1 Introduction
How to effectively use natural light in an indoor environment through reasonable design is a very important thing in terms of energy conservation and comfort, but increasing indoor natural lighting will also have the opposite negative effect. Excessive natural illuminance in sunny conditions is not only bad for health, but also increases lighting energy consumption by turning on artificial lighting while using shading measures. The 2009 Green Building Pioneer Award (LEED) Certification Regulations 2009 requires more than 75% of the new building's space to have sufficient natural light, that is, at least 250lx on September 21st (autumn) from 9am to 3pm. The natural illuminance, and the illumination under sunny conditions does not exceed 5000lx.
In the 1970s, China promulgated the "Design Standard for Lighting of Industrial Enterprises", which was revised in 1991. In 2001, it was renamed "Design Standard for Architectural Lighting" GB/T50033-2001. The standards stipulate the quantity and quality requirements for natural lighting of industrial and civil buildings and simple calculation methods. However, after investigation and interviews by the Institute of Building Physics of China Academy of Building Research, it is found that this method is not used in a large number of architectural design practices in China. Get seriously implemented.
2 Overview of calculation and evaluation methods based on daylighting coefficient
Since the “Industrial Lighting Design Standards†developed in the 1970s to the 2001 edition of “Architectural Design Lighting Standardsâ€, the lighting factor has been used as the core parameter for assessing the quantity and quality of lighting. The concept of daylighting coefficient is based on the typical sky model. Although it has certain limitations, it can easily and easily express the common problems in architectural lighting design. Since its introduction in 1907, it has played an irreplaceable role.
China has a wide geographical area and complex and diverse climate types. The evaluation system based on dynamic meteorological parameter model (CBDM) proposed since 2003 is more accurate than the evaluation method of daylighting coefficient, but for China's nationwide lighting design standards, the method is still Immature (and not really applied in the new standards of the United States and the United States). At present, in the international arena, the evaluation method based on the daylighting coefficient is still the most popular. In recent years, there have been many improvements, mainly focusing on the study of the average daylighting coefficient.
2. 1 lumen method
Lumen method is a simplified estimation method for indoor natural illumination. For side window lighting, a simplified “five-point method†can be used to predict the illumination levels at different depths in the room. The standard simplified model specifies the indoor reflectance under normal conditions, 70% for the roof, 50% for the wall, and 30% for the ground. The position of the five points in the room is 0. 1D, 0. 3D, 0. 5D, 0. 7D and 0. 9D, where D is the room depth. Calculated as follows:
Where Ei represents the illuminance value at the reference point, Exv is the outdoor vertical illuminance, Ï„ is the transmittance of the side window glass, and CU is the utilization factor of the room (determined by looking up the table). The most important parameter of this method is the utilization factor CU, which is based on an empirical summary of a large number of actual measurements. By looking up the table and determining the utilization factor corresponding to a certain point, the illuminance value of the point can be directly calculated by substituting the formula. The introduction of the utilization factor includes geometric information such as depth/window height, window width/window height, and measurement point position, which can effectively reflect the influence of spatial geometry on the daylighting coefficient. However, this method is slightly cumbersome to calculate, and the determination of the outdoor vertical illumination Exv is complicated, and it is difficult to use this method for lighting estimation in the architectural design stage.
2. 2 lighting coefficient method
The daylighting coefficient method is a method for estimating the daylighting coefficient at any point on the horizontal working surface of an indoor space under the premise that the sky brightness distribution is known. This method eliminates the direct sunlight, and usually uses the CIE standard full-sky model, which is suitable for areas where the weather is more common. The method determines that the illuminance value at any point on the horizontal working surface of the indoor space is determined by three factors: the sky component (SC), the outdoor reflection component (ERC), and the indoor reflection component (IRC). The sky component refers to the daylighting coefficient component obtained by a certain point directly affected by the sky. The outdoor reflection component and the indoor reflection component respectively refer to the daylighting coefficient components of the outdoor and indoor environment reflections acting on the point. The final daylighting coefficient value at this point is obtained by adding these three components (Figure 1):
DF = SC + ERC + IRC ( 2)
The process of determining the three kinds of daylighting coefficient components is also cumbersome. It is necessary to determine the geometric proportional information of the basic proportional relationship of the space to be estimated and the position of the point to be estimated. The most important part is the determination of the sky component, which will not be described here. . This method is based on the full cloudy model. It can estimate the lighting coefficient of a certain point in the indoor working plane at the design stage. The disadvantage is that it is not accurate enough to reflect the overall lighting level of the indoor space.
2. 3 Proposal and theoretical study on the calculation formula of approximate average lighting coefficient
The calculation method for the specific reference point on the indoor working surface is of great significance for summarizing the variation law of the daylighting coefficient on the plane, and is also the empirical basis for the calculation formula of the average daylighting coefficient. In recent years, the international lighting design standards have adopted the average lighting coefficient as an important parameter for assessing the quantity and quality of indoor lighting. The second part of the UK design standard BS 8206 describes: Under normal lighting requirements, if there is no artificial lighting, the indoor average lighting factor should not be less than 5%. If there is artificial lighting supplement, the average lighting factor should not be less than 2%. The North American Lighting Manual, 9th Edition, developed by the North American Institute of Lighting Engineering (IESNA) recommends that: when the average lighting factor of the indoor space is 5% or more, the lighting requirements can be met. If it is less than 2%, the indoor space will appear gray. And the lighting is insufficient.
In the 1970s, foreign scholars proposed a formula for calculating the average daylighting coefficient based on a large number of on-site measurements and the compilation of model experimental data. In 1979, Lynes proposed the following calculation expression for the average daylighting coefficient of the rectangular side window lighting space:
ADF is the average daylighting coefficient, Ag is the net surface area of ​​the window, At is the total surface area of ​​the room including the window, Ï„ is the glass transmittance, θ is the sky occlusion angle, and Ï is the average reflection coefficient of the indoor surface. The method for determining the sky occlusion angle θ is shown in Figure 2. The θ value is 90 when there is no occlusion outside:
In terms of, it is not the average daylighting coefficient on the work surface. It is an important significance of this expression to propose the sky occlusion angle parameter to measure the outdoor occlusion factor. In the subsequent study, a number of revised versions of the formula for the average daylighting coefficient appeared.
In 1984, Crisp and Littlefair revised the formula for Lynes. Through the model experiments under the artificial sky, they found that the Lynes formula underestimated the actual situation of the average daylighting coefficient in the model space. Based on the new research data, the formula for Lynes is revised to:
The calculations for this formula are more consistent with the measurements in the model experiments and were eventually affirmed and applied in the North American Institute of Lighting Engineering (IESNA) and many other versions of the specification. Harvard University's CF Reinhart in his recent research paper
A verification evaluation of the above two average lighting coefficient expressions using the computer simulation tool Radiance is demonstrated. The verification results are shown in Figure 3.
The graph on the right shows the comparison between the correction formula and the Radiance analog value. The left graph shows the comparison between the original formula of Lynes and the simulated value. The agreement of the latter can be summarized as the function y = 1. 1323x. The former can be used with the function. y = 0. 813x means. This comparative analysis shows that the modified formula is closer to the Radiance simulation calculation.
2. 4 Four-step calculation method for lighting by CF Reinhart et al.
CF Reinhart et al. proposed and validated the natural light design steps for the side window lighting space. Because the design step is based on the lighting factor
The parameters are determined, so it is suitable for receiving diffuse skylight-based space, and needs to supplement the glare analysis and energy consumption analysis of direct sunlight, which is suitable for early design process. The design process was verified by Radiance simulation of 2,300 side window lighting spaces as follows: (1) Determining the basic condition of the estimated target; (2) Natural light feasibility verification; (3) Setting the room ratio and surface reflectance; (4) Accurately evaluate the windowing area based on the Lynes lighting factor formula.