## 1 Introduction  
In general, sustainability is related to natural materials and ecological issues. For example, in the past [PERSON] and [PERSON] compared energy consumption and carbon dioxide emissions to build different types of typical buildings (commercial, industrial, and residential) in New Zealand [1]. More recently, the circular economy paradigm was the basis for designing a facade for a building rehabilitation, quantifying the improvement in energy efficiency with respect to a conventional facade [2]. A review of current and alternative systems to reduce the environmental impacts of road refrigerated transport was proposed to minimize energy consumption and greenhouse gas emissions [3]. Concerning energy savings in metro systems, [PERSON] et al. [4] recently proposed an integrated energy efficient train operation method with a jointly optimized driving strategy and train timetable. On the same topic, [PERSON] et al. [5] proposed an energy efficient train operation method that used the cooperative control approach to minimize the difference between the traction energy consumption and the reused regenerative energy. A perspective can be proposed that is more focused on buildings and sustainability. For example, [PERSON] et al. [4] developed a method for the life cycle analysis (LCA) of a residential building in New Zealand, based on the embodied and operating energy requirements and life cycle cost over the useful life of the building [6]. [PERSON] and [PERSON] [7] analyzed the characteristics and requirements of sustainable buildings in China and recommended different technical approaches for designing and implementing more efficient buildings. [PERSON] and his group provided an LCA of a three-bedroom semi-detached house in Scotland [8]. The authors analyzed five construction materials (ceramic, aluminum, wood, concrete and glass) determining their respective embodied energy and environmental impacts. They found that concrete consumed the greater part of the total energy embodied in the building. Professor [PERSON]'s interest in environmental sustainability dates to the 1970s of the past century at the beginning of his academic career with the first studies on solar cooling [9; 10]. As its main innovation, this paper describes the research studies and results in sustainable energy systems developed by Professor [PERSON] (now retired) and his research group during the 2000-2015 period, when he was working at the Department of Management and Engineering (DTG) of the University of Padova (Italy). To the best knowledge of the authors, no similar studies have been proposed in the literature that go through a long period of work from a research group of a university department on the topic of sustainable energy systems. Many different technologies were objects of the authors' interest and work. During the first years of 2000, Prof. [PERSON] and his research group focused on the topic of thermo-insulating materials [11]. The latter can be chlorofluorocarbon (CFC) and hydrofluorocarbon (HCFC) free, with a great positive effect preventing ozone depletion; nevertheless, an indirect negative effect is expected to increase the energy consumption of a building as a result of lower insulating properties. As a more mature approach to sustainability, the authors proposed to evaluate the environmental effect of thermo-insulating materials along their useful life. Therefore, the research work focused both on the environmental impact during their life (LCA) and on an economic evaluation (life cycle cost, LCC) to evaluate the optimum thickness in different climates (Section 2.1). At the beginning of 2000, another topic related to sustainable energy technologies in building applications was the subject of Professor [PERSON] and his research group's interest, concerning the evaluation of the energy performance of green roofs [12; 13]. As is well known, this is a useful technique to reduce the energy requirements of buildings and to ensure acceptable indoor comfort conditions. Only a few studies had been conducted before.