Numerical analysis of passive strategies for energy retrofit of existing buildings in Mediterranean climate: thermal mass and natural ventilation combination

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Table 1

Physical and non-physical elements in natural ventilation.

Physical elements

NVS	VT	HET	Description
Openings on the building envelope
CV, SSV, DV, UV	BV, SV, RV	Microclimatic	They represent a context-based necessary feature for NV optimisation and they are widely used, especially in hot and middle season. Openings trigger and define the path of air flow inside the building, being at the same time an effective source of natural light. Wing walls may be used in combination with openings to increase the pressure difference between two windows and therefore the induced natural ventilation [44]
Ventilated facade and roof
CV, SSV, DV, UV	BV, SV, RV	Microclimatic and radiative	Used all year around, they are composed by wall two layers separated by an air gap with openings to the outside and/or inside, allowing the air flow to and from the air gap. During summer, buoyancy air circulation subtracts heat from the building by convection. During winter, the gap may be closed, to exploit the insulating effect of static air, or opened, to dissipate the vapour in the gap, thus avoiding condensation. Passive systems, i.e. Trombe-Michel wall, can allow specific outside-inside circulation
Courtyards, bioclimatic atria, sunspaces and buffer spaces
VP, CV, UV	SV, RV	Microclimatic	Used all year around, these systems act as thermal mediators. Courtyards are more efficient on low rise buildings, while bioclimatic atria are more efficient over a certain building height [45]. Buffer spaces and sunspaces are both thermal and ventilative intermediate environments between the inside and the outside. During winter, they can be used as thermal store for pre-heated supply air, whilst in summer they can be used as air extractors [39].
Wind towers
DV, UV	BV, SV, RV	Geothermal, evaporative, radiative	Used all year around, wind towers are morphological-constructive elements with a vertical development that allows either upward or downward NV. They are usually associated with other ventilation systems, generally at the bottom (such as underground earth pipes or openings on the building), or at the top of the tower (i.e. windcatchers), or in synergy with particular heat exchange strategies, i.e. evaporative system or vegetation.
Buried earth pipes
DV, UV	BV, SV, RV	Geothermal	They are used all year. During the passage in the ducts, air temperature come to be close to ground temperature, which is warmer than the outside temperature in winter and cooler in summer thanks to its thermal stability during the year. This system can be independently used in summer and in winter as air thermal pre-treatment for other heating systems.

Non physical elements

NVS	VT	HET	Description
Manual control
CV, SSV, DV, UV	BV, SV, RV	Microclimatic	Manual control, generally applied to the openings on the building envelope, allows users to adjust the indoor conditions for thermal comfort and air quality according to their subjective perception. If a user can have the direct control on the environment is generally willing to accept a wider range of comfort conditions [46].
Automatic control
DV, UV	BV, SV, RV	Microclimatic	Automatic control requires the installation of sensors capable of measuring the necessary parameters required for the application of control strategies from actuators [47]. Even if automatic control is usually more effective than manual control in terms of energy consumption reduction [40,46], the design of these systems must take into great consideration users' needs without forcing any occupants behaviour.

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