A Raised Solar Dryer Sized from Airflow: Capacity in Kilograms per Day for Three Climates

This paper sizes a raised solar dryer from its airflow and from the energy needed to evaporate water, rather than from rules of thumb. The stated design has a collector of 2.0 m2, a tray area of 1.6 m2 under a mesh cover, a chimney 1.5 m high and a vent of 0.06 m2. Buoyancy drives air through the vent at 0.718 m/s in hot-arid conditions, 0.589 m/s in hot-humid conditions and 0.533 m/s in temperate conditions, and that air could in principle carry away 83.3, 17.88 and 9.83 kg of water per day. The energy balance is stricter in every climate: at a stated evaporation demand of 2.45 MJ per kilogram of water, stated daily irradiation of 20, 15 and 12 MJ per m2 and a collector efficiency of 0.40, the sun supplies enough energy to evaporate only 6.53, 4.90 and 3.92 kg of water per day. Throughput is therefore set by energy, not by airflow, and comes to 8.48 kg of fresh fruit per day in hot-arid conditions, 6.36 kg in hot-humid conditions and 5.09 kg in temperate conditions, with fish at 8.66, 6.50 and 5.20 kg per day and meat at 9.75, 7.31 and 5.85 kg per day. The collector area required per kilogram of fresh fruit per day is 0.236 m2 in hot-arid conditions, 0.314 m2 in hot-humid conditions and 0.393 m2 in temperate conditions. The complete dryer costs USD 150 at stated unit prices, or USD 17.69 per kilogram-per-day of fruit capacity in hot-arid conditions and USD 29.47 in temperate conditions. In a stated rainy-season case for the humid climate (29 C, relative humidity 0.90, a temperature rise of only 2 K and 2 solar hours), the computed moisture uptake of the air is zero and the dryer removes no water at all. SIMULATED, NOT MEASURED.