Evaluating the efficacy of a hybrid solar-thermophilic heat collector for food drying application

Abstract

Many farmers in Africa and in most of the developing nations depend on natural sun drying to preserve perishable products since it is cheap source of energy ,simple to use and renewable. However, the sun is not always available especially during the rainy season & at night. Open air-drying is unhygienic as it exposes foods to soil/dust, foreign matter, rain & other airborne contaminants that increase the danger of biodegradation and shorten shelf life. Delayed and insufficient dehydration contributes significantly to the accumulation of harmful mycotoxins/aflatoxins in stored produce. The main objective of this study was to evaluate the efficacy of a hybrid solar thermophilic heat collector for food drying applications. Specifically, it was sought firstly to characterize the suitability of locally available farm wastes for hot composting and secondly to quantify the energy and food drying potentials of a prototype solar thermophilic dryer that has been developed for use on small farms. The study was conducted on a farm in Kitale, Trans-Nzoia County from April 2019 to July 2019. Green grass clippings and weeds, cow and chicken manure, shredded maize stover and sawdust were sampled locally and analysed at Kenya Agricultural andLlivestock Research Organization (KALRO) in Nairobi. Total Carbon, Nitrogen, moisture content pH and micronutrients were analyzed using the Test method for the examination of composting and compost(TMECC) methods. The C/N ratio was then calculated to optimize the composting process feedstocks mixture. To profile energy and drying performance, temperature and humidity data loggers were mounted at different locations within the compost reactor and the dehydration chamber. Data analysis was done using the Origin Pro 2018 graphing software. Feedstock characterization established that most of the farm wastes feedstocks had a pH of 5.9-8.0, C/N range of 22:1 to 29:1, moisture content 74 - 92% wet basis (wb).The average pile temperatures reached maximum of 68 o C and average of 65 o C within the first 48 hours of composting. The compost heat extractor (COHE), yielded 174 kWh in 205 hours from 1350kg of blended biomass (C/N 28:1) resulting into 0.13 kWh/kg. The coefficient of Performance (COP) of the compost heat extractor was established to be a mean of 74.22 and maximum of 489.9. High COP was achieved because of the low energy consumption of the system auxiliary equipment. The drying chamber temperature and relative humidity conditions of 40 to 60 o C and 40 to 65 %, respectively, were attained therefore providing best conditions for low temperature drying. Cowpea (Vigna unguiculata) vegetables leaves were dried from a moisture content of 85 %wb to 10 %wb within 18 hours with thermal efficiency of 62 % comparable to other demonstrated drying technologies. The results affirm the efficacy of food drying with heat from composting of farm wastes. The hybrid solar- thermophilic dryer promises to be a sustainable and cost-effective renewable energy solution to mitigating post-harvest losses on farms in Africa. Further research is recommended to optimize the design of the compost heat exchanger in relation to the dryer sizing.