Abstract
In some tubular daylight guidance systems (TDGSs), one or more elbows have to be incorporated to change the direction of light transmission for adapting to complex building structures, although an elbow will cause a more light loss than for the equivalent length of a straight pipe and reduce the whole daylighting efficiency. Generally, one elbow’s light transmission efficiency (LTE) is calculated by comparing the measured or calculated LTE values of one straight pipe and the other pipe with this elbow inserted into the former straight one. In this article, firstly through some formula derivations, it is noted this method is based on the assumptions that the insertion of an elbow does not affect the LTEs of its sequent sections, and the identical straight sections or elbows have the same LTEs, regardless of their positions in the whole light pipe; Nevertheless, the assumptions have been refuted after the straight pipes’ LTEs from related literature being deeply analyzed; Moreover, the other conclusions have been drawn that the LTE of a certain straight section or an elbow is not a constant value and also be affected by its position in the whole pipe, so the present LTE calculation method of elbows is wrong and should be modified.
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