I. Visa, M. Moldovan, M. Comsit, A. Duta
Transilvania University of Brasov,
Keywords: solar-thermal facade, solar-thermal collector, solar-thermal infield efficiency, solar-thermal materials
Summary:Increasing the share of renewables in the today’s energy production formulated specific targets for the built environment as one of the largest consumer. The concepts of (Nearly) Zero Energy Buildings emerged thus as a need to meet the sustainability goal. Solar energy convertors are key actors in the renewable-based energy mixes. Building integrate solar-thermal systems represents a well-accepted solar technology; however, increasing the share of thermal energy for domestic hot water and for heating asks for using also the highly visible places on a building, as the facades. Solar-thermal facades require good conversion efficiency, considering the inherent losses in solar radiation incident on the modules, due to the vertical mounting; however, this can also be an advantage during hot seasons, by shortening the stagnation periods, thus increasing durability. An additional key pre-requisite is the architectural acceptance, as most of the traditional flat plate collectors have rather uniform geometry and colors, hardly acceptable if extensively implemented. This is why a new flat plate collector was designed and developed, as a building block for “lego-type” solar-thermal facades, with various geometries. The collector has isosceles trapeze shape (0.67 m2), and can be manufactured in a broad variety of colors (red, blue, green, granite, or their combinations). Environmentally friendly techniques (sol-gel) are used to obtain the dispersion that is sprayed at room temperature on the conditioned aluminum substrate. Additionally, self-cleaning thin films (SiO2/TiO2/Au nanoparticles) were deposited on the glazing and allowed the photocatalyitic decomposition of organics, while slightly enhancing the optical properties of the solar glass substrate. Combinations of commercial and novel dispersions used to develop the colored spectral selective absorber plates allow to develop a set of nine different prototypes, with nominal efficiencies up to 61.5% measured on the indoor testing rigs. These prototypes were further used to develop a solar thermal façade, implemented in a temperate-continental mountain climate (Brasov, Romania: 45°40'08.6"N, 25°32'57.8"E, 500 m above the sea). The façade allows various interconnection architectures of the collectors (serial/parallel), aiming at validating a concept that supports easy mounting/dismounting and maintenance: groups of three trapeze collectors serially connected, delivered as such by the manufacturer and customized according to the façade design. This paper presents the results of the infield monitoring of the new solar-thermal façade. The analysis of the experimental data outlines: - The infield thermal energy response is close to the nominal efficiency, particularly during the cold and mild months (November – March). - Groups of three serially interconnected collectors are efficient; the addition of a fourth, fifth... collector results in a lower overall efficiency. This is particularly true for the cold seasons, thus a further concept on seasonal changes in the interconnection architecture is proposed. Based on these results, novel façades are proposed, parallel connecting groups of three trapeze collectors serially connected; the coverage degree and the thermal energy output are comparatively analyzed with the same facades covered with commercial, rectangle collectors; increases up to 260% are demonstrated for South-facing facades of single family houses.