**3. The Double Light Pipe**

270 Solar Radiation

The main goal of this work is to propose an architectonical space equipped with a certain number of VDLP, in which the geometry is generated by the availability of daylight and the

The lack of daylight in buildings is often a cause of large amount of electric energy consumption. This problem is particularly present in the underground buildings or in industrial or commercial large plant area edifices in which electric light is used all the time in the spaces occupied. In recent years many technological devices, called light pipes, sky lights or tubular sky lights, have been proposed in order to offer a solution to the problem. They are able to collect natural light with a mobile or fixed collector and redirect it into the

Light pipes with mobile collectors can be equipped by heliostats that are solar tracking devices able to rotate with respect to one or two axis, so optimizing the direct solar radiation

Fixed collectors are cheaper than mobile collectors. They are usually constituted by a transparent polycarbonate dome with the function of collecting natural light from the sun and the sky and redirect it into the tube. In many cases they also have Fresnel lenses able to concentrate direct solar radiations coming from various directions in the direction parallel to

The entrance point is in most cases located on the roof-top of the building or alternatively on one of the external walls. A free collection of light is needed for optimizing the efficiency of the device so that any obstruction of solar radiations from adjacent structures may be

The tube is internally covered by a highly reflecting film along its length from the collector to the diffuser for long-distance transport of natural light in the underground areas or spaces without any direct interface to outdoor. The larger the diameter of the collector, the longer the distance that can be covered by light with minimum dissipation of energy.

Light pipes on the market have diameters of the collector varying between 0,25 to 1 m. The bigger light pipes are usually adopted in commercial or industrial applications. The smaller

Natural light is more efficiently transported with straight pipes, but curved or angled tubes can be adopted if necessary. Light transmission losses are reduced by the highly reflecting

In recent years many authors carried out numerical and experimental analysis with the aim to determine the performances of light pipes (Carter 2002, Zhang et al. 2002, Jenkins et al. 2003) and theoretical or empirical calculation methods were set up by various researchers for the prediction of the illuminance distribution inside a room equipped by light pipes (Jenkis & Muneer 2004, Jenkis & Zhang 2004), but standard design methods have not yet been developed. The difficulty in standardizing the calculation methods is mainly connected with the influence on internal illuminance distribution of the temporary weather variations that commonly occur during a day, a season or a year, and it is very difficult to describe the

film applied on the internal surface of the tube. Reflectivity ranges from 0,98 to 0,995.

ones are used for residential installations and they are typically 1-5 m long.

possibility of effecting an efficacy of natural ventilation by the VDLP.

**2. Traditional light pipes** 

inlet into the tube.

the pipe.

avoided.

interior spaces away from the collection point.

The Double Light Pipe (DLP) is an innovative device developed by the authors to illuminate a two - level underground room. It is an improvement of the traditional light pipe, moving from the idea that the installation of a traditional light pipe in the centre of a two - floor building, with the aim of illuminating the lower underground spaces, is impossible because of the encumbrance of the device.

Since the DLP is able to distribute daylight both in to the passage and the final room, it is proposed to be a solution for the problem. In fact, if a double light pipe is used instead of a traditional one, it is in good agreement with the architectural principle because it illuminates the passage room, so justifying its bulkiness.

The DLP is an improvement on the traditional light pipe. It consists of two concentric tubes, the internal one which illuminates the final room like the traditional light pipe. The same reflective film (=99,5 %) covers both the internal and the external surfaces of the inner pipe while the second one, concentric to the first, is made of a transparent material such as polycarbonate and it is installed so as to create a hole between the two pipes. It allows transmission of a portion of daylight, captured by the collector on the rooftop, into the interior spaces crossed by the system.

In figure 1 a picture is shown of a reduced scale (1:2) prototype of the DLP set up by the authors in the laboratory of Technical Physics of the University "G. D'Annunzio" of Pescara and the concept on which the device is based.

In order to diffuse light in the passage room, avoiding too intense reflections from the interior pipe, a thin plastic diffusing film is applied on the interior surface of the transparent external pipe. It is characterized by very precise 90° micro-prisms on one side and is made smooth on the other so that it may distribute light in a more diffuse way in the crossed room.

Thanks to its optical properties, it can either reflect or transmit, depending on the angle of incidence of the light. radiation. Light is reflected if the angle is less than about 27° with respect to the axis of the prisms, and transmitted if the angle is greater than 27°.

The diffusing film may be applied on the upper portion of the pipe (30-50 cm) in order to avoid glare. Figure 2 shows the construction steps of the system and its illuminating function in the passage and final room.

Fig. 1. Reduced scale prototype of a DLP and the concept of the system

Fig. 2. Evolution from a traditional light pipe to a double light pipe and its lighting function in a two floor underground room

smooth on the other so that it may distribute light in a more diffuse way in the crossed

Thanks to its optical properties, it can either reflect or transmit, depending on the angle of incidence of the light. radiation. Light is reflected if the angle is less than about 27° with

The diffusing film may be applied on the upper portion of the pipe (30-50 cm) in order to avoid glare. Figure 2 shows the construction steps of the system and its illuminating

> **Traditional light pipe**

 **Double light pipe** 

respect to the axis of the prisms, and transmitted if the angle is greater than 27°.

Fig. 1. Reduced scale prototype of a DLP and the concept of the system

Fig. 2. Evolution from a traditional light pipe to a double light pipe and its lighting function

room.

function in the passage and final room.

in a two floor underground room

Since the DLP provides light in the room, it can be installed in the centre of the area. The technological design of the DLP and data about its performances are shown in Chella et al. (2007), Baroncini et al. (2008), Baroncini et al. (2009), Baroncini et al. (2010).

The DLP can be installed either in underground rooms or large plant area spaces in which the windows are placed on the perimeter walls and they are not able to guarantee an efficient distribution of natural light in the whole environment. In such cases, the DLP is very suitable, particularly if a soft light distribution is required, characterized by a certain degree of uniformity, such as in exhibition rooms, museums and the like, in which direct solar radiations must be avoided because of the risk of glare and deterioration of the artworks sensitive to light. For this reason, in museums or other similar places, even if not in hypogeal rooms, any direct visual contact with outdoor is usually avoided and artificial light is used all the time in such environment.

The collector of a DLP is larger than a traditional one. It is able to collect light and redirect it both to the inner pipe and to the hole between the two pipes. It can be a dome or plane cover made of transparent material such as polycarbonate, equipped by Fresnel lenses or not.

In the reduced scale prototype set up by the authors, the simplest type of collector was adopted, a plane transparent polycarbonate device placed on the rooftop, with the intention of evaluating the performances of the transmitting and diffusing devices in the least favourable conditions. Any enhancement of shape and materials, with the aim of optimizing the possibility of collecting daylight, will obviously increase the performances of the DLP.

The inner pipe consists of an aluminium sheet folded like a tube which is covered by a multilayer highly reflective film both over its internal and external surfaces. Due to its reflecting characteristics, very efficient multiple reflections take place in the pipe; direct and diffuse natural light are channelled downward to the diffusion point in the final room.

The outer pipe, concentric to the inner one, is made of a transparent polycarbonate tube, internally covered partially or completely on its surface by a diffusing material, which distributes light into the passage spaces.

At the end point of the light path, in the final room, a conventional polycarbonate diffuser with a regular prismatic geometry was adopted similar to the traditional light pipe. In all respects the inner pipe performs like a traditional light pipe.

Figure 3 shows a two-dimensional section of a two - level underground building equipped by a DLP, in which the used materials are evident, while in Figure 4 an example of application of a DLP to an underground museum is shown. [Chella et al. 2007]
