Daylighting technologies

An array of knowledge has been developed in the science of light transport for daylighting in buildings. A variety of lighting devices have been designed and researched to improve penetration of daylight and to increase user acceptance.

A recent overview of daylighting technologies under IEA Task 21 (Kischkoweit-Lopin, 2002) divided up daylighting devices into logical categories based on their primary operating principles and the kind of light they were designed to utilise. This system of categorization was adopted for the review of daylighting technologies in the thesis in order to promote a standard approach to identifying and assessing innovative daylighting systems. It was also used to highlight the place of tubular light guides in the array of daylighting devices available. For the sake of brevity and relevance, only a few of the devices in categories other than light transport were reviewed.

2.3.1 Shading systems – diffuse and direct light

Some devices in this double category are illustrated in Fig. 2 - 6 (Kischkoweit-Lopin, 2002) and the category includes among others; prismatic panels (a), Venetian blinds and prisms (b), sun protecting mirror elements (c), lightshelves for sunlight redirection (d), and louvers (e).

(a) (b) (c) (d) (e)

Fig. 2 - 6: Shading systems, diffuse and direct light, selected schematics

These devices are not exclusively for use in window openings; anidolic zenithal openings which fall into this category, for example, are intended to bring diffuse light into a room from above. Venetian blinds were integrated into windows to redirect light and reduce glare by researchers at Cardiff University and their thermal and optical properties measured and modelled (Breitenbach, Lart et al, 2001). The device with variable-angle slats was found to control the distribution of redirected light according to the angle selected and could be used to send light further into the room. In the direct light redirecting category were laser cut panels (LCP), which use slots cut in transparent polymeric materials to redirect light using diffraction and total internal reflection. A variety of applications exist for such technology, including the production of pyramid skylights that reduce glare at times of peak illuminance and irradiance, thus lowering cooling loads in the tropics (Edmonds and Greenup, 2002).

2.3.2 Optical systems – diffuse and direct light

A number of devices fall into several categories because they perform more than one function. Lightshelves and LCP, for example, can both shade and redirect light according to design. Lightshelves were investigated in Spain using scale models (Claros and Soler, 2002; Soler and Oteiza, 1997) and models developed describing their performance. High levels of internal illuminance greater than 750lux were measured during office hours, dependent on room and shelf reflectance values. This demonstrated the energy saving potential of the devices at that latitude and in addition shading benefits were found for devices that were not so effective at daylighting.

Considerable effort has been spent on developing anidolic facades for buildings, based on the principles of non-imaging optics, that collect and deliver light further into a room than is possible with conventional glazing (Altherr and Gay, 2002; Courret, Francioli et al, 1998; Courret, Scartezzini et al, 1998; Molteni, Courret et al, 2001; Scartezzini and Courret, 2002). Based on the principles of non-imaging optics (Welford and Winston, 1989) and in particular on compound parabolic concentrators (CPC), anidolic facades are intended to increase light penetration.

The work by Scartezzini and Courret applied three different principles of design to anidolic facades to generate three different optimised systems. The first was a ceiling-based system that aimed to integrate with the building design by a reduced protuberance, the second was optimised for cost-reduction and the third, using micro-louvers, was designed for clear sky climates. The geometry of each design was specified according to these principles. The anidolic ceiling has most relevance to the current work, as it was intended to increase the daylight factor further from the window.

When compared to a room with standard glazing, it was found to increase the average daylight factor at the rear half of the room by 1.7 times. It was concluded that an

improvement in daylight penetration had been achieved without loss of visual comfort and that the cost-optimised device in particular had potential for widespread use commercially.

2.3.3 Optical systems – scattering

Systems such as light diffusing glass, capillary glass and frosted glass fall into this category, which has little technologically in common with tubular light guides. The field of advanced glazing into which these devices fall also includes such innovations as dimming glass (Inoue, 2003) and aerogel systems (Reim, Beck et al, 2002), which are helping to increase the amount of light provided by the glazed area of the building fabric without compromising occupant thermal and visual comfort. Several reviews of advanced glazing cover these topics (Citherlet, Di Guglielmo et al, 2000; Littlefair and Roche, 1998; Lorentzen, 1997), including life cycle assessment of the technologies.

Since light pipe and rod systems are generally installed where there are no windows, or far from windows, there is little cross-over between the systems. Only glazing which redirects light towards the rear of the room would compare with as light pipes or rods, and such devices fall into the direct and diffuse light categories above.

2.3.4 Optical systems – light transport

An early work on light transport (Whitehead, Nodwell et al, 1982) described a hollow light tube developed in 1978 and constructed from a prismatic polymer material that combined the high reflection efficiency of total internal reflection with the low cost and practicality of a hollow system. This device was intended both for electric and daylight transport, but only accepted light from a cone of 27.6°, precluding day-long passive solar collection for daylighting. It had the advantage over reflective hollow guides that

any light lost was emitted along the length of the system and so could still be used for illumination. The same author gave a more recent summary of daylighting technologies, with particular emphasis on prismatic tubular guides (Whitehead, 1998).

A more cost-effective method of manufacture had been developed, allowing the production and use of such guides for general lighting applications. It was highlighted that since the first publication describing prism lighting guides, the system had been refined to make more use of the length-emitting quality of the device that had been mentioned briefly in the earlier work. In this format, the prism system was used to convert a point source of light to a linear source. To encourage light loss at the desired rate along the length of the tube, a diffuser was fitted that caused scattered light to exceed the maximum angle of total internal reflection and so be emitted. The use of such devices with daylight, however, was highlighted as a problem by the author, due to the limited angle of acceptance. It was concluded that prism guides were not practical as daylight transporting devices without concentration, which was found to be prohibitively expensive. The author described the recent emergence of light pipe sky-lights, but limited their use to applications requiring low light levels.

An example of light-transport daylighting is the ‘Heliobus’ device, which employs a hollow light guide similar to light pipes and ducts (Aizenberg, 1997). Aizenburg highlighted the three parts of light transport as collection, transport and distribution and went on to describe the Heliobus system in these parts. The collector was a heliostat, which was a concave mirror that collected sunlight and delivered it to the reflective duct below, of square cross-section. This section had emitters fitted to allow the removal and use of light at various heights through the building, followed by a diffuser at the end of the duct to emit the remainder of the light. The system also had three efficient electric lamps to act as backup at times of insufficient external illuminance. Monitoring

of the system in the building showed that without electric lamps the system increased room illuminance by 1.5 to 3 times and overall was calculated to give energy consumption 3-4 times lower than a standard electric installation, as well as reducing the installed electric lighting capacity by half.

A more experimental area of light transport is solar fibre optic concentration, in which solar energy is collected and concentrated before being transmitted down efficient fibre optic bundles. A number of researchers are investigating the transport of solar energy using fibre optics (Andre and Schade, 2002; Ciamberlini, Francini et al, 2003;

Feuermann and Gordon, 1999; Jaramillo, del Rio et al, 2000; Liang, Fraser Monteiro et al, 1998; Zik, Karni et al, 1999), an idea that has been around for some time (Cariou, Dugas et al, 1982; Fraas, Pyle et al, 1983). Recent work on the transmittance efficiency of available fibre optic cables, however, has found the measured values to be considerably lower than those predicted theoretically and by manufacturers (Feuermann, Gordon et al, 2002). The use of fibre optics for the transport of daylight commercially has generally been prevented by high costs, with notable exceptions (Andre and Schade, 2002; Mori, 1979). The hybrid lighting system currently in development (Cates, 2002;

Earl and Muhs, 2001; Muhs, 2000a; Muhs, 2000b) makes use of fibre optics as a transport medium, but adds high-efficiency electric light to maintain light levels in a similar way to the Heliobus above. Cates commented that the USA spends $100 million per day on electric lighting and went on to state that peak availability of daylight coincided with peak demand for electric light. The hybrid lighting system described by Cates also made use of the unwanted IR radiation by converting it to electricity using thermovoltaic technology. The remaining visible light was then conducted down the fibre optics for use in the building, shown in Fig. 2 - 7, (Cates, 2002).

Fig. 2 - 7: Schematic of hybrid lighting system

In his description of the same system, Muhs highlighted the likely cost per kWh of displaced light using the hybrid system in three regions; sunbelt, average location and suboptimal location. He found that the lowest projected cost for the best situation was 1.9cents/kWh, giving a 2 year payback period. In the suboptimal location, however, with system use on only 259 days of the year, the projected cost was 4.5cents/kWh, giving a 4.9 year payback. The current costs of the system are far higher than the projected costs, which are based on a fully optimised system with cost savings. Muhs also referred to a US Department of Energy (DOE) technical assessment of hybrid lighting, which compared it to ‘the most energy efficient conventional daylighting strategy available (tubular skylights)’. The assessment concluded that the hybrid lighting system could become more cost-effective even than light pipes. A final comment was made which underpins all daylighting effort: the collection and use of natural light to illuminate buildings has a high end-use efficiency of perhaps 20-30% for the hybrid system, and similar figures for other devices, whereas using photovoltaic panels to convert natural light into electricity for use in lighting has a net efficiency of only 1-5%. It is much more efficient to use the light directly than to subject the energy it contains to numerous processes before final use.

Like the above systems, light pipes and rods also fall into the IEA light transport category, but are presented in Section 2.4 below because of their importance to the work.