INFRARED PHOTOVOLTAICS FOR COMBINED SOLAR LIGHTING AND ELECTRICITY FOR BUILDINGS L. M. Fraas*, W. E. Daniels*, J. Muhs† *JX Crystals Inc., 1105 12 th Ave NW, Issaquah, WA 98027 USA †Oak Ridge National Laboratory, PO Box 2009, MS-8058, Oak Ridge, TN 32831 USA ABSTRACT: A concept for indoor illumination using sunlight is described. For this system, a tracking concentrator on the building roof follows the sun and focuses sunlight into a light guide. A system of transparent light guides distributes the sunlight to interior rooms. In this system, a cold mirror splits the solar spectrum into visible light for indoor illumination and infrared radiation which is directed to an array of GaSb infrared sensitive photovoltaic cells which then generate electricity. It is shown that each Watt of visible sunlight displaces two Watts of electricity which otherwise would be used for florescent lighting and air conditioning. The savings from this displaced electricity can then pay back the cost of the concentrator, tracker, and light guides in approximately three years. Meanwhile, the GaSb cell array converts the concentrated infrared energy to electricity with an electric power density of one Watt per square cm. This power density is one hundred times higher than available from a planar silicon cell, thus easily allowing PV electric power production at a capital cost of under $1 per Watt. Keywords: GaSb – 1: Concentrators – 2: Buildings – 3 1. INTRODUCTION It is well known that solar energy can be converted to heat or electrical energy and that these forms have value. It is also well known that the sun’s light energy can be used directly for illumination. However, the economic value of sunlight is not generally appreciated. To appreciate the value of sunlight, imagine two alternative solar energy systems. In the first system, solar cells capture sunlight at a desert utility site and convert it to electricity with an efficiency of 10%. Then, the electricity is routed through electrical cables to a building. In the building, the electricity is converted back to light with an efficiency of 20%. For this system, only 2% of the solar energy is delivered as useful illumination energy. For the second system, imagine that the sunlight is captured on the building roof and concentrated and routed with optical cables to overhead lamp fixtures with only 50% transmission loss. The sunlight is used twenty-five times more efficiently in the second system. Since a solar illumination system can displace electricity, the energy in sunlight has more value as light than as electricity. The value of sunlight has been appreciated qualitatively for centuries through the use of windows and skylights. However, electric lighting is commonly used in buildings even during the day because electric lights have some special and very desirable qualities not available from window lighting. Some of these features are: (1) overhead illumination, (2) illumination well into the interior of the building, (3) adequate illumination levels, (4) constant illumination levels, (5) illumination control (on/off, high/low, portability). Window lighting does not allow for overhead illumination or illumination well into the interior of the building. Furthermore, the illumination level supplied from a window can vary by orders of magnitude throughout a day. For example, the direct sunbeam entering a window facing east in the morning is much more intense than the diffuse skylight entering the same window in the afternoon. These problems with window lighting are rectified by the concept shown in figure 1, in which direct light from the sun is collected by a tracking concentrator and focused into a light guide for distribution into overhead lighting fixtures in the building interior (ref. 1). One can install light guides in a manner analogous to installing a fire- extinguishing sprinkler system or an electrical conduit distribution system. Referring to figure 1, note that the concentrated solar radiation is incident on a beam splitter where the visible sunlight is reflected into the light guides and the infrared portion is transmitted through to a solar photovoltaic (PV) array where it is converted to electricity. A consortium headed by Dr. Jeff Muhs from Oak Ridge National Laboratory (ORNL) and Prof. Byard Wood from the University of Nevada is now developing this concept for indoor illumination using concentrated and piped sunlight. JX Crystals is developing the IR sensitive photovoltaic array to be used in this application. The consortium is funded by the US Department of Energy. Figure 1: Concept for concentrated and piped lighting for indoor illumination, with infrared energy directed to a panel of infrared-sensitive PV cells.