http://hdl.handle.net/10174/5270 2026-06-22T19:37:59Z http://hdl.handle.net/10174/42200 Title: Secondary mirrors for homogenization of parabolic trough radiative flux distributions on cylindrical receivers Authors: Martínez-Manuel, Leopoldo; Córdova-Gallegos, Jael; Santos, André; Riveros-Rosas, David; Canavarro, Diogo; Horta, Pedro; Arancibia-Bulnes, Camilo Abstract: For high-pressure applications, like direct steam generation or hydrothermal liquefaction, homogenizing the radiative flux on the tubular receiver of the parabolic trough concentrators can help avoid thermal stresses and potential material degradation. In the present study Monte Carlo ray tracing is used to analyze different secondary mirrors to enhance the flux uniformity level over the receiver tube of four parabolic troughs with different rim angles. Elliptical, V-shaped, Lambda-shaped (inverted V), and Compound Elliptical Concentrator (CEC) mirrors are considered. The design of these mirrors is optimized for each case using a Differential Evolution Algorithm. Solutions are sought with the maximum possible uniformity while keeping high optical efficiency. Results of the optical modeling show reductions in peak flux over the tube wall by approximately 60% when secondary mirrors are implemented with PTC systems. Furthermore, optical efficiencies of 90% and flux uniformity levels of 86.6% can be achieved by integrating the Elliptical and Lambda-shaped geometries as secondary mirrors. The effect of the secondary mirrors on the PTC efficiency for off-normal incidence was also studied. CECs are very good at increasing off-normal rays’ acceptance, followed by V-shaped mirrors, but at the cost of a worsened overall performance. 2025-01-01T00:00:00Z http://hdl.handle.net/10174/42199 Title: Molten Salt Mixtures as an Energy Carrier for Thermochemical Processes of Renewable Gas Production: Review and Perspectives Authors: D'Auria, Marco; Chiara Tizzoni, Anna; Rovense, Francesco; Sau, Salvatore; Turchetti, Luca; Canavarro, Diogo; Marchã, João; Horta, Pedro; Lanchi, Michela Editors: Marotta, Raffaele Abstract: This study provides a comprehensive review of molten salt technology, as well as electrochemical and thermochemical processes aimed at hydrogen and syngas production. First, this research illustrates the current types of molten salt mixtures, detailing their main applications and thermophysical properties. Then, the analysis delves into existing thermo-electrochemical cycles and their specific operating conditions for producing hydrogen and syngas. Moreover, this study assesses the compatibility of these processes with molten salt integration. This investigation involved a comprehensive review of the existing technical and scientific literature, blending insights and practical experiences to offer detailed data on the topics explored. The findings suggest that molten salts, with their medium–high operating temperatures, can markedly improve the efficiency and sustainability of hydrogen and syngas production. Furthermore, this study outlines the pivotal role these technologies can play in achieving the European Union’s ambitious goals by enhancing the use of renewable energy sources and advancing the shift to carbon-free solutions. 2025-07-18T23:00:00Z http://hdl.handle.net/10174/42197 Title: On optimum designs of linear Fresnel solar collectors Authors: Santos, André Vitor; Canavarro, Diogo; Horta, Pedro; Collares-Pereira, Manuel Abstract: This paper investigates the optimal design of linear Fresnel collectors using a two-objective optimization strategy aimed at maximizing annual energy efficiency while minimizing the specific direct cost of the solar field. The analysis then compares Pareto- optimal solutions for non-uniform designs (where geometric parameters such as width, shift, and radius vary) with solutions for variable radius configurations (constant width and shift). Additionally, the study evaluates whether the curvature radius can be effectively decoupled from other opti- mization parameters. The results in this paper show that non-uniform designs generally provide greater cost- effectiveness, particularly in certain regions of the objective space. While some areas show negligible differ- ences between configurations, others distinctly favor non-uniform designs, although the differences are not significant and are reduced as the number of mirrors in the primary field increases. Furthermore, an analysis through dimensionless parameters highlights clear trends among the Pareto solutions. The investigation also reveals that predefined curvature radius criteria, such as the sun reference or Iparraguirre’s model, yield out- comes nearly identical to optimization-based solutions for both uniform and non-uniform configurations. Consequently, curvature radius can be decoupled from other design parameters, significantly reducing compu- tational complexity without compromising performance. The insights presented are valuable for advancing cost- effective designs of linear Fresnel collectors. Nevertheless, the conclusions rely on a simplified cost model, highlighting the need for more comprehensive cost modeling. However, developing such detailed models re- mains challenging due to limited operational data stemming from the low commercial maturity of linear Fresnel technology. 2025-01-01T00:00:00Z http://hdl.handle.net/10174/38825 Title: The governance of the water-energy nexus: Co-produced narratives to take stock and address energy dependencies of the urban water cycle in Atlantic Europe Authors: Willaarts, Barbara; Novais, Maria Helena; González Navarro, José Maria; López Lara, Germán; Morais, Manuela; Zilliox, Erik; Peñate Suárez, Baltasar Abstract: Water and energy (WE) are key resources to support human well-being and are highly interconnected. Intensifying demands of both resources and increasing resource scarcity are exacerbating their interconnectedness and calling for the adoption of an integrated approach called “WE nexus”. This paper explores the barriers and opportunities to govern the WE nexus in the Urban Water Cycle (UWC), particularly, the energy dependencies of the water supply and sanitation services in Atlantic Europe, through the assessment of four contrasting and representative regions: Canary Islands (ES), Western Andalusia (ES), Alentejo (PT) and Brittany (FR). We applied a “Quantitative Story Telling (QST)” method to assess the discourses from 49 stakeholders from across the four regions on WE nexus challenges and opportunities in the UWC, and the evidence that exists on them. The result is a pluralistic narrative incorporating the views of different stakeholders on what are the issues at stake and why, what needs to be done and how, and sustained by available data. The resulting narrative explores the formal aspects underpinning WE nexus governance in the UWC, but also informal rules linked to political economy. Our results revealed that WE nexus challenges are context-specific, however, there are important commonalities across regions and phases of the UWC value chain, suggesting that these are relevant at the Atlantic Europe scale. QST is not instrumental in directly inducing policy change or decision-making, but might be a valuable means for knowledge mobilization to question and enrich the quality of dominant discourses, and thus paving the road for action towards sustainability. 2024-09-30T23:00:00Z