Design and Performance Evaluation of an Improved Solar-Biomass Greenhouse Dryer for Drying of Selected Crops in Western Kenya

Abstract

Solar drying systems are not able to achieve best drying rates given the intermittent nature of solar energy and poor airflows. To address this problem, a solar-biomass dryer measuring 8 m long, 4 m wide and 2.6 m high, with two layers of beds, was developed for use by medium scale processors in Kenya. It had a bimodal biomass heating system to back up the solar energy, and four provisions for air ventilation; chimney, turbo ventilators, lower opening and fans. It was constructed and preliminary tests on it undertaken at Khwisero in Kakamega County, Kenya. Six commonly grown crops; arrow roots (Maranta arundinacea), cassava (Manihot esculenta), sweet potatoes (Ipomoea batatas), kales (Brassica oleraceae var acephala), bananas (Musa spp.) and spider plant (Chlorophytum comosum) were utilised for the trials. Temperature, relative humidity, weight change and radiation data was collected in October 2017 to monitor performance of the dryer. This was done for three modes of the greenhouse dryer; natural ventilation, forced convection and solar-biomass (hybrid).

The difference between inside of dryer and ambient temperatures within the first three hours was 13.1, 20.8 and 17.9 °C under the natural, forced and the hybrid modes, respectively, with the inside of dryer temperature being 49.3, 53.8 and 53.2 °C respectively. The average solar radiation over the same time was 545, 668, 594 W/m², respectively. The monitored air velocity was controlled to 0.7 m/s for hybrid mode and 0.4 m/s for forced mode. The higher air velocity for the hybrid mode led to a lower inside air temperature, making it nearly equal to that of forced convection; but the hybrid mode drying rate was 18-19% higher than of the other modes. This implies that the hybrid system improved the drying conditions through higher heat supply. As expected, the relative humidity reduced during drying, with inside of dryer and the ambient relative humidity being 21.5% and 35.5%, 18.1% and 44.0%, and 19.5% and 35.3% for the natural, forced and hybrid modes, respectively. There was no marked difference in temperatures between lower and upper beds under the three modes. Overall, the study indicates that the efficiency of greenhouse solar dryers could be increased through improved ventilation and backup energy.