## 1 Introduction
Modifications of the typical solar drying application have also been investigated, such as the insulation of the north wall in a greenhouse-type solar dryer along with the installation of a solar collector on the floor of the structure [19]. A combined operation of solar collectors and greenhouse structures has also been investigated in Tunisia, with the difference that the collector was installed outside the structure [20]. The performance of solar drying systems has been investigated extensively for various types of agricultural products and has been proven to be at satisfactory levels [8; 10; 21; 22; 23; 24]. Solar drying systems have achieved a sufficient energy reduction by contributing mostly to increase the temperature in the dryer area without artificial means. However, there are some parameters that also need to be improved to further optimize the drying process, such as the reduction in relative humidity and ongoing air movement. These operation parameters are controlled by ventilation and air circulation. In the artificial drying process air movement and air exhaust is performed by fans that consume electricity for their operation [25]. Despite the energy source or the approach of the drying process, the air flow is usually performed by the assistance of a fan or blower [25]. In constant air velocity the increase in temperature leads to a lower energy consumption as drying time is reduced [26] It has been found that when the drying is performed in lower temperatures the energy consumed for air circulation is increased, while the increase in the air speed leads to a higher energy consumption [27]. The energy consumption when the air speed is increased from 1.0 m/s to 1.5 m/s and 2.5 m/s leads to an almost proportional increase in the energy consumption when the air-drying temperature remains at a constant level [28]. The energy that is consumed when a fan is operating in higher rotating speed is not the only reason for higher energy consumption of a drying process. It has been reported that high air speed is leading to higher energy consumption due to the cooling effect observed on the product to be dried. In this case the drying time and energy consumption are substantially increased [28]. Fans are often used in solar dryers, to enhance the drying process [29]. It has been found that by increasing the air flow inside a tunnel greenhouse-type dryer by a certain rate, the drying rate can be increased almost by 12-13% [29]. Increasing the flow rate above a certain rate does not have a positive effect on the drying process though [20; 29]. Different drying methods lead to different energy consumption amounts per product weight. Microwave driers present the lowest energy consumption, which is almost 70% lower compared to other drying methods [30], but the initial capital cost for these drying systems is significant higher. In this experimental work, a new drying system was created that exploits the proven advantages of indirect solar dryers without the use of conventional energy sources. The aim of this work was to examine whether the proposed systems can be proven as an efficient solution for drying, by studying the weight loss achieved by the constant air movement around the food product. Beyond the drying process, the aim of this work was to investigate whether the area of a greenhouse could be utilized for multiple uses and applications to enhance the depreciation of the total investment. This drying method could be proven vital in rural areas without access to the energy grid. It also promotes a different application for greenhouses located in regions with hot summers, where it is not economical to use them during these periods of high temperatures. More specifically, the object of the study was to evaluate the drying performance of the system and connect parameters such as the weather conditions and the air flow rate with the drying performance in different types of specially designed drying tubes.