Studies on thermal performance of the box solar cooker have resulted in the Indian standards IS: 13429:1992 & 2000 on solar cookers. Two figures of merit were proposed to quantify thermal performance of box solar cookers and to characterize the cookers. The second Figure of Merit, F{sub}2, takes care of the product of heat exchange efficiency factor (F'), optical efficiency (η{sub}o), and the heat capacity ratio (C{sub}R). F{sub}2 is obtained from the water heating test using the experimental data on water temperature (T{sub}w) and the climatic variables: insolation (I) and ambient temperature (T{sub}a). It takes into account the climatic variables through a dimensional group consisting of [(T{sub}w-T{sub}a)/I]. It was shown that F{sub}2 can be obtained by using an equation involving natural logarithm of a function with this dimensional group. The present study shows that F{sub}2 may be found by analyzing the experimental observations with the appropriate function on a semi-log plot. Linear regression of the experimental data on the semi-log plot is found to be excellent. The values of F{sub}2 obtained from the slope of linear regression are close to the values obtained by the equation.
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