Comparison between PCM filled glass windows and absorbing gas filled windows Kamal A.R. Ismail a, * , Carlos T. Salinas b , Jorge R. Henriquez c a Department of Thermal and Fluids Engineering, Faculty of Mechanical Engineering, State University of Campinas, P.O. Box 6122, CEP 13083-970 Campinas, SP, Brazil b Department of Mechanical Engineering, University of Taubate ´, Daniel Danelli s/n, CEP 12060-440 Taubate ´, SP, Brazil c Department of Mechanical Engineering, Federal University of Pernanbuco, Av. Acade ˆmico He ´lio Ramos s/n, CEP 50740-530 Recife, PE, Brazil Received 2 October 2006; received in revised form 10 May 2007; accepted 14 May 2007 Abstract From the thermal point of view, windows represent the weak link between the internal and external ambients of a room. In cold climates, they are responsible for 10–25% of the heat lost from the heated ambient to the external atmosphere. In hot climates, the excessive solar radiation entering the internal ambient through the windows leads to increasing the cooling load of the refrigeration system. The use of absorbing gases filling the gap between glass sheets appears to be an alternative solution for thermally insulated glass windows. The other options one may incorporate filling materials such as silica aerogel or a PCM. In this work, a comparison between the thermal efficiency of two glass windows one filled with an absorbing gas and the other with a PCM and exposed to solar radiation in a hot climate is done. To model double glass window filled with infrared absorbing gases, a CW real gas model is used. A radiative convective conductive model and a radiative conductive model were investigated. Three mixtures of gases were used; a strongly absorbing gas mixture, an intermediate absorbing gas mixture and a transparent to infrared radiation mixture. To model the double glass window filled with a PCM, a relatively simple and effective radiation conduction one dimensional formulation is used. Heat transfer through the window is calculated and the total heat gain coefficients are compared and discussed. # 2007 Elsevier B.V. All rights reserved. Keywords: Double glass window; PCM; Absorbing gases; Numerical modeling 1. Introduction One of the efficient means of reducing excessive cooling loads in hot climates is to eliminate or minimizing the solar heat gain through glass windows by using passive mechanisms of reducing the amount of direct heat radiation crossing the window without impairing the natural illumination. Curtains, persians and similar materials and methods are widely used as means of reducing incident direct energy radiation. Mills and McCluney [1], discuss the advantages of using shading materials especially internally installed ones as persians. These elements seem to be highly effective for thermal control if their surfaces are reflective and if their orientation is adjusted few times during the day. Simple glass windows are vonerable to incident solar radiation penetrating easily through a simple clear glass sheet whose transmittance is of the order of 90%. The development of new materials, new technologies and new strategies for reducing energy consumption led to enhance research and development of more efficient windows. Windows having selective solar radiation characteristics are examples of thermally efficient windows. The selective properties due to deposited films on the glass sheets allow changing the transmittance, reflectance and absorptance of the window. These films can be designed to absorb or reflect according to the wavelength of the incident radiation. A vast review of technologies of selective films is reported by Lampert [2]. High performance is also achieved by using evacuated glass panels where heat transfer by conduction and convection is greatly reduced [3–5]. The use of absorbing gases filling the gap between glass sheets appears to be an alternative solution for thermally insulated glass windows. The use of gases with strong infrared radiation characteristics was investigated and some www.elsevier.com/locate/enbuild Energy and Buildings 40 (2008) 710–719 * Corresponding author. Tel.: +55 19 35213376; fax: +55 19 37883213. E-mail addresses: kamal@fem.unicamp.br (K.A.R. Ismail), csalinas_99@yahoo.com (C.T. Salinas), rjorge@ufpe.br (J.R. Henriquez). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.05.005