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 a gradual increase of air humidity. Using the control system the capacity of the dryer is increased by 30%.
