Day-light-controlled artificial lighting : a potential energy saver: right interior light by sky luminance tracking

The energy consumption in office buildings can considerably be cut, if daylight is used as task lighting. A conservative estimate - starting from existing knowledge and calculation methods - gives a potential saving of 460 GWh a year or 46 % of the electricity costs for artificial lighting in Dutch office buildings, equivalent to 116 million Neth Fl/year or 51 million ECU/year. These substantial amounts justify the implementation of task lighting by daylight on a larger scale. To more accurately estimate the possible energy conservation by daylight, the designer lacks information on indoor daylight availability in the course of the year. The current way of tuning a daylight-controlled artificial lighting installation is too cumbersome and the human appraisal of these installations is still very low. because a daylight-factor-related control algorithm inherently has a very poor accuracy. The problem of comfortable photo-electric control of switched or dimmed artificial lighting upplementing the daylight illumination at the work'Place and correctly estimate the energy effectiveness of it, will require new research efforts to find a more accurate. prediction of the indoor daylight illumination level at points of the working area. The research objectives are: (i) for the designer: a frequency distribution of the indoor illuminance by daylight at various points, specified according to latitude, prevailing climate, part of the year, part of the day, and the geometry of the room and the window; (ii) for the artificial lighting engineer: an accurate conversion from the daylight environment outside to a control signal for the artificial lighting installation. The problem has to be tackled in a phenomenological way: the momentary indoor daylight illumination has to be related to the stochastical varying sky luminance patterns (in the direction of view from the window) by a function with a few measurable parameters. To this end it is necessary to measure and analyze the sky luminance patterns and the simultaneous indoor illuminances. In the running daylight research project of Eindhoven University an advanced sky luminance measuring instrument has been built.