No abstract

Technologies for solar steam generation with high performance can help solving critical societal issues such as water desalination or sterilization, especially in developing countries. Very recently, we have witnessed a rapidly growing interest in the scientific community proposing sunlight absorbers for direct conversion of liquid water into steam. While those solutions can possibly be of interest from the perspective of the involved novel materials, in this study we intend to demonstrate that efficient steam generation by solar source is mainly due to a combination of efficient solar absorption, capillary water feeding and narrow gap evaporation process, which can also be achieved through common materials. To this end, we report both numerical and experimental evidence that advanced nano-structured materials are not strictly necessary for performing sunlight driven water-to-vapor conversion at high efficiency (i.e. ≥85%) and relatively low optical concentration (≈10 suns). Coherently with the principles of frugal innovation, those results unveil that solar steam generation for desalination or sterilization purposes may be efficiently obtained by a clever selection and assembly of widespread and inexpensive materials.Developing sustainable technologies is fundamental for mitigating the anthropogenic impact on environment 1 : global warming and clean water scarcity are progressively impacting our economies and societies [2][3][4] . Therefore, international collaborative efforts have taken place to limit global temperature rise (up to 2 °C respect to pre-industrial levels 5 ) and address clean water scarcity in the most water-stressed areas, where nearly two-thirds of global population will live by the year 2025 6 . Several filtration or distillation processes have been developed for the generation of fresh water from either brackish or sea water. Most of these processes are highly energy intensive and powered by fossil fuels 7 , whereas more sustainable alternatives have been recently investigated to couple clean water generation with renewable energy 8 . Among these methods, solar distillation appears as one of the most economical and practical solutions for small scale desalination, particularly in remote and off-grid areas [8][9][10][11] . Steam generation by solar energy (solar steam) has been also recently investigated in a broad variety of other applications, for instance enhanced oil recovery 12,13 , power generation 14 , sterilization 15 , separation processes 16 or evaporation-driven engines 17 . The present challenge in solar steam generation is to develop robust and cost-effective technologies with enhanced solar-to-vapor conversion efficiencies 18,19 . Currently, large plants for steam generation from solar thermal energy rely on a cavity or surface absorbing solar radiation 20,21 , being the absorbed heat then used to evaporate water directly or by means of a carrier fluid. These solutions suffer from large surface heat and optical losses 18 ; therefore, nanofluid volumetric receivers have been rece...

A sensor for measuring small convective heat flows (<0.2 W/cm 2 ) from microstructured surfaces is designed and tested. This sensor exploits the notion of thermal guard and is purposely designed to deal with metal samples made by additive manufacturing, such as direct metal laser sintering (DMLS). For validation purposes, we utilize both experimental literature data and a computational fluid dynamic (CFD) model: Maximum and average deviations from CDF model in terms of the Nusselt number are on the order of ±13.7% and ±6.3%, respectively while deviations from literature data are even smaller. Similar works in the literature often have the necessity of maintaining onedirectional heat flows along the main dimension of a conducting bar using insulating materials. Such an approach can be critical for small fluxes due to the curse of heat conduction losses along secondary directions. As a result, for industrial applications (due to simplicity and robustness of the design);(ii) for high temperature measurements (unlike foil sensors, no delamination issues can be experienced); (iii) in the context of micro-electromechanical systems (MEMS) (easy to miniaturize).

In the present work, micro-protruded patterns on flush mounted heat sinks for convective heat transfer enhancement are investigated and a novel methodology for thermal optimization is proposed. Patterned heat sinks are experimentally characterized in fully turbulent regime, and the role played by geometrical parameters and fluid dynamic scales are discussed. A methodology specifically suited for micro-protruded patterns optimization is designed, leading to 73 % enhancement in thermal performance respect to commercially available heat sinks, at fixed costs. This work is expected to introduce a new methodological approach for a more systematic and efficient development of solutions for electronics cooling.

The choice of a suitable area to spiders where to lay eggs is promoted in terms of Darwinian fitness. Despite its importance, the underlying factors behind this key decision are generally poorly understood. Here, we designed a multidisciplinary study based both on in-field data and laboratory experiments focusing on the European cave spider Meta menardi (Araneae, Tetragnathidae) and aiming at understanding the selective forces driving the female in the choice of the depositional area. Our in-field data analysis demonstrated a major role of air velocity and distance from the cave entrance within a particular cave in driving the female choice. This has been interpreted using a model based on the Entropy Generation Minimization - EGM - method, without invoking best fit parameters and thanks to independent lab experiments, thus demonstrating that the female chooses the depositional area according to minimal level of thermo-fluid-dynamic irreversibility. This methodology may pave the way to a novel approach in understanding evolutionary strategies for other living organisms.