Daylight utilisation

The real level and the localised progression of daylight in a room depend on a multitude of influencing factors, e.g. size and location of the windows, their orientation relating to direction as well as type and extent of blocking of daylight by buildings opposite the windows and their reflectance (obstruction).

A simplified model of the daylight supply for an indoor room leads to determination of its daylight factor. For this purpose, the daylight outside of the room to be considered is assumed to be purely diffuse. Furthermore, it is assumed that no obstruction is affecting the incidence of light. Based on these assumptions, the only remaining factors the daylight supply within the room depends upon are room geometry, reflectivity of the surfaces in the room and the size and location of the windows. This model is widely established as a tool for determining relative daylight distribution in a room. The daylight factor specifies the ratio of resulting illuminance at any given point in the room compared to the generating illuminance outdoors.

With:
D_p        daylight factor at a point p inside the room
E_p        horizontal illuminance at point p inside the room
E_ext     horizontal illuminance outdoors.

The illuminance E_ext depends on time of day and season. This dependence is depicted in figure.

With these specifications, illuminance E_p can be determined for any point at any time. The localised distribution of illuminance within a room resulting from this model can then be determined and depicted, e.g. using isolux lines with commercially available lighting software (DIALux, Relux – see figure).

For a more detailed determination of daylight supply levels, obstruction, direction of the windows and further external parameters are generally used in addition.

Light control

To save energy, daylight can be utilised and the share of artificial lighting in overall lighting can be reduced or even switched off using an automatic control system. Here, the daylight factor D = 0 describes the windowless space (or also the centre area of an open-plan office or a production hall) which does not facilitate energy savings via daylight utilisation. Areas where the daylight factor on the working plane is at least 3% are referred to as indoor daylight zones. The usable share of daylight in a room is determined by

• the average daylight factor D in the area where the workstations are located,

• the period of operation, use or working time and

• maintained illuminance.

Figure shows a standard diagram for the usable amount of daylight for a workplace with a daily working time (period of use) from 7 a.m. to 6 p.m. with daylight-controlled artificial lighting for a daylight factor D = 0 to D = 20. The diagram applies to 51° latitude north and is representative for Germany.

The following figure displays an office with a workstation near a window. Illuminance is measured at the location of the workstation and, where necessary, held constant by supplementing the daylight with artificial lighting. The resulting light penetrates the room sufficiently in any situation to adequately illuminate the traffic area.

Figure uses the example of a classroom to show that workstations at a room depth of 5 m even with large window openings receive less than 30% of the daylight available at workstations near windows. However, it is mandatory to ensure that the quality criteria for lighting required by the lighting standard EN 12464-1 are met for all working areas. Accordingly, even at room depth the illuminance resulting from daylight and artificial lighting must be sufficient if workstations are located there. This must be considered when planning a daylight-dependent control system (see figure, see also chapter ).

Example

In an office, a specific electrical connection value of 15 W/m² has been determined for an average illuminance of 500 lx. Regular working hours are between 7 a.m. and 6 p.m., meaning 11 hours per day. The workstations are located in the window area. The daylight factor D refers to this area. The energy requirement without daylight utilisation (D = 0 %) at 11 hours per day and e.g. 200 working days per year amounts to a yearly average of 33 kWh/(m² · a). If the value is set to 100%, the energy requirement for artificial lighting with daylight-dependent control of the artificial lighting according to daylight factor D is reduced as follows:

The example shows the usable potential energy savings resulting from daylight-dependent control of artificial lighting, which can be up to 70% (the respective values are highlighted in figure).

Practice has shown that it is advisable to consider the possible savings in the order suggested above:

• Initially, efficient luminaires are sensibly selected and arranged to design a lighting installation which facilitates a high quality of lighting with optimum achievement of photometric quality criteria at minimum energy consumption.

• Then, electronic presence detection is considered, since it can access great savings potential in many application cases at relatively limited extra cost. Particularly in solutions that switch off, dimmable luminaires are not required. It may be necessary to ensure that the luminaires to be used are not unduly affected by frequent switching. Generally, LED luminaires should be favoured here.

• The final consideration is whether or not daylight-dependent control is suitable. It is important to consider that this generally means greater extra cost since besides sensors, dimmable luminaires are also required. This extra cost is only justified if sufficient savings remain after reducing operating periods through presence detection.