Facade Integration of Solar Thermal Collectors: Present and Future

Based on the results of a European survey on architectural integration quality, the paper presents a systematic analysis of the facade integration potential of solar thermal collectors provided by Swiss manufacturers, concentrating on glazed flat plates, unglazed flat plates and evacuated tubes systems. It demonstrates the poor “integrability” of most of them, indicating that novel collectors responding to facade integration issues should be developed. These new collectors should answer to the technical constraints of their specific solar thermal technology, but should also become architectural elements, conceived to be integrated into the building skin. They should provide an adequate level of flexibility in all the system characteristics affecting the building appearance (i.e. collector material and surface texture, absorber colour, shape and size of the modules, type of jointing). To ease the designer integration efforts and reduce overall costs, they should become multifunctional construction elements, facade cladding being the most relevant added function for glazed and unglazed flat plate collectors. Within this application mode, the use of dummy elements (non-active elements with a similar appearance, fulfilling only the construction function) is a key tool manufacturers should provide to make the geometric/architectural dimensioning of the system independent from the sole energetic sizing. A novel system concept responding to these requisites is presented for glazed systems. New glasses have been developed that are able to hide the absorber behind a coloured reflection while letting the solar energy pass through, providing a new freedom level to architects. The same glazing can in fact be used to cover areas of the facade equipped or not with absorbers, finally opening the way to proper multifunctional active facade systems.

Architectural integration and design of solar thermal systems

Maria Cristina Munari Probst

Although mature technologies at competitive prices are largely available, solar thermal is not yet playing the important role it deserves in the reduction of buildings fossil energy consumption. The generally low architectural quality characterizing existing building integrations of solar thermal systems pinpoints the lack of design as one major reason for the low spread of the technology. As confirmed by the example of photovoltaics, the improvement of the architectural quality of building integrated systems can increase the use of a solar technology even more than price and technique. This thesis investigates the possible ways to enhance the architectural quality of building integrated solar thermal systems, and focuses on integration into façade, where the formal constraints are major and have most impact. The architectural integration problematic is structured into functional, constructive and formal issues, so that integration criteria are given for each architectural category. As the functional and constructive criteria are already recognized by the scientific community, the thesis concentrates on the definition of the formal ones, yet underestimated or misunderstood. The results of a large European survey over architects and engineers perception of building integration quality are presented, showing that for architects formal issues are not a matter of personal taste, but that they relate to professional competences, and consequently can be described. The solar system characteristics having an impact on the formal quality of the integration are identified (formal characteristics), the related integration criteria are assessed, and finally integration guidelines to support architect integration design work are given. The limits imposed by the collectors available in the market are pointed out, showing that the lack of appropriate products is nowadays the main barrier to BIST (Building Integrated Solar Thermal) architectural quality. A methodology for the development of new solar thermal collectors systems responding at the same time to energy production needs and building integration requirements is defined. The importance to ensure, within the design team, the due professional competences in both these fields is stressed. Three progressive levels of system "integrability" are defined in the path leading to the concept of "active envelope systems" and the main role of facade manufacturers is highlighted. The methodology is applied to unglazed and glazed flat plate systems, and new façade system designs are proposed that show the relevance of the proposed approach.

EPFL2009

Reactively sputtered coatings on architectural glazing for coloured active solar thermal façades

Virginie Hody-Le Caër, Martin Joly, Stefan Mertin, Jean-Louis Scartezzini, Andreas Schueler

Covering a standard solar thermal collector with a coloured glazing, which is opaque to the human eye but highly transparent to solar energy, permits a perfect architectural integration of solar panels into glazed building façades. The colours are based on interference in the thin-film coating on the reverse side of the glass. Coloured thin-film filters with optimised energetic performance and angular stability in their coloured reflection were deposited by reactive magnetron sputtering. For substrates up to 100 mm in diameter the geometric configuration of the deposition chamber and the process parameters were optimised. The optical properties of the coatings were determined by spectroscopic ellipsometry and spectrophotometry. Furthermore, by means of a window test bench, the CIELAB colour coordinates of real-size glasses were determined as a function of the viewing angle. It was also demonstrated that the colour of solar collector glazing can be matched to colours of commercial windows. In comparison to uncoated glass panels, the presented coloured samples for solar thermal panels have an energy loss of only 2.8–4.5% at normal solar incidence. This difference reduces for higher angles of incidence. Thus, taking into account the angular distribution of solar radiation, the energetic losses are even lower.

Elsevier2014

Reactively Sputtered Nano-Structured Multilayer Coatings on Architectural Glazing for Active Solar Energy Façades

Stefan Mertin

Improving the aesthetic of solar thermal collectors would grant architects a huge potential for a perfect integration of solar panels into building façades: this can be achieved by coloured interference coatings on the cover glass. These coatings also open the possibility to match the colours of the collectors to those of other architectural components and design elements. An important boundary condition for the coloured filter production is however, to provide a sufficiently high solar transmittance. To deal with the latter, sophisticated numerical simulations are needed to improve the coloured filter design. In addition, a large variety of coating materials is important, ideally with huge differences in their refractive indices, while the absorption of all materials needs to be close to zero. This doctoral thesis focuses on the optical film design and the deposition of new coloured thin-film multilayers with an optimised energetic performance and an only little varying coloured reflection at different viewing angles. To access more directly the correlation between solar transmittance, light spectra and visual and energetic performance of the coatings, a thin-film simulation framework has been developed on the basis of Mathematica. New coating designs have reduced the number of parameters necessary to be adjusted and tuned during the coating-development process: this could help to shorten the prototype development phase on industrial coaters significantly as well as the whole coating-development cycle. Based on the new developed coloured filter designs, multilayer stacks were deposited via reactive magnetron sputtering. The deposited coatings exhibit a solar transmittance Tsol > 87%. For greenish colour hues even a Tsol up to 91% was reached, which corresponds to less than 1% loss in Tsol (respectively solar energy performance) in comparison to the uncoated glass substrate. Furthermore, the research deals with the deposition of low-refractive magnesium fluoride (MgF2) and MgF2 containing composites. Hereby, a novel deposition process by reactive magnetron sputtering was developed for MgF2 using a metallic magnesium target and a reactive gas mixture of oxygen (O2) and carbon tetrafluoride (CF4) diluted with argon. The obtained MgF2 films exhibit a very low refractive index of n = 1.382 at 550nm, which is congruent with the polycrystalline bulk, and only a very weak absorption in the whole solar range (300-2500nm). In addition, a novel approach to deposit Mg-F-Si-O composite materials by magnetron co- sputtering from compound MgF2 and silicon dioxide (SiO2) targets was performed. The obtained films show a two-phase system with nanocrystals of MgF2 embedded in a SiO2-rich amorphous matrix. The lowest achieved refractive index amounts to n = 1.424 at 550nm combined with a negligible extinction coefficient (k < 10-9). The investigated sputtered low-index materials open-up manifold possibilities for coloured filters. For instance, by adding a supplementary MgF2 inter-layer, a colour-invariant solar transmittance of Tsol = 85-85.7% could be achieved, as shown by numerical simulations. With this novel optical filter design it will be feasible to offer a whole colour palette for solar thermal collectors exhibiting all the same solar thermal energy performance.