DEVELOPMENT AND PERFORMANCE EVALUATION OF EVAPORATIVE COOLING SYSTEM FOR SHORT - TERM PRESERVATION OF FRUITS AND VEGETABLES

Abstract

The technology and applications of evaporative cooling in the storage of fruits and vegetables is not new and has been used in dry and windy climate. This research assesses its suitability in Ondo state, south western Nigeria. For this purpose an active evaporative cooling system using locally sourced cooling pads (Jute fibre, Shredded latex foam, palm fruit fibres and Charcoal) as evaporators were developed, experimentally evaluated and theoretically modelled. The major components include, cooling pad holder, three axial fans, water pump, water control switch and hexagonal storage chamber. The performance of the cooling pads were evaluated in terms of reduction of inlet air temperature, improvement of the inlet air relative humidity, cooling efficiency, energy efficiency ratio and cooling capacity at 3, 4, and 4.5m/s air inlet speeds to investigate their ability to provide a storage condition for fruits and vegetables. The thickness of the pads was maintained at 30mm with packing density ranging from 200 -220kg/m3. At no load condition, the temperature range of the storage room under the prevailing weather condition of 22.9 – 37.5oC and 29 – 76% relative humidity, for the four pads were 19.6 – 27.7 oC, 19.1 - 24.9 oC and 20.8 - 26.8oC at air inlet speed of 4.5, 4.0 and 3m/s respectively. Operating the cooling system with palm fruit fibre provided the lowest storage temperature of 19.1oC at 4.0 m/s. Averagely, the ranges of relative humidity for the four pads used were 63 -91%, 60 -85% and 57 – 70 at 4.5, 4.0 and 3m/s . The various pads used were able to drop the ambient temperature up to 0.6 - 15.3 oC at no load condition. The temperature reduction of 4 to 13 oC and relative humidity of 85.6 to 96.8% was achieved by the cooler under load condition. The results showed that the maximum cooling efficiency and cooling capacity were 98% and 2529 W respectively. The power consumption of the cooler based on energy efficiency ratio was 4 - 8 times less of a vapour compression refrigeration.Proximate food analysis, vitamin C and total soluble solid analysis after recommended storage life of seven days in the cooler for stored fruits and vegetables showed that the stored vegetables in the cooler has higher moisture and vitamin C content while the withered control has higher ash, protein, fibre and carbohydrate content. The research also showed that the fruits stored in the cooler, has lower vitamin C content than the control as a result of delay ripening while the control is over ripping and deteriorating at the same period. Analysis of variance with two factors between the stored fruits and vegetables and control shows a high significance at 5%. A mathematical model for experimental validation of the cooling efficiency of the evaporative cooling system was developed. The predicted values are in good agreement with the experimental values obtained from the field evaluation with the maximum deviation found to be in the range of 0.8 to 3.72%.