Heat Transfer, Friction Factor, and Performance Evaluation of U-Pipe Evacuated Tube Solar Thermal Collector with TiN Nanofluid and Twisted Tape Inserts

Evacuated U-pipe solar thermal collectors are among the most efficient and promising systems for solar thermal applications. This study investigates the enhancement of heat transfer, pressure drop, and overall performance of solar collectors using titanium nitride (TiN) nanofluid and twisted tape inserts. TiN nanoparticles of 40–50 nm were dispersed in distilled water at volume concentrations of 0.0%, 0.025%, 0.050%, 0.075%, and 0.1%. Outdoor experiments, conducted at 19.5761°N and 74.2070°E, involved twisted tape inserts (H/D = 5) and flow rates ranging from 0.25 to 1.25 LPM. The convective heat transfer coefficient, pressure drop, efficiency, performance index, energy absorbed, and heat removal factor were experimentally evaluated in accordance with ASHRAE Standard 93-2003. The results reveal a significant improvement in heat transfer performance, with a 475.37% enhancement in the convective heat transfer coefficient at 0.1% TiN nanofluid concentration and 1.25 LPM compared to water at 0.25 LPM. Pressure drop increased by 64% under identical conditions. The highest efficiency of 80.47% was achieved at 0.1% volume concentration with twisted tape inserts. The optimal performance index of 1.58 was observed at 0.025% volume concentration, 1.25 LPM flow rate, and twisted tape configuration. The novelty lies in the synergistic use of TiN nanofluid and twisted tape inserts to maximize collector performance. This study demonstrates the potential of TiN nanofluid and twisted tape inserts to significantly enhance the thermal performance of evacuated U-pipe solar collectors, paving the way for improved solar energy utilization.