New methods and results in the optimisation of solar power tower plants

Renewable energy technology has seen great advances in recent decades, combined with an ever increasing interest in the literature. Solar Power Tower (SPT) plants are a form of Concentrating Solar Power (CSP) technology which continue to be developed around the world, and are formed of subsystems that are open to optimisation. This thesis is concerned with the development of new methods and results in the optimisation of SPT plants, with particular focus on operational optimi- sation. Chapter 1 provides background information on the energy sector, before describing the design and modelling of an SPT plant. Here, the optical theory behind the transfer of incident radiation in the system is developed and the relevant equations presented. In Chapter 2, the cleaning operations of the heliostat eld are optimised for a xed schedule length using Binary Integer Linear Programming (BILP). Problem dimensionality is addressed by a clustering algorithm, before an ini- tial solution is found for the allocation problem. Finally, a novel local search heuristic is presented that treats the so-called route \attractiveness" through the use of a sequential pair-wise optimisation procedure that minimises a weighted attractiveness measure whilst penalising for overall energy loss. Chapters 3-6 investigate the aiming strategy utilised by the heliostat eld when considering a desired ux distribution pro le and operational constraints. In Chapter 3, a BILP model was developed, where a pre-de ned set of aim- ing points on the receiver surface was chosen. The linear objective function was constrained with linear equalities that related to distribution smoothing (to pro- tect receiver components from abnormal ux loads) via the use of penalisation. Chapter 4 extended this model by instead considering continuous variables with no xed grid of aiming points. This led to an optimisation problem with a non- linear, non-convex objective function, with non-linear constraints. In this case, a gradient ascent algorithm was developed, utilising a non-standard step-size selection technique. Chapter 5 further extended the aiming point optimisation topic to consider the dynamic case. In this sense, the aiming strategy across a period of time could be optimised, taking into account SPT plant technologi- cal limitations. Two algorithms were considered, Penalisation and Augmented Lagrangian, where theoretical properties for optimality and solution existence were presented. Finally Chapter 6 considered the efects of inclement weather on the optimisation model presented in Chapter 3. Stochastic processes were in- vestigated to determine optimal aiming strategies at a xed point in time when weather data could not be known for certain. All research presented in this thesis is illustrated using real-world data for an SPT plant, and conclusions and recommendations for future work are presented

Static optimal control: Real-time optimization within closed-loop aim point control for solar power towers

Many aim point optimization techniques exist to control Solar Power Towers (SPTs). However, SPTs exhibit optical losses that cannot be exactly modeled. Moreover, cloud passages cause transient incident flux distributions. Due to these modeling errors and disturbances, aim point optimization may exceed the Allowable Flux Density (AFD); consequently, these efficient aiming strategies are seldom applied at commercial plants. In this paper, an innovative closed-loop aim point control technique, the Static Optimal Control, is proposed. Flux density measurements close the open control loop of aim point optimization. Based on this feedback, the Static Optimal Control estimates weights that are embedded in the cost function of the aim point optimization. This GPU-based optimizer finds good aim point configurations in a few seconds even for large plants. Thus, the Static Optimal Control compensates for modeling errors and rejects disturbances to observe the AFD while maximizing the intercept. The performance of the Static Optimal Controller is evaluated for inaccurately modeled mirror errors and under a real cloud scenario. Aim of this control is not to exceed the AFD by more than 5% i.e. the accuracy of the flux density measurements. The aim is achieved for static modeling errors while improving the intercept by 1.7-8.6% compared to a heuristic control. In the cloud scenario, the Static Optimal Control reaches its limits. Even mapping all-sky-imager-based nowcasts in a feed forward manner on the heliostat field does not improve the control quality due to high prediction errors

Proceedings of the XIII Global Optimization Workshop: GOW'16

[Excerpt] Preface: Past Global Optimization Workshop shave been held in Sopron (1985 and 1990), Szeged (WGO, 1995), Florence (GO’99, 1999), Hanmer Springs (Let’s GO, 2001), Santorini (Frontiers in GO, 2003), San José (Go’05, 2005), Mykonos (AGO’07, 2007), Skukuza (SAGO’08, 2008), Toulouse (TOGO’10, 2010), Natal (NAGO’12, 2012) and Málaga (MAGO’14, 2014) with the aim of stimulating discussion between senior and junior researchers on the topic of Global Optimization. In 2016, the XIII Global Optimization Workshop (GOW’16) takes place in Braga and is organized by three researchers from the University of Minho. Two of them belong to the Systems Engineering and Operational Research Group from the Algoritmi Research Centre and the other to the Statistics, Applied Probability and Operational Research Group from the Centre of Mathematics. The event received more than 50 submissions from 15 countries from Europe, South America and North America. We want to express our gratitude to the invited speaker Panos Pardalos for accepting the invitation and sharing his expertise, helping us to meet the workshop objectives. GOW’16 would not have been possible without the valuable contribution from the authors and the International Scientific Committee members. We thank you all. This proceedings book intends to present an overview of the topics that will be addressed in the workshop with the goal of contributing to interesting and fruitful discussions between the authors and participants. After the event, high quality papers can be submitted to a special issue of the Journal of Global Optimization dedicated to the workshop. [...

Development of perovskite-like structures for hydrogen production via two-step thermochemical water splitting

Hydrogen powered technologies are proposed to help mitigate climate change as low carbonemitting technologies. Devices such as fuel cells convert the chemical energy stored within hydrogen molecules via electrochemical redox processes to electrical energy for work. These technologies have the primary benefit of not emitting carbon dioxide – one of the main contributing pollutants towards the greenhouse effect. However, current commercial hydrogen production technologies require fossil fuel reactants and emit carbon dioxide as a product. Therefore, research into ways of producing hydrogen from sustainable non-polluting sources has been of keen interest within the scientific community. One such technique is high temperature thermochemical water splitting. This process uses renewable concentrated solar power to heat up and thermally reduce metal oxide compounds and induce an oxygen nonstoichiometry within the lattice. The oxygen deficiency is then removed upon reoxidising with steam and producing hydrogen gas. Numerous thermochemical redox cycles have been proposed within the literature with the main aim to lower the reduction temperatures and increase the hydrogen production volumes. This has turned the attention of the field to investigate the ABO3 perovskite structures due to their ability to support a larger oxygen deficiency at lower temperatures compared to the benchmark material, cerium oxide, CeO2. This thesis combines theoretical first principle approaches and a wide range of experimental techniques to understand and discuss three different families of perovskite and perovskite-like metal oxide structures. The main findings of this thesis can be summarised as the following: Effect of antimony incorporation on the redox kinetics of SrCoO3-d - Thermal analysis techniques observe large oxygen production volumes onset between 300 and 400 °C under an inert gas flow with increased antimony content lowering total production. - Density Functional Theory (DFT) confirms the low reduction enthalpy in the region of 0.5 eV/O atom. Increased Sb concentration and proximity to the dopant increases vacancy formation energy. 6 - Low reduction enthalpy of the material was not favourable to drive thermochemical water splitting, however isothermal redox cycling demonstrated good performance for the alternative application of thermochemical oxygen separation compared to literature materials. - Antimony donor ions are postulated to lower the cobalt crystal field splitting to support an intermediate spin electron configuration with more favourable orbital filling for fast redox kinetics (eg=1). Effect of iron incorporation in (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites for thermochemical water splitting - Thermal analysis used to observed increasing Fe content coincides with an increase the oxygen production volumes and rates - DFT used to confirm lower vacancy formation energy in positions neighbouring Fe cations. Further predicted to have favourable thermodynamic properties for thermochemical water splitting. - Thermochemical water splitting observed hydrogen production rates similar to literature materials, Ce0.75Zr0.25O2-d. - Surface analysis techniques novel to this research field revealed increased strontium segregation towards the surface that prevented cyclability of the compounds. - Strontium-enriched perovskite surfaces can undergo reconstruction to form derivative phases such as Ruddlesden-Popper oxides, An+1BnO3n+1. Computational screening of n=1 Ruddlesden-Popper oxides for thermochemical water splitting - Screening study uses a combination of well-known crystallographic principles and DFT simulations to narrow down the field of this underexplored metal oxide family for use in thermochemical water splitting. - From an initial 27,899 structures, this study outlines a potential 30 A2BO4 Ruddlesden- Popper structures that have favourable reduction thermodynamics and “synthesisable” under laboratory conditions. - A new simpler and better fitting descriptor based on the lattice enthalpy is proposed to assist future screening work of Ruddlesden-Popper oxides at significantly reduced computational expense. Investigating Ca2MnO4 Ruddlesden-Popper oxide for thermochemical water splitting - Outputted compound from the prior screening study is explored further due its abundant constituent elements and favourable reduction thermodynamics. - Thermal analysis techniques observe similar oxygen production behaviour to the (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites investigated in a previous chapter. - Hydrogen was successfully produced via thermochemical redox reactions cycling between 1000 and 800 °C, thus experimentally verifying the screening study. - Further improvements are suggested by including doping ions to alter the thermodynamics or investigating the effect of perovskite/Ruddlesden-Popper heterostructures that have previously been observed to accelerate oxidation reactions.Open Acces

Annual Report 2016 of the Institute for Nuclear and Energy Technologies (KIT Scientific Reports ; 7742)

The annual report of the Institute for Nuclear and Energy Technologies of KIT summarizes its research activities and provides some highlights of each working group, like thermal-hydraulic analyses for nuclear fusion reactors, accident analyses for light water reactors, and research on innovative energy technologies: liquid metal technologies for energy conversion, hydrogen technologies and geothermal power plants. The institute has been engaged in education and training in energy technologies

Ultra-high temperature concentrated solar thermal energy

Given the extremely high surface temperature of the Sun (~5778 K), solar radiation has the theoretical potential, in accordance with the second law of thermodynamics, to heat a receiver on Earth up to ultra-high temperatures (specified in this thesis as >1300 K). However, there is a gap between theory and practice, as contemporary solar thermal energy systems are still limited to temperatures below 900 K due to material and mechanical limitations. Running solar thermal energy at ultra-high temperatures promises greater energy conversion efficiencies for power plants by upgrading their basic cycles to include more advanced power cycles. Furthermore, the provision of solar thermal energy at ultra-high temperatures can unlock a wide range of energy-intensive industrial applications, including hydrogen and cement production, which can contribute to decarbonising sectors which are difficult to electrify. This thesis proposes a novel concept of an ultra-high temperature solar cavity receiver based on an optically exposed liquid metal heat transfer fluid, which flows down a corrugated back plate. The concept is investigated using a quasi-steady-state analytical energy model, in addition to a radiation-coupled Computational Fluid Dynamics (CFD) solution. The developed analysis methods are tailored to the proposed class of receivers, nonetheless, they can be generalised for broad solar receiver analysis or for analysing similar problems involving volumetric radiation absorption in other thermal applications. The concept is shown implementable at its absorptive cavity configuration with an overall (optical and thermal) receiver efficiency exceeding 70%. The proposed concept is a step towards narrowing the technological mismatch, in terms of temperature and scale, between state-of-the-art thermal energy storage and concentrated solar thermal at ultra-high temperatures. A characterisation of prospective ultra-high temperature receivers is presented, which involved a review of state-of-the-art solar thermal technologies with the purpose of identifying the existing challenges to operating at ultra-high temperatures. Based on this characterisation, the proposed receiver is designed to address the literature concerns. The proposed receiver concept involved novel engineering features, including the use of refractory containment materials and a transparent ceramic window to seal the aperture. Therefore, the conceptual investigation attempted to address possible concerns that might be introduced by the new features. Finally, the proposed receiver is demonstrated in a concentrated solar power plant application to emphasise, using quantitative terms, the benefits of operating the receiver at ultra-high temperatures for large-scale applications

Energy: A continuing bibliography with indexes

This bibliography lists 1920 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System from July 1, 1980 through September 30, 1980