An exclusively solar heated dryer in modular design for medicinal plants and spices . Solar air heaters have been incorporated into the roof of a plastic film covered greenhouse in which a batch dryer is located. The dryer can be operated temporarily with recirculated air by closing the air inlet resulting in a rise of maximum temperature from 50°C to 70°C. A computer aided control system opens the air inlet as soon as the additional drying effect of the higher temperature is compensated by the reduction of drying rate caused by gradual increase of air humidity. Using the control system the capacity of the dryer is increased by 30%.