Optimization of the Performance of a Hybrid Solardesiccant Dryer

Abstract

A hybrid solar desiccant dryer (HSDD) was designed, developed and optimized with the aim of preconditioning the air for maize grain drying. The effect of four solar collector configurations in optimizing dryer performance was studied, comprising of (1) Radiation concentration lenses to increase solar radiation intensity and achieve high temperatures, (2) High density longitudinal finned elements for enhanced thermal contact and heat transfer rates, (3) Desiccant exhaust dehumidification conduits to enhance thermal recuperation of waste heat for regeneration into the dryer and (4) Combined effect of the three configurations. The collectors were tested and grain drying experiments performed on a loaded HSDD with the most efficient collector configuration and results compared with open sun drying method. Temperature changes of the solar collector configurations, heat transfer rates, collector efficiencies, grain drying rates and drying time were analysed. Moreover, the HSDD experimental moisture ratio data was fitted to 18 mathematical models of dying and regressed using MATLAB (Version R2016a) to evaluate goodness of fit by comparing coefficient of determination (R2), sum of square error (SSE) and root mean square error (RMSE). Results showed that collector configuration with finned elements, desiccant exhaust air regeneration conduits and radiation concentration lenses had average temperature change of 8°C, 17°C and 21°C above ambient respectively; while the combined collector had the highest average temperature change of 28°C. Similarly, changes in relative humidity were 6%, 16%, 19% and 25% for finned elements surfaces, desiccant exhaust air regeneration, radiation concentration lenses and the combined collector respectively. Analysis of variance using Stratigraphic16.1 software showed statistically significant differences in temperature changes under different test configurations at 95% level of confidence. Moreover, multiple range tests indicated significant differences between the means of temperatures from the contrasted collector configurations. The temperature and relative humidity changes increased linearly, and the rate of change was highest in the combined configuration and least in finned elements. Thermal efficiency increased with temperature changes as well as with useful heat gain and solar insolation. Useful heat gain increased to reach the maximum average values of 0.104 kJs, 0.19 kJs, 0.244 kJs and 0.289 kJs for the finned elements, desiccant exhaust air regeneration, radiation concentration lenses and the combined configurations respectively at maximum solar insolation time (13.30hrs). The average collector efficiencies were 17%, 36%, 45%, and 61% for the finned elements, desiccant exhaust air regeneration, radiation concentration lenses, and the combined collector configuration respectively. The integrated collector configuration improved thermal efficiency from individual configurations by 44%, 25% and 16% for the longitudinal finned elements, desiccant exhaust air regeneration and radiation concentration lenses alone respectively. The combined collector set up had lowest heat loss coefficient (7.1528) while the collector with radiation concentration lenses only manifested the highest value (12.336) despite achieving high temperatures than finned elements surfaces (10.008) and desiccant exhaust regeneration (9.0295). The HSDD achieved maize grain drying from 24.1% to 13.1% M.C (w.b) in 18 hours compared to 54 hours for open air sun drying method. Moisture removal rate increased from 0.162 kg/hr to 0.485kg/hr while the drying time was reduced by 67% using HSDD. Regressed moisture ratio datasets of the eighteen fitted mathematical models reviewed that the Two term model characterised the drying kinetics of maize grain in the HSDD with highest R2 (0.9676) and lowest SSE (0.05655) and RMSE (0.04078) values. The performance of the dryer was optimised by incorporating radiation concentration lenses, longitudinal finned element surfaces and desiccants exhaust regeneration system to increase drying temperatures, heat transfer and waste heat recovery for subsequent drying. This study is useful in scaling-up dryer design and prediction of tempering effects during conditioning of grain in planned drying schedules. It optimizes drying process parameters for improved dryer performance and efficiency enhancement to reduce time loss and possible grain damage during drying to benefit grain and seed industry for sustainable food security. Keywords: Solar collector, Grain dryer, Finned element, Exhaust regeneration, Desiccant, Radiation concentration lenses, Drying rate, Drying kinetics.