Thermal behavior of a novel type see-through glazing system with integrated PV cells Jun Han * , Lin Lu, Hongxing Yang Renewable Energy Research Group, Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China article info Article history: Received 4 November 2008 Received in revised form 5 March 2009 Accepted 5 March 2009 Keywords: Convective heat transfer Fluid flow Glazing system Semi-transparent photovoltaic (PV) Building integration abstract 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 were investigated for Rayleigh numbers in the range of 10 3  Ra  2  10 5 . Significant agreement for the Nusselt numbers was observed between numerical simulation results in this study 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 optimum thickness of the air layer in this research is found to be in the range of 60–80 mm. This novel glazing system type 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 in the indoor environment. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Advanced glazing systems have been substantially applied in both high-rise office buildings and commercial buildings in Hong Kong, over the past few years. The application of a double-skin façade can achieve energy savings through the reduction of air conditioning (A/C) energy consumptions. Natural lighting utiliza- tion and sound insulation can also be enhanced. A quiet and healthy working environment could thus be provided in a metropolitan city like Hong Kong. A new application of the glazing system is to integrate building facades with semi-transparent photovoltaic (PV) cells. These solar cells can, not only generate electricity, but also provide shade. The void between the two glass panes could serve as a thermal insulation layer and reduce the amount of energy consumed by the air-conditioning system in the building due to low conductivity of the air in the cavity. When the glazing is integrated with photovoltaic cells, an additional function of the building envelope is also created. This kind of solar energy applications is called building-integrated photovoltaics (BIPV). In this application, the windows are integrated with semi-transparent PV cells, which generates electricity as an energy source, thus contributing a dual function to the building envelope. Additional structural installation and maintenance cost could, therefore, be substantially reduced. Traditionally, windows play an important role in controlling heat transmission in indoor space during the summer period and external ambient heat loss during the winter period. In order to decrease solar transmission through windows, shading and blinds in the air cavities of the double-glazed window are, therefore, often employed. Avedissian and Naylor [1] investigated numerically, the free convective heat transfer in an enclosure with an internal louvered blind. The effect of Rayleigh numbers, enclosure aspect ratios, and blind geometry on the convective heat transfer, in this study is evaluated and compared with experimental results. Experimental results and other theoretical work related to heat transfer by natural convection or multi-mode heat transfer, for traditional double-skin facades, have been given by Todorovic and Cvjetkovic. [2] and Krishnan et al. [3]. Gosselin and Chen [4] investigated heat transfer and airflow through a dual-airflow window. Yang and Zhu [5–7] extensively investigated natural convective heat transfer in both inclined channels and tall cavities. Laminar or turbulent natural convection heat transfer and transi- tion of both flows were considered. This natural convective heat transfer in cavities has been of substantial interest owing to the practicality of its application, one example being the use of a double-skin façade to reduce heating and cooling consumption, others include heat loss from flat-plate solar collectors and so on. In addition to the results from theoretical studies, many well- recorded data from experimental tests are also commonly available in the literature for validation of the theoretical mode for evalua- tion of the buoyant cavity flow [8]. * Corresponding author. Tel.: þ852 2766 7958; fax: þ852 2774 6146. E-mail address: jun.han@polyu.edu.hk (J. Han). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.03.003 Building and Environment 44 (2009) 2129–2136