# Passive Solar Solutions for Buildings: Criteria and Guidelines for a Synergistic Design
## 2 Passive Solar Systems
### Synergy of Influencing Parameters  
A generic model for passive solar systems, like the one in Figure 1, is internally ruled by three main resistances related to heat exchange among room, ambient, and storage, and characterized by a capacitive massive element. It is also externally stressed by the incident solar radiation on the collector. In summer, shading systems represent the most relevant resistance, while in winter, all kinds of solar radiation contribute with specific resistances. In building applications, materials and structures work in synergy with the  
Figure 4: (a) Angles related to sun position; (b) maximum daily solar irradiance per month at different latitudes on a horizontal, h, and a vertical, \(\
u\), surface.  
elements previously analyzed to define passive system operativity. Glass properties are the first element that intervene. Once the incident solar irradiance has been determined with Equation (1), the energy transferred inside the building can be evaluated with the following balance:  
\[H\,\times\,A\,\,=\,\,Q_{t}\,\,+\,\,Q_{r}\,\,+\,\,Q_{a,out}\,\,+\,\,Q_{a,in} \tag{5}\]  
where \(H\) is global solar radiation on the window, \(A\) is windows surface, while subscript \(t\) represents the amount of energy transferred, \(r\)--the reflected share, \(a\),_out_--the energy absorbed and retransferred outside, whereas \(a\),_in_--the energy absorbed and transferred inside the building. The solar factor (\(g\)), the main parameter related to glass, can be defined as the percentage of solar energy incident on the glass that is transferred indoor, as showed in Equation (6).  
\[g\,\,=\,\,\frac{Q_{t}\,\,+\,\,Q_{a,in}}{H\,\times\,A} \tag{6}\]  
The solar factor depends on the composition and properties of the glass: from single to low emission glasses, the solar factor is almost halved, due to highly reduced transmittance.  
The type of building structure and envelope materials affect the system's integrability [11] and the opportunity of exploiting building elements for heat storage with no extra cost. Integrated heat storage reduces indoor temperature fluctuation and extends the performance of a passive system. Density, specific heat capacity, and thermal diffusivity influence the rate of heat storage and the useful thickness involved in the process: high density and mass guarantee high heat capacity, while the higher the thermal conductivity, thus the diffusivity, the more readily the storage system absorbs heat and the deeper the material thickness involved is. The minimum thermal storage surface recommended by Balcomb [28] is six times the solar collector glazing area. Passive solar design solutions can be grouped according to the integrability, as shown in Figure 5, based on the kind of structure, light or heavyweight, and the suitability to new or existing buildings.  
The building use and occupant's activity schedule are another set of parameters affecting the operation of a passive solar system: living pattern that varies according to the building use defines the period of comfort and that of possible setback. In office buildings, the main activity is focused on the middle of the day, from 7.00 a.m. to 5.00 p.m., while in residential buildings the time slot shifts to evening and morning. Educational buildings and offices, with a prevalent daytime use, are the most suitable for directly exploiting the heat provided by the sun. In residential buildings, the efficacy of passive solutions is related to heat storage effectiveness to deliver heat during night or cloudy days.  
Figure 5: Classification of passive solar design strategies.