Cette thèse s’inscrit dans le cadre d’un projet européen visant à améliorer la compétitivité des centrales solaires à concentration. Parmi les différents défis soulevés par ce projet, l’évaluation en temps réel de la disponibilité et de la variabilité de la ressource solaire est un point clé puisqu’elle permettrait une gestion optimisée du champ solaire et, par conséquent, une hausse de la productivité de la centrale. L’objectif de ce travail est donc de développer un outil d’évaluation de la ressource solaire destiné à la gestion de centrales CSP. Pour y parvenir, une étude approfondie des interactions entre le rayonnement solaire et l’atmosphère est tout d’abord menée. Cette étude révèle entre autres que l’éclairement normal direct (DNI) peut se scinder en deux composantes : le DNI par ciel clair et l’indice ciel clair. Le premier représente l’éclairement normal direct reçu au niveau du sol, lorsqu’aucun nuage ne vient occulter le Soleil. Le second traduit l’influence des nuages sur ce rayonnement par ciel clair. Évaluer ces deux composantes est essentiel pour l’opérateur de la centrale car elles lui permettent de connaître les marges de manoeuvre dont il dispose. D’une part, un modèle ciel clair permettant d’estimer et prévoir le DNI par ciel clair en temps réel est donc développé. Il permet de maintenir l’erreur quadratique moyenne sur l’estimation du DNI par ciel clair aux alentours de 30W/m². D’autre part, une caméra hémisphérique a été installée sur le site du laboratoire PROMES-CNRS afin de détecter les nuages et leur mouvement dans le but d’appréhender la variabilité de l’indice ciel clair. Ce système est notamment capable de fournir des images à haute dynamique, permettant de mesurer simultanément des informations dans la zone circumsolaire et dans les zones les plus sombres du ciel. Sur la base du modèle ciel clair et des images fournies par la caméra, un modèle de prévision du DNI pour tout type de conditions a été mis au point. Il permet de maintenir l’erreur quadratique moyenne sur la prévision du DNI aux alentours de 180 W/m², pour des horizons inférieurs à 30 min. En partenariat avec Acciona, l’outil développé est d’ores et déjà opérationnel sur la centrale solaire Palma del Rio II, en Espagne. / This thesis is part of a European research project which aims at improving the solar power plant efficiency. Among the different challenges pointed out by this project, the solar resource assessment and forecasting are essential tasks since they would allow a better real-time management of the solar field, and thus reduce the maintenance activities, while improving the expected benefits. Therefore, the purpose of this work is to develop a solar resource forecasting tool in order to improve the CSP plants management. An extensive review of the interactions between solar radiation and the atmosphere is firstly conducted. It reveals, among other things, that the direct normal irradiance (DNI) can be divided into two components : the clear sky DNI and the clear sky index. The former represents the direct normal irradiance received at ground level, when no clouds are occulting the sun. The latter reflects the influence of clouds on the clear sky DNI. Estimating these two quantities is essential for the plant operator, since it allows a better management of the solar field. As a consequence, a clear sky model able to estimate and forecast the clear sky DNI has been developed. The root mean squared error of the forecast is around 30 W/m². On the other hand, a sky imager has been installed at the PROMES-CNRS laboratory in order to detect clouds and their motion. The system is able to provide high dynamic range images, allowing the measurement of information both into the circumsolar area and into the darkest parts of the sky. Based on the clear sky model and the images provided by the sky imager, a DNI forecasting model is proposed. The root mean square error on the forecast is around 30 W/m², for 30 min forecasting horizon. One system is now operational at a solar power plant located in Palma del Rio II, Spain.

Ce travail de thèse porte sur l’étude de la réduction de CO 2 et H 2 O en CO et H 2 au moyen de cycles thermochimiques. Ces cycles utilisent des oxydes métalliques pour réaliser ces réductions en deux étapes, permettant de diminuer la température nécessaire. Dans une première étape endothermique, l’oxyde métallique est réduit à haute température (>1200°C) grâce à un apport d’énergie solaire concentrée. Dans une seconde étape exothermique réalisée à plus basse température (<1200°C), cette espèce réduite est ré-oxydée en présence d’eau ou de CO 2 , produisant H 2 ou CO et régénérant l’oxyde métallique pour un autre cycle. Le mélange de H 2 et CO (syngas), ainsi produit uniquement grâce à de l’énergie solaire peut ensuite être transformé en carburant liquide conventionnel par un procédé catalytique de type Fischer-Tropsch. Cette étude s’intéresse particulièrement aux cycles à base d’oxydes volatiles, ZnO/Zn et SnO 2 /SnO, dont le produit de la première étape de réduction est sous forme gazeuse à la température de réaction, puis se condense sous forme de nanoparticules. Tout d’abord, des moyens et méthodes ont été développés pour l’étude de la cinétique des réactions de réduction à hautes températures, en particulier une méthode inverse utilisant la mesure en ligne de l’oxygène produit dans un réacteur solaire, et un dispositif de thermogravimétrie solaire. Par ailleurs, différents moyens de diminuer la température des réactions de réduction ont été étudiés, à savoir la diminution de la pression et l’emploi d’un agent réducteur carboné. L’impact de la diminution de la pression sur la cinétique de réduction a été quantifié pour SnO 2 et ZnO.Une étude de l’évolution physico-chimique de poudres de SnO durant la deuxième étape d’oxydation du cycle a ensuite été réalisée, montrant l’importance de la réaction de dismutation de SnO en Sn et SnO 2 sur la réactivité des poudres dans la gamme de température étudiée. / This PhD thesis focuses on the study of the CO2 and H2O reduction into CO and H2 using thermochemical cycles. These cycles use metal redox pairs for stepwise reduction at lower temperature. The first step consists of the endothermic high temperature reduction of the metal oxide (>1200°C) using concentrated solar energy. The second step, operated at a lower temperature (<1200°C), uses the reduced specie to reduce CO2 or H2O, yielding CO or H2 and regenerating the metal oxide. The CO and H2 mixture (syngas), produced using solar energy, can then be converted into liquid fuel using a conventional Fischer-Tropsch catalytic process. The study considers more specifically the volatile oxide cycles, ZnO/Zn and SnO2/SnO, for which the reduced specie is obtained in gaseous phase at the reaction temperature, and is then condensed as nanoparticles. First, means and methods for studying the kinetics of reduction reactions at high temperatures were developed, namely an inverse method based on the online analysis of O2 production in a solar reactor and a solar-driven thermogravimeter. In addition, the study of reduced pressure operation and the use of a carbonaceous reducer were considered as efficient means to decrease the operating temperature and to promote a fast reaction. The impact of reduced pressure was quantified for SnO2 and ZnO reduction. A study of the evolution of the morphology and chemistry of the SnO powder during the second oxidation step was then conducted, emphasizing the importance of SnO disproportionation on the powder reactivity.

A heliostat is a device that reflects the sun’s rays to a stationary object. There are two different types of heliostat serial and parallel type. The most used is the serial type of heliostat with Azimuthal-Elevation(Az-El) drive. The parallel heliostat includes two or more actuators to accommodate for the movement, one such design is heliostat with Hydraulic Infinite Linear Actuator (HILA). In this design, the mirror assembly is supported by two HILA’s and a universal joint. The mirror thus has three support points making it a parallel heliostat. This thesis work includes setting up the geometry of the new concept for160 m2reflector area and formulating kinematics and optics equations for calculating the lengths of the actuators and rotations at the universal joint. For structural analysis, the application of wind loads on the structure in the form of pressure is calculated and the structure is designed to satisfy the deflection requirements of 2 mRads at a maximum allowable wind speed of 22 m/s. The symmetric structure is modelled and checked for deflection requirements in Ansys Workbench. The reaction forces are calculated which are then used for topology optimization of the base structure, which is performed Trinitas. When there is an established design for the heliostat with HILA benchmarking is done by designing a scaled-down model for 25 m2 and comparing it with pre-existing results for Az-El type heliostats in literature.

Solar Tower Power Plants with thermal energy storage are a promising technology for dispatchable renewable energy in the near future. Storage integration makes possible to shift the electricity production to more profitable peak hours. Usually two tanks are used to store cold and hot fluids, but this means both higher related investment costs and difficulties during the operation of the variable volume tanks. Another solution can be a single tank thermocline storage in a multi-layered configuration. In such tank both latent and sensible fillers are employed to decrease the related cost by up to 30% and maintain high efficiencies.  The Master thesis hereby presented describes the modelling and implementation of a thermocline-like multi-layered single tank storage in a STPP. The research work presents a comprehensive methodology to determine under which market structures such devices can outperform the more conventional two tank storage systems. As a first step the single tank is modelled by means of differential energy conservation equations. Secondly the tank geometrical design parameters and materials are taken accordingly with the applications taken into consideration. Both the steady state and dynamic models have been implemented in an existing techno-economic tool developed in KTH, in the CSP division (DYESOPT). The results show that under current cost estimates and technical limitations the multi-layered solid PCM storage concept is a better solution when peaking operating strategies are desired, as it is the case for the two-tier South African tariff scheme. In this case the IRR of an optimal designed power plant can be decreased by 2.1%. However, if a continuous operation is considered, the technology is not always preferred over the two tank solution, yet is a cheaper alternative with optimized power plants. As a result the obtained LCOE can be decreased by 2.4%.

In response to the extensive energy demands on national and global levels, concentrated solar power (CSP) plants are designed to harness and convert solar energy to electricity. For such green energy application, robust, reliable and durable materials for CSP constructions are required. The corrosion resistance is among many parameters to consider in these thermalelectrical stations such as for pipes and storage tanks in CSP. In this investigation, the corrosion behavior of AISI 304 stainless steel (18 wt. % Cr, 8 wt. % Ni) with the heat transfer fluid, also known as solar salt, has been examined. The ternary eutectic salt mixture with the composition, 53 wt. % KNO3, 40 wt. % NaNO2, and 7 wt. % NaNO3, that melts at 142°C, has a potential use in CSP as a heat transfer fluid. The solar salt was prepared for this corrosion study from reagent grades of high purity nitrites and nitrates. Samples of AISI 304 stainless steel were sectioned from a sheet stock of the alloy and exposed to solar salt at 530°C in air at 1 atmospheric pressure. After test intervals of 250, 500, and 750 hours in total immersion condition, AISI 304 stainless steel samples have developed a scale of corrosion products made up of multiple oxides. X-ray diffraction and scanning electron microscopy with X-ray energy-dispersive spectroscopy were employed to examine the extent of corrosion and identify the corrosion products. Transmission electron microscopy was used to verify the corrosion products identity via electron diffraction patterns. Oxides of iron were found to be the primary corrosion products in the presence of the molten alkali nitrates-nitrite salt mixture because of the dissolution of the protective chromium oxide (Cr2O3) scale formed on AISI 304 stainless steel coupons. The corrosion scale was uniform in thickness and made up of sodium iron oxide (NaFeO2), iron oxide, hematite (Fe2O3), and chromium-iron oxide (Cr,Fe)2O3 solid solution. The latter was iv found near the AISI 304 stainless steel. This indicates that the scale formed, particularly on the upper layers with presence of sodium iron oxide and iron oxide, hematite, is protective, and forms an effective barrier against penetration of fused solar salt. At the alloy interface with the bulk corrosion scale, the corrosion process induced a compositional modification in the grains located at the interface. There are iron rich and iron depleted grains at the interface if compared to the nominal iron content of the alloy. The mode of attack is identified as uniform at the test temperature of 530°C, showing a parabolic behavior with a parabolic rate constant (Kp) equals to (m2 /sec). By extrapolation, annual corrosion rate is estimated to reach 0.784 mils per year. Corrosion behavior of AISI 304 stainless steel is discussed in terms of thermodynamics and reaction paths.

A desirable feature of concentrated solar power (CSP) with integrated thermal energy storage (TES) unit is to provide electricity in a dispatchable manner during cloud transient and non-daylight hours. Latent heat thermal energy storage (LHTES) offers many advantages such as higher energy storage density, wider range of operating temperature and nearly isothermal heat transfer relative to sensible heat thermal energy storage (SHTES), which is the current standard for trough and tower CSP systems. Despite the advantages mentioned above, LHTES systems performance is often limited by low thermal conductivity of commonly used, low cost phase change materials (PCMs). Research and development of passive heat transfer devices, such as heat pipes (HPs) to enhance the heat transfer in the PCM has received considerable attention. Due to its high effective thermal conductivity, heat pipe can transport large amounts of heat with relatively small temperature difference. The objective of this research is to study the charging and discharging processes of heat pipe-assisted LHTES systems using computational fluid dynamics (CFD) and experimental testing to develop a method for more efficient energy storage system design. The results revealed that the heat pipe network configurations and the quantities of heat pipes integrated in a thermal energy storage system have a profound effect on the thermal response of the system. The optimal placement of heat pipes in the system can significantly enhance the thermal performance. It was also found that the inclusion of natural convection heat transfer in the CFD simulation of the system is necessary to have a realistic prediction of a latent heat thermal storage system performance. In addition, the effects of geometrical features and quantity of fins attached to the HPs have been studied. / Mechanical Engineering

Yes / Solar energy, amongst all renewable energies, has attracted inexhaustible attention all over the world as a supplier of sustainable energy. The energy requirement of major seawater desalination processes such as multistage flash (MSF), multi-effect distillation (MED) and reverse osmosis (RO) are fulfilled by burning fossil fuels, which impact the environment significantly due to the emission of greenhouse gases. The integration of solar energy systems into seawater desalination processes is an attractive and alternative solution to fossil fuels. This study aims to (i) assess the progress of solar energy systems including concentrated solar power (CSP) and photovoltaic (PV) to power both thermal and membrane seawater desalination processes including MSF, MED, and RO and (ii) evaluate the economic considerations and associated challenges with recommendations for further improvements. Thus, several studies on a different combination of seawater desalination processes of solar energy systems are reviewed and analysed concerning specific energy consumption and freshwater production cost. It is observed that although solar energy systems have the potential of reducing carbon footprint significantly, the cost of water production still favours the use of fossil fuels. Further research and development on solar energy systems are required to make their use in desalination economically viable. Alternatively, the carbon tax on the use of fossil fuels may persuade desalination industries to adopt renewable energy such as solar.

This thesis presents investigations on air-cooled air gap membrane distillation for desalination and the application of radial waveguides based on total internal reflection for solar thermal desalination. Using an air-cooled design for an air gap membrane distillation (AGMD) process may result in significantly lower energy requirements for desalination. Experiments were conducted on AGMD module to study the effect of air gap, support mesh conductivity and hydrophobicity, condensing surface hydrophobicity. A novel modular design was used in which modules could be used in a series configuration to increase the flux value for the distillate. The output from the series configuration was found to have about three times the production from a single pass water-cooled system with the same temperature difference between the saline and clear water streams. The results also indicated that the mesh conductivity had a favorable effect on the flux value whereas the hydrophobicity of the mesh had no significant effect. The hydrophobicity of the condensing surface was favorable on two accounts: first, it led to an increase in the flux of the distillate at temperatures below 60 °C and second, the temperature difference of the saline feed when it enters and leaves the module is lower which can lead to energy savings and higher yields when used in a series configuration. The second part of the thesis considers use of low-cost radial waveguides to collect and concentrate solar energy for use in thermal desalination processes. The optical-waveguide-based solar energy concentrators are based on total internal reflection and minimize/eliminate moving parts, tracking structures and cost. The use of optical waveguides for thermal desalination is explored using an analytical closed-form solution for the coupled optical and thermal transport of solar irradiation through a radial planar waveguide concentrator integrated with a central receiver. The analytical model is verified against and supported by computational optical ray tracing simulations. The effects of various design and operating parameters are systematically investigated on the system performance, which is quantified in terms of net thermal power delivered, aperture area required and collection efficiency. Design constraints like thermal stress, maximum continuous operation temperature and structural constraints have been considered to identify realistic waveguide configurations which are suitable for real world applications. The study provides realistic estimates for the performance achievable with radial planar waveguide concentrator-receiver configuration. In addition to this, a cost analysis has been conducted to determine the preferred design configurations that minimize the cost per unit area of the planar waveguide concentrator coupled to the receiver. Considering applications to thermal desalination which is a low temperature application, optimal design configuration of waveguide concentrator-receiver system is identified that result in the minimum levelized cost of power (LCOP). / Master of Science

Due to the vast fossil fuel wealth, the country of Saudi Arabia is experiencing a dramatic growth in both population and GDP. Therefore there is a growing demand for water and energy to meet these needs. All of the electricity that is generated is sourced from crude oil and natural gas. All natural gas production is used domestically and there are no net imports or exports. Due to many constrains on the natural gas supply, there is a slow shift in the generation mix going towards crude oil based power generation. This study assessed the viability of utility scale solar integration into the Saudi Arabian electric mix to potentially relieve some demand pressure for natural gas consumption as well as reduce green house gas emissions. Parabolic trough concentrated solar power technology was chosen as the primary technology for utility scale integration. A total of five scenarios were calculated. The scenarios include the following, base case, 5%, 10%, 15%, 20% solar integration in terms of installed capacity. Two sets of net present values were calculated. The net present values of each scenario were calculated. A second set of net present values was calculated with a projected increase in electricity prices. The natural gas and crude oil offset from the four solar integration scenarios were calculated using the base case forecasted natural gas and crude oil consumption from power generation. As expected, natural gas and crude oil consumption decreased when there was an increase in solar integration. The expected carbon dioxide offsets were calculated for each scenario. There was a decrease in carbon dioxide emission as solar integration was increased. Finally, all of these analyses were used as criteria for a decision analysis using the analytical hierarchy process. Depending on the decision maker’s importance on the determined criteria, solar integration in the Kingdom of Saudi Arabia is achievable. / text

In a proposed design for a concentrated solar power tower, sand is irradiated by solar energy and transfers its energy to another fluid stream by means of a finned tube heat exchanger. To maximize heat transfer and minimize potential damage to the heat exchanger, it is desired to have a very uniform flow through the heat exchanger. However, performing full scale flow tests can be expensive, impractical, and depending upon the specific quantities of interest, unsuitable for revealing the details of what it happening inside of the flow stream. Thus, the discrete element method has been used to simulate and study particulate flows. In this project, the flow of small glass beads through a square pyramid shaped hopper and a wedge shaped hopper were studied at the lab scale. These flows were also simulated using computers running two versions of discrete element modeling software – EDEM and LIGGGHTS. The simulated results were compared against the lab scale flows and against each other. They show that, in general, the discrete element method can be used to simulate lab scale particulate flows as long as certain material properties are well known, especially the friction properties of the material. The potential for increasing the accuracy of the simulations, such as using better material property data, non-uniform particle size distributions, and non-spherical particle shapes, as well as simulating heat transfer within a granular flow are also discussed.