Rice grains inside a solar bubble dryer. (Photo by Isagani Serrano, IRRI)
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Sun drying is the most common grain drying method in the Philippines, Thailand, Cambodia, Laos, and India, among other tropical countries, because it is cheap. It uses the sun as the heat source. But, it is laborious and unreliable. Farmers need to mix the grains at least every half hour for even drying. They need to cover the grains when the sun is too hot to minimize cracking from overheating and constantly keep animals away from the grains. Obviously, overcast skies extend the drying period. And, when it rains, farmers hurriedly collect the grains to avoid getting them wet.
The solar bubble dryer was designed to prevent these problems. It is made of two plastic sheets, a black one at the bottom where the grains are placed and a transparent one as roofing. Both sheets are connected by a zipper. The heat comes from the sun while the dome-like shape polyethylene plastic roof provides protection from the rain. A small ventilator inflates the bubble and circulates air. The airflow then removes water from inside the drying tunnel and prevents overheating. To make sure that the grains dry evenly, they are stirred from time to time using a metal roller underneath the dryer.
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