Experimental investigation on solar air heating system using evacuated tube collector with coaxial tube

Solar energy is a widely accessible, eco-friendly, and sustainable way to fulfil the global expanding energy needs. Solar air heaters are used in a variety of solar energy applications, including space heating, timber seasoning, and agricultural drying. The present work aimed to evaluate the performance of an evacuated tube solar air heater with coaxial tube by varying the mass flow rate of air and length of the coaxial tube at climatic conditions of Coimbatore, India. The experimental setup comprises of header, 20 evacuated tube solar collector and blower. Atmospheric air made to flow through the coaxial mild steel tubes under forced convection. The 1.5 m coaxial tube heated air to a maximum of 94 °C when we ran it at 50 kg/h, achieving 26% effective thermal efficiency. Meanwhile, the shorter 0.75 m tube reached 78 °C at the same flow rate but performed slightly better with effective thermal efficiency of 28%. It is concluded that the outlet air temperature is strongly dependent on the flow rate, coaxial tube length and solar intensity. Temperature range attained by this solar air heater is suitable for many solar thermal applications in small and medium scale industries.