Geometrical variation in receiver tube of SEGS LS-2 parabolic trough collector (PTC) based on heat flux distribution to improve the thermal performance
Introduction
Solar energy is one of the most tremendous sources of energy to produce sustainable and renewable energy for the future. Since the fossil energy sources (such as oil, coal and natural gas) could be finished in upcoming decades [1,2]. The consumption of energy is increasing gradually and this motivates researchers and scientists to develop the technologies of renewable energy [3]. Converting into heat or electricity is the significant property of solar energy as well as having various applications (e.g. desalination, steam generation, and cooling) [4]. Since the past decade, a great deal of research has investigated the various prospects of solar energy systems [5]. Among all the solar systems, the parabolic trough collector (PTC) is one of the most controversial topics of research [6]. One of the substantial proceedings in the increasing of PTC efficiency is the optimization of the collector geometry. In the optical analysis, the sun tracking, the optical and physical properties of the material, the concentrator shape, and some structural parameters must be considered before the evaluation of the performance of a solar concentrator. The aforementioned parameters can play a significant role in designing and analyzing thermal performance.
In the last years, some works have been concentrated on the application of new-designed geometries to enhance the heat transfer inside the receiver tube. Unconventional designs of the receiver tube have also been used to augment the turbulence of working fluid. Bellos et al. [7] conducted a study with several evaluation methods of a PTC utilizing 12 miscellaneous longitudinal fins in the tube. They analyzed the effects of fins on the thermal efficiency (), Nusselt number (Nu), and pressure drop (). Their results showed that the optimum receiver tube has the fin with 10 mm length and 2 mm thickness. The application of fin in the receiver tube enhanced the value of the Nu by 65% and the up to 0.82%. Bellos et al. [8] also investigated the effect of a wavy tube on the efficiency of a PTC. Their analysis presented that the presented design of the tube causes to the enhancement of the Nu and subsequently to 4.55% increasing in the . Zdaniuk et al. [9] fulfilled an experimental study to analyze both the Nu and friction factor (fr) in a helically finned tube. They could present a correlation for achievement of the Nu and fr in this kinds of tubes. Munoz and Abanades [10] applied the result of Zdaniuk et al. to have a numerical study in the helically finned tube in PTC using finite volume methods (FVM). Their result expressed a 2% enhancement in the of the PTC.
Kurşun et al. [11] presented and numerically analyzed a modified longitudinal fins which specified by its sinusoidal lateral surface. They concluded the sinusoidal lateral surfaces enhance the heat transfer rate in the longitudinal fins. Meanwhile, the effect of separated fin in the receiver tube has been analyzed by Kalidasan et al. [12] using one experimental equipment. The material of these fins was solid copper wire and twisted in helical pattern. Eventually, PTC with internal fins resulted high performance rather than conventional tube.
Divan and Soni [13] used an internal structure to make swirl flow which enhanced the Nu up to 330%. However, this structure increased the fr about 23 times greater than the base case-study. Shahin et al. [14] analyzed the utilization of metal foams and concluded a 3% more with 20 times more friction factor as a negative effect. Mwesigye et al. [15,16] surveyed the effect of perforated plate and resulted that the can augmented up to 8%, with 250% increase in the Nu, as well as 25 times more fr rather than the smooth tube. Furthermore, Chang et al. [17] inserted a rod in the center of tube and simulated the modified receiver tube. They concluded the Nu can be enhanced up to seven times and the parameter of fr rise up to 11 times, with the performance evaluation criterion (PEC).
Dravid et al. [18] studied helically coiled tubes with laminar flow to analyze the velocity profile of fluid flow on heat transfer characteristics. Also, Patankar et al. [19] studied the effect of Dean number on the Nu and fr in corrugated tubes affected by constant heat flux. Kurnia et al. [20] used computational fluid dynamics (CFD) techniques to survey the heat transfer and entropy generation of laminar flow in helical tubes with miscellaneous cross-sections. Meanwhile, effect of various variables such as geometry, wall temperature, Reynolds number and heating/cooling mode was elaborated at constant wall temperature. Kanatani et al. [21] defined a new model of receiver tube named helically coiled tubes and studied steady-state temperature distribution in its working fluid.
Generating miscellaneous heat flux distributions around the receiver tube can be realized using multi-piece reflectors. One commonly-used multi-piece reflector is linear Fresnel reflector (LFR). In LFR, a very large number of flat or curved mirrors with a common focal point are utilized to reflect the direct solar radiation onto a fixed receiver mounted on a tower along the linear reflectors. Zhang et al. [22] modeled and investigated the reflection of solar rays in direct vapor-generating LFR systems. They developed an economic alternative to PTC systems. Also, Abbas et al. [23] studied a multi-piece reflector and investigated the major disadvantages of LFR systems (e.g., lower optical efficiency. The field of linear Fresnel receivers (LFR) has been studied by experimental and numerical methods to investigate the optical properties of components such as primary reflectors in the Breault Research Organization [24]. In this work, the effect of the secondary reflector, acceptance angle, and focal position of the absorber on the optical performance of the LFR system have been scrutinized. Accordingly, it was concluded that the optical performance correlates with the geometric concentrator extremely. This correlation is commonly elaborated by the Intercept Factor, based on the definition of Bendt et al. [25]. Wu et al. [26] studied a three dimensional model of solar rays, convective and conductive heat transfer mechanism of PTC receiver. In this model, the temperature contours of PTC receiver under nonhomogeneous HF distribution have been elaborated.
Meanwhile, Hachicha et al. [27] compared their results with Jeter's paper to validate them. Wang et al. [28] applied a 3-D numerical model to analyze heat transfer in the PTC via the finite element method (FEM). According to Wang's results, the nonhomogeneous HF distribution influences the circumferential temperature of the receiver tube and has little effect on its thermal efficiency.
In current work, the simulation of solar radiation is conducted by Monte Carlo Ray Tracing (MCRT) technique which is a simulation method based on random numbers which are used for the accuracy solving of various problems (e.g. solar concentrators, wind energy modeling, rare gas modeling, etc.) [29]. Jeter had presented a mathematical model which has been applied as a index for the validation of a great number of research papers. These models are based on the Gaussian function to calculate HF distribution [30,31]. Cheng et al. [32,33], He et al. [34] and Peng et al. [35] adopted the MCRT method to simulate the HF around the receiver tube of PTC.
Muschaweck et al. [36] compared the performances of truncated symmetric and asymmetric edge ray reflectors by radiation analyses. They concluded the asymmetric edge ray reflectors may lead to the increase of the collector optical efficiency. The Soltrace software was used to trace the rays of one heliostat field to optimize its efficiency. The results of one mathematical model were validated by comparing with SolTrace.
Finally, due to the importance of heat transfer enhancement in the receiver tube, and the lack of study on changing the internal and external geometry of the receiver tube considering the HF distribution, the solar energy generating systems (SEGS) LS-2 collector will be analyzed in this work. To generate the different heat flux (HF) around the receiver tube, a conventional PTC and a PTC with segregated reflectors are considered in this study. The properties of these collectors will be elaborated in following section.
Section snippets
Model description
The miscellaneous solar collectors which exchange solar energy into thermal energy include the flat plate, parabolic, dish, and heliostat forms. A parabolic trough collector (PTC) consists of one parabolic-shaped reflector and a receiver tube placed along the focal axis of the parabola, and sun-tracking system. Solar rays are concentrated onto the receiver tube, where solar energy is absorbed by working fluid. A glass envelope covered the receiver tube to limit convection losses and further
Mathematical analysis
The applied equations are scrutinized in this section. First of all, the available solar energy () is obtained by multiplying the aperture area (Aa) by the direct normal incident (DNI).
Optical efficiency () of a solar collector is defined as the rate of optical energy hitting the receiver tube, divided by the . In this paper the amount of is considered equal with the nominal amount of in LS-2 collector (i, e., 75.5%) [39].
Collector thermal efficiency (
Definition of concepts
The concepts of some parameters and components including Mean value of HF, displacement of plug, eccentricity, the highest HF regions, and utilized longitudinal fins are separately explained in this section.
Heat flux distribution achieved by the sunrays tracing
To obtain the HF distribution via the MCRT technique, the incident points of the reflected rays on the outer wall of the receiver tube must be determined. The bundle of incident rays is considered conic shape, because of the difference between the sun and the earth diameters. The x', y', and z' are the components of the coordination system located at the tip of the mentioned cone. Fig. 5 illustrates the optical cone and one of the sunrays in the optical cone determined with two angles of and
Result and discussion
In this section, three effective variations are scrutinized including the central plug displacement, application of the fins in the highest HF regions, as well as the effect of the HF distribution created by multi-piece reflector. To analyze the effect of these three variations, some important variables are considered and studied comprising the outlet temperature (To), thermal efficiency (), Nusselt number (Nu), friction factor (fr), and two above-mentioned evaluation indices (i.e., ,
Conclusions
According to results, it can be concluded:
- •
Two indices of and can be applied in evaluation of heat transfer systems and gives the similar results.
- •
In view point of index, PTC with three-piece reflector and also with the central plug as well as the big fins can give the optimum results than the other case-studies. On the other hand, considering the index of , the conventional PTC with E.F = 2/3 and the big fins can give the optimum results.
- •
The devised concavity as fin in the highest
Declaration of competing interest
The Authors declare that there is no conflict of interest.
References (48)
Parabolic trough solar collectors integrated with a Kalina cycle for high temperature applications: energy, exergy and economic analyses
Energy Convers. Manag.
(2017)- et al.
Optimization of non-evacuated receiver of solar collector having non-uniform temperature distribution for minimum heat loss
Energy Convers. Manag.
(2014) - et al.
“Thermal enhancement of solar parabolic trough collectors by usingnanofluids and converging-diverging absorber tube
Renew. Energy
(2016) - et al.
“Experimental deter-mination of heat transfer and friction in helically-finned tubes.”
Thermal Fluid Sci.32
(2008) Abánades.“Analysis of internal helically finnedtubes for parabolic trough design by CFD tools
Appled Energy 88
(2011)Thermal performance assessment of internal longitudinal fins with sinusoidal lateral surfaces in parabolic trough receiver tubes
Renewal. Energy
(2019)- et al.
Absorber tube with internal hinged blades for solar parabolic trough collector
Energy Procedia
(2016) - et al.
Investigation of heat transfer enhancement in a new type heat exchanger using solar parabolic trough systems
Bar Int.
(2015) - et al.
Heat transfer and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts.Appl
Energy
(2014) - et al.
Multi-objective and thermodynamic optimisation of a parabolic trough receiver with perforated plate inserts. Appl. Therm
Eng. Times
(2015)
Zhao, Enhanced heat transfer in a parabolic trough solar receiver by inserting rods and using molten salt as heat transfer fluid
Appl. Energy
Prediction of laminarflow and heattransfer in helically coiled pipes, in: numerical Prediction of Flow
HeatTransfer, Turbulence and Combustion
Concentratedsolar power plants: review and design methodology
Renew. Sustain. Energy Rev.
Solarradiation concentration features in linear Fresnel reflector arrays
Energy Convers. Manag.
Three-dimensional numerical study of heat transfer characteristics of parabolic trough receiver
Appl. Energy
Heat transfer analysis and numerical simulation of a parabolic trough
Appl. Energy
A three-dimensional simulation of a parabolic trough solar collector system using molten salt as heat transfer fluid
J. of Appled Thermal Eng.
Integrated analysis on the volumetric absorption characteristics and optical performance for a porous media receiver
Energy Convers. Manag.
Calculation of the concentrated flux density distribution in parabolic trough collectors by a semifinite formulation
Sol. Energy
Analytical determination of the optical performance of practical parabolic trough collectors from design data
Sol. Energy
Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector
Int. Commun. Heat Mass Tran.
A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector,
Renew. Energy
A new modelling method and unified code with MCRT for concentrating solar collectors and its applications
J. of Applied Energy
Evaluation of heat flux distribution uniformity around the receiver tube of parabolic trough collector based on six statistical and geometrical indices
Int. J. Heat Mass Tran.
Cited by (9)
Effect of oval rib parameters on heat transfer enhancement of TiO2/water nanofluid flow through parabolic trough collector
2024, Case Studies in Thermal EngineeringExperimental performance evaluation of a solar parabolic dish collector using spiral flow path receiver
2023, Applied Thermal EngineeringHeat transfer enhancement of a parabolic trough solar collector using a semicircular multitube absorber
2022, Renewable EnergyCitation Excerpt :Providing fins at the bottom of the absorber enhanced the turbulence, further enhancing contact between HTF and the inner wall of the absorber. Heat transfer was intensified by inserting the plug into the center and positive displacement within the absorber tube [28]. A computational study was conducted to determine the thermal performance of a nanofluid-filled PTC with an acentric absorber tube.
Experimental study on heat transfer enhancement of parabolic trough solar collector using a rectangular channel receiver
2022, Journal of the Taiwan Institute of Chemical EngineersCitation Excerpt :Increasing the heat transfer area by providing fins and enhancing the fluid flow path can increase the heat transfer rate. The dual-functionality of the fins is preferred for enhancing the heat transfer with a lesser cost [72,73]. Increasing the heat transfer area of solar receivers increases the heat absorption and exchange rate [74].
Machine learning predictions for enhancing solar parabolic trough collector efficiency with corrugated tube receivers
2024, Sadhana - Academy Proceedings in Engineering Sciences