Appendix to chapter 1
2. Rebutting King’s argument
2.2 King’s argument
In order to see the impact of rain mesh on the internal air temperature of test-rigs, rain mesh was placed on top of two-layer and three-layers ETFE-foil panels. Figure 5-22, Figure 5-23 and Figure 5-24 illustrated relative internal air temperature, air-volume air temperature, mean radiant temperature of Test-rig 1 and Test-rig 2, surface temperature (both external and internal), incident and transmitted solar radiation recorded during 6th to 11th September, 19th to 24th September, 26th September to 2nd October 2015 respectively. Figure 5-22, Figure 5-23 and Figure 5-24 also showed the impact of mesh on test-rigs internal thermal environment as indicated. The configuration of ETFE-foil panel of Test-rig 1 and Test-rig 2 can be found from Table 5-7.
On 6th September rain mesh was placed on top of Test-rig 2 till 1.45pm, after that time it was removed and placed on top of Test-rig 1 at 2.00pm. It can be noticed from Figure 5-22 that from morning till noon (1.00pm) transmitted solar radiation through Test-rig 1 two-layer foil panel (with 75% fritted top layer) and Test-rig 2 three-layer ETFE-foil panel (with 25% fritted top layer) was 36.9% and 32.5% of that of the incident solar radiation respectively. Even though ETFE-foil panel of Test-rig 2 comprised with 25%
fritted three-layer system but due to the rain mesh, it transmitted 4.5% less solar radiation than Test-rig 1. It was also noticeable that during that period, on average Test-rig 2 air temperature was 1.5°C lower than Test-rig 1.
It can be noticed that during 6th September 2015, at night and in the morning (till 8.00am), the air temperature of Test-rig 2 was up to 2.7°C above Test-rig 1 air temperature. However, after 8.00am in the morning, solar radiation increased in each hour, causing an increase in internal air temperature of both test-rigs. Test-rig 1 air temperatures were above Test-rig 2 air temperatures until mesh was placed on it, at 1.45pm. During this period (between 8.00am and 1.45pm) the air temperature of Test-rig 1 went a maximum of 7°C above Test-Test-rig 2 air temperature. After the mesh was removed and placed on Test-rig 1, the average transmission of solar radiation (between 2.00pm and 7.00pm) of Test-rig 2 ETFE-foil panel increased by 6% than that of Test-rig 1 ETFE-foil panel. This also had an impact on the recorded internal air
5-146 temperatures and Test-rig 2 air temperature on average rose 5°C above Test-rig 1 air temperature in between 2.00 pm and 7.00pm.
Therefore, 75% fritted two-layer panel system with mesh performed better with 5°C less temperature than three-layer 25% fritted ETFE-foil panel system. It can be assumed that mesh reduced air temperature of the test-rigs by creating shading.
Mesh surface temperature was on average 4°C lower than outdoor air temperature during the day time. Whereas at night, its temperatures stayed relatively close to outdoor air temperatures similar to the ETFE-foil and test-rig’s internal air temperatures. Mesh surface temperature increased up to 5°C above the external surface temperature of ETFE-foil during the night and early morning. Moreover, between noon and late afternoon, this external surface temperature of ETFE-foil went to a maximum of 18°C above mesh surface temperature. The temperature difference between mesh surface and external ETFE foil surface was also identified in the Nottingham High School (see Figure 5-1) roof. This elevated temperature of ETFE-foil surface may be due to the influence of the outdoor temperature, heated air-volume air temperature of ETFE-foil panel adjacent to it. However, it was necessary to identify the impact of rain mesh individually on thermal environment of test rigs.
An investigation was carried out between 12th September and 14th September 2015 to determine the impact of rain mesh on the test-rig’s internal thermal environment.
During the experiment, Test-rig 1 was roofed over by two-layer transparent ETFE-foil panel (200μm top layer and bottom layer of 100 μm), also covered with mesh. While Test-rig 2 was roofed over by the similar configuration of ETFE-foil panel as Test-rig 1, but no mesh was placed on it. Relative air temperatures, surface temperatures and air-volume air temperatures of Test-rig 1 and Test-rig 2 are presented in Figure 5-32.
Comparison of thermal environment of two test-rigs showed no variation between measured surface temperatures, air temperatures of Test-rig 1 and Test-rig 2 at night and early morning. But particularly between 5.00am to 8.00pm, ETFE-foil panel of Test-rig 1 with mesh transmitted on average 12% less solar radiation than that of ETFE-foil panel of Test-rig 2. During this time, internal air temperature of Test-rig 2 went 2°C above Test-rig 1 air temperature, while the internal surface temperature of ETFE-foil panel of Test- rig 1 was 4°C less than same of Test-rig 2.
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Figure 5-32: Relative condition of surface temperature, air volume air temperature and air temperature of Test-rig 1 and Test-rig 2
5-148 Data recorded during 13th September 15 is presented in Figure 5-33, here grey and orange coloured boxes indicated the range of air temperature of test-rigs, air-volume air temperatures and surface temperatures recorded in Test-rig 1 and Test-rig 2 respectively. From Figure 5-33 it is also evident that presence of mesh reduced the range of air temperature and surface temperature in Test-rig 1 while comparing with Test-rig 2.
Figure 5-33: Comparison of Test-rig 1 and Test-rig 2 condition 13th Sep 15
From the study, it was apparent that incident solar radiation and surface temperatures of ETFE-foil panels (both external and internal) influenced the thermal environment of the test-rigs. This phenomenon is typical in a membrane enclosure which was also identified by Harvie (2015). Although air temperature ranges in each test-rig was significantly higher because of the air tightness, when comparison was made between air temperatures, air volume temperatures and surface temperatures of the ETFE-foil panels, it was evident that application of rain mesh reduced air-volume air temperatures, internal air temperatures of the test-rig and internal surface temperatures of the ETFE-foil panel. Therefore, it can be stated that rain mesh impacts the thermal environment of the test-rig by reducing solar gain and by creating shading, which also reduces surface temperatures of the ETFE-foil panel and air temperatures of the test-rig.
5-149 5.6 Summary
The fundamental purposes of this chapter were to analyse the thermal performance of single, two- and three-layer ETFE-foil panels. This was done in two phases-
• First phase: Measured the solar transmittance of single, two- and three-layer ETFE-foil panels through an in-situ method and compared results with values calculated according to BS EN410:2011.
• Second phase: Analysed thermal performance of single, two- and three-layer ETFE-foil panels and also evaluated the impact of fritting and rain-mesh on this performance.
In the first phase of the test-rig experiment solar transmittance of single, two- and three -layer ETFE-foil panels were determined following the similar method stated in ASTM (1986b) with slight alterations. Measured solar transmittance was validated with the results obtained in the laboratory experiment (Chapter 4) and calculated according to BS EN410:2011 (BSI, 2011). The results were in close agreement with 88% to 100%
accuracy.
The second phase of the experiment analysed the thermal environment of the rigs, and the thermal state of the ETFE-foil panels that were used to enclose the test-rigs. The results showed that the thermal environment of the test-rigs and thermal state of ETFE-foils changed rapidly as a result of the variations in the outdoor environment.
Comparison of the thermal environment enclosed with two-layer (75% fritted top layer) and three -layer (25% fritted top layer) ETFE-foil panels showed that air temperature of the test-rig enclosed with three-layer ETFE-foil panel was on average 5°C hotter than the test-rig when enclosed with two-layer ETFE-foil panel. This was because the extra layer of air and ETFE-foil increased the insulation properties of the test-rig enclosure, thus reducing heat transmission from the test rig’s internal environment towards the exterior. Hence in three-layer ETFE-foil panel systems heat was better preserved than in the two-layer ETFE-foil panel system. Therefore, increase in layer number of ETFE-foils in the panel increased insulation properties of the space it enclosed. Besides, under the same outdoor condition the air temperature of enclosed test-rig with three-layer ETFE-foil panel with 25% fritted top layer, was on average 1.4°C warmer than the test-rig, when enclosed with a three-layer panel with 75% fritted top layer. Thus, the density of fritting also influenced thermal environment enclosed with ETFE foil panel by reducing solar gain.
5-150 In ETFE-foil panels, air temperatures of the air-volume enclosed with ETFE-foil also varied depending on fritting and number of foil layers in the panel. Moreover, air-volume air temperatures were influenced by adjacent ETFE-foil surface temperatures.
In turn, both ETFE foil surface temperatures and air-volume air temperatures of the ETFE-foil panels affected air temperatures of the test-rigs they enclosed.
This variation of temperature in ETFE-foil surfaces and air temperatures of air-volume and test-rigs occurred by convective and radiative heat transfer mechanisms.
Convective heat flux depends on temperature difference, therefore heat was transferred between each layer of ETFE-foil panel and air adjacent to it by natural convection. Besides radiative heat transfer occurred on the ETFE-foil surfaces, and was dominated by short-wave radiation during the day and longwave radiation during the night. However, radiative heat transfer depends on the emissivity of materials.
Therefore, in two and three-layer ETFE-foil panels with fritted top layer and transparent middle and bottom layers, radiative heat transfer was more pronounced on transparent layers than fritted foil surfaces. Besides particularly at night and early morning, stable conditions occurred as external surface temperature increased over the internal surface, radiative and convective heat transfer was constant for all the surfaces, and increased as the temperature difference increased between the surface of ETFE-foil and adjacent air layers during the day.
In this experiment impact of rain-mesh on test-rigs thermal environment was evident.
The shading created by rain suppression mesh reduced surface temperatures as well as air temperatures inside the test-rigs. The experimental investigation also found that presence of mesh reduced solar radiation transmission by 12%. Air temperature of the test-rig and internal surface temperature of ETFE-foil also reduced by 2°C and 4°C respectively.
In this study the test-rigs’ thermal environment were evaluated based on its air temperatures and surface temperatures of ETFE-foils. This focused on the actual thermal performance of ETFE-foil but it is necessary to compare this thermal performance with that of glass, which is typically used in construction for similar purposes. In Chapter 7, a simulation model replicating Test-rig 1 and Test-rig 2 was developed using EDSL TAS version 9.3.3. The results obtained from this in-situ experiment was then used to verify the simulation model. After verification, this simulation model was used to simulate different types of glass panels, particularly used as overhead glazing. Thermal performance of single, two- and three-layer ETFE-foil panels was then compared with that of similar glass panels.
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