3.1 Numerical simulation details

Dynamic thermal simulations are performed using EnergyPlus. All simulations are carried out from April 1 to October 31, which covers the whole cooling season. The weather data is accessed based on the standard CSWD weather files provided by EnergyPlus [15]. The Conduction Finite Difference (CondFD) solution algorithm is adopted to depict the thermal process of phase change layer. The Fully Implicit Order is selected as the difference scheme for CondFD [16]. According to the suggestion of Tabares-Valesco et al. [17], time step for CondFD in EnergyPlus is set to 3 min, and node space is set to 3. The other components of the envelope are modeled with the default Conduction Transfer Functions algorithm. The heat conduction between the ground and the floor is calculated by Ground Heat Transfer: Slab module. TARP and DOE-2 are adopted for the interior and exterior surface convective heat transfer algorithms, respectively [18]. To perform NV strategy, the Zone Ventilation: Wind and Stack Open Area module is employed [19].

Therefore, to examine the climatic and seasonal suitability of PCT, Bio-PCMs with eight different PCT are adopted. The enthalpy-temperature graph of Bio-PCMs is

With the adaptive comfort theory, Evola et al. [24] proposed an index called Intensity of Thermal Discomfort (ITD), to evaluate the indoor thermal environment. It is defined as the time integral, over the occupancy period of the positive difference between the current operative temperature and the upper limit of thermal comfort range (see Figure 9). This index reveals well the duration

and the extent of discomfort thermal sensation perceived by the occupants within a long period. Therefore, it is employed to represent the thermal performance of PCMs coupled with NV strategy. The calculation is expressed

ºC–<sup>1</sup>

). Bio-PCMs are installed on the inner side of building envelope

. The density is

). The thermal conductivity is

depicted in Figure 8 [22]. The latent heat of Bio-PCMs is 219 kJkg<sup>1</sup>

860 kg<sup>m</sup><sup>3</sup> and the specific heat is 1.97 (kJkg–<sup>1</sup>

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

(external walls, internal walls and ceilings) in Ref. [23].

0.2 (W<sup>m</sup>–<sup>1</sup>

Figure 8.

as follows:

Figure 9. Definition of ITD.

19

ºC–<sup>1</sup>

H-T curves of eight different PCMs.

3.2 Evaluation index

Three-dimensional model and standard floor plan of the office building are depicted in Figure 7. The four-storey building is 3.6 m storey height, 31.9 m length, and 15.8m width. The total floor area is 2016 m2 . The size of each window is 1.8m height and 1.5 m width. The window-wall ratio is 16.3% of the north and south walls and 5.1% of the remaining walls, which satisfies local standard [20].

According to Ref. [21], it has been verified that phase change temperature (PCT) has great influence on thermal performance of PCMs in different seasons.

#### Figure 7.

(a) The 3D model and (b) the standard floor plan of the building for EnergyPlus simulation.

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

performance. It is critical to determine the phase change ratio in phase change process. In this part, it is determined based on experimental results. To provide references for PCM-based envelope thermal design in buildings, the phase change ratio of PCM-based envelope in different climate conditions could be obtained by

3. How to deal with issues on climatic and seasonal suitability of the

The purpose of the present work is to investigate the seasonal and climatic suitability for application of PCM-based envelope coupled with night ventilation (NV) strategy in naturally ventilated office buildings. For suitability analysis, the adaptive thermal comfort theory is applied and the adaptive comfort model in

Dynamic thermal simulations are performed using EnergyPlus. All simulations are carried out from April 1 to October 31, which covers the whole cooling season. The weather data is accessed based on the standard CSWD weather files provided by EnergyPlus [15]. The Conduction Finite Difference (CondFD) solution algorithm is adopted to depict the thermal process of phase change layer. The Fully Implicit Order is selected as the difference scheme for CondFD [16]. According to the suggestion of Tabares-Valesco et al. [17], time step for CondFD in EnergyPlus is set to 3 min, and node space is set to 3. The other components of the envelope are modeled with the default Conduction Transfer Functions algorithm. The heat conduction between the ground and the floor is calculated by Ground Heat Transfer: Slab module. TARP and DOE-2 are adopted for the interior and exterior surface convective heat transfer algorithms, respectively [18]. To perform NV strategy, the Zone Ventilation: Wind and Stack Open Area

Three-dimensional model and standard floor plan of the office building are depicted in Figure 7. The four-storey building is 3.6 m storey height, 31.9 m length,

height and 1.5 m width. The window-wall ratio is 16.3% of the north and south walls

According to Ref. [21], it has been verified that phase change temperature (PCT) has great influence on thermal performance of PCMs in different seasons.

and 5.1% of the remaining walls, which satisfies local standard [20].

(a) The 3D model and (b) the standard floor plan of the building for EnergyPlus simulation.

. The size of each window is 1.8m

series of further experiments.

Zero and Net Zero Energy

standard ASHRAE-55 is adopted [14].

3.1 Numerical simulation details

module is employed [19].

Figure 7.

18

and 15.8m width. The total floor area is 2016 m2

technology

Figure 8. H-T curves of eight different PCMs.

Therefore, to examine the climatic and seasonal suitability of PCT, Bio-PCMs with eight different PCT are adopted. The enthalpy-temperature graph of Bio-PCMs is depicted in Figure 8 [22]. The latent heat of Bio-PCMs is 219 kJkg<sup>1</sup> . The density is 860 kg<sup>m</sup><sup>3</sup> and the specific heat is 1.97 (kJkg–<sup>1</sup> ºC–<sup>1</sup> ). The thermal conductivity is 0.2 (W<sup>m</sup>–<sup>1</sup> ºC–<sup>1</sup> ). Bio-PCMs are installed on the inner side of building envelope (external walls, internal walls and ceilings) in Ref. [23].

#### 3.2 Evaluation index

With the adaptive comfort theory, Evola et al. [24] proposed an index called Intensity of Thermal Discomfort (ITD), to evaluate the indoor thermal environment. It is defined as the time integral, over the occupancy period of the positive difference between the current operative temperature and the upper limit of thermal comfort range (see Figure 9). This index reveals well the duration and the extent of discomfort thermal sensation perceived by the occupants within a long period. Therefore, it is employed to represent the thermal performance of PCMs coupled with NV strategy. The calculation is expressed as follows:

Figure 9. Definition of ITD.

$$\begin{aligned}ITD &= \int\_{\mathbf{p}} \Delta T^+(\tau) \cdot \mathbf{d}\tau \\ \text{where } \Delta T^+(\tau) &= \begin{cases} T\_{\text{op}}(\tau) - T\_{\text{lim}} & \text{if } T\_{\text{op}}(\tau) \ge T\_{\text{lim}} \\ \mathbf{0} & \text{if } T\_{\text{op}}(\tau) < T\_{\text{lim}} \end{cases} \end{aligned} \tag{16}$$

$$T\_{\rm lim} = \mathbf{0.31} \cdot T\_{\rm rm} + 2\mathbf{1.3} \tag{17}$$

where Top is the operative temperature,Tlim is the upper threshold of 80% acceptability limit of Adaptive Comfort Model in Standard ASHRAE-55 [14], and Trm is the mean monthly outdoor air dry-bulb temperature, which is the arithmetic average of the mean daily minimum and mean daily maximum outdoor air dry-bulb temperature for the month in sequence.

#### 3.3 Climatic and seasonal suitability analysis

A series of parametric studies over the selected eight Bio-PCMs are carried out, to investigate the suitability of PCT in transition and hot seasons. The optimum PCT for transition season and hot season is determined according to the minimum ITD, which are listed in Table 2. It can be found that in some cities, the optimal PCT for transition season does not match with the climate conditions in hot season, vice versa. Therefore, it is quite necessary to select different PCT based on the outdoor climatic characteristics of transition and hot seasons, respectively.

Based on the optimum PCT, the effect of PCMs coupled with NV strategy on reducing ITD in transition and hot seasons is compared with NV strategy and PCMs strategy. Figure 10 summarizes the ITD in both transition and hot seasons with different technologies in all cities. All strategies (with PCMs coupled with NV strategy, with NV strategy, and with PCMs strategy) can reduce ITD compared with reference group (without cooling strategy). The ITD of transition season is shown in Figure 10(a). For these cities in hot summer and warm winter zone, the effects with PCMs alone in reducing ITD are inferior to NV strategy. It is evident for Nanning and Hechi, where the ITD with PCMs is much higher than that with NV. When NV is introduced on the basis of PCMs strategy, ITD is reduced from 2231 to


1621°Ch for Nanning and from 1503 to 937°Ch for Hechi. It is due to NV strategy working well to introduce outdoor cool air into the room and to exclude the heat released from PCMs to outdoor during nighttime. Accordingly, PCMs can be fully solidified at night. Therefore, ITD can be effectively decreased during the occupancy period using PCMs coupled with NV. It further indicates the coupling use of NV strategy, and PCMs strategy is necessary for transition season under this climate condition. On the contrary, PCMs coupled with NV strategy is inferior to PCMs strategy for severe cold zones. It is clear that ITD is increased when PCMs coupled with NV strategy is adopted in Urumqi and Altay compared to PCM strategy. Such results indicate that PCM strategy is more suitable to severe cold zone in transition season rather than PCMs coupled with NV strategy. For other cities, PCMs coupled with NV strategy is more suitable in transition season in reducing ITD compared to

(a) Transition season and (b) hot season of the influence of different passive cooling technologies on ITD.

From Figure 10(b), PCMs coupled with NV is the most effective strategy to reduce ITD in hot season for all cities. However, the advantages of PCMs coupled

NV strategy and PCM strategy.

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

Figure 10.

21

#### Table 2.

Optimum PCT for transition season and hot season.

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

ITD ¼

temperature for the month in sequence.

Zero and Net Zero Energy

Table 2.

20

Optimum PCT for transition season and hot season.

3.3 Climatic and seasonal suitability analysis

ð

ΔTþð Þ� τ dτ

where <sup>Δ</sup>Tþð Þ¼ <sup>τ</sup> <sup>T</sup>opð Þ� <sup>τ</sup> <sup>T</sup>lim if <sup>T</sup>opð Þ<sup>τ</sup> <sup>≥</sup> <sup>T</sup>lim

where Top is the operative temperature,Tlim is the upper threshold of 80% acceptability limit of Adaptive Comfort Model in Standard ASHRAE-55 [14], and Trm is the mean monthly outdoor air dry-bulb temperature, which is the arithmetic average of the mean daily minimum and mean daily maximum outdoor air dry-bulb

A series of parametric studies over the selected eight Bio-PCMs are carried out, to investigate the suitability of PCT in transition and hot seasons. The optimum PCT for transition season and hot season is determined according to the minimum ITD, which are listed in Table 2. It can be found that in some cities, the optimal PCT for transition season does not match with the climate conditions in hot season, vice versa. Therefore, it is quite necessary to select different PCT based on the outdoor climatic characteristics of transition and hot seasons, respectively.

Based on the optimum PCT, the effect of PCMs coupled with NV strategy on reducing ITD in transition and hot seasons is compared with NV strategy and PCMs strategy. Figure 10 summarizes the ITD in both transition and hot seasons with different technologies in all cities. All strategies (with PCMs coupled with NV strategy, with NV strategy, and with PCMs strategy) can reduce ITD compared with reference group (without cooling strategy). The ITD of transition season is shown in Figure 10(a). For these cities in hot summer and warm winter zone, the effects with PCMs alone in reducing ITD are inferior to NV strategy. It is evident for Nanning and Hechi, where the ITD with PCMs is much higher than that with NV. When NV is introduced on the basis of PCMs strategy, ITD is reduced from 2231 to

Cities Optimum PCT/°C

Turpan 27 33 Nanning 29 29 Hechi 27 29 Chongqing 27 29 Xi'an 25 27 Chengdu 25 27 Urumqi 23 25 Altay 23 25 Guiyang 23 25 Kunming 23 23

0 if Topð Þτ , Tlim

( (16)

Tlim ¼ 0:31 � Trm þ 21:3 (17)

Transition season Hot season

P

Figure 10. (a) Transition season and (b) hot season of the influence of different passive cooling technologies on ITD.

1621°Ch for Nanning and from 1503 to 937°Ch for Hechi. It is due to NV strategy working well to introduce outdoor cool air into the room and to exclude the heat released from PCMs to outdoor during nighttime. Accordingly, PCMs can be fully solidified at night. Therefore, ITD can be effectively decreased during the occupancy period using PCMs coupled with NV. It further indicates the coupling use of NV strategy, and PCMs strategy is necessary for transition season under this climate condition. On the contrary, PCMs coupled with NV strategy is inferior to PCMs strategy for severe cold zones. It is clear that ITD is increased when PCMs coupled with NV strategy is adopted in Urumqi and Altay compared to PCM strategy. Such results indicate that PCM strategy is more suitable to severe cold zone in transition season rather than PCMs coupled with NV strategy. For other cities, PCMs coupled with NV strategy is more suitable in transition season in reducing ITD compared to NV strategy and PCM strategy.

From Figure 10(b), PCMs coupled with NV is the most effective strategy to reduce ITD in hot season for all cities. However, the advantages of PCMs coupled

#### Zero and Net Zero Energy

with NV strategy than NV strategy in Turpan, Nanning, Hechi, and Chongqing are greatly reduced compared to transition season due to the high temperature and small diurnal temperature difference in hot season. Additionally, with PCM strategy alone, ITD in these cities is much higher than that with NV strategy or PCMs coupled with NV strategy. It indicates that NV strategy is necessary for PCM strategy to obtain excellent performance. The use of PCMs coupled with NV strategy is critical to hot season.

A performance-based inverse design method is proposed to extract typical outdoor air dry-bulb temperature from typical meteorological year, which is favorable for application of PCMs coupled with NV. The typical air dry-bulb temperature curve (Tout) is obtained as Figure 11 shown (Nanning, China). Outdoor air drybulb temperature fluctuation range is much larger than the phase transition temperature range (27–31°C). Meanwhile, the maximum value Tmax (33°C) is 2°C higher than the upper limit of PCT, and the minimum value Tmin (25.5°C) is 1.5°C lower than the lower limit of PCT. In other words, PCMs could have an opportunity to solidify at low temperature phase and to melt at high temperature phase. Additionally, the average value Tave (28.6°C) is quite close to PCT 29°C, and the diurnal temperature difference (ΔT) is greater than 7.5°C. These conditions are beneficial to the utilization of latent heat of PCMs.

In order to verify the reliability of the method, indoor operative temperature using PCMs coupled with NV and NV in the typical week is compared in Figure 13. PCMs coupled with NV appear as a remarkable advantage in reducing indoor operative temperature than NV. Indoor operative temperature reduction is up to 1°C, even to 2°C in 6–8 June. It can infer that outdoor air dry-bulb temperature in the typical week is beneficial to PCMs coupled with NV. It further indicates that the method is reliable. Therefore, it can be used to estimate the relationship between PCMs coupled with NV strategy and local outdoor climatic characteristics.

Indoor operative temperature in the typical week: PCMs coupled with NV vs. NV.

4. How to improve thermal performance of phase change materials

Taking PCMs into building envelope significantly reduces building energy consumption and improves indoor thermal environment. The main categories of PCMs applied in building envelope are organic PCMs and inorganic PCMs. They both have solid–liquid phase flow properties, and the phase transition takes place at a certain temperature range. Paraffin is the most commonly used organic because it has chemically stable, high latent of fusion, and regular degradation in thermal properties after phase change cycles. However, paraffin has low thermal conductivity, flammable, and slow oxidation when exposed to oxygen. These problems present challenges in container design [25]. Salt hydrates as an important group of inorganic PCMs have extensively investigated in building envelope. The most serious limitation of the salt hydrates is phase segregation and supercooling compared with paraffin. Another problem is salt hydrates would cause corrosion in building materials and metal containers. These problems limit their applications in building

The integration of PCMs and building envelope can be divided into three types:

encapsulation (see Figure 14). Using PCMs in building envelope, one must keep in mind that liquid phase causes leaking to the surface out of the carrier materials. Encapsulation serves as barrier between PCM and surrounding environment. It provides long-term durability and structural requirements. Micro-encapsulation of PCMs provides faster charging and discharging rates because of the smaller distance for heat transfer compared to macro-encapsulation. However, the lower encapsulation rate of micro-encapsulation greatly reduced the energy storage and increases

direct incorporation and immersion, macro-encapsulation, and micro-

applied in building envelope

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

envelope [26].

Figure 13.

the cost [27].

23

Figure 12 is the comparison of Tout and outdoor air dry-bulb temperature in a typical week (1–7 June). Tout agrees well with outdoor air dry-bulb temperature curve. It means the approach can be used to determine Tout curve.

Figure 11. Characteristics of the typical outdoor air dry-bulb temperature curve.

Figure 12. Comparison between Tout and outdoor air dry-bulb temperature in a typical week.

Building Envelope with Phase Change Materials DOI: http://dx.doi.org/10.5772/intechopen.85012

with NV strategy than NV strategy in Turpan, Nanning, Hechi, and Chongqing are greatly reduced compared to transition season due to the high temperature and small diurnal temperature difference in hot season. Additionally, with PCM strategy alone, ITD in these cities is much higher than that with NV strategy or PCMs coupled with NV strategy. It indicates that NV strategy is necessary for PCM strategy to obtain excellent performance. The use of PCMs coupled with NV strat-

A performance-based inverse design method is proposed to extract typical outdoor air dry-bulb temperature from typical meteorological year, which is favorable for application of PCMs coupled with NV. The typical air dry-bulb temperature curve (Tout) is obtained as Figure 11 shown (Nanning, China). Outdoor air drybulb temperature fluctuation range is much larger than the phase transition temperature range (27–31°C). Meanwhile, the maximum value Tmax (33°C) is 2°C higher than the upper limit of PCT, and the minimum value Tmin (25.5°C) is 1.5°C lower than the lower limit of PCT. In other words, PCMs could have an opportunity to solidify at low temperature phase and to melt at high temperature phase. Additionally, the average value Tave (28.6°C) is quite close to PCT 29°C, and the diurnal temperature difference (ΔT) is greater than 7.5°C. These conditions are beneficial

Figure 12 is the comparison of Tout and outdoor air dry-bulb temperature in a typical week (1–7 June). Tout agrees well with outdoor air dry-bulb temperature

curve. It means the approach can be used to determine Tout curve.

Characteristics of the typical outdoor air dry-bulb temperature curve.

Comparison between Tout and outdoor air dry-bulb temperature in a typical week.

egy is critical to hot season.

Zero and Net Zero Energy

to the utilization of latent heat of PCMs.

Figure 11.

Figure 12.

22

Figure 13. Indoor operative temperature in the typical week: PCMs coupled with NV vs. NV.

In order to verify the reliability of the method, indoor operative temperature using PCMs coupled with NV and NV in the typical week is compared in Figure 13. PCMs coupled with NV appear as a remarkable advantage in reducing indoor operative temperature than NV. Indoor operative temperature reduction is up to 1°C, even to 2°C in 6–8 June. It can infer that outdoor air dry-bulb temperature in the typical week is beneficial to PCMs coupled with NV. It further indicates that the method is reliable. Therefore, it can be used to estimate the relationship between PCMs coupled with NV strategy and local outdoor climatic characteristics.
