On System Performance Of The PV-ETFE Cushion Roof

ETFE(ethylene tetrafluoroethylene) cushion structure with the capability of lightweight, high-transmittance and self-clean is one of the most important branches of the membrane structures. The ETFE cushion structure is generally used for stadiums and airport terminals where architectural aesthetics, thermal physics and structural behavior are indispensable. It has some essential characteristics in terms of building materials and structure types. In detail, the light transmittance of ETFE foil and air impermeability of ETFE cushion could lead to high temperature inside the cushion and standard prefabricated modular ETFE cushion could meet demands of complex spatical structures. In fact, these characteristics are consistent with high temperature performance of flexible amorphous silicon PV, resulting in membrane structures integrated photovoltaic. Therefore, the ETFE cushion roof integrated PV with less energy demand due to photovoltaic and photothermal effects could be near zero-energy, suatainable and environment-friendly buildings. Based on these considerations, a prototype composed of three-layer ETFE cushion roof and the PV(named as the PV-ETFE cushion roof) is proposed which could utilize photovoltaic electricity, collect thermal energy, avoid effects of external environment on the PV and reduce effect of high PV temperature on structural ETFE foils. As this PV-ETFE cushion roof could have some new characteristics due to the PV existence, attentions have been paid to these aspects in this thesis and useful conclusions for scientific research and engineering applications are drawn.Firstly, uniaxial tensile experiments are carried out for ETFE foils under a wide range of temperatures and loading speeds. A new method based on mathematical fitting is proposed for determination of yield point and elastic modulus. The feasibility and application of the method is validated by analysis results and comparison with corresponding results. On the basis of the phenomenological approach and small strain, the stress-strain relation of the ETFE foils is obtained under the condition of the Clausius-Duhem inequality. This constitutive equation is solved by a two-step method integrated in a newly-developed program. Unlike the uniaxial tensile mechanical properties, uniaxial cyclic tensile mechanical properties of ETFE foils are essential for estimating cyclic structural behavior of ETFE cushion. In this thesis, 8 groups with different loading stress amplitudes are carried out and corresponding programs for determination of mechanical constants are developed. The elastic modulus, yield stress, yield strain, ratcheting strain and hysteresis loop area evolving with loading number and the approximate stable values are obtained, which reveals the mechanism of viscoelastoplasticity.To investigate system performance of PV-ETFE cushion roof, a prototype composed of a three-layer ETFE cushion roof and flexible amorphous silicon PV and a corresponding complex measure system are designed and constructed. Typical winter and summer experiments are performed to acquire the electrical, thermal and structural parameters. The analysis focuses on the photovoltaic, thermal and pressure performance of the PV-ETFE cushion roof. The field experiments and corresponding analysis have validated the feasibility of the PV-ETFE cushion roof system, which reveals the essential technical characteristics.PV temperature distribution is indispensable to calculate temperature distribution of cushion roofs, which is fundamental for investigating building thermal physics and structural analysis concerning temperature. Therefore, a PV dynamic model is derived based on the energy balance method. The experimental results provide useful data for field analysis of the PV-ETFE cushion in terms of stable temperature boundary conditions and incompressible assumption. In detail, a two-dimensional model is established by considering the PV temperature and complex boundary conditions. Comparisons with the experimental results show that the numerical results are reasonable. Additionally, the heat transfer coefficients of the top and bottom layers of the three-layer ETFE cushion roof are obtained according to a recommended equation. It is concluded that the surface heat transfer coefficients of the PVETFE cushion roof are better than those of conventional single ETFE structures.Finally, structural behavior of PV-ETFE cushion roof needs to consider nonlinear constitutive model of ETFE foil under different temperature conditions and numerical temperature of ETFE cushion surfaces. In detail, the structural stress, strain and deformation are performed under the pressure of 250 Pa and pre-stress of 1.0MPa. To evaluate the variation within the parameter ranges, i.e. pressure of 250Pa-450 Pa and pre-stress of 1.0MPa-3.0MPa, a series of 25 cases are carried out with the help of software ABAQUS. These results quantify the structural performance of the PV-ETFE cushion roof. Furthermore, the safety factors and serviceability performance have verified the feasibility of the PV-ETFE cushion roof and revealed the significant structural behaviors.