A solar assisted high temperature refrigeration system for postharvest pre-storage fruit cooling

Abstract

Agriculture has emerged as a major economic activity in the African continent. Therefore, fruits and vegetables are considered as essential source of vitamins, minerals and proteins. However, fruits and vegetables are perishing rapidly. Thus, the adequate handling starts from the field, by applying appropriate method of storage and preservation in order to reduce post-harvest losses and extend its shelf life. Preservation by removing heat from the products is the most common method of preservation. In rural areas, the access to the grid is expensive or in some cases impossible. Therefore, researchers have been paying more attention to find alternative sources of power to run the cooling units. This study provides proof of a concept for the use of solar energy to cool down harvested fruits and vegetables at the farm level to an adequate farm storage temperature. The target storage temperature range is between 5 and 15 oC. A model refrigerator was designed and constructed in the mechanical engineering workshop of Cape Peninsula University of Technology. It was installed and tested in outdoor conditions to get the effect of different weather conditions. The model consists of a typical vapour compression system powered by a 12 V solar PV system. A DC compressor was used, and therefore, there is no inverter. The model was first tested without a product and then with 20 kg batches of different fruits for a period of two weeks in April 2016. Wind speed, ambient temperature and solar radiation intensity data were monitored and collected from a Campbell Scientific weather station mounted on the roof adjacent to the model. By monitoring the temperatures and compressor current hourly, the refrigeration effect, power consumption and coefficient of performance were determined. The overall COP based on input solar energy was 2.8. It was thus proved that a suitably sized PV system could be designed and implemented at farm level to cool harvested fruits from ambient to storage temperature without the use of an inverter. This could go some way to helping retard deterioration of fruits and vegetables before delivery to a marketplace or to storage.