In view of reducing the consumption of fuel and energy resources required for immobilizing the pupa of live silkworm cocoons, a high-performance solar thermal device designed for killing the pupa of cocoons is being proposed in this work. This portable device can be transported by any vehicle and handled by a single operator, and it does not require any special qualifications. Application of this device enables the saving of 100% of fuel and energy resources consumed in existing conveyor-type cocoon drying aggregates at the premises destined for the primary processing of cocoons. Experiments to study temperature conditions inside a solar thermal device with loaded living cocoons were carried out in industrial settings. Measurements of the ambient temperature inside the solar thermal unit and the flux of total solar radiation incident on the horizontal surface of the solar device were made by means of proven tools. Based on the findings from experimental investigations, a mathematical model of the rate of change in the temperature of cocoons inside a solar thermal device has been developed and proposed, taking into account the thermal characteristics of its enclosing elements, the arrival of solar radiation, and changes in ambient temperature. This mathematical model will enable the development of a solar thermal device for a similar purpose for the required temperatures inside the device.