room
Accepted Manuscript Title: Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in an intermittently heated room Authors: Yanru Li, Jing Zhou, Enshen Long, Xi Meng PII: DOI: Reference:
S2210-6707(17)30230-5 https://doi.org/10.1016/j.scs.2018.01.040 SCS 953
To appear in: Received date: Revised date: Accepted date:
7-3-2017 10-8-2017 22-1-2018
Please cite this article as: Li, Yanru., Zhou, Jing., Long, Enshen., & Meng, Xi., Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in an intermittently heated room.Sustainable Cities and Society https://doi.org/10.1016/j.scs.2018.01.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in
Yanru Li1, Jing Zhou1, Enshen Long1, 2,*, Xi Meng3
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an intermittently heated room
1. College of Architecture and Environment, Sichuan University, Chengdu, China
2. Institute of Disaster Management and Reconstruction, Sichuan University-Hong
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Kong PolyU, China.
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3. College of Architecture and Urban-Rural Planning, Sichuan Agricultural University, Chengdu, China.
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The research background of this study is the actual heating situation in the south of China, rather than the ideal continuous operation. An experiment was taken to study the heat storage and release process of building envelope integrated with PCM during intermittent heating process. In the first 1.3 hours, the PCM layer stored sensible heat, and it began to store latent heat after 1.3 hours heating. The heat release process of the PCM layer lasted 6-7 hours after the suspension of heating.
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Highlights
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Abstract: The intermittent heating is frequently used to save the building heating energy
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consumption. Both the heat storage and release processes of building envelope have coupling influences on the indoor thermal environment during intermittent heating process. In order to
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make the best of the heat storage and release process to adjust the indoor thermal environment, this paper integrated phase change material (PCM) with the building envelope and experimentally studied the thermal performance improvement under the four typical intermittent heating conditions summarized by questionnaires in China. The results show that the inner surface temperature of the PCM wall increased faster after heating, which was more favorable to ensure
that the indoor thermal environment recovers to the thermal comfort state quickly. The inner surface temperature of the PCM wall was higher than the reference wall after heating was off, which can maintain a constant indoor air temperature. In the heating process for the four
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conditions, the inner surface heat flux value was 18.48% lower than the reference wall. Integrating PCM with the building envelope can not only improve the indoor thermal environment, but also reduce the heating time and heating energy consumption.
Keywords: Intermittent heating; Phase change material; Questionnaire; Thermal performance; Experimental study
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1. Introduction
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In recent years, the building energy sector consumes more than 33% of the total energy in
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China [1]. People’s requirements for indoor thermal comfort have increased with the improving
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living standards, and the heating line, which is moving southwards, leads to the increase of
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building energy consumption. It is generally believed that intermittent heating can significantly reduce the heating energy consumption and cost compared with the continuous heating [2-6].
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However, in some cases the energy efficiency of intermittent heating is not obvious [7-8]. Due to the fact that the energy losses of walls accounts for about 25% of the total building energy
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consumption [9], it is very important to improve the wall thermal performance to increase the
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building energy efficiency during intermittent heating. The heating equipment starts and stops frequently during intermittent heating, so the
building envelopes always stores and releases heat, while the indoor thermal environment is changing. In the intermittent heating process, the thermal inertia of the building envelope is closely related to the indoor thermal environment variation [8, 10]. Previous studies on building
envelopes have mainly focused on the influence of different wall structures on the intermittent heating energy efficiency and the indoor thermal environment [11-12]. However, there is few study about using the heat storage of the building envelope’s internal part to regulate the
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intermittent heating process. The building envelope, as a thermal mass, is able to adjust the air temperature by storing heat when the temperature is high and releasing heat when the temperature is low. Ogoli et al. [13] found that a wall constructed of heavy materials could reduce the indoor air temperature effectively. Cheng et al. [14] comprehensively analyzed the effects of
different building heat storage materials, different surface colors of the exterior wall and different
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building orientations to indoor temperature. During the intermittent heating process, the building
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energy consumption can be reduced through rationally use of building heat storage to control the
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operation of heating equipment [15].
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The phase change material (PCM) is integrated with building envelope for building energy conservation and indoor thermal environment improvement due to its potential latent heat
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thermal storage [16-17]. Many scholars have integrated PCM with the building envelope to
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improve the indoor thermal environment of non-air-conditioned rooms and reduce energy
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consumption under continuous air-conditioning condition. Kuznik et al. [18] did experiments on the indoor thermal environments of the PCM room and the ordinary room in summer, winter and
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transition seasons, and their experimental results showed that the phase change wall could reduce the indoor temperature fluctuations. Behzadi and Faril [19] did computer simulations on
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PCMs impregnated in building materials, and found that the use of PCMs could effectively reduce the daily fluctuations of indoor air temperature to 4°C and maintain it at the desired comfort level for a long period of time without air conditioning. Yan [20] studied the thermal performance of PCM walls with different structures and found that the inner surface temperature and heat flux of PCM walls were better than traditional walls. Castell [21] did comparison experiments on two
cubicles integrated with different PCMs, and showed that PCM could reduce the peak temperature by up 1°C, and the air conditioning energy consumption by 15% in summer. Chan [22] found that an annual energy saving of 2.9% in air-conditioning system was achieved through
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integrating PCM with building facades. However, studies on the heat storage and release process of building envelope integrated with PCM during intermittent heating still lacks.
During the intermittent heating, the heat storage and release processes of building envelope have coupling influences on indoor environment variation, and the rational use of building heat storage can significantly improve the indoor thermal environment. In this paper, PCM is
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integrated to building envelope to improve the thermal performance. PCM with latent heat
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storage allows phase changing to reduce the wall load during the heating time and regulate the
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indoor air temperature during the heating suspension time. Experimental building with the PCM
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wall unit and the reference wall unit are built in this paper to study the thermal performance improvement of building envelope by integrating with PCM. The transmission and attenuation of
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temperature and heat flow in walls integrated with PCM are assessed under four typical
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intermittent heating conditions which are summarized by questionnaire.
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2. Experimental set-up
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2.1 Intermittent heating operation conditions based on questionnaire
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The intermittent heating conditions were determined based on residents’ living habits in
many studies [4, 6, 7, 8, and 23]. However, the intermittent heating operation mode is greatly influenced by the occupants, and an accurate determination of heating equipment operation modes can provide basic research for the room’s dynamic thermal response mechanism and building envelope optimization. In December 2015, the authors conducted an online
questionnaire survey on heating equipment operating time in residential buildings and office buildings located in the south of China. 420 questionnaires were issued and 420 were returned. The results show that in residential buildings the main operating time of bedrooms heating is
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21:00-7:00, while that of living rooms is 19:00-23:00. For office buildings, the heating equipment operation time is 9:00-17:00, which is the same as the working hours. However, some respondents go home to rest from 12:00 to 14:00, which leads to a cessation of the heating time.
Based on the respondents’ heating equipment usage habits and the research focus of intermittent heating, this paper choses four typical intermittent heating operating conditions through the
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results of the questionnaire survey. The test periods are shown in Table 1.
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2.2 Experimental methodology
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The experiment was carried out in the wall dynamic test experimental building. There are two rooms in the experimental test building, as shown in Figure 1, and the size of the rooms are
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both 3.5m (length)×3.0m (width)×2.2m (height). The building is located in Chengdu, southwest
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China. This location is a hot summer and cold winter area, with an average temperature of 5.6°C, and an extreme temperature of -3.9°C in winter. A split air conditioner (KFR-35GW/HFJ+3) is
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installed on the south wall of Room 1 wall to achieve the intermittent heating.
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Figure 1. The experimental system ((a) sketch map of the experiment test building; (b) experimental wall units; (c) architectural appearance)
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The experimental wall with the PCM wall unit and the reference wall unit are embedded in
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the north external wall of Room1 (Figure 1(b)). The two wall units with size of
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600mm×600mm×260mm are in the same indoor and outdoor environment, which makes the test results more comparable. Supported by steel frames, the two wall units are surrounded by 80mm
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EPS to reduce the heat transfer between units, ensuring one-dimensional heat transfer in the central area of each one. Figure 2 presents the schematic structures and measurement
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arrangements of walls, and Figure 3 shows the test wall units during construction process. From the inner side to the outer side, the PCM wall consists of one layer of cement mortar of 10mm, one layer of PCM of 20mm, one layer of solid brick of 220mm and one layer of cement mortar of 10mm. From the inner side to the outer side, the reference wall consists of one layer of cement mortar of 10mm, one layer of solid brick of 240mm and one layer of cement mortar of 10mm.The
physical properties of each layer of material are shown in Table 2. The PCM in this paper is the Natural TCM Energy Saver [24] produced by Tri-Y Enterprises Limited from Canada [25]. The phase-transition temperature range of the PCM is from 18°C to 26°C, which has phase change
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latent heat of 178.5kJ/kg.
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Figure 2. The schematic structures of walls and measurement arrangements
Figure 3. The test wall units during the construction process Note: a. The thermal conductivity of PCM decreases with the temperature increasing. b. The value is the specific heat of PCM in solid or liquid state.
For the test wall units, thermocouples are arranged at the center of the inner and the outer surfaces to measure the surface temperatures, and heat flux meters are arranged at the center of the inner surfaces to measure the heat flux of the inner surfaces. One thermocouple is located
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20cm from the wall inner surface to measure the indoor air temperature near the wall, while another sensor is arranged 20cm from the wall external surface and 1.5m above the ground to
measure the outdoor air temperature near the wall. In addition, there are thermocouples on the
center of both sides of PCM layer. The detailed measurement arrangements are shown in Figure 2.
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T-type thermocouples (with test error less than 0.5°C) and heat flux meters (JTC08A with
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accuracy of 5%) are used to measure the temperature and heat flux. All measurement data are
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recorded by a JTRG-II building thermal temperature automatic tester. The measurements of
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temperature and heat flux under different intermittent heating conditions were carried out every 10 minutes from 18:00 on 10th January 2016 to 24:00 on 18th January 2016. For each of the four
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working conditions in the questionnaire, two cycles of experiment were carried out. The test time
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is shown in Table 1. Because the phase-transition temperature range of the PCM is from 18°C to
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26°C, the heating thermostat setting was adjust to 30°C for winter space heating to ensure the phase change of PCM.
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2.3. The thermal properties of experiment walls
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The thermal characteristics of building envelope mainly depend on its thermal parameters.
According to thermal theory [26] and the phase-change energy storage theory [27], the thermal performance of PCM wall is evaluated by thermal resistance R and heat capacity HCA. The thermal resistance R (Eq. (1)) represents the total resistance amount of heat transferred from one side of
the wall to the other side, which reflects the wall’s resistance to heat flow. A larger R value means better thermal insulation performance of the wall and weaker heat conduction capability.
i
R1 R2
i i 1 i n
Rn
(1)
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R
Where, R1, R2… Rn—the thermal resistance of each layer material , m2·K/W; δ1, δ2… δn—the thickness of each layer material, m;
λ1, λ2… λn—the thermal conductivity coefficient of each layer material, W/(m·K).
The thermal resistance reflects the heat transfer performance of wall under given
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conditions. However, the heat storage and release process of wall are dynamic. In order to
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evaluate the wall’s heat storage and release performance objectively, the heat capacity per unit
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HCAn i ci i
(2)
i 1
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HCA HCA1 HCA2
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area (HCA) is introduced as:
Where, HCA1, HCA2… HCAn—the heat capacity of each layer, kJ/(m2·K);
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ρ1, ρ2… ρn—the density of each layer material, kg/m3; c1, c2… cn—the specific heat capacity of each layer material, kJ/(kg·K);
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δ1, δ2… δn—the thickness of each layer material, m.
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The specific heat capacity is an important thermal parameter in evaluating the heat storage performance of materials. In this paper, the equivalent heat capacity ceff is used to evaluate the
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heat storage performance of PCM. By making the specific heat of the PCM function of temperature, the latent heat effect can be simulated. The equivalent heat capacity of PCM in the whole temperature range is defined as Eq. (3) [28].
ceff
cs T Tl L c c s l Tl T Ts T T 2 s l cl T Ts
(3)
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where, ceff is the equivalent heat capacity of PCM, kJ/(kg·K); cs is the average specific heat capacity of PCM in the solid state, kJ/(kg·K); cl is the average specific heat capacity of PCM in the
liquid state, kJ/(kg·K); L is the phase change latent heat, kJ/kg; Tl is the solidus temperature of PCM, K; Ts is the liquidus temperature of PCM, K.
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Table 3 presents the calculation results of the thermal parameters. The R of the PCM wall is 11.63% larger than the reference wall when the PCM is in the liquid state, indicating that the
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thermal insulation performance of the PCM wall is better. However, when the PCM is in the solid
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state, the thermal resistance of the PCM wall is similar to the reference wall. The HCA of the PCM
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wall is 12.01% larger than the reference wall when the PCM is not phase changing, and it is nearly
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2.75 times larger than the reference wall when the PCM is phase changing, so the heat storage
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performance of the PCM wall is significantly improved because of the latent heat storage.
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3. Results and discussion
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3.1 Variation of indoor and outdoor air temperature Figure 4 shows the variations of indoor and outdoor air temperatures from 18:00 on 10th
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January to 24:00 on 18th January 2016. During the test period, the highest and the lowest values of outdoor air temperature was 14.55°C and 3.2°C respectively, while the temperature difference between day and night was 3.0-9.5°C. With the change of intermittent condition, the indoor air temperature changes significantly. For the four intermittent heating conditions, the indoor air
temperature increased quickly to the upper limit of the setting temperature in the first 1 hour after starting heating. The compressor was then suspended off because the air conditioner power was very large. When the indoor air temperature dropped to the lower limit of the setting
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temperature, the compressor started working and the indoor air temperature increased again. As a result, there was a cycle of reciprocating fluctuations.
The indoor air temperature was relatively stable after heating for 3 hours, and it fell rapidly after the heating was off. The indoor air temperature reduced and was equal to the indoor air
temperature at the beginning of heating in about 10 hours after stopping heating. The indoor air
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temperature fluctuation is influenced by the coupling effect of ambient condition and building
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envelopes. The longer the heating time, the more the heat stored by building envelope, and the
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smaller change rate of the indoor air temperature after stopping heating. However, the indoor air
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temperature drop range is great when the heating suspension time is long. Therefore, the temperature drop range in condition 3 heating suspension time at midday was much smaller than
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other conditions.
Figure 4. The variations of indoor and outdoor air temperatures from 18:00 on 10th Jan to 24:00 on 18th Jan 2016
3.2 The inner surface temperatures and the temperatures of PCM layer in the typical day The second test cycle of each condition is used to analyze to minimize the influence of the
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previous operating condition. Mean radiation temperature is an important factor of thermal comfort [29], and the inner surface temperature of building envelope is closely related to the
radiation temperature [30]. Figure 5 presents the variation of inner surface temperature on the typical day. The wall inner surface temperature increased with the increase of indoor air
temperature after heating, but its increase rate was much lower than that of the indoor air
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temperature due to the fact that the wall thermal inertia was far larger than air thermal inertia.
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Moreover, it can be clearly seen that the inner surface temperature changing rates of the PCM
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wall were larger under all four conditions compared with the reference wall. Table 4 shows the
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increased values of inner surface temperature in first 3 hours after heating. The increased values
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of inner surface temperature were 7.59°C, 8.49°C and 6.90°C in conditions 1, 2 and 4 for the PCM
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wall, while they were 5.70°C, 6.81°C and 5.31°C respectively for the reference wall. The inner surface temperature rise of the PCM wall was increased by 0.69°C - 1.89°C compared with the
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reference wall in 3 hours after heating. As for condition 3, the cessation of heating time at midday was short, and the heat storage of the wall inner part was not fully released, so that the inner
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surface temperature rise was small during the afternoon heating period (14:00-17:00) compared with the morning heating period. The heating time in condition 2 and condition 4 were 9 hours
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and 8 hours respectively, which means the heat storage of the inner part of wall is close to saturation with the long time heating. The inner surface temperature rise rate becomes slowly in 3 hours after heating while the heat storage process of the wall unit has not finished. For the four conditions, the inner surface temperature of the PCM wall increased more rapidly than the reference wall during heating, and the inner surface temperature of the PCM wall was about 2°C
higher than the reference wall. With the advantage of fast thermal response rate of inner surface after heating, the PCM wall is able to ensure that the indoor thermal environment recovers to the thermal comfort state quickly. Moreover, the PCM wall is more favorable to indoor thermal
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environment during intermittent heating because of the high inner surface temperature.
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Figure 5. The inner surface temperature variations under different conditions After the heating was off, the inner surface temperature decreased exponentially with the
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decrease of indoor air temperature. The inner surface temperature of the PCM wall was 2.2°C higher than the reference wall when heating was off, which means the PCM wall could release heat during the heating suspension time and was more beneficial in maintaining the indoor thermal environment in the comfort state. Under the four intermittent heating conditions, the PCMs in the PCM wall changed from phase changing state (or liquid state) to solid state, and
simultaneously released latent heat in the heating suspension time, which not only improved the indoor comfortable level but also extended the exothermic time of the PCM wall. The inner surface temperature variation of the PCM wall is much different from the
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reference wall in the intermittent heating start-stop process due to the PCM layer. Figure 6 presents the inside and outside temperatures of the PCM layer and the temperature difference
between them. Because the indoor air temperature is higher than outdoor air temperature in winter, the heat transfer direction in the wall is always from inside to outside, and the inside
temperature of PCM layer is higher than the outside. The inside and outside temperatures of the
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PCM wall were lower than the phase transition temperature before heating, and the PCM layer
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acted as an insulation with the temperature difference of 1°C. Then, the inside temperature of
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the PCM layer increased quickly after heating, and the heat transferred to the outside was less
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because the PCM layer was storing heat, so the outside temperature of the PCM layer was not affected by the indoor environment fluctuation in the first 1.3 hours since heating. However, in
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the first 1.3 hours after heating, the temperature difference between two sides of the PCM layer
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increased rapidly. The inside temperature of the PCM layer increased to above 18°C after 1.3
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hours heating, and the PCM in the inner part changed from solid state to phase transition state. Meanwhile some heat was transferred to the outside of the PCM layer to raise the temperature.
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The temperature rise rates of the two sides of the PCM layer became close to each other during 1.3 hours - 2 hours after heating, and the temperature difference was stable at about 6°C.
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However, during the heating process under four conditions, only the PCM near the indoor side changed phase because the outside layer temperature was always below 18°C which was the solidus temperature. After the suspension of heating, the influence of the indoor air temperature to the inside temperature of PCM layer was reduced, and the PCM changed from phase changing state to solid state with heat release. Moreover, the outside temperature of the PCM layer
continued to increase in the first 0.5 hour - 1 hour after heating was off due to the heat release, and then began to decrease. The temperature difference between the two sides of the PCM layer decreased after heating stopped. It reduced to the value before heating in 6 hours -7 hours after
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the suspension of heating, which indicated the end of the PCM exothermic process.
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Figure 6. The inside and outside temperatures of the PCM layer and the temperature differences
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between them
3.3 The inner surface heat flux variations and energy saving efficiency of PCM in the
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typical day
Figure 7 shows the variations of inner surface heat flux under different conditions, and the positive value of heat flux means that heat is transferred from indoor air to the wall’s inner surface. For the four conditions, the inner surface heat flux of the PCM wall and the reference
wall was about zero before heating, and there was no significant heat exchange between indoor air and the wall. After the heating was recovered, the indoor air temperature firstly increased, and then the heat was transferred to wall’s inner surface through convection heat transfer, so the
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inner surface heat flux increased rapidly. With the heating time increasing, the heat stored by the wall’s inner part was close to saturation and the heat flux reduced gradually. The inner surface
heat flux got close to stabilization 3 hours after heating in condition 2 and condition 4. Whenever
heating, the inner surface heat flux of the PCM wall was averagely reduced by 18.48% compared with the reference wall. And the low peak of the heat flux means the load in the building with
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PCM wall is more stable with no peak compared with the building with reference wall.
Figure 7. The inner surface heat flux variations under different conditions
However, during intermittent heating process, some of the heat transferred into the inner surface was stored by the wall, while the others was lose to the outdoor environment through the wall. The variations of outer surface temperature are shown in Figure 8. The outer surface
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temperature is mainly affected by the outdoor environment, while the indoor air temperature changes have a little effect on it. Heat is transferred to ambient through building envelope because the temperature difference between indoor and outdoor increases after heating. In
condition 1 and the first 3 hours after heating in the other 3 conditions, the differences between the PCM wall’s outer surface temperature and the reference wall’s outer surface temperature
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changed little compared with those before heating. The heat transferred to the walls from the
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indoor air through convection was stored in the inner part of the wall rather than transferred to
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wall’s outer surface through conduction at the beginning of heating. The reference wall’s inner
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part reached heat saturation after 3 hours heating, and then the heat was transferred to the outer part, so its outer surface temperature began to increase. As for the PCM wall, with increasing
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temperature, the PCM changed from solid to liquid and stored latent heat, which extended the
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wall heat storage after heating. The heat storage process of the PCM wall’s inner part did not finished in 3 hours after heating, so the heat transferred to outside was less and the outer surface
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temperature of the PCM wall was lower than the reference wall. The PCM wall’s outer surface
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temperature is 0.8°C lower than the reference wall when heating was off in condition 1, condition
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2, condition 3’s afternoon operation process and condition 4.
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4. Conclusions
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Figure 8. The outer surface temperature under different conditions
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The thermal performance improvement of building envelope by integrating with PCM in an
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intermittently heated room is assessed through experimental study in this paper. The results show that integrating PCM with the building envelope can improve the indoor thermal environment as
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well as reducing the heating energy consumption. In the beginning of heating, the inner surface temperature of the PCM wall increased rapidly, and its temperature rise was 0.69°C -1.89°C larger than the reference wall in the first 3 hours after heating. With the advantage of fast thermal response of inner surface, the PCM wall is able to ensure that the indoor thermal environment recovers to the thermal comfort state quickly. Moreover, after heating is off, the inner surface
temperature of the PCM wall is higher than the reference wall owing to the latent heat released by the PCM, which indicates that the indoor air would be favorable in maintaining a comfortable indoor thermal environment.
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Some of the heat transferred into the wall’s inner surface is stored by the wall, while the others is lost to the ambient through the wall during the initial period of intermittent heating.
During the heating process, the inner surface temperature of the PCM wall was always higher
than the reference wall while the outer surface temperature of the PCM wall was clearly lower than the reference wall after 3 hours heating, indicating the larger heat storage capacity of the
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PCM wall. The energy saving effect of the PCM wall is significant in the heating process. In
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addition, the PCM wall can store more heat during heating process and release it during the
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heating suspension time. Therefore, the heating time and heating energy consumption can be
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reduced to a certain extent through reasonably controlling the heating process when the indoor air temperature is maintained in the comfort range by the rational use of PCM wall’s exothermic
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process. Although the heating temperature in the experiment was higher than the indoor
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comfortable temperature, the experimental results, which revealed the thermal process of the
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PCM, are quite correct in the qualitative analysis. The PCM (with the phase transition temperature range of 18-26°C) did not change phase completely during the measurement even though the air
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condition was set at 30°C. It is very important to choose the phase-transition temperature range according to building envelope properties, heating conditions and climate characteristics, which
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will be studied in later works. This paper can provide guidance about the use of PCM in indoor thermal environment regulation and energy consumption reduction in intermittently heated rooms.
Acknowledgments This project is funded by the National Natural Science Foundation of China (No.51478280), and
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the National Key Research and Development Program (2016YFC00406).
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[21] Yan Quanying, Huo Ran, and Li Lisa. Experimental study on the thermal properties of the
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phase change material wall formed by different methods [J]. Solar Energy, 2012, 86: 309-3102.
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[22] A.L.S. Chan. Energy and environmental performance of building facades integrated with phase
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change material in subtropical Hong Kong [J]. Energy and Buildings, 2011, 43: 2947-2955.
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[23] XG Xu, etc. Thermal comfort in an office with intermittent air-conditioning operation [J]. Building Services Engineering Research and Technology. 2010, 31(1): 91–100.
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[24] http://www.tcm-energysaver.com/index.html
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[25] http://www.tri-y.com/ch/bj/main-gsjj.htm [26] Lu Yaoqing. Practical heating and air condition design handbook (the second edition) [M].
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Beijing: Chinese Building Industry Press, 2008. [27] Zhang Yinping, Hu Hanping, Kong Xiang, et al. Phase change heat storage: theory and application [M]. Hefei: Publishing House in University of Science and Technology of China, 1996. [28] Esam M, Alawadhi. Thermal analysis of a building brick containing phase change material [J].
Energy and Buildings, 2008, 40(3): 351-357. [29] P. O. Fanger. Comfort limits for asymmetric thermal radiation [J]. Energy and Buildings, 1985, 8(3): 225-236.
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[30] P. O. Fanger, A. K. Melikov, H. Hanzawa, J. Ring. Turbulence and draft [J]. ASHRAE Journal -
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American Society of Heating Refrigerating and Air-conditioning Engineers, 1989, 31(4): 18-&.
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Table 1. Operation conditions of intermittent heating Heating time
Test period
Condition 1
19:00~23:00
1.10-1.11
21:00~6:00
1.12-1.14
Condition 3
9:00~12:00, 14:00~17:00
1.15-1.16
Condition 4
9:00~17:00
1.17-1.18
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Operation condition
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Condition 2
Table 2. Physical properties of each layer
Density(kg/m3)
Thermal conductivity (W/(m·K))
Specific heat (kJ/kg·K)
Mortar
1406
0.3505
1.05
Solid brick
1536
0.7505
0.523
PCM
1300
0.25(liquid), 0.5(solid)a
1.785b
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Material
Table 3. Thermal parameters of two kinds of walls
HCA (kJ/(m2·K))
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R (m2·K/W)
Wall
252.67 (solid or liquid), 832.79
0.43 (liquid), 0.39 (solid)
0.38
(phase change) 222.32
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Reference wall
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PCM wall
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Table 4. The increased values of inner surface temperature in the first 3 hours after starting
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heating under four conditions
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Operation condition
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Condition 1
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Condition 2
Heating time
PCM wall (°C)
Reference wall (°C)
19:00~23:00
7.59
5.70
21:00-6:00
8.49
6.81
9:00-12:00
8.61
6.21
14:00-17:00
5.49
4.8
9:00-17:00
6.90
5.31
Condition 3
Condition 4
Figure Caption Figure 1. The experimental system ((a) sketch map of the experiment test building; (b)
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experimental wall units; (c) architectural appearance). Figure 2. The schematic structures of walls and measurement arrangements. Figure 3. The test wall units during the construction process
Figure 4. The variations of indoor and outdoor air temperatures from18:00 on 10th Jan to 24:00 on 18th Jan 2016.
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Figure 5. The inner surface temperature variations under different conditions.
Figure 6. The inside and outside temperatures of the PCM layer and the temperature differences
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between them.
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Figure 7. The inner surface heat flux variations under different conditions.
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Figure 8. The outer surface temperature under different conditions.
S2210-6707(17)30230-5 https://doi.org/10.1016/j.scs.2018.01.040 SCS 953
To appear in: Received date: Revised date: Accepted date:
7-3-2017 10-8-2017 22-1-2018
Please cite this article as: Li, Yanru., Zhou, Jing., Long, Enshen., & Meng, Xi., Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in an intermittently heated room.Sustainable Cities and Society https://doi.org/10.1016/j.scs.2018.01.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Experimental study on thermal performance improvement of building envelopes by integrating with phase change material in
Yanru Li1, Jing Zhou1, Enshen Long1, 2,*, Xi Meng3
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an intermittently heated room
1. College of Architecture and Environment, Sichuan University, Chengdu, China
2. Institute of Disaster Management and Reconstruction, Sichuan University-Hong
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Kong PolyU, China.
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3. College of Architecture and Urban-Rural Planning, Sichuan Agricultural University, Chengdu, China.
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The research background of this study is the actual heating situation in the south of China, rather than the ideal continuous operation. An experiment was taken to study the heat storage and release process of building envelope integrated with PCM during intermittent heating process. In the first 1.3 hours, the PCM layer stored sensible heat, and it began to store latent heat after 1.3 hours heating. The heat release process of the PCM layer lasted 6-7 hours after the suspension of heating.
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Highlights
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Abstract: The intermittent heating is frequently used to save the building heating energy
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consumption. Both the heat storage and release processes of building envelope have coupling influences on the indoor thermal environment during intermittent heating process. In order to
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make the best of the heat storage and release process to adjust the indoor thermal environment, this paper integrated phase change material (PCM) with the building envelope and experimentally studied the thermal performance improvement under the four typical intermittent heating conditions summarized by questionnaires in China. The results show that the inner surface temperature of the PCM wall increased faster after heating, which was more favorable to ensure
that the indoor thermal environment recovers to the thermal comfort state quickly. The inner surface temperature of the PCM wall was higher than the reference wall after heating was off, which can maintain a constant indoor air temperature. In the heating process for the four
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conditions, the inner surface heat flux value was 18.48% lower than the reference wall. Integrating PCM with the building envelope can not only improve the indoor thermal environment, but also reduce the heating time and heating energy consumption.
Keywords: Intermittent heating; Phase change material; Questionnaire; Thermal performance; Experimental study
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1. Introduction
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In recent years, the building energy sector consumes more than 33% of the total energy in
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China [1]. People’s requirements for indoor thermal comfort have increased with the improving
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living standards, and the heating line, which is moving southwards, leads to the increase of
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building energy consumption. It is generally believed that intermittent heating can significantly reduce the heating energy consumption and cost compared with the continuous heating [2-6].
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However, in some cases the energy efficiency of intermittent heating is not obvious [7-8]. Due to the fact that the energy losses of walls accounts for about 25% of the total building energy
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consumption [9], it is very important to improve the wall thermal performance to increase the
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building energy efficiency during intermittent heating. The heating equipment starts and stops frequently during intermittent heating, so the
building envelopes always stores and releases heat, while the indoor thermal environment is changing. In the intermittent heating process, the thermal inertia of the building envelope is closely related to the indoor thermal environment variation [8, 10]. Previous studies on building
envelopes have mainly focused on the influence of different wall structures on the intermittent heating energy efficiency and the indoor thermal environment [11-12]. However, there is few study about using the heat storage of the building envelope’s internal part to regulate the
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intermittent heating process. The building envelope, as a thermal mass, is able to adjust the air temperature by storing heat when the temperature is high and releasing heat when the temperature is low. Ogoli et al. [13] found that a wall constructed of heavy materials could reduce the indoor air temperature effectively. Cheng et al. [14] comprehensively analyzed the effects of
different building heat storage materials, different surface colors of the exterior wall and different
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building orientations to indoor temperature. During the intermittent heating process, the building
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energy consumption can be reduced through rationally use of building heat storage to control the
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operation of heating equipment [15].
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The phase change material (PCM) is integrated with building envelope for building energy conservation and indoor thermal environment improvement due to its potential latent heat
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thermal storage [16-17]. Many scholars have integrated PCM with the building envelope to
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improve the indoor thermal environment of non-air-conditioned rooms and reduce energy
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consumption under continuous air-conditioning condition. Kuznik et al. [18] did experiments on the indoor thermal environments of the PCM room and the ordinary room in summer, winter and
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transition seasons, and their experimental results showed that the phase change wall could reduce the indoor temperature fluctuations. Behzadi and Faril [19] did computer simulations on
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PCMs impregnated in building materials, and found that the use of PCMs could effectively reduce the daily fluctuations of indoor air temperature to 4°C and maintain it at the desired comfort level for a long period of time without air conditioning. Yan [20] studied the thermal performance of PCM walls with different structures and found that the inner surface temperature and heat flux of PCM walls were better than traditional walls. Castell [21] did comparison experiments on two
cubicles integrated with different PCMs, and showed that PCM could reduce the peak temperature by up 1°C, and the air conditioning energy consumption by 15% in summer. Chan [22] found that an annual energy saving of 2.9% in air-conditioning system was achieved through
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integrating PCM with building facades. However, studies on the heat storage and release process of building envelope integrated with PCM during intermittent heating still lacks.
During the intermittent heating, the heat storage and release processes of building envelope have coupling influences on indoor environment variation, and the rational use of building heat storage can significantly improve the indoor thermal environment. In this paper, PCM is
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integrated to building envelope to improve the thermal performance. PCM with latent heat
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storage allows phase changing to reduce the wall load during the heating time and regulate the
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indoor air temperature during the heating suspension time. Experimental building with the PCM
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wall unit and the reference wall unit are built in this paper to study the thermal performance improvement of building envelope by integrating with PCM. The transmission and attenuation of
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temperature and heat flow in walls integrated with PCM are assessed under four typical
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intermittent heating conditions which are summarized by questionnaire.
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2. Experimental set-up
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2.1 Intermittent heating operation conditions based on questionnaire
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The intermittent heating conditions were determined based on residents’ living habits in
many studies [4, 6, 7, 8, and 23]. However, the intermittent heating operation mode is greatly influenced by the occupants, and an accurate determination of heating equipment operation modes can provide basic research for the room’s dynamic thermal response mechanism and building envelope optimization. In December 2015, the authors conducted an online
questionnaire survey on heating equipment operating time in residential buildings and office buildings located in the south of China. 420 questionnaires were issued and 420 were returned. The results show that in residential buildings the main operating time of bedrooms heating is
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21:00-7:00, while that of living rooms is 19:00-23:00. For office buildings, the heating equipment operation time is 9:00-17:00, which is the same as the working hours. However, some respondents go home to rest from 12:00 to 14:00, which leads to a cessation of the heating time.
Based on the respondents’ heating equipment usage habits and the research focus of intermittent heating, this paper choses four typical intermittent heating operating conditions through the
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results of the questionnaire survey. The test periods are shown in Table 1.
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2.2 Experimental methodology
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The experiment was carried out in the wall dynamic test experimental building. There are two rooms in the experimental test building, as shown in Figure 1, and the size of the rooms are
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both 3.5m (length)×3.0m (width)×2.2m (height). The building is located in Chengdu, southwest
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China. This location is a hot summer and cold winter area, with an average temperature of 5.6°C, and an extreme temperature of -3.9°C in winter. A split air conditioner (KFR-35GW/HFJ+3) is
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installed on the south wall of Room 1 wall to achieve the intermittent heating.
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Figure 1. The experimental system ((a) sketch map of the experiment test building; (b) experimental wall units; (c) architectural appearance)
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The experimental wall with the PCM wall unit and the reference wall unit are embedded in
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the north external wall of Room1 (Figure 1(b)). The two wall units with size of
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600mm×600mm×260mm are in the same indoor and outdoor environment, which makes the test results more comparable. Supported by steel frames, the two wall units are surrounded by 80mm
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EPS to reduce the heat transfer between units, ensuring one-dimensional heat transfer in the central area of each one. Figure 2 presents the schematic structures and measurement
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arrangements of walls, and Figure 3 shows the test wall units during construction process. From the inner side to the outer side, the PCM wall consists of one layer of cement mortar of 10mm, one layer of PCM of 20mm, one layer of solid brick of 220mm and one layer of cement mortar of 10mm. From the inner side to the outer side, the reference wall consists of one layer of cement mortar of 10mm, one layer of solid brick of 240mm and one layer of cement mortar of 10mm.The
physical properties of each layer of material are shown in Table 2. The PCM in this paper is the Natural TCM Energy Saver [24] produced by Tri-Y Enterprises Limited from Canada [25]. The phase-transition temperature range of the PCM is from 18°C to 26°C, which has phase change
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latent heat of 178.5kJ/kg.
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Figure 2. The schematic structures of walls and measurement arrangements
Figure 3. The test wall units during the construction process Note: a. The thermal conductivity of PCM decreases with the temperature increasing. b. The value is the specific heat of PCM in solid or liquid state.
For the test wall units, thermocouples are arranged at the center of the inner and the outer surfaces to measure the surface temperatures, and heat flux meters are arranged at the center of the inner surfaces to measure the heat flux of the inner surfaces. One thermocouple is located
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20cm from the wall inner surface to measure the indoor air temperature near the wall, while another sensor is arranged 20cm from the wall external surface and 1.5m above the ground to
measure the outdoor air temperature near the wall. In addition, there are thermocouples on the
center of both sides of PCM layer. The detailed measurement arrangements are shown in Figure 2.
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T-type thermocouples (with test error less than 0.5°C) and heat flux meters (JTC08A with
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accuracy of 5%) are used to measure the temperature and heat flux. All measurement data are
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recorded by a JTRG-II building thermal temperature automatic tester. The measurements of
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temperature and heat flux under different intermittent heating conditions were carried out every 10 minutes from 18:00 on 10th January 2016 to 24:00 on 18th January 2016. For each of the four
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working conditions in the questionnaire, two cycles of experiment were carried out. The test time
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is shown in Table 1. Because the phase-transition temperature range of the PCM is from 18°C to
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26°C, the heating thermostat setting was adjust to 30°C for winter space heating to ensure the phase change of PCM.
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2.3. The thermal properties of experiment walls
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The thermal characteristics of building envelope mainly depend on its thermal parameters.
According to thermal theory [26] and the phase-change energy storage theory [27], the thermal performance of PCM wall is evaluated by thermal resistance R and heat capacity HCA. The thermal resistance R (Eq. (1)) represents the total resistance amount of heat transferred from one side of
the wall to the other side, which reflects the wall’s resistance to heat flow. A larger R value means better thermal insulation performance of the wall and weaker heat conduction capability.
i
R1 R2
i i 1 i n
Rn
(1)
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R
Where, R1, R2… Rn—the thermal resistance of each layer material , m2·K/W; δ1, δ2… δn—the thickness of each layer material, m;
λ1, λ2… λn—the thermal conductivity coefficient of each layer material, W/(m·K).
The thermal resistance reflects the heat transfer performance of wall under given
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conditions. However, the heat storage and release process of wall are dynamic. In order to
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evaluate the wall’s heat storage and release performance objectively, the heat capacity per unit
n
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HCAn i ci i
(2)
i 1
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HCA HCA1 HCA2
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area (HCA) is introduced as:
Where, HCA1, HCA2… HCAn—the heat capacity of each layer, kJ/(m2·K);
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ρ1, ρ2… ρn—the density of each layer material, kg/m3; c1, c2… cn—the specific heat capacity of each layer material, kJ/(kg·K);
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δ1, δ2… δn—the thickness of each layer material, m.
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The specific heat capacity is an important thermal parameter in evaluating the heat storage performance of materials. In this paper, the equivalent heat capacity ceff is used to evaluate the
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heat storage performance of PCM. By making the specific heat of the PCM function of temperature, the latent heat effect can be simulated. The equivalent heat capacity of PCM in the whole temperature range is defined as Eq. (3) [28].
ceff
cs T Tl L c c s l Tl T Ts T T 2 s l cl T Ts
(3)
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where, ceff is the equivalent heat capacity of PCM, kJ/(kg·K); cs is the average specific heat capacity of PCM in the solid state, kJ/(kg·K); cl is the average specific heat capacity of PCM in the
liquid state, kJ/(kg·K); L is the phase change latent heat, kJ/kg; Tl is the solidus temperature of PCM, K; Ts is the liquidus temperature of PCM, K.
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Table 3 presents the calculation results of the thermal parameters. The R of the PCM wall is 11.63% larger than the reference wall when the PCM is in the liquid state, indicating that the
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thermal insulation performance of the PCM wall is better. However, when the PCM is in the solid
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state, the thermal resistance of the PCM wall is similar to the reference wall. The HCA of the PCM
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wall is 12.01% larger than the reference wall when the PCM is not phase changing, and it is nearly
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2.75 times larger than the reference wall when the PCM is phase changing, so the heat storage
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performance of the PCM wall is significantly improved because of the latent heat storage.
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3. Results and discussion
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3.1 Variation of indoor and outdoor air temperature Figure 4 shows the variations of indoor and outdoor air temperatures from 18:00 on 10th
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January to 24:00 on 18th January 2016. During the test period, the highest and the lowest values of outdoor air temperature was 14.55°C and 3.2°C respectively, while the temperature difference between day and night was 3.0-9.5°C. With the change of intermittent condition, the indoor air temperature changes significantly. For the four intermittent heating conditions, the indoor air
temperature increased quickly to the upper limit of the setting temperature in the first 1 hour after starting heating. The compressor was then suspended off because the air conditioner power was very large. When the indoor air temperature dropped to the lower limit of the setting
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temperature, the compressor started working and the indoor air temperature increased again. As a result, there was a cycle of reciprocating fluctuations.
The indoor air temperature was relatively stable after heating for 3 hours, and it fell rapidly after the heating was off. The indoor air temperature reduced and was equal to the indoor air
temperature at the beginning of heating in about 10 hours after stopping heating. The indoor air
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temperature fluctuation is influenced by the coupling effect of ambient condition and building
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envelopes. The longer the heating time, the more the heat stored by building envelope, and the
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smaller change rate of the indoor air temperature after stopping heating. However, the indoor air
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temperature drop range is great when the heating suspension time is long. Therefore, the temperature drop range in condition 3 heating suspension time at midday was much smaller than
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other conditions.
Figure 4. The variations of indoor and outdoor air temperatures from 18:00 on 10th Jan to 24:00 on 18th Jan 2016
3.2 The inner surface temperatures and the temperatures of PCM layer in the typical day The second test cycle of each condition is used to analyze to minimize the influence of the
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previous operating condition. Mean radiation temperature is an important factor of thermal comfort [29], and the inner surface temperature of building envelope is closely related to the
radiation temperature [30]. Figure 5 presents the variation of inner surface temperature on the typical day. The wall inner surface temperature increased with the increase of indoor air
temperature after heating, but its increase rate was much lower than that of the indoor air
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temperature due to the fact that the wall thermal inertia was far larger than air thermal inertia.
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Moreover, it can be clearly seen that the inner surface temperature changing rates of the PCM
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wall were larger under all four conditions compared with the reference wall. Table 4 shows the
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increased values of inner surface temperature in first 3 hours after heating. The increased values
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of inner surface temperature were 7.59°C, 8.49°C and 6.90°C in conditions 1, 2 and 4 for the PCM
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wall, while they were 5.70°C, 6.81°C and 5.31°C respectively for the reference wall. The inner surface temperature rise of the PCM wall was increased by 0.69°C - 1.89°C compared with the
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reference wall in 3 hours after heating. As for condition 3, the cessation of heating time at midday was short, and the heat storage of the wall inner part was not fully released, so that the inner
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surface temperature rise was small during the afternoon heating period (14:00-17:00) compared with the morning heating period. The heating time in condition 2 and condition 4 were 9 hours
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and 8 hours respectively, which means the heat storage of the inner part of wall is close to saturation with the long time heating. The inner surface temperature rise rate becomes slowly in 3 hours after heating while the heat storage process of the wall unit has not finished. For the four conditions, the inner surface temperature of the PCM wall increased more rapidly than the reference wall during heating, and the inner surface temperature of the PCM wall was about 2°C
higher than the reference wall. With the advantage of fast thermal response rate of inner surface after heating, the PCM wall is able to ensure that the indoor thermal environment recovers to the thermal comfort state quickly. Moreover, the PCM wall is more favorable to indoor thermal
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environment during intermittent heating because of the high inner surface temperature.
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Figure 5. The inner surface temperature variations under different conditions After the heating was off, the inner surface temperature decreased exponentially with the
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decrease of indoor air temperature. The inner surface temperature of the PCM wall was 2.2°C higher than the reference wall when heating was off, which means the PCM wall could release heat during the heating suspension time and was more beneficial in maintaining the indoor thermal environment in the comfort state. Under the four intermittent heating conditions, the PCMs in the PCM wall changed from phase changing state (or liquid state) to solid state, and
simultaneously released latent heat in the heating suspension time, which not only improved the indoor comfortable level but also extended the exothermic time of the PCM wall. The inner surface temperature variation of the PCM wall is much different from the
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reference wall in the intermittent heating start-stop process due to the PCM layer. Figure 6 presents the inside and outside temperatures of the PCM layer and the temperature difference
between them. Because the indoor air temperature is higher than outdoor air temperature in winter, the heat transfer direction in the wall is always from inside to outside, and the inside
temperature of PCM layer is higher than the outside. The inside and outside temperatures of the
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PCM wall were lower than the phase transition temperature before heating, and the PCM layer
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acted as an insulation with the temperature difference of 1°C. Then, the inside temperature of
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the PCM layer increased quickly after heating, and the heat transferred to the outside was less
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because the PCM layer was storing heat, so the outside temperature of the PCM layer was not affected by the indoor environment fluctuation in the first 1.3 hours since heating. However, in
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the first 1.3 hours after heating, the temperature difference between two sides of the PCM layer
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increased rapidly. The inside temperature of the PCM layer increased to above 18°C after 1.3
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hours heating, and the PCM in the inner part changed from solid state to phase transition state. Meanwhile some heat was transferred to the outside of the PCM layer to raise the temperature.
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The temperature rise rates of the two sides of the PCM layer became close to each other during 1.3 hours - 2 hours after heating, and the temperature difference was stable at about 6°C.
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However, during the heating process under four conditions, only the PCM near the indoor side changed phase because the outside layer temperature was always below 18°C which was the solidus temperature. After the suspension of heating, the influence of the indoor air temperature to the inside temperature of PCM layer was reduced, and the PCM changed from phase changing state to solid state with heat release. Moreover, the outside temperature of the PCM layer
continued to increase in the first 0.5 hour - 1 hour after heating was off due to the heat release, and then began to decrease. The temperature difference between the two sides of the PCM layer decreased after heating stopped. It reduced to the value before heating in 6 hours -7 hours after
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the suspension of heating, which indicated the end of the PCM exothermic process.
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Figure 6. The inside and outside temperatures of the PCM layer and the temperature differences
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between them
3.3 The inner surface heat flux variations and energy saving efficiency of PCM in the
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typical day
Figure 7 shows the variations of inner surface heat flux under different conditions, and the positive value of heat flux means that heat is transferred from indoor air to the wall’s inner surface. For the four conditions, the inner surface heat flux of the PCM wall and the reference
wall was about zero before heating, and there was no significant heat exchange between indoor air and the wall. After the heating was recovered, the indoor air temperature firstly increased, and then the heat was transferred to wall’s inner surface through convection heat transfer, so the
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inner surface heat flux increased rapidly. With the heating time increasing, the heat stored by the wall’s inner part was close to saturation and the heat flux reduced gradually. The inner surface
heat flux got close to stabilization 3 hours after heating in condition 2 and condition 4. Whenever
heating, the inner surface heat flux of the PCM wall was averagely reduced by 18.48% compared with the reference wall. And the low peak of the heat flux means the load in the building with
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PCM wall is more stable with no peak compared with the building with reference wall.
Figure 7. The inner surface heat flux variations under different conditions
However, during intermittent heating process, some of the heat transferred into the inner surface was stored by the wall, while the others was lose to the outdoor environment through the wall. The variations of outer surface temperature are shown in Figure 8. The outer surface
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temperature is mainly affected by the outdoor environment, while the indoor air temperature changes have a little effect on it. Heat is transferred to ambient through building envelope because the temperature difference between indoor and outdoor increases after heating. In
condition 1 and the first 3 hours after heating in the other 3 conditions, the differences between the PCM wall’s outer surface temperature and the reference wall’s outer surface temperature
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changed little compared with those before heating. The heat transferred to the walls from the
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indoor air through convection was stored in the inner part of the wall rather than transferred to
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wall’s outer surface through conduction at the beginning of heating. The reference wall’s inner
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part reached heat saturation after 3 hours heating, and then the heat was transferred to the outer part, so its outer surface temperature began to increase. As for the PCM wall, with increasing
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temperature, the PCM changed from solid to liquid and stored latent heat, which extended the
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wall heat storage after heating. The heat storage process of the PCM wall’s inner part did not finished in 3 hours after heating, so the heat transferred to outside was less and the outer surface
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temperature of the PCM wall was lower than the reference wall. The PCM wall’s outer surface
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temperature is 0.8°C lower than the reference wall when heating was off in condition 1, condition
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2, condition 3’s afternoon operation process and condition 4.
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4. Conclusions
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Figure 8. The outer surface temperature under different conditions
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The thermal performance improvement of building envelope by integrating with PCM in an
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intermittently heated room is assessed through experimental study in this paper. The results show that integrating PCM with the building envelope can improve the indoor thermal environment as
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well as reducing the heating energy consumption. In the beginning of heating, the inner surface temperature of the PCM wall increased rapidly, and its temperature rise was 0.69°C -1.89°C larger than the reference wall in the first 3 hours after heating. With the advantage of fast thermal response of inner surface, the PCM wall is able to ensure that the indoor thermal environment recovers to the thermal comfort state quickly. Moreover, after heating is off, the inner surface
temperature of the PCM wall is higher than the reference wall owing to the latent heat released by the PCM, which indicates that the indoor air would be favorable in maintaining a comfortable indoor thermal environment.
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Some of the heat transferred into the wall’s inner surface is stored by the wall, while the others is lost to the ambient through the wall during the initial period of intermittent heating.
During the heating process, the inner surface temperature of the PCM wall was always higher
than the reference wall while the outer surface temperature of the PCM wall was clearly lower than the reference wall after 3 hours heating, indicating the larger heat storage capacity of the
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PCM wall. The energy saving effect of the PCM wall is significant in the heating process. In
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addition, the PCM wall can store more heat during heating process and release it during the
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heating suspension time. Therefore, the heating time and heating energy consumption can be
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reduced to a certain extent through reasonably controlling the heating process when the indoor air temperature is maintained in the comfort range by the rational use of PCM wall’s exothermic
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process. Although the heating temperature in the experiment was higher than the indoor
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comfortable temperature, the experimental results, which revealed the thermal process of the
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PCM, are quite correct in the qualitative analysis. The PCM (with the phase transition temperature range of 18-26°C) did not change phase completely during the measurement even though the air
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condition was set at 30°C. It is very important to choose the phase-transition temperature range according to building envelope properties, heating conditions and climate characteristics, which
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will be studied in later works. This paper can provide guidance about the use of PCM in indoor thermal environment regulation and energy consumption reduction in intermittently heated rooms.
Acknowledgments This project is funded by the National Natural Science Foundation of China (No.51478280), and
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the National Key Research and Development Program (2016YFC00406).
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Table 1. Operation conditions of intermittent heating Heating time
Test period
Condition 1
19:00~23:00
1.10-1.11
21:00~6:00
1.12-1.14
Condition 3
9:00~12:00, 14:00~17:00
1.15-1.16
Condition 4
9:00~17:00
1.17-1.18
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Operation condition
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Condition 2
Table 2. Physical properties of each layer
Density(kg/m3)
Thermal conductivity (W/(m·K))
Specific heat (kJ/kg·K)
Mortar
1406
0.3505
1.05
Solid brick
1536
0.7505
0.523
PCM
1300
0.25(liquid), 0.5(solid)a
1.785b
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Material
Table 3. Thermal parameters of two kinds of walls
HCA (kJ/(m2·K))
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R (m2·K/W)
Wall
252.67 (solid or liquid), 832.79
0.43 (liquid), 0.39 (solid)
0.38
(phase change) 222.32
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Reference wall
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PCM wall
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Table 4. The increased values of inner surface temperature in the first 3 hours after starting
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heating under four conditions
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Operation condition
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Condition 1
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Condition 2
Heating time
PCM wall (°C)
Reference wall (°C)
19:00~23:00
7.59
5.70
21:00-6:00
8.49
6.81
9:00-12:00
8.61
6.21
14:00-17:00
5.49
4.8
9:00-17:00
6.90
5.31
Condition 3
Condition 4
Figure Caption Figure 1. The experimental system ((a) sketch map of the experiment test building; (b)
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experimental wall units; (c) architectural appearance). Figure 2. The schematic structures of walls and measurement arrangements. Figure 3. The test wall units during the construction process
Figure 4. The variations of indoor and outdoor air temperatures from18:00 on 10th Jan to 24:00 on 18th Jan 2016.
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Figure 5. The inner surface temperature variations under different conditions.
Figure 6. The inside and outside temperatures of the PCM layer and the temperature differences
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between them.
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Figure 7. The inner surface heat flux variations under different conditions.
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Figure 8. The outer surface temperature under different conditions.