Development of a greenhouse-type solar dryer for medicinal plants and herbs

Abstract

The economy of medicinal plant production is burdened considerably by the energy costs of drying. To substitute fossil energy, a solar heated dryer was developed, taking a plastic film greenhouse as superstructure to lower the initial costs. The solar air heaters, being incorporated into the roof, consists of a black absorber tissue, which is placed between the transparent cover of the greenhouse and an insulating air-bubble foil. To obtain a modular design, the system is structured along the ridge into segments of 2 m width. Each segment forms an independent solar air heater/batch dryer unit and is operated by a fan with a power consumption of 500 W. A prototype was tested in Yugoslavia, drying mint, sage and hops. The drying process from an initial moisture content of 80% w.b. to a final moisture content of 11% w.b. took 3–4 days. The load is 250–500 kg fresh material per segment. Maximum temperatures have been within a range of 40–60°C. The total pressure drop is 50 Pa at an air flow rate of 3300 m 3 /hr per segment. Compared to conventionally dried crude drugs, the percentage of active ingredients was up to 40% higher. The low investment and operating cost and the high quality of the crude drugs permit the solar dryer to be used in agriculture.