A MODEL FOR NATURALLY VENTILATED CAVITIES ON THE EXTERIORS OF OPAQUE BUILDING THERMAL ENVELOPES
This paper describes a model for naturally ventilated cavities that are formed by lightweight baffles on the exteriors of opaque building thermal envelopes. The model can be used for building components that are slightly detached from the main envelope (but do not connect to the interior), such as photovoltaic panels, double roofs, rain screens, and unglazed transpired collectors. The model provides values surface temperature as well as exterior conditions for baffle boundary modeling heat transfer in the underlying surface. Two quasi-steady energy balances are used to for convection and radiation to use in solve for temperatures: one for the baffle surface and the other for the air in the cavity. The model assumes that the heat capacity in the baffle can be neglected, since it is much lower than the underlying mass surface. Natural ventilation rates are calculated with simplistic correlations that are based on user-provided discharge coefficients. Convection coefficients inside the cavity are based on those used for window gaps as documented in ISO (2003) Standard 15099. Radiation coefficients are linearized. The model has been implemented in EnergyPlus.
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