Abstract
In this paper, a novel flexible solar air heater (FSAH) has been rationally designed and fully investigated by practical experiments. The main part of the FSAH is an integrated flexible warp-knitted spacer fabric composite, in which spacer filaments are coated with solar energy absorbing materials by the air spraying method. In this system, flexible spacer fabric composite not only works as a solar thermal energy harvesting layer, but also provides airflow channels for energy transmission. By comparison test, it is found that the outlet temperature and thermal efficiency of FSAH were improved by introducing nano-black carbon particles as solar energy absorbing materials coated on the surface of spacer filaments. An analysis of net energy gain and effective efficiency was then carried out and the best thermohydraulic performance of the as-fabricated FSAH was found at the mass flow velocity of 0.037 kg m−2 s−1. The optimization experiments demonstrated that with the increase of heater length, the outlet temperature and energy gain of FSAH increased, while their thermal efficiency and effective efficiency decreased. Furthermore, to reduce the heat losses from the top layer to the surrounding ambient, another layer of transparent spacer fabric composite was assembled with the FSAH as a thermal insulation layer and its thermal efficiency and effective efficiency were found to be increased by 20%.
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