Because of their lightweight and thinness, the thermal behaviour of architectural textile membranes exhibits an extreme responsiveness to variations in external conditions. For this reason, it is argued that the reliable prediction of the thermal environment experienced in a space enclosed by a tensile membrane skin construction would require a detailed modelling of the dynamic thermal behaviour of the textile construction itself.
In order to assess the effect that different membrane constructions might have on thermal conditions inside the enclosed space, the thermal behaviour of two full-scale tensile enclosures, with different ventilation strategies and textile skin constructions, is monitored during an extensive period. The monitored data are compared with the environmental control strategy suggested at their design stage. In addition to the full-scale monitoring programme, the thermal response of a double-layer membrane construction is monitored on a test rig under real weather conditions.
The observed thermal behaviour of the tensile membrane constructions serves as a basis for the design of a numerical model capable of simulating the thermal response of single and double-layer membrane skin constructions and of predicting their surface temperatures and the solar radiation directed into the space they enclose. The approach is based on a detailed modelling of the radiative and convective heat transfer processes affecting the membrane surfaces.
The accuracy of this model is validated against the large set of environmental data monitored in full-scale enclosures and on the test rig. The proposed model is intended to be externally coupled with a flow analysis package in order to predict the thermal environment experienced inside tensile membrane enclosures.
- Tensile membrane enclosures
- The thermal behaviour of textile constructions
- Monitoring the thermal behaviour of textile enclosures
- Monitoring the thermal behaviour of a double-layer membrane skin construction
- Simulating the thermal behaviour of textile skin constructions
- Conclusions and suggested future research
(Extract from thesis)
Through a continuous research effort sustained over the last three decades, the application of tensile membrane structures has gradually evolved from small-scale semi-enclosed structures to become an integral part of permanent complex buildings.
The development of high strength coated fabric membranes and sophisticated structural analysis techniques have increased the scale and durability of these enclosures to a point where they could compete with more traditional structural systems.
As membrane enclosures started to penetrate the market for permanent buildings from the 1980s, however, the environmental consequences of enclosing an architectural space in a millimetre-thick textile skin became more critical. Substantial energy savings can be achieved by exploiting the daylighting possibilities of lightweight translucent membrane skins, but designers are increasingly expected to demonstrate the viability of tensile membrane enclosures, in terms of both energy performance and thermal comfort for the building occupants. These new requirements have brought to light a scarcity of knowledge regarding the thermal behaviour of tensile membrane enclosures.
The temporary nature of early textile structures, inherent to the limited life span of their materials, provided little incentive for the detailed investigation of their internal thermal environment. However, the lack of confidence with which the thermal performance of textile membrane enclosures can be predicted has recently become the most significant barrier to their wider acceptance by the architectural community.
The lightness and translucency of tensile membrane skins mean that they are best considered as passive environmental filters that can only moderate outdoor climatic variations, to which they are prone to respond very quickly. Designers are therefore faced with the challenge of approaching the climatic design process from an unusual angle. It is not possible to apply the environmental control strategies used in the traditional construction, namely to rely on the thermal resistance of the envelope materials themselves to regulate and damp thermal exchanges between the interior and the outdoor environment.
Because of the exceptional thermal responsiveness of membrane skins to fluctuations in their environment, the thermal behaviour of spaces inside a membrane structure is directly linked to that of the textile constructions enclosing it. An appropriate approach to the prediction of the internal thermal environment would thus require a detailed description of the thermal behaviour of the tensile membrane skin itself.
In addition to the complex double-curved geometry of membrane structures and the unusual topology of the spaces they enclose, the climatic design task may be daunting. The inability to predict the environmental performance of tensile membrane enclosures and the history of high energy consumption and poor thermal comfort have fuelled the mistrust of clients, leading to their rejection as a viable option for many projects.
Recent developments in the environmental performance of architectural textiles have mainly been focussed on multi-skin constructions, in an attempt to tackle the recurrent problems of condensation and thermal discomfort. On very few occasions have these benefits been quantified in completed buildings.
Nonetheless, the introduction of new membrane materials combining tensile strength, high translucency and durability has largely increased the potential for designers to adapt the environmental properties of the textile skin almost independently of the structural requirements.
Aspects relating to the complex structural design and fabrication of membrane enclosures have been thoroughly investigated, yielding a range of comprehensive analysis techniques and specialised numerical tools. However, the tools available for the prediction of the thermal performance of membrane constructions have merely been inadequately adapted from methods used for the analysis of conventional heavyweight buildings.
It appears that, despite an increase in the options of materials and constructions available, no proven tool exists for the comparison of their relative benefits in terms of thermal performance. It is intended that this research will contribute towards the development of such a tool.
This research aims to investigate the thermal response of tensile skin constructions and the environmental consequences that this thermal behaviour may have on the enclosed architectural space. The practical objective is the development of a modelling tool that can predict the thermal behaviour of textile constructions.
Objectives of the research
This investigation is organised in five stages:
a review the existing body of knowledge relating to the thermal analysis of tensile membrane skin constructions
an assessment of the effect that different membrane constructions might have on thermal conditions inside the enclosed space, through thermal monitoring of full-scale textile enclosures and on a test rig
the implementation of a numerical tool able to accurately predict the thermal behaviour of tensile membrane skin constructions in response to variations in external conditions.
the verification of the accuracy of this tool by comparing its predictions to the thermal behaviour monitored during this study
the use of this numerical model to further investigate the physical processes governing the phenomena observed in the monitored tensile membrane constructions.
Scope of the research
Considering the breadth of the subject and the limited existing body of knowledge available, it was decided to restrict the scope of the research presented in this thesis to the following aspects:
- Only frame-supported membrane enclosures were considered. Air-supported membrane enclosures were not included in this investigation, since their detailing and internal environment demonstrates features that are specific to pressurised structures.
- This research only considered woven fabric membranes, since they constitute the largest proportion of existing membrane enclosures. High translucency foils such as ETFE were not dealt with, as their applications tend to differ significantly from those of coated woven membranes.
- This research was restricted to single and double-layer membrane skin constructions and did not encompass the application of insulating materials such as quilts and fibrous insulation.
- Emphasis was put on the climatic aspects of Northern Europe.