Receiver Thermal Efficiency Research Articles

Investigating the performance of an affordable parabolic dish collector with double glass vacuum cavity cover

Parabolic dish collectors (PDC) are one of the most important ways to concentrate solar energy. In this study, the performance of a low-cost parabolic dish collector with high average output temperatures is investigated. The low-cost PDC has a 45 ° edge angle and a circular receiver. A vacuum double-glazed cavity is used to keep heat inside the receiver and reduce the rate of heat losses. On clear and cloudy days, tests were conducted with thermal oil as a heat transfer fluid (HTF) to analyze the performance of the PDC. The results show that the change in temperature of the heat transfer fluid between the open and closed ends of the cavity receivers affected the receiver heat gain, thermal efficiency, and exergy efficiency. It was also found that the HTF has the highest exit temperature of 221 °C, with an average radiation intensity of 1032 W/m2. With 48.4 % thermal efficiency and 9.1 % exergy efficiency, the results showed the best performance. When it came to total efficiency, PDC showed its highest value of 46.2 % at 13:17 p.m. on January 4th, 2024.

Reducing convective losses in a solar cavity receiver VoCoRec by creating a controlled vortex of returned air

The efficiency of air-type solar towers influences the minimal cost of electricity/heat produced by them, which limits their use despite the widespread use of many forms of concentrated sun power plants. Nonetheless, the temperature potential of this kind of concentrated plant is the highest. To improve the efficiency of an air-type solar tower, a technique for reducing convective energy losses with return air is therefore suggested. In order to increase a cavity receiver's thermal efficiency, a little-known concept called vortex creation will be discussed in this work. The article models different ways of creating an air vortex inside the cavity receiver. Using the VoCoRec design as an example, cases are shown where the vortex increases and decreases the thermal efficiency of the receiver. The concept of Air Return Ratio (ARR) is used to determine the convective losses in a solar collector. This coefficient indicates the convective losses of the receiver due to buoyancy forces and has a direct proportional dependence on the convective efficiency coefficients of the receiver. The VoCoRec receiver, which incorporates the directional vortex inside the receiver, increased the air return coefficient by 4 % (at the same air mass flow rate). The dependence of the air return coefficient on different angles of the air outlet to the absorber plane, including in the radial direction, was also investigated. Increasing the angle of inclination of the air outlet to the main absorber increases the air return coefficient in all cases, but also increases the aerodynamic drag of the receiver (pressure drop).

Study on the dynamic characteristics of a concentrated solar power plant with the supercritical CO2 Brayton cycle coupled with different thermal energy storage methods

The paper aims to study the impact of Thermal Energy Storage (TES) technology on the dynamic characteristics of Concentrated Solar Power (CSP). An integrated dynamic model of a CSP plant is firstly established, which combines the concentrating system, the TES system, and S–CO2 Brayton power cycle system. Three TES alternatives are considered: two-tank molten salt TES (TT-TES), packed-bed TES with solid fillers (PBS-TES), packed-bed TES with phase change materials (PBP-TES). Using this integrated dynamic model, the thermal performance and economic feasibility of different TES technologies applied to CSP are compared and analyzed. The results indicate that utilization of packed-bed TES primarily impacts the optical efficiency of the heliostat field and the thermal efficiency of the power cycle, while having minimal effect on the receiver's thermal efficiency. Furthermore, during the vernal equinox, the daily average system efficiencies of CSP configurations integrating TT-TES, PBS-TES, PBP-TES technologies are 26.0 %, 25.5 %, and 24.5 %, respectively. Meanwhile, the use of packed-bed TES systems significantly reduces the material cost of the TES. In comparison to the TT-TES, the PBP-TES and the PBS-TES can reduce cost by 21.2 % and 42.3 %, respectively, and decrease TES volume by 83.0 % and 63.8 %, respectively.

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.

Solar tower CSP plants with transcritical cycles based on CO2 mixtures: A sensitivity on storage and power block layouts

In this work three CO2-based binary mixtures, CO2 + C6F6, CO2 + C2H3N and CO2 + C4F8, are compared as innovative working fluids for closed power cycles in CSP plants. Adopted in transcritical cycles, they lead to cycle efficiencies higher than sCO2 cycles at minimum temperatures above 50 °C, a typical condition for arid regions with high solar radiation. The analysis considers four plant configurations: the first with direct storage, solar salts as HTF and cycle maximum temperatures of 550 °C, while the three other plants adopt sodium as HTF and an indirect storage system, designed for cycle maximum temperatures of 550 °C, 625 °C and 700 °C. Detailed models are used to characterize the solar fields optical performance, the receiver thermal efficiency and the HTF pump consumption, both at design and off-design conditions, for large scale plants located in Las Vegas. Different power block layouts are considered, spanning from the more efficient ones to cycles with a high heat recovery capacity. In addition, the impact of the mixtures on the design of heat exchangers is evidenced, with convincing results with respect to the heat transfer characteristics of CO2. Considering the resulting yearly performances and LCOE of each configuration, the adoption of indirect storage systems is considered a viable solution for high temperature solar plants. The three innovative mixtures allow for a reduction in LCOE with respect to sCO2 cycles (up to 10 $/MWh, depending on the configuration), capacity factors above 70% for the specific location, optimal solar multiples around 2.8 and 12 equivalent hours of TES.

Designing, Modeling, and Fabrication of a Novel Solar-Concentrating Spittoon against COVID-19 for Antibacterial Sustainable Atmosphere

Spreading infectious illnesses such as viral meningitis, hepatitis, and cytomegalovirus among people is facilitated by spitting in public. India is more prone to transferring infectious illnesses. Recent research discovered that the new Coronavirus may also be transmitted via an infected person’s saliva. Self-collected saliva from 91.7% of patients contains COVID-19. Numerous nations have prioritized preventing individuals from spitting in open or public areas such as hospitals, parks, airports, train stations, etc. The UVC range has a greater damaging effect on microbial cells because microorganisms’ intracellular components, such as RNA, DNA, and proteins, are sensitive to UVC photon absorption. In this article, the design and construction of a solar-concentrating spittoon is attempted. At its receiver, it can create a temperature of 390 K and 176 W of heat. At this temperature, most viruses (including Coronavirus), bacteria, and pathogens are inactivated. Daily, from 8:00 a.m. until 5:00 p.m., the solar-concentrating spittoon is functional. The solar-concentrating spittoon performance was best for nine hours. The receiver thermal efficiency was 80% and 20% of heat was lost to the surroundings. The overall efficiency was found to be 70%. During this time, most people spend their time outside, where this solar-powered spittoon can incinerate human cough and spit within one minute. The installation of this solar-concentrated spittoon will aid in preventing the spread of fatal dangerous diseases and cleaning the city.

Physical property design of multi-component particle systems as heat transfer medium for directly irradiated solar receiver

• New particle candidates considered as the HTM for directly irradiated SPSRs. • The different particles mixed to form novel multi-component particle systems. • The absorption property increases with the addition of dark colored materials. • Thermal performance of particle systems enhanced in a falling quartz tube SPSR. In directly irradiated solid particle solar receivers (SPSRs), the performance of the heat transfer and thermal energy storage medium is a hot research topic. Herein, the thermophysical properties, thermal stability, surface morphology, element compositions and absorptivities of 16 kinds of candidate particles are studied and evaluated. Considering that the performance of homogenous particles as heat transfer medium (HTM) is not ideal, a novel concept of multi-component mixed particle systems with different mass ratios is proposed for the first time. To explore the effect of the particle systems on the receiver performance, a quartz tube falling receiver numerical model is applied. The results indicate that the absorption performance of the particle systems have increased with the addition of dark colored materials, but this effect becomes less obvious with the amount of additive, especially for quartz sand-silicon carbide particle systems. A quartz sand-silicon carbide particle system with a mass ratio of 7:3 may be the optimal choice for high receiver outlet temperature and low mass flow rate. Compared to that of pure silicon carbide particles, this system can achieve a 64.47% reduction in the material cost with only a 4.11% reduction in receiver thermal efficiency. This work provides ideas for the design of multi-component particle systems, which may open a significant development for next-generation concentrated solar power station.

CFD analysis of the performance impact of geometrical shape on volumetric absorbers in a standard cup

The effect of three geometrical parameters of volumetric absorbers in a standard cup –height of the chamfer, the angle of the chamfer and the height of the straight protruding side– on the air temperature at the receiver outlet, the receiver's thermal efficiency and the temperature of the interface between the absorber and the cup are analysed. For the study, 126 different geometrical configurations have been checked in a two-dimensional CFD model. A silicon carbide (SiC) ceramic foam volumetric absorber was selected for the study, and the numerical model adopted includes the simulation of the performance of both the porous media and the cup body. The modelling strategy used was implemented in the commercial CFD code STAR-CCM+ v16.02.009® with a local thermal non-equilibrium model and the Rosseland approximation for the volumetric absorber. The results conclude that different sets of geometrical parameters can produce similar results in terms of air temperature at the cup outlet, thermal efficiency of the system and temperature at the interface between the volumetric absorber and the cup. Emphasizing that the larger the chamfer height the lower the air temperature at the outlet and that the effect of the protruding absorber parameter is influenced by two possible boundary conditions, inlet fluid velocity and wall, and the latter produces the higher air outlet temperature.

Biomass fueled chemical looping hydrogen generation, high temperature solar thermal and thermochemical energy storage hybrid system

Over the past decades, environmental and atmospheric pollution have become a significant global issue. Considering the non-renewable resources in the energy supply sectors that are responsible for a major part of these pollutions, makes the situation only worse. Therefore, switching to power generation systems operating on renewable resources with little to none environmental contamination seems more urgent and attractive than ever. In this study, an integrated multi-generation system utilizing two types of clean and renewable fuels is presented. A novel system design comprising a concentrated solar thermal subsystem integrated with a thermochemical energy storage subsystem and a zero-emission subsystem using chemical looping technology for cogeneration of electricity, hydrogen, and heat is investigated. The produced syngas is divided into two parts to fuel the solid oxide fuel cell subsystem for power generation and the chemical looping subsystem for hydrogen production. An extensive performance analysis is conducted to evaluate the effects of various parameters on the system's performance in order to determine the optimal values which are reported. The solar subsystem simulated using Engineering Equation Solver software resulted in 77.69 % and 79 % for the receiver thermal efficiency and field optical efficiency, respectively. Also, the storage subsystem simulated using Aspen Plus software resulted in a maximum round-trip efficiency of 79.34 %. More, the produced power in the SOFC subsystem and the Lower heating value of the generated hydrogen are found to be 5099 kW and 6124.45 kW, respectively. The values of the SOFC, electrical, hydrogen production, total, net total, and CO2 separation efficiency rates are realized as 71.2 %, 51.21 %, 92.19 %, 60.57 %, 54.72 %, and 100 % respectively.

Numerical and experimental studies on a pressurized hybrid tubular and cavity solar air receiver using a Scheffler reflector

• Coupled optical and thermal model of a pressurized tubular and cavity solar air receiver. • Modelled the dynamic variation of concentrated solar radiation input to the receiver cavity surface. • Three-dimensional CFD analysis is performed on the hybrid receiver model. • Validation with on-sun experimental results using a Scheffler dish. • Natural convection and radiation heat loss estimation. In this study, a pressurized tubular and cavity solar air receiver is analyzed numerically, and the results are validated experimentally using a Scheffler fixed focus concentrator. Transient numerical analyses for periods similar to experimental testing duration allows realistic performance prediction of new receiver designs and help in understanding the dominant heat loss mechanisms, thus mitigating or reducing them. A coupled optical and thermal model is developed for analyzing the receiver with a novel method of capturing the spatial and temporal variation in the heat input to the receiver. The concentrated heat flux input to the receiver is defined as the product of two separate functions. The first function defines the incident radiation to the receiver as a spatially resolved flux profile on the cavity inside surface, obtained from the ray tracing algorithm. The second function captures the transience in heat input by curve-fitting the measured direct normal irradiation variation with time corresponding to the experimental testing period. Receiver heat loss modelling involves natural convection heat loss from the inside cavity surface estimated using an existing correlation, while radiation heat loss is estimated by prescribing the surface emissivity and computing the ambient view factor. A three-dimensional transient CFD analysis is performed on the hybrid receiver model under identical heating conditions as experiments to predict the flow heat transfer. The result from the numerical model is subsequently compared with on-sun experimental results obtained using a test rig equipped with a Scheffler dish for heat input and supply of compressed air as the heat transfer fluid through the receiver. The deviation between numerical and experimental results are less than 16 °C for air outlet temperature and less than 9% for the receiver thermal efficiency, thus proving the effectiveness of the coupled optical and thermal model. To account for the transient nature of the receiver heating during the on-sun experiments, the receiver thermal efficiency definition has been modified to include the thermal inertia of the receiver material. It is also observed that natural convection is the dominant heat loss mechanism for the present configuration and can significantly reduce the overall thermal efficiency of a receiver. The present numerical model incorporating an optical model of the solar field and as-measured variation of solar radiation input can be effectively used for optimizing the design of any generic concentrator-receiver system with high-pressure heat transfer fluid, with the objective of minimizing thermal losses.

No-vacuum Mono-Tube Compound Parabolic Collector Receiver for Linear Fresnel Concentrator: Numerical and Experimental Approach for Dynamic Behavior Assessment

The present paper aims to develop a novel simplified transient model to investigate the dynamic behavior of a no-vacuum mono-tube receiver equipped with a compound parabolic collector. Considering the intermittency of solar radiation, predicting receiver thermal behavior is critical for linear Fresnel concentrator design and operation. From this standpoint, this research focuses on determining whether or not the steady-state assumption influences the receiver’s performance. Unlike the current models accounting for all the receiver components, the proposed model considers the absorber tube as the primary node and focuses on the heat transfer fluid temperature distribution. The remaining components form an equivalent thermal resistance through which heat loss occurs. The global heat losses are then characterized using an experimental receiver prototype with tube diameter of 70 mm, an aperture of 500 mm, and a secondary reflector. The overall receiver heat loss coefficient was determined at different absorber temperatures ranging between 60 and around 265 °C. The heat loss coefficients measured varied from 4.7 to 8.5 W/m2 K. The model computes receiver performance using the measured overall heat loss coefficient based on real solar data. The receiver response at steady and transient states has been conducted and discussed according to several design parameters, including the heat transfer fluid nature and mass flow rate, the receiver length, and the absorber tube thickness. For the same working conditions, synthetic oils fluid allows achieving higher efficiencies whereas molten salts enable reaching higher outlet temperatures. For the metal wall thicknesses ranging between 2 and 3 mm and the fluid outlet temperature varying between 250 and 300 °C, the receiver thermal efficiency during the day remains relatively close to 75 %. The impact of the absorber tube thermal inertia has been investigated by analyzing the dynamic behavior under various close-to-real-world scenarios. Results have shown that the steady-state assumption does not influence if the metal wall tube thickness is lower than 3 mm.

An experimental examination of a helically coiled conical cavity receiver with Scheffler dish concentrator in terms of energy and exergy performance

In this paper, the thermal performance of a helically coiled conical cavity receiver is investigated under actuation radiation conditions with a 16 m2 parabolic Scheffler dish concentrator. Under three different radiation scenarios, the receiver thermal energy efficiency, exergy efficiency, and overall heat transfer coefficient are investigated. Water is employed as a heat transfer fluid, and the receiver is tested at a flow rate of 2.5 L per minute in a temperature range of 30–100 °C. During the stagnation test, the receiver's maximum stagnation temperature was recorded as 395 °C for an incoming beam radiation of 654 W/m2, which demonstrates the receiver's ability to generate pressurised steam. The overall heat loss coefficient calculated from the stagnation test is 116 W/m2-K. The heat transfer fluid input and output temperature differences, as well as incoming beam radiation, are found to have a significant impact on the receiver's energy and energy efficiency. Under maximum beam radiation conditions during the second experiment (672 W/m2), the average thermal energy efficiency and exergy efficiency were determined to be 70.20% and 8.69%, respectively. Based on the findings, this receiver has the potential to be employed in temperature applications up to 100 °C and above for steam generation under pressurised conditions.

Experimental performance analysis of 16 m2 solar Scheffler system for jaggery making

The process of making jaggery involves heating and evaporating sugarcane juice with the addition of thermal energy. Dry bagasse is used as a thermal energy source in the traditional jaggery manufacturing process. Traditional methods of making jaggery have limitations in terms of energy loss due to poor combustion, energy loss via exhaust gases, and other losses related to the furnace wall. Additionally, burning bagasse pollutes the environment. To overcome these problems, the steam energy generated by the solar Scheffler system is used to power the jaggery-making process in this paper. The performance of a 16 m2 Scheffler is evaluated to estimate the amount of jaggery produced and the production time. The experiment was carried out with a fixed amount of sugarcane juice, i.e., 2500 ml, under three different radiation conditions. Under maximum beam radiation conditions, i.e., 754 W/m2 during the experiment, the receiver power and thermal energy efficiency are 5.2 kW and 63.5 %, respectively. The energy required to produce 1 kg of jaggery under maximum radiation conditions (754 W/m2) is 285 W for 130 min. More precisely, 60 min for steam generation and 70 min. for jaggery production. Under maximum radiation conditions (754 W/m2) with 63.5% receiver efficiency, this Scheffler system is capable of producing approximately 16.14 kg of jaggery in 130 min. of duration. The maximum amount of jaggery produced is clearly dependent on the receiver power available for steam generation, as evidenced by the summary of results obtained during the first, second, and third experiments.By improving the thermal efficiency of the receiver and reducing the amount of boiling power required, we can reduce the time required for the production of jaggery, also increasing the quantity of jaggery produced.

SolarReceiver2D: a Modelica Package for Dynamic Thermal Modelling of Central Receiver Systems

This paper describes the Modelica Package SolarReceiver2D which has been developed for the dynamic thermal analysis of central receiver systems. The package includes all the components necessary for the dynamic simulation of the solar receiver operation and it enables consideration of different receiver geometries, materials, HTFs, and operating conditions (e.g., HTF mass flow rate control strategies, heat flux, ambient conditions). Some of the package components are taken from existing Modelica libraries originally developed to model water and gas flows, while other are implemented as new components. The SolarReceiver2D package allows assessment of the receiver thermal efficiency and the temperature distributions experienced by the tubes and the HTF during its operation. The tubes temperature field is modelled in the axial, circumferential, and radial directions, and this makes the package suited for a thermo-mechanical analysis of the receiver. Moreover, the receiver response to passing clouds is dynamically simulated accounting for the effect of thermal transients. The SolarReceiver2D represents a useful tool for the design and optimization of solar tower receivers.

MDBA: An accurate and efficient method for aiming heliostats

In a solar power tower plant, the role of the heliostat aiming strategy is to control the radiative flux distribution at the receiver surface to avoid thermally induced damage, while minimising spillage losses and maximising the receiver thermal efficiency. Flux limitations arise from factors including the heat transfer fluid stability limits, and thermo-mechanical stress limits in receiver pipes. Maximised flux, as close as possible to local flux limits, facilitates an overall smaller receiver with lower thermal losses. Methods exist to sequentially optimise aiming points of single heliostats, using fast convolution-based optical simulations to evaluate individual flux maps. However, to accurately determine receiver flux distributions, ray-tracing is preferred. Ray-tracing is computationally expensive and determination of the aim-points for every heliostat independently potentially leads to impractical simulation times. In this study, a new parameterisation of heliostat aim-point locations is introduced that significantly simplifies the aiming problem. The new aiming model, named Modified Deviation-Based Aiming (MDBA), enables efficient use of ray-tracing to optimise the aiming strategy and, together with receiver thermal and mechanical models, is able to closely match the flux distribution to local values of allowable flux on the receiver. This new parameterisation enables accurate aiming strategy interpolation, used to dynamically predict full field aim-points at different sun positions and values of direct normal irradiance (DNI). A reference case with a surround field and a cylindrical external receiver compatible with the Gen3 Liquid Pathway project is presented to test the capability of the method developed in this study.

Evaluation of external tubular configurations for a high-temperature chloride molten salt solar receiver operating above 700 °C

Next-generation concentrating solar power (CSP) tower technologies target operating temperatures exceeding 700 °C to increase the thermal-to-electric conversion efficiency. Molten chloride salts are one possible alternative to current commercial molten nitrate salts to enable the higher operating temperature. This paper analyzes the predicted optical, thermal-fluids, and structural performance of traditional external tubular solar receiver configurations applied with a chloride salt heat transfer fluid (HTF) and inlet/outlet temperatures of 500 °C/735 °C, and considers sensitivity analysis and optimization relative to receiver sizing, tube sizing, number of panels, flow circuit configurations, and solar flux concentration under constraints on internal velocity, pressure drop, wall thickness, and required creep-fatigue lifetime. The high temperature conditions increase the significance of inelastic deformation mechanisms such as creep relative to that expected in commercial 565 °C nitrate salt designs. High-temperature creep and creep-fatigue damage in the metal alloy tubes are the key factors that limit allowable solar flux concentration and achievable receiver thermal efficiency at the near-800 °C wall temperature conditions. For a traditional external cylindrical receiver configuration, the design parameters and conditions capable of satisfying all constraints produced, at best, a design point receiver efficiency of 78.2%, or 80.5% when excluding receiver intercept efficiency. Variation in the optimal receiver performance relative to uncertainty in the binding maximum velocity, minimum wall thickness, and minimum lifetime constraints is presented.

Aiming Strategy on a Prototype-Scale Solar Receiver: Coupling of Tabu Search, Ray-Tracing and Thermal Models

An aiming point strategy applied to a prototype-scale power tower is analyzed in this paper to define the operation conditions and to preserve the lifetime of the solar receiver developed in the framework of the Next-commercial solar power (CSP) H2020 project. This innovative solar receiver involves the fluidized particle-in-tube concept. The aiming solution is compared to the case without the aiming strategy. Due to the complex tubular geometry of the receiver, results of the Tabu search for the aiming point strategy are combined with a ray-tracing software, and these results are then coupled with a simplified thermal model of the receiver to evaluate its performance. Daily and hourly aiming strategies are compared, and different objective normalized flux distributions are applied to quantify their influence on the receiver wall temperature distribution, thermal efficiency and particle outlet temperature. A gradual increase in the solar incident power on the receiver is analyzed in order to keep a uniform outlet particle temperature during the start-up. Results show that a tradeoff must be respected between wall temperature and particle outlet temperature.

A novel parabolic trough receiver enhanced by integrating a transparent aerogel and wing-like mirrors

In this paper, a novel evacuated receiver was proposed to improve the efficiency of the parabolic trough collector. In this receiver, a solar-transparent aerogel supported by a glass support was employed to insulate the absorber in the directly illumined vacuum zone, and wing-like mirrors on the support were designed to collect some irradiance wasted in the conventional receiver. An optical model developed using ray tracing and a heat transfer model developed using computational fluid dynamics were employed to study the receiver performance. Firstly, optical study indicates the receiver optical efficiency first increases then decreases with increasing aerogel thickness due to the combined effects of the aerogel and the mirrors. The peak receiver optical efficiency of 89.56% is achieved when the aerogel thickness is 20 mm. Moreover, thermal study indicates the receiver thermal efficiency just monotonically increases with increasing aerogel thickness before the aerogel touches the absorber. Then, the receiver with the aerogel thickness of 20 mm was selected as the suggested design after considering both the efficiencies and the safety requirements. Finally, performance evaluation reveals that the suggested receiver can improve the receiver efficiency at any direct normal irradiance or fluid temperature compared with the conventional receiver, which is the originality of this work. It is found the receiver efficiency can be improved by 0.32–5.04% under the typical direct normal irradiance of 300–1000 W·m−2 and the inlet fluid temperature of 573–823 K. Results from present study are beneficial for improving the performance of the parabolic trough collector.

The impact of oxidation on the optical properties of Si–SiC materials

Silicon carbide is an interesting candidate for high-temperature solar receiver due to its high solar absorptivity and its resistance to oxidation in air. Its oxidation stays passive with the formation of a protective silica layer up to 1900 K. Nevertheless, silica presents a lower spectral emissivity than silicon carbide in the solar range. Therefore the growth of a silica layer may affect the values of the spectral emissivity and therefore the surface receiver thermal efficiency. We have investigated here how high-temperature ageing in air affects the spectral emissivity and the thermal efficiency of commercial Si–SiC materials. We were able to correlate the variations of the spectral emissivities to the advancement of the oxidation using scanning electron microscopy, ellipsometry and 3D profilometry. Especially we observed a significant improvement of the surface receiver thermal efficiency due to the increase of the surface roughness that increases the solar absorptivity.

Experimental study on the influence of input solar flux, air flow rate, and absorber parameters on honeycomb ceramic air receiver performance

Honeycomb ceramic absorbers are considered to be one of the most practical options for the air receiver of a solar central receiver plant. To explore the influence of the average input solar heat flux, air flow rate, and absorber geometric parameters on the honeycomb ceramic air receiver performance, six samples were tested under different conditions and the outlet air temperature, thermal efficiency, and anti-interference characteristics of the receivers were determined. The investigation showed that as the input solar flux or air flow rate increased, the change in the outlet air temperature and thermal efficiency had opposite trends. The sample with the shortest length, thinnest wall, and highest porosity had the best performance. Among the three absorber geometry parameters, the porosity was the dominant factor affecting the receiver outlet temperature and thermal efficiency.