# Passive Solar Solutions for Buildings: Criteria and Guidelines for a Synergistic Design
## 6 Conclusions  
Passive solar systems hide a huge potential as energy saving measures for buildings. The integration of different cooperating solutions in buildings is a relevant asset towards a nearly zero-energy building perspective. The design process is ruled by the synergy of various parameters, both intrinsic and extrinsic, that affect the performance and integrability of the passive solar system. In the paper, the authors have tried to define a general model for the definition of the performance of a passive solar system. The model is based on the combined consideration of direct and indirect thermal resistance and capacitive elements: it takes into account the massive capacitance of passive solar strategies in buildings and solar radiation variable resistances due to shading systems in summer and surrounding condition in winter, affecting the exploitation of diffuse and reflected solar radiation. As analyzed in the paper:* passive solar systems' standard design depends both on intrinsic parameters such as latitude and orientation that influence the effectiveness of the solution, and on the correlation of extrinsic factors like building structure and destination, which affect their integration, heat storage, and cost, all operating in a synergistic way;
* as a result of the general analysis, direct systems showed the highest energy saving percentage for offices, 28.2% and 38.5% for Rome and Frankfurt, respectively, in the rotated configuration, while the indirect solution performed better in residential application, with the Trombe wall rotated setting that reached a saving of 12.8% in Frankfurt and 22.3% in Rome;
* building use also influenced the latitude impact on energy savings: while in offices the saving increased with the latitude, the opposite occurred in residential buildings. Direct systems showed higher sensitivity to latitude, around 10% variation of the energy saving, compared to the 2-6 percentage points in other cases, except for the indirect system in a residential building;
* the sensitivity to orientation variation decreased with increasing latitude, but generally south-oriented systems performed better than east-west ones: direct and indirect systems showed a higher range of variation, respectively, in Frankfurt for the office, 7.7%, and in Rome for the residential application, 12.8%;
* specific passive solar solutions on an existing public office building did not exceed 12%, while the synergy of different solutions showed a reduction of energy consumption of almost 20%. The application of synergistic passive solar systems can enhance the performance, saving more than 50% of the heating energy demand and up to 36% of the cooling load. The integration of passive solar systems is hampered by performance evaluation: a simple energy balance disadvantages solar energy, characterized by low available power and low conversion efficiencies, while in an economic analysis, low energy costs, subsidized by some local government, makes them less cost-effective. Due to higher reliability and easier design, the integration of active devices has become a more common practice in the building market. In a long-term time frame, considering energy and carbon payback times, solar energy-based interventions, even for energy retrofit, proved to be profitable [7]. Future activity will continue both quantifying the impact of influencing parameters on passive solar system performance to develop a general framework useful to building design, analyzing combined solar systems. Research will focus on a proper way to evaluate the benefits of a passive solar design accounting on energy evaluation and energy degradation, by developing a multicriteria optimization function that considers each contribution of passive solar strategies. Conceptualization, A.F. and F.F. ; methodology, A.F. ; software, G.C. ; writing--original draft preparation, G.C. ; writing--review and editing, A.F. and F.F. ; visualization and supervision, A.F. and F.F. All authors have read and agreed to the published version of the manuscript.