4. How to improve thermal performance of phase change materials applied in building envelope

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 envelope [26].

The integration of PCMs and building envelope can be divided into three types: direct incorporation and immersion, macro-encapsulation, and microencapsulation (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 the cost [27].

Time 0 min 10 min 20 min 30 min Legend

Tave 26°C 24.7°C 23.2°C 23.1 °C

Tave 26°C 24.1°C 23.0°C 22.8 °C

Time 0 min 5 min 10 min 15 min Legend

Tave 22.5°C 36.9°C 45.2°C 48.3°C

Tave 22.2°C 38.8°C 47.6°C 49.3°C

Time 45 min 60 min 75 min 90 min Legend

(°C)

Heat release Pure paraffin

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

Heat storage Pure paraffin

Non-optimized component (top and bottom)

Optimized component (top and bottom)

Table 5.

25

Table 3.

Table 4.

Composite PCM

Heat release comparison of pure paraffin and the composite PCM.

Composite PCM

Heat storage comparison of pure paraffin and the composite PCM.

Comparisons of optimized/nonoptimized component with PCMs.

Figure 14. Macro-encapsulation of PCMs applied in building envelope [28, 29].

#### 4.1 Heat transfer enhancement at material level

Among the different combinations of PCMs and building envelope, macroencapsulation (building component with PCMs) is considered as a promising option in building energy conservation. The advantages of the technique embodied at various packaging shapes and sizes includes avoiding leakage of material, providing sufficient thermal storage capacity, and reducing the adverse effects on structure, whereas unreasonable application and poor thermal conductivity would cause the regulating ability of indoor thermal environment untimely and incompletely. The building applications require fast absorption and release of latent heat. In addition, small temperature fluctuation is not beneficial to the utilization of PCMs in buildings. Hence, heat transfer enhancement of PCMs is significant for building envelope. The heat transfer performance of PCMs building component can enhance at material level and component level [30]. At material level, carbon-based materials are one of the most common additives due to high thermal conductivity, stable chemical performance, and low density. However, uniformly dispersion of additives in PCMs is the most important factor to improve thermal conductivity. Moreover, aspect ratio of additives is also crucial for thermal conductivity enhancement such as carbon fiber (CF) and carbon nanotubes (CNTs).

At material level, CNTs is selected as a heat transfer medium for the composite PCM in experiment. A group of the composite PCM is prepared with CNTs of 0.5, 1, 1.5, and 2 wt%. The thermal conductivity and latent heat are determined and compared. The experimental results show that thermal conductivity of the composite PCM with 2 wt% CNTs increased by 17 and 21% compared to pure paraffin in liquid and solid phase, respectively.

To evaluate the heat storage and release capacity of pure paraffin and the composite PCM, the infrared thermograph images of two samples are recorded in Tables 3 and 4. It can be found that the heat transfer rate of the composite PCM is greatly improved both in heat release and heat storage process. Especially, the average temperature of the composite PCM at different time is 2°C higher than that of pure paraffin in heat storage process. Moreover, the temperature distribution of the composite PCM is more uniform than that of pure paraffin in heat storage process.

#### 4.2 Heat transfer enhancement at component level

At component level, metal fins are widely used to increase the heat transfer areas and relatively easy to fabricate for building component with PCMs [31]. The main

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


#### Table 3.

4.1 Heat transfer enhancement at material level

Macro-encapsulation of PCMs applied in building envelope [28, 29].

such as carbon fiber (CF) and carbon nanotubes (CNTs).

4.2 Heat transfer enhancement at component level

liquid and solid phase, respectively.

process.

24

Figure 14.

Zero and Net Zero Energy

Among the different combinations of PCMs and building envelope, macroencapsulation (building component with PCMs) is considered as a promising option in building energy conservation. The advantages of the technique embodied at various packaging shapes and sizes includes avoiding leakage of material, providing sufficient thermal storage capacity, and reducing the adverse effects on structure, whereas unreasonable application and poor thermal conductivity would cause the regulating ability of indoor thermal environment untimely and incompletely. The building applications require fast absorption and release of latent heat. In addition, small temperature fluctuation is not beneficial to the utilization of PCMs in buildings. Hence, heat transfer enhancement of PCMs is significant for building envelope. The heat transfer performance of PCMs building component can enhance at material level and component level [30]. At material level, carbon-based materials are one of the most common additives due to high thermal conductivity, stable chemical performance, and low density. However, uniformly dispersion of additives in PCMs is the most important factor to improve thermal conductivity. Moreover, aspect ratio of additives is also crucial for thermal conductivity enhancement

At material level, CNTs is selected as a heat transfer medium for the composite PCM in experiment. A group of the composite PCM is prepared with CNTs of 0.5, 1, 1.5, and 2 wt%. The thermal conductivity and latent heat are determined and compared. The experimental results show that thermal conductivity of the composite PCM with 2 wt% CNTs increased by 17 and 21% compared to pure paraffin in

To evaluate the heat storage and release capacity of pure paraffin and the composite PCM, the infrared thermograph images of two samples are recorded in Tables 3 and 4. It can be found that the heat transfer rate of the composite PCM is greatly improved both in heat release and heat storage process. Especially, the average temperature of the composite PCM at different time is 2°C higher than that of pure paraffin in heat storage process. Moreover, the temperature distribution of the composite PCM is more uniform than that of pure paraffin in heat storage

At component level, metal fins are widely used to increase the heat transfer areas and relatively easy to fabricate for building component with PCMs [31]. The main

Heat release comparison of pure paraffin and the composite PCM.


#### Table 4.

Heat storage comparison of pure paraffin and the composite PCM.

Table 5. Comparisons of optimized/nonoptimized component with PCMs. influence factors on thermal performance of PCMs component are fins number and geometry. The current research works mainly focused on the effect of fins number or geometry separately. However, these two factors should be taken into consideration. To enhance heat transfer rate, the balance between fins number and geometry should be further investigated.

In the field of PCMs heat transfer, numerical method is widely employed by architects. To obtain the appropriate geometric of fins, the building component with PCMs is optimized by COMSOL Multiphysics. The results show that PCMs complete melting time in the building component and the temperature difference of heat transfer surfaces could be used to evaluate the fins effect. The optimized building component presents the best heat transfer performance with 8 mm length, 0.2 mm width, and 5 mm spacing of fins. The temperature comparison of building component with PCMs before and after optimization is shown in Table 5. It can be seen that the heat transfer performance of the optimized component is better than non-optimized component during heat storage process. The temperature difference between top and bottom in the non-optimized component is increasing over the time. It directly gives the fact that appropriate geometry of fins makes the heat transfer much more uniform and reduce the adverse effect of natural convection in heat storage process.

installation of the PCM panel is the same as that of the north wall. However, considering that there is a constructional column in the middle of the south wall protruding into the interior space, the exterior of the PCM panel is decorated with perforated aluminum plate. These tiny holes in the aluminum plate can promote the convection heat transfer between indoor air and the PCM panel (see

(a) Design sketch, PCM panel exposed to indoor air; and (b) design sketch, PCM panel decorated with

maximum relative error of 2.95% and average relative error of 0.96%.

The indoor thermal environment before and after the application of PCMs coupled with NV is compared with experiment and simulation results. It can be observed from Figure 18(a) that the interior surface heat flux of the wall with the PCM panel is significantly greater than that without the PCM panel. It demonstrates that PCMs absorb and release a lot of heat from indoor air during the melting or solidification process. Therefore, the interior surface peak temperature of the wall with the PCM panel is greatly reduced with respect to wall without the PCM panel as shown in Figure 18(b). Accordingly, the indoor air peak temperature is greatly reduced using the PCM panel. The maximum reduction of indoor air temperature is

Actual installation effect of the PCM panel: (a) exposed to indoor air and (b) decorated with perforated

Actual installation effect of the PCM panel is shown in Figure 17. To examine the application effect of PCMs coupled with NV strategy in hot season, full-scale field thermal environment test is conducted in a typical week of hot season. Indoor air temperature, surface temperature, and heat flux of the wall with the PCM panel are carefully investigated. To obtain the cooling potential of PCMs coupled with NV, the scenario of the wall without the PCM panel is modeled using EnergyPlus with the actual weather data during the test period. The accuracy and reliability of the method is validated by comparing the simulated and experimental results with

Figure 16(b)).

Figure 16.

perforated aluminum plate.

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

up to 1.1°C (see Figure 18(c)).

Figure 17.

27

aluminum plate.
