Receiver Tube Research Articles

Optical and thermal performance analysis of a micro parabolic trough collector for building integration

Medium temperature thermal energy (100–400 °C) is widely used in industrial processes and in large buildings. The presented work investigated the feasibility of a new solar collector which was designed to harvest solar energy from factory rooftops for industrial process heat applications. The proposed solar collector was comprised of parallel micro-parabolic troughs, vacuum tube receivers, and an internal tracking mechanical contained in a glazed box which can be easily mounted on large buildings. The system does not require external rotational tracking and achieves a concentration ratio of ~4.2 in a ~150 mm height, so it can be easily integrated with buildings. The optical performance of the presented collector is analysed and modelled theoretically and numerically, considering the effect of shading, inclination, and orientation. Furthermore, a transient thermodynamic model is developed to calculate its thermal efficiency and stagnation temperature, along with the effect of vacuum pressure, beam radiation, and emissivity of selective coatings. The theoretical analysis, verified by TRNSYS simulations and an outdoor experiment, revealed that the annual optical efficiency of the system was about 66.7% and the thermal efficiency was about 59.3% at 200 °C, if the collector was inclined to local latitude angle. These results reveal that the proposed design is competitive with evacuated flat plates (i.e., the TVP collector which has a thermal efficiency of ~36% at a normalized temperature difference of 0.2). Further, if the collector were to be installed on the vertical façade of a building, the theoretical model estimated that the optical and thermal efficiencies would be 44.1% and 37.5%, respectively. An economic analysis indicated that a levelized cost of heat energy of 0.51 $/kWh can be obtained. Overall, since the proposed collector has a simple structure and a low-profile, this study indicates it is promising for medium temperature solar thermal heat production for industrial processes and/or for multi-effect absorption chillers.

Finite Element Model and Design of Effective Focal Panel and Receiver Tube for Solar Steam Producer

Solar energy is an excellent alternative to replace conventional energy sources. Here solar energy is converted to electrical energy using a parabolic trough and a turbine. Incident rays of solar radiation falling on the trough get reflected and concentrated on the focal length of the parabolic collector. The receiver tube is placed on this axis. The thermal fluid is made to pass inside the tube (water). The concentration of solar energy on this tube raises the temperature of water converting it to steam. Steam is made to pass through a nozzle increasing its kinetic energy. It passes to the blades of an impulse turbine where the rotor rotates, and the shaft produces mechanical energy. The turbine is connected to a generator which converts mechanical energy into electrical energy, producing power. Thermal and structural stresses are determined using FEA software Ansys Workbench for the selection of appropriate sheet and tube material.

Experimental and numerical study of a solar collector using phase change material as heat storage

Abstract The present work present numerical and experimental investigations to study the performance of a small-scale parabolic trough solar concentrator integrated with thermal energy storage system. A new design of receiver which uses the phase change material to store the thermal energy is built. A parabolic-trough solar concentrator was proposed and constructed using a concentric absorber tube with phase change material. The absorber is considered as a temporary thermal heat storage component. The heat storage-recovery mechanism will act as a thermal compensator that allows the parabolic trough solar concentrator to stabilizing energy output during the absence of intense radiation or for short time cloudy conditions. The PCM used a binary salt composed of a mixture of 60% NaNO3 and 40% KNO3, filling the annular space of the absorber tube. Solar energy is converted to heat, stored in the phase change material and is discharged to cold water, which is the final heat transfer fluid in the inner pipe of the receiver. The simultaneous testing of the receiver with and without phase change material is investigated. A commercial computational fluid dynamics model of the whole system is presented and numerical results are compared to experimental results, which were conducted with and without phase change material in the receiver tube. The maximum outlet water temperature achieved using phase change material in the receiver tube is about 16.80% and 14.86% higher than the simple tube for 21 July and 21 October respectively. The daily thermal efficiency of the parabolic trough solar concentrator increases by 6.56% and 8.32 respectively for a both days using a receiver with a phase change material concentric tube. The phase change material showed good heat retention of 2 h to 1 h 40 min for water inlet temperature 30°C and 40°C respectively.

An experimental study on energy and exergy performance of a spiral tube receiver for solar parabolic dish concentrator

In this paper, energy and exergy performance of an external type spiral tube receiver for a solar parabolic dish concentrator is experimentally analyzed and presented. Performance evaluation is done at three different radiation conditions. The receiver is tested in the temperature range of 30 °C–100 °C with water as heat transfer fluid and at a flow rate of 1.5 L per minute. The overall heat loss coefficient of the receiver estimated from the stagnation test is found to be 182 W/m2K. The average thermal and exergy efficiencies of the receiver was determined to be 56.21% and 5.45% respectively under an average beam radiation of 750 W/m2. The variation trend of thermal efficiency of receiver is similar to that of difference between temperature of the heat transfer fluid at receiver inlet and outlet. The results show that this light weight, low cost receiver has potential to be employed with solar parabolic dish concentrator for process heating applications.

Allowable solar flux densities for molten-salt receivers: Input to the aiming strategy

Solar Power Tower technology requires accurate models and tools to assist in design and operation stages. The heliostat field aiming strategy seeks the maximization of the thermal output from the receiver, while preventing its permanent damage because of thermal stress and corrosion in molten salt receivers. These two limitations are translated into Allowable Flux Densities (AFD), which can be handled by the aiming strategy. This paper explains the methodology to determine AFDs, and analyzes the influence of tube geometry and material. AFD by corrosion is slightly lower in Haynes 230 than Inconel 625 and austenitic alloys. On the contrary, HA230 has better performance than In625 under thermal stress. Increment of tube wall thickness diminishes the AFD: slightly by corrosion, but significantly by thermal stress. The generated AFD databases feed the aiming model, herein applied to Gemasolar case study. In the cylindrical receiver, first northern panels are limited by thermal stress, while the last ones by corrosion. Under optimized aiming, HA230 receiver tubes produce equivalent thermal output than In625. • Corrosion AFD increases with mass flow rate, but decreases with salt temperature. • Stress AFD increases with salt temperature, but decreases with wall temperature. • Tube thickness has dramatic effect on thermal stress, while not on corrosion. • Receiver first panels restricted by thermal stress, while last ones by corrosion. • HA230 better mechanical performance is traded off by In625 higher corrosion tolerance.

Thermal-hydraulic and thermodynamic performances of liquid metal based nanofluid in parabolic trough solar receiver tube

Parabolic trough collectors (PTCs) are widely applied in concentrated solar energy utilization, and the further improvement of PTC's efficiency is usually restricted by heat transfer performance of parabolic trough solar receiver (PTR) tube. With the aim to enhance the heat transfer performance of PTR tube, liquid metal based nanofluid (i.e. the suspension of nano-scale powders in liquid metal) is proposed as working fluid due to its superior thermal transport properties. The thermal-hydraulic and thermodynamic performances of two types of liquid metal based nanofluids including gallium (Ga)-copper (Cu) and Ga-carbon nanotube (CNT) in PTR tube are numerically investigated. Monte Carlo Ray-Trace Method is used to obtain actual distribution of non-uniform heat flux on receiver tube wall, and four parameter turbulence model is adopted considering the low Prandtl number of liquid metal based nanofluid. The numerical method is validated by the experimental data. The results show that the presence of Cu and CNT can enhance the forced convection heat transfer of pure Ga in PTR tube, and the average enhancement degrees can reach up to 34.5% and 45.2% respectively under present conditions. Also, the frictional pressure drops of Ga–Cu and Ga-CNT nanofluids are larger than that of pure Ga at the same Reynolds number. With the increase of nano-powder concentration, the total entropy generation decreases and the exergetic efficiency increases. The thermodynamic performance of Ga-CNT nanofluid is better than that of Ga–Cu nanofluid under the same condition.

Flow and Thermal Mechanisms in Receiver Tube of Parabolic Trough Collectors with Rings Axially Connected Together and Radially Connected to the Inner Tube Surface

Artificial obstructions on the underside of receiver tubes can increase the heat transfer coefficient between the receiver tubes (Heat Exchanger, HX) of Parabolic Trough Collectors (PTCs) and water as a working fluid. In this study, we numerically and experimentally tested the behavior of laminar mixed convective heat transfer in HX tube installed with baffles. These baffles are rings that are connected together axially and connected radially to the inner tube surface. Using ANSYS fluent Version 15.0, we performed computation fluid dynamics modeling to achieve heat transfer enhancement in HX tubes equipped with turbulator baffles under laminar flow conditions. Moreover, the effects of Pitch ratio (P/D) = 3, 6 and 10 and Reynolds number Re#480 were recorded. The novel application of the rings that are connected axially together and radially to the inner tube surface contributes to the long-term storage of thermal energy and promotes heat transfer via. conduction from the tube surface to the center line of the water flow within a short period. In the study, the baffles generated a vortex to increase the Nusselt number (Nu) inside the HX. To simulate heat flux, we calculated the constant wall heat flux of the receiver tube using an electric heater. Results indicated that using 20 rings as baffles instead of plain tubes improves heat transfer by up to 75%. As P/D decreased and re increased, the heat transfer rate, friction factor (f) and Thermal Enhancement Factor (TEF) increased.

Optical Performance Analysis of Single Flow Through and Concentric Tube Receiver Coupled with a Modified CPC Collector Under Different Configurations

Compound parabolic concentrating (CPC) collectors have great potential to provide sustainable solar thermal energy for many applications operating in the medium temperature range. This paper presents the design, development and performance evaluation of a modified CPC collector integrated with an evacuated tube receiver. The optical performance of the designed CPC paired with concentric tube receiver is compared with that of a CPC coupled with single flow through evacuated tube receiver for stationary installation in the East-West and North-South directions. Ray tracing simulations of different configurations demonstrate that CPC coupled with single flow through receivers suffer high gap losses, especially at smaller incidence angles which are considerably alleviated by a concentric tube receiver arrangement. East-West installation of CPC paired with concentric tube receiver exhibited superior optical performance than all other configurations. The yearly average optical efficiency of CPC with concentric tube receiver was 5% higher than that of a single flow through receiver within the acceptance angle. A 60% truncated CPC coupled with concentric tube receiver emerged as the most effective design, which was fabricated for experimental testing. The tests conducted under actual outdoor tropical environmental conditions demonstrated that the experimental optical efficiency reached to about 69% in the case of N-S installation and 66.5% in an E-W arrangement. The experimental results closely match the simulation outcomes, which indicate the proposed performance prediction technique as instrumental for selecting the most effective configuration of CPC collectors for medium temperature heat supply.

Numerical and experimental study of solidification dangers in a molten salt receiver for cloudy conditions

Molten salts have high solidification temperatures, so they easily solidify in the receiver tube when the incident energy fluxes are very low due to clouds drifting and floating across the sky. Additional energy and time are then wasted to thaw the frozen solid with the salt freeze/thaw cycles eventually damaging the receiver. Therefore, operators need to adopt appropriate operating strategies to ensure that the salt remains molten during extended cloudy periods. The present study presents a thermo-hydraulic dynamic model of a molten salt receiver to investigate the heat transfer processes. The model is based on the Badaling 1 MWth receiver experiment with three experimental groups of tests modeled to validate the thermal hydraulic model in both the direct-filling mode and the S-type flow mode. The model results agree well with the experimental measurements with maximum relative errors of 0.4% for the outlet temperature and 0.9% for the surface temperatures. Then, a sensitivity analysis is conducted to better illustrate the flow distribution and surface temperatures, including the effects of the flow resistance, inlet temperature, cloudy duration, and valve abnormalities. Finally, the results indicate that the S-type flow mode should be used to prevent solidification for cloudy conditions.

Verification of the Numerical Algorithm for Parameter Analysis of the Tube Heat Receiver of the Solar Parabolic Trough System

This paper presents the results of the first stage of verification of the generalized computational algorithm for determining the geometrical, dynamical, and energy parameters of the “parabolic trough concentrator – tube heat receiver” solar receiver system. This algorithm is based on the dimensionless coupled mathematical model of heat and mass exchange in the “Sun – parabolic trough concentrator – tube receiver” system. The Monte Carlo ray tracing method was used to calculate irregular heat flux from the concentrator to the tube receiver surface. The 3D mathematical model of heat exchange in the tube receiver was solved using the finite volume method. The mathematical model took local climatic and geographical features into account, as well as micro- and macroscopic imperfections of the concentrator surface. The dimensional problem of heat exchange in the tube receiver of the solar parabolic trough system was solved at the first stage of verification. A simplified physical model was described. It was assumed that the glass envelope around the tube receiver was removed. The dimensional mathematical model of heat exchange in the tube receiver is based on the system of Navier–Stokes equations for viscous incompressible liquid with constant physical properties. The mathematical model with real boundary conditions was solved numerically. An analytical solution for the test problem with trivial boundary conditions was obtained. The results of the test numerical problem were compared with analytical solution results and were found to be in close agreement. Numerical experiments with real boundary conditions were conducted using the computational algorithm. The numerical data were compared with the results of field experimental studies and showed nearly total agreement, which proved the adequacy of the basic mathematical model and the resulting numerical algorithm.

Performance evaluation of compound parabolic concentrator with evacuated tube heat pipe

The keen interest of harvesting solar energy is increasing because of its clean, green and cheap nature. The pv modules collect the energy from the sun and produce electricity. The solar thermal collectors collect the solar energy in the form of heat. Compound parabolic concentrator is a type of solar thermal collector, which has a combined parabolic structure concentrating the solar radiation on a single line focus where the receiver tube is placed. The performance can be further increased by placing the evacuated tube, which traps the heat. The heat is efficiently conducted by using the heat pipe. A heat pipe is a heat-transfer device that transfer the heat between two solid interfaces. It combines the principles of both thermal conductivity and phase transition. The experimental study is carried out to find out the performance characteristics. The results according to the various parameters such as mass flow rate, tilting angles were plotted to find the better performance. The result shows that the maximum efficiency of the CPC is achieved between 12.00hrs. and 15.00hrs. The intensity of radiation during maximum efficiency is 1000 W/m2 to 1050 W/m2.

Performance Assessment and Improvement of Central Receivers Used for Solar Thermal Plants

In this work, the energy status and supply plans of Saudi Arabia are discussed with a focus on concentrated solar power (CSP) technologies. Subsequently, optimal designs for a 20 MWe solar power plant external receiver, combined with a 15 h thermal energy storage unit, operating under the weather conditions of Neom City, located in northeast Saudi Arabia, is proposed. The effects of receiver tube diameters, tube thicknesses, tube thermal conductivity and receiver’s performance are studied in detail and compared to those used in a well know operational CSP plant. Results show that a smaller tube diameter and thickness give higher receiver thermal efficiency but increase the annual cost of pumping energy. However, that increment in cost is negligible compared to the total energy gained. Furthermore, the aspect ratio is investigated and it was found that a higher aspect ratio gives a higher thermal efficiency. The thermal efficiency of the optimised receiver was increased by about 1% more than the reference plant. In addition, the new design decreases the total estimated cost of tube material by approximately 43%. It is anticipated that the reported results could pave the path for more efficient solar thermal power plants.

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.

A receiver geometrical details effect on a solar parabolic dish collector performance

Abstract In the present study, a parametric study of a parabolic dish receiver system is investigated in order to determine the best configuration to achieve an optimal system performance. A series of numerical simulations has been conducted for four receiver tubes and two positions of the receiver inlet/outlet configurations: position 1 and position 2. The thermal analysis proves that the receiver’s inlet/outlet configuration effect is influenced by the geometric properties of the receiver. Numerical results show that with a “low” height receiver (H = 0.075 m, H = 0.048 m), position 2 gives the highest thermal efficiency, while position 1 ensures the greatest thermal efficiency value for the “elevated” height cylindrical receiver (H = 1.2 m, H = 0.468 m). Indeed, for the configuration with H = 0.075 m, the thermal efficiency increases by 62.6% from position 1 to 2. Moreover, by changing the position from 1 to 2, the thermal efficiency decreases by 53% for a configuration with H = 1.2 m.

A numerical analysis of the energy and entropy generation rate in a Linear Fresnel Reflector using computational fluid dynamics

This work presents an energy and entropy generation analysis of a Linear Fresnel Reflector using the Computational Fluid Dynamics. It consists of 25 mirrors oriented to a receiver tube, which is located inside a Compound Parabolic Concentrator. The formulation of the entropy generation rate considers the phenomena of viscous dissipation, heat transfer and radiation, it is performed in a local and global way and implemented by user-defined functions. Results of the incident radiation, absorbed radiation, radiation temperature, temperature gradients, air velocity contours, Nusselt number and optical efficiency, are presented. Results show that the maximum values of the absorbed radiation (7800 W m−2), incident radiation (30,000 W m−2) and radiation temperature were located at the receiver tube. Also, the maximum value of the temperature gradient (39,000 K m−1) was obtained on the lower half of the receiver tube and the upper part of the secondary receiver. Moreover, the highest values of the entropy generation rate were located at the upper part of the secondary receiver for each phenomenon considered. It is concluded that the entropy generation rate due to heat transfer phenomenon is the most dominant (97.4% of the total), followed by radiation (2.59%) and then by viscous dissipation (negligible).

Design of a small size PTC: computational model for the receiver tube and validation with heat loss test

In EU, the residential sector is responsible for the 40% of the total energy consumption. The integration of solar technologies in buildings is therefore necessary in order to reduce the use of fossil fuels. Concentrating collectors could offer an interesting solution but nowadays their application in buildings is rare due to high costs, large dimensions and complexity of the system. A novel small parabolic trough collector (PTC) has been conceived to overcome these limits and to allow the integration in buildings. The main features of realized prototype are the compact dimensions, strongly reduced compared to the PTC standards, and modularity. The paper deals with the numerical analysis necessary to design the concentrating collector properly and its validation with experimental results. An optical analysis has allowed to select the optimum values for the parameters of the parabola, aperture and rim angle. A thermo-fluid dynamics finite element model has been developed with Comsol Multiphysics, to analyse the relevant physical characteristics and to predict the performance of the receiver tube. The efficiency curve of the collector has been extracted. Successively a receiver tube has been built based on the indications of FEM model for what concerns geometry and materials. In order to evaluate the heat loss of the receiver and to validate the finite element model, a test bench has been realized. The results of off-sun heat loss tests on the receiver tube are reported for several temperatures. The computational model is in good agreement with experimental results and therefore it is validated.

Design, construction and evaluation of a Fresnel Linear Concentrator for Oil Heating

El objetivo de este trabajo es describir el diseño y desarrollo de un concentrador lineal de Fresnel que pueda concentrar las radiaciones solares hacia un tubo receptor con una relación de concentración de 11,54. La metodología aplicada permitió analizar el espaciado, las inclinaciones y los anchos adecuados de los espejos más convenientes para obtener el mayor factor de concentración solar y las pérdidas ópticas y térmicas mínimas. El diseño del colector de concentración solar consistió en un sistema de 3.5 m x 3.0 m x 2.65 m con 15 espejos, cada uno de 0.12 m de ancho, y una altura central de recepción de 2.0 m. Los resultados experimentales confirmaron que se alcanzó la temperatura máxima del proceso 140 °C. Además, el almacenamiento de aceite térmico fue de 995 kJ-h disponibles en 6 horas (desde las 9: 00h hasta las 15: 00h). Las pruebas de rendimiento del sistema se realizaron en la ciudad de Barranquilla, con las condiciones ambientales locales. Las pruebas experimentales del prototipo permitieron validar el diseño propuesto en este trabajo.

Simplified heat transfer model for parabolic trough solar collectors using supercritical CO2

The main Concentrated Solar Power (CSP) plants type installed worldwide corresponds to parabolic trough (PT) technology transforming solar energy into thermal energy on the receiver tubes. The use of alternative Heat Transfer Fluids (HTF) in order to increase the solar-to-electric efficiency by means of either higher HTF temperatures or the use of supercritical cycles have opened a new line of research. In this framework, super-critical carbon dioxide (sCO2) seems to be a good candidate to replace current HTFs in PTs. This work implements a one-dimensional heat transfer model for a PT solar collector considering different HTFs such as synthetic oil, sub-critical carbon dioxide, and sCO2. The numerical results of the outlet fluid temperature are compared to experimental data and numerical results published in the literature, obtaining maximum temperature deviations of 0.9%, 2.9% and 2.7% respectively. Finally, a sensitivity analysis was conducted to evaluate the influence of the solar irradiation, mass flow rate and HTF inlet temperature on the thermal performance of a PT working with sCO2 showing that the solar irradiation produces the greatest variations.

Photothermal Energy Conversion Enhancement Studies Using Low Concentration Nanofluids

The present study aims to develop a compact experimental facility to trap solar energy. Line focusing concentrators, i.e., Fresnel lens and secondary reflectors, are coupled to enhance the photothermal conversion efficiency. Two types of receiver tubes are used, a plain copper tube and an evacuated glass tube embedded with a copper tube. Surfactant-free multiwalled carbon nanotubes–Therminol55 nanofluid with concentrations of 25, 50, 75, and 100 ppm are used in this study. The characterization of the nanoparticles and nanofluids is presented. In the visible range, a maximum absorbance and extinction coefficient of 0.75 and 1.7 cm−1 are obtained for 100 ppm concentration. The thermal conductivity is also enhanced by 6.29% compared to base fluid. A maximum fluid temperature of 78.15 and 89.58 °C is observed for plain receiver tube and receiver tube in evacuated space, respectively, and the corresponding efficiencies are 12.65 and 17.36%

Homogenization of solar flux distribution in a carbon aerosol entrapped cavity receiver

An uneven heat flux distribution on the receiver's surface can lead to a highly non-uniform temperature distribution and high local temperature on the receiver tubes which results in fatigue failure. In the present work, a carbon aerosol entrapped cavity receiver of “DAHAN” power plant was numerically simulated to achieve homogenized heat flux distribution with improved safety of the receiver. A three-dimensional Monte Carlo Ray Tracing (MCRT) and Finite Volume Method (FVM) coupled model was developed to simulate the radiation-conduction-convection heat transfer in the receiver. Firstly, the MCRT method is used to simulate the solar heat flux distribution on the surface of the receiver. Then, the thermal performance and heat losses in the receiver were investigated by the coupled model. Finally, the levelized cost of energy (LCOE) was calculated and stress analysis was performed to predict the lifespan of the receiver. Moreover, with this strategy, the peak solar heat flux on the back panel significantly dropped from 290 kW/m2 to 135 kW/m2, while the peak temperature dropped from 652K to 620K. Carbon aerosol particle slightly decreases the thermal performance of the receiver. However, it decreases stress concentration on the receiver panels. Also, the economic analysis revealed that carbon aerosol entrapped receiver is more economical.