performance of etfe in the indian conditions

L

0.10 to 0.15 L

100 to 200 microns thick film

ETFE AND ITS PERFORMANCE IN THE TROPICAL CLIMATES SMRITI MOHTA; BEM-484 Second semester; New Building Materials Masters in Building Engineering & Management 2010 School of Planning and Architecture, New Delhi

ABSTRACT

This paper focuses on the thermal performance of ETFE in building design in tropical conditions, such as India. Initially, a brief description of the ETFE material is given in order to reach a fundamental understanding of its performance.

1.0 INTRODUCTION -

ETFE (ethylene tetra flouro ethylene) is a fluorocarbon based transparent polymer, originally invented by DuPont for aeronautical insulation purposes, but has over the years gained immense popularity as a building material for architectural (tensile) membranes.

1.1 Characteristics of ETFE ETFE exhibits the following characteristics –

Light weight 95% light

transmittance Recyclable Durable Superior mechanical toughness Outstanding chemical inertness Easy processibility at relatively high rates Self cleanable Requires low maintenance Strong enough to bear 400 times its own

weight Can achieve longer spans Can be stretched to three times its length

without loss of elasticity Can be repaired by welding patches over

tears Unaffected by UV light, atmospheric

pollution and weathering Offers more green design opportunities Comes in variety of shapes and sizes

1.2 ETFE Usage in Architecture

ETFE sheets are usually used as follows-

A single layer stretched between two supports with reinforcement.

Figure 1 Two or more sheets

attached and inflated to form cushions by means of compressors for structural reasons.Such ETFE cushions can provide thermal insulation with reduced initial costs and less structural supports as compared with a conventional glazed roof. Figure 2

1.3 Characteristics of ETFE foil cushion

Maximum Size- Circular or square cushions – 7.5m x 40m Rectangular cushions- 4.5m x 40m

Weight- 2-3.5 kg/m2, less than 2% of equivalent glass cladding

Frame - Al extrusions (due to its flexibility and affordability), carbon, steel or stressed cables. Entire system including Al connections and steel frame weighs 10% - 50% of conventional glass structure

Figure 3; fixing technology of ETFE cushions

ETFE and its performance in the tropical climates; Smriti Mohta

Inflation system- Entire roof (1000 m2), single air handling unit which contains 2 fans powered by electric motors

Insulation properties U-value g-value (W/m2k)

6mm single layer glass 5.9 0.95Double layered glass 3.3 0.83

6-12-6 high performance double glazing unit

2.0 0.35

Double layered ETFE film 2.6 0.71-0.22 (with frit)

Triple layered ETFE film 1.7 0.71-0.22 (with frit)

Four layered ETFE film 1.4 0.71-0.22 (with frit)

Five layered ETFE film 1.1 0.71-0.22 (with frit)

Table 1; Insulation properties of glass and ETFE

Solar control - treated with translucent or opaque fluoro polymer ink in different ways (fritting, tinting, etc) to manipulate its light transmission properties.

Fire - foil softens, fails and shrinks away (due to hot smoke) to create natural ventilation.

Power failure- Maintain pressure for between 3 and 6 hours before deflating

1.4 Application

ETFE solutions initially found use on projects such as botanical gardens, zoological gardens, swimming pools, and exhibitions spaces but now it is also used in more traditional buildings as roofing for courtyards, shopping malls, atria and stores. The ETFE material has been used on prominent architectural projects such as the Eden Centre and the Water Cube and it is currently considered for a number of high profile international sports venues.

2.0 PERFORMANCE OF ETFE

All of ETFE’s properties point its advantages towards colder regions of the world, where

heating requirements are the main concern; whereas little research has been carried out in order to determine its performance in tropical/composite climates of countries like India. Hence it becomes difficult for designers to deliver energy performance optimized designs.ETFE is not entirely opaque to longwave radiations and cause greenhouse effect inside the building, hence allowing the high amount of heat entered inside the building to be trapped, absorbed and emitted back slowly, causing overheating. This is contrary to what is required in tropical climates.Currently available commercial software tools are not developed to take into account the long wave transmittance through the ETFE layers, therefore in practice ETFE foils are usually modeled as glazing units. Depending on the building use, the building design, the site, and geographical location of the building, this simplification may impact on the accuracy of the simulated building performance. Therefore, it becomes essential to develop methods to model this material in order to maximize performance (and minimize risk) in hot/humid/ composite conditions.

2.1 ETFE cushions in INDIA

Implementing ETFE cushions in building design in India is a complicated task due to the following reasons –

Unavailability of the specialized technology required for its installation

The unusual energy transmission characteristics of the material.


2.2 Shortwave and long wave radiation

All bodies emit and absorb energy in the form of electromagnetic radiation, which comprises heat and light. At a given temperature, the thermal radiation emitted from a surface varies for different

wavelengths, indicated by “spectral”, which depends on the characteristics and temperature of the emitting surface. In order to accurately quantify radiative heat transfer, the spectral and directional effects should also be taken into account.

Figure 4; bands of the electromagnetic radiation spectrum

2.3 Alterations to Thermal & Optical Properties of ETFE

2.3.1 General

The thermal and optical properties of the ETFE cushions can be altered significantly by application of coatings, print, geometry and the build-up in which they are applied. Energy transmission through an ETFE cushion (transmission, reflection and absorption) can be modified as follows:

Application of a reflective frit to intermediate cushion; the intermediate foils can be in an open or closed position allowing heat and daylight inside.

Figure 5; Frit in ETFE cushion

Application of coatings (low emissivity coating to reduce long wave transmission

losses i.e. solar control coating in order to reduce the solar transmittance).

Figure 6; Coatings in ETFE cushion

Usually ETFE cushions incorporate two or three air chambers. Convective heat transfer within these air chambers will influence the thermal performance of the cushion.


2.3.2 Transmission properties of ETFE & glass

Glazing is virtually opaque to long wave radiation, while ETFE transmits part of the long wave radiation as indicated in Figure 7.

The visual light transmittance of ETFE is 94-97% with ultraviolet transmittance being in the 83-88% range.


Figure 7; Comparison of the mechanism between a ‘triple glazed unit’ and a triple-layer ETFE cushion is presented

2.3.2.1 Triple Glazed Unit, exposed to solar radiation

Incident solar radiation reaches the 1st pane; part shortwave and part long wave radiation.

The shortwave radiation is transmitted, absorbed, and reflected. Absorption leads to an increase in temperature of the 1st pane. The same applies for the 2nd and 3rd pane.

The longwave radiation exchange is dominated by the incident longwave radiation from the sun. However, depending on the temperature difference between the panes and the emissivities of their surfaces, long wave radiation is exchanged between various panes and environment.

The resulting temperature of the panes depends on the long wave radiation exchange, convection between the panes and absorption (including the effect of multiple reflections).

The total solar transmittance (g-value) of the triple glazed system is the sum of the shortwave transmitted part, the net long wave radiation emitted from the 3rd pane to the indoor side, and the energy transfer by convection from the 3rd pane to the indoor side.

2.3.2.2 ETFE layers, exposed to solar radiation:

Incident solar radiation reaches the 1st layer; part shortwave and part long wave radiation

ETFE is not opaque to longwave radiation. Therefore, when solar radiation reaches the 1st layer, part of the longwave radiation is transmitted. A reduced part will be absorbed and re-emitted due to the transmission.

This impacts on the transmission of energy absorbed in layers (for instance a fritted layer) which is emitted and transmitted through other layers, as well

situations where long wave exchange occur between inside and outside across the ETFE. The significance of these effects will vary with the environmental conditions and the properties of the ETFE build-up.

By applying low emissivity coatings on one of the layers, the long wave radiative exchange and therefore the thermal transmittance of the system can be reduced. By filling the cavities with Argon or Krypton, the heat exchange due to convection between the panes can be reduced, achieving lower U-values.

2.3.3 Effect of long wave transmission on building performance

When exposed to solar radiation, during summers, the shortwave energy transmission will typically dominate, but long wave radiation exchange will potentially affect the resulting heat transfer, depending on the temperature differences between the different ETFE layers (which may include fritted and thus absorbing layers) the floor of the occupied space, the sky and any surrounding buildings.

2.3.3.1 Impact of frit on long wave transmission

The performance of ETFE cushions is influenced by the presence of a fritted intermediate layer. Fritting is done to introduce shading and reduce the transmitted solar energy into the occupied space. The solar transmission may be variable by means of multiple fritted layers, which can be inflated or deflated to vary the combined shading effect.

In winters, a frit will increase the ETFE’s opacity to longwave, hence increasing the thermal insulation


During summers, the frit will reduce the amount of shortwave radiation entering the space, by reflecting a part of the shortwave and absorbing another part, increasing the temperature of the layer. Higher the frit density, lower will be the direct shortwave penetration.

A highly absorbing frit will increase the temperature of the middle layer more than a highly reflecting frit, increasing the emitted long wave radiance (towards indoors and outdoors). On the other hand, since the frit is opaque to long wave radiation, a fritted intermediate layer would shield from transmission of long wave radiation from outdoors.

Figure 8; Impact of fritting on ETFE performance

The impact of fritting reduces the risk of overheating of a space, although the energy transfer mechanism is complicated.

2.3.3.2 Quantifying the impact of longwave transmission through an ETFE roof

For comparison, the performance of glass and ETFE systems chosen to represent a roof solution with a fritted interlayer in tropical climate, is as follows-

Table 2; Heat changes to the internal environment and the temperatures of each layer within the glazing and ETFE system.

From the above table:

The ETFE construction has a 12% increase in the longwave heat changes when compared to a glass construction.

The ETFE construction has a 2% increase in the total heat gain through the element.

In a scenario with solar radiation, the effects of longwave transmission through the ETFE are not considered significant.

For other configurations and different environmental conditions, the longwave transmission is likely to have a more significant impact on the thermal performance of ETFE building elements.

2.4 Impact of usage of ETFE in conjunction with another material; a case study

Suvarnabhumi Airport, Bangkok, Thailand

This structure was made of PTFE and glass but the principles would still apply for ETFE.


This structure uses a fabric cover to reflect sunlight and control the temperature beneath it- a three layer translucent membrane developed to mediate between the exterior and interior conditions, dealing with heat and noise transmission, while still allowing for natural daylight within the building.

Suvarnabhumi airport, Bangkok

Figure 9; Temperature depiction in the roof structure

The shape of the structure traps more heat near the roof and enables the floor and bottoms half to be cool. Hence, the building is flooded with controlled daylight in a tropical climate.

3.0 CONCLUSION

From the above study, the following conclusions can be drawn-

In comparison to a glass construction, ETFE construction leads to a 2% increase in the total heat gain in the building. Hence the building will require more cooling, which defies the purpose of ETFE’s energy efficiency. In such cases, an increase in the number of intermediate fritted layers can help. Also, the material can be successfully used in conjunction with other materials by careful designing to achieve the desired results for 1/100th of the cost of glass construction. Hence, further research is suggested with a view to evaluating in more

detail the performance of ETFE and increase confidence in its implementation during the design process.

4.0 REFERENCES

Incropera P.F., De Witt P.D., 2002. Fundamentals of heat and mass transfer (5th edition), John Wiley and Sons, ISBN 0-471-38650-2

Robinson A.L. 2005. Structural Oportunities of ETFE (ethylene tetra fluoro ethylene), Masters thesis at the Massachusetts Institute of Technology, USA

Robinson-Gayle S., Kolokotroni M., Cripps A., Tannob S., 2001. ETFE foil cushions in roofs and atria, Journal of Construction and Building Materials 15, p 323-327


performance of etfe in the indian conditions

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