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

Urban areas are facing a growing deployment of solar photovoltaic and thermal technologies on building envelopes, both on roofs and on facades. 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 m(2)/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.