Numerical and experimental analysis of heat transfer and airflow in double skin facades with integrated amorphous silicon PV cells

Abstract

A double skin facade (DSF) is a strategy utilized for improving building performance through using an inner and outer wall on a building with a cavity created between two walls. There is an increasing growing tendency for architects and engineers to use double skin facade in the building construction. If photovoltaic modules or solar cells are integrated with a DSF, this facade becomes a building-integrated photovoltaic (BIPV) facade. This thesis presents a numerical and experimental analysis of heat transfer and airflow in DSF system with integration of a-Si PV cells at the Hong Kong Polytechnic University, which generates both electricity and providing day lighting. A 2D numerical model is developed to explore the thermal behaviour and fluid dynamics in the air cavity of the DSF system for both open and closed inlet and outlet operational modes. The combined radiation and convection heat transfer in the air cavity are analyzed in detail in this thesis. Steady natural convective airflow in a novel type glazing system with integrated semi-transparent photovoltaic (PV) cells has been analyzed numerically using a stream function vorticity formulation. Based on the resulting numerical predictions, the effects of Rayleigh numbers on airflow patterns and local heat transfer coefficients on vertical glazing surfaces are investigated for Rayleigh numbers in the range of 10³ ≤ Ra ≤2 × 10⁵. Good agreement for the Nusselt numbers was observed between numerical simulation results in this thesis and those of earlier experimental and theoretical results available from the literature. In addition, the effect of air gap thickness in the cavity on the heat transfer through the cavity is evaluated. The effect of the thickness of the air layer between two glass panes on the heat flux through the PV window is investigated for different temperature differences (Tout -Tin). In Hong Kong, the outdoor design temperatures for A/C systems are 32°C for summer and 10°C for winter. The indoor air temperature in a typical office in Hong Kong is supposed to be maintained, for energy savings, at 25°C in summer and 22°C in winter. The effect of the thickness of the air layer in relation to two temperature differences, i.e. 12°C and 7°C has been considered. It is found that heat transfer through the window can be considerably reduced by optimizing the thickness of the air layer. The overall heat transfer is decreasing slightly when the thickness of the air layer is up to 60 mm. Therefore, the optimum thickness of the air layer could be chosen as 60 mm - 80 mm when energy saving due to less energy transport through the window is considered. Various turbulence models (RNG K-ε model, Standard K-ε model, Realizable K-ε model) were employed and velocity and temperature profiles predicted were compared and discussed in this thesis. A test rig was developed and calibrated for this study. An experimental study was carried out in the lab of the Department of the Building Services Engineering, and predicted results were then compared with experimental data to evaluate the numerical simulation accuracy. Experimental measurements taken in the full scale indoor test facility are in good agreement with predicted results. The experimental results indicated that the inside air temperature for PV DSF system is quite lower than temperature in conventional facade with internal curtain for shading purposes. It is found that the temperature in the PV facade system is more stable. The temperature variation in conventional facade system with internal curtain is larger. There are 4 temperature peak have been observed during the measurement. The maximum temperature for conventional facade is close to 34°C at 13:10 PM, 29°C for PV facade. This indicated that the effectiveness of the solar screening through the use of the naturally ventilation of air beneath the PV module. The temperature deviation between the two systems becomes smaller late in the afternoon. The results show that heat transfer and airflow in the cavity is a complex problem, which could influence the thermal performance of the PV DSF system. The results also show this novel glazing system type developed in the current thesis could not only generate electricity but also achieve potential energy savings by reducing the air conditioning cooling load when applied in subtropical climatic conditions and simultaneously provide visual comfort for occupants in the indoor environment.