This is the first part of a set of two papers, about the estimation of solar irradiance on the absorber plate and cover of a semi-cylindrical solar tunnel dryer (STD). The second part discusses the modelling and thermal performance results of the STD. A detailed analysis has been carried out in this paper for solar irradiance estimation for the general orientation of the STD with respect to reference positions (N-S, E-W). The obtained values of solar irradiance for the absorber plate and cover includes the internal reflection effect up to second-order. It is found that if the STD axis is in the E-W, the absorber plate receives more solar radiation as compared to the other reference position N-S. The amount of solar irradiation absorbed by the cover is also calculated and found very small compared to the absorber. All calculations have been made for Delhi climate (28.58 degrees N latitude).