# Thermal Performance and Energy Analysis of Phase Change Material-Integrated Building with the Auxiliary Heating System in Different Climate Regions

Authors: Teng Guo; Guochen Sang; Yangkai Zhang; Xiaoling Cui
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Phase change material (PCM) embedded in the building contributes to enhancing indoor thermal comfort and reducing the heating load in winter by absorbing/releasing solar energy. The application effects of PCM incorporated into a single-space passive solar building with the auxiliary heating system (AHS) were investigated numerically in the six representative regions of western China. In the calculation and analysis by the enthalpy model, it was found that the effectiveness of PCM greatly depended on local climate conditions, thickness, and location of PCM. The results indicated that it was necessary to adopt the AHS for winter heating of the passive solar building. On this basis of using AHS, it showed a favorable thermal energy storage effect after the application of PCM resulting in the minimum indoor air temperature being maintained above 15°C within the comfort range stably. The integrated discomfort degree in each region was decreased by more than 96% compared to the original building, and the average temperature in Lhasa was 2°C higher than that in Urumqi. Furthermore, the highest rate of energy saving and carbon dioxide emission saving in the six regions was 19.7% and 17.4% in Lhasa, and the lowest were 4.3% and 4.2% in Urumqi, while other regions were relatively close, respectively. As a result, it can be stated that for a passive solar building using the AHS during the heating period, PCM was the most suitable for application in Lhasa, unsuitable in Urumqi, and generally suitable in other regions.


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## Body

## 1. Introduction

The building sector accounts for more than one-third of global final energy consumption and is also a significant source of CO2 emissions [1–3]. Meanwhile, the rising energy consumption for building heating and cooling demand, as well as the excessive application of traditional fossil fuels, has led to serious environmental pollution issues [4]. The environmental crisis has just occurred, and the energy demand of the building sector is reported to increase by 50% by 2050 [5, 6]. About 21.2% of total energy consumption comes from the building sector, and carbon emission accounts for it nearly 19.4% in China [7]. The continuous increase in building energy consumption promotes the development of energy-saving technology [8]. Especially for the western region of China with large daily temperature difference climate conditions, the improvement of building thermal stability can achieve a preeminent energy efficiency effect [9]. Consequently, it is contributive to the fundamental promotion of energy conservation and resource rational utilization through research and development of new technologies for building [10, 11].Taking advantage of solar energy is an effective and positive measure to realize building energy conservation in the previous studies [12, 13]. Nevertheless, it is applicated limitedly by climate conditions as a kind of renewable natural energy. Indoor thermal comfort will be affected due to insufficient solar supply, thus increasing the heating load [14]. Thereby, achieving a continuous and stable energy supply requires a combination of energy storage technology [15]. In passive solar buildings such as the type of direct-gain and Trombe wall, heavy envelopes are required to store the heat energy during the day while releasing to indoor air in the absence of sunshine period, thus raising the indoor thermal condition to satisfy comfort requirements for the occupants [16]. However, these buildings are mainly based on sensible heat storage which requires heavy materials as the medium; the results cause a large fluctuation of indoor temperature. In contrast, as a new energy-saving means, phase change energy storage provides the characteristic of absorbing or releasing a great deal of heat energy in terms of latent heat under near isothermal conditions [8, 17]. The equivalent specific heat of phase transition latent heat is much greater within a certain range of temperature than the sensible heat of ordinary material. Based on the latent thermal storage technology, on the one hand, it can not only ease the contradiction adequately within energy supply and demand in time, intensity, and space mismatch but also play a role in shifting peaks and filling valleys; on the other hand, it can flatten temperature fluctuations in buildings and improve indoor comfort, which is a specific method to alleviate energy consumption as well as reduce negative environmental impact [18, 19]. Therefore, combining solar energy and phase change energy storage will get rid of the dependence on conventional energy and make it possible for passive solar buildings to transform from “passive climate response” to “active climate adaptation.”China is located in the eastern part of the Northern Hemisphere where the total annual solar radiation exhibits about 3,300~8,400 MJ/m2 with an average value of about 5,900 MJ/m2; it is provided more than 2,000 hours of sunshine in 2/3 areas of China regarded as one of the countries with abundant solar energy resources [20]. In fact, the distribution characteristics of solar energy resources in China present “plateau is larger than the plain, the western regions are larger than the eastern.” In particular, the development of passive solar buildings has great geographical advantages in western regions [21].With high heat storage density of PCM that can productively increase the heat capacity of the room when added to the building envelope, at the same time, the phase transition occurs in a small temperature range which contributes to improving thermal comfort [22]. Kuznik and Virgone [23] investigated experimentally the differentiated effects of composite wall building integrated with or without PCM on a winter day. The results showed that the addition of PCM effectively reduced room temperature fluctuations of the building and thermal energy charged in the material was discharged into indoor air when the temperature dropped to a minimum. Meng et al. [24] explored the thermal behavior of a composite PCM room by experimental and numerical methods, the authors concluded that with PCM application, indoor temperature was raised 6.93~9.48°C on a winter night compared to the reference room, and dampened temperature fluctuation decreased by 17.7~25.4% on shining days. Labihi et al. [25] conducted an experimental and numerical investigation combining a reduced scaled enclosure to evaluate the application effect of PCM in building walls on indoor thermal conditions. The findings indicated that the peak and stabilization time of indoor air temperature was prominently influenced by the melting temperature, latent heat, and thickness of the PCM. And the integration of PCM can contribute to 15% of energy conservation to maintain the comfort range. Zhang et al. [26] analyzed the effects of different thermophysical parameters on the indoor thermal environment of a passive solar building with PCM panels in Tibet. The optimal design parameters on enhanced building thermal performance were obtained by numerical simulation that presented a phase transition temperature of 18°C and thickness of 30 mm on the inner surface of the composite wall, respectively. Zhou et al. [27] simulated the thermal performance of shape-stabilized PCM plates applied in a direct-gain room in Beijing, and the phase transition temperature, latent heat, and thickness of the PCM plates were mainly optimized. They found that the PCM plate was suitable for placing on the inner surface of the interior wall, the latent heat is not less than 90 kJ/kg, and the optimal thickness and phase transition temperature were 20 mm and 20°C, respectively. The energy storage effect of PCM walls applied to a direct-gain building was discussed by Peippo et al. [28], the study proposed an approximate analysis method for optimizing temperature and thickness of PCM, and the building energy saving of 5-20% was achieved. Similar discoveries were obtained by Heim and Clarke [29]; the authors simulated a passive solar building with a large window-to-wall ratio where PCM panels were placed on the inner surface of the walls and estimated energy saving during the whole heating period that found a 90% reduction in energy consumption of the building. Furthermore, it is concluded from the incorporation of PCM also contributed to reducing carbon emissions in buildings. Arıcı et al. [30] discussed the energy-saving performance and CO2 emission reduction potential of incorporating two layers of PCM on building exterior walls. It was found that compared to the base wall, using two separate PCM layers achieved the largest annual energy savings of 17.2% and CO2 emission was 18.4% at the highest annual reduction. Abu-Hamdeh et al. [31] considered the relationship between energy consumption and carbon dioxide in PCM buildings under Arab climate; the results showed the phase change temperature between 17°C and 19°C can reduce CO2 by 6.8% a year. Through the findings of the above scholars, although it is conducive to boosting indoor thermal comfort and energy conservation for PCM, the application of materials mainly depends on solar heat sources for passive latent heat energy storage. However, it is not enough for heating in winter if only PCM is used due to insufficient solar gains under severe cold and cloudy weather conditions. The lack of scientific design of rural buildings may be unable to meet the human thermal comfort requirements within all day by latent thermal storage of PCM. Since the energy storage function of PCM can maximize only when the melting-freezing thermal cycle is completed [32], therefore, an AHS is required to provide heat for the PCM to better play the role of thermal energy charge and discharge.Some scholars have studied the performance of thermal and energy saving for building integration of PCM with using AHS. Onishi et al. [33] analyzed to simulate the energy storage effect of phase change Trombe wall building by developing a CFD program with a hybrid heating system in Sapporo of Japan; the result indicated that the room temperature can be sustained within the comfortable range after the application of PCM under the auxiliary heat source was turned on in winter, in which the efficacy of PCM and the feasibility of exploring low-energy buildings with the established distributed heating system were proven. A novel floor heating system with the incorporation of shape-stabilized phase change material (SSPCM) was proposed and experimentally investigated by Zeng et al. [34]. The study obtained that the room temperature swing was shaved by about 50% as well as raised by 2-3°C in the minimum value of temperature and proved the potential to efficiently utilize the energy storage of daytime for night heating. Lin et al. [35] also explored the thermal performance of underfloor heating system with SSPCM plates applied to the room by simulation analysis and validated with experimental data. The results indicated that the phase transition temperature and the thickness of air layer should be selected appropriately and the regulated underfloor heating area can maintain room temperature within the thermal comfort range during the entire winter. Farid and Kong [36] simulated the latent heat storage effects of PCM-integrated floor combined with the electric heating system and found that the electric heater was operated for 8 h at night when the thickness of PCM was 30 mm under the influence of given climate conditions; the heat absorbed by material can satisfy the heating demand for the whole day. In another study, there is a similar simulation in which two identical huts were modeled with EnergyPlus software in New Zealand by Devaux and Farid [37]. The authors concluded that PCM can transfer peak load effectively and save energy by 32% correspondingly. Additionally, Plytaria et al. [38] also used TRNSYS software to analyze the addition of PCM to a typical building with AHS in Athens of Greece and concluded that the heating load of PCM building was decreased by about 40% compared to an original building. However, the literature was mainly limited to the specific cases of PCM application where there are few attempts to address the effects in different climate regions. The thermal performance of buildings can perform significant variability even with the addition of the same PCM in different regions as the influence of climate conditions. This situation poses a challenge for the promotion of phase change storage solar buildings. Due to the geographical benefits of solar energy resources in western China, it is worth considering the variability in indoor thermal environment and energy consumption characteristics of phase change solar building with the AHS in western different regions in which related studies have been scarcely reported.The paper is aimed at studying the application effects of PCM embedded in a passive solar building with the AHS in western China. Six representative regions were selected from which the appropriate amount and location of PCM were determined firstly, followed by a series of studies on the dynamic variation law of indoor thermal environment in the winter heating period, the performance of thermal comfort, and the characteristics of energy conservation as well as carbon dioxide emission of building. This study for developing PCM solar buildings in different regions can supply an academic reference and has implications for promotion and implementation.


## 2. Methodology

### 2.1. Building Model Details

A single-space rural building was identified as the base analysis model presented in Figure1. The building is located on the north-facing south; it has a floor area of 30m2 (6m×5m) and a height of 3 m. On the south wall, the window was set as plastic steel hollow double glass and the area was 50% of the vertical wall. The size of the external door was 2m×1.1m and the flat roof was selected. These parameters correspond to the standard scope of the civil building thermal design code [39]. Furthermore, all building envelope components complied with the range recommended by the design standard for energy efficiency of rural residential buildings and the design standard for energy efficiency of residential buildings in severe cold and cold zones [40, 41]. Phase change materials (PCMs) were arranged on the walls and floor. On the one hand, the wall is the main component for collecting solar thermal energy; on the other hand, the floor is affected by solar direct radiation throughout the day. As a result, the application of PCM in these two locations of the building was considered in the present study. For the present work, the detailed information on thermophysical properties of different materials and building construction of the base model are shown in Tables 1 and 2, respectively.Figure 1
Schematic of base building model.Table 1
Thermophysical properties of different materials.
Materials typeThickness (m)Thermal conductivity (W/(m·K))Density (kg/m3)Specific heat (kJ/(kg·K))Thermal resistance (m2·K/W)Porous brick0.240.5814001.050.41EPS-insulating board0.040.04201.381.0Cement mortar0.020.9318001.050.02Reinforced concrete0.11.7425000.920.057Asphalt0.010.176001.470.059Plain concrete0.11.5123000.920.066Slag-insulating material0.50.2910000.921.72PCM0.040.58502.00.08Table 2
Details of the base building envelopes.
Building envelopeConstruction (from outside to inside)RoofCement mortar-protective layer, asphalt waterproof layer, cement mortar screed, EPS-insulating board, reinforced concrete, and cement mortar plasteringPCM-integrated wallCement mortar exterior plaster, EPS-insulating board, porous brick, PCM layer, and cement mortar interior plasterWallCement mortar exterior plaster, EPS-insulating board, porous brick, and cement mortar interior plasterPCM-integrated floorSlag-insulating material, plain concrete, PCM layer, and cement mortar surface layerFloorSlag-insulating material, plain concrete, and cement mortar surface layerThe minimum indoor thermal comfort temperature in rural buildings for determination was deliberated in the previous research. Zhu [42] analyzed indoor thermal conditions of rural dwellings during the winter period in Northwest China and obtained that the indoor thermal comfort temperature should not be lower than 15°C in this region. A related result by the study of passive solar buildings in the northwestern countryside was discussed by Rui [43] that the thermal comfort temperature of indoor for winter design should be taken as 15°C. The same temperature index was proven by the Building Energy Efficiency Research Center of Tsinghua University [44] ground on a great of field survey data. Hence, 15°C is taken as the thermal comfort evaluation standard of indoor temperature as well as the melting temperature of PCM.


### 2.2. Climate Conditions in the Six Regions of Western China

Taking advantage of the abundant local solar resources for passive solar building design is of great significance to enhancing indoor thermal environment quality during the winter period in western rural buildings [45]. Simulation analyses were carried out in the six representative regions of western China, namely, Lhasa, Urumqi, Xining, Lanzhou, Yinchuan, and Xi’an, as shown in Figure 2. [21, 46] provided detailed meteorological data and specific information on passive heating climate zoning, which can facilitate the study of the thermal environment of buildings in various regions. The details of representative regions are shown in Table 3, in which Lhasa belongs to the richest solar resource zone with annual solar radiation of 7,000 MJ/m2 more than twice that of eastern coastal areas [47]. The winter heating period plotted variability in different regions due to the climatic conditions, compared with the longest heating period in Urumqi while Xi’an and Lhasa performed the shortest heating period from November 15 to March 15 of the following year. To facilitate the study, the heating period of shortest was taken as the analysis duration in the six regions, and the related information of climatic data in each region is presented in Figure 3.Figure 2
Representative regions in western China.Table 3
Main meteorological information of representing regions.
CityHeating climate zoneSoutherly radiative temperature difference ratio (W/(m2·°C))Annual average temperature (°C)Annual average sunlight hours (h)Annual solar radiation (kWh/m2)Lhasa (91.13°E/29.67°N, 3650 m)Optimal climate A zone11.38.031002521.1Urumqi (87.65°E/43.8°N, 947 m)General climate zone3.67.42528.71872.9Xining (101.77°E/36.62°N, 2296 m)Suitable climate A zone7.85.92477.51976.3Lanzhou (103.88°E/36.05°N, 1518 m)Suitable climate C zone5.110.22469.91746.6Yinchuan (106.2°E/38.47°N, 1112 m)Suitable climate A zone7.59.52813.41939.3Xi’an (108.93°E/34.3°N, 398 m)Suitable climate C zone5.414.11949.41333.8Figure 3
Daily variation of outdoor temperature and solar radiation in different regions.
(a)
Lhasa(b)
Urumqi(c)
. Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anThe six representative regions were selected for this study based on outdoor thermal effects and administrative-geographical divisions [21]. The dynamic variation of solar radiation and outdoor temperature in various regions has obvious regional characteristics [48]; Xining and Urumqi belong to severe cold climatic zone performed low and long-lasting outdoor temperature resulting in the heating degree days based on 18°C (HDD18) is the highest (see Figure 3). In the six regions, the variation ranges from -19.3 to 11.6°C in outdoor temperature and 0 to 35.1 MJ/m2 of solar radiation intensity, separately. In addition, the average temperature in Lhasa is only 0.3°C lower than that in Xi’an, while its radiation intensity is optimal at 35.1 MJ/m2 in the six regions, which is 2.8 times that of Xi’an. In contrast, the outdoor temperature in Urumqi is the worst, with the lowest temperature reaching -19.3°C, which is 30.9°C different compared to the highest temperature in Xi’an. Meanwhile, the radiation intensity is only 9.3 MJ/m2 higher than that in Xi’an. Among different regions, the average outdoor temperature of highest while the weakest radiation intensity on average in Xi’an are 2.3°C and 8.8 MJ/m2, respectively. Lanzhou and Yinchuan have relatively similar climatic conditions during the heating period.


## 2.1. Building Model Details

A single-space rural building was identified as the base analysis model presented in Figure1. The building is located on the north-facing south; it has a floor area of 30m2 (6m×5m) and a height of 3 m. On the south wall, the window was set as plastic steel hollow double glass and the area was 50% of the vertical wall. The size of the external door was 2m×1.1m and the flat roof was selected. These parameters correspond to the standard scope of the civil building thermal design code [39]. Furthermore, all building envelope components complied with the range recommended by the design standard for energy efficiency of rural residential buildings and the design standard for energy efficiency of residential buildings in severe cold and cold zones [40, 41]. Phase change materials (PCMs) were arranged on the walls and floor. On the one hand, the wall is the main component for collecting solar thermal energy; on the other hand, the floor is affected by solar direct radiation throughout the day. As a result, the application of PCM in these two locations of the building was considered in the present study. For the present work, the detailed information on thermophysical properties of different materials and building construction of the base model are shown in Tables 1 and 2, respectively.Figure 1
Schematic of base building model.Table 1
Thermophysical properties of different materials.
Materials typeThickness (m)Thermal conductivity (W/(m·K))Density (kg/m3)Specific heat (kJ/(kg·K))Thermal resistance (m2·K/W)Porous brick0.240.5814001.050.41EPS-insulating board0.040.04201.381.0Cement mortar0.020.9318001.050.02Reinforced concrete0.11.7425000.920.057Asphalt0.010.176001.470.059Plain concrete0.11.5123000.920.066Slag-insulating material0.50.2910000.921.72PCM0.040.58502.00.08Table 2
Details of the base building envelopes.
Building envelopeConstruction (from outside to inside)RoofCement mortar-protective layer, asphalt waterproof layer, cement mortar screed, EPS-insulating board, reinforced concrete, and cement mortar plasteringPCM-integrated wallCement mortar exterior plaster, EPS-insulating board, porous brick, PCM layer, and cement mortar interior plasterWallCement mortar exterior plaster, EPS-insulating board, porous brick, and cement mortar interior plasterPCM-integrated floorSlag-insulating material, plain concrete, PCM layer, and cement mortar surface layerFloorSlag-insulating material, plain concrete, and cement mortar surface layerThe minimum indoor thermal comfort temperature in rural buildings for determination was deliberated in the previous research. Zhu [42] analyzed indoor thermal conditions of rural dwellings during the winter period in Northwest China and obtained that the indoor thermal comfort temperature should not be lower than 15°C in this region. A related result by the study of passive solar buildings in the northwestern countryside was discussed by Rui [43] that the thermal comfort temperature of indoor for winter design should be taken as 15°C. The same temperature index was proven by the Building Energy Efficiency Research Center of Tsinghua University [44] ground on a great of field survey data. Hence, 15°C is taken as the thermal comfort evaluation standard of indoor temperature as well as the melting temperature of PCM.


## 2.2. Climate Conditions in the Six Regions of Western China

Taking advantage of the abundant local solar resources for passive solar building design is of great significance to enhancing indoor thermal environment quality during the winter period in western rural buildings [45]. Simulation analyses were carried out in the six representative regions of western China, namely, Lhasa, Urumqi, Xining, Lanzhou, Yinchuan, and Xi’an, as shown in Figure 2. [21, 46] provided detailed meteorological data and specific information on passive heating climate zoning, which can facilitate the study of the thermal environment of buildings in various regions. The details of representative regions are shown in Table 3, in which Lhasa belongs to the richest solar resource zone with annual solar radiation of 7,000 MJ/m2 more than twice that of eastern coastal areas [47]. The winter heating period plotted variability in different regions due to the climatic conditions, compared with the longest heating period in Urumqi while Xi’an and Lhasa performed the shortest heating period from November 15 to March 15 of the following year. To facilitate the study, the heating period of shortest was taken as the analysis duration in the six regions, and the related information of climatic data in each region is presented in Figure 3.Figure 2
Representative regions in western China.Table 3
Main meteorological information of representing regions.
CityHeating climate zoneSoutherly radiative temperature difference ratio (W/(m2·°C))Annual average temperature (°C)Annual average sunlight hours (h)Annual solar radiation (kWh/m2)Lhasa (91.13°E/29.67°N, 3650 m)Optimal climate A zone11.38.031002521.1Urumqi (87.65°E/43.8°N, 947 m)General climate zone3.67.42528.71872.9Xining (101.77°E/36.62°N, 2296 m)Suitable climate A zone7.85.92477.51976.3Lanzhou (103.88°E/36.05°N, 1518 m)Suitable climate C zone5.110.22469.91746.6Yinchuan (106.2°E/38.47°N, 1112 m)Suitable climate A zone7.59.52813.41939.3Xi’an (108.93°E/34.3°N, 398 m)Suitable climate C zone5.414.11949.41333.8Figure 3
Daily variation of outdoor temperature and solar radiation in different regions.
(a)
Lhasa(b)
Urumqi(c)
. Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anThe six representative regions were selected for this study based on outdoor thermal effects and administrative-geographical divisions [21]. The dynamic variation of solar radiation and outdoor temperature in various regions has obvious regional characteristics [48]; Xining and Urumqi belong to severe cold climatic zone performed low and long-lasting outdoor temperature resulting in the heating degree days based on 18°C (HDD18) is the highest (see Figure 3). In the six regions, the variation ranges from -19.3 to 11.6°C in outdoor temperature and 0 to 35.1 MJ/m2 of solar radiation intensity, separately. In addition, the average temperature in Lhasa is only 0.3°C lower than that in Xi’an, while its radiation intensity is optimal at 35.1 MJ/m2 in the six regions, which is 2.8 times that of Xi’an. In contrast, the outdoor temperature in Urumqi is the worst, with the lowest temperature reaching -19.3°C, which is 30.9°C different compared to the highest temperature in Xi’an. Meanwhile, the radiation intensity is only 9.3 MJ/m2 higher than that in Xi’an. Among different regions, the average outdoor temperature of highest while the weakest radiation intensity on average in Xi’an are 2.3°C and 8.8 MJ/m2, respectively. Lanzhou and Yinchuan have relatively similar climatic conditions during the heating period.


## 3. Mathematical Model

Numerical simulation of the effect of PCM in walls and floor on building thermal performance using the enthalpy model [27]. The following assumptions are made to facilitate the analysis: (1) heat transfer through the walls, roof, and floor is one-dimensional; (2) natural convection and the super-cooling effect during the freezing process of the PCM can be ignored; and (3) except for the equivalent specific heat of PCM, thermophysical properties of building materials are constant.


### 3.1. Heat Transfer Model of PCM Wall

A great deal of direct solar radiation is received by the exterior wall of the building as the major part of the building envelope. The heat transfer schematic of the PCM wall is shown in Figure4.Figure 4
Heat transfer schematic of PCM wall. (I, V: cement mortar; II: EPS-insulating board; III: porous brick; IV: PCM).Based on the above assumptions, the enthalpy equation of PCM wall is(1)ρk∂H∂τ=λk∂2t∂x2.The enthalpy of the PCM layer and other material layers is, respectively, defined as(2)H=∫t0t1cp,sdt+∫t1t2cp,mdt+∫t2tcp,ldt,(3)H=∫t0tcp,kdt.The parameters of each material layer of the wall are defined as(4)ρk=ρm1,λk=λm1,cp,k=cp,m1,0≤x<x1,ρk=ρi,λk=λi,cp,k=cp,i,x1≤x<x2,ρk=ρb,λk=λb,cp,k=cp,b,x2≤x<x3,ρk=ρp,λk=λp,x3≤x<x4,ρk=ρm2,λk=λm2,cp,k=cp,m2,x4≤x<x5.The initial condition is(5)tx,ττ=0=tinit.The analysis uses the third type boundary conditions with the following expressions:(6)qr,out+houttout−tm1,out=−λm1∂t∂xx=0,(7)qr,in+hintin−tm2,in=λm2∂t∂xx=x5,whereρm1, ρi, ρb, ρp, and ρm2 and λm1, λi, λb, λp, and λm2 are the density and thermal conductivity of the outer cement mortar, the insulation layer, the porous brick, the PCM layer, and the inner cement mortar, respectively; cp,m1, cp,i, cp,b, and cp,m2 are the specific heat capacity of the outer cement mortar, the insulation layer, the porous bricks, and the inner cement mortar, respectively; cp,s, cp,m, and cp,l are the solid phase specific heat, equivalent specific heat, and liquid phase specific of PCM, respectively; tinit, τ, qr,in, qr,out, hin, hout, tin, tout, tm1,out, and tm2,in are initial temperature, time, radiant heat flux, and convective heat transfer coefficient on the inner and outer surfaces, indoor and outdoor air temperature, outer surface temperature, and inner surface temperature, respectively; and t0, t1, and t2 denote the temperature at which the enthalpy is zero, the lower limit temperature of phase change, and the upper limit temperature of phase change, respectively.


### 3.2. Heat Transfer Model of PCM Floor

As the sun continuously transmitted radiation energy into the floor through the window during the day, PCMs are applied to the inside of the floor. The heat transfer schematic of the PCM floor is shown in Figure5.Figure 5
Heat transfer schematic of PCM floor. (I: slag-insulating material; II: plain concrete; III: PCM; IV: cement mortar).The enthalpy equation of PCM floor is written as(8)ρk∂H∂τ=λk∂2t∂y2.The enthalpy of PCM and other materials is the same as those defined in the PCM wall.The parameters of each material layer are as follows:(9)ρk=ρs,λk=λs,cp,k=cp,s,0≤y<y1,ρk=ρc,λk=λc,cp,k=cp,c,y1≤y<y2,ρk=ρp,λk=λp,y2≤y<y3,ρk=ρm,λk=λm,cp,k=cp,m,y3≤y<y4.The initial condition is(10)ty,ττ=0=tinit.The analysis still used the third type boundary conditions as follows:(11)λs∂t∂yy=0=0,y=0,(12)qm+hmtin−tm=λm∂t∂yy=y4,y=y4,where ρs, ρc, ρp, and ρm and λs, λc, λp, and λm are the density and thermal conductivity of the slag-insulating material, plain concrete, the PCM layer, and the cement mortar surface layer, respectively; cp,s, cp,c, and cp,m are the specific heat capacity of the slag-insulating material, plain concrete, and the cement mortar surface layer, respectively; qm is the radiant heat flux of the ceiling and walls to the floor; hm is the convective heat transfer coefficient of indoor air to the floor surface; tm is the surface temperature of floor; and other symbols mean the same as above.


### 3.3. The Heat Balance Equation of Inner Surface of Building Envelope

Each inner surface of the building envelope is affected by thermal conduction, direct radiation heat, radiation heat transfer between surfaces, and convective heat transfer with indoor air. The heat balance equation for the internal surface of building envelope is calculated by the following equation [49].
(13)qiτ+αictaτ−tiτ+∑i=1NiCbεikφiktkτ1004−tiτ1004+qirτ=0,where qiτ is the heat transfer obtained by the inner surface of the ith envelope at time τ; αic is the convective heat transfer coefficient of the ith inner surface of the envelope; Ni is the total number of inner surfaces of different envelopes in the room; Cb is the blackbody radiation constant (5.67 W/(m2·K4)); φik is the radiation angle coefficient of the inner surface i to inner surface k of the envelope; tiτ and tkτ are the inner surface temperature of the ith and kth envelope, respectively; and εik is the system emissivity between the ith and kth inner surfaces of the envelope, which is equal to the product of the emissivity of the ith and kth surfaces itself.


### 3.4. The Heat Balance Equation of Indoor Air

The energy conservation equation of indoor air is(14)ρacaVBdtadτ=QW+QF+QR+QA+QC+Qwin,(15)QW=hintW,i−taAW,i,(16)QF=hintF,i−taAF,i,(17)QR=hintR,i−taAR,i,(18)QC=caρaVBACHtout−ta3600,(19)Qwin=Uwintout−taAwin,(20)ΔT=ta,max−ta,min,whereρa,ca, and ta are the density, specific heat capacity, and temperature of indoor air, respectively; QW and QF are the convection heat transfer rate on inner surfaces of walls and floor, respectively; QR is the convection heat transfer rate between air and inner surface of the roof; QA is the convection heat transfer rate from the indoor heat sources; Qwin is the heat transfer rate through window; QC is the heat transfer rate by air leakage; VB is volume of building; tW,i,tF,i, and tR,i are the inner surface temperature of walls, floor, and roof, respectively; ACH is the air change per hour; ΔT is the amplitude of indoor air temperature fluctuation; ta,max and ta,min are the maximum temperature and minimum temperature of indoor air, respectively;Uwin is the heat transfer coefficient of window; and AW,i,AF,i,AR,i, and Awin are the area of walls, floor, roof, and window, respectively.


### 3.5. Simulation Method

Numerical simulation was carried out by DesignBuilder software which is a program using the EnergyPlus engine developed by the US Department of Energy [50]. In addition, it is the most comprehensive and powerful graphical user interface for EnergyPlus [51]. The Conduction Finite Difference algorithm (condFD) in EnergyPlus accurately simulates the thermal performance of PCMs by combining it with an enthalpy-temperature function based on the law of energy conservation of room air and room surface [52]. This difference scheme employed the fully implicit first order, the space discretization constant was 3, and the relaxation factor was 1. The equation is given as follows:
(21)cρΔxTij+1−TijΔt=λWTi+1j+1−Tij+1Δx+λETi−1j+1−Tij+1Δx,where Δx represent the finite difference layer thickness; Δt is the time step of calculation; cand ρ are the specific heat capacity and density of material, respectively; Tis the node temperature; i−1, i, and i+1 are, respectively, the adjacent node to exterior of construction, the node being modeled, and the adjacent node to interior of construction; jand j+1 are the current time step and new time step; and λw and λE are the thermal conductivities of interfaces between nodes i and i+1 and nodes i and i−1, respectively.The equivalent heat capacity of PCM is updated according to Equation (22) at each time step:
(22)Cp=hij−hij−1Tij−Tij−1,where Cpis the equivalent heat capacity of material and his the specific enthalpy that user defined a function of temperature as follows:
(23)hiTi=HTFTi,where HTFTi refers to the enthalpy-temperature function.


## 3.1. Heat Transfer Model of PCM Wall

A great deal of direct solar radiation is received by the exterior wall of the building as the major part of the building envelope. The heat transfer schematic of the PCM wall is shown in Figure4.Figure 4
Heat transfer schematic of PCM wall. (I, V: cement mortar; II: EPS-insulating board; III: porous brick; IV: PCM).Based on the above assumptions, the enthalpy equation of PCM wall is(1)ρk∂H∂τ=λk∂2t∂x2.The enthalpy of the PCM layer and other material layers is, respectively, defined as(2)H=∫t0t1cp,sdt+∫t1t2cp,mdt+∫t2tcp,ldt,(3)H=∫t0tcp,kdt.The parameters of each material layer of the wall are defined as(4)ρk=ρm1,λk=λm1,cp,k=cp,m1,0≤x<x1,ρk=ρi,λk=λi,cp,k=cp,i,x1≤x<x2,ρk=ρb,λk=λb,cp,k=cp,b,x2≤x<x3,ρk=ρp,λk=λp,x3≤x<x4,ρk=ρm2,λk=λm2,cp,k=cp,m2,x4≤x<x5.The initial condition is(5)tx,ττ=0=tinit.The analysis uses the third type boundary conditions with the following expressions:(6)qr,out+houttout−tm1,out=−λm1∂t∂xx=0,(7)qr,in+hintin−tm2,in=λm2∂t∂xx=x5,whereρm1, ρi, ρb, ρp, and ρm2 and λm1, λi, λb, λp, and λm2 are the density and thermal conductivity of the outer cement mortar, the insulation layer, the porous brick, the PCM layer, and the inner cement mortar, respectively; cp,m1, cp,i, cp,b, and cp,m2 are the specific heat capacity of the outer cement mortar, the insulation layer, the porous bricks, and the inner cement mortar, respectively; cp,s, cp,m, and cp,l are the solid phase specific heat, equivalent specific heat, and liquid phase specific of PCM, respectively; tinit, τ, qr,in, qr,out, hin, hout, tin, tout, tm1,out, and tm2,in are initial temperature, time, radiant heat flux, and convective heat transfer coefficient on the inner and outer surfaces, indoor and outdoor air temperature, outer surface temperature, and inner surface temperature, respectively; and t0, t1, and t2 denote the temperature at which the enthalpy is zero, the lower limit temperature of phase change, and the upper limit temperature of phase change, respectively.


## 3.2. Heat Transfer Model of PCM Floor

As the sun continuously transmitted radiation energy into the floor through the window during the day, PCMs are applied to the inside of the floor. The heat transfer schematic of the PCM floor is shown in Figure5.Figure 5
Heat transfer schematic of PCM floor. (I: slag-insulating material; II: plain concrete; III: PCM; IV: cement mortar).The enthalpy equation of PCM floor is written as(8)ρk∂H∂τ=λk∂2t∂y2.The enthalpy of PCM and other materials is the same as those defined in the PCM wall.The parameters of each material layer are as follows:(9)ρk=ρs,λk=λs,cp,k=cp,s,0≤y<y1,ρk=ρc,λk=λc,cp,k=cp,c,y1≤y<y2,ρk=ρp,λk=λp,y2≤y<y3,ρk=ρm,λk=λm,cp,k=cp,m,y3≤y<y4.The initial condition is(10)ty,ττ=0=tinit.The analysis still used the third type boundary conditions as follows:(11)λs∂t∂yy=0=0,y=0,(12)qm+hmtin−tm=λm∂t∂yy=y4,y=y4,where ρs, ρc, ρp, and ρm and λs, λc, λp, and λm are the density and thermal conductivity of the slag-insulating material, plain concrete, the PCM layer, and the cement mortar surface layer, respectively; cp,s, cp,c, and cp,m are the specific heat capacity of the slag-insulating material, plain concrete, and the cement mortar surface layer, respectively; qm is the radiant heat flux of the ceiling and walls to the floor; hm is the convective heat transfer coefficient of indoor air to the floor surface; tm is the surface temperature of floor; and other symbols mean the same as above.


## 3.3. The Heat Balance Equation of Inner Surface of Building Envelope

Each inner surface of the building envelope is affected by thermal conduction, direct radiation heat, radiation heat transfer between surfaces, and convective heat transfer with indoor air. The heat balance equation for the internal surface of building envelope is calculated by the following equation [49].
(13)qiτ+αictaτ−tiτ+∑i=1NiCbεikφiktkτ1004−tiτ1004+qirτ=0,where qiτ is the heat transfer obtained by the inner surface of the ith envelope at time τ; αic is the convective heat transfer coefficient of the ith inner surface of the envelope; Ni is the total number of inner surfaces of different envelopes in the room; Cb is the blackbody radiation constant (5.67 W/(m2·K4)); φik is the radiation angle coefficient of the inner surface i to inner surface k of the envelope; tiτ and tkτ are the inner surface temperature of the ith and kth envelope, respectively; and εik is the system emissivity between the ith and kth inner surfaces of the envelope, which is equal to the product of the emissivity of the ith and kth surfaces itself.


## 3.4. The Heat Balance Equation of Indoor Air

The energy conservation equation of indoor air is(14)ρacaVBdtadτ=QW+QF+QR+QA+QC+Qwin,(15)QW=hintW,i−taAW,i,(16)QF=hintF,i−taAF,i,(17)QR=hintR,i−taAR,i,(18)QC=caρaVBACHtout−ta3600,(19)Qwin=Uwintout−taAwin,(20)ΔT=ta,max−ta,min,whereρa,ca, and ta are the density, specific heat capacity, and temperature of indoor air, respectively; QW and QF are the convection heat transfer rate on inner surfaces of walls and floor, respectively; QR is the convection heat transfer rate between air and inner surface of the roof; QA is the convection heat transfer rate from the indoor heat sources; Qwin is the heat transfer rate through window; QC is the heat transfer rate by air leakage; VB is volume of building; tW,i,tF,i, and tR,i are the inner surface temperature of walls, floor, and roof, respectively; ACH is the air change per hour; ΔT is the amplitude of indoor air temperature fluctuation; ta,max and ta,min are the maximum temperature and minimum temperature of indoor air, respectively;Uwin is the heat transfer coefficient of window; and AW,i,AF,i,AR,i, and Awin are the area of walls, floor, roof, and window, respectively.


## 3.5. Simulation Method

Numerical simulation was carried out by DesignBuilder software which is a program using the EnergyPlus engine developed by the US Department of Energy [50]. In addition, it is the most comprehensive and powerful graphical user interface for EnergyPlus [51]. The Conduction Finite Difference algorithm (condFD) in EnergyPlus accurately simulates the thermal performance of PCMs by combining it with an enthalpy-temperature function based on the law of energy conservation of room air and room surface [52]. This difference scheme employed the fully implicit first order, the space discretization constant was 3, and the relaxation factor was 1. The equation is given as follows:
(21)cρΔxTij+1−TijΔt=λWTi+1j+1−Tij+1Δx+λETi−1j+1−Tij+1Δx,where Δx represent the finite difference layer thickness; Δt is the time step of calculation; cand ρ are the specific heat capacity and density of material, respectively; Tis the node temperature; i−1, i, and i+1 are, respectively, the adjacent node to exterior of construction, the node being modeled, and the adjacent node to interior of construction; jand j+1 are the current time step and new time step; and λw and λE are the thermal conductivities of interfaces between nodes i and i+1 and nodes i and i−1, respectively.The equivalent heat capacity of PCM is updated according to Equation (22) at each time step:
(22)Cp=hij−hij−1Tij−Tij−1,where Cpis the equivalent heat capacity of material and his the specific enthalpy that user defined a function of temperature as follows:
(23)hiTi=HTFTi,where HTFTi refers to the enthalpy-temperature function.


## 4. Validation

Lin et al. [53] established an experimental room for the application of SSPCM plates in Beijing. The related parameters of indoor thermal environment were measured by instrumentations in this experiment. The room had dimensions of 3mdepth×2mwidth×2mheight and had a size of double-glazed window facing south which was 1.6m×1.5m. The walls and roof were constructed according to 100 mm thick polystyrene. The detailed floor was composed of different thicknesses 8 mm wood floor, 10 mm air layer, 15 mm PCM, and 120 mm polystyrene insulation. It should be mentioned that the experiment was conducted in the Tsinghua University of Beijing, so the software adopted consistent meteorological data.For the present work, the performance of the algorithm and PCM module used in the DesignBuilder was validated by comparing the simulation results with the experimental data. As can be seen from Figure6, the simulation results agreed well with the measured data, and the average deviation percentage is about 2.5%. In addition, Mi et al. [51] have verified the use of DesignBuilder for building energy consumption analysis. Therefore, the paper carried out this software to simulate the performance of thermal and energy conservation in PCM buildings.Figure 6
Comparison of experimental and simulated results.


## 5. Results and Discussion

### 5.1. Analysis of Temperature Dynamic Variability without AHS

The six representative regions are classified into optimal climate zone, suitable climate zone, and general climate zone according to the passive heating climate zoning [21]. Then without the AHS, the indoor air temperature of PCM building and original building was analyzed during the heating period (see Figure 7).Figure 7
Hourly variation of indoor temperature without AHS in the six regions.
(a)
Original building(b)
PCM buildingFigure7 shows significant variability of indoor air temperature in different regions no matter whether PCM was added or not. Obviously, a basic comfort level of 15°C reached for the most period in Lhasa was satisfactory while the other five regions had low room temperature, especially the worst indoor comfort in Urumqi. The average temperature during the heating period of the original and PCM buildings in Lhasa was 15.8°C and 16.2°C, respectively, whereas the lowest temperature in November reached 16.6°C and 17.4°C, respectively. However, there were periods below the thermal comfort level in other months; thus, Lhasa does not need auxiliary heating equipment only in November. With the two conditions (with or without PCM), the average temperature of Urumqi was not only the lowest presented at -0.6°C and -1.2°C, respectively, but also had the largest temperature fluctuation which was 19.9°C and 20.7°C. In addition, the variation of close average temperature ranged from 6.49~7.99°C to 7.17~8.58°C, respectively, which still made it difficult to satisfy the thermal comfort requirements for occupants in the other four regions; it should be mentioned that the outdoor climatic conditions presented similarly. Hence, it was seen that thermal demand was the most urgent in Urumqi of all regions while other regions had an analogous thermal environment where the average temperature was below the basic thermal comfort temperature. Lhasa and Urumqi were divided into different solar heating climate zones due to the local climate condition influenced. From this perspective of outdoor thermal disturbance, the hourly indoor air temperature in Lhasa is greatly better than that of Urumqi regardless of whether PCM is added to the building. Given the analysis above, even with the addition of PCM in the building, indoor temperature below 15°C in most periods can still be found because the melting-freezing thermal cycle of PCM may be incomplete. It can contribute to a mismatch between building heat gain and human comfort level. Therefore, to take full advantage of the energy storage and release functions of PCM, an auxiliary heating system is necessary to be used during the heating period in the six regions.


### 5.2. Effect of PCM Thickness on Building Thermal Performance

To facilitate the analysis, the whole heating period was simulated and selected results from December 11 to 14 for four consecutive days to analyze. The heating capacity of AHS was adjusted by stepped regulation, and the heat supply can be controlled by simulating the experiment through the trial and error method [54]. Following that, the building thermal performance effect of various PCM thicknesses was mainly discussed to obtain a suitable amount of material for subsequent analysis (see Figure 8).Figure 8
PCM thickness on thermal performance of building in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure8 shows that the thickness of PCM on building thermal performance varied in different regions given the opening of AHS. In this section, temperature fluctuation reduction rate (TFRR) is defined as the difference between the indoor air temperature fluctuation value in original building minus that of the PCM building and then the ratio of the indoor air temperature fluctuation value of the original building. TFRR correlates with the thermal stability of buildings. Along with the larger of TFRR, the better indoor thermal comfort achieved. It can be observed that the PCM-integrated building provided a beneficial heat storage effect. With the thickness of PCM increased from 20 mm to 80 mm, the average temperature and minimum temperature gradually improved in different regions. It was expected equivalent to supplement the extra thermal resistance of building envelopes with the addition of PCM layer [55]. However, the degree of change in TFRR showed a difference with the growing thickness of PCM. TFRR presented a positive correlation with thickness in Lhasa, but the impact degree of per unit thickness progressively enervated when exceeded 40 mm. Since the total thermal energy of outdoor provided to the PCM layer during the day is limited, the melting-freezing thermal cycle of PCM is completed at a certain critical thickness, even if excessively increasing the thickness will not directly affect the heat storage capacity of the material. Conversely, TFRR exhibited a negative correlation with the increase of PCM thickness in Xining, Yinchuan, and Xi’an. Taking Yinchuan for instance, on the one hand, the increasing degree of per unit thickness on average temperature growth rate (ATGR) and minimum temperature growth rate (MTGR) with thickness variations from 20 mm to 40 mm was 1.73% and 2.62%, while TFRR on per unit thickness dropped by only 0.45%; on the other hand, the temperature fluctuation of 20 mm and 40 mm was 3.99°C and 4.01°C in relatively close values, yet it was the minimum temperature of 15.02°C at 40 mm that can satisfy the basic thermal comfort. The results in Xining and Xi’an were in agreement with this finding. In addition, the changes in TFRR performed a trend of increasing and then decreasing in Urumqi and Lanzhou. Although it seemed that the highest TFRR was at 60 mm in Urumqi, the close effects to meeting the indoor thermal environment were achieved with a minimum temperature of 15.14°C at 40 mm, and the cost is increasing linearly with the increase of PCM thickness. Similar results have also been obtained by other scholars [56]. From the standpoint of ATGR, MTGR, and TFRR in Lanzhou, the slope of ATGR and MTGR curves tended to be gentle when surpassing 40 mm, and it presented the highest TFRR at 40 mm of thickness; thus, this amount of material can provide a better indoor thermal condition. In summary, it can be inferred that the thickness of PCM has an optimal value in different regions based on indoor thermal comfort and economic considerations. For the present study, the 40 mm of PCM thickness was determined as an appropriate selection for the later analysis.


### 5.3. Effect of PCM Location on Building Thermal Performance

In a normal scenario, the inner surface of the roof is generally not affected by direct solar radiation. This section took the PCM to integrate into the wall (PCMW), floor (PCMF), and both locations (PCMWF). Thus, the appropriate amount of PCM obtained in the previous section was applied to different locations, and then, the building thermal performance variability characteristics were analyzed coupled with AHS (see Figure9).Figure 9
PCM location on thermal performance in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anAs seen in Figure9, PCM placed in different locations exhibited differentiation after the AHS was turned on in the six regions. Referring to the original building, the PCMF in raising the indoor air temperature was weaker than in the other two locations. For instance, the average indoor temperature improvement of PCMF and PCMWF in Lanzhou was 0.3°C and 1.9°C, respectively, and the application effect of PCMF was reduced by 84% compared to PCMWF. Additionally, PCMF in most portions of the period was below the basic thermal comfort level in each region which cannot reach the heating demand of the human body. Due to the limited amount of PCM integrated into the floor, the change of outdoor solar heat source with the movement path contributes to the lowest heat storage and thus the weakest thermal regulation performance of the PCM. In contrast, the distinct from its effect was PCMWF, such as the average temperature in Xi’an also reached a minimum of 15.8°C which reduced the indoor discomfort. Although there were few regions where the indoor temperature variation of the PCMWF building was close to that of the PCMW, as a whole, the temperature wave amplitude was reduced obviously by the arrangement scheme of PCMWF. It would be shown following that PCMWF as the optimal location was selected for analysis.


### 5.4. Thermal Analysis of PCM Solar Building

From the previous sections, optimal choice in thickness and location of PCM solar building was mainly acquired. In this section, the dynamic law of room temperature of solar buildings whether to add PCM under AHS turned on was exhaustively investigated during the heating period in different regions as seen in Figure10.Figure 10
Variation of building thermal performance in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anAs can be seen in Figure10, the addition of the PCM played an active role in the level of indoor thermal comfort regardless of whether the AHS was turned on or not. However, there was indoor uncomfortable in Lhasa part periods and the whole day in the other five regions. The building thermal performance had been significantly improved after using the AHS, which was below the set temperature value in some periods, along with the incorporation of PCM enabled the room temperature to be maintained in the comfort range and effectively shaved the temperature swing. Due to the benefit heat gain effect produced by the comprehensive effect of solar radiation and auxiliary heat energy, the thermal energy was stored by PCM absorbing during the daytime convected with the indoor air and raised the temperature despite the lack of solar heat energy input during the absence of sunshine period. The results were consistent with the findings of the previous research work [54]. Nevertheless, it was observed that there is a positive effect of PCM thermal storage in different regions, yet there were also arguments. Notably, after the AHS was turned on, compared to the original building, the highest average temperature increase was 1.93°C in the PCM building of Urumqi. However, with the outdoor climate conditions being worst and located in the general heating climate zone, thus the auxiliary heat energy mainly was absorbed by PCM which would lead to the relatively high extra heating load. In contrast, although the elevated of indoor temperature was low in Lhasa, the abundant solar energy resources of local presented the room temperature reached thermal comfort demand easily and also reduced the heating energy consumption of AHS. Comparing the variation of building thermal performance in these two regions, it also can be found that the average temperature in Lhasa was 2°C higher than that in Urumqi. In addition, the thermal performance analysis results of buildings in other regions are similar.


### 5.5. Analysis of Indoor Thermal Comfort Performance

The section was to gain an insight into the indoor thermal comfort demand of occupants in the building adopting the AHS. For a passive solar building, the thermal comfort may be estimated by a specific index of the fixed temperature field [57]. The parameter Iwin integrated discomfort degree (IDD) for the indoor temperature to evaluate the thermal environment and human thermal comfort in winter was proposed by Zhang et al. [58]. It described the degree to which the indoor air temperature deviated from the critical temperature and stipulated Iwin if the variation range is close to 0, indicating that the indoor thermal environment met the thermal demand. It can be expressed by the following formula:
(24)Iwin=∫yeartL−tadτ,where tL and ta are 15°C of lower temperature limits within thermal comfort range and indoor air temperature (°C), respectively.Based on the IDD and using the AHS, the indoor thermal comfort performance of building whether added with PCM was compared and assessed in the six regions. The variation of IDD during the heating period in each region is shown in Figure11.Figure 11
Variation of IDD during heating period in the six regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure11 shows that the addition of PCM in buildings exploited differentiated effects on IDD in an indoor thermal environment, with a reduction of more than 96% IDD compared to original buildings. It was observed that the most significant reduction of IDD was in December in Urumqi, with a decrease of 587.8°C·h. On the contrary, there were situations where the IDD increased for certain months in individual regions. Taking the results of March as an instance in this region, it may be since the PCM in the state of solid or phase transition along with the AHS was activated, the material is prepared to charge heat by melting; thus, the room temperature was reduced, causing discomfort. It was apparent that the PCM played an efficient energy storage effect in most months during the heating period and improved indoor thermal comfort under the systematic influence of thermal energy of solar and auxiliary.


### 5.6. Analysis of Energy Saving and Carbon Dioxide Emission

#### 5.6.1. Energy-Saving Performance

To further explore the impact of PCM building energy performance, after the AHS was turned on, a comparative analysis of building energy consumption (BEC) during the heating period was performed in the six regions for both the original building and PCM building, and the minimum daily indoor air temperature was controlled above 15°C using AHS. It is evaluated by energy consumption and energy-saving rate [59], and the calculation formula is as follows:
(25)Qs=Qno,pcmheating−Qpcmheating,(26)η=Qno,pcmheating−QpcmheatingQno,pcmheating×100%,where Qs and η are the energy-saving amount and energy-saving rate, respectively, and Qno,pcmheating and Qpcmheating are the energy consumption of original building and PCM building, respectively.Repeat all the simulations presented in Section5.5 to calculate the corresponding BEC as shown in Figure 12. It can be noted that the maximum energy saving in December is124.0 kW·h in Xi’an. Since the average outdoor temperature in Xi’an was the highest during the heating period in all regions, the PCM effectively carried out the heat storage and release process in that month; thus, heating energy consumption was saved. However, BEC increased in March of Urumqi and February of Lanzhou when PCM was added to the building which may be because of PCM absorbed heat by melting leading to the decrease of the room temperature in the building, and therefore, it required auxiliary heat supply, thus resulting in the higher BEC; the results were in agreement with the simulation of the reasons for the increase of IDD in Urumqi in March. It can be seen that regardless of whether PCMs existed or not, the dynamic change of BEC presented the law of increasing first and then decreasing. The regions where the optimal month of PCM with energy saving was 40% in Lhasa December and 22% in Urumqi November, and the energy saving rate of the other four regions within 30% ~40% in November.Figure 12
Variation of monthly BEC in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure13 illustrates that BEC was saved dramatically by the appearance of PCM under the AHS during the heating period, with the highest energy-saving rate reaching 19.7% in Lhasa, only 4.3% in Urumqi, and other regions close to that. The rich solar energy resources are provided sufficiently in winter as Lhasa is located in the optimal heating climate zone, and the PCM mainly absorbs and releases latent heat by solar radiation. Consequently, it was unnecessary to need the great auxiliary heat energy which in turn can reduce the heating energy consumption. The outdoor climate conditions are harsh in the winter of Urumqi, so more auxiliary heat energy must be provided to satisfy the demand for thermal comfort; indeed, the energy consumption of auxiliary heating increases. Furthermore, the effects of energy conservation were similar in other regions ranging from 7.0% to 8.2%.Figure 13
Variation of building energy-saving performance in different regions.


#### 5.6.2. Carbon Dioxide Emission Characteristic

Household CO2 emission (CE) is one of the major sources of carbon emissions, and the reduction of CE could provide a significant contribution to mitigating global climate change [60]. Based on the analysis in the previous section, to illustrate the influence of PCM on building carbon emission, the CE of the original building and PCM building under the AHS during the heating period was analyzed in this section and revealed the law of CE of buildings in different regions (see Figures 14 and 15.Figure 14
Variation of monthly CO2 emission in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure 15
Variation of CO2 emission in heating period.Figure14 presents that the characteristic of CE was consistent with the variable trend of BEC performed. The incorporation of PCM can reduce CE depending mainly on the outside temperature, the indoor temperature, and the phase transition temperature [31]. In the climatic conditions of Xi’an, the CE saving in December showed the highest at 75.4 kg. The reason was in line with the performance analysis of energy saving. Along with the AHS being turned on, the PCMs in the building were stored productively under the comprehensive influence of solar energy and auxiliary thermal energy. It can effectively increase the room temperature and reduce the heat load of the AHS, thus reducing the generation of carbon dioxide. On the contrary, the CE of Urumqi in March and Lanzhou in February increased by 17.8 kg and 6.0 kg, respectively, by adding the PCM. It may be due to the insufficient utilization of PCM during this period, and the heat absorption began to decrease temperature after the AHS operated. Consequently, maintaining the indoor temperature within the comfort range contributed to the heating demand being increased, which in turn generated carbon dioxide.Figure15 exhibits the variation of CE in the entire heating period. It was evident that the results of the integrated PCM can reduce the building CE and weaken the greenhouse effect on the outdoor environment. Compared to only 4.2% of CE-saving rate in Urumqi, the optimal was 17.4% in Lhasa. In addition, the other four regions had similar CE characteristics ranging from 6.9% to 8.1% as well as the results consisted with the finds of BEC.


## 5.1. Analysis of Temperature Dynamic Variability without AHS

The six representative regions are classified into optimal climate zone, suitable climate zone, and general climate zone according to the passive heating climate zoning [21]. Then without the AHS, the indoor air temperature of PCM building and original building was analyzed during the heating period (see Figure 7).Figure 7
Hourly variation of indoor temperature without AHS in the six regions.
(a)
Original building(b)
PCM buildingFigure7 shows significant variability of indoor air temperature in different regions no matter whether PCM was added or not. Obviously, a basic comfort level of 15°C reached for the most period in Lhasa was satisfactory while the other five regions had low room temperature, especially the worst indoor comfort in Urumqi. The average temperature during the heating period of the original and PCM buildings in Lhasa was 15.8°C and 16.2°C, respectively, whereas the lowest temperature in November reached 16.6°C and 17.4°C, respectively. However, there were periods below the thermal comfort level in other months; thus, Lhasa does not need auxiliary heating equipment only in November. With the two conditions (with or without PCM), the average temperature of Urumqi was not only the lowest presented at -0.6°C and -1.2°C, respectively, but also had the largest temperature fluctuation which was 19.9°C and 20.7°C. In addition, the variation of close average temperature ranged from 6.49~7.99°C to 7.17~8.58°C, respectively, which still made it difficult to satisfy the thermal comfort requirements for occupants in the other four regions; it should be mentioned that the outdoor climatic conditions presented similarly. Hence, it was seen that thermal demand was the most urgent in Urumqi of all regions while other regions had an analogous thermal environment where the average temperature was below the basic thermal comfort temperature. Lhasa and Urumqi were divided into different solar heating climate zones due to the local climate condition influenced. From this perspective of outdoor thermal disturbance, the hourly indoor air temperature in Lhasa is greatly better than that of Urumqi regardless of whether PCM is added to the building. Given the analysis above, even with the addition of PCM in the building, indoor temperature below 15°C in most periods can still be found because the melting-freezing thermal cycle of PCM may be incomplete. It can contribute to a mismatch between building heat gain and human comfort level. Therefore, to take full advantage of the energy storage and release functions of PCM, an auxiliary heating system is necessary to be used during the heating period in the six regions.


## 5.2. Effect of PCM Thickness on Building Thermal Performance

To facilitate the analysis, the whole heating period was simulated and selected results from December 11 to 14 for four consecutive days to analyze. The heating capacity of AHS was adjusted by stepped regulation, and the heat supply can be controlled by simulating the experiment through the trial and error method [54]. Following that, the building thermal performance effect of various PCM thicknesses was mainly discussed to obtain a suitable amount of material for subsequent analysis (see Figure 8).Figure 8
PCM thickness on thermal performance of building in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure8 shows that the thickness of PCM on building thermal performance varied in different regions given the opening of AHS. In this section, temperature fluctuation reduction rate (TFRR) is defined as the difference between the indoor air temperature fluctuation value in original building minus that of the PCM building and then the ratio of the indoor air temperature fluctuation value of the original building. TFRR correlates with the thermal stability of buildings. Along with the larger of TFRR, the better indoor thermal comfort achieved. It can be observed that the PCM-integrated building provided a beneficial heat storage effect. With the thickness of PCM increased from 20 mm to 80 mm, the average temperature and minimum temperature gradually improved in different regions. It was expected equivalent to supplement the extra thermal resistance of building envelopes with the addition of PCM layer [55]. However, the degree of change in TFRR showed a difference with the growing thickness of PCM. TFRR presented a positive correlation with thickness in Lhasa, but the impact degree of per unit thickness progressively enervated when exceeded 40 mm. Since the total thermal energy of outdoor provided to the PCM layer during the day is limited, the melting-freezing thermal cycle of PCM is completed at a certain critical thickness, even if excessively increasing the thickness will not directly affect the heat storage capacity of the material. Conversely, TFRR exhibited a negative correlation with the increase of PCM thickness in Xining, Yinchuan, and Xi’an. Taking Yinchuan for instance, on the one hand, the increasing degree of per unit thickness on average temperature growth rate (ATGR) and minimum temperature growth rate (MTGR) with thickness variations from 20 mm to 40 mm was 1.73% and 2.62%, while TFRR on per unit thickness dropped by only 0.45%; on the other hand, the temperature fluctuation of 20 mm and 40 mm was 3.99°C and 4.01°C in relatively close values, yet it was the minimum temperature of 15.02°C at 40 mm that can satisfy the basic thermal comfort. The results in Xining and Xi’an were in agreement with this finding. In addition, the changes in TFRR performed a trend of increasing and then decreasing in Urumqi and Lanzhou. Although it seemed that the highest TFRR was at 60 mm in Urumqi, the close effects to meeting the indoor thermal environment were achieved with a minimum temperature of 15.14°C at 40 mm, and the cost is increasing linearly with the increase of PCM thickness. Similar results have also been obtained by other scholars [56]. From the standpoint of ATGR, MTGR, and TFRR in Lanzhou, the slope of ATGR and MTGR curves tended to be gentle when surpassing 40 mm, and it presented the highest TFRR at 40 mm of thickness; thus, this amount of material can provide a better indoor thermal condition. In summary, it can be inferred that the thickness of PCM has an optimal value in different regions based on indoor thermal comfort and economic considerations. For the present study, the 40 mm of PCM thickness was determined as an appropriate selection for the later analysis.


## 5.3. Effect of PCM Location on Building Thermal Performance

In a normal scenario, the inner surface of the roof is generally not affected by direct solar radiation. This section took the PCM to integrate into the wall (PCMW), floor (PCMF), and both locations (PCMWF). Thus, the appropriate amount of PCM obtained in the previous section was applied to different locations, and then, the building thermal performance variability characteristics were analyzed coupled with AHS (see Figure9).Figure 9
PCM location on thermal performance in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anAs seen in Figure9, PCM placed in different locations exhibited differentiation after the AHS was turned on in the six regions. Referring to the original building, the PCMF in raising the indoor air temperature was weaker than in the other two locations. For instance, the average indoor temperature improvement of PCMF and PCMWF in Lanzhou was 0.3°C and 1.9°C, respectively, and the application effect of PCMF was reduced by 84% compared to PCMWF. Additionally, PCMF in most portions of the period was below the basic thermal comfort level in each region which cannot reach the heating demand of the human body. Due to the limited amount of PCM integrated into the floor, the change of outdoor solar heat source with the movement path contributes to the lowest heat storage and thus the weakest thermal regulation performance of the PCM. In contrast, the distinct from its effect was PCMWF, such as the average temperature in Xi’an also reached a minimum of 15.8°C which reduced the indoor discomfort. Although there were few regions where the indoor temperature variation of the PCMWF building was close to that of the PCMW, as a whole, the temperature wave amplitude was reduced obviously by the arrangement scheme of PCMWF. It would be shown following that PCMWF as the optimal location was selected for analysis.


## 5.4. Thermal Analysis of PCM Solar Building

From the previous sections, optimal choice in thickness and location of PCM solar building was mainly acquired. In this section, the dynamic law of room temperature of solar buildings whether to add PCM under AHS turned on was exhaustively investigated during the heating period in different regions as seen in Figure10.Figure 10
Variation of building thermal performance in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anAs can be seen in Figure10, the addition of the PCM played an active role in the level of indoor thermal comfort regardless of whether the AHS was turned on or not. However, there was indoor uncomfortable in Lhasa part periods and the whole day in the other five regions. The building thermal performance had been significantly improved after using the AHS, which was below the set temperature value in some periods, along with the incorporation of PCM enabled the room temperature to be maintained in the comfort range and effectively shaved the temperature swing. Due to the benefit heat gain effect produced by the comprehensive effect of solar radiation and auxiliary heat energy, the thermal energy was stored by PCM absorbing during the daytime convected with the indoor air and raised the temperature despite the lack of solar heat energy input during the absence of sunshine period. The results were consistent with the findings of the previous research work [54]. Nevertheless, it was observed that there is a positive effect of PCM thermal storage in different regions, yet there were also arguments. Notably, after the AHS was turned on, compared to the original building, the highest average temperature increase was 1.93°C in the PCM building of Urumqi. However, with the outdoor climate conditions being worst and located in the general heating climate zone, thus the auxiliary heat energy mainly was absorbed by PCM which would lead to the relatively high extra heating load. In contrast, although the elevated of indoor temperature was low in Lhasa, the abundant solar energy resources of local presented the room temperature reached thermal comfort demand easily and also reduced the heating energy consumption of AHS. Comparing the variation of building thermal performance in these two regions, it also can be found that the average temperature in Lhasa was 2°C higher than that in Urumqi. In addition, the thermal performance analysis results of buildings in other regions are similar.


## 5.5. Analysis of Indoor Thermal Comfort Performance

The section was to gain an insight into the indoor thermal comfort demand of occupants in the building adopting the AHS. For a passive solar building, the thermal comfort may be estimated by a specific index of the fixed temperature field [57]. The parameter Iwin integrated discomfort degree (IDD) for the indoor temperature to evaluate the thermal environment and human thermal comfort in winter was proposed by Zhang et al. [58]. It described the degree to which the indoor air temperature deviated from the critical temperature and stipulated Iwin if the variation range is close to 0, indicating that the indoor thermal environment met the thermal demand. It can be expressed by the following formula:
(24)Iwin=∫yeartL−tadτ,where tL and ta are 15°C of lower temperature limits within thermal comfort range and indoor air temperature (°C), respectively.Based on the IDD and using the AHS, the indoor thermal comfort performance of building whether added with PCM was compared and assessed in the six regions. The variation of IDD during the heating period in each region is shown in Figure11.Figure 11
Variation of IDD during heating period in the six regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure11 shows that the addition of PCM in buildings exploited differentiated effects on IDD in an indoor thermal environment, with a reduction of more than 96% IDD compared to original buildings. It was observed that the most significant reduction of IDD was in December in Urumqi, with a decrease of 587.8°C·h. On the contrary, there were situations where the IDD increased for certain months in individual regions. Taking the results of March as an instance in this region, it may be since the PCM in the state of solid or phase transition along with the AHS was activated, the material is prepared to charge heat by melting; thus, the room temperature was reduced, causing discomfort. It was apparent that the PCM played an efficient energy storage effect in most months during the heating period and improved indoor thermal comfort under the systematic influence of thermal energy of solar and auxiliary.


## 5.6. Analysis of Energy Saving and Carbon Dioxide Emission

### 5.6.1. Energy-Saving Performance

To further explore the impact of PCM building energy performance, after the AHS was turned on, a comparative analysis of building energy consumption (BEC) during the heating period was performed in the six regions for both the original building and PCM building, and the minimum daily indoor air temperature was controlled above 15°C using AHS. It is evaluated by energy consumption and energy-saving rate [59], and the calculation formula is as follows:
(25)Qs=Qno,pcmheating−Qpcmheating,(26)η=Qno,pcmheating−QpcmheatingQno,pcmheating×100%,where Qs and η are the energy-saving amount and energy-saving rate, respectively, and Qno,pcmheating and Qpcmheating are the energy consumption of original building and PCM building, respectively.Repeat all the simulations presented in Section5.5 to calculate the corresponding BEC as shown in Figure 12. It can be noted that the maximum energy saving in December is124.0 kW·h in Xi’an. Since the average outdoor temperature in Xi’an was the highest during the heating period in all regions, the PCM effectively carried out the heat storage and release process in that month; thus, heating energy consumption was saved. However, BEC increased in March of Urumqi and February of Lanzhou when PCM was added to the building which may be because of PCM absorbed heat by melting leading to the decrease of the room temperature in the building, and therefore, it required auxiliary heat supply, thus resulting in the higher BEC; the results were in agreement with the simulation of the reasons for the increase of IDD in Urumqi in March. It can be seen that regardless of whether PCMs existed or not, the dynamic change of BEC presented the law of increasing first and then decreasing. The regions where the optimal month of PCM with energy saving was 40% in Lhasa December and 22% in Urumqi November, and the energy saving rate of the other four regions within 30% ~40% in November.Figure 12
Variation of monthly BEC in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure13 illustrates that BEC was saved dramatically by the appearance of PCM under the AHS during the heating period, with the highest energy-saving rate reaching 19.7% in Lhasa, only 4.3% in Urumqi, and other regions close to that. The rich solar energy resources are provided sufficiently in winter as Lhasa is located in the optimal heating climate zone, and the PCM mainly absorbs and releases latent heat by solar radiation. Consequently, it was unnecessary to need the great auxiliary heat energy which in turn can reduce the heating energy consumption. The outdoor climate conditions are harsh in the winter of Urumqi, so more auxiliary heat energy must be provided to satisfy the demand for thermal comfort; indeed, the energy consumption of auxiliary heating increases. Furthermore, the effects of energy conservation were similar in other regions ranging from 7.0% to 8.2%.Figure 13
Variation of building energy-saving performance in different regions.


### 5.6.2. Carbon Dioxide Emission Characteristic

Household CO2 emission (CE) is one of the major sources of carbon emissions, and the reduction of CE could provide a significant contribution to mitigating global climate change [60]. Based on the analysis in the previous section, to illustrate the influence of PCM on building carbon emission, the CE of the original building and PCM building under the AHS during the heating period was analyzed in this section and revealed the law of CE of buildings in different regions (see Figures 14 and 15.Figure 14
Variation of monthly CO2 emission in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure 15
Variation of CO2 emission in heating period.Figure14 presents that the characteristic of CE was consistent with the variable trend of BEC performed. The incorporation of PCM can reduce CE depending mainly on the outside temperature, the indoor temperature, and the phase transition temperature [31]. In the climatic conditions of Xi’an, the CE saving in December showed the highest at 75.4 kg. The reason was in line with the performance analysis of energy saving. Along with the AHS being turned on, the PCMs in the building were stored productively under the comprehensive influence of solar energy and auxiliary thermal energy. It can effectively increase the room temperature and reduce the heat load of the AHS, thus reducing the generation of carbon dioxide. On the contrary, the CE of Urumqi in March and Lanzhou in February increased by 17.8 kg and 6.0 kg, respectively, by adding the PCM. It may be due to the insufficient utilization of PCM during this period, and the heat absorption began to decrease temperature after the AHS operated. Consequently, maintaining the indoor temperature within the comfort range contributed to the heating demand being increased, which in turn generated carbon dioxide.Figure15 exhibits the variation of CE in the entire heating period. It was evident that the results of the integrated PCM can reduce the building CE and weaken the greenhouse effect on the outdoor environment. Compared to only 4.2% of CE-saving rate in Urumqi, the optimal was 17.4% in Lhasa. In addition, the other four regions had similar CE characteristics ranging from 6.9% to 8.1% as well as the results consisted with the finds of BEC.


## 5.6.1. Energy-Saving Performance

To further explore the impact of PCM building energy performance, after the AHS was turned on, a comparative analysis of building energy consumption (BEC) during the heating period was performed in the six regions for both the original building and PCM building, and the minimum daily indoor air temperature was controlled above 15°C using AHS. It is evaluated by energy consumption and energy-saving rate [59], and the calculation formula is as follows:
(25)Qs=Qno,pcmheating−Qpcmheating,(26)η=Qno,pcmheating−QpcmheatingQno,pcmheating×100%,where Qs and η are the energy-saving amount and energy-saving rate, respectively, and Qno,pcmheating and Qpcmheating are the energy consumption of original building and PCM building, respectively.Repeat all the simulations presented in Section5.5 to calculate the corresponding BEC as shown in Figure 12. It can be noted that the maximum energy saving in December is124.0 kW·h in Xi’an. Since the average outdoor temperature in Xi’an was the highest during the heating period in all regions, the PCM effectively carried out the heat storage and release process in that month; thus, heating energy consumption was saved. However, BEC increased in March of Urumqi and February of Lanzhou when PCM was added to the building which may be because of PCM absorbed heat by melting leading to the decrease of the room temperature in the building, and therefore, it required auxiliary heat supply, thus resulting in the higher BEC; the results were in agreement with the simulation of the reasons for the increase of IDD in Urumqi in March. It can be seen that regardless of whether PCMs existed or not, the dynamic change of BEC presented the law of increasing first and then decreasing. The regions where the optimal month of PCM with energy saving was 40% in Lhasa December and 22% in Urumqi November, and the energy saving rate of the other four regions within 30% ~40% in November.Figure 12
Variation of monthly BEC in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure13 illustrates that BEC was saved dramatically by the appearance of PCM under the AHS during the heating period, with the highest energy-saving rate reaching 19.7% in Lhasa, only 4.3% in Urumqi, and other regions close to that. The rich solar energy resources are provided sufficiently in winter as Lhasa is located in the optimal heating climate zone, and the PCM mainly absorbs and releases latent heat by solar radiation. Consequently, it was unnecessary to need the great auxiliary heat energy which in turn can reduce the heating energy consumption. The outdoor climate conditions are harsh in the winter of Urumqi, so more auxiliary heat energy must be provided to satisfy the demand for thermal comfort; indeed, the energy consumption of auxiliary heating increases. Furthermore, the effects of energy conservation were similar in other regions ranging from 7.0% to 8.2%.Figure 13
Variation of building energy-saving performance in different regions.


## 5.6.2. Carbon Dioxide Emission Characteristic

Household CO2 emission (CE) is one of the major sources of carbon emissions, and the reduction of CE could provide a significant contribution to mitigating global climate change [60]. Based on the analysis in the previous section, to illustrate the influence of PCM on building carbon emission, the CE of the original building and PCM building under the AHS during the heating period was analyzed in this section and revealed the law of CE of buildings in different regions (see Figures 14 and 15.Figure 14
Variation of monthly CO2 emission in different regions.
(a)
Lhasa(b)
Urumqi(c)
Xining(d)
Lanzhou(e)
Yinchuan(f)
Xi’anFigure 15
Variation of CO2 emission in heating period.Figure14 presents that the characteristic of CE was consistent with the variable trend of BEC performed. The incorporation of PCM can reduce CE depending mainly on the outside temperature, the indoor temperature, and the phase transition temperature [31]. In the climatic conditions of Xi’an, the CE saving in December showed the highest at 75.4 kg. The reason was in line with the performance analysis of energy saving. Along with the AHS being turned on, the PCMs in the building were stored productively under the comprehensive influence of solar energy and auxiliary thermal energy. It can effectively increase the room temperature and reduce the heat load of the AHS, thus reducing the generation of carbon dioxide. On the contrary, the CE of Urumqi in March and Lanzhou in February increased by 17.8 kg and 6.0 kg, respectively, by adding the PCM. It may be due to the insufficient utilization of PCM during this period, and the heat absorption began to decrease temperature after the AHS operated. Consequently, maintaining the indoor temperature within the comfort range contributed to the heating demand being increased, which in turn generated carbon dioxide.Figure15 exhibits the variation of CE in the entire heating period. It was evident that the results of the integrated PCM can reduce the building CE and weaken the greenhouse effect on the outdoor environment. Compared to only 4.2% of CE-saving rate in Urumqi, the optimal was 17.4% in Lhasa. In addition, the other four regions had similar CE characteristics ranging from 6.9% to 8.1% as well as the results consisted with the finds of BEC.


## 6. Conclusion

The study simulated the application effect of PCM in a passive solar building with the AHS taking six representative regions in western China as instances. The variation laws of PCM that affected building thermal performance, indoor thermal comfort, energy saving, and carbon dioxide emission were revealed. Some conclusions can be summarized as follows:(1)
The variability of meteorological parameters in different regions made it insufficient to provide the heat gain of buildings during the heating period by solar energy. It was difficult to retain the indoor thermal comfort level in the building with or without the addition of PCM in the absence of AHS, so using AHS was necessary to better utilize the heat storage and release function of PCM(2)
A single-space passive solar building applied PCM with the AHS was analyzed by numerical simulation in the six regions. The results showed significant differences in the influence of PCM dosage and arrangement scheme on the indoor thermal environment. The optimum thickness obtained in terms of raising the indoor temperature and shaving the temperature fluctuation was 40 mm, and the most suitable location was PCMWF. The addition of PCM showed a beneficial heat gain effect, and the comprehensive effect of solar radiation heat and auxiliary heat can maintain above 15°C of the minimum indoor air temperature during uncomfortable periods(3)
The application effects of PCM embedded in building with the AHS presented differentiations in regions of western China. The average temperature in Lhasa was 2°C higher than that in Urumqi, and the IDD in each region was decreased by more than 96% compared to the original building. The thermal conditions of indoor and outdoor directly affected the energy saving and carbon dioxide emission of PCM-integrated building. The highest rate of energy saving and CE saving in the six regions was 19.7% and 17.4% in Lhasa, and the lowest was 4.3% and 4.2% in Urumqi. The effects were similar in other four regions ranging from 7.0% to 8.2% of energy-saving rate and 6.9% to 8.1% of CE-saving rate. Based on the division of passive heating climate zone, if the passive solar buildings use an auxiliary heating system in winter, the most suitable region with the application of PCM is Lhasa, the unsuitable region is Urumqi, and other regions are generally suitable(4)
The study mainly explores the performance of thermal and energy conservation in the PCM-integrated single-space passive solar building with the AHS which provides a reference for the promotion and application in regions of western China. To better optimize the effect of PCM for energy storage and temperature regulation in buildings. Future research can analyze in detail the variation law of thermal performance in different time and space of PCM incorporated into multizone buildings and establish an experimental room for verification. Furthermore, an economic analysis will also be performed to determine a suitable evaluation method



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*Source: 2518180-2023-02-07.xml*