The system pipes sunlight throughout a building, providing useful levels of light whenever the sun is out. But on cloudy days, or at night, the lighting fixtures turn on fluorescent tubes to supplement the output -- that's the "hybrid" part. The system can even capture light for power generation.
That last part is where it really gets cool. According to ORNL's web site:
The concept, originated by ORNL's Jeff Muhs, separates and uses different portions of sunlight for two applications, interior lighting and distributed power generation. The concept takes advantage of two facts. First, the luminous efficacy (or light output per unit of energy, expressed as lumens per Watt) of the visible part of the spectrum is more than double that of electric lamps. Second, photovoltaic cells, especially thermo-photovoltaic cells, are very efficient in converting the infrared portion of the spectrum to electricity.
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