# Solar Energy in Urban Planning: Lesson Learned and Recommendations from Six Italian Case Studies
## 1 Introduction  
Cities represent the place where 80% of the gross domestic product is generated and where around half of the world population lives, with expected growth to two-thirds by the middle of the current century [1]. The combination of these two factors demonstrates why urban areas are considered responsible for up to 75% of the global energy consumption and more than 70% of energy-related carbon dioxide (CO\({}_{2}\)) emissions [2]. In this context, the decarbonization of the energy sector, in line with the Paris Agreement's goals to keep the temperature increase below 2 \({}^{\circ}\)C, requires a substantial contribution from cities. A growing number of cities, mainly located in the European Union (EU) and North America, have set renewable energy targets to support the green energy transition [3]. At the municipal level, the adoption of regulations, economic incentives, and the direct financial involvement of municipalities and public authorities, combined with the storytelling of successful practices, proved to be crucial in raising public awareness and in steering private developers toward sustainable strategies [4,5]. Similarly, in the past decades, national level measures constituted a strong enabler for the exploitation of renewable energy sources (RES) [6]. Solar photovoltaic (PV) and solar thermal (ST) had respectively the first (36%) and the fourth (10.5%) highest expansion rate among the RES in the past 30 years [7]. In one of the preconfigured future scenarios, the International Energy Agency (IEA) is expecting that PVs would cover almost one-third of the new electricity demand within 2030, with an average annual growth rate of 13% [8]. Utility-scale ground-mounted systems dominate the market [9]; however, several potential issues related to their impact on the ecosystem, landscape, and competition with agricultural land are emerging [10,11]. The major alternative is distributed rooftop systems (i.e., applied and integrated systems), representing a market share of 40% in 2020 [12]. This technology has the highest potential use in the built environment, especially for photovoltaic systems fully integrated into the building envelopes (i.e., facades and roofs), which perform the double function of energy producer and building cladding. The advantages of building-integrated photovoltaics (BIPV) and solar thermal (BIST) are manifold: unused surfaces can be turned into active energy generators [13], losses associated with transmission and distribution of electricity are reduced thanks to on-site production [14], and higher energy flexibility toward extreme weather conditions is guaranteed [3]. On the other hand, the installation of such systems in urban areas, especially on vertical surfaces (i.e., facades), requires considering complex phenomena such as inter-building solar reflections and overshadowing effects [15, 16, 17], and other energy and climate-related issues, such as high surface temperature [18] and fire hazard [19]. The visibility and the socio-cultural sensitivity impact of the systems should also be considered [20, 21, 22, 23]. In Europe, BIPV accounted for a cumulative installed capacity of 6.9 GWp at the end of 2019, with Italy covering about 38% of the total, due to the early campaign of statal incentives to boost the adoption of PV technology [24]. Other two viable alternatives, representing small market niches in rapid expansion, are floating solar and agrivoltaics [12]. These technologies allow to reduce land use for solar installations and through appropriate design can provide a series of benefits. Floating solar has the advantage of reducing the evaporation of water reservoirs. Furthermore, the presence of water helps to keep the panels' temperature low, stabilizing the efficiency of the system [25,26]. Similarly, agricultural PV can improve crop yield, limit evaporation, provide shade to livestock or crops, and protect against extreme weather conditions and soil erosion [12,27].