# Passive Solar Solutions for Buildings: Criteria and Guidelines for a Synergistic Design
## 1 Introduction  
Energy use in the residential sector represents a great share of the global energy demand, attested between 20% and 40% of the total [1]. Each country has developed specific energy efficiency strategies for the civil sector. Firstly, the main attention concerned energy conservation, focusing on the building envelope insulation that can reduce energy demand by up to 28% [2]. Then the focus moved to increasing the efficiency of Heating Ventilation and Air Conditioning system (HVAC) systems, that are the main building energy consumption source and commonly do not operate efficiently [3]. In a nearly zero-energy building (nZEB) perspective, the integration of different strategies regarding envelope insulation, heat generation, shading, and control devices is fundamental to minimize building conditioning loads, so as to be able to maximize the share of energy demand covered by renewable energy sources. It must also be taken into account that residential space heating is responsible for 86% of building energy demand and can easily take advantage from renewable sources [4]. Among the various alternatives, solar energy represents the best option for buildings as one of the most accessible and easily exploitable, even if its current contribution to global energy supply is still imperceptible [5]: it can be directly exploited by heating up the building through solar irradiation, or indirectly by means of photovoltaic (PV) or solar thermal systems. As Table 1 data from International Energy Agency (IEA) [6] show, in a scenario where mandatory building energy codeswill almost totally cover construction activity by 2030, with a nZEB share forecast of 50%, the sales of heat pumps, which can be coupled with PV as a renewable production system, are slowly but constantly increasing, between 0.3 and 0.5% on a yearly basis, with fossil fuel-based equipment that, after a peak in 2014, is gradually decreasing. These data highlight the core role that solar energy plays in building design in the near future: by properly combining photovoltaic and solar thermal systems, indeed, solar energy can provide at least 76% of the primary energy demand of a residential building, with a short payback period [7]. Despite the main contribution in a modern building coming from active solar systems commonly integrated in a new building [8], passive solar strategies represent an energy reduction design feature with a high potential to dramatically reduce building energy demand [9] through energy promotion, buffering and prevention, influencing heating, cooling, ventilation loads, and building's system size. The main advantages of passive solutions include variety, versatility, simplicity, generally low initial and maintenance cost, and long lifetime [10] involving different building components, from walls to roofs. The effectiveness of passive solar solutions depends on a wide variety of design parameters [11]. Like all solar based devices, the performance is mainly related to the latitude, as it influences the average weather conditions and solar radiation on the collector surface. The maximum impact is expressed at medium latitudes, where cold winter temperature and long sunshine create the ideal conditions to exploit passive solutions. The benefit is also related to the interaction with the building and its users as heat delivery, shading, or ventilation must comply with the comfort requirements of the occupants: the magnitude of passive systems' gains becomes more relevant in highly energy consuming buildings like public or office ones. The aim of the present paper is to provide a theoretical background of the parameters influencing the applicability and integrability of the various passive solar solutions in buildings and analyze how their synergy affects building energy performance.