Heat transfer enhancement techniques using different inserts in absorber tube of parabolic trough solar collector: A review

Various collector technologies are prevalent for harnessing of solar energy. Some of the most common types are flat plate and evacuated tube collectors. Parabolic trough collectors are not commercialized other than large industrial set up because these collectors occupy larger land area along with higher cost of installation. It becomes important for the researcher to device new technologies that would reduce the cost and hereby promote sustainable renewable energy technologies. Heat transfer enhancement techniques like tube turbulators mainly twisted tape, wire coils, metal foam, corrugations, fins, and use of nano fluid are among the several alternatives. The present review focuses on research done in enhancement of heat transfer rate in PTC using above techniques considering the work done from 2015-2023. An effort has been made to compare the output of such techniques after exhaustive literature survey. The enhancement in heat transfer rate by dimensionless Nusselt number Nu/Nup. Where, Nu is the Nusselt number for PTC absorber tube when use with turbulator and Nup is the Nusselt number for plain absorber tube. Similar comparison is made for f/fp which is related to friction factor encountered and when compared to smooth tube. From the literature it is evident that Nu/Nup has a range from 1.2-10 when using different turbulator and f/fp has a range of 1.02-10.2 which is mainly dependent on geometry. These results prove efficacy of using tube turbulators in PTC absorber thereby promoting interest for further exploration especially with experimental investigation which is seldom reported in the literature. Moreover, previous research is not sufficient as direct comparison among these techniques is not reported. This critical study reports direct comparison of above techniques.