Investigation on heat transfer enhancement of conventional and staggered fin solar air heater coated with CNT-black paint—an experimental approach

The present study aims at a detailed experimental study of a passive-type solar air heater (SAH) on the effect of coating the absorber plate with higher thermal conductive black paint under forced circulation method and studied under the climatic conditions of Chennai. Furthermore, to enhance the thermal performance index of conventional SAH, additional enhancement such as coating and staggered fins are fixed to the absorber plate of duct which simultaneously increases the turbulent intensity, kinetic energy to enhance the temperature of outlet air. Comparisons are made with a solar air heater without coating and staggered fin in order to assess the thermal performance. Experiments were conducted on a continuous basis and the flow rates of air flowing through the duct are varied. Experimental results revealed that the effect of coating improved the plate temperature to a maximum of 102 °C while the modified SAH coated with ordinary black paint and staggered fin arrangement is found as 95 °C for the flow rate of mf = 0.03 kg/s. The difference in temperature between exit and inlet of conventional SAH with coating alone is found as 13.09 °C at a flow rate of mf = 0.03 kg/s whereas the average thermal efficiency is found as 22.3%. Similarly, increasing the mass flow rate from 0.13 to 0.22 kg/s has no significant improvement in average daily thermal efficiency, whereas the temperature difference decreases. The coating of absorber plate with higher thermal conductivity paint and increased turbulence created between the duct by using staggered fin improved the temperature of exit air by 63, 64, 38 and 35% for air flow rates of 0.03, 0.04, 0.13 and 0.22 kg/s respectively. On a flat absorber with coating, the average increase in temperature is found at 6.3% compared to that of SAH coated with ordinary black paint. The hourly thermal efficiency of the conventional type SAH with coating exhibited an enhancement of about 5% in thermal efficiency as compared to that of conventional type with black paint coating alone for the same climatic condition, whereas the thermal efficiency of staggered fin SAH with coating alone is enhanced by 7.5%. With staggered fin arrangement in the absorber plate, heat absorption by the air is increased with CNT-coated absorber and excessive turbulence produced by the fins enhanced the average temperature difference from 13.5 to 20.3 °C. The cost per unit kW of conventional type SAH with CNT-doped black paint reduced from 0.01754 $ to 0.00832 $/kW while varying the flow rate from 0.031 to 0.22 kg/s.