Este trabalho dimensionou um receptor de cavidade para uso como reator químico de um ciclo de conversão de energia solar para energia química. O vetor energético proposto é o hidrogênio. Isso implica que a energia solar é concentrada em um dispositivo que absorve a radiação térmica e a transforma em energia térmica para ativar uma reação química endotérmica. Essa reação transforma o calor útil em gás hidrogênio, que por sua vez pode ser utilizado posteriormente para geração de outras formas de energia. O primeiro passo foi levantar os pares metal/óxido estudados na literatura, cuja finalidade é ativar um ciclo termoquímico que possibilite produção de hidrogênio. Esses pares foram comparados com base em quatro parâmetros, cuja importância determina o dimensionamento de um receptor de cavidade. São eles: temperatura da reação; estado físico de reagentes e produtos; desgaste do material em ciclos; taxa de reação de hidrólise e outros aspectos. O par escolhido com a melhor avaliação no conjunto dos parâmetros foi o tungstênio e o trióxido de tungstênio (W/WO3). Com base na literatura, foi determinado um reator padrão, cujas características foram analisadas e suas consequências no funcionamento do receptor de cavidade. Com essa análise, determinaram-se os principais parâmetros de projeto, ou seja, a abertura da cavidade, a transmissividade da janela, e as dimensões da cavidade. Com base nos resultados anteriores, estabeleceu-se um modelo de dimensionamento do sistema de conversão de energia solar em energia útil para um processo químico. Ao se analisar um perfil de concentração de energia solar, calculou-se as eficiências de absorção e de perdas do receptor, em função da área de abertura de um campo de coleta de energia solar e da radiação solar disponível. Esse método pode ser empregado em conjunto com metodologias consagradas e dados de previsão de disponibilidade solar para estudos de concentradores de sistemas de produção de hidrogênio a partir de ciclos termoquímicos. / This work aimed to design a cavity receptor for purpose of chemical reactor for cycles of energy conversion of solar energy to chemical energy. The proposed chemical agent is hydrogen gas. Solar energy is concentrated in a device that absorbs thermal radiation, transforming it in thermal energy, used to activate chemical reactions. This reaction transforms the heat in hydrogen gas and the last, in its turn, can be used to generate other forms of energy. The first step oh this work was an assessment of metal/oxides pairs studied in literature, which can be used to activate thermochemical cycles for hydrogen production. These pairs were compared based in four parameters, important to cavity receptor design: reaction temperature, physical state of the reactants and products, material resistance to several cycles; hydrolysis reaction rate and other aspects. The chosen pair, rated as the higher average in all parameters, was the pair tungsten and tungsten trioxide. (W/WO3). Based in the literature, it was determined a standard reactor, which was studied regarding cavity reactor performance. By such analysis, it was possible to determine the main design parameters, therefore, cavity aperture, window transmissivity, and the cavity geometric dimensions. The results allowed to establish a mathematical model in which solar energy can be converted in useful energy for chemical processes, inside a cavity receptor. Given a profile of solar energy concentration, it was calculated absorption and energy lost efficiencies, related to a solar concentration field and radiation available. This method can be used in tandem with available methodologies and data of solar predictions for hydrogen production by concentration systems via thermochemical cycles.

Este trabalho dimensionou um receptor de cavidade para uso como reator químico de um ciclo de conversão de energia solar para energia química. O vetor energético proposto é o hidrogênio. Isso implica que a energia solar é concentrada em um dispositivo que absorve a radiação térmica e a transforma em energia térmica para ativar uma reação química endotérmica. Essa reação transforma o calor útil em gás hidrogênio, que por sua vez pode ser utilizado posteriormente para geração de outras formas de energia. O primeiro passo foi levantar os pares metal/óxido estudados na literatura, cuja finalidade é ativar um ciclo termoquímico que possibilite produção de hidrogênio. Esses pares foram comparados com base em quatro parâmetros, cuja importância determina o dimensionamento de um receptor de cavidade. São eles: temperatura da reação; estado físico de reagentes e produtos; desgaste do material em ciclos; taxa de reação de hidrólise e outros aspectos. O par escolhido com a melhor avaliação no conjunto dos parâmetros foi o tungstênio e o trióxido de tungstênio (W/WO3). Com base na literatura, foi determinado um reator padrão, cujas características foram analisadas e suas consequências no funcionamento do receptor de cavidade. Com essa análise, determinaram-se os principais parâmetros de projeto, ou seja, a abertura da cavidade, a transmissividade da janela, e as dimensões da cavidade. Com base nos resultados anteriores, estabeleceu-se um modelo de dimensionamento do sistema de conversão de energia solar em energia útil para um processo químico. Ao se analisar um perfil de concentração de energia solar, calculou-se as eficiências de absorção e de perdas do receptor, em função da área de abertura de um campo de coleta de energia solar e da radiação solar disponível. Esse método pode ser empregado em conjunto com metodologias consagradas e dados de previsão de disponibilidade solar para estudos de concentradores de sistemas de produção de hidrogênio a partir de ciclos termoquímicos. / This work aimed to design a cavity receptor for purpose of chemical reactor for cycles of energy conversion of solar energy to chemical energy. The proposed chemical agent is hydrogen gas. Solar energy is concentrated in a device that absorbs thermal radiation, transforming it in thermal energy, used to activate chemical reactions. This reaction transforms the heat in hydrogen gas and the last, in its turn, can be used to generate other forms of energy. The first step oh this work was an assessment of metal/oxides pairs studied in literature, which can be used to activate thermochemical cycles for hydrogen production. These pairs were compared based in four parameters, important to cavity receptor design: reaction temperature, physical state of the reactants and products, material resistance to several cycles; hydrolysis reaction rate and other aspects. The chosen pair, rated as the higher average in all parameters, was the pair tungsten and tungsten trioxide. (W/WO3). Based in the literature, it was determined a standard reactor, which was studied regarding cavity reactor performance. By such analysis, it was possible to determine the main design parameters, therefore, cavity aperture, window transmissivity, and the cavity geometric dimensions. The results allowed to establish a mathematical model in which solar energy can be converted in useful energy for chemical processes, inside a cavity receptor. Given a profile of solar energy concentration, it was calculated absorption and energy lost efficiencies, related to a solar concentration field and radiation available. This method can be used in tandem with available methodologies and data of solar predictions for hydrogen production by concentration systems via thermochemical cycles.

In order to maximize the e ciency of power generation in concentrated solar power systems (CSP) it is de- sired to achieve as high a tempera- ture in the receiver as possible due to the use of the Sterling cycle to gen- erate power. This report investigates three di↵erent geometries for cavity receivers in CSP systems; cylindrical, conical and truncated conical. The goal has been to minimize the heat loss due to radiation. This was achieved through mathematical mod- eling with the help of MATLAB. Five di↵erent cases with regards to oper- ating temperature and proportions of the receiver where investigated for each of the three chosen geometries. It was found that the conical geometry minimized this heat loss in all except one case.

The principal objective of this study is to develop, investigate and optimise the Heteroconical Tubular Cavity receiver for a parabolic trough reflector. This study presents a three-stage development process which allowed for the development, investigation and optimisation of the Heteroconical receiver. The first stage of development focused on the investigation into the optical performance of the Heteroconical receiver for different geometric configurations. The effect of cavity geometry on the heat flux distribution on the receiver absorbers as well as on the optical performance of the Heteroconical cavity was investigated. The cavity geometry was varied by varying the cone angle and cavity aperture width of the receiver. This investigation led to identification of optical characteristics of the Heteroconical receiver as well as an optically optimised geometric configuration for the cavity shape of the receiver. The second stage of development focused on the thermal and thermodynamic performance of the Heteroconical receiver for different geometric configurations. This stage of development allowed for the investigation into the effect of cavity shape and concentration ratio on the thermal performance of the Heteroconical receiver. The identification of certain thermal characteristics of the receiver further optimised the shape of the receiver cavity for thermal performance during the second stage of development. The third stage of development and optimisation focused on the absorber tubes of the Heteroconical receiver. This enabled further investigation into the effect of tube diameter on the total performance of the Heteroconical receiver and led to an optimal inner tube diameter for the receiver under given operating conditions. In this work, the thermodynamic performance, conjugate heat transfer and fluid flow of the Heteroconical receiver were analysed by solving the computational governing Equations set out in this work known as the Reynolds-Averaged Navier-Stokes (RANS) Equations as well as the energy Equation by utilising the commercially available CFD code, ANSYS FLUENT®. The optical model of the receiver which modelled the optical performance and produced the nonuniform actual heat flux distribution on the absorbers of the receiver was numerically modelled by solving the rendering Equation using the Monte-Carlo ray tracing method. SolTrace - a raytracing software package developed by the National Renewable Energy Laboratory (NREL), commonly used to analyse CSP systems, was utilised for modelling the optical response and performance of the Heteroconical receiver. These actual non-uniform heat flux distributions were applied in the CFD code by making use of user-defined functions for the thermal model and analysis of the Heteroconical receiver. The numerical model was applied to a simple parabolic trough receiver and reflector and validated against experimental data available in the literature, and good agreement was achieved. It was found that the Heteroconical receiver was able to significantly reduce the amount of reradiation losses as well as improve the uniformity of the heat flux distribution on the absorbers. The receiver was found to produce thermal efficiencies of up to 71% and optical efficiencies of up to 80% for practically sized receivers. The optimal receiver was compared to a widely used parabolic trough receiver, a vacuum tube receiver. It was found that the optimal Heteroconical receiver performed, on average, 4% more efficiently than the vacuum tube receiver across the temperature range of 50-210℃. In summary, it was found that the larger a Heteroconical receiver is the higher its optical efficiency, but the lower its thermal efficiency. Hence, careful consideration needs to be taken when determining cone angle and concentration ratio of the receiver. It was found that absorber tube diameter does not have a significant effect on the performance of the receiver, but its position within the cavity does have a vital role in the performance of the receiver. The Heteroconical receiver was found to successfully reduce energy losses and was found to be a successfully high performance solar thermal tubular cavity receiver.