A COMPARISON OF FOUR DAYLIGHTING METRICS IN ASSESSING THE DAYLIGHTING PERFORMANCE OF THREE SHADING SYSTEMS

The assessment of the daylighting performance of a design solution is a complex task due to the changing nature of daylight.A few quantitative metrics are available to designers to assess such a performance,among them are the mean hourly illuminance(MHI),the daylight factor(DF),the daylight autonomy(DA)and the useful daylight illuminance(UDI).Each of these metrics has a purpose,a set of criteria and limitations that affect the outcome of the evaluation.When to use one metric instead of another depends largely on the design goals to be achieved.Using Design Iterate Validate Adapt(DIVA)daylighting simulation program,we set out to examine the performance behavior of these four metrics with the changing dimensions of three shading devices:a horizontal overhang,a horizontal louver system,and a vertical fin system,and compare their performance behavior as the orientation changes of the window to which these devices are attached.The context is a typical classroom of a prototypical elementary school.Our results indicate that not all four metrics behave similarly as we vary the size of each shading device and as orientation changes.The lesson learned is that not all daylighting metrics lead to the same conclusions and that it is important to use the metric that corresponds to the specific goals and objectives of the design and of the daylighting solution.The UDI is the metric that leads to outcomes most different than the other three metrics investigated in this paper.

Daylight and thermal performance of a switchable ethylene tetra-fluoro-ethylene cushion with dynamic control in different climates

Switchable multi-layer ethylene tetra-fluoro-ethylene(ETFE)cushion controls the natural light and heat flux passing through the cushion with varying outdoor conditions by dynamically modifying its properties.In this paper,the switchable ETFE cushions with ink printing of different optical and thermal properties were adopted as the window in a typical office model,and the indoor daylight and energy consumption were simulated by using Grasshopper software.Experimental model was built to validate the numerical model.Five locations representing five climate zones in China were selected to analyze the feasibility of the switchable ETFE cushion in different climates.The hourly indoor daylight and heat gain in a single day revealed the effects of the dynamic mechanism of ETFE cushion in improving indoor natural light and thermal environment.In addition,the annual daylight performance at the working area(1.5 m from the window)was simulated.ETFE cushion with printing of the lowest transmittance(ETFE1)was the optimal option for most cities and window-to-wall ratios,with the percentage of annual useful daylight hours up to 78.6%,except for the cases where the window-to-wall ratio(WWR)was 0.35 in Harbin,Beijing,and Hefei.The distribution of useful daylight hours revealed that ETFE cushions with low,medium,and high printing transmittance were suitable for offices where people work in the front,medium,and back region of the room,respectively.Energy consumption calculation revealed that ETFE1 showed advantages over other windows in most cities except for cities highly dominated by heating.ETFE1 delivered up to 33%of energy saving over a year in Kunming compared with the conventional double glazing but was not superior in cities with high heating and low cooling demands,such as Harbin.