A comparative study of the performance of a novel helical direct flow U-Tube evacuated tube collector

The direct flow evacuated tube collector (ETC) is a highly efficient solar energy collector. It has the advantage of using forced convection, which increases its efficiency compared to other ETC systems. In the present study, a helical tube ETC is proposed and compared to the traditional U-tube ETC. The helical tube was designed with three, seven, and eleven helical steps over the 1.8 m length of the ETC and was tested at flow rates of 10, 20, and 30 l/h. The helical tube ETC achieved energy and exergy efficiency enhancements over the traditional U-tube ETC of 6.1%, and 3.8%, respectively, at the 10 l/h flow rate using the eleven-step helical tube ETC. However, the maximum average energy and exergy efficiencies of 38.6% and 18%, respectively, were achieved in the seven-step helical tube ETC at the highest flow rate of 30 l/h. The cost analysis revealed that using the three-step helical tube instead of a traditional U-tube will produce hot water with a cost of 0.012 $/kWh at the 10 l/h flow rate, while the maximum cost will reach 0.15 $/kWh for the eleven-step helical tube using the 30 l/h flow rate.