Progress in the development and deployment of solar drying techniques in Nigeria

Abstract Open sun drying (OSD) is a common food preservation practice in Nigeria, particularly in the northern regions, where high solar radiation, low relative humidity, and abundant sunshine for much of the year provide favourable conditions for drying agricultural products. While drying is an effective way to reduce losses after harvest and prolong the storage life of agricultural and forestry products, the traditional OSD method has notable drawbacks. These include product damage by insects, birds, microorganisms, and sudden rainfall. To address these challenges, Nigerian researchers have fabricated, tested, and assessed different solar dryer designs, such as direct, indirect, hybrid, and mixed-mode models, which have demonstrated promising performance outcomes. This paper reviews selected studies conducted over the past two decades on solar dryers in Nigeria. It presents an overview of solar drying technology within the context of Nigerian agro-processing industry, while also examining their benefits in sustainable food security. In addition, the review explores factors that influence the efficiency of solar dryers and the role of thermal storage devices in improving performance. It is found that the drying rate and time depend foremost on the product being dried and on the maximum temperature attainable in the dryer, which ranges between 30 and 60 °C, 50 and 66 °C and 30 and 90 °C for direct, mixed and forced-convection mode dryers. The drying efficiency is in the range of 7 and 30%, and 30 and 67% in the direct and forced-convection mode dryers. However, an enhanced drying performance can be achieved by using heat energy storage materials such as the phase-change materials and improved technical design of the dryers. Finally, it discusses future prospects and recommendations for scaling up solar drying in Nigeria, with emphasis on its potential to reduce preservation costs, boost exports, and strengthen food security.