Test procedures have been proposed to evaluate thermal performance of solar cookers: box type [1] and concentrator type [2]. The performance of a box-type solar cooker can be represented in terms of two figures of merit, F1 and F2, which can be determined by simple thermal tests and to characterize the cookers [1]: studies have resulted in the Indian Standards [3,4] on solar cookers. F2 is obtained from the water-heating test using the experimental data on water temperature (Tw) and the climatic variables: insolation (I) and ambient temperature (Ta). It takes into account the climatic variables through a dimensional group consisting of [(Tw-Ta)/I]. It was shown that F2 could be obtained by using an equation involving natural logarithm of a function with this dimensional group. The Second figure of merit can also be determined by using a semi-log plot [5].
Outdoor experiments have been performed on a box type solar cooker with wooden board casing having size 1.185m long, 1.175m wide and 0.28m thick. In the present work, the effect of the climatic variables: insolation (I) and ambient temperature (Ta) on F2 is studied using the semi-log plot method. The values of F2 are evaluated by taking (i) the values of climatic variables recorded at an interval of 5 minutes and (ii) the values of ambient temperature and solar radiation averaged over entire interval of analysis. The R2 values of regression through the data points with the averaged values of climatic variables are higher than that obtained with using the instantaneous values. The values of F2 obtained using the instantaneous values and the averaged values of Ta and I are compared. It may be preferable to use the averaged values of the climatic variables in the calculation of F2 using semi-log plot method.