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    Enhancing the overall thermal performance of a large aperture parabolic trough solar collector using wire coil inserts
    (Elsevier, 2020) Yilmaz, Ibrahim Halil; Mwesigye, Aggrey; Goksu, Taha Tuna
    With the use of large apertures (higher concentration ratios) in parabolic trough solar collectors, increased temperature gradients, increased heat losses and increased heat transfer irreversibilities become inevitable. As such, means of reducing the magnitude of these operating parameters to enhance the overall thermal and thermodynamic performances become crucial. In this study, the use of wire coil inserts in the receiver's absorber tube to improve the parabolic trough solar collector's performance and to lessen the associated temperature gradients is presented. The parabolic trough solar collector having an aperture width of 9 m and a rim angle of 80 degrees was modeled. Using Monte-Carlo ray tracing, the realistic heat flux profile on the receiver's absorber tube was obtained. The resulting non-uniform heat flux profile was later coupled to a finite volume based computational fluid dynamics model. The working fluid properties were considered to be temperature-dependent. The wire coil inserts with a pitch of 0.076, 0.114 and 0.152 m and widths of 0.03, 0.033 and 0.036 m were examined in this study. The wire coil has a triangular cross-section of 0.0076 m in size. Results show significant improvements in receiver thermal performance with the use of wire coil inserts owing to the improved fluid mixing, disruption of the thermal boundary layer and reduction in the absorber tube temperatures. The heat transfer performance is increased up to 183% whereas the thermal efficiency improves between 0.4 and 1.4% when the flow rate is below 13 m(3)/h.
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    Modeling, simulation and performance analysis of parabolic trough solar collectors: A comprehensive review
    (Elsevier Sci Ltd, 2018) Yilmaz, Ibrahim Halil; Mwesigye, Aggrey
    Solar thermal systems are advantageous since it is easier to store heat than electricity on a large scale. As such, concentrated solar power is receiving considerable interest among researchers, developers and governments. Several concentrated solar power technologies have been developed including the solar tower, the parabolic trough technology, solar dish and linear Fresnel systems. Among them, the parabolic trough solar collector is a proven technology used dominantly for both industrial process heat and power generation. This technology has matured over the years, and its advancement has become the topic of numerous research studies which were the counter driving force of the field. Particularly in recent years, a significant amount of theoretical and numerical studies have been conducted to assess and improve the performance of parabolic trough solar collectors. This review methodologically holds colossal knowledge of current and past studies to assess the optical and thermal performances of parabolic trough solar collectors, modeling approaches and the potential improvements proposed on behalf of the parabolic trough solar collector design. The optical modeling approaches are identified to be analytical and ray-tracing. The review of thermal modeling approaches presents the steady and transient heat transfer analyses of single and two-phase (with direct steam generation) flows. Also, the computational fluid dynamics models used to analyze the physics of parabolic trough solar collectors with a better insight are reviewed and presented. Finally, the studies conducted on the performance improvement of parabolic trough solar collectors are separately examined and presented, these include novel designs, passive heat transfer enhancement, and nanoparticle laden flows.
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    Numerical analysis of the thermal and thermodynamic performance of a parabolic trough solar collector using SWCNTs-Therminol®VP-1 nanofluid
    (Pergamon-Elsevier Science Ltd, 2018) Mwesigye, Aggrey; Yilmaz, Ibrahim Halil; Meyer, Josua P.
    In this paper, energetic and exergetic performances of a parabolic trough solar collector using single walled carbon nanotubes (SWCNTs)-Therminol (R) VP-1 nanofluid were numerically investigated and presented. The main objective of this investigation was to determine the influence of high thermal conductivity SWCNTs suspended in the widely used heat transfer fluid, Therminol (R) VP-1 on the performance indicators of the parabolic trough solar collector. A parabolic trough system with a high concentration ratio of 113 was analyzed in this study. The thermo-physical properties of SWCNTs were taken as functions of nanotube length, nanotube diameter, and temperature, while the properties of Therminol (R) VP-1 were considered to be temperature dependent. The study involved determination of the actual heat flux profile through Monte Carlo ray tracing and the subsequent coupling of this heat flux profile to a computational fluid dynamics tool using user defined functions. The computational fluid dynamics tool was finite volume based, and the realizable k-epsilon model together with enhanced wall treatment were used for turbulence modeling. The entropy generation rates were obtained directly from the local velocity and temperature fields of the computed domain and later used in the exergy analysis. Results showed that although the heat transfer performance significantly improved with the use of SWCNTs, the increase in the thermal efficiency was not substantial. For the considered range of parameters, while the heat transfer performance increased up to 234%, the thermal efficiency increased around 4.4% as the volume fraction increased from 0 to 2.5%. The corresponding reduction in the entropy generation was about 70%. (C) 2017 Elsevier Ltd. All rights reserved.
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    Prioritization of heat transfer fluids in parabolic trough solar systems using CFD-assisted AHP-VIKOR approach
    (Pergamon-Elsevier Science Ltd, 2023) Yilmaz, Ibrahim Halil; Mwesigye, Aggrey; Kılıç, Fatih
    Parabolic trough solar collectors (PTSCs) are proven technologies and are used in various fields including concentrating solar power, integrated solar combined cycle, industrial process heat, air conditioning, and desalination. The selection of heat transfer fluid (HTF) is a part of the design consideration of concentrating solar technologies (CST) since its technical specifications have a significant effect on the system configuration, operating conditions, and levelized energy cost. Several HTFs including water, thermal oils, molten salts, and gases are already in use for PTSC systems, and others including liquid metals, ionic liquids, and nanofluids are still being tested. Each HTF has pros and cons; thermal and thermodynamic evaluations are practical to compare the performance of an HTF with that of its counterparts however they are insufficient to attribute a parabolic trough CST in terms of design concerns. This study has presented an integrated AHP-VIKOR (Analytical Hier- archy Process - VIekriterijumsko KOmpromisno Rangiranje) multi-criteria decision making (MCDM) approach assisted by computational fluid dynamics (CFD) to prioritize HTFs, particularly for CST and the results are compared to AHP-TOPSIS (Analytical Hierarchy Process - Technique for Order Preference by Similarity to Ideal Solution) for validation. While AHP is chosen to decide the criteria weights in the multi-criteria process, VIKOR is used as decision making to rank and prioritize alternatives. Water, Therminol VP-1, Solar salt, Hitec, Hitec XL, Liquid sodium, Lead-bismuth eutectic, Carbon dioxide, Air, and Helium are evaluated as the HTF inventory and a comprehensive CFD study is conducted to benchmark their thermal and thermodynamic properties in a high concentration PTSC in terms of solidification temperature, upper thermal stability, collector efficiency, pumping power consumption, receiver temperature gradient, and irreversibilities. In addition, other critical design con- siderations of HTFs including material cost, heat storage capability, material compatibility, operational safety aspects, operational maturity, and solar field control sophistication are taken into account in the MCDM process. Results show that the current parabolic trough CST should have priority for the HTFs of water and molten salt (Hitec) but molten metals and gaseous fluids would gain more insight in near future. This study presents an original perspective for deciding suitable HTFs in PTSC applications where thermal and thermodynamic analyses are limited.
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    Thermal and thermodynamic benchmarking of liquid heat transfer fluids in a high concentration ratio parabolic trough solar collector system
    (Elsevier, 2020) Mwesigye, Aggrey; Yilmaz, Ibrahim Halil
    The thermal oil-based heat transfer fluids (HTFs) used in parabolic trough solar collector (PTSC) systems suffer from degradation at temperatures above 400 degrees C, limiting the thermal efficiencies of these systems. As such, several researchers have investigated various HTFs for high-temperature applications of PTSCs. In this study, the thermal and thermodynamic performance of a PTSC system with a geometrical concentration ratio of 113 is numerically investigated. The developed and thorough validated numerical model combines Monte-Carlo ray tracing and computational fluid dynamics for optical analysis, and thermal and thermodynamic studies, respectively. Ten HTFs with temperature-dependent thermal physical properties are considered. They are -Liquid metals: liquid sodium, Lead-Bismuth Eutectic (LBE), -Molten salts: Solar Salt, Hitec, Hitec XL, a ternary salt mixture: LiNO3 + NaNO3 + KNO3 (18 wt%, 52 wt%. and 30 wt%), a quaternary salt mixture: NaNO3 + KNO3 + LiNO3 + Ca(NO3)(2) (9 wrI, 54 wt%, 18 wt%, and 18 wt%), a new salt mixture: NaCI+KCI + ZnCl2 (7.5 wt%, 23.9 wt%, and 68.6 wt%), and -Thermal oils: The rm inol VP-1 and Dowtherm A. Results show that liquid sodium gives the best thermal, hydraulic, and thermodynamic performance of the considered HTFs at all flow rates and inlet temperatures. LBE gives the second highest heat transfer performance, however, its thermal and thermodynamic performance degrade as flow rates increase above 32.75 m(3)/h owing to the high pumping power and fluid flow irreversibilities. As liquid sodium and LBE are expensive, LiNO3 + NaNO3 + KNO3 (18 wt %, 52 wt%, and 30 wt%) shows better overall performance as compared with other molten salts. Moreover, it possesses a low melting point and high thermal stability temperature. In addition, the thermal efficiency is within +/- 0.4% for molten salts at flow rates between 16 and 36 m(3)/h that give optimal performance. (C) 2020 Elsevier B.V. All rights reserved.
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    Thermal and thermodynamic optimization of the performance of a large aperture width parabolic trough solar collector using gaseous and supercritical CO2 as heat transfer fluids
    (Elsevier, 2023) Mwesigye, Aggrey; Yilmaz, Ibrahim Halil
    The degradation of thermal-oil-based working fluids in parabolic trough solar collector (PTSC) systems at temperatures above 400 degrees C has accelerated the search for cost-effective and efficient heat transfer fluids (HTFs) for high-temperature applications. Carbon dioxide is abundant and has the potential to be used as a working fluid in PTSCs. In this work, the performance of a larger aperture width PTSC with CO2 as the working fluid (either in gaseous or supercritical phase) is investigated. A PTSC system with an aperture width of 9 m (geometric con-centration ratio of 113) was considered. The operating pressure is in the range of 40-100 bar, the HTF inlet temperatures are between 650-1000 K, and the flow rate varies from 32.6 to 653 m3/h. A thoroughly verified and validated computational model using a combination of Monte-Carlo ray tracing and computational fluid dynamics was used for the numerical investigations. In addition, the Realizable k-epsilon model was used for turbu-lence modeling. The results show that CO2 at 80 bar has comparable thermal performance relative to current thermal oil-based HTFs. Results further show that supercritical CO2 (sCO2) has the best thermal performance compared to gaseous CO2. Moreover, the entropy generation minimization method was used to determine the Reynolds number that minimizes the irreversibilities at a given operating pressure. At low pressures, much higher flow rates are required to achieve high heat transfer rates and reduce temperature gradients in the absorber tube. Furthermore, correlations have been developed for the optimal Reynolds numbers at which the entropy generation rate is minimum and the collector efficiency is maximum.