Solar thermal absorber coatings play a key role in the thermal efficiency of receivers for applications in the field of Concentrated Solar Power (CSP). The development of stable spectral selective coatings with a high solar absorptance alpha sol and a low thermal emittance epsilon th is often desired to reduce thermal losses. However, quantitative indicators describing selectivity and the trade-off between solar absorptance and thermal emittance is seldom discussed in the literature. In this review, relevant opto-thermal figures of merit are analyzed for the comparison of reference solar thermal absorber coatings, including real and ideal coatings, both black and spectral selective. The comparison is made for a temperature ranging from 25 to 1000 degrees C and for a concentration factor ranging from 20 to 1000, based on spectral data measured at room temperature from 0.25 to 20 mu m. New figures of merit are introduced, i.e. a normalized selectivity ratio Si*, a trade-off factor Ztrade-off, a normalized solar reflectance index SRI* and a peak efficiency temperature Tpeak,opt. These metrics are derived from existing figures of merit and adapted for CSP. The set of figures of merit analyzed in this review offer a complementary perspective for the detailed characterization of any coating opto-thermal performance. For solar thermal absorber coatings, thermal efficiency eta thermal and peak efficiency temperature Tpeak,opt are respectively deemed more insightful than opto-thermal efficiency eta opt-th and maximum steady-state temperature TSST,max, when comparing the relative opto-thermal performance of two coating formulations.

The solar receiver coating opto-thermal efficiency has a significant impact on a central receiver system thermal final system efficiency. The development of durable high solar absorptance coatings with simple application process and minimal thermal treatment can directly improve the receiver efficiency, thus reducing the levelized cost of electricity. During the past years, innovative receiver coatings for solar thermal tower plants have been developed on various substrates and tested under isothermal load at different temperature levels. In this paper, eight commercial black coating formulations are sprayed on Haynes 230 metal coupons. Solar absorptance and thermal emittance are monitored before and after isothermal exposure. Mass deviations are also measured to pinpoint any oxidation or coating outgassing. Isothermal testing is performed at 700, 750 and 800 °C in a muffle furnace for 1000 hours. After 1000 hours isothermal exposure, Coterill 750 leads the benchmark in front of Pyromark 2500, while other black coatings degrade optically. Uncoated samples oxidize significantly and appear darker than some aged black coatings.

The growing concern for the climate change has led to an increasing research effort in renewable energy technologies in order to achieve a more sustainable electricity production. Concentrating Solar Power (CSP) is identified as a promising technology to deal with part of the future electricity production. In CSP technologies, a solar receiver converts the concentrated sunlight into high temperature heat. The solar receiver is one of the most critical CSP components as it must provide high thermal power collection efficiencies while operating under very high temperatures and heat fluxes. Thereby, improving the solar receiver efficiency and endurance would benefit the technical and economic viability of CSP.

This PhD thesis aims at improving the efficiency and endurance of a typical solar cavity receiver for the dish-Stirling CSP technology. This research work includes new experimental and numerical analyses contributing to the state of the art of solar receiver design. The efficiency is improved through the analysis of the receiver cavity shape, geometry, operating conditions, and radiative properties, whereas the durability improvement is achieved through the study of advantageous receiver support structures using Finite Element Analysis (FEA). Moreover, a solar laboratory was developed and characterized to conduct representative experiments of the cavity receiver. Multiple parametric experiments were conducted in order to perform a comprehensive validation of the simulations.

During the development of the solar laboratory, it was observed that the commonly utilized flux mapping system (CMOS camera-Lambertian target) should not be used for the characterization of Fresnel lens-based solar simulators. Due to this, the lab characterization was approached combining measurements from a thermopile sensor (radiometer) and a self-designed flat plate calorimeter. Furthermore, a detailed Monte Carlo uncertainty analysis allowed an accurate evaluation of the uncertainty propagation. All the experiments were designed and conducted to increase the accuracy of the final results.

Regarding the cavity receiver design for a dish-Stirling system, the aperture diameter is the most important parameter towards improving the cavity receiver efficiency. The reverse-conical cavity shape provided higher efficiencies (up to 2%) than the cylindrical shape. Additionally, a potential efficiency increase of 0.6% could be achieved by using a cavity material/coating with optimal radiative properties(high emissivity/absorptivity ratio). Finally, the studies suggested that convection has a negligible influence on determining the optimum aperture diameter, whereas the Direct Normal Irradiance (DNI) has little influence. The simulations yielded a cavity receiver with a maximum total receiver efficiency of 91.5%.

Experimental measurements of the receiver displacements under thermal expansion allowed finding realistic mechanical boundary conditions of the receiver. Further structural simulations suggested that thermomechanical stresses can be reduced by setting the receiver supports to certain positions, which can be achieved with the application of external forces and torques. Moreover, the peak stresses can be moved to colder regions to improve the lifetime of the receiver. By shifting the support positions, the receiver simulations calculating creep lifetime under no relaxation showed a potential lifetime improvement of 57%.

Den växande oron för den globala uppvärmningen har lett till en ökad forskningsinsats i förnybar energiteknik mot en hållbarare elproduktion. Koncentrerad solenergi (CSP) identifieras som en lovande teknik för att hantera en del av den framtida elproduktionen. I CSP-tekniken konverterar en solfångare det koncentrerade sollujset till högtemperaturvärme. Solfångaren är en av de kritiska CSPkomponenterna eftersom den arbetar under väldigt höga temperaturer och termiskflöde. Därigenom har solfångarens verkningsgraden och uthålligheten en direkt påverkan på CSP-tekniken och dess ekonomiska genomförbarhet.

Denna doktorsavhandling syftar att förbättra verkningsgraden och hålligheten hos en typisk solkavitetsfångare för dish-Stirling CSP-tekniken. Detta forskningsarbete innehåller nya experimentella och numeriska analyser som inriktar på att förbättra designen. Verkningsgraden förbättras genom analys av solfångarkavitets form, geometri och strålningsegenskaper. Hållfasthetsförbättringen uppnås genom studier av fördelaktiga stödstrukturer för solfångare. Dessutom har ett sollaboratorium utvecklats och karakteriserats för att genomföra representativa experiment. Multipla parametriska experiment andvändes för att validera de numeriska simuleringarna.

Under sollaboratoriets utveckling konstaterades att det allmänt använda CCD-kamera-Lambertian-mål systemet inte kunde användas för sollaboratorikarakteriseringen med Fresnel-linser. På grund av detta utfördes laboratoriekarakteriseringen med en termopil-sensor (mätning av termisk flöde) och en platt kalorimeter. Dessutom gjorde en detaljerad Monte Carlo-osäkerhetsanalys det möjligt att utvärdera osäkerhetskedjan. Experimenten utformades för att öka noggrannheten i de slutligaresultaten.

I den studerade kavitetsfångaren var öppningdiametern den viktigaste parametern för dess verkningrad. Den koniska kavitetsformen gav den högsta verksningsgraden medan verkningsgraden potentiellt kan ökats 0.6% genom idealiska kavitetstrålningsegenskaper (hög emissivitet/absorptivitet förhållande). Studierna antyder att konvektion har en försumbar inverka för att bestämma den optimala öppningsdiametern och DNI-värdet har liten påverkan. Slutligen gav simuleringarna en kavitetsfångare med en maximal total verkningsgrade av 91.5%.

Experimentella mätningar av solfongarens utböjning andändes för att hitta realistiska mekaniska randvillkor. Ytterligare strukturella simuleringar antydde att de termomekaniska spänningarna kan minskas genom justering av solfångarens stödpunkter. Detta kan uppnås med tillämpningen av krafter och vridmoment. Dessutom kan toppspänningarna flyttas till kallare regioner för att förlänga solfångarens livslängd. Med nya stödpositioner kan livslängden mot creep öka 57% för det studerade fallet.

This work presents the comprehensive development of a solar receiver for the integration into a micro gas-turbine solar dish system. Special focus is placed on the thermo-mechanical design to ensure the structural integrity of all receiver components for a wide range of operating conditions. For the development, a 3-dimensional coupled multi-physics model is established and is validated using experimental data. Contrary to previous studies, the temperature of the irradiated front surface of the absorber is included in the comprehensive validation process which results in a high level of confidence in the receiver design.

Finally, a full-scale solar receiver for the integration into the OMSoP solar dish system is designed and its performance determined for a wide operating range to define its safe operating envelope using the validated model. It is shown that the receiver is capable of operating at 803_C with an efficiency of 82.1% and a pressure drop of 0.3% at the nominal operating point, while at the same time functioning effectively   for a wide range of off-design conditions without compromising its structural integrity. At the nominal operating point, the maximum comparison stress of the porous absorber is 5.6 MPa compared to a permissible limit of 7.4 MPa.

The solar receiver performance has a direct impact on the CSP power plant performance and, thereby, its levelized cost of electricity. Improved receiver designs supported by new advanced numerical tools and experimental validation campaigns directly help to make CSP technology more competitive. This paper presents an experimental and numerical investigation of the influence of the cavity receiver radiative properties and the thermal power input on the Dish-Stirling performance. Three cavity coatings are experimentally investigated: the original cavity material (Fiberfrax 140), Pyromark 2500 and Pyro-paint 634-ZO. Moreover, simulations validated with the experimental measurements are utilized to define a higher performance cavity receiver for the Eurodish system. The results indicate that the absorptivity of the cavity should be as low as possible to increase the receiver efficiency whereas the optimum emissivity depends on the operating temperatures. If the cavity temperature is lower than the absorber temperature, low emissivities are recommended and vice-versa. All material/coatings analyzed for the cavity provide similar receiver efficiencies, being Fiberfrax 140 slightly more efficient. Finally, a total receiver efficiency of 91.5% is reached by the proposed Eurodish cavity receiver when operating under the most favorable external conditions. 

The solar receiver tubes work under the highest temperatures and heat flux conditions, being their thermo-mechanical design critical to assure a safe and durable operation. Finite Element Analyses are traditionally utilized to assess the stresses for lifetime calculations. However, the real boundary conditions for these analyses are not well known yet. Thereby, this paper presents an experimental and numerical study to determine more realistic boundary conditions. Firstly, four deflection measurements are measured simultaneously by high-accuracy laser meters. Secondly, three types of boundary conditions are simulated trying to fit the experimental deflections: fixed, elastic and remote displacement. Finally, the stresses at critical regions are compared for each simulation. The results show that, unlike fixed support, remote displacement boundary conditions can obtain realistic deflection results but must be re-adjusted for each specific support, and elastic support fails to capture the manifold rotations. Using remote displacement stress results as reference for the case under study, fixed support leads to deviations in the stresses of at least 50% whilst elastic support can provide some similar stress results.

This paper presents the experimental results of the Cleanergy's C11S solar engine-generator tested in the KTH solar simulator. The paper focuses on the analysis of the thermal performance of the cavity receiver used in the C11S module. Multiple temperature measurements were taken on the tubes of the receiver, inside the cavity and on the internal surface of the cavity. These values allowed characterizing the temperature distribution all around the cavity receiver for the validation of thermal models and the estimation of the thermal losses. Moreover, this paper shows a comparison of the operating characteristics of the C11S module under the real operating conditions and the laboratory ones. It was observed that the temperatures of the receiver in the High Flux Solar Simulator (HFSS) resemble well the real temperatures. Thereby, the KTH solar lab provides proper irradiance levels to operate solar receivers at representative working conditions.

Higher performance cavity receivers are needed to increase the competitiveness of solar power plants. However, the design process needs to be improved with more relevant experimental and numerical analyses. Thereby, the performance of four different Dish-Stirling cavities is investigated experimentally analyzing the influence of the cavity aperture diameter and shape at various operating temperatures. Temperatures inside the cavity receiver were collected together with the electrical power produced by the engine-generator. Then, a thermal system simulation was modelled and a comprehensive multi-parameter and multi-operation validation was performed. To improve this validation, the temperature distribution across the receiver tubes was analyzed in order to relate temperatures on the irradiated region with the non-irradiated one, where thermocouples can measure. The simulations were later used to obtain cavity receiver efficiencies, temperatures and loss breakdowns. The results show that the cavity receiver must be studied in optimization processes in conjunction with the other system components. Moreover, the reverse-conical cavity was found to be more efficient than a nearly cylindrical shape. Regarding the cavity receiver thermal losses, radiation and natural convection present similar contributions in the system under study. Finally, it was found that thermocouples installed on a non-irradiated region can be used to obtain peak receiver temperatures if the measurements are rectified by a correction value proportional to the DNI.

This work presents the experimental evaluation of a novel pressurized high-temperature solar air receiver for the integration into a micro gas-turbine solar dish system reaching an air outlet temperature of 800°C. The experiments are conducted in the controlled environment of the KTH high-flux solar simulator with well-defined radiative boundary conditions. Special focus is placed on providing detailed information to enable the validation of numerical models. The solar receiver performance is evaluated for a range of operating points and monitored using multiple point measurements. The porous absorber front surface temperature is measured continuously as it is one of the most critical components for the receiver performance and model validation. Additionally, pyrometer line measurements of the absorber and glass window are taken for each operating point. The experiments highlight the feasibility of volumetric solar receivers for micro gas-turbine based solar dish systems and no major hurdles were found. A receiver efficiency of 84.8% was reached for an air outlet temperature of 749°C. When using a lower mass flow, an air outlet temperature of 800°C is achieved with a receiver efficiency of 69.3%. At the same time, all material temperatures remain below permissible limits and no deterioration of the porous absorber is found.

Laboratory-scale component testing in dedicated high-flux solar simulators is a crucial step in the developmentand scale-up of concentrating solar power plants. Due to different radiative boundary conditions available inhigh-flux solar simulators and full-scale power plants the temperature and stress profiles inside the investigatedreceivers differ between these two testing platforms. The main objective of this work is to present a systematicscaling methodology for solar receivers to guarantee that experiments performed in the controlled environmentof high-flux solar simulators yield representative results when compared to full-scale tests. In this work theeffects of scaling a solar air receiver from the integration into the OMSoP full-scale micro gas-turbine based solardish system to the KTH high-flux solar simulator are investigated. Therefore, Monte Carlo ray-tracing routines ofthe solar dish concentrator and the solar simulator are developed and validated against experimental characterizationresults. The thermo-mechanical analysis of the solar receiver is based around a coupled CFD/FEManalysislinked with stochastic heat source calculations in combination with ray-tracing routines. A geneticmulti-objective optimization is performed to identify suitable receiver configurations for testing in the solarsimulator which yield representative results compared to full-scale tests. The scaling quality is evaluated using aset of performance and scaling indicators. Based on the results a suitable receiver configuration is selected forfurther investigation and experimental evaluation in the KTH high-flux solar simulator.