Parabolic Trough Receiver Research Articles

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.

A model for hot mirror coating on solar parabolic trough receivers

Parabolic solar troughs are amongst the most widely studied solar thermal technologies available today. Methods for improving efficiency include the use of selective coatings on the absorber which reduces dominant thermal radiation losses. In this paper, we model a different approach of reducing radiation losses: the glass cover around the absorber can be coated with a hot mirror film, which reflects infrared radiation back onto the absorber. In order to describe such a mechanism, it becomes necessary to model theoretically the long range thermal radiation interactions inside the receiver unit. Our model uses discretization of the active surfaces to account for all the dominant radiation interactions, and can be used in a simulation to establish a temperature profile for the receiver unit, from which thermal properties can be inferred. The results of the simulation are compared to existing simulations and experimental data, wherever possible.

Heat transfer performance enhancement and thermal strain restrain of tube receiver for parabolic trough solar collector by using asymmetric outward convex corrugated tube

With the aim to increase the overall heat transfer performance and reliability of tube receiver for parabolic trough solar collector system, asymmetric outward convex corrugated tube is introduced as the metal tube of parabolic trough receiver. An optical-thermal-structural sequential coupled method was developed to study the heat transfer performance and thermal strain of tube receiver for parabolic trough solar collector system. Heat transfer performance and thermal strain comparisons between conventional tube receiver and asymmetric outward convex corrugated tube receiver are conducted. The researches indicated that the usage of asymmetric outward convex corrugated tube as receiver can enhance the heat transfer performance and reduce the thermal strain effectively. By using asymmetric outward convex corrugated tube as receiver, the maximum enhancement of overall heat transfer performance factor is 148% and the maximum restrain of von-Mises thermal strain is 26.8%.

Flow field optimization for the solar parabolic trough receivers in direct steam generation systems by the variational principle

Direct steam generation (DSG) systems with parabolic trough solar receivers have higher operation temperature and lower investment than double-loop systems. However, they have not been widely employed in commercial plants due to some reasons including the unreliability of solar receivers. In this contribution, in order to improve the reliability, we apply the minimum circumferential temperature difference on the absorber of a solar parabolic trough receiver as the optimization objective, and set the continuity equation, the energy conservation equation and prescribed viscous dissipation rates as constrains to construct a Lagrange function. Then, the variational principle is used to derive the optimization equations that the optimal flow field should satisfy to minimize the circumferential temperature difference. Finally, the convective heat transfer processes in a typical receiver are optimized by the newly derived optimization equations to show the applications. The results show that generating longitudinal vortexes along the main flow in the receiver reduce the circumferential temperature difference effectively, e.g. from 69K to 42K (39% reduction), which provides a guidance for the structural optimization of practical receivers to improve the reliability.

Influence of optical errors on the thermal and thermodynamic performance of a solar parabolic trough receiver

This paper presents results of an investigation covering the influence of optical errors on the thermal and thermodynamic performance of a solar parabolic trough system. The optical performance was evaluated by using Monte Carlo ray tracing for slope errors and specularity errors in the range 0–5mrad. A parabolic trough collector (PTC) with a rim angle of 80° and a concentration ratio of 86 was considered in this study. The heat flux profiles obtained from the optical analysis were coupled with a computational fluid dynamics tool (ANSYS FLUENT) to numerically investigate the thermal and thermodynamic performance of the receiver. In the numerical analysis, inlet temperatures and Reynolds numbers ranging from 350K–650K and 6800–1,140,000 were considered, respectively. Results show that, slope and specularity errors influence the heat flux distribution on the receiver’s absorber tube. Results further show that the optical performance and thermal performance reduce significantly as the slope errors increase. For the range of parameters considered, the intercept factor reduces by up to 21% while the overall thermal efficiency reduces by up to 17% as optical errors increase. Receiver thermodynamic performance is shown to deteriorate as slope errors increase.

In-situ heat loss measurements of parabolic trough receivers based on transient infrared thermography

The design lifetime of parabolic trough receivers is at least 25years. However, depending on the specific operating conditions, this expected lifetime may be shortened. A periodic inspection of receivers mounted in a solar field is thus recommended. Several field methods with distinct depth of information and field throughput exist to estimate receiver thermal performance. This paper gives an overview of these field methods and presents the application of a transient infrared thermography method, which combines dynamic temperature measurements with a numerical heat transfer model in order to derive specific receiver heat losses and separate internal heat loss mechanisms. This method has been previously investigated under laboratory test conditions and is here applied under field test conditions for in-situ thermal measurements on mounted receivers.The transient thermography method allows a correct estimation of specific heat losses within a precision range below ±10% for all tested receivers under field test conditions. Relevant separation of heat loss mechanisms could be derived for evacuated PTRs with solar selective absorber coating, while partially biased diagnoses could be observed for partially or non-evacuated PTRs with highly emissive absorber coatings. Active thermography is considered as a relevant tool for receiver field inspection, in combination with a complementary field screening measurement technique.

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.

Thermal Performance of a Single-Pass All-Glass Parabolic Trough Receiver

AbstractA novel single-pass all-glass parabolic trough receiver (SPAG-PTR) was developed and the parabolic trough solar collector with concentration ratio of 7.5 was built to investigate the thermal performance of the SPAG-PTR. The indoor heat loss tests were settled as well as the outdoor experiments in quasi-steady-state and transient-state test conditions. The heat loss per unit absorber area of the SPAG-PTR in the average absorber temperature from 100 to 287°C was within the range of 85–674 W/m2, which represents a better heat loss performance (−15%) upon the Sanle-3 receiver (Nanjing Sanle Electronic Information Industry Group, Nanjing, China). The experimental efficiency in transient state was within the range of 0.53–0.73 with the experimental relative error of approximately 3.6–6.5%. In addition, the experimental efficiency curve in quasi-steady-state was obtained. In quasi-steady-state, the experimental efficiency was approximately 0.65–0.71 when the inlet heat transfer fluid temperature was in ...

Comparative analysis of long-term solar resource and CSP production for bankability

Feasibility analysis for CSP projects requires quantifying the economic risk associated with the inter-annual variability of the solar resource and uncertainties in the annual energy production estimates. To assess this risk, statistical methods may be applied either before the system simulation (e.g. to create solar resource data in terms of exceedance probabilities such a P50 or P90) or on the system performance to determine the likelihood that a power plant will generate a certain amount of energy in any given year over the plant's life (e.g. the P50 or P90 annual productions). This paper presents and illustrates the long-term analysis of four CSP plants (parabolic trough and central receiver technology) in four sites within the Mediterranean and North Africa region; the analysis includes the exceedance probabilities estimations. Satellite-derived data for 20 years have been used for solar resource datasets in hourly basis, and multiyear simulations of the plant output have been performed with System Advisor Model. The associated uncertainties have been included in the long-term analysis of both DNI and CSP plant production. The results remark the advantages of multiyear analysis of CSP output for long-term analysis against the use of artificial meteorological years which represent the long-term.

Parabolic trough receiver with corrugated tube for improving heat transfer and thermal deformation characteristics

A symmetric outward convex corrugated tube design is introduced for parabolic trough receivers with the aim of increasing their heat transfer performance and reliability. An optical–thermal–structural sequential coupled method was developed to analyze the heat transfer performance and thermal deformation of the glass cover and metal tube of the parabolic trough receiver. The developed coupled method has been validated with experimental results conducted in the DISS test facility in Spain. The numerical results indicated that the introduction of a symmetric outward convex corrugated tube design for the metal tube of the parabolic trough receiver can effectively enhance the heat transfer performance and decrease the thermal strain. The effective heat transfer coefficient can be increased up to 8.4% and the maximum thermal strain of metal tube can be decreased up to 13.1% when symmetric outward convex corrugated tube is used at Re=81728, p/D=4.3. In addition, regression correlations are put forward in order to find an effective heat transfer coefficient and effective Nusselt number for the fluid flow in the parabolic trough receiver.

Performance investigation of parabolic trough solar receiver

The influences of some parameters on the performance of parabolic trough solar receiver are investigated in the present work. When the mass flow rate of working fluid, ambient temperature and solar incident angle increase, the heat losses of solar receiver decrease. The exergy losses of solar receiver increase as the inlet temperature of working fluid, wind velocity, and the inner diameter of glass cover increase. The convective heat loss of glass cover predominates in the heat losses of solar receiver based on thermal efficiency, but the exergy lost from absorber ends takes the largest proportion. The optical heat loss of solar collector is far more than the heat losses of solar receiver, especially in the larger solar incident angle. There exists an optimal mass flow rate of working fluid for exergy efficiency, and the thermal efficiency and exergy efficiency have opposite changing tendencies under some conditions. Therefore, the selection of evaluation criteria is crucial to the performance optimization of solar collector system. This study is of great significance to guide the design and optimization of parabolic trough receivers.

Heat transfer and entropy generation in a parabolic trough receiver with wall-detached twisted tape inserts

In this paper, heat transfer enhancement in a parabolic trough receiver using wall-detached twisted tape inserts was numerically investigated. The resulting heat transfer, fluid friction and thermodynamic performance were determined and presented. The flow was considered fully developed turbulent, with Reynolds numbers in range 10 260 ≤ Rep ≤ 1 353 000 depending on the fluid temperature. The twisted tape's twist ratio and width ratio vary in the range 0.50–2.00 and 0.53–0.91, respectively. The numerical investigations are based on a finite volume method, with the realisable k–ɛ model for turbulence closure. The study shows considerable increase in heat transfer performance of about 169%, reduction in absorber tube's circumferential temperature difference up to 68% and increase in thermal efficiency up to 10% over a receiver with a plain absorber tube. An entropy generation analysis shows the existence of a Reynolds number for which there is minimum entropy generation for each twist ratio and width ratio. The optimal Reynolds number increases with increasing twist ratio and reducing width ratios. The maximum reduction in the entropy generation rate was about 58%. Correlations for heat transfer and fluid friction performance for the range of parameters considered were also derived and presented.

Vacuum lifetime and residual gas analysis of parabolic trough receiver

The vacuum characteristics and lifetime are the key problems of parabolic trough receiver. Heat loss of the receiver will greatly increase when the vacuum has been lost. Especially, if hydrogen is inside the annulus space of the receiver, heat loss at a level is approximately a factor of four higher than the loss for a receiver with good vacuum. Suitable vacuum levels and residual gases should be maintained in the receiver to ensure performances during its projected lifetime. In this paper, the variations of composition and partial pressure of residual gases with temperature in the receiver were measured by a high sensitivity quadrupole mass spectrometer gas analyzer. The effects of residual gas and getter on the vacuum lifetime of receiver were analyzed. The results showed that hydrogen was the main residual gas in the annular space of receiver without getter, and the nitrogen was the main gas released in the receiver with getter. It can be confirmed that the residual gas analysis was a very effective way to predict and evaluate the vacuum lifetime of the receiver.

Numerical optimization of catalyst configurations in a solar parabolic trough receiver–reactor with non-uniform heat flux

A three-dimensional numerical model of a solar parabolic trough receiver–reactor (SPTRR) with methanol steam reforming reaction was developed. Based on this model, the characteristics of hydrodynamics, heat transfer and chemical reaction of SPTRR were studied. Also, the effects of non-uniform heat flux boundary condition, catalyst layout and mass flow rate on the performances of SPTRR were analyzed. The results showed that SPTRRs partially filled with catalyst have better comprehensive performance than those fully filled with catalyst, and the SPTRR with horizontal cylindrical segment shaped catalyst in the lower part of receiver tube is recommended to substitute the fully catalyst-filled SPTRR. The chemical energy conversion per unit pump power of the proposed partially catalyst-filled SPTRR is one order of magnitude larger than that of the fully catalyst-filled SPTRR, and the mean outer surface temperature and maximum outer surface temperature difference of the proposed partially catalyst-filled SPTRR can be as low as that of the fully catalyst-filled SPTRR, when the mass of the catalyst in the partially catalyst-filled SPTRR is 0.4 times as much as that in the fully catalyst-filled SPTRR.

Thermal and Thermodynamic Performance of a Parabolic Trough Receiver with Syltherm800-Al2O3 Nanofluid as the Heat Transfer Fluid

With the recent advances in technology, the use of higher concentration ratios has become feasible and shown potential to reduce the cost of electricity from parabolic trough collector systems. With this, efficient and improved heat transfer performance of parabolic trough receivers is becoming essential. Nanofluids are known to increase the heat transfer capabilities to levels higher than ordinary heat transfer fluids. In this work, the thermal and thermodynamic performance of a parabolic trough receiver using Syltherm800-Al2O3 nanofluid as the heat transfer fluid is presented. Fluid inlet temperatures in the range 350K to 650K, nanoparticle volume fractions in the range 0 - 8% and Reynolds numbers in the range 3 560 to 1 334 000 depending on the inlet temperature were considered. Results from this study show improved thermal and thermodynamic performance of a parabolic trough receiver with the use of nanofluids. The thermal efficiency increases by up to 8% for the range of parameters considered. Results further show that there is a Reynolds number beyond which the use of nanofluids becomes thermodynamically undesirable at a given inlet temperature.

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.

Transient Infrared Thermography Heat Loss Measurements on Parabolic Trough Receivers under Laboratory Conditions

Abstract This paper describes a transient infrared thermography heat loss measurement method for parabolic trough receiversand its development under laboratory conditions. The method is designed for future field measurements and represents a complementary approach to steady-state heat loss measurement methods. The receiver specific heat loss is determined by applying a thermal excitation to the absorber tube and by measuring both the absorber and glass temperature response signals with infrared pyrometers. These transient signals are processed to derive a mean absorber temperature, a mean glass temperature, an amplitude ratio and a phase shift at a mean air temperature. Temperature signals are used to identify receiver thermal properties with the help ofanumerical heat transfer model coupled to a parameter identification algorithm. Parabolic trough receivers with distinct absorber coating and annulus properties have been tested with two different transient excitation profiles, i.e. sinusoidal and ramp-and-hold signals. The identified receiver thermal properties have been compared to steady-state heat loss experiments conducted withDLR THERMOREC test bench.The observed specific heat loss deviations between steady-state and transient laboratory measurements respectively range from 2% to 4% for evacuated receiverswith selective coating and from 1% to 9% for receivers with black painted absorbers.

A Solar Simulator for the Measurement of Heat Collection Efficiency of Parabolic Trough Receivers

Abstract A solar simulator consisting oflinear focus unitsand a fluid circulation systemhas been constructed, and abasicschemeforevaluating heat collection efficiency of parabolic trough receivers has been demonstrated. The linear focal areaalong the 4 m long receiverwas formed byan array of twenty linear focusunits each with a 5kWXenon-short arc ramp. The flux distribution was measured by scanning a heat flux sensor, and the average flux of 24.3 kW/m2 was achieved along the half-round surface of the receiver tube. A parabolic trough receiver with the optical efficiency of 79.3% has been fabricatedand applied tothe demonstration.During irradiating the linear focal flux to the receiver, thecollectedthermal output was measured from the increase in temperature of water running through the receiver.The absoluteheat collection efficiency of 73.9%,defined as the ratioof the collectedthermal outputto theirradiated optical input,was obtained under the thermal equilibrium condition.

Durability Testing of Parabolic Trough Receivers – Overheating, Thermal Cycling, bellow Fatigue and Antireflex-Coating Abrasion

Abstract This paper describes accelerated aging tests on parabolic trough receivers. Overheating of parabolic trough receivers about 80-100 K over the nominal operating temperature is applied for accelerated aging of the absorber coating.A lifetime prediction based on the Arrhenius parameters requires extensive testing; A reduced test with heating receivers to 478 °C for 1000 hours is applied on receivers for oil fluid in order to compare heat loss and optical efficiency before and after such accelerated aging. In a further test set, such receivers are thermally cycled between 200 °C and 478 °C for 100 times. A bellow fatigue test has been developed for testing of the bellows stability for the compression and expansion. Receivers are subjected to 20,000 cycles and rested for a 24 hour waiting period. The receiver heat loss during the bellow fatigue test shall not increase more than 30%. Durability of the antireflex coating on the glass is tested with the Taber linear abrasor using the CS-10F rubber (¾”) and MIL 12397 Eraser (¼”). Both abrasive materials proved to be suited to obtain reproducible results, although the discussion is still ongoing on which material should be used. The larger diameter of the CS-10F rubber offers the advantage that transmittance measurement with the Lambda 950/1050 spectrophotometer can be performed without the necessity for multiple abrading stripes.