Parabolic Trough Receiver Tubes Research Articles

In-situ heat losses measurements of parabolic trough receiver tubes based on infrared camera and artificial intelligence

Thermal problems occurring in glass cover, getters, vacuum, expansion bellows, and HTF lead to significant thermal losses due to convection-conduction, which accelerates the absorber degradation and implies replacing immediately these tubes. The replacement of the absorbers tubes is very expensive and interrupts the normal production of concentrated solar power plants. Therefore, the maintenance of the absorber tubes is one of the most critical challenges. In this paper, the Heat Loss Out System (Heat LOS) is proposed for regular evaluation and monitoring of absorbers tubes to maintain their efficiency and reduce operation and maintenance costs through immediate intervention. The proposed system is based on the intelligent processing of photos and videos in real time taken by a robust infrared camera with exact GPS coordinates and implemented on a vehicle. The Heat LOS evaluates in real-time, precisely, and rapidly, the temperature distribution of the absorbers tubes without contacting the surface or interrupting the normal production. First, an analytical study is performed to determine the appropriate speed of the vehicle, which can reach up to and the distance between the camera and the absorber tube that meet a high accuracy of measurement and do not disturb it which should not exceed 5 m. In addition, several absorbers tubes installed in the Green Energy Park (GEP, Morocco) research center are tested and evaluated using the Heat Loss Out system. The results obtained show a promising performance of the proposed system up to an accuracy of 93% based on deep learning with the CNN method, which will be valid when compared to manual analysis. This solution was assembled on a vehicle to rapidly evaluate several tubes and goes through many loops.

Effect of Different Arrangements of V-Shaped Ribs on the Performance of an Optically Enhanced Parabolic Trough Solar Collector

Abstract In this article, a V-shaped ribbed tube is utilized to improve the thermal performance of a parabolic trough collector (PTC). Six different rib arrangements are employed, and a detailed analysis is presented. In addition, the effect of adopting a secondary reflector (SR) on the temperature distribution around both a smooth and a ribbed parabolic trough receiver (PTR) tube is conducted. A computational fluid dynamics model is employed to study the heat transfer and fluid flow characteristics inside the tube. Results show that V-shaped ribs are an effective tool to stir up the flow and increase the velocity gradient of the fluid near the inner surface of the tube. This helps increase the convective heat transfer rate and reduce the tube’s maximum circumferential temperature. Moreover, results from the study show that the secondary reflector contributes to a further decrease in the tube surface temperature and hence improves the overall thermal efficiency of the collector. The combination of a V-shaped ribbed PTR tube and a secondary reflector is thus shown to be beneficial for the PTC system, especially at high Reynolds numbers.

In-Situ Heat Losses Measurements of Parabolic Trough Receiver Tubes Based on Infrared Camera and Artificial Intelligence

In-Situ Heat Losses Measurements of Parabolic Trough Receiver Tubes Based on Infrared Camera and Artificial Intelligence

Experimental investigation on a solar parabolic trough receiver tube enhanced by toroidal rings

The experimental investigation with the incorporation of toroidal rings in an absorber tube of a lab-scale solar parabolic trough collector is carried out in the present study. Five different cases of the absorber tube including the reference smooth absorber tube and the absorber incorporated with 2, 4, 6 and 8 mm thick toroidal rings are investigated. The influence of performance parameters like thermal efficiency, heat loss, thermal enhancement index and so on with different absorber tube configurations are evaluated, compared and illustrated to determine the best case. The outcomes indicate that the absorber tube with 6 and 8 mm thick toroidal rings are the best cases, with the 8 mm case exhibiting a higher exergy efficiency in the range of 38.57%-24.64%. The thermal efficiency for the best absorber cases is found to be 69.43% and 70.34% with the pressure drop being 1.67 and 0.93 kPa, respectively. Also, the cost of heat gain produced with the efficient absorber case is seen to be ₹ 3.43/kWh (US$ 0.045/kWh and € 0.040/kWh), which is nearly 15.51% lower than the reference case. Hence, the incorporation of toroidal rings could be effectively utilized in a large-scale solar thermal system for improved thermal performance.

Numerical Investigation of the Thermal Performance of an Axially Rotating Internally Finned Receiver Tube of a Parabolic Trough Concentrator

Enhancement of the thermal performance of the parabolic trough receiver tube is one of the approaches to energy sustainability. In the present work, the thermal performance of an axially rotating receiver tube equipped with internal flat longitudinal fins is studied. The effects of both the fin height and the rate of axial rotation are investigated at low values of axial Reynold’s number. The numerical analysis is held at various rotation rates using ANSYS Fluent. The numerical findings showed that the effect of the axial rotation on the internally finned receiver tube is not significant yet negative where a maximum reduction of 6% in the outlet temperature is reached in the 2mm height internally finned tube at rotation rate of N=21. However, the analysis showed that as the rotation rate increases, the temperature homogeneity between the fluid layers also increases and thus the liquid stratification phenomenon between the fluid layers is eliminated. The percentage of temperature difference between the fluid layers near the pipe center and the layers near the pipe wall reaches an optimum value of 58.4% at N=21 which is confirmed by an optimum increase of 110% in Nusselt number at the same rotation rate. However, a maximum loss of 81.6% in pressure coefficient is found in the case of the 2mm internally finned tube due to the increased turbulence. Thus, the integration of pipe axial rotation and internal fins can yield an enhancement in the heat transfer to the parabolic trough concentrator receiver tube and thus its thermal performance.

Thermal performance enhancement in a parabolic trough receiver tube with internal toroidal rings: A numerical investigation

The present work investigates the incorporation of internal toroidal rings in an absorber tube of a solar parabolic trough collector (SPTC) with an objective to enhance the thermal performance. To produce substantial effect on the heat transfer rate, nine different cases of absorbers have been considered. The fully developed turbulent heat transfer characteristics of the absorber tube have been numerically studied and validated under varying inlet temperatures and flow rate of the heat transfer fluid (HTF). Realizable k-ε two-equation turbulence model with enhanced wall treatment has been used with ANSYS FLUENT 15 commercial codes. The influence of toroidal rings on heat transfer and fluid flow are evaluated and presented. The absorber with toroidal ring having a diameter ratio (H) of 0.88 and pitch size (p) of 2d is the thermally efficient optimal case. While the absorber with H = 0.92 and p = 2d is the energy efficient optimal case, with H being the ratio of inner to outer diameter of the toroidal ring and 2d being the distance between the adjacent rings, which is equal to two times the inner diameter of the absorber tube. When the inlet temperature of HTF is 600 K, the increase in efficiency for the thermally efficient and energy efficient optimal case are found to be 3.74% and 1.88%, while the increase in Nusselt number is found to be 2.33 and 1.49 times higher than the smooth absorber tube (SAT), respectively.

Thermal performance assessment of internal longitudinal fins with sinusoidal lateral surfaces in parabolic trough receiver tubes

In this study, the effect of the internal longitudinal fins with the flat and sinusoidal lateral surface on the thermal performance of the parabolic trough receiver tube was investigated. Nusselt number (Nu), friction coefficient (f), thermal enhancement factor (ψ) and circumferential temperature difference (ΔTmax) were evaluated by the numerical analyzes. Analyzes were carried out using the different amplitude (a) and periodic length (p) values of the sinusoidal geometry for the different Reynolds numbers (Re). It was observed that the most important factors affecting the heat transfer were the fluid inlet temperature (Tin) and the a value. The highest enhancement in the Nu was found to be 25% and 78% for the flat and sinusoidal fin, respectively. The ΔTmax with the sinusoidal lateral surface geometry decreased in the range of 66–164 K. With the increase in the Re, the ψ remained approximately constant for the flat fin geometry (ψ ≈ 1,43). For the sinusoidal lateral surfaces, the ψ fell below 1 in high amplitude and Re values. For the Tin in the range of 300–600 K, the ψ value in the range of 0.85–2.32 were obtained. The results of the analysis revealed that the sinusoidal lateral surface geometry contributes to the heat transfer enhancement.

Failure Study of Compression Glass-Metal Joint for Parabolic Trough Receiver Tube application

The concentrated solar power plant technology has been proven to be a latest source for generation of renewable energy. Its performance depends on efficiency of the parabolic trough receiver tube or heat collecting element (HTC). In this paper, experimental study on fabrication of glass metal joint has been elaborated for parabolic trough receiver tube. Borosilicate glass tube was joined with austenitic stainless steel (SS304) metal ring using compression glass-metal type assembly with the help of induction heating and using 40SiO2-54B2O3-4Na2O-2Al2O3 material as the glass sealant. The process parameters for joining were selected on the basis of wettablity, pre-oxidation and microstructure studies. The generated residual stresses in glass tube after fabrication process was measured experimentally with the help of photo elastic technique using polariscope ‘PS-100-BS’. From the experimental results, it was observed that, the intensity of residual tensile stress in glass tube increases as thickness of metal ring increases. The cracking of glass just 1mm above the interface was also observed corresponding to maximum metal thickness ring.

Development and interface characterization of unmatched glass-metal joint

An unmatched glass-metal joint between borosilicate glass and austenitic stainless steel SS 304 was developed for parabolic trough receiver tube application. 40SiO2-54B2O3-4Na2O-2Al2O3 material was used as the glass sealant to join the SS304 metal ring and borosilicate glass tube. The research work reported was carried out with objective of investigating the role of glass sealant, joint design and pre-oxidation conditions on the wettability and adhesion strength of the glass-metal joint. The work also aimed at analysing the types of oxides formed at the glass-metal joint interface. The pre-oxidation behavior of SS304 was studied by varying the pre-oxidation temperature and pre-oxidation time. The oxide formed on the surface of SS304 substrates were examined using x-ray diffraction and energy dispersive x-ray spectroscopy. During the glass-metal sealing process the oxides at the glass-metal interface were characterized using x-ray diffraction and scanning electron microscopy. In-situ wetting behavior of glass sealant 40SiO2-54B2O3-4Na2O-2Al2O3 over austenitic stainless steel SS 304 was examined using the in-house developed image acquisition system. The optimum condition for joining 40SiO2-54B2O3-4Na2O-2Al2O3 glass sealant with SS304 is obtained by pre-oxidizing metal specimen at 700 °C for 3 min, where optimum oxide layer mass gain of 5.8666 × 10−04 g/cm2 and optimum oxide layer thickness of 87.57 μm was achieved. The maximum strength of joint during adhesion test was measured around 2.57 N/mm2 (MPa) corresponding to pre-oxidation temperature of 700 °C and 3 min pre-oxidation time. The FeO and ((SiO2),AlNa(SiO4)) oxide phases were observed at the interface of glass and the metal during joining. During in-situ wettability analysis the contact angle between the glass sealant pallet and SS304 substrate was measured at different temperatures. The contact angle of the glass sealant pallet decreased during heating process and the contact angle increased as the glass sealant pallet cooled from sealing temperature.

Numerical investigations on fully-developed mixed turbulent convection in dimpled parabolic trough receiver tubes

The fully-developed mixed turbulent convective heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2×104 and different Grashof numbers ranged from 0 to 3.2×1010 to produce substantial surface heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-ε two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall heat flux distributions and dimple depth on flow resistance and heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-uniform heat flux (NUHF) are larger than those under uniform heat flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/Di=0.875) are far superior to the shallow dimples (d/Di=0.125) at a same Grashof number.

Vacuum evaluation of parabolic trough receiver tubes in a 50 MW concentrated solar power plant

The solar receiver tube is a key component in the parabolic trough solar thermal power system. The loss of vacuum or degradation of the receiver has a significant impact on the receiver’s thermal performance and is the single largest cost factor in parabolic trough solar power plants. This paper presents a procedure to evaluate the annulus gas and thermal performance of receivers in the solar field. It is based on the glass and absorber temperatures of receivers as well as on plasma generation in the annulus space. The procedure has been executed in a 50MW commercial plant after five years of operation. 9% of receivers operating in the solar field showed vacuum loss to some degree. A sample of 70 receivers showing degradation were individually evaluated, of which: 34% of receivers showed progressive oxidation of the absorber coating, one receiver was measured to have argon in the annulus space at 10−2mbar pressure, and the remainder showed glass temperatures corresponding to those with air being the gas in the annulus. Temperatures measured in the receivers’ glass envelope did not match those with hydrogen being the gas in the annulus. Hydrogen was not found either by plasma generation. Heat losses under operating conditions added up by the found vacuum losses have been estimated over the reference performance of a solar field keeping its initial vacuum. The full procedure, measurements, results and discussion are described in this paper, aiming to understand as fully as possible the cause of the loss of vacuum experienced by the receivers.

Thermal analysis of a solar parabolic trough receiver tube with porous insert optimized by coupling genetic algorithm and CFD

In this paper, the heat transfer enhancement in a solar parabolic trough receiver tube with porous insert and non-uniform heat flux condition was investigated. A new optimization method, which couples genetic algorithm (GA) and computational fluid dynamics (CFD) based on Socket communication, was proposed to optimize the configuration of porous insert. After the acquisition of the optimal porous inserts, some performance evaluation criterions such as synergy angle, entransy dissipation and exergy loss were introduced to discuss the heat transfer performance of the enhanced receiver tubes (ERTs) with optimal and referenced porous inserts. The results showed that, for a large range of properties of porous insert (including porosity and thermal conductivity) and Reynolds number, the heat-transfer performance of ERT with porous insert optimized by GA is always higher than that of the referenced ERTs. Better heat-transfer performance can further improve the solar-to-thermal energy conversion efficiency and mechanical property of the solar parabolic trough receiver. When some porous materials with high thermal conductivity are adopted, ERT can simultaneously obtain perfect thermal and thermo-hydraulic performance by using the same optimized porous insert, which cannot be achieved by using the referenced porous insert. In the view of those introduced evaluation criterions, using the optimized porous insert can obtain better synergy performance and lesser irreversibility of heat transfer than using the referenced porous insert. Entransy dissipation per unit energy transferred and exergy loss rate have equivalent effects on the evaluation of irreversibility of heat transfer process. These evaluation criterions can be used as optimization goals for enhancing the comprehensive performance of the solar parabolic trough receiver.

A Novel Portable Device to Measure Transmittance and Reflectance of Parabolic Trough Receiver Tubes in the Field

The performance of parabolic trough (PT) receiver tubes (RTs) has a direct impact on concentrated solar power (CSP) plant production. As a result, one major need of operation and maintenance (O&M) in operating plants is to monitor the state of the receiver tube as a key element in the solar field. In order to fulfill this necessity, Abengoa Solar has developed the first existing portable device for measuring the transmittance and reflectance of parabolic trough receiver tubes directly in the field. This paper offers a description of the technical features of the instrument and reviews the issues related to its usability as a workable portable device in operating solar fields. To evaluate its performance, laboratory studies have been carried out using two patterns to determine the accuracy and standard deviation of the measurements, obtaining excellent results. This information is complemented with data collected by O&M using this instrument in solar power plants. Studies have been carried out to determine the effect of both rainfall and artificial cleaning on the increase of transmittance. These values are then compared to those obtained from hand-cleaning, and show important differences. The results are discussed in this paper.

Degradation of receiver tube optical performance after four years of operation

The performance of parabolic trough receiver tubes has a direct impact on concentrated solar power plant production and thus must be maintained at as high a level as possible during the life cycle of the plant. In this paper, in-the-field degradation of commercial plant receiver transmittance and absorptance is studied after four years of operation. Since no records of similar measurements were available regarding the reception of the receivers, the results are compared with both the manufacturer’s technical specifications and with the guaranteed values. For this purpose, the Mini Incus portable spectrophotometer developed by Abengoa is used to measure the optical properties of an 80-receiver sample. This sample is built up considering the two main factors most likely involved in transmittance and absorptance degradation: interactions with external agents and operating temperature. Therefore, receivers from different locations both in the field and within loops are chosen. A statistical study is performed on the data collected in order to obtain representative values for the whole population of evaluated receivers as well as for every group differentiated. The results reveal no significant degradation of the absorptance or transmittance of the receivers, with measured average transmittance and absorptance values of 96.6±0.2% and 95.9±0.2%, respectively, higher than the 96% and 95% indicated in the technical specification for new first generation receivers. Receiver location in the field and position within the loop does not influence optical property degradation, since the appreciated differences fall within the uncertainty of the measurement device. The obtained results are then compared with the technical specification values and with those guaranteed by the manufacturer after 4years of operation, and the suitability of the guarantee in matching the reality experienced by the receivers in the field is discussed.

Thermodynamic optimisation of the performance of a parabolic trough receiver using synthetic oil–Al2O3 nanofluid

In this paper, results of a thermodynamic analysis using the entropy generation minimisation method for a parabolic trough receiver tube making use of a synthetic oil–Al2O3 nanofluid as a heat transfer fluid are presented. A parabolic trough collector system with a rim angle of 80° and a concentration ratio of 86 was used. The temperature of the nanofluid considered was in the range of 350–600K. The nanofluid thermal physical properties are temperature dependent. The Reynolds number varies from 3,560 to 1,151,000, depending on the temperature considered and volume fraction of nanoparticles in the base fluid. Nanoparticle volume fractions in the range 0⩽ϕ⩽8% were used. The local entropy generation rates due to fluid flow and heat transfer were determined numerically and used for the thermodynamic analysis. The study shows that using nanofluids improves the thermal efficiency of the receiver by up to 7.6%. There is an optimal Reynolds number at each inlet temperature and volume fraction for which the entropy generated is a minimum. The optimal Reynolds number decreases as the volume fraction increases. There is also a Reynolds number at every inlet temperature and volume fraction beyond which use of nanofluids is thermodynamically undesirable.

Second law analysis and optimization of a parabolic trough receiver tube for direct steam generation

Entropy generation in the receiver tube of a parabolic trough solar collector can mainly be attributed to the fluid friction and finite temperature differences. The contribution of each of these components is investigated under different circumstances. Mass flow rates, tube diameters and operating pressures are investigated to obtain good guidelines for receiver tube and plant design. Operating pressures between 3 MPa (saturation temperature of 233.9 °C) and 9 MPa (saturation temperature of 303.3 °C) were investigated. Results show that small diameters can result in excessive fluid friction, especially when the mass flow rates are high. For most cases, tube diameters beyond 20 mm will exclusively be subject to entropy generation due to finite temperature differences, and entropy generation due to fluid friction will be small to negligible. Increasing the concentration ratio will decrease entropy generation, due to a higher heat flux per unit meter. This will ultimately result in shorter receiver tube lengths. From a simulated annealing optimization it was seen that if the diameter is increased, the entropy generation can be lowered, provided that the concentration ratio is kept constant. However, beyond a certain point gains in minimizing the entropy generation become negligible. The optimal operating pressure will generally increase if the mass flow rate is increased. Finally it was seen that higher operating pressures are more advantageous when the entropy generation minimization is considered in conjunction with the work output.

Simulation of thermal fluid dynamics in parabolic trough receiver tubes with direct steam generation using the computer code ATHLET

Abstract In the present feasibility study the system code ATHLET, which originates from nuclear engineering, is applied to a parabolic trough test facility. A model of the DISS (DIrect Solar Steam) test facility at Plataforma Solar de Almería in Spain is assembled and the results of the simulations are compared to measured data and the simulation results of the Modelica library “DissDyn”. A profound comparison between ATHLET Mod 3.0 Cycle A and the “DissDyn” library reveals the capabilities of these codes. The calculated mass and energy balance in the ATHLET simulations are in good agreement with the results of the measurements and confirm the applicability for thermodynamic simulations of DSG processes in principle. Supplementary, the capabilities of the 6-equation model with transient momentum balances in ATHLET are used to study the slip between liquid and gas phases and to investigate pressure wave oscillations after a sudden valve closure.

Solar Selective Coatings with Enhanced Thermal and Corrosion Stability: Electrochemically Deposited Black Chrome on Stainless Steel in the Presence of Graphite Encapsulated FeCo Nanoparticles

Black chrome coatings were electrodeposited under galvanostatic conditions on SS substrates. To enhance the thermal stability of the coatings for viable usage in parabolic trough receiver tubes in solar thermal applications beyond 250°C, graphite encapsulated FeCo nanoparticles had been introduced in the electrolyte solution at different weight percentages (0.025, 0.05, 0.1). An increase in the thermal stability of the coatings in the range of 400°C to 600°C is noticed from the STA analysis. The corrosion analysis (Tafel) and cyclic voltammetric studies of the coatings in the presence and absence of nanoparticles have been carried out in 3.5% NaCl solution to understand the stability of the coating in saline environment.

Hydrogen permeation model of parabolic trough receiver tube

Hydrogen gas formed by the thermal decomposition of organic heat transfer fluids can permeate through parabolic trough receiver tubes into the vacuum annulus, leading to significant heat losses in the receiver which reduces the parabolic trough power plant efficiency. This problem is quite important as these systems are being developed to drive down the cost of electricity. Thus, hydrogen permeation in parabolic trough power plants must be understood to develop strategies to reduce or prevent hydrogen gas accumulation in the receiver annulus. Hydrogen permeation model for parabolic trough receivers was developed based on measurements of hydrogen permeability of solar-selective coatings. The model was then used to study the effects of the hydrogen generation rate, the hydrogen pressure in the receiver tubes and the hydrogen barrier coating on the hydrogen permeation into the annulus. The hydrogen generation rate plays a significant role in the hydrogen permeation process, with the hydrogen pressure, permeability and adsorb area all related to balancing the hydrogen permeation rate with the hydrogen generation rate.