## 1 Introduction  
Food preservation by the drying method is a very old technique that is still relevant today. Dehydrated food is resistant to bacteria, yeasts, and fungi growth, as water is essential for their propagation. Dried agricultural products have many advantages over fresh ones. Long shelf life is one of these advantages because nutrients from off-season agricultural products can be provided throughout the entire year [1]. Moreover, the fact that they are more easily handled, packaged, and stored than fresh food makes them ideal for transportation from the region of production to even long distant markets [1]. During the drying or dehydration process a percentage of the water content is removed by fruits or vegetables until the desired water level is reached. The removal of water can be performed by various physical (e.g., solar drying) or artificial process, such as evaporation and reverse osmosis [2]. Drying agricultural products is a fully developed process with commercial applications, and it is energy demanding. Important parameters during the drying process are the air temperature of the space where the drying occurs and the air relative humidity level [3]. An effective technique to retain the relative humidity at a low level is air recirculation [4]. So,energy is required mainly for heating, dehumidification, and air circulation in conventional applications to achieve suitable conditions for the food drying process. It has been reported that the final cost of the drying process is strongly dependent on the initial capital, labor, energy, and handling costs [5]. In an examined case study, among these costs, energy occupies 30% of the drying cost when the price for propane is almost at 0.18 \(\upxi\)/L, and the electricity cost is about 0.06 \(\upxi\)/kWh, while 46% of the total investment cost belongs to the initial capital [5]. Therefore, energy has a crucial role in the price formation of the final product. Previous research has focused on reducing the energy needed for drying to make it cost efficient and environmentally friendly [6; 7; 8; 9]. One approach is the utilization of renewable energy sources, such as solar drying or natural ventilation systems, where the drying energy cost is lower than the use of conventional energy sources (i.e., electricity). The effect of solar drying on the qualitative characteristics of the final product was previously studied by many authors [10; 11]. Solar drying systems are divided into two types: direct and indirect, based on the application method of solar energy [12]. In direct solar drying systems, thermal efficiency has been found to be higher than the indirect systems [13]. In fact, the thermal efficiency of direct solar drying systems has been found almost between 15% and 40% over the years [12; 14; 15]. This rate is not constant, and it is strongly dependent on the region, type of solar dryer, and the product to be dried. On the other side, indirect systems have the advantage of a better-quality final product, as vitamins and color are better protected since they are not exposed to direct solar radiation [12]. Even in these cases, the periodical characteristics of solar radiation affect the process. According to an experimental and numerical heat transfer analysis on an indirect solar drying system, the heat transfer coefficient of the product varies during the day [16]. This is due to the dependence of the process on the solar radiation intensity, which affects the solar collector efficiency and, therefore, the drying rate [16]. Other factors that might affect the performance of indirect systems with solar collectors are the inclination of the collector [17], which has been proven to affect the drying time in an experimental drying system for bananas [17]. Hybrid systems are also mentioned in the literature. In these cases, solar drying is enhanced using other systems that can also be renewable energy sources (biomass), conventional systems (electrical heaters), low energy consumption systems (heat pumps), or thermal storage systems [13; 18].