**4. The Ventilated Double Light Pipe**

People living or working in underground environments of buildings need both natural light and ventilation for well being. When air is made to flow by mechanical ventilation a considerable amount of electric energy has been consumed. Light pipes are often equipped by mechanical ventilation devices in order to assure comfortable optical and hygienic conditions for the occupants. For this need, the authors propose a modification of the DLP so that it can be used as an efficient tool for natural ventilation in the passage area in a twolevel underground building, allowing a significant energy saving. This device is named Ventilated Double Light Pipe (VDLP).

The Ventilated Double Light Pipe (VDLP) is a further transformation of the DLP which geometry is modified to optimize the air coming in and out. To be more precise, in the VDLP, the inner tube is narrowed at the top, in order to generate a convergent section at the top to improve the air extraction, while the outer tube is narrowed at the bottom, so the hole between the two tubes has a convergent section downward for the air to come in. In this way each device can be alternatively used as an inlet or an outlet system. Particularly, in the inlet VDLP the inner pipe is closed at the top and the bottom, while at the outlet the air cavity between the two pipes is closed at the top and bottom, as shown in Figure 9.

Fig. 9. Qualitative air circulation in an inlet and outlet VDLP
