Development of advanced materials for solar receivers:

Short description

One of the greatest challenges today is the design of efficient high temperature solar receivers. The solar receiver can be considered as the key component of a solar thermal power plant, as it is responsible for a highly efficient conversion of solar energy into heat that is subsequently fed into the working fluid of the attached power cycle. Especially in the case of power tower plants working at temperatures above 600 ºC efficient energy conversion is not easy to achieve, and requires particular optical properties such as high solar absorptance and low thermal emittance. In cases where the bulk receiver material itself does not provide these properties, the application of surface coatings, paints and/or surface structures is crucial.

The Solar Thermal Energy Department at CENER explores the possibility of increasing the solar absorptance by surface textures without the application of paint or coatings. The idea is to increase the solar absorptance of the solar receiver material by increasing its apparent absorptivity. In particular, the geometries designed and manufactured are based on cone dumps, Wood’s horn and compound parabolic concentrators (CPCs) using hexagonal honeycomb-like shapes for efficient distribution over the surface. The material under consideration is a high-temperature metal alloy (cobalt-chrome), a typical candidate material for tubular or opaque-heat-exchanger-type solar receivers. The manufacturing technique is 3D printing.

F. Zaversky, F. Sallaberry, M. Ezquer-Mayo, I. Pagola, L.S. Rios, A.M. Espiago, A parametric study of innovative light-trapping surface structures to increase solar absorption of high-temperature solar receiver material, A. Méndez-Vilas (Ed.) EMR 2015, BrownWalker Press, Madrid, Spain, 2015.

Main technical features

The underlying idea of increasing the apparent absorptivity is to design beam dumps or light traps, that is, specially shaped surface structures that reflect the incoming light multiple times, thus trapping it deep inside the structure, ensuring that only a small fraction of the incoming rays can escape. The original beam dump is most probably Wood’s horn, named after the American physicist and inventor Robert Williams Wood. It is a gently curved and tapered tube where the gentle taper traps the specular reflections and the curved shape leads to multiple bounces before light may escape.

Innovative aspects

Novel light-trapping geometries investigated are (1) a compound parabolic concentrator (CPC) in the form of a surface of revolution. The geometry is defined by the aperture radius (R) of the CPC and its angular acceptance half angle (Theta). (2) A combination of a CPC shape in hexagonal arrangement and a simple cone. (3) A two-dimensional cone dump as presented by Hobbs. It has been extended to a three-dimensional shape, having a hexagonal base area on that a simple cone is placed (the cone’s base circle touches the hexagon). The cone is surrounded by a hexagonal hollow prism having the same height.

It has been shown that surface structures can significantly enhance solar absorptance. Based on a reference absorptance value of roughly 40% for the base material without surface structure, the examined light-trapping surface structures can double the solar absorptance, reaching values around 80% (of course depending on the parameter combinations).

Applications

The advanced light-trapping structures are typically applied at opaque-heat-exchanger-type solar receivers, which can use pressurized air as HTF (solar combined cycle).

Under the framework of the project MIRASOL the topic was studied in detail (funded by Spanish National program and ended in December 2015)

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Type of partner sought

Companies,Research organizations

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Testing of prototype in the relevant environment.