Für den ökonomischen Erfolg konzentrierender Solarkraftwerke und für die Effizienz-steigerung der Industrie durch Weiterverwendung von Abwärme sind skalierbare Hochtemperatur-Wärmespeicher zu vertretbaren Kosten unabdingbar. Bisher sind für dieses Anwendungsgebiet nur sensible Speicher kommerziell verfügbar. Denen gegenüber besitzen chemische Speicher zahlreiche Vorteile. Sie bieten höhere Speicherdichten, geringere Wärmeverluste, die Möglichkeit zur Wärmetransformation durch Variation des Reaktionsdrucks und eine Vielzahl von Reaktionssystemen für eine optimale Prozess-integration. Jedoch befinden sie sich noch in der Entwicklungsphase. Die reversible Gas-/Feststoffreaktion von Calciumoxid und Wasserdampf zu Calcium-hydroxid geschieht bei Temperaturen von 400 – 600 °C und ist damit optimal für solarthermische Anwendungen geeignet. Für die Entwicklung eines Speichers ist neben der thermochemischen Charakterisierung des Speichermaterials ein effizientes, skalierbares Reaktorkonzept nötig. Ein Reaktor mit bewegtem Reaktionsbett ermöglicht die Trennung der zwei charakteristischen Speichergrößen Leistung und Kapazität und stellt damit einen wirtschaftlichen Speicher in Aussicht. Die vorliegende Arbeit befasst sich mit Aufbau und Inbetriebnahme eines neuen Teststandes, in welchem ein innovatives Reaktordesign erprobt werden soll. Sie beschreibt die Auslegung einer planaren Reaktorgeometrie, die einen Schwerkraftfluss des Bettes und die Modularisierung für größere Anlagen gewährleistet. Bei Vorversuchen stellt sich die homo-gene Bewegung des Reaktionsbettes aufgrund dessen Kompressibilität als schwierig heraus. Der angestrebte homogene Massenfluss des Reaktionsmaterials kann durch die ursprünglich eingesetzten Feindosiereinheiten nicht erzielt werden. Sie zeigen sich jedoch für die Temperierung des Speichermediums und die Gasdichtheit des Reaktionsraumes als geeignet. Das homogene Ausfließen wird einer separaten Austragshilfe zugeteilt, welche konstruiert und umgesetzt wird. Experimente mit einem Schaureaktor identifizieren eine Zahnwelle als beste Option. Für einen kommerziellen Speicher wird ein Schlitzschieber empfohlen. Ebenso erfolgen Auslegung und Errichtung der peripheren Anlagenteile, wie z.B. die Fertigung eines Druckhalters zur Steuerung der Reaktionstemperatur. Am Teststand werden somit alle Vorbereitungen abgeschlossen, um Heißversuche bei Reaktionstemperatur durchzuführen.:Kurzfassung.....................................................................II Aufgabenstellung ..............................................................III Inhaltsverzeichnis ..............................................................V Nomenklatur ...................................................................VII Abbildungs- und Tabellenverzeichnis ............................................IX Vorwort ........................................................................XI 1 Einleitung ................................................................... 1 2 Theorie thermischer Energiespeicher .......................................... 3 2.1 Beschreibung von Wärmespeichern ............................................ 3 2.2 Sensible Wärmespeicher ..................................................... 4 2.3 Latente Wärmespeicher....................................................... 9 2.4 Sorptive Wärmespeicher .....................................................12 2.5 Chemische Wärmespeicher ....................................................14 3 Spezifikation des thermochemischen Speichersystems ...........................17 3.1 Thermochemische Grundlagen .................................................17 3.2 Motivation der Aufgabenstellung ............................................20 3.3 Charakterisierung des Reaktionssystems .....................................21 4 Systembeschreibung des Speicherkonzepts ......................................26 4.1 Kurzdarstellung der Ausgangssituation ......................................26 4.2 Weiterentwicklung zum bewegten Reaktionsbett ...............................27 4.2.1 Theorie des bewegten Reaktionsbettes .....................................27 4.2.2 Konstruktion des Reaktors ................................................28 4.2.3 Förderung des Speichermaterials ..........................................31 4.3 Periphere Anlagenteile .....................................................33 4.3.1 Anlagenschema ............................................................33 4.3.2 Entwurf des Druckhalters .................................................35 INHALTSVERZEICHNIS VI 4.3.3 Ausführung der Elektro- und Messtechnik ..................................37 5 Experimentelle Untersuchungen ................................................39 5.1 Versuchsdurchführung .......................................................39 5.2 Betrieb der Fördereinheiten ................................................40 5.3 Optimierung der Fördereinheiten ............................................44 5.3.1 Inaktive Mischpaddel .....................................................44 5.3.2 Modifizierte Mischpaddel .................................................47 5.4 Erkenntnisse ...............................................................49 6 Finales Konzept des Versuchsstandes ..........................................50 6.1 Lösungsansätze für den Massenfluss .........................................50 6.2 Gestaltung der Austragshilfe ...............................................54 7 Zusammenfassung und Ausblick .................................................57 Eidesstattliche Erklärung ......................................................59 Literatur- und Quellenverzeichnis ..............................................60 Anlagen ........................................................................63 A.1. Parametrierung des Temperaturwächters (Kapitel 4.3.3) .....................63 A.2. Inhalt des beigelegten Datenträgers (Einband) .............................63 A.3. Berechnung der Aufheizstrecke des Stickstoffstroms (Kapitel 4.3.1) ........64 A.4. Konstruktionszeichnung des Druckhalters (Kapitel 4.3.2) ...................65 A.5. Dampftafel: Sättigungsdampfdruck von Wasserdampf (Kapitel 4.3.2) ..........66 A.6. Stromlaufpläne und Baugruppenliste des Teststandes (Kapitel 4.3.3) ... ....67 A.7. Ermittlung der Kabelquerschnitte für Stromlaufplan (Kapitel 4.3.3) ........73 A.8. Parametrierung der Frequenzumrichter (Kapitel 5.1) ....................... 74 A.9. Ergebnisse der Kalibiermessungen (Kapitel 5.2) ............................75 A.10. Berechnungen zur Dynamik des Schlitzschiebers (Kapitel 6.1) ............. 76 A.11. Konstruktionszeichnungen der Austragshilfe (Kapitel 6.2) .................77

Les procédés thermochimiques solaires étudiés concernent la conversion de charges hydrocarbonées solides ou gazeuses en syngas, ainsi que la réduction d’oxydes en métaux en utilisant l’énergie solaire concentrée pour effectuer les réactions endothermiques, permettant ainsi le stockage de l’énergie solaire intermittente en carburants sans émissions de CO2. Ce travail a pour objectif l’étude expérimentale de trois procédés solaires incluant la gazéification de biomasse, le reformage de méthane en boucle chimique, et la carboréduction de ZnO et MgO. La gazéification et le reformage permettent la valorisation de biomasse bois et de méthane en syngas, tandis que la carboréduction permet de produire Zn et Mg à partir de ZnO et MgO. Ces procédés ont été étudiés dans des réacteurs solaires de 1.5 kWth, en utilisant le rayonnement concentré fourni par des systèmes à concentration du laboratoire PROMES, Odeillo, France. L’impact des paramètres opératoires de chaque procédé sur les mécanismes réactionnels, conversion, rendement, et performances énergétiques a été évalué en détail. Ces procédés ont permis d’améliorer la conversion chimique, les rendements en syngas, les efficacités énergétiques tout en permettant un stockage de l’énergie solaire en combustibles transportables, avec des performances globales supérieures aux procédés conventionnels. De plus, leur faisabilité, fiabilité et robustesse pour la conversion de méthane et biomasse en syngas et la production de Mg et Zn en fonctionnement batch ou continu sous pression réduite ou atmosphérique en conditions solaires réelles ont été démontrés. / The investigated solar thermochemical processes consist of the thermochemical conversion of solid and gaseous carbonaceous feedstocks into syngas as well as metal oxides reduction into metal commodities utilizing concentrated solar energy to drive endothermic chemical reactions, thereby enabling intermittent solar energy storage into solar fuels and avoiding CO2 emissions. This work aims to experimentally investigate three key solar thermochemical conversion approaches regarding biomass gasification, chemical looping reforming of methane, and carbothermal reduction of ZnO and MgO. Solar gasification and solar chemical looping reforming allowed valorizing wood biomass and methane into syngas, while solar carbothermal reduction was applied to produce Zn and Mg from ZnO and MgO. Such solar thermochemical processes were performed in 1.5 kWth prototype solar chemical reactors, utilizing highly concentrated sunlight provided by a solar concentrator at PROMES laboratory, Odeillo, France. The impact of controlling parameters of each process on the reaction mechanism, conversion, yields, and process performance, during on-sun testing was investigated and evaluated thoroughly. Such processes were proved to significantly improve the chemical conversion, syngas yields, energy efficiency, with solar energy storage into transportable fuels, thereby outperforming the conventional processes. Moreover, their feasibility, reliability, and robustness in converting both methane and biomass feedstocks to syngas as well as producing Mg and Zn metals in batch and continuous operation under vacuum and atmospheric conditions during on-sun operation were successfully demonstrated.

No description available.

Solar energy is of huge interest due to the current situation with global warming. A number of solar technologies have been developed, such as photovoltaic, concentrated solar power, and solar water heating systems, to name a few. However, the technologies are being slowly implemented, and Botswana, with generous amounts of sun, is particularly suitable for solar energy. The opportunities were investigated due to the overwhelming and horrifying warnings echoed by many of the world's scientists regarding global warming. First, a handful of solar technologies were thoroughly investigated. A field study at Gakgatla village was then conducted, where the locals were interviewed. The interviews were analyzed, and solutions were suggested based on the identified needs of the locals. The biggest challenges identified were the lack of electricity in many of the households, parental figures not being supportive of the children's education, and theft. Additionally, many locals cooked with firewood. The most helpful solution but difficult to implement is photovoltaic panels. Solar water heating systems would also be helpful to the locals who predominantly had cold water. Additionally, solar cookers could help minimize exposure to hazardous smoke. Lastly, solar lamps could help students study during nighttime. Some locals also showed huge enthusiasm for what the future partnership with Botho University will bring. It is encouraged to develop a prototype of a solar cooker, particularly the parabolic solar cooker, in order to install solar cookers in Gakgatla village. Solar dryers would also greatly benefit the locals and are also suggested developments. Both of the mentioned solutions would help minimize the locals' exposure to the hazards of cooking with firewood. The solar dryers will also help eliminate bacteria by drying the food in a protected space inside a container.

One of the key challenges the world will need to face during the 21st century is global warming and the consequent climate change. Its presence is indisputable, and decarbonizing the gird emerges as one of the required pathways to achieve global sustainable objectives. Solar energy power plants have the potential to revert this situation and solve the problem. One way to harness this energy is through Concentrated Solar Power plants. The major advantage and potential of this technology is its ability to integrate cost-effective Thermal Energy Storage (TES), which is key with such an inherently intermittent resource. On the other hand, the drawback is the high current Levelized Cost of Energy (LCOE). The other main way to harness that highlighted solar energy is the use of Photovoltaic panels, which have recently achieved very competitive LCOE values. On the other hand, the storage integration is still a very pricey option, normally done with Battery Energy Storage Systems (BESS). As a conclusion, a hybrid power plant combining the LCOE of the PV and the TES of the CSP emerges as the key way of achieving a very competitive solution with a big potential. This master thesis aims at exploring the possibilities of a hybrid CSP and PV power plant with a sCO2 power cycle, integrated in the primary, secondary and tertiary electricity markets. To achieve this purpose, firstly, a Python-based Energy Dispatcher was developed to control the hybrid power plant. Indeed, the Dispatcher is the tool that decides when to produce, when to store… following an optimization problem. This can be formulated mathematically, and that was done and integrated into the Python code using Pyomo, a software for optimization problems. As a result, the Dispatcher achieved an effective control of the plant, showing intelligent decisions in detailed hourly analyses. The results were very promising and included optimization functions as maximizing the profitability of the plant or the total production, among others. To proceed with the Techno-economic assessment of the hybrid plant, the electricity markets were studied. The main source of income of any power plant is normally the revenue from selling electricity to the grid, but since there are several markets, there are also other possibilities. In this thesis, it was assessed from a Techno-Economic perspective how the performance and optimal design of the plants vary when providing different services extra to selling electricity to the grid. The conclusion was that even though the Net Present Value (NPV) achieved working on the spot market was already very high, the extra value added from participating in the secondary or tertiary markets was indisputable. Indeed, the profits attained in those markets were between two and four times higher than the ones of the spot market. This is a specific case, but a trend was identified: these hybrid power plants have a huge possibility and a bright future on the service markets. As a consequence, this thesis shows the huge potential of hybrid power plants integrated in the grid participating in several markets. It also lays the foundation for future studies in other locations, under different conditions and with different technologies, among others.

<p>  </p> <p>Concentrated solar power (CSP) is a technology that utilizes focused sunlight to heat a high-temperature medium (such as a molten salt). Heat from this medium can be transferred to a working fluid (such as supercritical CO2) that is then used to drive a turbine to generate electricity. Alternatively, the hot medium/fluid can be pumped into tanks for thermal energy storage (TES), for heat extraction later to generate dispatchable electricity and/or for electricity production at night or on cloudy days. By increasing the fluid temperature to <u>></u>750oC and utilizing TES, CSP can become more cost competitive with fossil-based electricity production. Current CSP systems utilize molten nitrate salts for heat transfer and TES that are known to thermally degrade at temperatures >600oC. To achieve temperatures <u>></u>750oC, molten chloride salts, such as ternary MgCl2-KCl-NaCl compositions, are being considered as heat transfer and thermal energy fluids for next generation CSP plants due to their higher temperature stability, low cost, and availability. </p> <p>In this work, it was demonstrated that MgCl2-containing molten salts are prone to oxidation in ambient air at 750oC, which can enhance corrosion of the containment materials and alter the thermophysical properties of the fluid. An alternative, low-cost, earth-abundant, MgCl2-free, oxidation-resistant molten salt, a eutectic CaCl2-NaCl composition, was developed, along with a corrosion mitigation strategy, to enable the slow growth of protective oxide layers on metals that are resistant to dissolution by such MgCl2-free molten chloride salts. </p> <p>This strategy was expanded to other low-cost, oxidation resistant compositions, such as eutectic BaCl2-CaCl2-KCl-NaCl with tailored chemical and thermophysical properties for CSP and TES. The melting temperature, heat capacity, oxidation resistance, and crystallization behavior were measured for eutectic a BaCl2-CaCl2-KCl-NaCl(17.5-47.8-3.3-31.4 mol%) (BCKN) salt and a MgCl2-KCl-NaCl (40-40-20 mol%) salt. BCKN salt was shown to have a similar melting temperature while having a higher heat capacity and far better oxidation resistance. </p> <p>The corrosion of the nickel-based superalloy Haynes 214 was studied in molten MgCl2-KCl-NaCl (40-40-20 mol%) salt at 750oC under inert atmosphere conditions using a custom-built rotating-disc corrosion testing apparatus that maintained laminar fluid flow on the sample. Non-protective external Cr-, Al-, and Mg- oxide layers were formed on Haynes 214 that were prone to spallation. Internal oxidation of Al was also observed along with Cr depletion zones within Haynes 214.  Corrosion kinetics were evaluated to quantify the role of fluid flow for application of this alloy for use in containment and transportation of this molten chloride salt. </p>

Recently, the supercritical carbon dioxide (s-CO2) Brayton cycle has been identified as a promising candidate for solar-thermal energy conversion due to its potentially high thermal efficiency (50%, for turbine inlet temperatures of ~ 1000 K). Realization of such a system requires development of solar receivers which can raise the temperature of s-CO2 by over 200 K, to a receiver outlet temperature of 1000 K. Volumetric receivers are an attractive alternative to tubular receivers due to their geometry, functionality and reduced thermal losses. A concept of a ceramic pressurized volumetric receiver for s-CO2 has been developed in this work. Computational Fluid Dynamics (CFD) analysis along with a Discrete Ordinate method (DOM) radiation heat transfer model has been carried out, and the results for temperature distribution in the receiver and the resulting thermal efficiency are presented. Issues regarding material selection for the absorber structure, window, coating, receiver body and insulation are also addressed. A modular small scale prototype with 0.5 kWth solar heat input has been designed. The design of a small scale s-CO2 loop for testing this receiver module is also presented in this work. There is a lot of ongoing investigation for design and simulation of different configurations of heat exchangers and solar receivers using s-CO2 as the working fluid, in which wall temperatures up to 1000 K are encountered. While CO2 is considered to be transparent as far as solar radiation spectrum is concerned, there may be considerable absorption of radiation in the longer wavelength range associated with radiation emission from the heated cavity walls and tubes inside the receivers. An attempt has been made, in this study, to include radiation modelling to capture the effect of absorption bands of s-CO2 and the radiative heat transfer among the equipment surfaces. As a case study, a numerical study has been performed to evaluate the contribution of radiative heat transfer as compared to convection and conduction, for s-CO2 flow through a circular pipe. The intent is to provide a guideline for future research to determine the conditions for which radiation heat transfer modelling inside the pipe can be significant, and what errors can be expected otherwise. The effect of parameters such as Reynolds number, pipe diameter, length to diameter ratio, wall emissivity and total wall heat flux has been studied. The effect of radiation modelling on wall temperatures attained for certain amount of heat flux to be transferred to s-CO2 is also studied. The resulting temperature distribution, in turn, affects the estimation of heat loss to the environment

Depuis les années quarante, la consommation énergétique mondiale n'a cessé d'augmenter. Cette énergie étant majoritairement d'origine fossile, il en résulte une augmentation globale de température terrestre. De ce fait, il est devenu urgent de réduire les émissions de gaz à effet de serre pour stopper le changement climatique. Dans ce contexte, le développement de la production d'électricité à partir d'énergie solaire concentrée par voie thermodynamique est une solution prometteuse. Les efforts de recherche visent à rendre cette technologie plus efficace et plus compétitive économiquement. Dans ce but, ce manuscrit présente une méthode de conception optimale pour les centrales solaires à récepteur central. Elle tire parti des méthodes développées depuis de nombreuses années par le groupe de recherche StaRWest, regroupant notamment des chercheurs des laboratoires RAPSODEE (Albi), LAPLACE (Toulouse) et PROMES (Odeillo). Couplant des algorithmes de Monte Carlo à hautes performances et des algorithmes stochastiques d'optimisation, le code de calcul implémentant cette méthode permet la conception et l'optimisation d'installations solaires. Il est utilisé pour mettre en évidence les potentialités d'un type de centrales à récepteur central peu répandu : les centrales à réflecteur secondaire, également appelées centrales de type "beam down". / Since the early 40's, world energy consumption has grown steadly. While this energy mainly came from fossil fuel, its use has included an increase in temperatures. It has become urgent to reduce greenhouse gas emissions to halt climate change. In this context, the development of concentrated solar power (CSP) is a promising solution. The scientific community related to this topic has to focus on efficiency enhancement and economic competitiveness of CSP technologies. To this end, this thesis aims at providing an optimal design method applied to central receiver power plants. It takes advantage of methods developed over many years by the research group StaRWest. Both RAPSODEE (Albi), LAPLACE (Toulouse) and PROMES (Odeillo) researchers take an active part in this group. Coupling high performance Monte Carlo algorithms and stochastic optimization methods, the code we developed allows an optimal design of concentrated solar systems. This code is used to highlight the potential of an uncommon type of central receiver plants: reflective towers, also called "beam down" central receiver systems.

<p dir="ltr">To decrease the use of fossil fuels that generate greenhouse gases, there has been a push to find alternative processes for electricity generation. An attractive renewable alternative is to use solar-thermal energy for grid level electricity production. One method used to generate electricity from the conversion of solar-thermal energy is concentrated solar power (CSP) via the power tower paradigm, which involves an array of mirrors that concentrate sunlight to a spot on a tower. The light heats up a heat transfer fluid which later transfers the thermal energy to a working fluid that expands so as to spin a turbine to generate electricity. Current CSP plants have a peak operation temperature of 550℃, but improvements to the heat exchanger are integral to increasing the peak operation temperature of such plants to a 750℃ target. Ceramic/metal composites (cermets) have been proposed for use as heat exchangers in these CSP plants due to the creep resistance of the ceramic component and toughness of the metal component. One potential material that has an attractive combination of properties for this application is the alumina/chromium (Al2O3/Cr) cermet, given the rigidity and creep resistance of the Al2O3 component and the high-temperature toughness of the Cr phase. Compared to other oxidation-resistant oxide/metal cermets, the Al2O3 and Cr components of this cermet have a relatively close average linear thermal expansion match from 25℃ to 750℃, which is advantageous due to the thermal gradients and thermal cycling of the heat exchanger during operation.</p><p dir="ltr">In this dissertation, the Al2O3/Cr cermet was produced via reaction forming (RF) or reactive melt infiltration (RMI). The RF method involves the reaction of Cr2O3 and Al constituent powder mixtures at high temperature and modest pressures to obtain dense Al2O3/Cr plates. The RMI method involves immersing a shaped porous Cr2O3 preform into an Al or Al-Cr alloy bath to infiltrate and react to form Al2O3/Al-Cr plates. For both methods, the plate microstructure was analyzed for the various reaction conditions. The adiabatic temperature increase for the reaction between Cr2O3 and Al liquid or Al-Cr liquid alloys was calculated. Thermal properties (linear coefficient of thermal expansion, heat capacity, thermal diffusivity, thermal conductivity) and mechanical properties for the RF Al2O3/Cr plates were also measured. Lastly, the reaction kinetics between dense, polycrystalline Cr2O3 and a liquid Al-35at% Cr alloy were experimentally determined at various temperatures and compared to models based on different rate-limiting steps.</p>

No description available.