Energy Performance of Curtain Wall

1. Introduction

Building envelope is used to separate the conditioned and unconditioned environment, the material used in envelop will affect the indoor environment of the building. For a commercial building, the electricity consumption of HVAC system and lighting system is related to the features of the curtain wall system, such as U-value, shading coefficient and visible transmittance. The energy performance of each curtain wall system will be studied in this chapter and it can separate into three sections: 1) U-value and Energy Consumption of HVAC system, 2) Shading Coefficient and Energy Consumption of HVAC system and 3) Visible Transmittance and Energy Consumption of Lighting system.

2. U-value and Energy Consumption of HVAC system

According to Bougdah (2009), U-value, a thermal transmittance coefficient, is used to measure the insulation property of a building element. As thermal insulation of curtain wall may affect the energy consumption of the building, the relationship between U-Value and energy consumption of HVAC systems is studied in order to analyze the energy performance of various curtain wall system.

Based on table 6.2.1, the relationship between U-value and energy consumption of HVAC systems can be shown. The U-value of curtain wall system is directly proportional to the annual energy consumption of HVAC system. With higher values of U-value, the energy performance of the curtain wall system becomes poorer and it requires more electricity as the heat can more easily transfer from outdoor to indoor. On the contrary, the lower in U-value, curtain wall system can perform better energy consumption in HVAC system.

Table 6.2.1 U-value and Energy Performance of HVAC System of Targeted Curtain Wall

Figure 6.2.1 Relationship between U-Value and Energy Consumption of HVAC System

Figure 6.2.1 demonstrates the directly proportional relationship between U-value and energy consumption of HVAC system obviously. With the saw-tooth pattern of the line chart, there are some types of curtain wall system which have three layers of glazing require more energy than two layers one. It means that there are other factors, such as shading coefficient, which may also affect the energy performance of the HVAC system. The relationship between shading coefficient and annual energy consumption of HVAC system will be discussed in the next section.

3. Shading Coefficient and Energy Consumption of HVAC system

According to Ander (2014), the shading coefficient of glass is a measure to determine the solar heat gain through the glazing. Radiation is a method to transfer heat from the outdoor environment to the indoor environment. For commercial buildings with curtain wall system, shading coefficient becomes another major factor that affects the amount of heat transferred to the building. The external envelope of the building is glazing which has lower insulation for heat transmission through radiation. Since the shading coefficient will affect the energy consumption, the relationship of shading coefficient and energy consumption of HVAC system is studied.

Based on the table 6.3.1 below, the shading coefficient of curtain wall is directly proportion to the energy consumption of HVAC system. It means that the amount of annual energy consumption of HVAC system becomes higher with a relatively high shading coefficient of curtain wall system. With the lower of the shading coefficient, the lower the energy consumption of HVAC system, less heat transfer from external environment to internal environment. The major factor affected the shading coefficient of curtain wall system is the type of glazing. In this study, clear glasses and tinted glasses are used in various types of curtain wall. Tinted glass is a common glazing that used to construct the curtain wall system and Pfrommer (1995) stated that the tinting can produce various colours, such as green, bronzer and tinted glass can absorb solar radiation and reduce those heat and light transmission. Tinted glass can perform a better energy saving performance compared with clear glass which do not contain any chemicals to absorb radiation. However, the price of curtain wall system with tinted glass is more expensive than clear glass. Besides, the number of layers of curtain wall system is another reason affected the shading coefficient of that curtain wall system. It is easy to understand that the radiation transmission becomes lower with a higher number of layers of curtain wall system. Therefore, triple glazed curtain wall with tinted glass has the lowest annual energy consumption while single glazed curtain wall with clear glass has the highest annual energy consumption.

Table 6.3.1 Shading Coefficient and Energy Performance of HVAC System of Targeted Curtain Wall

Figure 6.2.1 Relationship between shading coefficient and Energy Consumption of HVAC System

From the figure 6.2.1 above, a straight proportional relationship between shading coefficient and annual energy consumption of HVAC system can be identified. Type I triple glazed curtain wall system with spectrally low-e tinted glass which has the lowest shading coefficient has the best performance in annual energy consumption of HVAC system. However, Type A single glazed curtain wall system with clear glass which has the highest shading coefficient has the poorest performance in annual energy consumption of HVAC system.

4. Visible Transmittance and Energy Consumption of Lighting System

Apart from the attractive appearance, a high portion of natural lighting is another characteristic of curtain wall system. Unlike other external wall construction, curtain wall systems are constructed by glazing elements and more daylighting can pass through to the indoor environment of the building. Efficient Windows Collaborative (2014) stated that visible transmittance is the amount of light in the visible portion of the light spectrum that passes through glasses and it will be influenced by the glazing type, the number of panels, and any glass coatings.

Based on the table 6.4.1 and figure 6.4.1, lighting transmittance is inversely proportional to the annual energy consumption of the lighting system. With a higher value of lighting transmittance, the amount of annual energy consumption of lighting system becomes lower. It means that more visible light transmitted from the external environment to internal environment and the target building can save more energy in lighting system. However, more energy is used in lighting system in case the value of lighting transmittance is in a small value. In this study, two factors affect the lighting transmittance of curtain wall can be identified. The first reason is the number of layers. More layers can increase the thickness of curtain wall, thus less visible daylighting can pass through curtain wall. This is easy to understand that triple glazed curtain wall system blocks more lighting than single glazed curtain wall and double glazed curtain wall system. The second reason is the type of glazing. From table 6.4.1, clear glasses always have higher lighting transmittance than tinted glasses. As mentioned above, tinted glasses can reduce light transmission. Although it performs good energy saving in HVAC system, it consumes more energy in lighting system.

Table 6.4.1 Visible Transmittance and Energy Performance of Lighting System of Targeted Curtain Wall

Figure 6.4.1 Relationship between Visible Transmittance and Annual Energy Consumption of Lighting System

From the above figure 6.4.1, the inversely proportional relationship between visible transmittance and annual energy consumption of lighting system is shown. Type I triple glazed curtain wall system with spectrally low-e tinted glass which has the lowest visible transmittance has the poorest performance in annual energy consumption of lighting system. Nevertheless, Type A single glazed curtain wall system with clear glass which has the highest visible transmittance has the best performance in annual energy consumption of lighting system.

5. Overall Performance in Energy Performance

After assessment, the performance of each curtain wall system can be assessed. Using the ranking as the score to calculate the performance of curtain wall, curtain wall system with a higher performance score means they perform better in energy consumption of Building A. The performance ranking is converted to score from 1 (the poorest performer) to 10 (the best performer). As around 70% of electricity is consumed by HVAC system normally, the over score is equal to “HVAC System Score x 70% + Lighting System Score x 30%”. Based on the following table, Type J obtains the highest score in this study, whereas type B obtain the lowest score in this study.

Table 6.5.1 Score of Targeted Curtain Wall System

1. Type A Curtain Wall System

Type A curtain wall is constructed by single glazed with clear glass. As there is only one layer of glass, the U-value of curtain wall is the lowest. Clear glass used in Type A cannot absorb radiation but it can provide more visible lighting to Building A. From the above reason, Type A obtains the highest score in lighting system and the lowest score in HVAC system.

2. Type B Curtain Wall System

Single glazed with tinted glass curtain wall system is similar to Type A curtain wall system. Nevertheless, tinted glass is used in this curtain wall system. Type B curtain wall system has a relatively low shading coefficient and visible transmittance compared with Type A curtain wall system.

3. Type C Curtain Wall System

Type C curtain wall system is double glazed with low-e tinted glass. The low-e coating can minimize the amount of ultraviolet and infrared light that pass through glass. As there is air space between two glasses, the U-value of curtain wall is lower than curtain wall system with one layer. And less heat can be transferred from the external environment to the internal environment since tinted glasses can reduce the heat transmittance through radiation.

4. Type D Curtain Wall System

Type D curtain wall system is double glazed with low-e clear glass. Compared with Type C, it has better performance in annual energy consumption of lighting system as it has a higher value in visible transmittance. However, clear glass has a poor performance in thermal insulation in which the amount of heat transmitted through radiation is higher than Type C.

5. Type E Curtain Wall System

The emissivity level of Type E curtain wall system is lower than 0.1. With a spectrally low-e value, U-value and shading coefficient of the curtain wall system will be reduced and the energy consumption of HVAC system also can be reduced. However, the visible transmittance will be reduced because of the low-e coating.

6. Type F Curtain Wall System

Like Type E curtain wall system, it performs a better performance than Type D in saving energy of HVAC system because of spectrally low-e coating. On the contrary, the annual energy consumption of Type F curtain wall system in lighting system is higher than Type D which has the same number of layers as Type E curtain wall system

7. Type G Curtain Wall System

Type G curtain wall system is constructed by three layers of tinted glass and its U-value is higher than previous curtain wall systems. It has a better thermal insulation compared to one layer and two layers because of increasing in thickness and number of air space. It ranks no.3 in this assessment because of its low value of shading coefficient and U-value.

8. Type H Curtain Wall System

Triple glazed with low-e clear glass has same U-value as Type G. Nevertheless, the visible transmittance of Type H curtain wall system is higher than Type G since Type H curtain wall uses clear glasses which cannot absorb radiation. As mentioned before, the thermal insulation of clear glass is relatively lower than tinted glass used in Type G.

9. Type I Curtain Wall System

In this assessment, Type I has the highest score. The emissivity of Type I curtain wall system is lower than 0.1. The U-Value and shading coefficient of Type I are lower than Type G system and it has the best performance in annual energy consumption of HVAC system.

10. Type J Curtain Wall System

Triple glazed with spectrally low-e clear glass has a better performance than Type I in the energy consumption lighting system. However, more heat can be transferred to indoor as shading coefficient of clear glasses is higher than Type I. It still ranks no.2 in energy consumption because of three layer glass and spectrally low-e coating of curtain wall.

In figure 6.5.1, U-value and shading coefficient are direct proportion to the annual energy consumption of HVAC system and it means that both of them may affect the energy consumption directly. For energy consumption of lighting system, visible transmittance can influence the curtain wall performance.

However, an interesting results is identified. The influence of shading coefficient is more obvious compared to U-value. Using Type B and Type D curtain wall system as an example, although Type B and Type D have similar amounts of annual energy consumption in lighting system, the U-value of type B is much higher than type D. It shows that the shading coefficient of curtain wall system is in the first priority during making decision on the types of curtain wall system.

Figure 6.5.1 Overall Performance of targeted building in HVAC System