Evaluation of Electrical Energy Demand for Artificial Lighting in Non-residential Buildings

Summary

Daylighting the work place and residential area saves electrical energy: First, there is no need for artificial lighting, second, artificial lighting (al) causes a rising need for cooling energy. On the other hand, windows for daylighting allow the incidence of solar energy which causes internal loads in summer. Shading devices become necessary. The amount of electrical energy for artificial lighting in respect to generating electrical by fossile energy, can be 2 to 10 times the value for residential buildings. Computer programs which calculate the need for electrical energy for artificial lighting exist. A simplified method does not. In this paper a simplified method is proposed.

The Daylight Coefficient describes sufficient daylighting. Artificial lighting is switched on if the present illuminance falls below a given par value of illuminance on workplane. Parameter studies with varied geometry and material properties for room and window are carried out. Occupation time is between 7:00 AM and 4:00 PM. With the simulation program HAUSer which uses the daylight algorithms given in DIN 5034 700 values of switch-on-periods of artificial lighting are determined.

The main concept of the simplified method for estimating the switch-on-period of artificial light for south-facing windows without obstructions per year (Fig. 1) with the characteristic parameter X_r,w as follows :

The characteristic parameter X_r,w is given as a function of input values: factors for pollution of glazing, frame ratio (k₁ , k₂), window-to-facade ratio (r_r), height, depth and width of the room, par value of illuminance on a reference point on the workplane (E_n) and reflectance of walls (n_i are empirical exponents).

The x-axis of Figure 1 shows values for ni of window-size ascending from window-breast. For other window placement exponents n_i vary.

Correction for differing orientation and obstructions:

The switch-on-period of artificial lighting is higher for differing orientation (from south) and obstructions (from none) and can be corrected by correction functions:

with correction functions in polynominal form with c_i als coefficients:

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