# Passive Solar Solutions for Buildings: Criteria and Guidelines for a Synergistic Design
## 2 Passive Solar Systems
### An Overview of the Common Solutions  
Common categorization classifies passive solar systems into three classes. Direct gain systems collect solar energy through wide south-facing glazed envelopes: low construction cost, widely available and common technology represent the main advantages. No thermal storage is needed when these systems are sized to provide a small fraction of the heating load, but this could generate high indoor air temperature fluctuations. The most relevant disadvantage is related to glare discomfort or possible degradation from ultraviolet radiation [16]. The integration of insulation systems, thermal storage, external reflective surfaces, and shading systems (Figure 2) are the main possible improvements but can hugely impact the total cost. Indirect gain systems include a thermal mass placed between the solar collector and the indoor space that prevents indoor glare and ultraviolet degradation. A main disadvantage that can affect the system's lifespan is the maintenance, as proper access to the wall cavity for dust and condensation removal may be hard to achieve. In isolated gain systems, the collector and the storage are thermally isolated from the building: the main advantage is the independence of the system from the building. Integration of shading systems and reflective surfaces are simple strategies for both new and existing buildings with a low economic impact. At low latitudes, blinds on the east and west windows reduce heat gain by around 20% more than on the south-faced windows, whose contribution increases with latitude [17]. Investigation carried out by [PERSON] et al. [18] shows that self-shading envelope in a multistory building has a lower payback period, 2 years, than additional insulation and low-e windows, with an energy consumption decrease of 20%. In direct systems, U value and solar factors are the most relevant design parameters: for common glass, U values around 1-1.5 W/m\({}^{2}\)K and a minimum solar factor of 0.3 is needed to have positive gain [19]. Low thermal resistance of the glass represents a critical weakness: use of triple glasses is a relevant energy saving  
Figure 1: Conventional equivalent thermal circuit for passive solar heating building systems. Figure 2: Example of a passive solar system, external adjustable blinds applied in the case study building: (**a**) external view, (**b**) interior view, (**c**) louvres detail, (**d**) connection detail. measure in cold climates. Night insulation methods, such as roller-shade devices, are profitable but determine a not negligible extra cost. In massive wall systems, the wall works as a storage mass buffering heat transfer. The greenhouse effect guaranteed by the external glazing improves the performance of the system. The thickness of the wall must be chosen according to the wall's material properties such as periodic and steady-state thermal transmittance, which define heat transfer rate, time lag, and decrement factor. Trombe wall adds convective heat exchange to the massive wall system by adding bottom and top openings to the wall: with the integration of outdoor vents, it can be used as a solar chimney [20]. This kind of configurations have good performance both in warm climates, cutting off 71.7% and 36.1% of heating and cooling demand, respectively, and in warm weather [14]. By exploiting fluid in the storage, water wall's heat transfer occurs by convection and the system is quite isothermal. This solution presents lower heat loss during night-time due to a lower surface temperature, and faster achievement of steady-state operating conditions, compared to other massive systems. Tests on a passive solar house with integrated water thermal storage walls showed an energy consumption reduction of 8.6% [21]. Roof pond systems, Figure 3, integrate water bags inside the roof structure: the large ceiling surfaces assure a more uniform heating distribution, but the effectiveness is limited to single-story buildings with an efficiency around 45%, as less than half of the collected heat is transferred downward.