Optical and Thermal Analysis of a Heteroconical Tubular Cavity Solar Receiver

The principal objective of this study is to develop, investigate and optimise the Heteroconical Tubular Cavity receiver for a parabolic trough reflector. This study presents a three-stage development process which allowed for the development, investigation and optimisation of the Heteroconical receiver. The first stage of development focused on the investigation into the optical performance of the Heteroconical receiver for different geometric configurations. The effect of cavity geometry on the heat flux distribution on the receiver absorbers as well as on the optical performance of the Heteroconical cavity was investigated. The cavity geometry was varied by varying the cone angle and cavity aperture width of the receiver. This investigation led to identification of optical characteristics of the Heteroconical receiver as well as an optically optimised geometric configuration for the cavity shape of the receiver. The second stage of development focused on the thermal and thermodynamic performance of the Heteroconical receiver for different geometric configurations. This stage of development allowed for the investigation into the effect of cavity shape and concentration ratio on the thermal performance of the Heteroconical receiver. The identification of certain thermal characteristics of the receiver further optimised the shape of the receiver cavity for thermal performance during the second stage of development. The third stage of development and optimisation focused on the absorber tubes of the Heteroconical receiver. This enabled further investigation into the effect of tube diameter on the total performance of the Heteroconical receiver and led to an optimal inner tube diameter for the receiver under given operating conditions. In this work, the thermodynamic performance, conjugate heat transfer and fluid flow of the Heteroconical receiver were analysed by solving the computational governing Equations set out in this work known as the Reynolds-Averaged Navier-Stokes (RANS) Equations as well as the energy Equation by utilising the commercially available CFD code, ANSYS FLUENT®. The optical model of the receiver which modelled the optical performance and produced the nonuniform actual heat flux distribution on the absorbers of the receiver was numerically modelled by solving the rendering Equation using the Monte-Carlo ray tracing method. SolTrace - a raytracing software package developed by the National Renewable Energy Laboratory (NREL), commonly used to analyse CSP systems, was utilised for modelling the optical response and performance of the Heteroconical receiver. These actual non-uniform heat flux distributions were applied in the CFD code by making use of user-defined functions for the thermal model and analysis of the Heteroconical receiver. The numerical model was applied to a simple parabolic trough receiver and reflector and validated against experimental data available in the literature, and good agreement was achieved. It was found that the Heteroconical receiver was able to significantly reduce the amount of reradiation losses as well as improve the uniformity of the heat flux distribution on the absorbers. The receiver was found to produce thermal efficiencies of up to 71% and optical efficiencies of up to 80% for practically sized receivers. The optimal receiver was compared to a widely used parabolic trough receiver, a vacuum tube receiver. It was found that the optimal Heteroconical receiver performed, on average, 4% more efficiently than the vacuum tube receiver across the temperature range of 50-210℃. In summary, it was found that the larger a Heteroconical receiver is the higher its optical efficiency, but the lower its thermal efficiency. Hence, careful consideration needs to be taken when determining cone angle and concentration ratio of the receiver. It was found that absorber tube diameter does not have a significant effect on the performance of the receiver, but its position within the cavity does have a vital role in the performance of the receiver. The Heteroconical receiver was found to successfully reduce energy losses and was found to be a successfully high performance solar thermal tubular cavity receiver

Geometrical Optimisation of Receivers for Concentrating Solar Thermal Systems

In concentrated solar thermal technologies, the receiver converts concentrated solar radiation into high-temperature heat. Solar receivers are commonly simulated with a stochastic integration method: Monte-Carlo ray-tracing. The optimisation of the geometry of receivers is challenging when using existing optimisation methods for two reasons: each receiver evaluation using Monte-Carlo ray-tracing requires significant computational effort and the outcome of a simulation involves uncertainty. A series of novel optimisation techniques are proposed to enable gradient-free, stochastic and multi-objective optimisation adapted to such problems. These techniques address the computational load difficulty and the challenge of conducting stochastic optimisation based on uncertain evaluations by introducing the concepts of “Progressive Monte-Carlo Evaluation (PMCE)”, “Intermediate Ray Emission Source (IRES)” and adaptive view-factor calculation. A new “Multi-Objective and Evolutionary PMCE Optimisation (MOEPMCE-O)” method is then built around PMCE to enable multi-objective geometrical optimisation of receivers. PMCE is shown to be able to reduce the computational time of a random search optimisation by more than 90% and is used in the geometrical design of a new receiver for the Australian National University SG4 dish concentrator that achieved 97.1% (±2.2%) of thermal efficiency during on-sun testing. MOE-PMCE-O is applied to a multi-objective tower receiver problem where liquid sodium is used as the receiver heat-carrier in a surround configuration heliostat field. A series of useful geometrical concepts emerge from the results, with geometrical features able to maintain high efficiency while keeping acceptable incident peak flux values with a moderate receiver total mass. Finally, a more fundamental look at the impact of the interaction of concentrating optics on the exergy of radiation available at the receiver location highlights the major role played by concentrator surface slope error in lowering the exergy in concentrated solar thermal systems and quantifies the exergy loss associated with non-ideal match between flux and surface temperature in receivers

Energy and exergy analysis of microchannel central solar receivers for pressurised fluids

Within the new generation of advanced central solar receivers, microchannel pressurised gas receivers are emerging as reliable and efficient alternatives to operate at high temperatures and pressures. This paper presents an optimisation and comparative analysis of different compact plate-fin type structures, constituting the receiver’s absorber panels, classified according to the type of fin arrangement inside: plain rectangular, plain triangular, wavy, offset strip, perforated, and louvred fin. A versatile thermo-fluid receiver model is implemented, allowing simple variation of characteristic geometric parameters of each structure. Exergy efficiency is chosen as the optimisation function, as it considers both heat and pressure losses. The framework of the analysis is set by the receiver’s boundary conditions, operating at the design point conditions of a solar thermal power plant. For each compact structure, the optimal configuration is determined, providing interesting findings that have not been reported in the state-of-the-art to date. Although all geometries show good thermal performance, the perforated and plain rectangular configurations demonstrate the best exergy efficiencies of 59.21% and 58.80%, respectively, favouring taller and narrower channels. This analysis methodology could be seamlessly extrapolated to other gases and working conditions, owing to the thermo-fluid model’s versatility, to reveal the optimal configuration for each case.This work has been developed within the framework of the ACES2030-CM project, funded by the Regional Research and Development in Technology Programme 2018 (ref. P2018/EMT-4319). The authors would like to thank the support of the Spanish Ministry of Economy and Competitiveness through the PID2019-110283RB-C31 project

Impacts of thermal aging and associated heat losses on the performance of a Pyromark 2500-coated concentrated solar power central receiver

Pyromark 2500 is a widely used coating for concentrated solar power central receiver systems due to its high absorptivity, ease in application, and relatively low cost. Pyromark's performance is quantified by its figure of merit (FOM), which relates the coating's heat losses to its solar-to-thermal conversion efficiency. After long-term exposure to high temperatures (>750{\deg}C) and irradiance levels, Pyromark's absorptivity and FOM decrease. The aim of this research is to evaluate changes in Pyromark's absorptivity, heat losses, and FOM as a function of thermal aging. This work also compares the most common FOM expression, which neglects convection losses, to an FOM that includes all heat losses experienced by a central receiver. Isothermal aging experiments are conducted on Pyromark-coated Inconel 600 substrates at 750{\deg}C. The spectral, hemispherical absorptivity of the samples is measured at room temperature with a spectrophotometer and input into a finite element analysis model that includes radiation and convection boundary conditions. The heat flux and temperature output by the model are used to determine the heat losses and FOM of the Pyromark samples. After 151 h of thermal aging, the sample with the thinnest Pyromark coat maintains the most stable total, hemispherical absorptivity. Conversely, the total, hemispherical absorptivity of the sample with the thickest Pyromark coat drops by a maximum of 1.73%, and the corresponding maximum drop in FOM is 1.90% when windy conditions (which are expected around central receivers) are assumed. In windy conditions, convection losses constitute between 21% and 24% of the samples' total heat loss; thus, the most common FOM expression in the literature overestimates the samples' FOM by ~4.40%. An analysis of the samples' heat losses indicates that reflection losses exceed emission losses when the absorptivity declines significantly.Comment: 36 pages, 6 figures, 2 table

Transient thermal performance prediction method for parabolic trough solar collector under fluctuating solar radiation

As the effect of the global warming is becoming noticeable, the importance for environmental sustainability has been raised. Parabolic trough solar thermal collector system, which is one of the solutions to reduce the carbon dioxide emission, is a mature technology for electricity generation. Malaysia is a tropical country with long daytime, which makes suitable for solar thermal applications with parabolic trough solar thermal collectors. However, the high humidity causes the solar radiation to fluctuate. In order to simulate the solar thermal collectors’ performance at an early design stage of solar thermal power generation systems, fast still accurate transient thermal performance prediction methodis required. Although multiple transient thermal simulation methodologies exist, they are not suited especially at an early design stage where quick but reasonably accurate thermal performance prediction is needed because of their long calculation time. In this paper, a transient thermal prediction method is developed to predict exit temperature of parabolic trough collectors under fluctuating solar radiation. The method is governed by simple summation operations and requires much less calculating time than the existing numerical methods. If the radiation heat loss at the parabolic trough collector tube surface is small, the working fluid temperature rise may be approximated as proportional to the receiving heat flux. The fluctuating solar radiation is considered as a series of heat flux pulses applied for a short period of time. The time dependent solar collector exit temperature is approximated by superimposing the exit temperature rise caused by each heat flux pulse. To demonstrate the capabilities of the proposed methodology, the solar collector exit temperature for one-day operation is predicted. The predicted solar collector exit temperature captures the trend of a finite element analysis result well. Still, the largest temperature difference is 38.8K and accuracy is not satisfactory. Currently, the accuracy of the proposed method is being improved. At the same time, its capabilities are being expanded

Economic and thermo-mechanical design of tubular sCO2 central-receivers

Supercritical CO2 central-receivers must withstand high temperatures and pressures combined with cyclic operation, which makes the solar receiver susceptible to creep-fatigue failure. In this work, a creep-fatigue analysis of a sCO2 Inconel 740H tubular receiver of a 2 MWe solar tower plant has been accomplished to study the influence of the tube size on the receiver and solar field design. A 2D numerical model of the tubular receiver that accounts for the thermal conduction in both radial and circumferential directions was developed to determine the sCO2 and wall temperature profile, which is crucial for the creep-fatigue calculations. The receiver flux distribution, which is an input to the model, was obtained with SolarPILOT, while a conventional recompression model was used to calculate the cycle efficiency and inlet temperature to the receiver. Comparison of the results of the 2D model with those of a 1D model showed that the 1D model overestimates the creep fatigue rupture time by two orders of magnitude. Furthermore, the efficiency and costs of the heliostat field and receiver were calculated for different receiver tube sizes. Smaller tubes allowed a higher maximum heat flux leading to smaller receiver and heliostat field designs, which resulted in higher overall efficiency of the power plant and lower material costs. For a design ensuring 25 year receiver lifetime the minimum sCO2 solar receiver cost, 345 €/kWth, was obtained for the smallest pipe diameter.This research is partially funded by the Spanish government under the projects RTI2018-096664-B-C21 (MICINN/FEDER, UE) and RED2018-102431-T (AEI, MICINN) and the fellowship “Programa de apoyo a la realizaci on de proyectos interdisciplinares de I+D para j ovenes investigadores de la Universidad Carlos III de Madrid 2019e2020” under the project ZEROGASPAIN-CM-UC3M (2020/00033/001), funded on the frame of “Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid 2019e202”

A three-dimensional ring-array concentrator solar furnace

PD/BD/142827/2018. PD/BD/128267/2016. SFRH/BPD/125116/2016. CEECIND/03081/2017. UID/FIS/00068/2019. Sem PDF conforme despacho.A single ring-array concentrator solar furnace unit was firstly modeled analytically, and then optimized numerically by ZEMAX® and ANSYS® software, reaching a temperature of 3778 K, nearly equivalent to that of a medium size solar furnace with 3.14 m2 collection area. A novel three-dimensional ring array concentrator solar furnace was subsequently proposed and analyzed. It consisted of five single ring array concentrators, forming a compact box-shaped solar furnace with an opening at the rear side for an easy access to a common focal spot in the center. Based on the edge-ray principle of non-imaging optics, 30,960 solar concentration ratio was analytically calculated for this solar furnace, leading to significantly enhanced thermal and optical efficiencies. The temperature performance of the three-dimensional ring-array concentrator furnace as a function of receiver size and collector area was analyzed numerically and compared to that of the medium size solar furnace. For a 5.68 mm diameter spherical receiver and large collection area varying from 3.14 m2 to 100 m2, 1.1 times gradual enhancement in the maximum attainable temperature was calculated for the ring array concentrator furnace. More importantly, its average and minimum temperatures were significantly improved by 870 K and 1140 K, respectively, as compared to that of the medium size solar furnace. In addition, the three-dimensional ring-array concentrator also presented a significant tracking error compensation capacity in relation to that with the medium size solar furnace.publishe

Numerical model of solar external receiver tubes: Influence of mechanical boundary conditions and temperature variation in thermoelastic stresses

Failure in solar external receivers is mainly originated from the thermal stress, caused by the high non-uniform transient solar flux. The heat-up and cooldown of tube receivers in daily cycles produce low-cycle fatigue that limits the lifetime of tubes. The corrosion of tube materials produced by incompatibility between the decomposed heat transfer fluid and tube material may increase this issue. The temperature spatial distribution in these tubes has strong variations in radial, circumferential, and axial directions. The stress field, produced by the temperature gradients, has been commonly analyzed using bidimensional models in isolated tube cross sections, without taking into account the axial temperature variation, the mechanical boundary conditions, and the temperature-dependent thermomechanical properties. In this work, a three-dimensional finite element model has been developed in order to calculate the stress field distribution, without performing any geometrical simplification. In addition, appropriate mechanical boundary conditions have been imposed in order to adequately simulate the tube behavior. Besides, radial, circumferential and axial temperature variations have been studied separately to analyze how each of them influences the maximum stress distribution. This 3D modelhas been compared with analytical solutions for the two-dimensional thermal stress problem incircular hollow cylinders. The results show that the boundary conditions have a significant effect on the tube stresses, increasing the axial stress component and therefore the equivalent stress. The analysis of each of the temperature variations showed that the circumferential variationtemperature is the one that produces most of the stress, since it tries to strongly bend the tube, which is impeded by the boundary conditions. The results also present that 2D models are not capable of obtaining the correct stress distribution along the tube, since they are not taking into account the loThis work has been supported by the Iberdrola Foundation Spain under the fellowship "Ayudas a la investigación en energía y medio ambiente". M.R. Rodríguez-Sánchez and D. Santana would like to thank the Ministerio de Economía y Competitividad the support of the project ENE2015-69486-R (MINECO/FEDER, UE)