Chimney assisted passive cooling of ODAN transformer radiators under hot ambient conditions Sandra Sorte, Diogo Ventura, Nelson Martins Results in Engineering, 2026 • Validated conjugate CFD for ODAN radiator bank (1.8% Q error, 0.1% Tout). • Radiation adds about 14% of heat rejection and improves natural convection. • Cooling capacity drops 25 to 35% per 10°C rise in ambient temperature (20 to 50°C). • Straight chimneys raise cooling up to 16.2% at 5 m via buoyancy draft. • Convergent, deflector and unified designs reduce Q; radiator upgrades cost less. Power transformers are key grid assets whose reliability depends on effective heat rejection. Oil Directed Air Natural (ODAN) radiator banks rely on buoyancy driven airflow and are therefore sensitive to hot ambient conditions. This study validates a conjugate computational fluid dynamics (CFD) model of an industrial ODAN radiator bank against manufacturer tests, then uses it to quantify hot ambient performance losses and to evaluate passive chimney extensions. The model predicts total heat dissipation within 1.8% and a weighted oil outlet temperature within 0.1% of measurements and shows that surface to surface radiation contributes about 14% of the total heat rejection under natural convection. Parametric simulations over ambient temperatures from 20 to 50°C and oil inlet temperatures from 54.8 to 105°C show a strong monotonic reduction in cooling capacity with ambient temperature, with losses of about 25 - 35% per 10°C increase at fixed oil inlet conditions. Straight chimneys increase buoyancy draft and raise cooling capacity approximately linearly with height, reaching a 16.2% gain for a 5 m extension. Convergent, deflector equipped, and unified chimney variants reduce performance because added minor losses and recirculation weaken plume coherence. A first order cost screening indicates that conventional radiator geometry upgrades provide greater thermal gains per unit cost than stainless steel chimneys under the assessed assumptions. These results provide design guidance for improving the robustness of ODAN passive cooling under hot ambient conditions.
Advancing Domestic Freezers With Phase Change Materials: Experimental Study Towards Commercialization Daniel Marques, Vitor Silva, Nelson Martins, Fernando Neto Heat Transfer, 2025 The urgency for more efficient and sustainable domestic refrigeration systems (DRSs) is intensifying due to climate change events like more frequent heat waves. Such challenges impose reducing greenhouse gas emissions, increasing renewable energy storage rates, meeting the perishable food needs for cooling, and mitigating food wastage through power outages. While previous investigations contributed to these goals by studying the potential benefits of adding phase change materials (PCMs) to DRSs, our study extends their application to chest freezers: a type of system still underexplored. Additionally, it seeks to enhance industrialization and design decision‐making towards tailoring different solutions to distinct markets. Namely, by adopting test procedures closely adhering to the European Standard EN 62552:2013 for experimentally testing four prototypes. The analysis of the novel systems' performance focuses on two metrics internationally recognized but scientifically overlooked by previous peer research: the temperature rise time and the daily energy consumption. A novel approach for filling the top‐mounted door with PCMs and an industrialization technique for simultaneous wrapping PCM bags and evaporator tubes around the freezer compartment are introduced to incorporate PCMs, with melting temperatures (Tm) of −21°C and −12°C. Our findings reveal the potential to extend blackout autonomy by 7%–40% and to reduce daily energy consumption by 13%. Furthermore, the results demonstrate that higher Tm values enhance the commercial attractiveness of DRSs in regions with unstable electricity grids where significant autonomy gains are appreciated, while lower Tm values suit sophisticated markets where extended energy storage capacity and compressor lifetime can be prioritized.
Residential Buildings at Climate Crossroads: Insights from Portugal for South European Energy Performance Alexandre Castro, Sandra Sorte, Vera Rodrigues, Nelson Martins Energies, 2025 This study evaluates the impact of climate change on the energy performance of residential buildings across Portugal’s diverse climatic regions, providing a representative reference for Southern European contexts. Dynamic energy simulations using EnergyPlus were conducted for standardised residential building models in five cities: Bragança, Porto, Lisbon, Évora, and Faro. Three climate scenarios were analysed: present-day conditions (TMY2021), the current regulatory scenario (LNEG-EPW), and a projected mid-century scenario (CCW-EPW). Results indicate substantial regional variations, with significant increases in cooling demands and corresponding reductions in heating needs, exposing limitations in the regulatory climate files currently used in energy certification processes. These findings emphasise the critical need to incorporate predictive climatic scenarios into building design standards and energy policies. Adopting such an approach will enhance residential building resilience, ensure thermal comfort, reduce energy consumption, and contribute to sustainable development goals. These insights offer practical guidance for policymakers, urban planners, architects, and engineers aiming to effectively adapt residential buildings to anticipated climatic shifts, facilitating proactive and informed decision-making to address future energy challenges.
Powering the future: Releasing the potential of phase change materials in domestic refrigeration systems to store renewable energy D. Marques, N. Martins, F. Neto Applied Thermal Engineering, 2025 • 2 billion home refrigeration units represent 4% of global electricity consumption. • This comprehensive review considers leveraging subzero PCMs for storing renewables. • Nanoparticles to boost PCMs’ properties of thermal conductivity and supercooling. • Rating domestic VCR units with PCMs needs uniform criteria to disclose potential. • Simplified numerical tools needed for virtual test benches and parametric studies. Globally, two billion domestic refrigeration systems (DRSs) represent 4 % of electricity consumption and stimulate Demand Side Management (DSM) actions like smart load shifting to balance energy supply and demand. Additionally, they offer the potential for Thermal Energy Storage (TES), which is crucial to revolutionizing thermal batteries for Renewable Energy Sources (RES). Explicitly, leveraging Phase Change Materials (PCMs) can also enhance the coefficient of performance (COP) and resilience to power outages in refrigerators/freezers. Despite extensive research, a comprehensive review addressing the barriers still hindering PCMs’ adoption in DRSs for storing renewables remains absent. This paper fills that gap by covering existent subzero cold storage PCMs, debating corrosion and leakage issues, and summarizing findings from experiments on DRSs with PCMs at the evaporators and compartments. It concludes by reviewing state-of-the-art numerical tools for system design, optimization, and control. Our findings highlight that novel PCMs have improved thermal conductivity and reduced supercooling but require further development toward long-term chemical stability. Experimental studies project up to fourfold autonomy extensions and 50 % reductions in energy consumption, operating costs, and emissions in PCM-enhanced DRSs. Literature reveals that systems with up to 2500 kJ of additional latent storage capacity have been investigated, primarily relying on exhaustive experimental/empirical studies. To advance this field, this review proposes future research directions to unleash the PCMs’ potential for accelerating DRSs’ transformation into advanced thermal batteries for renewable energy storage. Specifically, we advocate for developing simplified dynamic models to enable virtual test benches that support parametric studies and avoid complex CFD simulations.
Development of a framework for decarbonising the electricity supply under climate change scenarios M.A. Russo, D. Carvalho, N. Martins, A. Monteiro Sustainable Energy Technologies and Assessments, 2025 When developing a decarbonisation scenario for future electricity supply, a holistic approach to the electricity generation mix should be taken while considering the climate-induced changes to resource availability and variability. The objective of this study was to develop a framework for a resource-based analysis of the optimal renewable mix to decarbonise future electricity supplies. The Weather Research and Forecasting model (WRF) was used to run very high resolution (≈1 km 2 ) simulations using the latest climate change scenarios (CMIP6 SSPs), to then calculate the wind power density, solar photovoltaic power density and future changes to water availability. A fully decarbonised electricity supply scenario was proposed based on a site selection algorithm that selects the most cost-effective source between wind and solar combined with changes to hydroelectricity. The results show that most of the study area is viable for solar photovoltaic while floating wind is the most cost-effective solution. Overall, wind power has the highest potential contribution to total electricity production, with 54 % for SSP2-4.5 and 46 % for SSP5-8.5. Additionally, the results suggest that the amount of land required for the implementation of renewable installations to meet future demand is equivalent to less than 2 % of the study domain area.
Advancing Power Transformer Cooling: The Role of Fluids and Nanofluids—A Comprehensive Review Sandra Sorte, Alexandre Salgado, André Ferreira Monteiro, Diogo Ventura, Nelson Martins, Mónica S. A. Oliveira Materials, 2025 The ongoing pursuit of enhanced efficiency and sustainability in power transformer cooling systems has spurred extensive research into the properties and performance of insulating fluids. This review explores the evolution of transformer cooling technologies, focusing on traditional mineral oils and the emerging roles of alternative fluids, such as natural and synthetic esters, and nanofluids. Mineral oils, though widely used, degrade over time, leading to a reduction in breakdown voltage (BDV) from 46 kV to 30 kV, exhibiting low fire resistance. Natural and synthetic esters provide improved biodegradability, fire safety but have higher viscosities—potentially limiting convective cooling. Nanofluids, have demonstrated BDV enhancements of up to 47.8%, reaching 88.7 kV in optimised formulations, alongside increases in partial discharge inception voltage (PDIV) of 20–23%. Additionally, thermal conductivity improvements of 5–20% contribute to enhanced heat dissipation. Moreover, it addresses challenges such as nanoparticle agglomeration, sedimentation, ageing, and compatibility with transformer materials. The analysis provides critical insights into the trade-offs between technical performance and economic feasibility. Concluding with an outlook on future research directions, the review identifies key parameters across various categories, establishing a roadmap for nanofluid integration with existing transformer systems.
Power Transformers Cooling Design: A Comprehensive Review Sandra Sorte, André Ferreira Monteiro, Diogo Ventura, Alexandre Salgado, Mónica S. A. Oliveira, Nelson Martins Energies, 2025 Efficient cooling technologies for power transformers are critical to modern power systems, ensuring reliability, performance, and AN extended lifespan. This review systematically analyses advancements, challenges, and opportunities in cooling systems for power transformers. Oil-immersed transformers, widely used due to their superior insulation and effective cooling, require efficient thermal management to prevent overheating and ensure operational stability. This review evaluates key cooling strategies across oil-natural air-natural (ONAN), oil-natural air-forced (ONAF), oil-directed air-forced (ODAF), and oil-forced air-forced (OFAF) systems. It highlights innovations in radiator design, such as top-mounted radiators and chimney caps, and explores sustainable alternatives, including biodegradable esters, nanofluids, and hybrid ventilation methods. Advanced computational tools like Computational Fluid Dynamics (CFD) and artificial intelligence (AI), particularly neural networks, are identified as transformative for optimising cooling performance, predicting thermal behaviour, and enabling real-time monitoring. Despite progresses, challenges persist in radiator optimisation, airflow dynamics, and scalability of innovative cooling methods. By offering a comprehensive review and identifying critical areas for improvement, this study provides a foundation for developing cost-effective, reliable, and environmentally sustainable cooling systems, aligning with the growing demand for efficient energy infrastructure.
Geometry Optimisation of a Wave Energy Converter Susana Costa, Jorge Ferreira, Nelson Martins Energies, 2025 The geometry optimisation of a point-absorber wave energy converter, focusing on the increase in energy absorption derived from heave forces, was performed. The proposed procedure starts by developing an initial geometry, which is later evaluated in terms of hydrodynamics and optimised through an optimisation algorithm to tune the shape parameters that influence energy absorption, intending to obtain the optimal geometry. A deployment site on the Portuguese coast was defined to obtain information on the predominant waves to assess several sea states. NEMOH and WEC-Sim (both open-source software packages) were used to evaluate the interaction between the structure and the imposed wave conditions. The results extracted and analysed from this software included forces in the six degrees of freedom. Under extreme wave conditions, the highest increase in the relative capture width between the initial and final shapes was around 0.2, corresponding to an increase from 0.36 to 0.54, while under average wave conditions, the increase only reached a value of around 0.02, corresponding to an increase from 0.22 to 0.24, as calculated through the relative capture width values.
Enhanced Hg(II) removal using thiourea-functionalized graphene oxide: Lab to pilot scale evaluation Gil Gonçalves, Eddy M. Domingues, Nicole Ferreira, Khuzaim Ranawadia, Bruno Henriques, Ana Bessa, Daniela Tavares, N. Martins, Eduarda Pereira, Paula A.A.P. Marques Separation and Purification Technology, 2024 Here, a thiourea-formaldehyde functionalized graphene oxide (G3DTF) is shown to effectively remove mercury (Hg) from various water sources, including ultrapure, bottled, and seawater. Over 99 % of the Hg in each water source is removed with only 10 mg/L of G3DTF resulting in a residual Hg concentration < 1 μg L−1. This concentration falls within the permissible limits established by the European drinking water guidelines. Notably, the presence of chlorocomplexes in seawater doesn’t reduce the sorption efficiency of G3DTF. The sorption process across all water matrices can be accurately described by pseudo-second-order kinetics, with an R2 > 0.99 suggesting chemical interactions between Hg ions and G3DTF functional groups. The equilibrium isotherms demonstrate a remarkably high maximum adsorption capacity (qm) of 1039 mg g−1, exceeding the values reported in literature for the sorption of Hg on carbon-based materials. Remarkably, G3DTF retains its performance in the presence of other metal ions such as Cu, Cd, and Pb. Incorporating G3DTF into commercially activated carbon (AC) at a concentration as low as 2 wt% significantly enhances the efficiency of Hg(II) removal in fixed bed adsorption. The breakthrough curve exhibits enhanced absorption, attaining a 99.7 % removal of Hg(II) within a span of 2 h, surpassing the efficiency of AC. This relevant result represents a substantial advancement towards the adoption of graphene-based nanocomposites as effective commercial remediation materials.
Code generation for embedded predictive control of gas water heaters André Quintã, Cheila Conceição, Nelson Martins, Jorge A. F. Ferreira Science and Technology for the Built Environment, 2024 Conventional control strategies usually employed in tankless gas water heaters present difficulty in controlling the hot water temperature when subjected to sudden changes in water flow rate. Inadequate control leads to temperature overshoots and undershoots with long settling times that severely affect the user comfort, increasing water and energy wastage and associated gas emissions. A strategy based on model predictive control is presented to reduce the impact of changes in hot water demand. A semi-empirical model, parameterized with experimental data and compatible with real-time simulation, is used for the heat cell. A tailored state observer is proposed, considering time-varying delays characterizing this thermal process. An automatic code generation software tool was developed for the embedded implementation of gas water heater predictive controllers. Numerical simulations and hardware-in-the-loop experiments were established to evaluate conventional and predictive control strategies. It was demonstrated that embedded model predictive control could be successfully implemented on computationally limited microcontrollers, even for thermal systems with extensive varying time delays. Predictive control has shown significant performance improvements, with decreased temperature fluctuations, a gain in comfort index from 36% to 75% and a reduction of up to 32 s in the settling time.
Hydrogen production from salinity gradients Eduardo Durana, Francisco José Almeida Loureiro, Nelson Martins, Duncan P. Fagg Hydrogen Technology Fundamentals and Applications, 2024
Evidence-Based Calibration of an Energy Simulation Model: Dealing with Practical Issues of Data Availability and Granularity in an UK Apartment Block PLEA 2020 35th PLEA Conference on Passive and Low Energy Architecture Planning Post Carbon Cities Proceedings, 2020
Heat flux as a parameter for diagnostic and control of industrial thermal systems International Journal of Heat and Technology, 2000
Heat flux a design, diagnostic and control parameter for thermal equipment American Society of Mechanical Engineers Heat Transfer Division Publication HTD, 1999
