The objective is to study the dependence of different parameters of solar thermal collectors on the radiative regime. This study is focused on the influence of the state of sky, weather stability and operation regime. The work focuses on two possible research directions to improve the efficiency of the solar air collector: first, by using another working fluid (water) to recover the heat losses from the air channel, and the second research direction is by using different shapes of the absorber. The detailed objectives and the used methods are as follows: 1. The thermal performance of the Hybrid Solar Collector (HSC) for water and air heating was studied under different weather conditions (clear sky day, overcast sky day, low cloudiness, medium cloudiness and high cloudiness). 2. An analysis was conducted regarding the performance of a Hybrid Solar Collector (HSC) for water and air heating with different mass flow rates, for different shapes of the air channel and three volumes ofthe storage tank which is used to accumulate the thermal energy gained from the sun by the collectors. 3. Two mathematical models that evaluate the performance of Hybrid Solar Collector.