Abstract

Concentrating solar power (CSP) systems use solar absorbers to convert sunlight into thermal electric power. In CSP systems, a high reflective surface focuses sunlight onto a receiver that captures the solar energy and converts it into heat. The operation of high efficiency CSP systems involves improvements in the performance of the coatings of the solar absorption materials. To accomplish this, novel, more efficient selective coatings are being developed with high solar absorptance and low thermal losses at their operation temperature. Heat losses in a CSP system occur by three mechanisms: conduction, convection and radiation. It has been widely documented that energy losses increase with increasing operating temperature of CSP systems, and the precise knowledge of the thermophysical properties of the materials involved in CSP systems may allow us to increase the efficiency of systems.

In this work, we applied the pulsed photoradiometry technique (PPTR) to evaluate the changes in the thermophysical properties of selective coatings on a variety of substrates as a function of temperature. Three types of coatings deposited with two different techniques on three types of substrate were examined: commercial coatings based on titanium oxynitride deposited by sputtering on substrates of copper and aluminum, coatings based on black nickel deposited by electrochemical methods on substrates of steel, and coatings based on black cobalt deposited by electrochemical methods on substrates of steel and copper. Values of the thermal diffusivity and thermal conductivity were obtained in the temperature range of 25 to 550 °C. Optical reflectance measurements have been performed in order to provide an estimate of the dependence of the thermal emittance on temperature using the black body radiation theory. REFERENCES

Abstract

The optical quality of concentrators has a direct impact on the thermal efficiency of concentrating solar power plants. There is a need to evaluate the quality of the mirrors before installation and during operation. A review of the optical characterization techniques that have been developed for solar concentrators is presented. A brief description of the operation and methodology of each technique is done. The strengths and possible vulnerabilities of the techniques are also discussed. A classification of the different techniques in families according to their underlying principles of operation is proposed. Finally an analysis of the available information about the accuracy and precision of the different methods is carried out.

Abstract

In the literature for photocatalytic reaction modeling engineering, several simplified schemes applicable to ambient temperature radiative transfer for scattering media are available. A popular strategy is the Six-Flux method because it is simple and is not computer time demanding but its accuracy is not always explicit. In the present work we assess the accuracy of low order methods, including six flux case, by solving the radiative transfer equation in dimensionless form within a cubic enclosure, for i) a collimated beam and ii) a diffuse beam. Radiation enters the cube through a square window centered on one face and with an area one quarter the area of the face. The simulation considers fully absorbing walls, and isotropic scattering. The deviations of simplified models based on Discrete Ordinate or Finite Volume schemes, are compared to the mesh independent solution. The results indicate that for a collimated beam boundary condition the Six-Flux model and more refined models are moderate. The error of the total rate of energy absorbed (TREA) and that of the local volumetric rate of energy absorption (LVREA) with respect to a mesh independent solution are below 5% and 22% respectively. In contrast, it is found that for diffuse boundary conditions the Six-Flux model is very inaccurate since the corresponding errors are larger than 120%.

Abstract

A multiscale model is presented to describe the radiation absorption field in photocatalytic reactors with supported catalyst. The characteristic matrix method is applied at the photocatalyst layer scale, and is embedded within a Monte Carlo ray tracing method, applied at the photoreactor scale. This approach allows to account for important design parameters, such as photocatalyst layer thickness, location of supporting surfaces, and incoming radiation profiles, among others. To resolve the validity of the characteristic matrix method for the description of the optical properties of the catalyst, modeled transmittance and reflectance of the supported films is compared to experimental data. This comparison is carried out for different wavelengths and film thicknesses. Afterwards, the model is applied to a solar reactor with anatase catalyst films supported on multiple surfaces.

The reactor consists of a compound parabolic concentrator with a tubular borosilicate glass receiver. Smaller glass tubes coated with the catalyst are located inside this receiver. With the developed model, a study is conducted to analyze the reactor optical performance as a function of two important design variables: film thickness and radius of the absorber tubes. The results of the model indicate directions for the improvement of the current design.

Abstract

A three-dimensional transient heat transfer model is presented to predict the start-up operation of a multi-tubular cavity reactor under concentrated irradiation in a solar furnace. The reactor consists of a cavity containing nine absorber tubes, through which a suspension of CeO2 in Argon flows. An iterative splitting scheme coupling a Continuous Random Walk, a Finite Volume, and a Ray-Tracing Monte Carlo methods, is implemented to estimate the temperature gradients in the tubes and gas-particle media. The radiation heat transfer among the tubes and cavity walls is considered, as well as conduction and convection in the tubes and the particle suspension. During the initial heating stage, gradients are mainly angular, while in steady-state they are primarily axial. The former may cause tube bending or cracking, and strategies to reduce them are examined. In particular, different heating ramps were simulated, which was found to reduce these initial thermal gradients.

Abstract

Solar reactors designed and constructed for thermochemical applications present different configurations and general performance. The selection of a solar reactor that optimizes a particular process is always a difficult challenge. This work studies two types of reactor configuration by means of a comparative experimental analysis. It was employed a solar device, which is able to operate as fixed reactor with packed bed samples and as rotary kiln. The reduction of CuO into Cu2O was tested under both operation modes, due to its proved potential and interest as thermochemical storage material. It was found that heat transfer was hindered in static experiments limiting the fraction of reactive sample. Thermal gradients of about 200 °C were found in the packed bed through thermocouple and IR camera measurement. Heating rates and total fed energy must be restricted at the risk of front of the sample to melt, resulting in several operation drawbacks. In contrast, mixing conditions in rotary kilns allowed for higher heating rates and led to homogenous sample temperature. Maximum reaction yields in stationary mode did not overpass 14% while it was achieved more than 80% in rotary mode at temperatures about 860 °C. Thermal efficiencies were very limited in both operation modes due to the high thermal inertia of the solar reactor. Because rotary mode admitted much more energy, its thermal efficiency was even lower than static. A solution to increase rotary kilns thermal efficiency is working in continuous mode.

Abstract

Hydrogen is a promising energy carrier for transportation, domestic and industrial applications. Nowadays hydrogen is consumed basically by the chemical industry, but in long term its demand is expected to grow significantly due to emerging markets. Hence production of hydrogen with sustainable methods is a relevant issue. This work presents a review of the different CSP- aided thermochemical processes for hydrogen and syngas production. For each process, some relevant solar-tested reactor prototypes are described. In a second part, the developed solar furnaces for investigation of thermochemical process are also discussed. In addition, relevant research on hydrogen or syngas production in solar tower installations is presented. Finally the current challenges of the technology and the process for its future commercialization are also analyzed.

Abstract

Solar reactors designed and constructed for thermochemical applications present different configurations and general performance. The selection of a solar reactor that optimizes a particular process is always a difficult challenge. This work studies two types of reactor configuration by means of a comparative experimental analysis. It was employed a solar device, which is able to operate as fixed reactor with packed bed samples and as rotary kiln. The reduction of CuO into Cu2O was tested under both operation modes, due to its proved potential and interest as thermochemical storage material. It was found that heat transfer was hindered in static experiments limiting the fraction of reactive sample. Thermal gradients of about 200 °C were found in the packed bed through thermocouple and IR camera measurement. Heating rates and total fed energy must be restricted at the risk of front of the sample to melt, resulting in several operation drawbacks. In contrast, mixing conditions in rotary kilns allowed for higher heating rates and led to homogenous sample temperature. Maximum reaction yields in stationary mode did not overpass 14% while it was achieved more than 80% in rotary mode at temperatures about 860 °C. Thermal efficiencies were very limited in both operation modes due to the high thermal inertia of the solar reactor. Because rotary mode admitted much more energy, its thermal efficiency was even lower than static. A solution to increase rotary kilns thermal efficiency is working in continuous mode.

Abstract

Rotary kilns have a long history of use in classical industries. They are able to achieve high temperatures with higher thermal efficiencies than other reactor types. Their performance has been widely studied and classified according to different parameters. Since it is a well-known technology, rotary kilns have been selected for high temperature solar processes. This article initially presents a brief review of the rotary kiln technology and it focuses on the employment of these devices for thermal and thermochemical processes conducted by concentrating solar energy. Among the solar devices, a novel rotary kiln prototype for thermochemical processes is presented and compared with a static solar reactor. Finally, some practical conclusions on the design and operation of solar rotary kilns are remarked and an analysis of their main limitations is presented.