a b s t r a c tIn the last decade, different absorber geometries, such as foams and honeycombs, have been tested at laboratory or industrial scale in order to achieve high performance in the conversion of the solar radiation into usable heat, with the current state-of-the-art, the HiTRec-II monolithic honeycomb, characterized by a square-channel section and made out of siliconized silicon carbide (SiSiC). Such geometry has been so far the best compromise for large-scale application thanks to the low production costs, easy manufacturability through extrusion procedure and overall acceptable performance. However, it does present some drawbacks, since the geometry is not able to contain the radiative heat losses, especially from the front surface. An optimized absorber geometry, capable to reduce overall thermal losses, is presented in this work, being able to increase the final thermal efficiency of more than 12% compared to the current state-of-the-art and showing the presence of the so-called volumetric effect, since the outlet fluid temperature is higher than the solid inlet temperature. A test sample has been produced for laboratory-scale experiments, in the form of a 3:1 scaled prototype through additive manufacturing procedure, using a titanium-aluminium alloy (Ti6Al4V) and the experimental results were in good agreement with the numerical calculation, with a deviation of 3%, computed considering a 3:1 Ti6Al4V scaled-up sample. As the manufacturing technology will progress and become cheaper in the near future, it will be possible to improve the overall Solar Power Tower (SPT) plants performance using advanced micro-geometry for open volumetric receivers.
a b s t r a c tSolar Tower Technology is a promising way to generate sustainable electricity from concentrated solar radiation. In one of the most effective variants of this technology, a so called volumetric air receiver is used to convert concentrated radiation into heat. This component consists of a high temperature resistant cellular material which absorbs radiation and transfers the heat to an air flow which is fed from the ambient and from recirculated air. It is called volumetric, because the radiation may penetrate into the "volume" of the receiver through the open, permeable cells of the material. In this way a larger amount of heat transfer surface supports the solid to gaseous heat transfer in comparison to a tubular closed receiver. Finally the heated air is directed to the steam generator of a conventional steam turbine system. In this study an advanced cellular metal honeycomb structure has been designed, manufactured and tested for use as an open volumetric receiver. It consists of winded pairs of flat and corrugated metal foils. The technology is based on a one which has been primarily developed for the treatment of combustion engine exhaust gases. A number of variations of the pure linear honeycomb structure have been introduced to increase local turbulence and radial flow. Firstly, a set of samples has been tested in laboratory scale experiments to determine effective properties and the solar-to-thermal efficiency. After that, results have been compared with theoretical predictions. Finally, the three most promising materials have been used for a 500 kW test on the research platform of the Solar Tower Jülich. Air outlet temperatures of more than 800 C have been achieved with efficiencies of about 80%, which is about 5% more than the state-of-the-art technology, which is currently used at the main receiver of the Solar Tower. Next to this, lifetime models will be developed to increase the overall reliability of the technology.
The complex response characterizing elastomeric isolation bearings is reproduced by employing a novel uniaxial hysteretic model that has been recently formulated and successfully implemented in OpenSees. Such a novel OpenSees material model offers several advantages with respect to differential models typically available in commercial software products for structural analysis, such as 3D-BASIS and CSi programs. Firstly, it is based on a set of only five model parameters that have a clear mechanical significance; such a property not only allows one to drastically simplify the parameters identification process, but it also allows the model to be used in practice. In addition, the model does not require numerical methods for the evaluation of the restoring force since the latter is computed by solving an algebraic equation. To encourage researchers and designers to adopt the proposed model for research and practical purposes, we demonstrate its accuracy by performing some numerical tests in OpenSees. In particular, we first employ the recently implemented model to compute the nonlinear dynamic response of a seismically base-isolated structure with elastomeric bearings and, subsequently, we compare the results with those obtained by modeling the seismic isolators with the OpenSees BoucWen uniaxial material model, that is one of the most popular and accurate hysteretic models currently available in OpenSees.
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