Tubular Receiver Research Articles

An efficient 3D tube-level flux-thermal model of external tubular receivers in solar power tower systems

Accurate and efficient radiative flux simulation and thermal analysis of liquid tubular receivers are essential for safe and stable operations of solar power tower systems. However, the complicated structure and photothermal processes on these receivers pose significant challenges for simulation. Although existing models have made great simplifications, the simulation results are still inefficient and inaccurate. To address these issues, a novel and universal 3D flux-thermal model is proposed. Firstly, a flux model is proposed via a tube-level Monte Carlo ray tracing method based on GPU parallel computing, enabling real-time and accurate radiative flux density distribution simulation for each absorber tube. Secondly, a high-resolution multi-tube thermal model is proposed, providing accurate 3D thermal analysis for each absorber tube. Through pre-factorization and highly parallelized designs, the proposed thermal model achieves a 600 times acceleration in efficiency, even with tens of millions of discrete elements. Finally, the novel flux-thermal model is validated against publicly published measured data from Solar Two, indicating a mean deviation of only 0.1% and a root mean square error of 0.006, demonstrating its high accuracy. Furthermore, the proposed flux-thermal model reveals complex and asymmetrical flux and temperature distributions on the absorber tube, in contrast to the symmetrical distributions obtained by the simplified models. Such inaccuracies in the simplified simulations may cause operational misjudgements and affect system safety potentially. Therefore, the proposed 3D tube-level flux-thermal model provides an accurate and efficient solution to radiative flux simulation and thermal analysis, enhancing the safe and stable operation of solar power tower systems.

Numerical investigation of the thermal-hydraulic performance of a novel parabolic trough wavy tubular receiver by using FMWNCT/oil-nanofluid coupled with fins

This study employs a computational fluid dynamics (CFD) approach to assess the performance of a novel wavy profile tube receiver. It integrates rectangular fins and an Oil-based nanofluid, specifically utilizing Thermodyne GG-03 synthetic oil combined with Functionalized Multi-Wall Carbon Tubes (FMWNCT). The synthesis involves rigorous experimental procedures to characterize the thermophysical properties of the oil and nanofluid. Empirical correlations, validated against Bitam et al.'s model [1], guide the numerical analysis. The investigation, utilizing a steady-state κ - ω SST turbulence model with a non-uniform Gaussian thermal flux distribution, explores thermohydraulic performance in fully developed turbulent flow (Reynolds number range: 2 × 104 – 5 × 104). Comparative analysis with Bitam et al.'s work, employing Syltherm 800 oil as a reference, reveals a 25.76% average Nusselt number enhancement with Thermodyne GG-03. Additional improvements are observed: 124.69% with FMWNCT/oil-nanofluid, 137.04% with fins, and 236.54% with combined FMWNCT/oil-nanofluid and fins. These enhancements lead to increased heat transfer coefficients and rates. The friction coefficient rises by less than 60%, mainly attributed to the Nusselt number improvement. At RE = 20000, the Maximum Performance Evaluation Criteria (PEC) is 5.5, signifying significant improvement. Notably, a 300% PEC enhancement is observed compared to Roger and Mayhew's curved tube model [1]. In conclusion, these findings offer valuable insights for efficiently designing wavy tubular receivers in PTR collectors.

The Effect of Uneven Metal Foam Distribution on Solar Compound Parabolic Trough Collector Receiver Thermal Performance

Solar energy is a key player among other renewable energies that reduce greenhouse gases and replace conventional fuel, i.e., to solve global warming and fossil fuel descending issues. However, the thermal solar systems’ performance should be enhanced to cope with intermittent solar radiation. Metal foam can be used as an enhancer in solar collectors’ receivers. However, metal foams are associated with pressure drop. To benefit from the metal foam as a thermal enhancer and overcome the pressure drop issue, the present study numerically and experimentally investigates a novel Compound Parabolic Solar Collector (CPC) receiver with innovative uneven metal foam inserts of varying thickness. Two pores per inch (PPI) Cu-foam inserts, PPI10 and PPI20, were tested. These inserts were strategically placed at three different positions along the receiver, with thicknesses of 3 cm, 2 cm, and 1 cm starting from the inlet side. In the experimental part, three tubular receivers were tested, empty, i.e., without metal foam, inserted with Cu-foam of PPI10, and inserted with Cu-foam of PPI20. The experiments were conducted from 09:00 AM to 04:00 PM. The investigation involved water volume flow rates from 0.2 to 0.6 l/min. The numerical part included solving the governing equations, i.e., mass, momentum, and energy conservation, simulating conditions similar to the experiments. The Brinckman model described the fluid flow through the metal foam. The thermal performance of the CPC system was evaluated using the Nusselt number (Nu), thermal efficiency, water bulk temperature, and water outlet-inlet temperature difference. Inserting Cu-foam of PPI20 resulted in the hourly maximum Nu and thermal efficiency compared to the empty and PPI10 cases. Experimentally, the hourly maximum Nu was 8.9, 8.4, and 7.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. The average thermal efficiency was 88.3, 85, and 81.9 for PPI20, PPI10, and empty receivers, respectively, at 0.6 l/min. As for the outlet-inlet water temperature difference, the highest values were at 0.2 l/min. Again, PPI20 recorded the best results, i.e., 23.3 K, 21.5 K, and 20.2 K for PPI20, PPI10, and empty cases, respectively.

Performance evaluation of a partially-filled porous foam cylindrical tubular receiver realizing Ni foam material reduction

The optimization of metal foam structure parameters is crucial for enhancing the heat transfer and pressure drop performance of cylindrical tube heat exchangers. In this study, thermal output performance of 16 different nickel foam-filled solar receivers with varying thicknesses and positions is numerically simulate. An optical-thermal conversion model is established using the Monte Carlo ray tracing (MCRT) method and user-defined functions. The influences of foam filling thickness and position on heat absorber, wall hot spot distribution, heat loss type, thermal efficiency, pressure drop, thermohydraulic performance, exergy loss, and outlet temperature were analyzed. Furthermore, a multi-objective optimization algorithm called non-dominated sorting genetic algorithm II (NSGA II) is used to generate the best trade-off solutions between improvement of thermal efficiency and pressure drop. The results show that enhancing the thickness of the filled foam within the CTR tube enhances thermal efficiency and elevates pressure drop. However, optimizing the length and filling position of the foam can effectively mitigate excessive pressure drop caused by longer foam, without compromising thermal efficiency. This approach results in a reduced pressure drop while maintaining optimal performance. The optimal solution of multi-objective optimization is that the 123 mm thickness foam is filled at 0 mm from the inlet of the tube, and the pressure drop and thermal efficiency of the solar receiver are 830.2Pa and 83.1 %, respectively, when the mass flow rate is 0.01 kg/s.

Heat transfer and thermal stress of a parabolic trough solar collector with secondary reflector using microencapsulated phase change material slurries

The traditional parabolic trough solar collectors (PTSC) often face the problems of high thermal stress and the deformation of the tubular receiver. In this paper, a secondary reflector (SR) is put into the traditional PTSC, and the microencapsulated phase change material (MEPCM) slurries with 7 vol%, 12 vol%, and 15 vol% are used as the operating fluids. By associating the Monte Carlo ray-tracing (MCRT), finite volume (FVM), and finite element (FEM) methods, the optical-thermal–mechanical coupling problem of the absorber tube is settled, the flow and heat transmission of MEPCM slurries in the collector tube are numerically investigated by applying the Eulerian-Eulerian multiphase flow model. The results show that the solar flux and wall temperature of the absorber tube with the SR are more uniform along the circumferential direction. The use of SR enhances the local heat transfer of the MEPCM slurries at the phase change section in the absorber tube, while when the phase changes finish, the local heat transfers of the slurries in the PTSC with SR have no difference with the traditional PTSC. Compared with the water, the MEPCM slurries obviously enhance the heat transfer of the absorber tube due to the phase change and the interactive motion of the microencapsulated particle, and the higher the slurry concentration, the larger the mean Nusselt number, the smaller the thermal stress. The addition of SR significantly decreases the thermal stress of the absorber tube, the maximal equivalent stress is reduced by 81.8%

Buoyancy influencing convective heat transfer characteristics of supercritical CO2 in a serpentine solar receiver tube at low Reynolds number

The tubular receiver is one of the most promising forms of supercritical CO2 (S-CO2) receiver. Due to the high-temperature and high-pressure working condition of S-CO2, the pressure resistance of solar receiver tube faces great challenges. The convective heat transfer performance of S-CO2 will be significantly improved by using the serpentine solar receiver tube instead of the traditional straight solar receiver tube in the S-CO2 tubular solar receiver, which will offset the negative effects of increased thermal resistance caused by increasing wall thickness to improve pressure resistance. Centrifugal buoyancy and gravitational buoyancy are important factors affecting the convective heat transfer characteristics of S-CO2 in serpentine tube which are numerically studied in this paper. The results show effect of buoyancy in the serpentine tube can significantly improve the convective heat transfer performance with a maximum value of 23% and reduce the entropy generation with a maximum value of 22% compared with straight tube. The convective heat transfer performance of S-CO2 for the downward flow is stronger than the upward flow due to the effect of gravitational buoyancy on the intensity of turbulence kinetic energy near the wall. The dominant position of centrifugal buoyancy and gravitational buoyancy affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The synergistic/competitive relationship between centrifugal buoyancy and gravitational buoyancy will promote/suppress the change of the flow field which affects the convective heat transfer characteristics of S-CO2 in serpentine tube. The conclusions of this work can provide theoretical guidance for the design and optimization of tubular receiver using S-CO2 as heat transfer fluid.

Integrated optical–thermal modelling and techno-economic evaluation of a ‘bladed’ solar-thermal receiver

The receiver in a concentrating solar thermal system is a costly component, and its contribution to system-level techno-economics must be carefully evaluated. This study examined the strategy of adding ‘blades’ to conventional tubular receivers, in the form of additional tube-banks perpendicular to the primary aperture surface, to improve light trapping and reduce size. An integrated optical–thermal model was used, incorporating geometrical configuration, flow path design and aiming strategy, and optical and thermal performance assessed followed by an economic analysis of its dynamic annual performance. A 65MW˙th polar heliostat field and molten nitrate salt working fluid were assumed, and design iterations conducted to maximise overall optical–thermal receiver efficiency. The final design achieved a 30% reduction in optical and thermal losses, equivalent to 2.5% increased efficiency. Even though the final design can sustain higher peak flux, its size could not be reduced without incurring excessive spillage loss. Meanwhile, the added blades did not increase the intercepted radiation. As a result, in the techno-economic evaluation, the approximately doubled tube mass increased the receiver cost by 16%–25%, and only 1–3USD/MWh˙e savings were achieved in the levelised cost of electricity (LCOE). The LCOE of bladed receivers was evaluated for the first time in this study, confirming that bladed receivers in their current form are not promising. However, the study provides insight into the benefits of the related ‘STAR’ concept, where vertical blades are added to cylindrical receivers, which increase the receiver aperture while decreasing the tube cost per aperture area.

Numerical Modeling and Experimental Validation of Heat Transfer Characteristics in Small PTCs with Nonevacuated Receivers

The development of small-sized parabolic trough collectors (PTCs) for processing heat production at medium temperatures (100–250 °C) represents an interesting approach to increase the utilization of solar thermal technologies in industrial applications. Thus, the development of simplified models to analyze and predict their performance under different operative and climatic conditions is crucial for evaluating the application potential of this low-cost technology. In this paper, we present a numerical method that by combining three-dimensional finite element simulations (implemented with COMSOL Multiphysics software version 6.1) with a one-dimensional analysis (based on a MATLAB script) allows for the theoretical determination of the power output of a small-PTC with a nonevacuated tubular receiver operating at a medium temperature. The finite element model considers both the nonuniformity of the concentrated solar flux on the receiver tube (evaluated using Monte Carlo ray-tracing analysis) and the establishment of natural convection in the air gap between the glass envelope and absorber tube. The model calculates, for several values of direct normal irradiance (DNI) and inlet temperatures, the thermal power transferred to the heat transfer fluid (HTF) per unit length. The data are fitted using the multiple linear regression method, obtaining a function that is then used in a one-dimensional multi-nodal model to estimate the temperatures and the heat gains along the receiver tube. The outputs of the model are the outlet temperature and the total thermal power transferred to the HTF. In order to validate the developed methodology for the assessment of the heat transfer characteristics in the small-PTC with a nonevacuated receiver, an experiment at the ENEA Trisaia—Solar Thermal Collector Testing Laboratory was carried out. This work compares the theoretical data with those acquired through experimentation, obtaining a good agreement, with maximum differences of 0.2% and 3.6% for the outlet temperatures and the power outputs, respectively.

Geometric characteristics and design optimization of parabolic trough concentrators with a flat mirror as the secondary reflector designed based on two components of a solar source

Parabolic trough concentrators with a tubular receiver (PTCs) are widely used for the production of solar thermal power. However, the geometric concentration ratio (CR) of PTCs falls short of thermodynamic concentration limit (C0) by a factor larger than π, and restricted by the angle subtended by solar sources (Δs = 2θa). To increase CR, a novel idea by dividing a solar source into two components in the open angle is proposed for the design of PTC equipped with a secondary reflector (SR), and PTC with a flat mirror (FM) is designed as a trial of the idea. For such design idea, rays incident at rims of primary parabolic reflector (PR) from one component (S1) are required to directly redirect onto on the absorber tube (AT) and those from another component (S2) are required to arrive on the AT after reflections from PR first then from FM. To investigate effects of the angular division factor (k) of solar sources (Δs2 = kΔs) and rim angle (ϕe) of PR on the effective CR (Cg), a mathematical procedure to characterize the geometry is suggested and examined by ray-tracing technique. The analysis shows that the geometry of PTC-FM is dependent on k, ϕe and θa, and a set of optimal k and ϕe for maximizing Cg can be obtained when θa is given. Calculation results show that the optimal k for maximizing Cg is 0.5 regardless of θa, but the optimal ϕe of PTC-FM for maximizing Cg, is sensitive to θa. As compared to conventional PTCs, the Cg of optimized PTC-FM is increased more than 80%, larger than 56.5% of C0, for k = 0.5 and θa < 1.0°. Empirical expressions for a quick estimation of optimal ϕe are also proposed based on numerical calculations. Results by ray-tracing analysis indicate that the optimal ϕe for maximizing CR and average solar concentration ratio are almost identical when the reflectivity of PR and FM is higher than 0.8. In principle, the novel idea is applicable for the design of other solar concentrators, therefore a further increase of Cg of PTCs is possible by using other imaging and non-imaging optics as the SR.

Enhancing the Thermal Performance of a Central Tower Tubular Solar Receiver With Direct Steam Generation by Using Internal Fins and Thicknesses Variation

Abstract This study provides a comprehensive analysis of the thermal behavior of tubular solar receivers used for direct steam generation, focusing on the effects of different wall thicknesses and longitudinal rectangular fins on the internal surface. The study evaluated six tubes, including two tubes with distinct wall thicknesses (5 mm and 3 mm) without fins and four tubes with fins but varying configurations. These tubes represent external tubular receivers used in solar central tower plants and operate at 4.5 MPa with a nonuniform solar flux averaging 0.8 MW/m2. Adding longitudinal fins to the tubes significantly reduced the maximum temperature on the irradiated surface and improved heat transfer to the fluid. The study concluded that the 45F5 receiver, which has three fins on the internal section, outperforms the other receivers, presenting a higher vapor generation. Furthermore, the 45F5 geometry enhances heat transfer, allowing the lowest maximum Biot number. These results are crucial for designing tubular solar receivers with direct steam generation systems and improving their efficiency in generating energy from renewable sources.

Nonimaging Behavior of Circular Trough Concentrators With Tubular Receivers

Abstract This paper presents a detailed analysis of the optical performance of circular trough concentrators with tubular receivers. First, a simple analytical formula for the achievable geometric concentration ratio as a function of the rim angle and acceptance angle is developed. Notably, the development reveals the existence of three distinct concentration ratio regimes: a first regime where the receiver is sized based on the reflection of the edge rays from the rim alone, a second regime where the receiver is sized based on the rim and the edge ray caustics, and a third regime where two reflections from the mirror are permitted. Several exemplary designs are proposed and further analyzed using Monte Carlo ray tracing to obtain transmission angle curves and receiver flux distributions. For an acceptance angle of 1 deg, the circular trough concentrator with circular receiver is found to achieve a maximum geometric concentration ratio of 7.695× with a peak flux of 30 suns. For large acceptance angles (10 deg), the circular trough achieves a geometric concentration ratio as high as 82% of that of a parabolic trough. This noteworthy performance, along with the fact that a circular mirror is amenable to an inflated polymer construction, makes this configuration promising for low-cost, low-concentration solar thermal applications.

Numerical study on the start-up strategy of cylindrical tubular receiver filled with nickel foam

A cylindrical tubular receiver filled with nickel foam (CTRNF) appears to be a promising receiver concept due to its capability to provide temperatures over 900 °C at a high pressure. Nevertheless, CTRNF is challenged by the unsteadiness and high intensity of radiant flux may cause inaccurate start-up time predictions, providing unrealistic and unstable outlet temperatures. In this article, these matters are addressed by a numerical model coupling the Monte Carlo ray tracing (MCRT) method and discrete coordinate (DO) radiative transfer model. The model considers the cold, semi-cold, semi-hot, and hot start-up strategies on the CTRNF using N2 and Ar as the working medium. Three preheating methods and their influence on the thermal characteristic of the CTRNF are proposed. Temperature and equivalent stress distribution of internal heat absorber is simulated and analyzed. The results indicate that the duration for four start-up strategies takes 1711 s, 2189 s, 1212 s and 1032 s as the outlet temperature reaches to 1200 K, respectively, when Ar is used as the working medium (10 g/s) and radiative power of 10.5 kW incidents on the solar receiver. Compared with the 68.44 kg Ar consumption of cold start-up strategy, the Ar working medium consumption decrease by 24.44 kg, 51.68 kg, and 63.16 kg for semi-cold start-up, semi-hot start-up, and hot start-up strategy. However, the maximum equivalent stress of the CTRNF under the hot start-up strategy is 100 MPa higher than under the cold start-up strategy, which requires accurate monitoring of the cavity temperature in solar receiver.

Structural design challenges and implications for high temperature concentrating solar power receivers

High operating temperatures along with diurnal cycling and high operating stresses bring many material and engineering challenges for concentrated solar power (CSP) receivers. Current design rules in ASME Boiler and Pressure Vessel (B&PV) Code for high temperature structural components were not intended for CSP components. The paper reviews the design rules in several sections of ASME B&PV Code in terms of their applicability in designing high temperature CSP receivers. With some restrictions and modifications, the conditions assumed in the design rules in Section III, Division 5 of ASME B&PV Code are found to closely resemble the loading conditions of CSP receiver, including accounting for the interaction between creep and fatigue damage. The paper applies the proposed design rules to an external tubular receiver design and discusses several lessons learned from the design study. The creep damage accumulation due to high thermal stresses resulting from circumferentially non-uniform flux distribution is found to control the design of CSP receivers. Several mitigation actions to improve the design life of receivers are also discussed.

Computational analysis of passive strategies to reduce thermal stresses in vertical tubular solar receivers for safety direct steam generation

When exposed to high non-uniform heat fluxes on the receiving surface, significant temperature gradients develop in the tubes, producing thermal stress and deformations in the receivers. This paper reports a detailed investigation of the thermal and structural performance of a solar tower system that operates with direct steam generation tubes when the fins are added to the internal surface and thickness varies. Six tubes were evaluated, two without fins varying the wall thickness (5 mm and 3 mm) and four with fins under different configurations. The thermal system corresponds to a representative tube of an external tubular receiver of a central tower system. The thermal receiver operates with direct steam generation under a non-uniform concentrated solar flux with a maximum value of 0.93 MW/m2. The best configuration of the tube was with fins on the frontal surface of the tube, maintaining a receiving wall thickness of 3 mm and an adiabatic wall thickness of 5 mm. It could reduce maximum thermal gradients and temperature by 31% and 14%, respectively, maximum thermal stresses by 38%, and maintain vapor mass flow at the output (276 kg/h).

Influence of eccentricity on the thermomechanical performance of a bayonet tube of a central solar receiver

This work numerically evaluates the thermal and mechanical behaviors of eccentric bayonet tubes to be used in external central receivers of solar power tower plants. A bayonet tube is composed of two tubes, one inside the other, creating circular and annular sections, through which the molten salt of the receiver sequentially flows. Eccentricity in the annular section is achieved by displacing the axis of the interior tube with regard to the exterior one. For comparative purposes, two examples of conventional tubes (single tubes with circular cross-sections with diameters of 25mm and 50mm) are also investigated in this work to compare their performances with those of bayonet tubes. The results obtained with the eccentric configurations show an enhancement of the heat transfer to the molten salt and a reduction of the tube wall overheating compared with the concentric bayonet tubes and the largest simple tube. For conditions representative of the normal operation of a solar power tower, eccentric bayonet tubes could reduce the pressure drop by 30.8% and increase the convective heat transfer achieved in a concentric configuration of the bayonet tube by 26.1%. Nevertheless, this pressure drop was considerably higher than those obtained in the smallest and largest simple tubes, which were 1.28 bar and 0.13 bar, respectively. To investigate whether the enhancement of the convection heat transfer experienced by bayonet tubes compensates for their higher pressure drop or not, a Performance Evaluation Criterion (PEC) was proposed and used to compare the global performance of bayonet tubes with that of conventional tubes. The bayonet tubes with eccentricity 0.45 obtained the largest PEC, which was up to 13% higher than reference conventional tubes. Enhancement of the tube wall refrigeration produced when increasing the eccentricity is reflected in the maximum tube temperature and thermal stresses, which are found to diminish by approximately 8.8% with the highest eccentricity. In addition, the largest eccentric bayonet tube layout obtains the smallest peak temperatures compared to conventional tubes. The lower inertial moment of the smallest conventional tube indicates that its thermal stress is 2.1% lower than the stress obtained in the most eccentric layout analyzed in this work. Nevertheless, the time to rupture associated with creep damage of the eccentric bayonet tube is 1.04 times higher than that obtained in the smallest simple tube, demonstrating that bayonet tubes could be a potential alternative to the current tubes of external tubular receivers.

Development of Nusselt number correlation for solar CPC with a tubular receiver

Development of Nusselt number correlation for solar CPC with a tubular receiver

Thermal performance of a supercritical CO2 based central receiver

This study investigates the thermal performance of a supercritical carbon dioxide (sCO2) based central receiver for a 3rd generation concentrated solar thermal (CST) plant having 10 MW thermal capacity. A thermo-optical modelling methodology is deployed to evaluate the thermal performance of tubular receivers with outer radii of 6.2 mm, 12.4 mm, and 24 mm. Solar PILOT ray tracing software is used to obtain the solar flux arrangement around the central receiver. The maximum heat flux permitted in the smaller tube receiver (ro = 6.2 mm) results in a smaller receiver and heliostat field for a particular power output. Heat flux concentration is found to be 77 % higher in a smaller tube (ro = 6.2 mm) receiver than in a large tube (ro = 21 mm). However optical efficiency is 3% higher in large tube receivers compared to smaller tubes. Computational fluid dynamics (CFD) simulation is performed to investigate the receiver performance considering Inconel 640 as a receiver material. Tube having ro = 21 mm has the highest solar to thermal efficiency (58.14%) i.e., 4.5% higher than the large tube (55.6%). Hence, further investigation is performed for the smallest tube to examine the variation of temperature, pressure drop, and Nusselt number as a function of Reynold’s number along the tube. There is a marginal change (1%) in the local Nusselt number from the receiver tube’s entry to exit.

A novel lightweight tubular receiver for drone wireless charging system

AbstractIn the research of drone wireless charging, the lightweight design of the wireless charging device has a large impact on the overall performance of the drone. To realize the lightweight, the tubular coil, which caused by high‐frequency excitation to produce skin effect in the conductor becomes a way to optimize the weight of the receiver coil. This paper proposes an aluminum tube with a thin copper layer plated on the surface as a drone receiver coil and landing gear. This solution can not only replace the original bracket but also be lightweight and obtain high coupling coefficient. Experiments show that this novel receiver can receive power up to 30 W at 250 kHz excitation source with 78% average transmission efficiency and has good antimisalignment ability. This new tubular coupler is more suitable for the demand of lightweight wireless charging for drones.

A novel asymmetric compound parabolic collector under experimental and numerical investigation

In this study a novel compound parabolic collector (CPC) with an asymmetric reflector and an evacuated U-pipe tubular receiver was examined numerically and experimentally. The thermal operation of the system was tested in a range of 22–70 °C inlet temperature in the city of Athens in Greece, using water as the working fluid. The collector was oriented towards the sun in each case, while the measurements where conducted for 10 min regarding each operating point. The data acquisition as regards the solar irradiation intensity, the inlet, the outlet and the ambient temperature was conducted every 30 s. The experimental results were also validated by developing a numerical model in SolidWorks Flow Simulation environment. Particularly, it was found that the numerical results diverge from the experimental one by 3% on average, with the maximum declination to take the value of 5.17%. By this simulation it was possible to estimate the temperature of the receiver and the convection regime at the interior of the U-pipe. The convective coefficient at the interior of the flow tube was also validated through a theoretical model. Last but not least, the collector was compared with three other designs taken from literature as regards the optical performance.

Exploring efficiency limits for molten-salt and sodium external cylindrical receivers for third-generation concentrating solar power

Third-generation concentrating solar-thermal power (CSP) systems are proposed which aim to halve the cost of electricity from CSP. This work demonstrates that a transition from state-of-the-art cylindrical molten nitrate salt receivers with a peak concentrated solar flux of 1MW/m2 and a heliostat field optical error of 3mrad to future cylindrical sodium receivers with a peak flux of 1.8MW/m2 and a heliostat field optical error of 1mrad could enable the exergy output of CSP receivers to be increased significantly, by 25% at design point, while keeping the general system design very similar. This comparison is conducted on a large dataset established with a detailed field and receiver energy balance model considering field aiming, peak receiver flux and hydrodynamics. It is based on design point simulations, and is independent from cost considerations. At high temperatures, the optimal configurations achieve their gains only with very accurate heliostats and when the peak flux is increased, while at low temperatures, there is nothing to be gained from very high peak flux limits on external tubular receivers. This underscores the critical importance of high heliostat accuracy for high-temperature CSP systems design. For the third-generation, high-temperature (740 °C) sodium receiver configuration considered, it is found that the upper limit to thermal–optical efficiency of optimised receivers at design-point is around 88 %.