Volumetric Solar Receiver Research Articles

Impact of HRSG characteristics on open volumetric receiver CSP plant performance

The use of near-ambient pressure air as the heat transfer fluid in central receiver concentrating solar power (CSP) plants operating the water/steam cycle presents a number of potential benefits that have attracted research attention to the concept. In such plants, an open volumetric solar receiver (OVR) is used to heat air entrained from the atmosphere and a heat recovery steam generators (HSRG) is employed to transfer heat from the air to the water/steam cycle. An aspect of this technology that has not been previously addressed in the literature is the impact that the selection of HRSG configuration and operating parameters has on overall plant performance, and in particular, which HRSG characteristics allow for the best utilisation of solar energy. The results presented in this paper address these questions by evaluating the impact of HRSG operating characteristics on the performance of a 100MWe OVR central receiver plant. Investigations are carried out on the basis of coupled OVR/HRSG/water/steam cycle models implemented and solved in the EES© programming environment. Single and multi-pressure HRSG arrangements with and without reheating are examined. In specific terms, sensitivities to the variation of receiver outlet temperature, air return strategy, air return ratio, HRSG pinch-point temperature difference, deaerator outlet temperature and duct velocity are evaluated and discussed.

Optical Transmission Characteristics of Concentrated Solar Rays in the VSR using Cup-shaped Porous Absorber

The optical transmitting characteristics of curved quartz window fitted with a Volumetric Solar Receiver (VSR) are studied. The VSR adopts a cup-shaped alumina ceramics as the receiver for modeling and testing. The transmission model of window assembled with VSR is established considering multiple reflections, refraction, and absorptions. The flux distribution on the receiving surface is simulated by Monte-Carlo ray tracing method (MCRTM). When the relative positions of the focal point, glass window and porous are changed, not only the 2-D flux distribution, but also the total energy of input and central porous are compared. The experimental system for the high-temperature VSR is established based on the sixteen dish concentrator (SDC). Through experimental studies, the highest temperatures of central porous and the time spent at different parameters are tested, instead of testing the optical flux distribution inside VSR which is hardly to be obtained. The results can provide important reference and practical value for the design and construction of high-temperature VSR in future.

Towards the development of simple methods for determining normal absorptances of open-cell foams based on opaque materials

The knowledge of the normal spectral absorptances of open-cell foams used as volumetric solar receivers is required to finely compute their thermal efficiencies. For this purpose, absorptances of a set of virtual open-cell foams of varying porosities, beforehand generated by using a numerical generator, are computed thanks to a ray tracing code. This work details the contribution of the intrinsic optical properties of the solid phase to the normal spectral absorptance of open-cell foams, through the statistical analysis of both the path and the events undergone by a ray, which is permitted by the ray tracing code. This study allows us to propose a robust and linear relationship that links the normal spectral absorptance to the porosity and the intrinsic optical properties of the solid phase, which is considered as optically thick.

Coupled heat transfer performance of a high temperature cup shaped porous absorber

In this article, a newly designed volumetric solar receiver (VSR) using cup-shaped Al2O3 porous absorber is studied through numerical simulations and experimental verification. The absorber achieves several advantages including lower heat loss and higher uniformity of heat flux. The ray tracing method is adopted to obtain the external radiation source. The inner radiative exchange within the cavity is solved with the aid of User Defined Function (UDF). The heat transfer performance has been compared based on different parameters including window reflectivity, porosity and mass flow rate. The theoretical advantages of using cup-shaped Al2O3 absorber are presented. The experimental verification is adopted based on sixteen-dish concentrator (SDC) platform.

Optical properties of ZrB2 porous architectures

Porous ceramic materials are currently used as volumetric sunlight absorbers in concentrating solar power systems. As the efficiency of thermodynamic cycles rapidly increases with the operating temperature, the favorable characteristics of so-called ultra-high temperature ceramics (UHTCs) can be successfully exploited in novel solar absorbers. The present work reports, for the first time to the best of our knowledge, on optical properties and microstructural analysis of novel ice-templating porous ZrB2 UHTCs, to evaluate their potential as volumetric solar receivers. We demonstrate that the different complex structures that can be obtained with the freeze casting technique show promising optical properties. The idea of conjugating an highly tailorable morphology, useful for optimizing gas fluxes and heat exchanges between absorber and gas, to the spectral selectivity which is a characteristics of ZrB2 can be a promising route for increasing the efficiency of thermal solar systems.

Inspection and Structural Health Monitoring techniques for Concentrated Solar Power plants

Parabolic trough concentrators are the most widely deployed type of solar thermal power plant. The majority of parabolic trough plants operate up to 400 °C. However, recent technological advances involving molten salts instead of oil as working fluid the maximum operating temperature can exceed 550 °C. CSP plants face several technical problems related to the structural integrity and inspection of critical components such as the solar receivers and insulated piping of the coolant system. The inspection of the absorber tube is very difficult as it is covered by a cermet coating and placed inside a glass envelope under vacuum. Volumetric solar receivers are used in solar tower designs enabling increased operational temperature and plant efficiency. However, volumetric solar receiver designs inherently pose a challenging inspection problem for maintenance engineers due to their very complex geometry and characteristics of the materials employed in their manufacturing. In addition, the rest of the coolant system is insulated to minimise heat losses and therefore it cannot be inspected unless the insulation has been removed beforehand. This paper discusses the non-destructive evaluation techniques that can be employed to inspect solar receivers and insulated pipes as well as relevant research and development work in this field.

Efficiency potential of indirectly heated solar reforming with open volumetric solar receiver

In solar reforming, the heating value of natural gas is increased by utilization of concentrated solar radiation. Hence, it is a process for storing solar energy in a stable and transportable form that also permits further conversion into liquid fuels like methanol. The feasibility of solar reforming is proved. However, its overall process efficiency potential has not been studied systematically. In this work, an indirectly heated solar reforming process with air as heat transfer fluid is designed and modelled to produce syngas suitable for subsequent methanol synthesis. For provision of solar high temperature heat, an open volumetric receiver is implemented into the process model and the overall performance is investigated. Results show the paramount significance of the air return ratio of the receiver and its ability to achieve high efficiencies at temperatures above 850 °C. For realistic air return ratios, design point process efficiencies of 19% can be achieved, for an increased air return ratio, values up to 23% are feasible. The determined corresponding annual efficiencies are 12% and 14% respectively. Considering its relative technical simplicity this makes indirectly heated solar reforming a promising technology to overcome the current limitations of solar energy in the medium term.

Numerical Simulation of Quartz Tube Solid Particle Air Receiver

Abstract The quartz tube solid particle air receiver is a new type of solar receiver in which fluidized particles absorb the solar radiation directly and heat the air effectively, improving the efficiency of solar thermal power generation and reducing costs. In this article, transient numerical simulation was conducted to simulate the heat transfer and flow processes in single quartz tube under concentrated solar radiation. The results showed that the distribution of solid particles temperature was uniform in the fluidized region, which could overcome the problem of overheating in the volumetric solar receiver. The temperature difference between solid particles and air was no more than 25K, indicating that heat transfer between particles and air was very effective. Further, as the direct solar radiation increased, the average air temperature in the outlet increased while the thermal efficiency decreased. The high tube wall temperature caused heat loss to the environment by radiative and convective heat transfer. With the air inlet velocity increasing, the averaging air temperature in the outlet decreased while the efficiency of the receiver increased. The simulation results provided important reference for improving the performance of the quartz tube solid particle air receiver.

Thermal performance analysis on a volumetric solar receiver with double-layer ceramic foam

Using porous materials has become an effective means to enhance heat transfer. Volumetric receiver is a key component inside the solar thermal systems. The novel concept using double-layer ceramic foam holds great potential for improving the efficiency. The current study aims to investigate the effects of geometric properties of each porous layer on the thermal performance. The local thermal non-equilibrium model is adopted for energy equations of the fluid and solid phases. Radiative transfer in the foam combined the concentrated solar radiation absorption is solved with the modified P1 approximation. The results indicate that the thickness of the first porous layer has significant effect on the temperature field and pressure drop. Compared with the increasing-porosity configuration, the decreasing-porosity configuration tends to achieve higher air outlet temperature and lower solid inlet temperature. In addition, the increasing design of mean cell size shows improved performance compared to the decreasing design.

Diagnosing a solar volumetric receiver combining NN-based modelling with online parameter identification and rule-based techniques

The lack of detectability when model-based techniques are applied to intelligent fault detection and isolation tasks, which mainly occur in most non-linear processes so far is an unresolved problem. Generally, all types of non-linear processes will also suffer from lack of detectability due to the inherent ambiguity in discerning faults in the process, sensors and/or actuators. The aim of this work is to develop and apply a strategy to detect and isolate process and/or sensor faults by combining a neural network-based functional approximation procedure associated with an online identification algorithm, both processed by recursive rule-based techniques using parity space approaches. A case study dealing with the supervision of a solar volumetric receiver was performed using the proposed intelligent techniques. The conducted study produced reliable and acceptable intelligent fault detection and isolation results on the basis of heuristic knowledge-based rules.

Preliminary design and analysis of a novel solar receiver for a micro gas-turbine based solar dish system

The solar receiver is one of the key components of hybrid solar micro gas-turbine systems, which would seem to present a number of advantages when compared with Stirling engine based systems and photovoltaic panels. In this study a solar receiver meeting the specific requirements for integration into a small-scale (10kWel) dish-mounted hybrid solar micro gas-turbine system has been designed with a special focus on the trade-offs between efficiency, pressure drop, material utilization and economic design. A situation analysis, performed using a multi-objective optimizer, has shown that a pressurized configuration, where the solar receiver is placed before the turbine, is superior to an atmospheric configuration with the solar receiver placed after the turbine. Based on these initial design results, coupled CFD/FEM simulations have been performed, allowing detailed analysis of the designs under the expected operating conditions. The results show that the use of volumetric solar receivers to provide heat input to micro gas-turbine based solar dish systems appears to be a promising solution; with material temperatures and material stresses well below permissible limits.

Process modelling and heat management of the solar hybrid sulfur cycle

Thermochemical cycles for water splitting are considered as a promising example of emission-free routes for large-scale hydrogen production – with potentially higher efficiencies and lower costs compared to low temperature electrolysis of water. The hybrid–sulfur cycle was chosen as one of the most promising cycles from the ‘sulfur family’ of processes. A process model has been established to study the main parameters influencing efficiency with specific attention paid to dynamic effects when coupled to solar heat. The process is separated into two sections – one at steady-state, and the other one fictively imposed by transients. This allows a first analysis with respect to reasonable energy and mass flow management, while considering concepts of coupling such a process to a concentrating solar system in a later step. Process efficiencies are calculated based on conservative assumptions, revealing the most important development tasks for the future. The extensive usage of recoverable high temperature heat – including the heat from the highly corrosive condensing phases – is a key factor to attain reasonable efficiencies for industrial application. With idealised heat recovery rates – limited by thermodynamic considerations, but excluding heat exchange between stationary and dynamic sections – and for decomposer temperatures appropriate for volumetric solar receiver operation, a thermal process efficiency close to 30% is predicted for stationary operation based on the lower heating value of hydrogen. Subsequent investigations will derive annual yields, taking into account the effect of coupling to solar energy.

Performance Characteristics of a Volumetric Solar Receiver: Presence of an Absorber Plate with a Selective Surface

Performance characteristics of a concentrated solar volumetric absorber are examined numerically. The thermal system considered consists of parabolic trough, glass tube, absorbing plate, and slurry containing 7% lauric acid as a phase change material and water as a carrier fluid. To assess the effect of the absorbing plate on performance characteristics, two locations of the absorbing plate on the glass tube surface are incorporated in the analysis. A selective surface is considered at the absorber plate surface for improved absorption of solar radiation and reduced thermal emission due to temperature increase at the surface. Temperature ratio, gain parameter, and pump power loss parameters are introduced to quantify the performance characteristics of the volumetric receiver. The study is extended to include the effect of Reynolds number on the receiver performance characteristics. A heating model incorporating radiation, convection, and conduction is adopted to simulate the thermal process. It is found that the gain parameter of the concentrated solar volumetric receiver improves by 15% when the absorber plate is located at the left face of the glass tube opposing the trough surface. The effect of Reynolds number on gain parameter is found to be inconsiderable.

Modelling and testing of a solar-receiver system applied to high-temperature processes

Solar volumetric-receivers have been successfully used in both electricity production and thermochemical applications. This paper studies the applicability of this technology to the production of process heat for high-temperature uses (573–1073 K).As such, a volumetric-receiver system was designed, installed and tested in the Plataforma Solar de Almería's Solar Furnace (SF-60). The facility consisted of an open-volumetric-receiver module connected to a concentrated-solar-energy driven prototype, whose design was based on a previous three-dimensional CFD model.This work focuses on the validation of the CFD model and on the experimental evaluation of the high-temperature solar prototype, taking into account the uncertainty of the experimental and simulation results. Numerical results were in appreciable agreement with the experimental data, which determined that the prototype was able to reach the high-temperature range (850 K) with a homogeneous thermal profile.

Cyclic production of syngas and hydrogen through methane-reforming and water-splitting by using ceria–zirconia solid solutions in a solar volumetric receiver–reactor

For cyclic production of syngas and hydrogen by methane reforming (reduction) and water splitting (re-oxidation) under simulated solar-light irradiation, foam devices coated with ceria–zirconia (CexZr1−xO2, where x=0.3, 0.5 and 0.8) solid solutions and CeO2 were used. Reduction characteristics of the foam devices were investigated in the temperature range from 873 to 1173K. The solid solutions were found to be reduced more easily than pure CeO2 especially at low temperatures. The gas yields by the solid solutions increased with increasing Ce content. Depending on temperature and composition of the solid solutions, the optimum operating times for methane reforming, during which carbon deposition was negligible, were determined. Among the solid solutions, Ce0.8Zr0.2O2 showed the best stability and gas yields during repeated operations of the redox cycle at 1173K, and it was confirmed from the XRD analysis that sintering of Ce0.8Zr0.2O2 was the least. To find the effect of metallic foams on the gas productivity, a Ni foam coated with Ce0.8Zr0.2O2 was investigated, and it rendered significantly higher gas yields than the SiC foam device; however, its long-term stability was assessed to be inferior to the latter.

Heat flux and temperature prediction on a volumetric receiver installed in a solar furnace

This paper presents a prediction procedure of the solar irradiance distribution on a plane parallel to the focal one of the Plataforma Solar de Almería’s Solar Furnace, within defined limits, as well as its application to test a sample of a volumetric solar receiver exposed to concentrated solar radiation. From initial heat-flux measurements at different focal planes of the Solar Furnace, this procedure is able to reliably predict the irradiance distribution on any plane and under different operating conditions, such as direct normal irradiance, shutter aperture, and mirror reflectance. As a result, this study allowed predicting the heat flux distribution at different Furnace axis positions selected for testing a sample of a volumetric solar receiver. Furthermore, the temperature reached on the sample surface, which receives the concentrated solar radiation, could be obtained from the flux features previously analysed. It makes possible to preserve the absorber material from overheating areas modifying the operating variables.

Numerical Analysis of Radiation Attenuation in Volumetric Solar Receivers Composed of a Stack of Thin Monolith Layers

In a volumetric receiver installed in solar tower power plants, the absorber operates as a convective heat exchanger, absorbing concentrated solar radiation and transferring thermal energy to a fluid flowing through it. Radiation absorption is related to the absorber geometry, the optical properties of the absorber surface, and the incident radiation intensity distribution, which depends on the heliostat field configuration. In order to minimize light reflection and thermal emission from the receiver frontal surface, a well-designed volumetric absorber requires a geometry that promotes radiation penetration. In addition, it should encourage a high heat transfer to the fluid to increase the thermal efficiency of the absorber. This paper analyses the radiation attenuation in an original volumetric absorber, applying a Monte Carlo ray-tracing method. The proposed absorber consists of a stack of thin multi-channel monoliths with square cross section channels in which the relative position between consecutive layers is shifted in the transversal direction. The study includes the influence of surface optical properties (absorptivity and reflectivity), the geometrical characteristics of the structure (length, wall thickness and spacing between elements) and the incident radiation profile (related to different heliostat field configurations). As result, a general expression for describing the transmittance inside the absorber as a function of the depth length is proposed.

Performance optimisation of solar receivers that use oil as a heat transfer fluid

This article presents the use of a wire mesh in optimising the performance of two volumetric solar receivers that use oil as a heat transfer fluid. Computational fluid dynamics models have been used to optimise the receivers. Varied parameters (including the use of a wire mesh) of the receiver models were changed in the optimisation process. Based on the models, prototype receivers were developed and tested. After that, the models were validated against experiments and the results compare well. The results indicate that the use of a wire mesh placed inside a receiver improves its performance. An optimal wire mesh porosity was found as ≈0.95 mainly because the efficiency is increased without inducing an adverse pressure drop inside the receiver.

Thermal analysis and design of a volumetric solar absorber depending on the porosity

The thermal evaluation of different absorber configurations for a volumetric solar receiver designed for a solar furnace has been carried out by means of commercial Computational Fluid Dynamics (CFD) software in a 2D numerical model. Simulation results for proposed configurations depending on the porosity are discussed and compared to find the optimum configuration for which flow instabilities and thermal stresses are minimized and higher efficiencies are reached. The results obtained from the comparison of air velocity and thermal profiles at the absorber outlet propose a gradual-porosity configuration as an alternative to a previous design of a porous silicon-carbide honeycomb structure in order to heat an air stream up to temperatures suited for several high-temperature industrial processes.

Numerical investigation of flow and heat transfer in a volumetric solar receiver

Volumetric solar receivers are used in solar power plants to convert concentrated solar radiation into high temperature heat to operate a thermal engine. In general, porous high temperature materials are used for this purpose. Since the pore geometry is important for the efficiency performance of the receiver, current R&D activities focus on the optimization of this quantity. In this study, the influence of slight geometry changes of this component on its temperature distribution and efficiency has been investigated with the objective of an overall improvement. A numerical analysis of the mass and heat transfer through the receiver has been performed. The investigated receiver was an extruded honeycomb structure made out of Silicon Carbide. Additionally, experimental tests have been performed. In these tests, selected receiver samples have been exposed to concentrated radiation. From these tests solar-to-thermal efficiency data have been derived, which could be compared with the calculated data. Two numerical models have been developed. One makes use of the real geometry of the channel (single channel model), the other one considers the receiver to be “porous continuum”, which is described by homogenized properties such as permeability and effective heat conductivity. The experimental parameters such as the average solar heat flux and the mass flow were taken into account in the models as boundary conditions. Various quantities such as the average air outlet temperatures, the temperature distributions and the solar-to-thermal efficiency were used for the comparison. The correspondence between the experimental and numerical results of both numerical models confirms the capability of the approaches for further studies.