Parabolic Trough Receiver Research Articles

Extended Heat Loss and Temperature Analysis of Three Linear Fresnel Receiver Designs

Heat loss prediction models for parabolic trough receivers do not consider the thermal effect of a secondary mirror. As an extension a Thermal Resistance Model (TRM) has been developed at Fraunhofer ISE for the prediction of the heat loss of three different Linear Fresnel Collector (LFC) receiver configurations. In previous investigations we have found the energy balance of a LFC receiver to be strongly influenced by the amount of solar radiation absorbed by the secondary mirror. This absorption provokes an increase of temperature of the secondary mirror and hence a decrease in the total amount of heat loss of a LFC. The size of this effect depends on the receiver geometry and diverse ambient parameters. Investigated parameters are wind velocity, ambient temperature and Direct Normal Irradiance (DNI). This dependency and its effect on heat loss and secondary mirror temperatures are analyzed for three different LFC receiver configurations. As the radiation absorbed by the secondary mirror is affected by the aperture area of the LFC, studies are performed for small-scale and for large-scale collectors.

Linear Focus Solar Simulator Test Bench for Non-destructive Optical Efficiency Testing of Parabolic Trough Receivers

Abstract This paper describes a new2 nd generation linear focus solar simulator test bench. The test bench is used for the non-destructive calorimetric measurement of relative optical efficiency of parabolic trough receivers. Compared to the predecessor, the test bench has been optimized for faster measurement, better long term stability and easier operation. The test bench optics is an elliptical shaped glass mirror trough with flat end mirrors. The receiver and six solar simulator lamps, metal halide lamps with 2.5 kWeeach are located in their respective focal lines. Absorbed power is measured via temperature increase of the water flowing through the receiver and the mass flow rate. By comparison to the absorbed power of a reference receiver, the optical efficiency of a sample can be determined relative to the optical efficiency of the reference receiver.

Modelling, Simulation and Identification of Heat Loss Mechanisms for Parabolic Trough Receivers Installed in Concentrated Solar Power Plants

This paper describes a thermodynamic model library developed with the object-oriented language Modelica, which is both implemented for steady-state and transient heat transfer analyses of Parabolic Trough Receivers (PTRs) installed in Concentrated Solar Power (CSP) plants. For the identification of PTR heat loss mechanisms, this heat transfer model is coupled to a derivative-free hybrid optimization routine developed in Matlab, combining a Particle Swarm Optimization (PSO) algorithm with a Nelder-Mead Simplex (NMS) algorithm with search space boundary constraints.

Multi-objective and thermodynamic optimisation of a parabolic trough receiver with perforated plate inserts

In this paper, multi-objective and thermodynamic optimisation procedures are used to investigate the performance of a parabolic trough receiver with perforated plate inserts. Three dimensionless perforated plate geometrical parameters considered in the optimisation include the dimensionless orientation angle, the dimensionless plate diameter and the plate spacing per unit meter. The Reynolds number varies in the range 1.02 × 104 ≤ Re ≤ 1.36 × 106 depending on the fluid temperature. The multi-objective optimisation was realised through the combined use of computational fluid dynamics, design of experiments, response surface methodology and the Non-dominated Sorted Genetic Algorithm-II. For thermodynamic optimisation, the entropy generation minimisation method was used to determine configurations with minimum entropy generation rates.

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.

A numerical study of parabolic trough receiver with nonuniform heat flux and helical screw-tape inserts

Effect of solar incidence angle was analyzed in order to accurately simulate the heat flux distribution around the absorber tube outer surface. Helical screw-tape inserts was proposed to homogenize the absorber tube temperature distribution and improve the thermal efficiency. Three dimensional periodical models of flow and heat transfer were established and solved with the heat flux of different transversal angle (β). The results show that β affects the flux distribution more greatly than longitudinal angle (φ). Transversal angle (β) of 11.567 mrad increases the relative change of heat loss (Qloss) as inlet temperature rises, and also increases the maximum temperature on absorber tube (Tmax) and the maximum circumferential temperature difference (ΔT). But its effect reduces as Reynolds number rises. Within the range of studied Re, the helical screw-tape inserts of given geometrical parameters greatly reduce the Qloss, Tmax and ΔT, which indicates that helical screw-tape inserts is a feasible way to enhance the heat transfer inside the receiver.

Minimum entropy generation due to heat transfer and fluid friction in a parabolic trough receiver with non-uniform heat flux at different rim angles and concentration ratios

In this paper, Monte Carlo ray-tracing and computational fluid dynamics are used to numerically investigate the minimum entropy generation due to heat transfer and fluid friction in a parabolic trough receiver. The analysis was carried out for rim angles in the range 40°–120°, concentration ratios in the range 57–143, Reynolds numbers in the range 1.02 × 104–1.36 × 106 and fluid temperatures in the range 350–650 K. Results show existence of an optimal Reynolds number at any given combination of fluid temperature, concentration ratio and rim angle for which the total entropy generation is a minimum. The total entropy generation was found to increase as the rim angle reduced, concentration ratio increased and fluid temperature reduced. The high entropy generation rates at low rim angles are mainly due to high peak temperatures in the absorber tube at these low rim angles.

Validation of a Novel Method for Thermal Performance Evaluation of Parabolic Trough Receivers

Parabolic trough receivers are a kind of key components of a solar trough power plant, which absorb and transfer the high flux solar energy to the heat transfer fluid flowing in it. A Receiver Impedance Heating (RIH) method is put forward by analyzing the inadequacies of traditional methods. Voltage is imposed on both ends of the receiver and then the receiver is self-heated via a large electrical current flowing through it based on the Joule Effect. Once the receiver reaches a thermal equilibrium the product of voltage imposed on the receiver and electrical current flowing through the receiver is equal to the heat loss of the receiver to ambient environment at corresponding temperature difference of receiver temperature above ambient. The experiment system is simple, low-cost, and easy to operate. Experiment results show that curve of heat loss is smooth and measurement uncertainty is low, which means accuracy of the experiment results is high, while time period to reach a thermal equilibrium at every absorber temperature reduces to 2-3hours which is one fifth of that needed in traditional methods.

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.

Heat transfer and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts

In this paper, a numerical investigation of thermal and thermodynamic performance of a receiver for a parabolic trough solar collector with perforated plate inserts is presented. The analysis was carried out for different perforated plate geometrical parameters including dimensionless plate orientation angle, the dimensionless plate spacing, and the dimensionless plate diameter. The Reynolds number varies in the range 1.02×104⩽Re⩽7.38×105 depending on the heat transfer fluid temperature. The fluid temperatures used are 400K, 500K, 600K and 650K. The porosity of the plate was fixed at 0.65. The study shows that, for a given value of insert orientation, insert spacing and insert size, there is a range of Reynolds numbers for which the thermal performance of the receiver improves with the use of perforated plate inserts. In this range, the modified thermal efficiency increases between 1.2% and 8%. The thermodynamic performance of the receiver due to inclusion of perforated plate inserts is shown to improve for flow rates lower than 0.01205m3/s. Receiver temperature gradients are shown to reduce with the use of inserts. Correlations for Nusselt number and friction factor were also derived and presented.

Structural reliability analysis of parabolic trough receivers

Parabolic trough receivers typically account for 30% of the cost of a solar field. Furthermore, experiences show the cost of operation and maintenance of parabolic trough receivers over the lifetime of the system can be quite high as regular maintenance is required. Therefore, the receiver’s cost and structural reliability is crucial to guarantee the parabolic trough plants work steadily, safely and above all, economically. The structure of a parabolic trough receiver is principally a stainless steel inner tube surrounded by a glass outer tube. Because the stainless steel tube has 4m long and is easy to get bent, the glass outer tube is quite fragile and therefore prone to break. Experiences in field operations show the bending of stainless steel tube is the main reason of parabolic trough receiver’s structural failure. This study mainly investigates why and how the stainless steel tubes get bent. In this paper, an indoor experiment, numerical simulations (CFD and FEA) and field measurements are conducted to jointly investigate the causes of stainless steel tube’s bending. An indoor experiment is conducted on a heat loss test-rig, and the deflection of the stainless steel tube under different isothermal conditions is studied. Numerical simulations are used to investigate the detailed temperature distribution, and its corresponding structural deformation of the stainless steel tube of parabolic trough receivers. The simulations adapt a MCRT code, FLUENT software and ANSYS workbench. Finally, field measurements are conducted on a 36m long parabolic trough collector to investigate the stainless steel tube bending under different working conditions. This study shows that improper installation and operational practices of parabolic trough receivers are the main causes for the structural failure of the parabolic trough’s receiver. This study is of great importance to guide the installation and operation of parabolic trough receivers.

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.

Modeling and Analysis of Stress in High Temperature Molten Salt Trough Receivers

SkyFuel has investigated the stresses and deformations occurring in parabolic trough receivers operating at temperatures above 425°C. Operating at these temperatures allows for direct molten salt storage and higher efficiency conversion from thermal to electric energy. However, at these temperatures, the typical stainless steels used in receiver construction are susceptible to chromium carbide precipitation. After the precipitation has occurred, the steel is vulnerable to intergranular corrosion, and the fatigue strength of the steel is reduced. Corrosion increases the stresses in the receiver walls, and the reduced fatigue strength lowers the stress limit where failure will occur. This paper presents the results of an analysis of these stresses and an evaluation of the receiver material at these operating temperatures. It is shown that parabolic trough receivers can be designed to mitigate the negative effects of chromium carbide precipitation and operate above 425°C without risk of premature failure.

CFD Analysis of a Receiving Cavity Suitable for a Novel CSP Parabolic Trough Receiver

The aim of this work was to study, by means of accurate 3D steady-state CFD simulations, the thermo-fluid dynamics behavior of a helically coiled heat exchanger (HCHE) constituting the receiving cavity of the novel CSP receiver based on Airlight Energy technology. In this innovative receiver design, air is used as heat transfer fluid (HTF), which, besides being inexpensive and environmentally friendly, is optimally suited for high temperature operation well beyond the limit of conventional HTFs.According to preliminary information related to the collectors orientation of the first 3.9 MWth Airlight Energy pilot plant, under construction in Ait Baha (Morocco), two reference skew angles of the incoming solar radiation were considered and the receiving cavity performance were evaluated in terms of thermal efficiency and pressure drop. Among all, one of the main requirements was to achieve, at the outlet section of the HCHE, an air temperature of 650°C; hence the mass flow rate was tuned accordingly. In order to minimize the pumping power requirements, the HCHE was designed to guarantee a laminar flow regime under all the operating conditions. Navier-Stokes, energy and radiation transport equations, the latter accounted for by the Discrete Ordinates (DO) model, were numerically solved, using the finite-volume method approach, with Fluent code from ANSYS.A meticulous experimental proof of concept was then carried out in Biasca (Switzerland) by the Swiss company Airlight Energy Manufacturing SA. The analysis of the experimental results, detailed in this paper, allowed to assess the reliability and effectiveness of this novel CSP receiver design in the solar energy harvesting.

Characterization of High Temperature Solar Thermal Selective Absorber Coatings at Operation Temperature

In cooperation between the Physikalisch-Technische Bundesanstalt (PTB) and the SCHOTT Solar CSP GmbH (SCHOTT Solar) measurements have been carried out with the goal to assure, that the directional-hemispherical reflectance measurements, done as production control by SCHOTT Solar, lead to applicable results for calculating the emittance of absorber tubes for parabolic trough receivers at their operation temperatures above 350°C.A significant influence of the temperature on the directional spectral emittance could not be found. Due to the spectral characteristics of the emittance with a rising spectral emittance towards shorter wavelength the spectral integrated quantity: directional total emittance becomes larger with an increasing temperature due to the shift of the Planckcurve.Furthermore, the measurements of PTB confirmed the emittance values calculated from the reflection measurements and the directional-hemispherical reflectance measurements performed by SCHOTT.

Progress on Integrated Compound Concentrator Design

Nonimaging optics has been playing an important role in both tracking and non-tracking devices used for solar applications. We will report on our progress on implementing the integrated design within a glass vacuum tube for a non-tracking solar thermal collector. Such a design allows a thermodynamically efficient concentrator to be used for any convex shaped absorber. The acceptance angle of the design is also optimized for the seasonal change of sun. To minimize the material cost of our design, the reflector is integrated with the shape of the evacuated tube. Compared to from the conventional parabolic trough receiver which utilizes bellows and double end seals, our design uses a single end metal glass seal and allows the other end to freely expand. A 0.7 meter prototype is constructed as a prototype. The optical efficiency and incident angular modifier are simulated using a ray tracing program to show that within the designed acceptance angle, the collector performs at around 78% optical efficiency.

Three-dimensional numerical study of heat transfer characteristics of parabolic trough receiver

Parabolic trough receivers are the key component of parabolic trough solar plants, and they typically account for 30% of the cost of the construction of a solar field. The receiver’s reliability is still a major item which affects the plant’s cost. The temperature distribution of the parabolic trough receivers is required to identify the causation of parabolic trough receiver’s failure, and is the prerequisite to design and optimize the parabolic trough receiver’s structure. In this study, the detailed temperature distribution of a parabolic trough receiver is successfully simulated by combining a MCRT code and FLUENT software. The heat transfer fluid flow, conduction and radiation heat transfers are jointly considered. Temperature-dependent properties of the heat transfer fluid, the wavelength-dependent optical properties of the receiver surfaces and the glass envelope’s absorption of the solar radiation energy are also taken into account. Comparison with indoor experimental results show the average difference is within 6%. In addition, the transient behaviors of parabolic trough receiver under direct concentrated solar irradiance are investigated. The information from this study is of great importance to the design and the optimization of the structure of parabolic trough receiver, as well as to identify the causation of parabolic trough receiver’s failure.

A Transient Thermography Method to Separate Heat Loss Mechanisms in Parabolic Trough Receivers

This paper describes a transient thermography method to measure the heat loss of parabolic trough receivers and separate their heat loss mechanisms. This method is complementary to existing stationary techniques, which use either energy balances or glass envelope temperature measurements to derive overall heat losses. It is shown that the receiver heat loss can be calculated by applying a thermal excitation on the absorber tube and measuring both absorber tube and glass envelope temperature signals. Additionally, the emittance of the absorber selective coating and the vacuum quality of the annulus can be derived. The benefits and the limits of the transient method are presented and compared to the established stationary method based on glass envelope temperature measurements. Simulation studies and first validation experiments are described. A simulation based uncertainty analysis indicates that an uncertainty level of approximately 5% could be achieved on heat loss measurements for the transient method introduced in this paper, whereas for a conventional stationary field measurement technique, the uncertainty is estimated to 17–19%.

Numerical investigation of entropy generation in a parabolic trough receiver at different concentration ratios

This paper presents results of a numerical analysis of entropy generation in a parabolic trough receiver at different concentration ratios, inlet temperatures and flow rates. Using temperature dependent thermal properties of the heat transfer fluid, the entropy generation due to heat transfer across a finite temperature difference and entropy generation due to fluid friction in the receiver has been determined. Results show a reduction in the entropy generation rate as the inlet temperature increases and an increase in the entropy generation rate as the concentration ratio increases. Results further show that, there is an optimal flow rate at which the entropy generated is a minimum, for every combination of concentration ratio and inlet temperature. The optimal flow rates at which the entropy generated is minimum are presented for different flow rate and concentration ratio, and the results are the same irrespective of the inlet temperature considered. For the range of inlet temperatures, flow rates and concentration ratios considered, the Bejan number, which measures the contribution of entropy generation due to heat transfer irreversibility to the total entropy generation rate is about 1 at low flow rates and is between 0 and 0.24 at the highest flow rate.

An experimental study of thermal characterization of parabolic trough receivers

The receiver is a key component of the parabolic trough solar station. The receiver requires the most challenging technology and has a decisive influence on the thermal and economic performance of a power plant. The Institute of Electrical Engineering Chinese Academy Sciences (IEECAS) and Himin Solar Co., Ltd. (HSC) cooperated to develop solar receivers for the first 50MW parabolic trough project in Inner Mongolia, China. This paper examines overall heat loss, end loss and thermal emittance of the coating of a newly designed receiver in order to evaluate its thermal characterization. A series of heat loss tests are conducted in a newly developed test stand following the steady state equilibrium method. The tests provide a correlation between heat loss and the absorber temperature. This paper presents a new testing method to accurately test the coating emittance. The method uses a receiver with a high vacuum system and a vacuum gauge to maintain continuous exhaust and high vacuum throughout the heat loss testing. A heat loss comparison between the receiver and other existing receivers provides a reference that enabled further optimization. Theoretical and experimental analysis examines the effects of end loss both with and without a heat insulator and a coil heater. The emittance curves of different coatings are acquired and the reasons for initial emittance decrease and then remarkable increase versus temperature are analyzed.