Towards a GIS-based Multiscale Visibility Assessment Method for Solar Urban Planning

Urban areas are facing a growing deployment of solar photovoltaic and thermal tech-nologies on building envelopes, both on roofs and on façades, essential for the realization of the Swiss Energy Strategy 2050. This process often occurs regardless of the desirable archi-tectural integration quality in a given urban context, which depends on socio-cultural sensitivi-ty and on the visibility of the solar modules from the public space. Visibility and visual impact are recurrent decisional factors in spatial planning processes, with practical implications in-cluding touristic and real estate promotion, outdoor human comfort, way finding, public feeling of security and advertisement. In this thesis, the definition of visibility under a geometrical, physical and psycho-physiological perspective is explored, several quantitative indicators being described and test-ed. The objective is to provide a scale-dependent methodology to assess the visibility of build-ing envelope surfaces exposed to solar radiation, which could host solar modules, in urban areas. A visibility index is determined for inclusion as a variable in a multi criteria method, cover-ing areas from the strategic broad territorial scale to the district level, including neighborhoods and clusters of buildings. Accomplished research includes the estimation of public visual inter-est on the basis of crowd-sourced photographic databases, complementing geometry-based parameters such as cumulative viewsheds and solid angles. At each scale, the visibility index is systematically overlapped on an urban sensitivity layer issued from land use and on a spatial representation of the solar energy generation potential, at an appropriate level of detail. Results indicate that stakeholders can reasonably expect to harvest a serious amount of solar energy by means of building integrated solar systems without crucially affecting public perception. In the study area located in the city of Geneva (Switzerland), more than 50 m2 / building of non-visible envelope surface receiving sufficient solar radiation for an economically viable solar re-furbishment is available over half of the buildings. Solar thermal collectors or PV panels in-stalled on scarcely visible surfaces, mainly situated in courtyards, far from the streets or in deep urban canyons, could cover about 10% of the annual heating demand or alternatively, the same share of electricity needs on a district basis. At the same time, plenty of highly visible areas remain available for high-end solar deployments, which could also serve pilot and demonstration purposes.

Assessing visibility in multi-scale urban planning: A contribution to a method enhancing social acceptability of solar energy in cities

Pietro Florio, Maria Cristina Munari Probst, Christian Roecker, Jean-Louis Scartezzini, Josef Andreas Schuler

Urban areas are facing a growing deployment of solar photovoltaic and thermal technologies on building envelopes, both on roofs and on façades. An effective solar energy planning process considers social acceptance, in relation to the landscape alteration induced by the solar modules. “Visual impact” is often considered as a major component of social acceptance but comprehensive visibility assessment models are lacking at the scale of the city. This paper presents a scale-dependent methodology to assess the visibility of building envelope surfaces exposed to solar radiation, which could host solar modules, in urban areas. A match between annual solar radiation, visibility and socio-cultural sensitivity of the built environment are proposed in a multi-criteria decision framework. Results are illustrated for the city of Geneva (Switzerland), as a case study: a partial overlap between highly sensitive urban areas and high visual interest is identified at the broad, strategic planning scale. In a second more detailed phase, a frequency breakdown of buildings is provided, according to the (non-) visible share of useful roof area for solar energy production. Less visible roofs are more likely to be situated in courtyards, far from the streets, in deep urban canyons or on low-pitched roofs. The outcomes indicate that stakeholders can reasonably expect to harvest a serious amount of solar energy by means of building integrated solar systems without crucially affecting public perception. In Geneva, more than 50 m2/building of non-visible roof surface receiving sufficient solar radiation for an economically viable solar refurbishment is available over half of the buildings. This method is valuable for large districts or cities (i) to spot more/less visible building sets and to estimate adapted precinct refurbishment strategies; (ii) to compare visibility on a common conventional basis and to detect zones deserving further investigations at the finer scale.

2018

Optimisation of Urban Form by the Evaluation of the Solar Potential

Marylène Montavon

What will the cities of tomorrow look like? This is a crucial question because in the four and a half billion years of its existence, the Earth has never been so threatened, as it is today. Since 2008 half of the world's population has lived in towns – more than 70% in rich nations. Three quarters of CO2 emissions are produced in towns, 75% of the world's energy is consumed in towns. The battle for our climate, for the continuation of our species and all other life, will be largely waged in cities. Will it be possible to create the ideal ecological town? The objective of this study is to compare actual and theoretical urban forms in order to explore the diverse effects of daylighting and solar potential on densely built-up sites. Indeed, the search for ideal solar orientations has been a persistent problem from the 19th century until the beginning of the 20th. Even today, for all our scientific knowledge and empirical experience of solar energy, making the correct choice has still not crossed the boundary from an enigmatic, tricky art into the field of an exact science. As a first step, six existing urban sites in Switzerland and Brazil, as well as two projects in the United Kingdom, a few generic models and a utopian city, were modelled and analyzed. The goal was not merely the construction of new, exemplary districts but also the transformation of existing towns. The findings have been used to assess the potential of façades and roofs located in urban areas for active and passive solar heating, photovoltaic electricity generation and daylighting. The results obtained from these different case studies revealed large variations of the potential for solar energy collection on the buildings' façades and roofs. Ideally, these investigations are expected to help architects and town planners understand how to optimise solar collection on buildings design. In addition, a study was made of the Contemporary City of Three Million Inhabitants created by Le Corbusier. According to the latter, the Contemporary City could increase the urban capacity and at the same time improve the urban environment and the efficiency of the city. Therefore the present study extracted the solar potential of this utopian city in order to assess Le Corbusier's propositions. Since 1922, Le Corbusier's studies have influenced architects and town-planners in their choice of orientations and urban forms: it was therefore crucial to discover if this reliance on Le Corbusier was justified. Solar irradiation and illuminance values obtained through numerical simulations form the core part of the method. The method adopted for the entire set of computer simulations consisted in modelling — via a computer-aided drawing piece of software, namely AutoCAD — the digital model of each building or group of buildings prior to exporting them to an urban performance analysis software (PPF/RADIANCE software package). Spatial distributions of solar irradiation and daylight fluxes over the overall building façades and roofs were calculated using ray-tracing simulation techniques to determine the appropriate placement of different solar technologies (passive and active solar, photovoltaic and daylighting). Performance indicators were used to assess the solar utilisation potential of the urban sites (e.g. statistics of sky view factors and daylight factors) in order to determine the optimal solar strategies for a given urban context. In addition, various works and sun-related recommendations that have appeared mainly since the end of the nineteenth century, together with the various scientific models and software tools pertaining to this topic, are presented in this study.

EPFL2010

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.

EPFL2015