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ETFE is a copolymer and a relatively new building material in the construction industry. It has been significantly and increasingly being used in buildings, particularly large span buildings that require minimal support. Commonly used as a roofing material in place of traditional ones, ETFE also outperforms glass in several areas including weight, light transmittance, insulation and cost. Despite transmitting nearly 95% of light, ETFE is not as transparent as glass. Since ETFE are usually used in large spans, the amount of heat lost through the material needs to be considered. Solar radiation provides natural heat source in the building. The sun emits shortwave radiation which is then absorbed by the ETFE surface and transformed into solar energy. It then reemits the energy as longwave radiation. Study has been made and it was found that both glass and ETFE behave similarly with shortwave radiation but differently with longwave radiation. Robinson-Gayle et al. (2001) and Saltz et al. (2006) both mentioned that ETFE is almost transparent to longwave radiation while glass is opaque to it. Poirazis et al. (2010) produces a graph of spectral transmittance of glass and ETFE and concludes that for ETFE "largest part of longwave radiation transmitted, decrease reabsorption and reemission". Due to this differing behaviour for longwave transmission, ETFE cannot simply be modelled as glass when used in building calculations to avoid significant design errors. Thus in this chapter, the review of literature and previous work that had been done which are related to transmission properties of ETFE will be discussed and analysed to help progressing with computational modelling that will be done. In this chapter, only literature discussing ETFE transmittance properties, structural capabilities and heat transfer will be reviewed. Studies on heat transfer mechanism provides in depth understanding of feasible mechanism(s) occurring. Previous studies on transmittance properties will allow a prediction or a comparison of expected results and structural capabilities will allow an enhanced understanding of the behaviour of material and practicality in the building.
ETFE provides a better alternative than glass when large volume and high light levels are needed. ETFE foil has an embodied energy of 27 MJ/m2 while 6m float glass has 200 MJ/m2. Brauer (1999) and Robison-Gayle (2003) states that the lower embodied energy of ETFE opened up its possibility as a replacement for glass which in return could also reduce the cost of transparent structure and enhances flexibility in building geometry. Other materials such as thermoplastics polymers (Plexiglas and polystyrene) and fluorocarbons (including PTFE and PE) were examined and found unsuitable as a replacement for glass as they were lacking in several areas (visual, transmittance and performance).
ETFE foil is typically between 100 Âµm - 200 Âµm thick with an average U-Value of 2.6 Wm2/K which is almost equivalent to double glazing. Larger ETFE cushion are able to reach a U-Value of 2.1 Wm2/K. It transmits 94 - 97% of visible light which is 5 - 8% higher than 6mm single glass which is also agreed upon by Leslie A. Robinson (2004) and added that ETFE transmits 83 -88% of UV light. The main mechanism of heat transfer is radiation although other mechanisms do occur. In ETFE cushions, the air restricts convection as it acts as an insulator and this reduces the U-Value. Leslie A. Robinson (2004) also mentioned that refraction occurs on the curved proportion of the cushion surface. Robinson-Gayle et al. (2003) state that when ETFE is used horizontally, there is a larger area for convection to occur hence increasing the U-Value. This statement will be modelled using IES-ve to analyse the feasibility of ETFE to be used as cladding material.
Total energy performance of ETFE is greatly influenced by heat transfer through conduction, convection and radiation while transmission of longwave radiation is dependent on the temperature of panels and surroundings, orientation in relation to change in exterior and interior temperature. Ghoshdastidar (2004) defines radiation as heat transmitted in the form of electromagnetic waves through a bounding medium in which Jones (2000) and Poirazis (2010) defines them as vacuum, gases or transparent materials. The 4th power of the body temperature of the bounding medium is directly proportional to radiative flux. Modest (2003) mentioned that the radiative properties is also directly dependent upon direction, wavelength (m) and temperature (degrees Kelvin).
Although the modelling of ETFE cushions is not easy, Saltz et al. (2006) found that the U-Value of 3 layers ETFE cushion is approximately equal to 6-12-6 High Performance Double Glazing while Leslie A. Robinson (2004) found that 3 layers ETFE cushion has U-Value: 1.96 W/m2K which lower than a triple glazed glass. For multilayer membrane structure it was found that 2 layers of ETFE cushions has a U-Value of 2.49 W/m2K while 5 layers of ETFE cushions has a U-Value of 1.8 W/m2K. This shows that increasing the number of layers reduces the U-value.
A comparative study was done by Poirazis et al. (2006) comparing space with glazed roof for winter night and summer day scenario. From winter night study, it has been found that several factors contribute to the resultant heat transfer through ETFE membrane. These factors include change in temperature, temperature of ETFE layers and longwave transmission properties. During summer day, the resultant heat transfer is mainly affected by longwave radiation despite dominated by shortwave. From the summer study, Poirazis et al. (2006) concludes that "ETFE gain more heat and longwave radiation flux compared with glass" although "longwave transmission not as significant as solar radiation". These studies opened up the possibility of new studies under different conditions and situations.
In a field test and computational modelling done by Antretter (2008) it was revealed that there is an uneven distribution of heat in the interior of membrane cushions. There was a 30 degrees Celcius difference in temperature where 30% was due to convection and 70% was due to radiation. From this, Atretter also concludes that the distribution of heat is not dependent on the inclination of membrane but rather the temperature difference. In a comparative study by Poirazis (2010) there was an increase in 12% of heat flux due to longwave radiation which is considered to be insignificant compared to heat loss during night time.
Dimitridou et al. (2012) carried out an experiment by placing two identical boxes on roof. One was covered in ETFE sheet and the other was in glass. Different types of ETFE sheet was used including clear, clear fritted, matt and white fritted. The longwave radiation is a significant indication to show the existence or the lack of clouds above the cladding material. Zhang (1996) mentioned that there is more heat lost during clear sky hence cloud cover is very important especially in winter.
Outcome From Previous Studies
Poirazis et al. (2006) identified several problems during their research. Even though a spectral transmission graph was produced, the extent to which the graph is accurate is still unconfirmed. It was "difficult to obtain information on physical properties of ETFE for longwave spectrum" and "precise knowledge of the ETFE spectral behaviour is essential for increasing the confidence in predictions for impact of longwave radiation on building performance". It was observed that glass also emits longwave radiation but this depends on surface emissitivity and temperature change. More information on the material properties of ETFE particularly on energy transfer characteristics was also identified as a need. Currently it is difficult to accurately assess ETFE system in terms of how it influences the energy performance of buildings as there is still no certainty on the physical properties of ETFE under longwave radiation and simulation software does not usually consider the spectral properties of ETFE (scarcity of information available). Further research was suggested in order to give a more detailed evaluation of ETFE performance and to develop more confidence when implemented in the design process. In their work, it was suggested that longwave radiation behaviour is analyse using other available building software particularly during cold night.
The performance of ETFE cushions in terms of thermally and optically is also greatly influenced by frits as mentioned by Poirazis et al (2006). Increasing frits encourages shading and this will cause a decrease in shortwave penetration and reduces the amount of solar transmission. Other factors affecting its performance are by applying low-e coating (Robinson-Gayle (2003) 2003, Leslie A. Robinson 2004), adding prints and geometry. According to Robinson-Gayle (2003) low e-coatings reduce the emissivity and the thermal flow of heat through the glazing. Leslie A. Robinson (2004) proposed an innovative solution to reduce solar gain by actively controlling "differentially pressurised air chambers" in an ETFE cushion.
Poirazis et al. (2009, IBPSA Paper) states that current commercial software needs to be developed more in order to allow the modelling of longwave transmittance. Thermal radiation ranges from 0.1 Âµm to 100Âµm in the electromagnetic radiation range and gives out energy in the form of heat and light while Jones (2000) mentioned that thermal radiation wavelength ranges from 300 - 50 000nm. Poirazis et al. (2009) also mentioned that thermal insulation is related to longwave heat transfer as increasing thermal insulation will reduces amount of heat transfer due to longwave radiation during cold night. Shortwave radiation decreases during warm day.
In Dimitridou et al. (2012)'s experiment, both materials absorb and trap shortwave solar radiation which was identified by the increase in operative temperature while the internal conditions was monitored and when it reached cooler temperature during the night, it was proven that ETFE is transparent to longwave radiation. There was a significant loss in heat when there was no solar radiation and clear ETFE was found to be the most unsuccessful type of ETFE sheet used when compared with glass as it failed to provide the desirable comfort condition under clear sky condition for both heating and cooling. During overcast sky and relatively high eternal air temperature, both materials perform steadily and satisfactorily with the operative temperature inside ETFE box reaching the desirable comfort condition (20 degrees Celcius). ETFE was proven to be more successful than glass under overcast sky conditions despite consuming more energy. When used as a single layer, ETFE consumed more 18% more energy than glass but in practice ETFE is rarely used as single sheet. Glass also absorbed and retained more heat than ETFE.
Dimitridou et al. (2012) concludes from their experiment that energy consumption is dependent on external air temperature and longwave radiation where low external temperature increases energy consumption and decreases longwave radiation in the absence of clouds. They also suggested a comparison study between double glazed and inflated ETFE cushions on larger scale. Dimitriadou et al. (2012) also mentioned about the lacking published data regarding thermal behaviour. They did an experimental testing using scale models in the aims to study the behaviour of thermal performance of ETFE. According to Poirazis et al. (2010), when the largest part of longwave radiation was transmitted there is a decrease in both reabsorption and reemission.
Hamasaki et al. (n.d) states that thermal radiation is dependent on temperature and gave the longwave transmittance value of ETFE film to be 0.45 which lies between PE and PVC.
Using metal framing system has been found to also increase the U-Value of the façade. Alternative suggestion is to use uPVC or wood as framing material. ETFE has an effective emissivity of 0.89 and to reduce this value, new framing system needs to be designed and applying shading inside the cavity. This will reduce thermal and solar transmission as well as unwanted solar gain. In a research article produced by Marucci et al. (2012), it was mentioned that "longwave infrared radiation energy losses from protected environments depend on the tranmissitivity of the covering material in the wavelength value less than 3000 nm" and that energy losses increases as emissivity increases.
From the various previous studies that had been done, it enables a creation of another computational modelling that will compare and contrast the previous findings in several ways particularly in areas that were uncertain (graph of spectral transmission of glass and ETFE). Data form this review of literature will aid the computational modelling to predict the results.