Aggrey Mwesigye

Thermal Performance of a Receiver Tube for a High Concentration Ratio Parabolic Trough System and Potential for Improved Performance With Syltherm800-CuO Nanofluid

In this paper, the thermal performance of a high concentration ratio parabolic trough system and the potential for improved thermal performance using Syltherm800-CuO nanofluid were investigated and presented. The parabolic trough system considered in this study has a concentration ratio of 113 compared with 82 in current commercial systems. The heat transfer fluid temperature was varied between 350 K and 650 K and volume fractions of nanoparticle were in the range 1–6%. Monte-Carlo ray tracing was used to obtain the actual heat flux on the receiver’s absorber tube. The obtained heat flux profiles were subsequently coupled with a computational fluid dynamics tool to investigate the thermal performance of the receiver. From the study, the results show that with increased concentration ratios, receiver thermal performance degrades, with both the receiver heat loss and the absorber tube circumferential temperature differences increasing, especially at low flow rates. The results further show that the use of nanofluids significantly improves receiver thermal performance. The heat transfer performance increases up to 38% while the thermal efficiency increases up to 15%. Significant improvements in receiver thermal efficiency exist at high inlet temperatures and low flow rates.Copyright

HEAT TRANSFER ENHANCEMENT IN A PARABOLIC TROUGH RECEIVER USING WALL DETACHED TWISTED TAPE INSERTS

In this paper, heat transfer and fluid friction performance of a parabolic trough receiver with twisted tape inserts detached from the absorber tube’s wall is numerically studied. The numerical investigations were conducted for twist ratios in the range 0.30 ≤ y ≤ 2.40, width ratios in the range 0.53 ≤ w ≤ 0.91 and Reynolds numbers in the range 10,260 ≤ Re ≤ 320,000. The numerical simulations were performed using a finite volume method with the realisable k-e turbulence model and Syltherm 800 as the heat transfer fluid. The use of twisted tape inserts shows a significant increase in the heat transfer and fluid friction performance of the receiver. The study also reveals significant reduction in absorber tube’s circumferential temperature difference due to the improved heat transfer performance. For the range of parameters considered, the Nusselt number, fluid friction and thermal enhancement factor are 1.01–3.36, 1.32–21.8, and 0.74–1.25 times those in a receiver with a plain absorber tube respectively. The absorber tube’s circumferential temperature difference reduces between 4–76% compared with a plain absorber tube. Correlations for Nusselt number and fluid friction are also reported for the range of parameters considered.Copyright

Thermal Performance of a Parabolic Trough Receiver With Perforated Conical Inserts for Heat Transfer Enhancement

Heat transfer enhancement in receivers of parabolic trough collectors offers several benefits including reduction in absorber tube circumferential temperature differences, reduced emissivity of the absorber tube selective coating, thus improved thermal and thermodynamic performance of the receiver. In this work, heat transfer enhancement in a parabolic trough receiver using perforated conical inserts was numerically investigated. The analysis was carried out for dimensionless insert’s cone angles in the range 0.40–0.90, dimensionless insert spacing in the range 0.06–0.18 and dimensionless insert size in the range 0.45–0.91. The flow was considered fully developed turbulent with Reynolds numbers in the range 1.02 × 104 ≤ Re ≤ 7.38 × 105 depending on the temperature of the heat transfer fluid. The heat transfer fluid temperatures used were 400 K, 500 K, 600 K and 650 K. The numerical solution was obtained using the finite volume method together with the realizable k-e model for turbulence modeling. From the study, there is a range of Reynolds numbers and geometrical parameters for which the gain in performance is more than the increase in pumping power due to heat transfer enhancement. The use of perforated conical inserts in the receiver’s absorber tube increases the thermal efficiency in the range 3–8% for some range of geometrical parameters.Copyright

A numerical analysis and optimization of the dynamic performance of a multi-purpose solar thermal system for residential applications

This article presents results of a multipurpose solar thermal system that provides hot service water, space heating, and space cooling for residential use during all seasons in Pretoria, South Africa. A pressurized system utilizing evacuated tube solar collectors with internal heat pipes was considered for hot water production. Space cooling is achieved using a micro single-effect LiBr–H2O absorption chiller. The focus of the study was on the prediction of seasonal hourly performance trends and determination for optimum performance parameters. The solution was obtained by assembling the system’s mathematical model, for which the solution was obtained numerically using Engineering Equation Solver (EES). A solar field consisting of 15 (1.959 m2), 20 (2.615 m2), and 25 (3.266 m2) evacuated tube collectors connected in parallel with three, four, and five arrays was considered. The developed model was validated using data available in literature and found to be valid within ±3% of the available data. Results showed that the 20-tube collector with a five-array configuration gave the most favorable and optimal system performance.

A numerical model for optimal receiver array and mass flow rate in residential solar water heating systems

ABSTRACT In this paper, the performance of a pressurised evacuated tube solar collector system using internal heat pipes is presented. The system was optimised for the seasonal supply of hot service water for residential use in Pretoria, South Africa. The prediction of seasonal hourly performance trends along with the maximum thermal performance at the optimal receiver array and manifold mass flow rate was of major concern in this investigation. A mathematical model representing the thermal performance of the system was developed and numerically implemented in Engineering Equation Solver. The dynamic performance of collectors with 15, 20 and 25 tubes was determined throughout all the seasons. Moreover, the performance of the residential solar water heating system with a 20-tube collector was investigated in detail for mass flow rates of 0.03, 0.05 and 0.07u2005kg/s.

Experimental study on the thermal performance of R600a, R290 and R600a/R290 mixtures in a retrofit R134a refrigeration system

Refrigeration systems play paramount roles in life quality of human beings and social development in terms of food security, environmental impact, and energy efficiency. The traditional CFC, HCFC, and HFC refrigerants have been in the process of phase-out and being replaced by the sustainable working fluids. In this paper, the experimental thermodynamic performance evaluation of the hydrocarbons R600a, R290 and their mixtures used in a vapour compression refrigeration system that utilizes R134a as a working fluid was carried out. Firstly, a theoretical analysis was developed to evaluate the feasibility of the retrofit by employing the vapour compression refrigeration cycle. The evaporation temperatures were ranging from -25 °C to 3 °C, and the condensation temperatures ranging from 25 °C to 65 °C with superheating and subcooling degrees constant at 5 °C. The thermodynamic and thermophysical properties were obtained using REFPROP software. Lastly, based on the results obtained from the theoretical analysis, the experimental comparison of the refrigeration cycle performance was conducted using a refrigeration system designed for R134a. The results show that both pure R600a and R290 cannot be recommended as drop-in substitutes for R134a due to their significant differences in thermophysical properties. A mixture of R600a/R290 50%/50% composition was found to be the most appropriate alternative refrigerant for the R134a system retrofit with a comparative thermal performance.