title: Performance Enhancement of a High Concentration Ratio Parabolic Trough Solar Collector using Supercritical CO2 as the Working Fluid with Modified Twisted Tape Inserts and Rotating Receivers
Creators: name: Oketola, Temitayo
Publisher: name: Graduate Studies
Year: 2024
Subjects: subject: parabolic trough solar collector
subject: supercritical carbon dioxide
subject: thermal and thermodynamic analysis
subject: entropy generation minimization
subject: rotating receiver
subject: modified twisted tape inserts
subject: thermal gradients
subject: computational fluid dynamics
subject: Engineering--Mechanical
subject: Engineering--Chemical
subject: Engineering--Environmental

description: Supercritical carbon dioxide (sCO2) is considered a next-generation working fluid for advanced high-temperature and high-concentration ratio parabolic trough solar collectors (PTSC) due to the fluid’s low cost, availability, and thermal stability at high temperatures. However, the lower thermal transport properties of supercritical CO2 compared to common liquid working fluids used in concentrating solar power systems necessitate heat transfer enhancement of receivers in high heat flux systems . In this thesis, the optical, thermal, and thermodynamic performance of a high-concentration ratio parabolic trough collector using sCO2 with twisted tape inserts and rotating receiver tubes as passive and active heat transfer augmentation techniques, respectively, are investigated. The optical model was developed using SolTrace® and thoroughly validated to obtain the accurate non-uniform heat flux on the absorber tube of the parabolic trough collector. The thermal and thermodynamic models for the passive and active heat transfer augmentation technique were developed and implemented using ANSYS Fluent. For the passive augmentation, a novel modified twisted tape (MTT) insert is proposed, and the effect of twisted tape geometry, including varying twist ratios and width ratios, on the thermal and thermodynamic performance of a high-concentration ratio parabolic trough solar collector is investigated. The extensively validated study considers both the first law and second law performance using the entropy generation minimization method. MTT inserts with twist ratios in the range of 2.0 to 4.0 and width ratios in the range of 0.75 to 0.95 were investigated. While in the active heat transfer augmentation study, this thesis proposes the use of a rotating receiver tube as a means of reducing absorber tube temperature gradients and improving both the thermal and thermodynamic performance. The effect of receiver tube rotation on the thermal and thermodynamic performance of PTSCs with varying concentration ratios and rim angles has, therefore, been considered. Angular velocities were varied from 0 to 0.8 rad/s; rim angles were varied from 40º to 80º, and the concentration ratios were varied from 110 to 140. For the twisted tape inserts, results show enhancement in the heat transfer performance by up to 73% and thermal efficiency by up to 6%, and reduction of the receiver heat losses and circumferential temperature gradients when the MTT inserts are used. Moreover, the results of an entropy generation minimization study show that the use of MTT inserts reduces the total entropy generation rates, with the lowest irreversibility occurring at a Reynolds number of 1,650,000 for a twist ratio of 2.0 and width ratio of 0.95. For the rotating receiver tube studies, when compared to a plain receiver tube without the effect of rotation, results show that the use of a rotating absorber tube increased the thermal performance by 2.16% without significantly contributing to increased friction factor. In addition, a reduction in absorber tube thermal gradients by 53.3%, glass cover thermal gradients by 49.9%, and entropy generation by 27.1% were achieved with the use of a rotating receiver. In addition, PTSC with higher rim angles and concentration ratios were reported to have higher heat fluxes and thermal efficiency. However, with the higher heat fluxes, there are increased absorber tube wall temperatures, heat losses, and entropy generation rates. As such, heat transfer enhancement will play a critical role in enhancing the thermal and thermodynamic performance of high-concentration ratio systems.