A focusing collector for air heater, the concentrator being Fresnel lenses, is here examined. The sunlight is focalized on black-body metallic sheets, contained in an insulated box, through which the energy is transferred to the fluid (dry air). The sheets, having the same height, are arranged at a distance one from another increasing from the center to the edges in order to trap all the incident energy. The energy reflected by the sheets is also trapped. A thermotechnical analysis of the system in order to determine the optimal conditions for the heat transfer is reported.