Putting lightpipes to the test
Lightpipes for bringing daylight to the heart of a buildng can present a range of appearances at roof level.
Lightpipes that bring daylight into the heart of a building are an effective way to use renewable energy. Their efficiency has increased to over 50%, and moves are afoot to combine lightpipes with PV energy to provide lighting day and night, as TERRY PAYNE explains.Ten years ago lightpipes were unheard of in the UK but who would have thought that in eight short years, lightpipes, also known as tubular skylights or SunPipes would have had such an impact on architects and designers considering refurbishing buildings and the development of new ones? Lightpipes, mirror-finish aluminium tubes that carry sunlight and daylight from the rooftop down to the ceiling below, were invented and patented by an Englishman, Stephen Sutton, nearly 17 years ago in Australia. The technology then moved to the USA. There are now more than 10 companies making variations on the original design. Monodraught first imported lightpipes from Canada in 1996 but quickly saw the potential for this energy-free system of natural lighting and developed the range to 10 sizes, from 230 mm to 2 m diameter. Monodraught has filed more than eight patent applications, including Diamond dome SunPipes and Square SunPipes; sales account for more than 90% of the lightpipes sold in the UK — over 25 000 systems per year. Other makes are mainly imported from the USA and Australia. Three basic types
There are three basic types of lightpipe available in the UK. • An anodised aluminium tube, which formed the basis of theoriginal Monodraught range. • An aluminium tube with a 3M multi-layer polymetric film bonded internally. • Aluminised flexible spiral-wound tube, similar to a tumble-dryer hose extract. Increasing interest in lightpipe technology from architects has led to competition to find the most reflective material to give the best performance. One US manufacturer produces a product with the 3M multi-layer film, which proved hard to beat. A drawback was that the polymeric film is prone to delamination, particularly under humid conditions. Monodraught recognised the challenge and last year launched the Super Silver SunPipe, an aluminium tube impregnated with pure silver and stabilised with PVD (physical vapour deposition) to give colour fastness and a 25-year guarantee Lighting tests
Recognising the importance of establishing independent criteria for assessing lightpipe performance and the impact it may have on the future development of lightpipes, Monodraught commissioned the environment division of the Building Research Establishment (BRE) to carry out tests under simulated Sun and overcast conditions. The tests were carried out during the autumn of 2004 by Dr Paul Littlefair, the principal lighting consultant of the Daylighting Division for the last 13 years and recognised as the UK’s leading expert on daylight design. These tests established an improvement between the original anodised-aluminium SunPipe and the new Super Silver SunPipe of a 68% on the 300 mm-diameter SunPipe, which is the most popular system. The tests also showed that the SunPipe was 23% better than its nearest rival under overcast conditions. Full copies of the report can be obtained from Monodraught free of charge. Tests were also carried out to find out if lightpipes can be applied horizontally and to measure their transmittance in this orientation. Future tests are scheduled to assess the thermal properties of lightpipes and test the fundamental claim of all lightpipe manufacturers that there is no heat loss in winter and no heat gain in summer. Monodraught works with four universities to assess the performance of its products. Whereas the academics accept that 99% of UV penetration is absorbed by polycarbonate or acrylic domes, they insist that there must be some IR penetration, although manufacturers point out that this is partially absorbed by the external dome and also by the aluminium pipe itself on each reflection of the light through the tube. In a tube over 2 m long, there may be as many as 14 or 16 reflections so that negligible IR reaches the ceiling diffuser. Nevertheless, it is important that this technical argument is resolved to ensure the confidence of lighting designers. Application
The two leading manufacturers have lightpipes with 99.6% reflectance. However, that is a 0.4% loss on every reflection, so the longer the lightpipe, the greater the loss. There are also losses through the roof dome or light collector and the ceiling diffuser. Nevertheless, the performance of a lightpipe is really quite remarkable and, typically, the 300 mm-diameter lightpipe can light up a room of 10 m2 to a normal daylight level without the need for electric lighting during normal daytime. Lightpipes, of 450 and 530 mm diameter, are used in larger offices and buildings with higher ceilings, but for sports halls, dining halls, leisure centres etc., the 750 and 1000 mm-diameter lightpipes are proving the most valuable. In Dubai, we are negotiating to supply 750 mm-diameter SunPipes to all five exhibition halls at the Dubai World Trade Centre to save the cost of electric lighting throughout the day during exhibition periods. This contract is typical of many projects throughout the world where a good level of lighting is required throughout the day but, even in countries with maximum sunlight, electric lighting has to be used extensively to internal areas. Energy savings
Monodraught is sponsoring a 3-year lectureship at Nottingham University, to look at the environmental benefits of daylighting through lightpipes and more specifically to assess the energy savings that are directly related to a building’s use. In schools, for instance, Government sources say that 70% of electricity is lighting. Since schools are used mainly during daytime, lightpipes will result in substantial energy savings. A comprehensive schedule of energy savings will be produced by this study. Where lightpipes are used instead of traditional fluorescent lighting, a payback of six years can be established. The environmental benefits will also be studied at Nottingham University to determine improvements in health, the greater production rates and the general improvement in human awareness and reflexation that can be achieved with lightpipes. Studies in the USA by Heschong Mahone’s Consulting Group in 1999 on three schools with a total of 21 000 students, found that those with the most daylighting in their classrooms progressed 20% faster on maths tests and 26% faster on reading tests over the course of the year. These results were confirmed in a further study in 2000.
Lightpipes bring natural daylight into the heart of a building, with no heat loss or heat gain. The future for lightpipes?
So what of the future? The great challenge for the lightpipe industry is to produce 24 hour energy-free lighting by using small solar panels during the day to charge solar batteries to provide power to LEDs embedded into the surround of a lightpipe diffuser. As external light levels drop, the batteries will illuminate the LEDs. PIRSs will ativate the lighting only if there is movement below. The impact of such energy-free lighting will be enormous, not just for the UK but also for third-world and developed countries throughout the world. Monodraught is sponsoring research by Prof. Mike Wilson of London Metropolitan University to develop a system, and prototypes have been made. Much depends on the development of more powerful LEDs for success. All the technology is in place awaiting the development of more efficient LEDs. PV technology is already in used with Monodraught’s Sola-Vent, which uses a 600 x 300 mm roof-mounted solar panel to charge internal solar batteries that supply power to a 100 mm-diameter extract fan integrated with a SunPipe. A PIR in the bathroom triggers the fan whenever required With 99.6% reflectivity for lightpipes, architects and specifers are making greater use of the technology for bespoke systems. Typical applications are hexagonal and octagonal lightpipe systems for offices, reception areas etc., that can even be fitted with coloured glass or special-effect diffusers.
Tests have shown that the best lightpipes achieve over 50 transmittance with simulated sunlight and over 30% with an overcast sky.
The worldwide demand for reducing electricity from fossil fuels and a dramatic improvement in sustainable engineering is well served by lightpipes. With eight years’ experience in the UK, lightpipes have proved a dependable, predictable, energy-free system, with added health and environmental benefits. Lightpipes have come a long way and are here to stay. Terry Payne is managing director of Monodraught Ltd, Halifax House, Cressex Business Park, High Wycombe, Bucks HP12 3SE.