Solar Chimney
A solar chimney is a vertical, partially glazed box with an absorbing surface inside. With a solar chimney the so called “stack effect” can be used to boost natural ventilation.
The stack effect is based on the density difference of air at different temperature levels. As the temperature increases, the density of the air decreases. If the outside air temperature is lower than the air temperature in the solar chimney, thermal buoyancy occurs and leads to negative pressure in the chimney. This forces the outside air, which is at a higher pressure-level, through the building and into the solar chimney, where it rises and is exhausted through an opening at the top. This effect can happen without solar radiation, as long as the air that is to be exhausted is warmer than the outside air. Solar radiation and increasing the height of the chimney can notably strengthen this effect. Solar radiation first falls on the absorber, which heats up. The air in the vertical shaft heats up by convection. This increases the temperature difference between the chimney and the outside air further, and the pressure in the chimney continues to drop, increasing the volume flow through the building and the chimney.
The solar chimney of the demonstration unit is an enclosed space about 15cm deep and 1.5 m long, which overhangs the two floors in height by about 50 cm. It is attached to the western facade and is completely glazed on this side; the back and the interior are equipped with highly absorbent materials.
The interior of the demonstration unit is connected to the solar chimney by an operable opening, which is controlled based on temperature and CO2 levels. When it is open, air is drawn out of the room into the open due to the thermal buoyancy described above. The result is a higher ventilation rate of the interior. If this increased air exchange is not desired, for example due to low outside temperatures, the opening remains closed. The increase in comfort that follows the operability and control strategy of the openings, can be seen in the following diagram. Significantly more hours stay in the comfortable area between the blue borders.
On days that feature high outside temperatures and appropriate humidity levels, the supply air can be cooled down by the adiabatic cooling elements in front of the windows.
