High latitude application of laser cut panels for enhanced daylighting of commercial and domestic buildings via light guides

Northern European houses are frequently built as row houses to minimise wall area and thereby heat loss. The row houses are where possible oriented on an East West axis so that the window wall gains maximum exposure to the South. If the windows are carefully designed radiant heat gain through South facing windows is, for at least part of the year, larger than the energy lost and South facing windows may be effective in reducing the energy cost of heating the home. However as the energy lost through windows in winter greatly exceeds any possible energy gain and it is usually necessary to minimise the area of conventional windows with a consequent reduction in view, natural daylighting as well as radiant heat gain during that time of year when window gain can exceed window loss. In Northern Europe this is the 6 month period centred about the equinox. During the earlier and the later part of the day during this period direct sunlight falls on South facing windows at high angles of incidence and the radiant heat gain is small. As mid winter approaches the shading of South facing windows by neighbouring buildings becomes significant. For row houses of the form shading of the lower level windows occurs from October through to March depending on the road spacing between row houses. It is possible to reduce window losses during this period by using transparent insulating material such as silica aerogel or capillary material. Both offer very high thermal resistance and good radiant heat transmission. However the viewing transparency is lost and there may be overheating problems during summer. This presentation investigates the possibility of obtaining radiant heat gain from a light pipe which directs radiation from the roof line to the lower level of row houses. Light pipes can be made very efficient at high latitudes if a light deflecting system is used to deflect low elevation light near axially down the pipe. The efficiency can be increased by tracking the deflector to follow the sun. At high latitudes, close to the polar axis, the path of the sun is close to a simple constant 24 hour rotation so that tracking the sun about a vertical axis can be accomplished with a simple 24 hour clock mechanism. With the input aperture at the roof line a light pipe receives direct radiation at all daylight times and is never shaded. Additionally a roof aperture receives a very much higher proportion of the diffuse radiation from overcast skies than a vertical window. The advent of three key materials opens the possibility of using light pipes in cold climates. These materials are: (a) Capillary transparent insulating material (TIM), which provides high thermal resistance coupled with high radiant energy transmission. Importantly light transmitted by this material retains the angular orientation to the capillary axis. (b) Laser cut light deflecting panels which provide an inexpensive method of light transmission with deflection through large angles. (c) Silver coated acrylic reflecting films with 95 % reflectance of solar radiation (reference). The combination of efficient light deflection by laser cut panels with the high reflectance of silver films allows a high fraction of sunlight to be transferred through very long light pipes. The inclusion of capillary TIM minimises heat loss via the pipe without reducing transmission efficiency. It can be shown that a light pipe from the roof to the lower level of a building admits radiant heat at as efficiently as South glazing during the equinox period at high latitudes. Practical aspects of applying this concept are discussed. (orig.)