الفهرس 
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Abstract
Renewable energy resources are limitless and reflect natural cycles. Solar energy is the promising and most recent energy source in terms of permanent power supplies, and it has been widely used in a variety of areas. Solar air heaters (SAHs) are one of the most important solar uses that have become essential in order to preserve a clean environment and minimize the use of fossil fuels. Therefore, SAHs have gained the interest of many researchers, and it has been developed in recent years to improve their performance through the flat plate and evacuated tube solar collector. Recently, the interest in evacuated tube solar collectors has increased due to their advantages in transferring heat and reducing radiation losses, in addition to their high efficiency.
Despite the common use of evacuated tubes in previous literature, most of them were studied on water heating, while solar air heaters through evacuated tubes did not receive sufficient study, especially the connected in series technique with the addition of storage materials.
The purpose of this study is to investigate the performance of a solar air heater using five units connected in series. Each unit includes a glass-evacuated tube and two concentric aluminum pipes have been welded from below and inserted into the evacuated tube, one to enter the air and the other to load the storage materials. The thermal storage system makes use of phase change material (PCM) to store and retrieve when radiation reduces or completely disappears, mostly through solid-to-liquid and vice versa conversions. PCM was strengthened by combining copper oxide nanoparticles (CuO ) to improve its thermal conductivity. The hybrid storage material was referred to in this research as nano enhancer phase change material (NE-PCM).
This work was carried out on two tests, the first without NE-PCM and the second after it was included. The findings were compared under comparable boundary conditions such as ambient temperature, solar radiation , wind speed and airflow rate . Also, the thermal performance of the system was evaluated as a whole and for each separate unit at varied five flow air rates (0.006-0.008-0.01-0.03-0.05 kg/s).
Experience has shown that evacuated tube solar collectors are a suitable choice for applying the solar air heater because they more effective efficient in capture of solar radiation and the lack of energy gained is minimal. The results showed that the proposed design achieved a maximum thermal efficiency of 64 and 31 % with and without NE-PCM respectively, at a flow rate of 0.05 kg/s. The pressure DROP was 0.06 bar for the system as a whole. Additionally, the highest outlet temperature values were recorded 116, 108, 102, 95 and 93 °C at 13 :00, at air flow rate of 0.006, 0.008, 0.01, 0.03, and 0.05 kg/s respectively without NE-PCM adding. After adding NE-PCM, the outlet temperature is reduced by about 5-12 °C due to absorbing some energy during the phase change process. However, thermal efficiency rises and the operating time of the device also increases for 5 hours after sunset.
This work achieves an alternative energy supply with low-cost equipment and an easy installation system, making it suitable for solar air heater applications in low and medium temperatures , such as heating systems, drying, and greenhouses. This research is also a reference for researchers and those interested in the field of solar energy, especially solar air heaters with evacuated tube solar collectors connected in series.