Using Eq. (1 ) and Fig. 2a, the variation of solar radiation normal to the aperture plane of a parabolic trough
collector is presented in Fig. 2b. Using Fig. 2b the calculation period can be divided into smaller time steps in a way
that in each sub step the variation of direct normal irradiance can be neglected which answers the first question. Also
the values of DNI in each sub step can be calculated using Fig. 2a which answers the second remark. It should be
mentioned that SolTrace software uses direct normal irradiance as an input to calculate the distribution of solar heat
flux on the absorber tube. Based on the above statements, Table 2 indicates, the number of time steps and related
values of direct normal irradiance considered for each of the four above mentioned days. Note that irradiance is
symmetrical with respect to noon hour.