Color-rendering of daylight from water-filled light pipes

Abstract

Water-filled light pipes can be used to transport concentrated beam solar radiation from a solar collection system to a utilization system, altering the spectral distribution of the radiation in a beneficial way in the process. One use for such a system would be for the daylighting of the core interior spaces of buildings, spaces that are far removed from outside walls or the roof and are therefore not amenable to conventional daylighting with sidelights or toplights. The filtering action of the water can be used to remove unwanted infrared and ultraviolet radiation while affecting the visible portion of the radiation only slightly. There are limits, however, to the distance such light can be propagated in water without undesirable color shifts. Comparing with warm white fluorescent light this distance appears to be about 6–8 m. Comparing it to daylight, the distance extends to about 10–12 m. The distance for a significant drop in the color-rendering index is approximately 10 m. Filters can be added to the light pipe to partially compensate for unwanted color shifts and to extend the maximum distance for transfer, but substantial transmission losses are thereby incurred. Due to these limits, it is suggested that the most likely use of such light pipes would be in a hybrid water/air system, where the water-filled portion of the light pipes would be of limited length and would be used primarily for infrared heat removal. It is possible that separate color-correcting filters can be replaced by appropriately selected dyes added to the water. Another possible use for a version of the water-filled light pipe is for spectral shaping and heat removal in concentrating PV solar energy conversion systems.