LRT Digest 2 Tubular daylight guidance systems: Fid-Bau Portal

LRT Digest 2 Tubular daylight guidance systems

Carter, David

This Digest concerns passive tubular daylight guidance systems (TDGS), the most commercially successful method of daylight guidance. TDGS were introduced some 20 years ago and thus now may be considered a mature technology. They are now manufactured in large numbers worldwide and installed in a wide range of building types. This Digest summarises the research effort on the devices themselves, design methods for, and economics of, the systems, and their relationship with the buildings they light. TDGS comprise a device for collecting light, a transport section lined with a highly efficient optical material and a means of distributing the light within the interior. Collectors consist of a clear polycarbonate or acrylic dome located at roof level which captures of the order of 80% of unobstructed global illuminance (the combined total of sunlight and skylight) when mounted horizontally. There are a number of methods of illuminance distribution prediction within rooms in a building lit by a TDGS. These vary in sophistication depending on the available photometric data and the accuracy required. They include ‘first principles’ illuminating engineering calculation processes; ‘rules of thumb’; methods based on tabulated transmission tube efficiencies and a utilisation factor calculation; and spacing criteria for regular arrays of guides to give a uniform distribution of daylight. Two pieces of specialist software which permit the analysis of daylight delivered to interiors by TDGS of various configurations are publicly available. There has been research on the thermal properties of TDGS. Heat loss depends on length, diameter and insulation properties of the guides, and steady-state heat loss to the exterior of a building via TDGS may be estimated using published U values. Similarly, published solar gain factor data may be used to estimate heat gain via TDGS. Generally, the thermal performance of a building equipped with TDGS compares well with that of a similar building lit using conventional windows or roof-lights. TDGS potentially offer paths for spread of fire within buildings. A number of design techniques and devices, such as fire stopping and fireproof ducts, may be used to prevent spread of fire between building compartments via TDGS. Buildings equipped with TDGS present no greater risk of surface spread of flame or propagation of fire between buildings than buildings equipped with conventional lighting systems.