The performance evaluation of shape-stabilized phase change materials in building applications using energy saving index

Applied Energy 113 (2014) 1118–1126

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The performance evaluation of shape-stabilized phase change materials in building applications using energy saving index Hong Ye a,⇑, Linshuang Long a, Haitao Zhang a, Ruqiang Zou b a b

Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui 230027, PR China College of Engineering, Peking University, Beijing 100871, PR China

h i g h l i g h t s  Energy saving equivalent (ESE) and energy saving index (ESI) are presented.  ESI and ESE can evaluate a passive component/material from an energy standpoint.  A kind of PCM’s performance is demonstrated in a full size lightweight room.  PCM’s performances in a residential room are simulated and evaluated via ESI.

a r t i c l e

i n f o

Article history: Received 5 March 2013 Received in revised form 17 June 2013 Accepted 24 August 2013 Available online 15 September 2013 Keywords: Building application Performance evaluation Energy saving index Phase change material Insulation material

a b s t r a c t The performance of a kind of shape-stabilized phase change material (PCM) was demonstrated in the Testing and Demonstration Platform for Building Energy Research. The results indicate that the use of PCM could lower the indoor temperature fluctuation and slow the indoor temperature’s decline rate. The PCM’s performance was also simulated in BuildingEnergy, a modeling software developed by the authors and validated via experiments, and evaluated via energy saving index (ESI), an evaluation index presented by the authors. The ESI is the ratio of a particular material or component’s energy saving equivalent (ESE) to the corresponding value of the ideal material or component that can maintain the room at an ideal thermal state in passive mode, where the ESE represents the hypothetical energy that should be input to maintain a passive room at the same thermal state as that when a particular material or component is adopted. The ESI can be used to characterize the performance of an actual building material or component from a common standpoint and be used to evaluate the performance of materials or components in different climatic regions or under different operating situations. The performance of the insulation material, represented by expanded polystyrene (EPS), was also simulated to give a comparison. The results show that the PCM has a better performance in the summer and a worse performance in the winter, while the EPS has a better performance over an entire year. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Phase change material (PCM) has been extensively studied as a kind of thermal storage materials in building applications [1,2]. They can store or release a large quantity of heat within nearly an isothermal process, i.e., phase change process. As a latent heat storage material, PCM has, in general, a larger heat storage capacity than the sensible heat storage material. When applied in a passive room, its heat gain is mainly from the solar radiation, which is a time-dependent source. With the adoption of PCM, the solar radiation can be stored when it is adequate and released when it is inadequate [3,4]. Kuznik et al. have studied the thermal performance of a PCM wallboard in a full scale test ⇑ Corresponding author. Tel./fax: +86 0551 63607281. E-mail address: [email protected] (H. Ye). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.08.067

room [5]. For organic PCMs, e.g., paraffin, they usually undergo a solid–liquid phase change process. Particular containers are necessary to encapsulate such PCMs. Microencapsulated PCM has been widely studied due to its high thermal conductivity and stability [6–8]. Barreneche et al. have compared the thermal properties of the microencapsulation, suspension and impregnation technology [9]. Shape-stabilized PCM, consisting of PCM and supporting material, is a kind of compound material and can be applied without containers [10,11]. The supporting materials can be the high density polyethylene (HDPE) [12], metal foam [13], graphite [14,15], carbon nanotubes, carbon nanofibers [16], etc. A lot of experiments have been conducted to demonstrate the PCMs’ performance in the building application. Castell et al. have built two house-like cubicles with inner dimensions of 2.4 m  2.4 m  2.4 m to analyze the benefits of PCMs [17]. Lin et al. have used a 3 m  2 m  2 m experimental house to study

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the shape-stabilized PCMs’ performance [18]. It is difficult to demonstrate the PCM’s performance in different buildings and regions only through experiments. Therefore, numerical methods also have been extensively used by the researchers. There are two main methods used to deal with the phase change problem: the effective heat capacity method [19], in which the PCM’s temperature and specific heat capacity are taken as variables, and the enthalpy method [20–23], in which the PCM’s heat of fusion was assumed to be shared equally over its phase change range. In general, the effective heat capacity of the PCM at each temperature is needed to be obtained through differential scanning calorimeter in the former method. And the latter method can be used to carry out a parameter analysis on a PCM that has hypothetical properties. An appropriate index is essential to evaluate the PCM’s performance in both experimental demonstrations and numerical studies. Zhang et al. have adopted the integrated discomfort degree for indoor temperature as a new parameter for determining the ideal thermophysical properties of building envelope materials, ideal natural ventilation rates, and minimal additional space heating or cooling energy consumption [24]. Pisello et al. have proposed the thermal deviation index to evaluate buildings’ thermal performance based on dynamic simulation [25]. The time lag, the decrement factor [26] and the total equivalent temperature difference [27] also have been used as the parameters for evaluation. However, these parameters were commonly based on a perspective of the building’s thermal state parameters. In this study, we presented two concepts, energy saving equivalent (ESE) and energy saving index (ESI), to evaluate a building material or component’s application performance to a passive building directly from an energy consumption’s standpoint. A kind of shape-stabilized PCM’s performance is demonstrated in a full scale room and simulated in a modeling software. Using the ESI as an evaluation index, its performance was compared with that of an insulation material.

2. The concepts of ESE and ESI The energy saving equivalent (ESE) represents the hypothetical energy that should be input to maintain a passive room at the same thermal state as that when a particular material or component is adopted. To elucidate the definition, the schematic representation of ESE is shown in Fig. 1. The original thermal state of a passive room, which is set as the indoor temperature in this paper, can be described as S1 (represented by the black solid line in Fig. 1). And the indoor temperature state after using a building component

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or material can be described as S2 (represented by the red dotted line in Fig. 1). When this building component or material is not adopted in the passive room, a cooling quantity (described as Qc, the light-colored area in Fig. 1) and heating quantity (described as Qh, the dark-colored area in Fig. 1) are needed to be delivered into the room to change the indoor temperature state from S1 into S2. From the perspective of changing the indoor temperature state, the use of the building material or component is equivalent to the effect of the Qc and Qh input, i.e., Qc and Qh can be used to represent the effect of the building material or component applied to the passive room. If the building material or component is adopted in the summer, the equivalent cooling quantity input, Qc, is desirable and can be taken as the energy ‘‘saved’’ by the use of the material or component. Considering the energy efficiency ratio (EER) of the refrigeration facility, the cooling quantity can be converted into electricity consumption using Qc/EER. However, the equivalent heating quantity input, Qh, is undesirable. To remove such undesirable heating quantity, the same cooling quantity is needed to be delivered into the room and can be viewed as the energy ‘‘wasted’’ by the use of the material or component. When converted into electricity consumption, the value of ‘‘energy wasted’’ is Qh/EER. Therefore, the net energy ‘‘saved’’ by the material or component in the summer is:

ESE ¼

Qc Q  h EER EER

ð1Þ

where ESE is referred to as the energy saving equivalent (ESE). Similarly, the net energy ‘‘saved’’ by the material or component in the winter is:

ESE ¼

Qh Q  c COP COP

ð2Þ

where COP is the heating facility’s coefficient of performance. Based on the ESE’s definition, the energy saving index (ESI) is defined as:

ESI ¼

ESE ESEmax

ð3Þ

where ESE is a particular material or component’s energy saving equivalent over a particular time period; ESEmax is the ideal building material or component’s ESE over the same time period. The ideal building material or component is a hypothetical material or component that can maintain a passive room’s indoor temperature at a constant comfortable level, and its performance is represented by the red dotted line in Fig. 1. The indoor temperature at the constant comfortable level is set here as 23 °C, which mostly satisfies the demand for thermal comfort in both the summer and winter (Table A.5 in [28]). According to Eq. (3), the ESI is a value less than unity. A positive ESI value means more energy is ‘‘saved’’ than ‘‘wasted’’. In other words, using the material or component in a passive room may ‘‘save’’ energy, indicating that the material or component is ‘‘energy-saving’’. However, a negative ESI value means more energy is ‘‘wasted’’ than ‘‘saved’’, which means the material or component is ‘‘energy-wasting’’. The thermal comfort level of a passive room with an ‘‘energy-wasting’’ material or component is lower than that without the material or component, and such an ‘‘energywasting’’ component or material should not be recommended. 3. Performance demonstration in a lightweight room 3.1. The demonstration system

Fig. 1. Schematic representation of the energy saving equivalent.

Testing and Demonstration Platform for Building Energy Research was built in Hefei, China to analyze the application

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performance of a building material or component. The platform, whose schematic diagram is shown in Fig. 2, contains two identical testing rooms: Room A and Room B. One of the testing rooms can be set as an experiment unit by adopting a building component or material that needs to be demonstrated, while the other one can be set as a control. The testing rooms have internal dimensions of 2.9 m  1.8 m  1.8 m (length  width  height). Their south walls are glass curtain walls, and the glass has a size of 1.65 m  1.65 m. The non-transparent envelopes of the rooms are made of polyurethane wrapped with metal boards, and the polyurethane is 37 kg/m3 in density, 1385 J/(kg K) in specific heat and 0.0228 W/(m K) in thermal conductivity. The thicknesses of the walls and the roofs are 10 cm. The rooms’ floors are thickened to 15 cm, and the testing rooms are put on two steel frames to attenuate the heat transfer through the floor.

3.2. A kind of shape-stabilized PCM In this kind of shape-stabilized PCM, paraffin wax was chosen as phase change material. And the graphite foam was chosen as the supporting material to enhance the composite’s thermal conductivity. The weight ratio of the paraffin wax is 40%. Crosslinking agent was also used to make the paraffins embraced well by the graphite foam. For such shape-stabilized PCM, its shape can be stabilized no matter the paraffin is in its liquid or solid state. The composite was formed as a plate of 290 mm  290 mm  25 mm (length  width  thickness), and its photo is shown in Fig. 3. To facilitate the discussion, the composite is represented as PCM rather than shape-stabilized PCM in this study. The density of the PCM was measured to be 675.5 kg/m3. Its phase change temperature, Ts, and heat of fusion, Hm, were measured by differential scanning calorimeter (DSC), and the instrument used was Q2000 DSC offered by TA Instruments company. DSC analysis was carried out at a heating/cooling rate of 10 °C/ min, and the sample’s weight was 9.02 mg. The DSC curve is shown in Fig. 4. The PCM’s thermal conductivity was measured by LFA 457 MicroFlash, a laser flash apparatus offered by NETZSCH company, and the value is 1.54 W/m K and 1.15 W/m K when the sample’s temperature was 20 °C and 50 °C, respectively. During the demonstration, Room B was set as the experiment unit, and Room A was set as the control. As shown in Fig. 5, the PCM plates were laid over Room B’s floor, and covered with thin steel plates. The rooms’ indoor temperatures and envelops’ surface temperatures were measured by thermal resistors and recorded by dataloggers to analyze the rooms’ thermal behaviors. The meteorological data (solar irradiance, wind speed, ambient temperature, etc.) were also measured and recorded.

Fig. 2. Schematic diagram of Testing and Demonstration Platform for BuildingEnergy Research.

Fig. 3. Photo of a shape-stabilized PCM plate.

3.3. The experiment results The demonstration began on November 11th, 2012, and lasted for 13 days. Some of the meteorological data are shown in Fig. 6. It can be seen from the figures that 6 days (the third, eighth to tenth, twelfth and thirteenth days) among the 13 days were cloudy or rainy, and other days were sunny. The variations in the two rooms’ average indoor temperatures are shown in Fig. 7. The figure indicates that on the sunny days, the indoor temperature of the room with PCM (Room B) was 15 °C lower and 10 °C higher than that of Room A in the daytime and nighttime, respectively; and on the cloudy/rainy days, Room B’s indoor temperature was a little higher than that of Room A. For the rooms without an inner heat source, the indoor air was warmed up by the solar radiation transmitted through the room’s envelopes. However, some of the solar radiation transmitted into Room B was absorbed by the PCM plates of larger heat storage capacity compared with the other envelopes, leading to a less heat gain of the indoor air in Room B. At night, when there was no solar radiation, the heat stored in the PCM plates was released due to the PCMs’ temperature was higher than that of the indoor air. Therefore, the use of PCM led to a decrease in the indoor temperature in the daytime, and an increase in the nighttime. In other words, the indoor air temperature fluctuation was lowered by the PCM. On cloudy/rainy days, the solar radiation was weak (shown in Fig. 6(b)), which led to a lower temperature of the indoor air than

Fig. 4. DSC curves for the PCM sample.

H. Ye et al. / Applied Energy 113 (2014) 1118–1126

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Fig. 5. Photos of the experiment unit (Room B). (a) Shows the view when the PCM plates were not covered with steels plates yet. (b) Shows the view when the preparatory work was completed.

that of the PCM plates. As a result, heat stored in the PCM plates during the sunny days was released to the indoor air, and made Room B’s indoor temperature higher than that of Room A. Fig. 7 also shows that on the cloudy/rainy days, Room B’s indoor temperature was higher in both the daytime and nighttime, which means the PCM did not absorb heat from the indoor air and was always releasing heat. Comparing the temperature curves in Fig. 7, it can be found that the slopes of the Room A’s temperature curve were consistently high in both rising and declining periods, while the slope of Room B’s temperature curve was consistently high only in rising period, and that in declining period seemed to be irregular: Room B’s temperature curve dropped sharply at first, then dropped gradually. Such phenomenon was caused by the PCMs’ effect. When the solar

(a)

irradiance was increasing, the indoor air and the PCM plates were warmed up rapidly, and the PCM absorbed heat quickly. When the solar irradiance began to decrease, the indoor air’s and the PCM plates’ temperatures decreased, and the latter decreased more slowly due to the PCM’s large thermal capacity. When the indoor temperature was lower than the PCMs’ temperature, the PCM plates began to release heat, which led a decrease in the indoor temperature’s drop rate. This phenomenon means that the adoption of the PCM can slow the indoor temperature’s decline rate.

3.4. Performance evaluation using ESI Energy saving index was used as an index to evaluate the PCM’s performance during the demonstration from an energy standpoint. According to Eq. (3), the key issue to obtain an ESI is the calculation of the PCM’s ESE and that of the ideal material or component. For the demonstration, the original state, S1, and the state when using the building material or component, S2, were Room A’s and Room

(b)

Fig. 6. Meteorological data (a: ambient air temperature; b: solar irradiance) during the demonstration.

Fig. 7. Variations in average indoor temperatures.

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B’s indoor temperature state, respectively. The heating and cooling quantities that should be delivered into the room without PCM to transfer S1 into S2, Qc and Qh can be simulated by building energy analysis computer programs (e.g., TRNSYS, DOE2, EnergyPlus, DeST, etc.), and with the appropriate EER or COP values, the ESE can be calculated using Eq. (1), (2). A type of modeling software developed by the authors, ‘‘BuildingEnergy’’, was used in this study. BuildingEnergy, mentioned in our previous work [29,30], has high credibility and has been validated by ANSI/ASHRAE Standard 140–2004 (Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs). However, the thermal physical properties of the building materials in the standard’s cases were assumed to be constant, which is not reasonable when there were PCMs in the buildings. PCM’s specific heat and thermal conductivity vary with its temperature. Using DSC analysis, the PCM’s effective specific heat was calculated and shown in Fig. 8. In BuildingEnergy, the effective heat capacity method was used. The indoor temperatures simulated by BuildingEnergy were compared with the measured ones in Fig. 9. The figures show that there was little difference between the simulated and measured results for both Room A and Room B, which means it is credible to use BuildingEnergy to analyze PCM’s performance. The procedure for calculating the ESI over this demonstration with Building Energy is introduced as follows. First, the variations in the indoor temperature of the testing room with and without the PCM plates, S1 and S2, were simulated, and are shown in Fig. 9. Second, by setting the room as active without the PCM plates, the set point of the thermal control strategy was S2, meaning that the temperature of the room in active mode was exactly the same as that represented by S2. The cooling or heating load needed to convert S1 into S2 is called the equivalent cooling or heating load, and are shown in Fig. 10. Qc and Qh could be obtained by integrating the equivalent cooling and heating loads, respectively, and their values are 27.0 and 42.9 MJ, respectively. Third, by choosing the appropriate EER or COP value according to the season and facility, the ESE of the PCM plates can be obtained using Eq. (1), (2). Considering that the demonstration was performed in the winter, only the COP needed to be determined. The heating equipment used in the platform is the electric heater, which means the COP is unity. Using Eq. (2), the PCM plates’ ESE during the demonstration is 4.42 kWh. The ESE of the ideal component or material, ESEmax, is calculated by setting the temperature control point in the second step at a constant value of 23 °C, and its value is 38.07 kWh. Using the determinate values for ESE and ESEmax, the ESI is obtained using Eq. (3), and its value is

Fig. 8. Effective specific heat of the PCM plates.

Fig. 9. Comparison between the simulated and measured (with error bars) results.

0.116. The positive value means the use of PCM plates in the testing room is ‘‘energy saving’’. 4. Application performance evaluation in a residential room Simulations are used in this section to discuss PCM’s performance in a residential room. In Section 3.4, the effective heat capacity method was used and the PCM’s effective specific heat was obtained through DSC. However, a hypothetical PCM whose Ts or Hm is set to a particular value will be studied to discuss its properties’ effects on the application performance. For such PCM, the enthalpy method will be adopted in this section. The equivalence of these two methods has already been verified [31]. Here we compared these two methods in BuildingEnergy. For the PCM discussed above (its DSC curve can be seen in Fig. 4), its Hm was 101.1 J/g, and its Ts was set to 38.79 °C and its phase change radius was 5.38 °C (38.79–33.41 °C). The results simulated using enthalpy method and effective heat capacity method are compared in Fig. 11, and it shows that these two methods are equivalent. 4.1. A typical room in a residential building A mid-floor room in a multi-story residential building is shown in Fig. 12, and has internal dimensions of 4 m  3.3 m  2.8 m

Fig. 10. Equivalent load variation during the demonstration.

H. Ye et al. / Applied Energy 113 (2014) 1118–1126

Fig. 11. Indoor temperature simulated using different methods.

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To discuss the PCM’s application performances in different climate regions, three typical cities in China were chosen: Beijing in the cold zone, Shanghai in the hot summer and Guangzhou in the hot summer and warm winter zone. Table 2 gives the climate conditions and cooling/heating periods of each city. The climate data used to simulate the ESE in BuildingEnergy are the typical meteorological year data offered by the Chinese Architecture-specific Meteorological Data Sets for Thermal Environment Analysis. As a result of the differences in local heating and cooling facilities, the COP and EER needed to calculate ESE were also different in each city. In Guangzhou, the EER is set to the recommended 2.7 [33]. For most households in Shanghai, the cooling and heating facilities were both room air conditioners, and the EER and COP are set to the recommended 2.3 and 1.9, respectively [32]. Central heating systems are adopted in Beijing, which is in a cold zone, in the winter. However, the COP is not applicable to central heating systems. An indirect approach was used to transfer the heating quantity offered by the heating system into electric power: the residential electricity price in Beijing is 0.4883 CNY/kWh and the initial heating price from the factory is 25 CNY/GJ or 0.09 CNY/kWh, so the heat is converted to electricity using a price ratio of 5.4 : 1. In summary, the EER is 2.7 in Guangzhou and 2.3 in both Beijing and Shanghai, while the COP is 1.9 in Shanghai and 5.4 in Beijing. 4.2. Performance evaluation using ESI

Fig. 12. Schematic diagram of a typical room.

(length  width  height). The room has only one exterior wall oriented to the south, and contains a 1.5 m  1.5 m window in the center of the exterior wall. The indoor temperatures of this room and the adjacent ones are assumed to be the same. Therefore, there is no heat transfer occurs via the interior walls. And the heat transfer via the exterior wall and glazing is assumed to be one-dimensional. The properties of the walls, roof and floor are listed in Table 1. According to the design standards [32–34], the inner heat gain from the occupants and equipment is taken to be 4.3 W per unit floor area, and that from lighting is 3.5 W per unit floor area when the lights are on from 18:00 till 22:00 every day. The ventilation rate is set as 1.0 air change per hour (ACH) in the winter, and 10.0 ACH in the summer.

Table 1 Properties of the room’s envelop. Item

Thickness [mm]

Density [kg/m3]

Specific heat [J/(kg k)]

Thermal conductivity [w/ (m k)]

Walls

240 (exterior) 100(interior) floor

1400

1050

0.58

100

2500

920

Roof & 1.74

4.2.1. Application performance in the winter During the demonstration, the PCM plates were placed on the floor’s upper surface. While in the simulation, the PCM plates are also placed on the exterior wall’s inner surface or the north wall’s inner surface to discuss the effect of the plates’ location on their application performance. The PCM’s ESIs in Beijing and Shanghai are shown in Fig. 13. The positive ESIs mean the use of PCMs could save energy. Fig. 13 also shows that the ESI trends are similar in the two cities: the ESIs when the PCM plates are on the exterior wall’s inner surface are the highest, and those on the north wall’s inner surface are the lowest. Such trends mean that for the PCM plates discussed in this study, the best performance is obtained when the PCMs are placed on the exterior wall’s inner surface. It is known from the demonstration that the PCM plates could lower the indoor temperature fluctuation by absorbing heat in the daytime and releasing it in the nighttime. For the exterior wall, it receives the most solar radiation, and had a higher surface temperature than the other walls. However, for the north wall, which is an interior wall, it receives little solar radiation, and has a lower surface temperature. Therefore, the PCM plates on the exterior wall can absorb the most heat, and has a more outstanding performance in the winter than those on the interior walls or on the floor. The phase change temperature Ts of the PCM discussed above is about 38 °C, and such Ts may be too high for the winter application. The optimal Ts is influenced by many factors (e.g., the room’s air change rate, inner heat source, the range of the comfortable temperature, etc.), and Jiang et al. have proposed a simple analytical method to estimate the optimal Ts and latent heat of interior PCM thermal mass in a passive solar room [35]. In this study, only Ts is set to a variable to simplify the issue, and the ESI variation according to Ts is shown in Fig. 14. The figure shows that with the increasing Ts, ESI first increases to a peak, then drops and begins to level off. It can be seen from Fig. 8 that PCM’s heat storage capacity is large only when its temperature is near Ts. A lower Ts makes the PCM stay at its liquid state longer and less heat is released. While a higher Ts makes the PCM stay at its solid state longer and less heat is stored. And at an optimal Ts, the PCM can undergo its phase change process to store and release its latent heat. The optimal Ts in Beijing and Shanghai are about 16 °C and

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Table 2 Climatic conditions and the cooling/heating periods in each typical city. City

Average temperature/°C

Beijing Shanghai Guangzhou

Coldest month

Hottest month

3.6 3.7 13.3

25.3 29.2 28.4

Cooling period (summer)

Heating period (winter)

June 12–September 1 June 24–September 15 May 13–October 17

November 1–March 15 of the following year December 6–March 22 of the following year –

0.16 Winter 0.14 0.12

ESI

0.1 0.08 0.06 0.04 0.02 0 Beijing PCM (on the exterior wall's inner surface)

Fig. 13. ESI of the PCM plates in Beijing and Shanghai.

Shanghai EPS

Fig. 15. ESI comparison of the room with PCM and EPS in the winter.

Fig. 14. ESI variation according to Ts when PCM on the exterior wall’s inner surface.

10 °C, respectively. During the daytime in winter, the average solar irradiance on a south-facing vertical surface in Beijing (285.74 W/m2) is more than that in Shanghai (169.70 W/m2), which means the PCM in Beijing absorbs more heat and has a higher temperature, thus a higher optimal Ts is needed in Beijing. In many Chinese cities, the insulation materials are used on the exterior wall’s outer surface to obtain energy saving benefit. One of the most widely used insulation materials is expanded polystyrene (EPS), and whose density, thermal conductivity and specific heat are 55 kg/m3, 0.027 W/(m K) and 1210 J(kg K), respectively (from Table A.3 in [36]). We calculated here the EPS’s ESIs in Beijing and Shanghai to give a comparison to the PCM. Their ESIs are shown in Fig. 15, in which the PCM used is that discussed in Section 4 and the EPS’s thickness is 30 mm. The figure shows that EPS has better performance than PCM in the winter. The indoor temperature variations in Beijing are shown in Fig. 16 to give further information about EPS’s and PCM’s performances. It can be seen that the indoor temperature with EPS is always higher than the original state, because the use of EPS can block the heat transfer from the room to the environment, while the indoor

Fig. 16. Indoor temperature variations in Beijing from Jan 1st to Jan 7th.

temperature with PCM fluctuates above and below the original state. Because the PCM’s performance dependeds on the temperature difference between the PCM and indoor air: if the PCM’s temperature is higher than that of the indoor air, PCM releases heat and warms up the air; if the PCM’s temperature is lower than that of the air, PCM absorbs heat and cool down the air.

4.2.2. Application performance in the summer When the PCM plates were placed on the exterior wall’s inner surface, the ESIs in the summer were 0.0020, 0.0024 and 0.0035 in Beijing, Shanghai and Guangzhou, respectively. And the EPS’s ESIs are also negative in both Beijing and Shanghai. Fig. 17 shows the indoor temperature variations in Beijing, which indicates that the indoor temperatures are close to each other. It means that the EPS cannot block the heat transfer from the environment to the room as the solar radiation is too strong in the summer and the PCM cannot regulate the indoor temperature

H. Ye et al. / Applied Energy 113 (2014) 1118–1126

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shown in Fig. 19. Because there is no heating period in Guangzhou, the ESI over an entire year is equal to that in the summer. The figure shows that the use of PCM could ‘‘save’’ energy in Beijing and Shanghai, and could ‘‘save’’ energy in Guangzhou only when it is not placed on the exterior wall, and the EPS’s ESIs are much higher than those of PCM, which means the EPS had a better performance than PCM over an entire year.

5. Conclusions

Fig. 17. Indoor temperature variations in Beijing from Aug 1st to Aug 7th.

0.004 0.002

Summer

0

ESI

-0.002 -0.004

Beijing

Shanghai

The concepts of ESE and ESI are presented in this paper. The ESE can be used to intuitively evaluate the performance of a component or material from an energy standpoint. It can also be used for the performance evaluation of all types of building components or materials in passive applications. The ESI, which is based on the ESE, can be used to characterize the performance of an actual building component or material from a common standpoint. A kind of shape-stabilized PCM’s application performance was estimated using the ESI as the evaluation index. The demonstration results showed that the PCM could improve the indoor thermal comfort degree through lowering the indoor temperature fluctuation and slowing the indoor temperature’s decline rate. The simulation results show that the PCM’s application performance to a residential room depends on the PCM’s location and application season, and the insulation material has a better performance than PCM over an entire year.

Guangzhou

-0.006

Acknowledgment

-0.008 -0.01 -0.012 -0.014 PCM on exterior wall's inner surface

PCM on north wall's inner surface

PCM on floor's upper surface

EPS

Fig. 18. ESI comparison of the room with PCM and EPS in the summer.

0.05 An entire year 0.04

ESI

0.03 0.02 0.01 0 Beijing

Shanghai

Guangzhou

-0.01 PCM on exterior wall's inner surface

PCM on north wall's inner surface

PCM on floor's upper surface

EPS

Fig. 19. ESI comparison of the room with PCM and EPS over an entire year.

as the PCM’s and indoor air’s temperatures are close to each other. It can be inferred that if the PCM had a lower or higher temperature, it would absorb or release heat. Therefore, it can be seen from Fig. 18, ESIs of PCM placed on the interior wall or floor are higher than that on the exterior wall. 4.2.3. Application performance over an entire year The ESE over an entire year can be obtained by adding those in the winter and summer together, then the ESI over an entire year can be obtained using Eq. (3). The ESIs over an entire year are

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