Effect of absorption of solar radiation in glass-cover(s) on heat transfer coefficients in upward heat flow in single and double glazed flat-plate collectors Naiem Akhtar a,⇑ , S.C. Mullick b a Department of Mechanical Engineering, Z.H. College of Engineering & Technology, Aligarh Muslim University, Aligarh 202 002, Uttar Pradesh, India b Centre for Energy Studies, Indian Institute of Technology, New Delhi 110 016, India article info Article history: Received 9 September 2008 Received in revised form 10 June 2011 Accepted 15 July 2011 Available online 24 September 2011 Keywords: Absorption of solar radiation in glass cover(s) Heat transfer coefficients Single and double glazed flat plate collectors abstract Absorption of solar radiation in the glass cover(s) of a flat plate solar collector increases the temperature of cover(s) and hence changes the values of convective and radiative heat transfer coefficients. The gov- erning equations for the case of single as well as double glazed collector have been solved for inner and outer surface temperatures of glass cover(s) with/without including the effect of absorption of solar radi- ation in the glass cover(s), with appropriate boundary conditions. The effects of absorption of solar radi- ation on inner and outer surface temperatures and consequently on convective and radiative heat transfer coefficients have been studied over a wide range of the independent variables. The values of glass cover temperatures obtained from numerical solutions of heat balance equations with and without including the effect of absorption of solar radiation in the glass cover(s) are compared. For a single glazed collector the increase in glass cover temperature due to absorption of solar radiation could be as high as 6°. The increase in temperatures of first and second glass covers of a double glazed collector could be as high as 14° and 11°, respectively. The effect on the convective heat transfer coefficient between the absor- ber plate and the first glass cover is substantial. The difference in the values of the convective heat trans- fer coefficients between the absorber plate and the first glass cover (h cp1 ) of a double glazed collector for the two cases: (i) including the effect of absorption and (ii) neglecting the effect of absorption in glass cover, could be as high as 49%. Correlations for computing the temperatures of inner and outer surfaces of the glass cover(s) of single and double glazed flat plate collectors are developed. The relations devel- oped enable incorporation of the effect of absorption of solar radiation in glass cover(s) in the relations for inner and outer surface temperatures in a simple manner. By making use of the relations developed for inner and outer surface temperatures of glass cover(s) the convective and radiative heat transfer coeffi- cients can be calculated so close to those obtained by making use of surface temperatures of glass cov- er(s) obtained by numerical solutions of heat balance equations that numerical solutions of heat balance equations are not required. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Solar radiation incident on the aperture of a solar collector is transmitted through the glass cover(s) to the absorber plate. A fraction of the solar radiation is absorbed by the glass cover(s). Absorption of solar radiation in a glass cover leads to an increase in its temperature and consequently affects the values of heat transfer coefficients. The values of the convective and radiative heat transfer coefficients are often of interest. For example, a method to obtain the emittance of a selective coating by experi- mentation has been presented by Nagar et al. [1]. The emittance has been calculated from the top heat loss coefficient (U t ) using an empirical relation for U t that was popular at that time. Simi- larly, Francey and Papaioannou [2] have obtained the wind heat transfer coefficient (h w ) experimentally by calculating h w from U t using an empirical relation for U t . However, the empirical rela- tion for U t can lead to large errors as shown in the analytical study by Mullick and Samdarshi [3], Samdarshi and Mullick [4] and by Akhtar and Mullick [5,6]. In a study on upward heat flow in solar collectors exposed to solar radiation and in such cases where the values of convective and radiative heat transfer coeffi- cients are of interest the effect of absorption of solar radiation in the glass cover(s) has to be incorporated in the set of heat balance equations (and then numerical solutions have to be obtained). Correlations for inner and outer surface temperatures of the glass cover for estimation of heat transfer coefficients in upward heat flow in solar collectors with single glazing have been proposed by the authors [7]. 0017-9310/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2011.08.048 ⇑ Corresponding author. Tel.: +91 0571 2721335; +91 9457694976 (mobile); fax: +91 0571 2701895. E-mail address: akhtar_n@rediffmail.com (N. Akhtar). International Journal of Heat and Mass Transfer 55 (2012) 125–132 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt