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Pt.14: Mixing Daylight And Electric Lighting By JULIAN EDGAR Electric Lighting Using natural light to illuminate building interiors during daylight hours could significantly reduce energy consumption and cut power bills. The concept is simple: collect the sunlight falling on the roof and use light pipes to distribute it throughout the building to provide natural lighting. 82  Silicon Chip T RY THIS QUICK QUIZ: when, during the 24 hours of a day, would you expect the greatest power consumption due to the use of electric lighting? If you said “at night” you would be wrong. The greatest demand for artificial lighting is at the very time of day when the Sun is at its highest and natural light is most abundant! The cost, in both energy and dollar terms, of switching on a light instead of making use of daylight is considerable. In the US, the power bill for electric lighting is about $US100 million every day and electric lighting uses about one-quarter of all the electricity generated. In addition to the direct energy cost, electric lighting also has an indirect energy cost. Electric lighting generates heat and about 10% of total cooling and ventilation costs go towards removing this heat. One obvious way to reduce the cost of lighting is to supplement artificial light with natural light. In the past, this meant using large windows and skylights. However, these traditional forms of natural lighting do not distribute light to remote locations. One way around this is to “pipe” natural light to dim locations and add artificial lighting as necessary. This approach, which relies on the use of “light pipes”, is called “hybrid lighting”. A hybrid lighting system consists of four main parts: (1) natural light collectors; (2) artificial light sources; (3) transport and distribution systems for both light types; and (4) a control system. Natural light collection On a cloudless day and with the Sun high in the sky, the amount of sunlight falling on a square metre of the Earth’s surface is more than 1kW. All this power is in the form of visible radiation – a quite different situation to a 1kW incandescent lamp that might emit only 180W of visible radiation. One square metre of bright sunlight is therefore equivalent to about 55 100W light bulbs. FACING PAGE: Oak Ridge National Laboratory’s Mike Cates (left) and Jeff Muhs with a light pipe of the type that could be used in commercial hybrid lighting systems. (Photo: ORNL). This means that a square metre’s worth of bright sunlight could theoretically light about 20 rooms. Or to put it another way, enough sunlight falls on the roof area of a multi-storey building to light every room in the building – even if it’s more than 100 storeys high! However, this assumes that the light can be both efficiently collected and then transported without loss to where it is needed. The most efficient method of collecting sunlight is to use a collection mechanism that tracks the movement of the Sun across the sky. Solar furnaces and solar energy plants take this type of approach, using large mirrored reflectors. However, such tracking systems are mechanical in nature, with moving parts. They require energy to operate and often use sophisticated and relatively expensive electronics to maintain their tracking position. For these reasons, moving collectors are not frequently used in hybrid lighting systems. Instead, efficiency is traded off for reliability and cost-effectiveness. Solar collectors for lighting systems are not required to have optical quality reflective surfaces. Instead, coated plastic collectors (concentrators) can be cast, moulded or extruded into the appropriate shapes. In addition, a system can use three such collectors in a passive arrangement – one facing east, one west and the other north (in the southern hemisphere), so that morning, afternoon and midday sunshine can be caught. Although much less efficient than an active tracking system, the system can be easily scaled up in size to more than compensate for the reduced efficiency. However, some systems do use tracking reflectors. One such system is claimed to provide enough interior light on sunny days to make electric lighting unnecessary from one hour after sunrise to one hour before sunset. Artificial light sources If artificial light is to be used with daylight, its colour temperature should be about the same. However, achieving this is very difficult, especially if light sources with high efficacies are to be used. As one commentator put it, if we are to exactly duplicate daylight, the “artificial lights would have to look like a 5750K black Hybrid lighting systems use rooftop collectors and light transmission pipes to gather and distribute natural light within a building. Either fixed or tracking collectors can be used, although the lower cost and greater reliability of fixed collectors makes them the preferred option for most applications. (Photo: ORNL). body shining through several miles of atmosphere made up mostly of nitrogen, oxygen, and water vapour!” That said, the human eye quickly adapts to light sources of varying colours (and, of course, the colour temperature of daylight varies during the course of the day, anyway). As a result, “daylight white” fluorescent lamps are usually used in hybrid systems. Transport and distribution Hybrid lighting systems use light pipes to “transport” the natural light from the roof to various rooms. Often called “hollow light guides”, they must be highly efficient in order for hybrid lighting systems to work effectively. VN Chakolev in Russia and Professor William Wheeler in the US invented hollow light guides in the 1880s. They were motivated by the introduction of the electric carbon arc lamp, a light source too powerful for normal indoor illumination. However, if the light from the arc lamp could be piped to each room, it could become a practical means of domestic illumination. Unfortunately, the mirrors used in these early light guides were both expensive and inefficient. The metalon-glass mirrors had an absorption of more than 10%, a figure which becomes significant when it is realised that a great many reflections can occur within a light guide. Subsequently, in 1946, Henry Pear­ son of the Rohm and Haas Company AUGUST 1999  83 Either light-pipes or direct radiation can be used to distribute any artificial light that’s being used to complement natural lighting. New developments in artificial lighting (for example, microwave sulphur lamps) also lend themselves to lightpipe technology. (Photo: ORNL). used acrylic rods and sheets to transmit light from one place to another. Unlike metallic mirrors, this material guides light with high efficiency because it employs a technique called “total internal reflection” (TIR). This means that very little light is lost through the walls as the light travels along the guide. Another important development was the advent of low-cost optical surfaces in the mid-1960s, made possible by the mass-production of optically-treated polymeric films. Vacuum metallisation of polyester film can produce a flexible mirror 84  Silicon Chip that is as specular as an ordinary glass mirror but costs far less. These films were commonly used in light guides installed in the former USSR and similar films are now employed in many current commercial light pipes. In 1978, Lorne Whitehead at the University of British Columbia developed the prism light guide. This also employs the total internal reflection technique, with the guide’s transparent walls containing precise longitudinal rightangle prisms. Light rays incident on the inside surface of the wall undergo total internal reflection at the prismatic exterior surface, re-entering the central airspace or gel filling to continue propagating along the pipe. While commercially successful, these light guides were expensive due to the precision required for the prisms along the walls. Most recently, researchers at the 3M company have developed a technology known as “micro-replication”. This allows the large-scale manufacture of micro-prismatic structures with surface irregularities substantially smaller than the wavelength of light. The 0.5mm thick prismatic polymethylmethacrylate film developed by 3M is now widely used in light guides. Incidentally, the use of glass or silica optical fibre is generally not considered viable for this application. That’s because of the high expense of the fibres, which would have to be quite large to carry the luminous flux required for conventional illumination. Light guides are capable of transporting large amounts of light. The bright sunlight from one square metre can be focused into and transported by a guide with a cross-sectional are of just 1cm2. This guide, in turn, can feed a number of smaller guides, each about the size and weight of electrical wiring. However, even the best currently-available hollow light guides still require improvement if multi-storey buildings are to be effectively illuminated using light collected at roof level. Today’s light guides have a loss of 1% in 30cm and researchers are currently trying to reduce that by a factor of 10, to 1% in three metres. Using current technology, the maximum effective length of a hollow light guide carrying sunlight is about 30 metres. Some hollow light guides are used to distribute as well as transport the light. In these designs, the light is allowed to “leak” at a controlled rate as it travels along the guide. This is achieved by lining the pipe with longitudinal strips of “extractor film”. In operation, the extractor film changes the incidence of the light so that total internal reflection no longer occurs. If necessary, a uniform light distribution can be achieved along the entire length of the guide by varying the widths of the extractor strips. Incidentally, hollow light guides are also a very important part of microwave sulphur lamps, a lighting Oak Ridge National Laboratory’s Mike Cates with a light pipe. The efficiency of light pipes needs further improvement if their use is to become widespread, especially in multistorey buildings. (Photo: ORNL). technology that’s currently undergoing major research and development. Control systems Electronic systems are used to automatically control the electric lighting part of a hybrid installation (the natural lighting always works at full power). These systems use light level sensors and control circuits with adjustable hysteresis to prevent the lights from rapidly cycling on and off due to small or momentary changes in ambient light conditions. This can easily occur when clouds pass overhead, for example. Some controllers rely on one or more strategically placed sensors to operate all the lights within a room, while others use one sensor per fixture. The latter system is the most energy efficient. That’s because it only turns on those lights that are necessary to compensate for natural light variations (eg, through windows) as the Sun moves across the sky. Hybrid lighting systems In the US, hybrid lighting systems are now being installed in new buildings. One recent example is the Durant Middle School in Raleigh, North Carolina. However, instead of using hollow light guides, this single-sto- rey building uses special skylights and carefully orientated windows to provide daylight illumination of the classrooms. The school is built on an east-west axis and has north and south-facing solar roof collectors of various sizes. The collected sunlight is diffused by a series of baffles within each collector, so that good-quality natural light is spread evenly throughout the classrooms. The windows on the north and south walls allow further light from the outside to illuminate the rooms. The electric lighting controls are equipped with motion and light level sensors and operate automatically. Despite adding to the building cost, the economic benefits of the new system are impressive. The advanced hybrid lighting system itself cost around $US230,000, much of this spent designing and testing the new systems. This was offset by a reduction of $US115,000 in the cost of the cooling system (it no longer had to remove much of the heat generated by artificial lighting), leaving a net additional cost of $US115,000. This extra outlay was recouped in less than a year by the energy saving, estimated at around $US165,000 per annum! Another recent hybrid lighting system can be found in the Bay de Noc Community College in Michigan, USA. This system uses 14 x 330mm dia­meter light pipes in its Extension Center Building. The sunlight is collected through clear roof-mounted acrylic domes and is reflected down mirrored tubes to ceiling-mounted diffusers. The light pipes were installed as part of a complete lighting refit in the building, which also involved replacing the existing standard fluorescent luminaires with more energy-efficient T8 fluorescent bulbs and electronic ballasts. This new electric lighting system, on its own, reduced annual power consumption by 29%, with consumption subsequently dropping a further 15% after the installation of the light pipes. The efficiency of the system could be further improved by fitting an automatic control system to the fluorescent lights. At present, the electric lighting is switched off manually when sufficient natural light is available. Future goals The US Government is preparing to pour a great deal of money into making hybrid lighting a commercial success. For example, the Department of Energy’s Oak Ridge National Laboratory has developed a Hybrid Lighting Partnership with 10 private companies which are expected to contribute some $US5 million for research. A further $US3-6 million is expected from the Department of Energy. The aims of the Hybrid Lighting Partnership are as follows: (1). Successfully deploy a working, first generation proof-of-concept hybrid lighting system by the end of financial year 2001; (2). Begin introducing commercial hybrid lighting systems by 2003; (3). Create a multi-billion dollar industry by 2010; (4). Reduce electric light energy consumption by about 50 billion kWh in the year 2020 and save electricity users $US7 billion annually by 2020. Although the concept of hybrid lighting is quite simple, it has the potential to drastically reduce the amount of electrical energy used for lighting! And that can only be good news for consumers and for the enSC vironment. AUGUST 1999  85