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Optical properties of solar absorbers : results on Round Robin and guidelines

Publication . Carvalho, Maria João; Páscoa, Soraia; Sallaberry, Fabienne; Wilson, Helen Rose; Fischer, S.; Loren, Anders; Bohren, Andreas; Nielsen, Jan Erik

ABSTRACT: The scope of the standard ISO 22975-3:2014 is the determination of the long term behavior and service life of selective solar absorbers for use in vented flat plate collectors used in typical solar domestic hot water systems or combisystems. With this scope, this standard can be the basis for the certification of solar absorbers in the framework of Solar Keymark European Certification Scheme. Measurements of optical properties are fundamental to support this certification. Six test laboratories participated in a Round Robin test for the measurement of solar absorptance and thermal emittance of different absorber coatings. The samples used were one PVD coating and two selective paint coatings all on aluminum substrates. The results were submitted anonymously and were analyzed according to standard ISO 13528:2015. The results obtained showed good agreement among the measurements made by the participants and allowed the preparation of guidelines to guarantee quality of the measurements of optical properties of solar absorbers. A proposal to include absorber coatings in the Solar Keymark certification scheme was prepared.

2018Conference object

Open access

Tintas seletivas para aplicação em sistemas de energia solar térmica

Publication . Alves, Isabel Nascimento; Carvalho, Maria João; Cunha Diamantino, Teresa

RESUMO: As tintas espectralmente seletivas são atualmente uma potencial alternativa a outros revestimentos seletivos, como os obtidos por métodos de deposição a vácuo, para sistemas de energia solar térmica, tendo como benefícios a utilização de técnicas de aplicação e manutenção mais simples, disponibilidade comercial a mais baixo custo e elevada durabilidade. Existem apenas algumas tintas comerciais capazes de produzir revestimentos com elevado desempenho ótico e simultaneamente elevada estabilidade térmica, para algumas aplicações. Estas tintas são referidas como seletivas sensíveis ou insensíveis à espessura (TSSS e TISS). Este trabalho apresenta estas tintas seletivas, com referência aos constituintes que têm merecido maior atenção, a relação entre eles e as características óticas dos revestimentos e desempenho obtido nos diversos estudos efetuados, tanto em termos de eficiência das propriedades óticas, como de durabilidade de materiais. Pretende-se ainda dar uma visão atual da possibilidade de aplicação das tintas seletivas em sistemas de energia solar térmica a baixas-médias e altas temperaturas.

2020-11Conference object

Open access

Accelerated aging tests to selective solar absorber coatings for low temperature applications

Publication . Cunha Diamantino, Teresa; Gonçalves, Rita; Páscoa, Soraia; Alves, Isabel Nascimento; Carvalho, Maria João

ABSTRACT: The need of a higher role of solar energy within the energy mix in the coming decades obliges the collectors to increase their performance and reliability. It is demanded that the solar absorbers, as the key component of solar thermal collectors, should be low cost with high efficiency for extended lifetimes under different kinds of environments. Commercially, there are presently two main types of solutions as selective solar absorbers coatings for low temperature applications: coatings obtained by physical vapour deposition (PVD) and by paint coatings (PC). These coatings present different physical and chemical characteristics. Therefore, it is important to know how these commercial coatings degrade over time in different environments. Results obtained with two different PVD coatings and three PC, under different accelerated aging tests, are presented. The aging tests performed included different environmental stress corrosion conditions: temperature, humidity, chlorides, sulfur dioxide and nitrogen oxides. The chlorides and the gases SO2 and NO2 affect distinctly the different selective coatings. Cyclic variation of corrosion promoting gases (sulfur dioxide and nitrogen dioxide), higher humidity, salt spraying and drying seem to be an aging test that reflects the different environments where the solar thermal collectors are exposed. In addition to the contaminants, drying/wetting cycles also play an important role in degradation mechanisms of absorber coatings. So, the international standard ISO 22975-3 needs to be revised to include cyclic conditions with these contaminants.

2021-10Journal article

Open access

The importance of developing accelerated tests on the reliability of solar absorber and solar reflector coatings

Publication . Cunha Diamantino, Teresa; Gonçalves, Rita; Páscoa, Soraia; Alves, Isabel N.; Carvalho, Maria João; Fernández-García, Aránzazu; Wette, Johannes; Sutter, Florian

ABSTRACT: Selective absorber coatings for solar thermal collectors and reflector coatings for Concentrating Solar Power (CSP) plants are the key components of these technologies and their durability is one of their most important characteristics. They should be low cost and withstand 20-30 years under different kinds of environments without significant loss of optical performance. Commercially, there are different physical vapour deposition (PVD) coatings for aluminium absorbers and reflectors. Results obtained with two commercial PVD solar absorber coatings (SA) and two solar reflector coatings (SR), under different accelerated aging tests (AAT), are presented and correlated with outdoor exposure. An Outdoor Exposure Testing (OET) site with maritime and industrial influence was used for an exposure campaign of the absorbers and reflectors for two years. Alternative artificial aging tests are proposed for absorbers and reflectors that better reproduce the corrosion mechanism obtained in natural conditions with maritime and industrial influence. The characterization of the coatings and degradation mechanisms of different aluminium absorbers and reflectors were evaluated optically, morphologically and chemically. The results obtained in an atmosphere with high corrosivity as in marine and/or industrial areas are a reliable way to verify the corrosion resistance of new materials in a short time and are a valuable tool to validate the different methodologies of accelerated aging tests.

2022-11Conference object

Open access