Volumetric Receiver Research Articles

A novel solar tower simulator for hydrogen regeneration by thermo-catalytic cracking of Ammonia: Energy, Exergy, and CFD investigations

This paper presents a novel concept of a central solar tower-based hydrogen regeneration system by thermo-catalytic cracking of ammonia. This includes a closed volumetric receiver for ammonia heating and cracking, a 2.45 kWe high-concentration radiation source, and an air-cooling system for the radiation source. The thermodynamic analysis shows that the net solar-to-hydrogen generation efficiency, first law efficiency, and exergy efficiency are 31 %, 82 %, and 79.8 %, respectively, for the optical efficiency of 35 %, the conversion efficiency of 99 %, and the waste heat recovery fraction of 20 %. The simulations with an ammonia flow rate of 1 kg/hr and flux concentration of 1200 Suns show the regeneration of 1.27 kg H2/day at Jodhpur and 1.57 kg H2/day at Ladakh. Monte Carlo ray tracing simulations revealed that the developed radiation source provides an average flux of ∼ 2 MW/m2 onto a spot size of 20 mm with an efficiency of about 31 %. Reynolds-averaged Navier Stokes-based analyses are used to design a novel air-cooling system for the radiation source. The computations show that (a) an unequal distribution of air mass flow rates at the front mḟ and back mḃ, should be preferred with mḟ>mḃ, and (b) for a total mass flowrate of 0.1 kg/s with mḟmḃ = 4, the maximum surface temperature of reflectors and Xe-arc lamps is less than 315 K and 610 K, respectively. The investigations revealed the feasibility of the concept and provided valuable insight for the planned experimental assessment and scale-up of the solar tower simulator.

Numerical Assessment of Enhanced Coatings in Wire Mesh Volumetric Absorbers

Two different wire mesh open volumetric receivers (OVRs) are studied together with six different coatings and two different working conditions in order to assess their effect on the performance of the OVR comparing them to the two baseline OVRs, uncoated and state-of-the-art. The results show that selective coatings produce the best OVR thermal results, having the best results when the solar absorptance is as high as possible and the thermal emittance is as low as possible.

A Real-Time Capable Simulation of Open Volumetric Receiver Surface Temperatures With Spatially High Resolution

The performance of open volumetric receivers can profit from advanced control of the heliostat aim points and the air mass flow. Advanced control techniques like model predictive control and state observer are based on dynamic system models. A suitable model for this purpose is introduced and validated with data recorded at the solar tower Jülich. The model shows good performance with respect to the dynamics of individual absorber cups. The simulation of the whole receiver requires an advanced optimization of the model parameters.

Pressurized concentrated solar power receiver designed to operate with closed Brayton cycles

Parabolic through and parabolic dish concentrators (PDC) followed by a volumetric receiver is proposed, studying the performance behavior of a closed Brayton cycle (CBC) operating with He and N2. A pressurized gas (helium, nitrogen) circulates along the volumetric receiver capturing the concentrated thermal solar energy which is converted into mechanical energy by a CBC. The overall efficiency of the plant (PC-Receiver + CBC) is being computed under variable parameters to determine the operating conditions for which efficiency is acceptable. As consequence of the proposed analysis, it is concluded that direct coupling between volumetric receiver and thermal engine renders high efficiency while avoiding a heat exchanger.

Hydrogen Production using Solar Hierarchical Volumetric Receivers and Steam Gasification Reactor

Hydrogen Production using Solar Hierarchical Volumetric Receivers and Steam Gasification Reactor

Manufacturing and Thermal Shock Resistance of 3D-Printed Porous Black Zirconia for Concentrated Solar Applications

A novel approach for manufacturing porous materials, foreseen as solar receivers for concentrated sun radiation, used in the power tower technology is presented. In such applications, materials are subjected to steep thermal gradients and thousands of cycles. Yet, materials consisting of honeycombs and ceramic foams showed insufficient thermal performance. By using the fused filament fabrication process, one can design printed parts meeting the requirements for solar receivers, namely dark color and high solar absorptance. This exploratory study unveils data on the retained crushing strength of newly developed 3D-printed porous Black Zirconia cubes after thermal cycling under similar conditions to those experienced by volumetric receivers and catalyst substrates for solar fuels (H2 and/or CO) production via the thermochemical cycle. Unlike dense ceramics, the resistance to thermal shock of 3D-printed cubes underwent a gradual decrease with the increase in the thermal gradient. The thermal shock cycles were performed between 800 °C and 1100, 1200, and 1300 °C, corresponding to a ΔT of 300, 400, and 500 K, respectively. Additionally, water quenching tests were performed at ΔT = 300 K up to 400 K. Crushing strength measurements carried out to evaluate the retained mechanical strength after exposure up to 100 cycles showed that the Black Zirconia cubes can withstand thermal gradients up to at least 400 K.

Flux augmentation and temperature enhancement through secondary parabolic concentrator and air recirculation for a central receiver tower system

Flat volumetric receivers for central receiver tower (CRT) systems face two critical shortcomings: light spillage nearby the receiver and low exit air temperature due to suction of air in ambient condition. Here, a thorough design modification is proposed and analyzed to address both the issues by introducing a secondary optical addendum based on the principle of compound parabolic concentrator (CPC). As a case study, existing and modified designs of PS10 plant with flat heliostats are compared. For the same receiver, the modified plant increases the intercept efficiency by 6% whereas the incorporation of CPC augments the concentrated flux by 18%. A glass covered secondary optical element not only reduces operational difficulties due to absorber-soiling, but also provides an opportunity of heated air recirculation. If the heated air is partially recirculated, it can provide a control parameter, i.e. the air return ratio (ARR), by which the exit air temperature can be enhanced. It is demonstrated here that temperature as high as 1775 K is achievable for the modified PS10 plant when significant fraction of heated air is recirculated. Hence, the present study opens up optimization routes for CRT based plants based on limited area heliostat field and volumetric porous absorber through CPC based secondary optical element and fractional recirculation of heated air.

Modeling a solar pressurized volumetric receiver integrated in a parabolic dish: Off-design heat transfers, temperatures, and efficiencies

Concentrated solar power plants are commonly recognized as one of the most attractive options within carbon-free power generation technologies because of their high efficiency and feasible hybridization and/or storage implementation. In this work, a complete heat transfer analysis for an air volumetric receiver coupled to a parabolic dish focused on distributed generation (in the range of kWe) is carried out. It includes most relevant heat losses. Dish collector optical efficiency is computed by means of a ray-tracing software while the thermal performance of the solar receiver is modeled under steady-state conditions using a comprehensive set of equations with a clear physical origin and meaning. Detailed information on the temperatures and heat transfers along the different inner and outer receiver zones are computed with a built from scratch in-house code programmed in Mathematica®. The model considers the main losses from convection, conduction and radiation and through the surrounding insulator. The resulting thermal efficiency mainly depends on the incoming solar irradiance at the glass window, the receiver geometry and the type of materials considered, as well as on the ambient temperature. Explicit numerical results are given at two locations under different meteorological conditions. Optical efficiencies reach values of about 84%. For irradiance values around 800–900 W/m2, at the receiver outlet, air can reach temperatures of about 1200K and receiver thermal efficiency is over 80%. It is expected that this model (precise but not too expensive from the computational viewpoint) could help to identify the main efficiency bottlenecks, paving the way for optimization when designing this type of concentrated solar plants through further coupling with a power block, as Brayton or other cycles.

Geometric design of micro scale volumetric receiver using system-level inputs: An application of surrogate-based approach

Concentrating solar thermal power is an emerging renewable technology with accessible storage options to generate electricity when required. Central receiver systems or solar towers have the highest commercial potential in large-scale power plants because of reaching the highest temperature. With the increasing solar chemistry applications and new solar thermal power plants, various receiver designs require in micro or macro-scale, in materials, and temperature limits. The purpose of the article is computing the geometry of the receiver in various conditions and provide information during the conceptual design. This paper proposes a surrogate-based design optimization for a micro-scale volumetric receiver model in the literature. The study includes creating training data using the Latin Hypercube method, training five different surrogate models, surrogate model validation, selection procedure, and surrogate-based design optimization. Selected surrogates have over 98% R2 fit and less than 4% root mean square error. In final step, optimization performance compared with the base model. Because of the model complexity, surrogate models reached better objective values in a significantly shorter time.

Exploring and enhancing the volumetric behaviour in solar receivers through specular reflectivity and simplified design

One of the remaining challenges of point focusing concentrating solar power systems is the realization of a true volumetric receiver, one whose entire volume is utilized for the absorption of irradiance. Commercial state-of-the-art receivers (e.g., HiTRec-II and SolAir-200) have not demonstrated the volumetric effect, because of low radiation penetration within the absorber. Then, literature state-of-the-art receivers were able to demonstrate a volumetric behaviour (toward achieving the volumetric effect) through axially varied macro-scale porosity (void fraction), but at the cost of design complexity and manufacturability. This study presents the conceptual design and computational assessment of an improved volumetric receiver by employing varied reflectivity on the irradiated surfaces, thereby enhancing radiation penetration. This approach maintains the simplicity, manufacturability, and commercial viability of structured receivers. Three reflectivity distributions were initially investigated using a 3D heat transfer mathematical model, verified with literature experimental data, which incorporated a source term ascertained through Monte-Carlo Ray Tracing (MCRT). The findings from these three design cases revealed that while diffuse reflectivity distributions could improve thermal efficiency, they were unable to enhance radiation penetration. Consequently, an improved volumetric solar receiver was realized by integrating specular reflectivity, leading to a more uniform source term distribution and achieving a thermal efficiency of 96%, as opposed to the 85% thermal efficiency of the commercial state-of-the-art HiTRec-II receiver. The discovery of the specular receiver as a true volumetric receiver facilitated an exploration of quantitative definitions of the volumetric effect. The exploration concluded that volumetric effect metrics would only target designs that maximizing thermal efficiency when the receiver is exhibits a truly volumetric nature, characterized by a uniform source term distribution.

The volumetric effect indicator – A new dimensionless characteristic number for the optimum design and operation of volumetric solar receivers

This work introduces a new dimensionless number Z that should aid in the design phase of volumetric receivers. The number Z is able to indicate whether the volumetric effect is achievable for a specific combination of absorber geometry and thermal boundary conditions. It can be seen as an indicator of the slope of the solid-phase temperature profile in direction of gas flow. The volumetric effect is expected to occur at values of Z smaller than 1, when the convective plus radiative cooling (Pc) outweighs the absorbed solar power (Pa). An experimentally validated numerical model is used for a parametric simulation study, which sheds light on the absorber’s operating conditions (incident solar flux density and operating temperature) that favor the ideal temperature profile.

Off-design performance of micro-scale solar Brayton cycle

A novel methodology to design a micro-scale, solar-only, air-breathing, open Brayton cycle and assess its on- and off-design performance. The methodology is applied to generate and assess six thermodynamic layouts over a range of solar irradiation levels. All plants have the same on-design requirements to create a baseline to compare their off-design performance. PyCycle, a thermodynamic cycle modeling library to model jet engine performance, is revised to transform the jet engine performance modeling to solar thermal plant performance modeling and used to create a volumetric receiver component. A response surface surrogate model of the receiver is created for design optimization to maximize the component-level efficiency. The compressor and turbine maps are scaled for the balance of the plant. Off-design efficiency, mass flow rate, operation range, turbomachinery maps, and maximum power output are presented. Since the methodology can be adapted to all plant sizes, the results are normalized to on-design condition. The outcome of this study demonstrates the impact of the thermodynamic configuration on off-design performance and provides a methodology to design plants that are more robust across a range of solar irradiation levels and can be operated in a more flexible manner. Compared to single shaft configuration, solar radiation operation range is improved by 5%, with 6% less mass flow, and operates more efficiently than the benchmark case over 85% of the operating regime.

Numerical study of a nanofluid-based receiver for linear Fresnel collectors

In concentrated solar technologies, the use of volumetric receivers instead of surface receivers may help to achieve higher solar collection efficiencies. This paper reports a simplified model that computes the concentrated solar radiation absorption and energy transport by advection in the volumetric receiver of a linear Fresnel collector. The proposed linear Fresnel collector is intended for use in industrial solar heat processes. The receiver consists of a rectangular channel whereby flows a nanofluid composed of thermal oil and graphite nanoparticles. The bottom wall of the channel is a glass, while the upper wall is a highly reflective surface that is well insulated. The light absorption into the nanofluid was solved using the Rayleigh dispersion, while the convection was solved with a simplified velocity profile for turbulent flow (power law) and incorporating a radiative source term in the energy equation. All equations were discretized by using finite volumes and were coded in Python language. Receiver efficiencies in the range of 92–96% were found for temperatures of 403–343K. The results show that the volumetric fraction (optical depth) drives the way the radiation is absorpted, and in consequence, the temperature profile and the receiver efficiency. For low optical depths, the solar absorption is low, resulting in high reflected radiation losses. Contrarily, high optical depths mean high solar absorption, consequently, the temperature close to the glass increases, and the convection losses are higher. The results suggest that values of optical depth close to 2.3 are optimum to obtain the highest receiver efficiency.

Analysis on the Effects of Different Receiver Structures and Porous Parameters on the Volumetric Effects and Heat Transfer Performance of Porous Volumetric Solar Receiver

The volumetric solar receiver is an important heat transfer component of the concentrated solar power (CSP) system. Moreover, in order to improve the absorption of concentrated solar radiation, the porous media are widely used in volumetric solar receiver. In recent years, many studies were concerned with the effects of porosity, pore number density and size, Reynolds number, and Darcy number on heat and flow performance in volumetric solar receiver. However, there are few studies on the effects of structure type and geometric parameters on the volumetric effects and the radiation characteristics of a porous volumetric solar receiver with nonuniform heat flux boundary condition. In this contribution, in order to analyze the effects of structures on the volumetric effects and heat transfer performance, we design six types of porous volumetric receiver structures, whereas the volume of different structures keeps consistent. Then, we apply the local non-thermal-equilibrium model and also consider the effect of structure shape on the concentrating solar radiation transport characteristic. Furthermore, we apply the Gaussian distribution model (GDM) to simulate the actual nonuniform heat flux boundary condition. The result shows that drum-type structure (RT-III) has the best heat transfer performance among the six structures; e.g., the outlet average temperature of fluid is up to 851 K when the pore size and porosity of porous media are 2 mm and 0.7, respectively, which is 41 K increased than the scaled cone-type structure. In addition, the pore diameter has less influence on the outlet temperature of fluid, but it has greater influence on the pressure drop. The contribution can provide a reference for this type of solar receiver design and reconstruction.

Dynamic Performance Characteristics of a Porous Volumetric Solar Receiver Under Transient Flux Conditions

Abstract The solar flux incident on a volumetric receiver is inherently unsteady, resulting in high thermal stresses, fatigue failures, and reduced component life. The knowledge of transient response characteristics of a porous volumetric receiver used in concentrating solar technologies is cardinal for its reliable and safe working. The dynamic controlling of the solar-to-thermal conversion process is also possible with the prior prediction of the output variations. The present study aims to investigate the transient behavior of a porous volumetric receiver subjected to flux variations approximations occurring in real working scenarios with the help of a coupled transient model. The solid and fluid temperature fields, output fluid temperature, and pressure drop variations are determined for transient flux conditions during start-up, shut-down, clear sky, and cloud passage. The results are used to analyze the thermal response of the receiver during various operating conditions. In addition, the effects of structural parameters of the porous absorber are also investigated. The results indicate that the receiver transient performance is comparatively more affected by the variation in porosity than in pore size for all conditions. Smaller porosities and pore sizes show slower thermal response to transient fluctuations and less temperature changes during cloud passage. Conversely, higher values help in the faster restoration of the steady-state output conditions without dynamic control.

Gradient-based optimization of micro-scale pressurized volumetric receiver geometry and flow rate

This study focuses on the design optimization of a micro-scale pressurized volumetric receiver by changing geometry and flow rate constrained by the volume, outlet air temperature, and outer surface temperature. The pressurized volumetric receiver model is replicated from an existing model, which assumes constant air pressure and neglects the convection loss from the cavity. The existing model is revised from a solver to a design optimizer.The replicated model is restructured using OpenMDAO (Open-source MultiDisciplinary Analysis and Optimization) framework, and analytical derivatives are implemented for efficient derivative calculation to increase optimization performance. The replicated model is verified, and the maximum outlet air temperature difference is less than 0.05%. Optimization performance, selection of optimizers, the effect of the domain size, and radiative methods are discussed. The combined impact of the design variables is observed by selecting SLSQP (Sequential Least SQuares Programming) and trust-region optimizers. Optimization performance is tested in different domain sizes and compared with a design of experiments analysis. For testing the impact of radiative heat transfer methods to design optimization, the Rosseland approximation, and P1 method are selected. Depending on the design domain, a solution methodology is suggested for future receiver design optimizations applicable for macro-scale pressurized volumetric receivers.

A scaled-up, CSP integrated, high-temperature volumetric receiver with a semi-transparent packed-bed absorber

• Explores annual system-integrated performance of a semitransparent solar absorber. • Investigates performance of some alternative gaseous HTFs to air. • Clarifies the effect of operating pressure on the system's overall performance. • Modular receiver design reduces LCOE by ∼38% compared to the conventional system. • The novel receiver can feed a gas turbine with an annual temperature above 700°C. Conventional solar thermal receivers are limited to operating at temperatures below 600°C due to the operational limitations of liquid heat transfer fluids (HTFs). In addition, pushing to temperatures above 600°C leads to a steep trade-off between the thermodynamic availability of the outlet fluid and receiver efficiency due to the fact that radiation losses are ∝ T 4 . This paper investigates a nascent gas-phase receiver incorporating a semi-transparent packed bed absorber, which is capable of mitigating these issues. To date, this new class of receiver design has only been modelled on the small-scale under design-point direct normal irradiance. To obtain an understanding of the annual performance, the novel design was scaled up and integrated with a CSP system by linking MATLAB with SolarPilot to conduct a transient analysis using real-time solar irradiance data. The results demonstrated that the gas-phase receiver can feed a gas turbine with an annual temperature above 700°C (i.e., a >700°C gas can be delivered to the turbine for ∼65 % of the annual operating hours). In terms of selecting a gaseous HTF, a previously unexplored area, helium was found to provide the best performance, with ∼1 % and ∼9.2 % increases in the annual power and outlet temperature, respectively, as compared to air. As another alternative, CO 2 provided almost the same annual output as air. A non-dimensional analysis also revealed that excessive operating pressure would not lead to an actual gain in the overall receiver performance. The developed method also provided insight into the economic viability of the scaled-up receiver design. It was found that this design is most economic for small, modular designs. For example, a modular receiver design with a single tower integrated with a 9 MWe turbine outperformed the conventional system (i.e., a Rankine cycle operating at 574°C) with a ∼38 % decrease in the LCOE. Overall, this research provides the first theoretical integration of a semi-transparent packed-bed receiver design within a high-temperature, small-scale CSP system.

Experimental evaluation of wind induced pressure fluctuations in cavity shaped open volumetric air receivers

In this study wind induced surface pressure fluctuations during operation of an open volumetric cavity receiver were investigated in wind tunnel experiments. Due to the process design of the open volumetric receiver, surface pressure fluctuations impact the receiver mass flow which is crucial for cooling the highly irradiated receiver. For this study a model of a solar tower was constructed under the concept of similitude which allows wind tunnel experiments at Reynolds numbers up to 13.17 × 106. Transient pressure measurements were conducted under 17 different flow directions to evaluate the impact of separation induced pressure fluctuations. The experimental design further allows a rotation of the model to evaluate the impact of changes in wind direction on the surface pressure at the receiver. For cases under a constant wind direction, a critical angle range could be identified in head on wind with a horizontal inclination of 45.0 to 78.8° relative to the central receiver axis. The results further show that surface pressure fluctuations are more sensitive to changes in wind direction as the amplitudes with maximum 20Pa for a Reynolds number of 13.17 × 106 are about one order of magnitude higher compared to the cases with a constant flow direction. Furthermore, a frequency analysis shows the shielding effect of the cavity design for side-wind as no peaks in the pressure spectrum appear. For flow directions within the critical range of 45.0 to 78.8° a main vortex shedding frequency under constant Strouhal numbers of 0.25 to 0.50 can be observed.

Improvements in the measurement of high solar irradiance on a 300 kWth volumetric receiver

The current solar tower plants demand the measurement of high solar irradiance on the receiver. This measurement would facilitate the operation of the plant, the estimation of the receiver efficiency and would contribute to the safety of the receiver. Nowadays, these plants don't have a system to measure the flux because there are some limitations, as the difficulty to add new elements to the design and the structure of the tower. It is necessary to include a flux measurement system in the planning and construction of the commercial plants. This work presents a validation of the new indirect method to measure the flux which only use the surface of the receiver, a radiometer and a digital camera. In the context of an international project, the new method to measure the flux has been tested with a different receiver and with different working conditions. The receiver tested in this project is a volumetric receiver made of silicon carbide. The flux measurements have been done in the solar tower facility of the Plataforma Solar de Almería, the SSPS-CRS. The results obtained with this system have been compared to the results obtained with the reference flux measurement system based on a white lambertian target, obtaining a good level of agreement. This reference method can't be extrapolated to the commercial plants because it has moving parts that can't be added to the design of the towers. The new method avoids these moving elements and simplify the measurement of the flux distribution on the receiver.

Multi-dimensional numerical analysis of flow instabilities in 3D-shaped honeycomb absorbers

Considered a valid alternative to other receiver types, open volumetric receivers for central tower power plants have been developed and researched for multiple decades. 3D-Shaped absorber geometries, made possible by modern manufacturing technologies, particularly offer the promise of high thermal efficiencies. Yet, volumetric absorbers carry the inherent risk of flow instabilities, which can lead to a total failure of the receiver due to local overheating. Therefore, a numerical, multi-dimensional methodology for the flow stability analysis of volumetric absorbers is proposed. The first stage of the methodology applies an 1D LTNE continuum model to quantify the general pressure loss characteristic of an absorber and identifies critical pressure drop levels with potential for flow instability. These levels are tested as part of the second analysis stage using a discrete 3D CFD model against different irradiation and flow perturbations. If the absorber is able to compensate the disturbances, its flow behavior is deemed stable. Otherwise, the absorber geometry is considered susceptible to local overheating due to flow instability.The new methodology has been validated against two reference absorbers with known stable resp. unstable flow behavior. Further, the new approach was applied to analyze the flow stability of two 3D-shaped honeycomb absorber geometries. It was shown that the geometric features of 3D-shaped absorbers can directly improve the flow stability, but do not prevent instabilities automatically. Therefore, a flow stability analysis should be included in design/optimization processes of new volumetric absorbers alongside a maximization of the thermal efficiency.