Because hybrid lighting systems require the use of state-of-the-art electric lights when sunlight is not available, their cost is additive. As such it is not fully appropriate to compare them directly. However, in a "head-to-head" comparative analysis, the estimated additive cost of installed hybrid solar lighting systems (a clean energy alternative) in terms of ¢/kwh displaced (5 - 11 cents/kWh) is typically lower than the cost of running electric lighting systems in a deregulated market considering time of day rates (10 - 15 cents/kWh) during peak demand periods on hot, sunny days.

A complete study of all types of topside daylighting is not warranted for the purposes of a comparative analysis with adaptive full-spectrum solar energy systems. We limit this discussion to skylights, generally accepted as the most cost-effective form of conventional topside daylighting. On average, incident sunlight does not enter skylights normal to the horizontal plane. Depending on the type and configuration of skylight, light transmission varies dramatically and is attenuated significantly. This is due to several factors but is predominately determined by the efficiency of the light well and glare control media. The typical transmittance of state-of-the-art tubular, domed skylights varies widely, depending on lighting requirements, but for commercial applications is typically well under 50%.

Comparatively speaking, several other factors must also be considered. First, the coefficient of utilization (CU) of a single 1-m² tubular skylight will inherently be much lower than a system that distributes light from the same square meter to six or more luminaires. Assuming that the room cavity ratio and other room parameters are identical, the CU of the more distributed hybrid system is significantly better. If the single 1-m² skylight were replaced by ~6 much smaller skylights, the two systems CUs would compare equally, yet the cost of the skylights would increase prohibitively.

Skylights are typically not designed based on the maximum amount of light that can be supplied but rather designed to approximate that which is produced by the electric lighting system when the total exterior illuminance is 3000 footcandles. This reduces over-illumination and glare. Because of this, all light produced by skylights beyond this value is typically wasted. As such, preliminary estimates suggest that on average, depending on location, approximately 30% of the total visible light emerging from skylights on a sunny day is excess light not used to displace electric lighting. Conventional skylights are also plagued by problems associated with heat gain and do not harvest non-visible light. Finally, conventional skylights are not easily reconfigured during floor-space renovations common in today's commercial marketplace. Once all factors are considered, the simple payback (typically >8 years) and energy end-use efficiency of even the best topside daylighting systems is considerably worse than projected adaptive, full spectrum solar energy systems.

To date, the United States has invested billions of dollars in systems capable of converting solar energy into electricity. The most relevant examples include solar PV modules and solar thermal technologies. The advantages of these systems are obvious. First, PV modules require no moving parts to convert sunlight into direct-current electricity, and they can be conveniently used for any electrically-powered end use. Unfortunately, these advantages come with a steep price in terms of overall efficiency. For example, commercial solid-state semiconductor PV modules typically have a total conversion efficiency of < 15%. Solar thermal systems typically have a conversion efficiency somewhat higher (< 25%), depending on system design and complexity. Further, losses attributed to electric power transmission/distribution (~8%) and dc-ac power conversion (10 - 15%) further reduce the overall efficacy of conventional solar technologies. Because of these and other reasons, conventional solar technologies have not displaced significant quantities of nonrenewable energy and are expected to be used in the United States for residential and commercial buildings, peak power shaving, and intermediate daytime load reduction. The PV modules currently sell for between $3 - $5/Wp. The projected peak performance of adaptive full spectrum solar energy systems ($3,200 per 1,940 Wp or $1.65/Wp) have the immediate potential to more than double the affordability of solar energy when compared to these solar technologies. For more information on system performance, see our Papers/Presentations page.

If the intended uses of solar technologies are for reductions in energy use in buildings, peak power shaving, and intermediate daytime load reduction as recent U.S. DOE documents suggest¹, we suggest that our approach reflects a more effective way of using solar energy to reduce nonrenewable energy consumption in developed countries like the United States that have a well-established electrical grid.

¹National Laboratory Directors for the U.S. Department of Energy, "Technology Opportunities:  to Reduce U.S. Greenhouse Gas Emissions," and "Technology Opportunities:  to Reduce U.S. Greenhouse Gas Emissions:  Appendix B; Technology Pathways," October 1997.

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