Beijing University of Technology Wu Yanpeng Ma Chongfang
Abstract Light pipe technology is a way of utilizing solar light and belongs to green lighting technology, which provides a possible way for efficient transmission of light energy. This paper summarizes the development history and research status of building light pipe technology, and points out the future development trend of the technology. The combination of light pipe technology and natural ventilation can make the function of the light pipe more perfect, and maintain good natural ventilation in the room while lighting, which is of positive significance for building energy saving and improving indoor air quality.
Key words light pipe, green lighting, natural lighting, natural ventilation
1 Introduction
Energy and the environment are the focus of current global attention. Lighting power has accounted for 10-20% of total power generation with the development of society. At present, China's lighting power consumption accounts for 10-12% of the country's total power generation. In 2001, China's total power generation was 143.325 billion kWh, and annual lighting power consumption reached 143.32 to 171.99 billion kWh, about twice the annual power generation capacity (84 billion) after the construction of the Three Gorges Hydropower Project. The rapid increase in lighting power will not only increase the investment in large amounts of electricity, but also contain the development of the national economy, but also generate a lot of pollution.
Natural light is a precious treasure that nature has given to mankind. It can be said that it is inexhaustible and inexhaustible. Compared with other energy sources, it has the characteristics of being clean and safe. Making full use of natural light can save a lot of lighting electricity and save electricity for lighting. It can also indirectly reduce the consumption of natural resources and the emission of harmful gases. At present, China's electric energy is mainly produced by burning fossil fuels. When electric energy is obtained, a large amount of harmful gases such as carbon dioxide, sulfur dioxide and nitrogen oxides are emitted into the atmosphere, which are important causes of various environmental problems around the world. In January 1991, the US Environmental Protection Agency (EPA) first proposed the concept of implementing "Green Lights" and promoting the "Green Lights Program." In November 1993, China’s State Economic and Trade Commission started the China Green Lighting Project. On November 16, 1995, eight units were convened to discuss the “China Green Lighting Implementation Planâ€, one of which was to promote natural lighting. Therefore, natural lighting is an important aspect of green lighting.
For a long time, people have always had a misunderstanding about natural light, and believe that natural light enters the room and brings more heat than the artificial light source. In fact, if the same illumination is provided, natural light will generate less heat than most artificial light sources. That is to say, if natural light is used instead of artificial light source illumination, the air conditioning load can be greatly reduced, which is conducive to reducing building energy consumption.
The light-guided green lighting system that collects sunlight can effectively transmit the daytime sunlight to indoor dark rooms or rooms that are inflammable and explosive and unsuitable for electric light sources. Many buildings are now "outdoor sunny, indoor lights are brilliant" The situation can effectively reduce power consumption. Light pipes can also be used for natural lighting or auxiliary lighting in the basement or corridor of buildings such as office buildings, houses, shops, hotels, etc., and can achieve good lighting effects, which is an effective way to utilize solar light.
2 Basic structure and type of light pipe system for construction
The light pipe system for building is mainly divided into three parts, one is the lighting part; the other is the light guiding part, which is generally composed of three sections of light guiding tubes. The inner wall of the light pipe is highly reflective material, and the reflectivity is generally above 95%. The bending and overlapping can be rotated to change the light guiding angle and length; the third is the astigmatism part. In order to make the indoor light distribution uniform, the bottom of the system is equipped with astigmatism components to avoid glare.
In terms of lighting, the light pipes are active and passive. Actively collects sunlight through a concentrator that tracks the sun. This type of light pipe works well for collecting sunlight, but the concentrator is expensive and is rarely used in buildings. At present, the most widely used passive light-collecting light pipe, the concentrating cover and the light pipe itself are fixed together, and the concentrating cover is mostly injection molded from PC or plexiglass, and the surface has a triangular total reflection concentrating edge. This type of light guide is mainly composed of a concentrating cover, a flashing plate, a dimmable guide tube, an extension light guide, a seal ring, a support ring and a diffusing plate, as shown in FIG. 1 .
Fig.1 Schematic diagram of light-emitting green lighting system for collecting sunlight
There are two main types of light pipes from the way they transmit light: slitted light pipes and prismatic light pipes. The slitted light pipe has a long cylindrical shape, and the inner surface is coated with a specular reflection coating, and a long light exit slit is left to make the light hit the working surface. The light guide processing process is complicated, and the light is in the process of propagation. The large losses are large, making the entire light pipe device inefficient, so this type of light pipe is rarely used in light pipe systems that collect sunlight. The prismatic film hollow light guide is developed by 3M Company of the United States. The light pipe is manufactured according to the principle of total reflection of light radiation in a light-tight medium. The thickness of the prism film is 0.5mm. The product of 3M Company is made of PMMD plastic, and the primary reflectivity It can reach 99.99%, which is the world's highest reflectivity and best performance light pipe material. One side of the film is flat and the other side has a uniform distribution of longitudinal corrugations. The cross section of these corrugations is a triangular prism with a apex angle of 90°. The film is characterized in that if light incident on the flat side thereof is not reflected, it will be injected into the material and the prism film will be rolled into a cylindrical tube. A beam of light that travels along the length of the tube can enter through the end of the light pipe and is reflected multiple times to the other end of the tube. The selection and fabrication process of prismatic film hollow light guide film materials is a key issue. Non-standard optical surfaces and impure optical materials can lead to an increase in the loss of light during propagation, and even some of the light is scattered from the light pipe, and The longer the propagation path, the greater the loss. These issues are subject to further research and resolution.
From the way of installing the light pipe, it is divided into two types: top lighting and side lighting. At present, most of the light pipe systems for collecting sunlight in foreign countries use top lighting, and side lighting has not been reported in the literature.