Preheating the ventilation air by a ventilated double window

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The need to ensure indoor air quality in all buildings it requires air renovation through ventilation. The outgoing air removes warm from inside the building in the winter and the income cold air may result in local thermal discomfort. These thermal losses due to ventilation, results in energy consumption. One way to reduce the inconvenience of undesirable and uncomfortable cold ventilation, associated to heating energy consumption needs is to pre-heat de incoming air with passive systems.
To reduce thermal loss and cold air infiltration in colder zones ofPortugal, traditional Portuguese fenestrations are being doubled by a second single glazed window on the outside of the pre-existing window. The air channel between windows is therefore not ventilated. Adding vents at the bottom of the outer window and at the top of the inner window creates a path for incoming airflow through the gap. This ventilated double window is a passive heating system that provides pre-warmed air for winter ventilation, then otherwise would enter the building at outdoor’s temperature.
A study was carried out at the UBI to investigate the performance of a ventilated double window to preheat ventilation air. This air is pre-heated within the air channel between the windows by heat that is lost from the building and also by solar gains. It is delivered inside warmer than it is outside. Tests were conducted under outdoor weather conditions and considering exclusively natural ventilation.
Experimental work on a south-facing test cell facility tested several full-scale configurations of the ventilated double window being monitored for a range of weather conditions. Furthermore a parametric analysis was used to determine the performance of this passive heating system. The outlet temperature and useful heat gains of the system were plotted as a function of ambient temperature difference, incident wind and solar radiance. This had shown that the results are particularly sensitive to the modelling of the inlet temperature and incident solar radiation.
The case study was used to understand the principles behind operation of this ventilated double window, which works as a heat recover and also as solar collector.However, it is a highly dynamic passive system, reacting to the climatic changes that come across; namely the air temperature, wind and solar radiation, so the system’s success depends on several associated characteristics, like place and the design of the building.

Doctoral Thesis in Civil Engineering (UBI) of Jorge Manuel da Silva Carlos, 2010.

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