Absorber Tube Research Articles

Analytic optical evaluation of linear aplanatic solar Fresnel reflectors

Linear Aplanatic Fresnel Reflectors (LAFRs) were recently presented as a novel and viable dual-mirror solar concentrator. Within a vast parameter space, a few promising designs were identified and evaluated via raytracing. This cumbersome, time-intensive exploration left unresolved whether configurations with superior optical performance remained undiscovered. Here, we present analytic methods for all inherent optical losses for the LAFR - results that are equally valid for almost all sources of optical loss in all linear Fresnel reflector solar concentrators. The loss modes include the separate contributions linked to the transverse and longitudinal projected incidence angles. This permits exact rapid comparisons among the wide range of possible aplanatic designs in a physically transparent manner that quantifies each mode of optical loss. These analytic derivations subsume blocking of rays reflected from the primary mirror segments, radiation that misses the primary and strikes the ground, shading of the primary by the secondary, end losses, and rays reflected from the primary that either miss the secondary (spillage) or enter it and miss the absorber tube (ray rejection). LAFR designs superior to those determined previously, in terms of higher intercept factor and higher concentration, are found and evaluated.

Experimental investigation of a parabolic trough collector-thermoelectric generator (PTC-TEG) hybrid solar system with a pressurized heat transfer fluid

Concentrating solar power (CSP) plants that used thermal parabolic trough collectors (PTC) are the most suitable technology in the clean power production. Several efforts have been done for enhancing the performance of PTCs. In this research, a parabolic trough collector-thermoelectric generator (PTC-TEG) hybrid solar system is proposed. In this system, two approaches to enhancing efficiency were implemented and conducted, and its thermal and electrical performances were experimentally investigated. The first, the heat transfer fluid (HTF) into the absorber tube was pressurized using a pressure control unit under relative pressures of 0.3 and 0.5 bar. The second, it was proposed that a TEG module was installed on the backside of the reflector for generating the additional electrical power from the absorbed solar radiation by it. These two approaches were implemented on a parabolic trough collector was equipped with polar N–S axis and E-W tracking mechanism. The results of the thermal performance evaluation of this system showed that the thermal efficiency increased 6.88% and 14.64% by increasing the HTF pressure of 0.3 and 0.5 bar relative to conventional PTC, respectively. The generated electrical power by a series array 70 TEG module has a 0.96–1.11% contribution to the increasing total efficiency of the proposed system. By increasing the pressure of the working fluid, the rising rate of working fluid temperature increased. Generally, the total efficiency of the proposed hybrid solar system 15.75% enhanced.

Dynamic mathematical heat transfer model for two-phase flow in solar collectors

In the present study, a dynamic heat transfer model is developed for two-phase flow through the absorber tube of linear solar collectors, where the heat transfer fluid is water. The model considers that thermophysical properties, such as viscosity, thermal conductivity, densities, and specific heat, depend on temperature; this dependency is reflected in the value of the convective heat transfer coefficient. The governing partial differential equations for the fluid are solved using the finite difference method in an explicit scheme, the heat transfer equation for the absorber tube uses an implicit scheme, whose solution is implemented in C++ compiler. The model is validated with experimental data from a solar collector using a Solar Fresnel Reflector type, with an error related to the steam quality lower than 4.28% at the outlet of the collector and a better fit with the temperature profile through the collector in comparison with previous studies. The results show that as the phase change occurs, increasing the quality of steam in the absorber tube, the collector efficiency decreases. This is due to that the convective heat transfer coefficient of the absorber decreases, since the thermophysical properties of the liquid-steam mixture do not favor heat transfer.

Combined Thermal Performance Enhancement of Parabolic Trough Collectors Using Alumina Nanoparticles and Internal Fins

Parabolic trough collectors are the currently dominant technology for concentrated solar power systems, employed to produce thermal energy at low to medium temperatures (up to 400°C). Extensive research has been carried out to enhance the thermal efficiency and reduce the power production costs of these concentrators. However, there is a lack of studies on combined passive performance enhancement using alternative fluids and absorber designs. In this study, the thermal performance of a full-sized parabolic trough collector is analyzed with the presence of internal longitudinal fins in combination with the use of oil-based nanofluid (Al2O3-Syltherm 800) of different volume fractions. The governing equations are numerically solved using ANSYS FLUENT 17.1 software and the Monte-Carlo ray-tracing (MCRT) model was used to apply the non-uniform heat flux profile over the external surface of the solar receiver. The results show that both techniques enhance thermal energy utilization and reduce radiative and convective thermal losses, resulting in higher thermal efficiency, but also larger pressure losses. The thermal performance is enhanced by 0.1-1.16 % with nanofluid, up to 6.8 % with internal fins, and by up to 7.25 % when both techniques are adopted. These enhancements are attributed to the reduced mean circumferential temperature of the absorber tube.

Investigations of thermo-structural instability on the performance of solar parabolic trough collectors

Solar parabolic trough collectors are a mature technology among other concentrated solar power technologies. However, improvements in the structural stability and the accuracy in the shape of the reflectors and the receiver are essential for making them at par with other solar power technologies. A methodology is proposed in the present work for the investigations of the combined effect of the thermo-structural instability in the reflector and the receiver on the performance of trough collectors. The circumferential temperature distribution and wind loads lead to bending in the receiver and the reflector, respectively. The analysis involves temperature and bending effects at four mass flow rates of 3 kg/s, 6 kg/s, 9 kg/s, and 18 kg/s in the absorber tube and three slope errors 1 mrad, 2 mrad, and 3 mrad at the reflector. The optical efficiency decreases from 86% to 67.24% for the bending from 0 mm to 10 mm, and the loss is as high as 17.7% for 3 mrad and 10 mm deflections at the reflector and the receiver, respectively. The present study shall help in the design and condition monitoring of the trough collectors.

Design study for the lifetime of double-sided irradiated absorber tubes

The world is in need of sustainable and cost efficient supply of electrical energy. One important technology could be solar power towers, because of their base load capability. Even though this technology has seen a significant decrease in installation and operation costs, further cost reduction is necessary to reach a higher market share.This study investigates double-sided irradiated absorber tubes to reduce installation costs of molten salt receivers. With a re-arrangement of the tubes towards a star-shaped layout, the required number of the expensive absorber tubes could be halved. This study introduces and uses a vivid methodology, based on linear damage accumulation that allows for the calculation of damage proportions, as well as the expected lifetime of receiver components in years. The calculations show the lifetime of double-sided irradiated tubes to be about ten times higher than for those irradiated from one side. This enables a change in material towards cost efficient austenitic stainless steels, offering an additional option for cost savings. However, it is also outlined that particular attention has to be payed to the aiming point optimization of the heliostat field, because the irradiation has to be quite smooth and uniform on both sides of the tubes to extend the lifetime.

Thermal Analysis of a Parabolic Trough Collectors System Coupled to an Organic Rankine Cycle and a Two-Tank Thermal Storage System: Case Study of Itajubá-MG Brazil

This study examined an Organic Rankine Cycle powered by a parabolic trough collector and a two-tank thermal storage system based on the development of a mathematical model, for the conditions of the city of Itajubá in Brazil. First, geometrical optics and heat transfer models of the collector–receiver set were used to determine the thermal equilibrium of the solar thermal collector system and parameters such as the efficiency of the solar field, heat and optical losses, and thermal energy of the outlet fluid. Next, the thermal equilibrium of the Organic Rankine Cycle was found in order to establish its operational parameters. Finally, the behavior of the thermal storage system was analyzed through its modeling. Once the characterization of the storage system was completed, the integrated operation of the proposed system was evaluated. Given Itajubá’s weather conditions, the results indicate that an electricity generation system can be implemented with the Solel UVAC Cermet selective coating for the absorber tube, water as the heat transfer fluid, and R-245fa as the working fluid. Based on the solar irradiation profile (1 March 2019), the parabolic trough collectors provided 63.3% of the energy required by the Organic Rankine Cycle to generate 7.4 kW, while the thermal storage system provided 36.4% of the energy demanded by the power generation block. Additionally, the results demonstrate the main conclusions that the turbine’s efficiency was influenced by parameters such as rotational speed, which is affected by the turbine inlet temperature, which, in turn, depends on the behavior of the solar irradiation profile onsite.

Investigation of the effect of twisted tape turbulators on thermal-hydraulic behavior of parabolic solar collector with polymer hybrid nanofluid and exergy analysis using numerical method and ANN

In the present work, a study has been performed on increasing the heat transfer of parabolic solar collectors through modeling of twisted tapes inside the absorber tube containing nanofluid, using ANSYS19.2 software with Finite Volume Method (FVM). In this regard, two specimens of twisted tape with different number of channels is installed in the absorber tube having a constant wall heat flux (1200 W) in the range of Re numbers (25,000–5000) flows with different volume fractions (2–6%). Performance Evaluation Criterion (PEC), Nusselt Number (Nu) and solar collector efficiency (η) have been defined and their changes have been investigated. And achieve high efficiency (2.18) at Re =25,000, φ=2%. Therefore, model two turbulator is more desirable from the point of view of thermal fluid dynamics. The exergy efficiency of the liquid containing nanoparticles with a volume fraction of 2% declined for Cases 2, and 1 by about 0.023% and 0.026%, respectively, with increasing velocity. The neural network was used to trend the effects of perturbation on Nu, PEC, and exergy efficiency. The neural network trended three parameters with high accuracy so that the maximum error was estimated below 3.8%.

Exergy, Economic and Environmental Analysis of a Direct Absorption Parabolic Trough Collector Filled with Porous Metal Foam

A direct absorption parabolic trough solar collector (DAPTC) integrated with porous foam as a volumetric absorber has the potential to be applied as an energy conversion integrant of future renewable energy systems. The present study comprehensively analyzes a DAPTC in terms of exergy, economic, and environmental analysis for different porous configuration inserts in the absorber tube. Ten different arrangements of porous foam are examined at several HTF flow rates (40–120 L/h) and inlet temperatures (20–40 °C). The exergy efficiency, entropy generation, Bejan number, and pumping power are investigated for all cases. Obtained results indicate that fully filling the absorber tube with porous foam leads to a maximum exergy efficiency of 20.4% at the lowest inlet temperature (20 °C) and highest flow rate (120 L/h). However, the Bejan number reaches its minimum value due to the highest pumping power in this case. Consequently, all mentioned performance parameters should be considered simultaneously. Finally, the environmental and economic analyses are conducted. The results show that fully filling the absorber tube with porous foam reflects the best heat production cost, which can reduced the embodied energy, embodied water, and CO2 emission by 559.5 MJ, 1520.8 kL, and 339.62 kg, respectively, compared to the base case at the flow rate of 120 L/h.

4E study of experimental thermal performance enhancement of flat plate solar collectors using MWCNT, Al2O3, and hybrid MWCNT/ Al2O3 nanofluids

Nanofluid is a cutting-edge heat transfer fluid with the potential to significantly improve the heat transfer performance of conventional fluids. The most valued aspects to increase the overall solar collector's effectiveness are to improve design factors and the convection heat transfer coefficient between the absorber tubes and fluid. Water nanofluids including MWCNTs, Al2O3, TiO2, SiO2, and CuO are the most often utilized nanofluids in solar collectors. In this paper, the Flat Plate Solar collector thermal efficiency was experimentally investigated for MWCNT, Al2O3, and hybrid MWCNT/Al2O3 50:50% as a working fluid, under controlled circumstances. The effects of several parameters, including solar radiation intensity, volume proportion of nanoparticles, and volumetric flow rate on the efficiency, are investigated. Four volume concentration percentages were studied for each type of nanofluid (0.5%, 0.025%, 0.01% and 0.005%) with three different mass flow rates for each concentration. The results show that using hybrid MWCNT/Al2O3 (50:50%) offers an increase in efficiency by 26%, 29%, and 18% for 1.5 L/m, 2.5 L/m, and 3.3 L/m, respectively, recommending the replacement of 50% of the MWCNTs by the more economic and environmentally friendly Al2O3.

Thermal Performance Estimation of Nanofluid-Filled Finned Absorber Tube Using Deep Convolutional Neural Network

Numerical simulations are usually used to analyze and optimize the performance of the nanofluid-filled absorber tube with fins. However, solving partial differential equations (PDEs) repeatedly requires considerable computational cost. This study develops two deep neural network-based reduced-order models to accurately and rapidly predict the temperature field and heat flux of nanofluid-filled absorber tubes with rectangular fins, respectively. Both network models contain a convolutional path, receiving and extracting cross-sectional geometry information of the absorber tube presented by signed distance function (SDF); then, the following deconvolutional blocks or fully connected layers decode the temperature field or heat flux out from the highly encoded feature map. According to the results, the average accuracy of the temperature field prediction is higher than 99.9% and the computational speed is four orders faster than numerical simulation. For heat flux estimation, the R2 of 81 samples reaches 0.9995 and the average accuracy is higher than 99.7%. The same as the field prediction, the heat flux prediction also takes much less computational time than numerical simulation, with 0.004 s versus 393 s. In addition, the changeable learning rate strategy is applied, and the influence of learning rate and dataset size on the evolution of accuracy are investigated. According to our literature review, this is the first study to estimate the temperature field and heat flux of the outlet cross section in 3D nanofluid-filled fined absorber tubes using a deep convolutional neural network. The results of the current work verify both the high accuracy and efficiency of the proposed network model, which shows its huge potential for the fin-shape design and optimization of nanofluid-filled absorber tubes.

Conductive and convective heat transfer augmentation in flat plate solar collector from energy, economic and environmental perspectives - a comprehensive review.

The primary objective of the paper is to identify the effective way to enhance the conductive and convective heat transfer of the FPSC. The performance enhancements of different FPSC components such as absorber plate, absorber tube, and heat transfer fluid are reviewed in detail. The influence of absorber plate configurations, material properties, a center-to-center distance of the absorber tube, plate thickness, coatings, and tube geometry have been assessed to increase the conduction heat transfer. Also, the augmentations of convective heat transfer using different nanofluids in FPSC such as Al2O3/water, CuO/water, CNT/water, TiO2/water, SiO2/water, graphene oxide/water, MgO/water, CeO2/water, WO3/water, ZnO/water, and hybrid nanofluids are evaluated in detail. The performance improvements using both conductive and convective (combined) passive technique have been elaborated. The table representation has been used to describe the activities performed in each paper which include FPSC type, passive technique detail, properties of heat transfer fluid, Reynolds number, heat transfer aspects, pumping power, energy, exergy, environmental aspects, and inference. These data will help the researcher to identify existing activities and the potential gap. This review paper also deals with the suggestions for the research work which can be carried out in the direction of heat transfer from solar flat plate collectors.

Performance analysis of a solar parabolic trough collector fitted with a helical coiled tube absorber

The SPTC is used for medium- and high-temperature applications. Several works are carried out for the enhancement of heat transfer of SPTC using size variation, absorber with inserts, nanofluid with working medium and different configurations of the absorber. The development of turbulence effect in the working fluid is important for heat transfer enhancement. Therefore, a novel idea is introduced to use a helical coiled tube absorber in the PTC, which enhances the heat transfer between the tube and the flowing fluid by providing swirling motion. The current research work uses the helical coiled tube absorber which consists of 42 turns and 2 m in length, 10 mm in internal diameter and 12.5 mm in outer diameter. It is enclosed by a glass envelope to reduce thermal losses. The experiments are conducted by varying the mass flow rate of water from 0.01 kg s−1 to –0.03 kg s−1. The maximum achieved useful heat gain, water temperature and instantaneous efficiency are 800 W, 64°C and 56%, respectively while using the flow rate of 0.01 kg s−1.

Heat Transfer Enhancement in a Receiver Tube of Solar Collector Using Various Materials and Nanofluids

The solar flux distribution on the Parabolic Trough Collector (PTC) absorber tube is extremely non-uniform, which causes non-uniform temperature distribution outside the absorber tube. Therefore, it generates high thermal stress which causes creep and fatigue damage. This presents a challenge to the efficiency and reliability of parabolic trough receivers. To override this problem, we have to homogenize the heat flux distribution and enhance the heat transfer in the receiver’s absorber tube to improve the performance of the PTC. In this work, 3D thermal and thermal stress analyses of PTC receiver performance were investigated with a combination of Monte Carlo Ray-Trace (MCRT), Computational Fluid Dynamics (CFD) analysis, and thermal stress analysis using the static structural module of ANSYS. At first, we studied the effect of the receiver tube material (aluminium, copper, and stainless steel) on heat transfer. The temperature gradients and the thermal stresses were compared. Second, we studied the effect of the addition of nanoparticles on the working Heat Transfer Fluid (HTF), employing an Al2O3-H2O based nanofluid at various volume concentrations. To improve the thermal performance of the PTC, a nanoparticle volume concentration ratio of 1%–6% is required. The results show that the temperature gradients and thermal stresses of stainless steel are significantly higher than those of aluminium and copper. From the standpoint of thermal stress, copper is recommended as the tube receiver material. Using Al2O3 in water as an HTF increases the average output temperature by 2%, 6%, and 10% under volume concentrations of 0%, 2%, and 6% respectively. The study concluded that the thermal efficiency increases from 3% to 14% for nanoparticle volume fractions ranging from 2% to 6%.

DESIGN AND NUMERICAL SIMULATION OF A SOLAR ENERGY DRIVEN SMALL-SCALE MILK PASTEURIZATION SYSTEM

The conventional sources of heat generation are widely prevalent in the food industry like the electricity and usage of fossil fuels. However, they have some associated drawbacks with them like rising fuel prices, associated pollution level etc. These clearly mandate the use of renewable sources of energy like wind, hydrothermal or solar energy. Solar energy is the one of the major sources of energy, which is renewable, inexhaustible, and clean. It can significantly contribute the world energy requirement with available conversion technology. The present study is about to designing and numerical modeling a solar energy driven small-scale milk pasteurization system. The design and operating parameters were evaluated first and then computer-aideddesigning (CAD) software was used to create a 3-D diagram of the system. Then a prototype has been developed and its performance evaluation was carried out. Collector of focal length 12 cm, aperture width 31.5 cm, rim angle 131.5 ° was fabricated with a 3 m long absorber tube. The milk was passed through absorber tube at flow rate 30 L/h giving an outlet temperature of 72 °C. Computation fluid dynamics (CFD) analysis of the data showed good correlation between experimental data and simulation.

Thermal performance of nanofluids based on tungsten disulphide nanosheets as heat transfer fluids in parabolic trough solar collectors

Nanofluids are considered as a new generation of heat transfer fluids since they exhibit thermophysical properties improvements compared with conventional heat transfer fluids. The high thermal conductivity of nanofluids and even the isobaric specific heat enhancements over conventional liquids make these colloidal suspensions very attractive in many research areas, including solar energy. In this work, nanofluids based on tungsten disulphide (WS 2 ) nanosheets have been prepared from the thermal oil currently used as heat transfer fluid in Concentrating Solar Power (CSP) plants. The high aspect ratio of WS 2 bidimensional nanostructures provides high long-term colloidal stability to the nanofluids and facilitates heat transport. Cetyltrimethylammonium bromide and polyethylene glycol have been used as surfactants to improve the exfoliation process and enhance the colloidal stability of the nanomaterial dispersions. Some properties such as density and viscosity of the base fluid have not been significantly altered by the presence of WS 2 nanosheets in the base fluid. However, studies on the thermal properties of nanofluids have shown promising results with increases in thermal conductivity of up to 33% and heat transfer coefficient by 21% over the base fluid. Furthermore, it has been estimated that the overall efficiency of the CSP system could be improved by 31% by replacing the conventional thermal fluid with 2D-WS 2 -based nanofluids. • The efficiency of PTC solar collectors is increased by 31% by using WS 2 nanofluids. • Nanofluids improved the heat transfer coefficient of the base fluid by 21%. • Thermal conductivity was enhanced by 33% in WS 2 -based nanofluids. • WS 2 nanofluids favours the elimination of selective coatings on the absorber tube.

Thermo-hydraulic analysis of EuroTrough-150 solar receiver with porous foam by using CFD and MCRT techniques

The application of porous medium for enhancing heat transfer within parabolic trough solar system (EuroTrough-150-PTSC) is investigated. Several configurations of porous foam with step and uniform porosity are studied. The Monte Carlo Ray Tracing (MCRT) method, the surface-to-surface model, and the Darcy-Brinkman-Forchheimer model are used for heat flux profile, radiation, and the porous medium, respectively. The simulation results show that the rebuilding of the velocity field and the absence of hotspot formation on the absorber tube are achieved by using porous insert. Moreover, the usage of porous medium decreases the thermal resistance. Using porous medium leads to an increase in heat transfer and more pressure drop. If the porous foam is used in the bottom of the absorber tube, the Nusselt number can be increased 3 times as much as the clear absorber tube, but the Darcy friction factor increases by 2.2 times, so the PEC value would be above 1. Moreover, if the absorber tube is partly filled with the porous medium, it has a better performance compared to full-porous. In addition, if the foam is made of SiC, the heat transfer is achieved more effectively in comparison to bronze.

Design and analysis of an innovative photovoltaic-thermal collector with embedded tank

This paper deals with a new design of a photovoltaic-thermal (PVT) collector which incorporates a storage tank at the back of the thermal module to store heat as well as an electrical resistance to increase thermal production when necessary. This work investigates modeling and CFD study with the aim to provide a dynamic model which reflects a realistic behavior of the system. Simulations are performed to validate the model and to highlight the usefulness of the proposed design. Moreover, the modeling proves to be useful in analyzing the influence of internal geometrical parameters, such as thickness of insulation and absorber, type of fluid and also of materials used for absorber and serpentine tube.

Optimization and modelling of linear Fresnel reflector solar concentrator using various methods based on Monte Carlo Ray–Trace

A Linear Fresnel Reflector Solar Concentrator (LFRSC) consists of three main parts primary mirrors, a secondary reflector, and an absorber tube. However, the primary mirrors, as well as the secondary reflector, need to be more effective than conventional ones for optimal results. In this paper, an LFRSC is considered to optimize its construction to obtain higher efficiency. First, the primary mirrors are considered slightly curved to more concentrate on the rays, and then, the secondary reflector should forward the received rays to the absorber tube. Therefore, a novel structure of the secondary reflector is also proposed that is made of the composition of two simple reflectors. For simulation of LFRSC behavior, a robustness method based on the Monte Carlo Ray Tracing (MCRT) is used, which could model its performance, with more than 97 % precision. In the next step, three scenarios as small and medium-size, and full optimization modules, have self–advantages and disadvantages. Then, the Improved Prey Predator Optimization Algorithm (IPPOA) is introduced and used for problem optimization. In addition, other algorithms are investigated and the results are compared. The results of the small-size module with various methods are similar, however, the efficiency obtained by IPPOA is 47.13 %. In the case of medium size, the IPPOA has reached an efficiency of 42.5 %, which is lower than the efficiency of small size. Moreover, the width of the mirrors in the medium case is higher than the small size. In the third optimization case, the final efficiency with IPPOA is calculated as 47.88. As a result, case one, small size, is chosen as the cost-efficient model, and case three, full optimized, is considered as the high-efficiency model.

Thermal performance enhancement in parabolic trough solar collectors by using an absorber tube with spherical pins

ABSTRACT Improving the overall performance of parabolic trough collectors (PTCs) represents the best method to reduce the cost of energy produced. The goal is to increase their potential to generate more useful heat, particularly at high ranges of temperatures. Therefore, various techniques have been thoroughly examined in the literature. Indeed, the non-uniform heat flux on the PTC receivers induces overheating of the absorber tube and significant thermal stress, which requires the use of expensive materials. It also reduces the overall thermal performance of the system. To overcome these issues, practical solutions must be introduced. In this paper, integrated spherical pins are used and different configurations have been proposed and investigated. To do so, a numerical model that combines optical and thermal aspects has been developed. For this, the Monte Carlo ray tracing technique (MCRT) is coupled with the finite volume method (FVM). Also, experimental tests on the MicroSol-R platform have been used to validate the numerical models. A number of energetic and exergetic performance indices are used to analyze the potential improvements due to the use of spherical pins. The analysis showed that the use of pins increases the heat transfer coefficient by up to 27.2% and reduces the heat loss by up to 9.32% compared with conventional smooth tubes. Moreover, it is found that the performance evaluation criterion (PEC) can be enhanced by up to 24.92%. Consequently, when using a solar receiver with five pins (which was selected as the best configuration among the examined configurations), the overall efficiency and the exergetic efficiency are increased by up to 4.1% and 2.2%, respectively. Indeed, the use of pins inside the solar receiver is a practical solution to improve the efficiency of the PTC system and has the potential to reduce the thermal stress on the receiver thanks to the high heat transfer coefficient. It also helps prevent overheating and reduce thermal losses by keeping the maximum temperature within a suitable operational range.