Prediction of interior daylight under clear sky conditions
Most available techniques for predicting internal daylight illuminance do not take into account reflected sunlight, nor the fact that under clear sky conditions the direction of the illuminance is usually upwards, not downwards from the sky. The general goal of this study is to investigate the issue of predicting the internal illuminance from natural light in clear sky conditions. This includes the possibility of proposing a method based on the concept of the average daylight factor for use in sunny climates. This thesis is divided into eight chapters. After introducing the problem in Chapter One, Chapter Two is a literature review of problems associated with utilising natural light in sunny regions. Chapter Three is the statement of the problem and how it will be solved. Existing equations for finding the average daylight factor have one thing in common: they assume that the incident light on the window comes directly from an overcast sky or by external reflection from it. If any of these equations are to be used under clear sky conditions, or a new method is to be developed based on the same concepts, the sensitivity of average internal illuminance to the direction of external light needs to be tested. A study of this is described in Chapter Four. Chapter Five tests, by numerical simulation, the performance of existing average daylight factor methods under clear sky conditions. It is concluded that they are not appropriate for sunny regions. The tests, and the conclusions from Chapter Four, do, however, suggest a new approach. This has two bases. The first is that it has been shown to be possible to relate incident light on the window plane to horizontal sky illuminance, and this sets a minimum condition for window design. In practice this can be used in conjunction with a limiting maximum window size based on heat gain and other environmental issues. The second basis is a new formula for relating average internal illuminance to external window plane illuminance. The overall result is a formula for predicting internal illuminance as a ratio of external horizontal sky iluminance. In Chapter Seven this approach is tested. Two methods are used: field measurements under real sky conditions, and comparison with detailed calculations.