Lighting a building with a single bulb : toward a system for illumination in the 21st c.; or, A centralized illumination system for the efficient decoupling and recovery of lighting related heat

Abstract

Piping light represents the first tenable method for recovery and reutilization of lighting related heat. It can do this by preserving the energy generated at the lamp as radiative, departing from precedent and avoiding absorption of and re-emission of radiant heat as convection and conduction. Within thermodynamic limits, the radiant heat generated by an electric lamp or the sun is available for optical concentration and for reconstitution as a high thermodynamic quality power reservoir. Piping light from a large central lamp facilitates the decoupling of lighting related heat at the source, and also means that the efficiency of one central fixture can be stringently optimized instead of the multitude of fixtures it replaces in buildings. Luminous efficacy for a full-spectrum lamp decoupled of its infrared can be shown to approach theoretical limits of 250 lumens/watt. UV generated by the lamp, if coupled along with the illumination into the transport fibers, can be converted into visible radiation at the emitting end of the fiber, supplementing the light output. Fiber optics are used to carry information over long distances (actually encoded pulses of radiative IR), but certain fiber optics can carry tremendous amounts of energy. As fiber optics become more and more plentiful in telecommunications, their prices will come down. Cost and operating expense studies included in the final chapter of this thesis indicate that a large single source with light that is efficiently coupled and piped throughout a building's interior could reduce electric light consumption to one-fourth, and that even at current fiber pricing levels some systems can be competitive in initial cost to conventional lighting. Certain aspects of centralization suggest further reductions in cost and operating expenses such as centralized, instead of localized, relamping and cleaning, and eliminated requirements for thermal, electrical, and structural hardware at room fixture locations. The economic and technical feasibility of a central system depends on the simultaneous minimization of fiber aperture area and energy losses. Thermodynamically, the concentration of light for transport cannot surpass the energy density of the source. So such a system employs, at best, an optical process that preserves the extent of the source. That is, a high brightness source must be used to drive the system, regardless of the lamp's lumen output. High brightness lamps, then, can be viewed as an alternative to high efficacy lamps for increasing the energy performance of lighting systems in buildings. This thesis anticipates the existence of high brightness, high lumen lamps. The sun's 10,000 footcandles in peak conditions can be a potent contributor to the energy efficacy of buildings if a collection and utilization strategy is properly devised. At 100 sq. ft of available illumination for each sq. ft of collected sunlight, a scenario including simultaneous collection and distribution of electric light and heat and sunlight and solar heat in a building could reduce to near zero the energy consumed for lighting during peak sun conditions. Studies in this thesis indicate that an economically driven future role of solar energy in the lighting, heating, and cooling of buildings could very well revolve around keeping sunlight in the form of illumination and sunheat in the form of radiative heat, instead of converting both into electricity via photovoltaics and reconversion of this electricity back into electric light. Conventional lighting is an inefficient process, essentially using heat sources for the light they provide. Not only is lighting related electricity generating predominantly waste heat, this heat must be removed from the building's envelope by an additional input of energy. Even energy saving fluorescent lamps and fixtures produce at least 80% heat. This might serve to explain why 30% of the country's electricity is consumed by lighting. This thesis proposes a method for decoupling and recovery of lighting related heat, and transporting light in lieu of electricity to lighting fixtures (Chapters 2 and 6). Each of the optical components that would comprise such a system is examined. Chapter 7 investigates the radiation source. Chapter 8 develops the source reflector which will direct the source's radiative output in a particular direction. Chapter 9 studies a mirror that will separate the source's radiation beam into a light beam and a heat beam for subsequent processing. Chapter 10 looks at the heat collector that will convert the heat beam into a usable high-temperature power reservoir. Chapter 11 devises the light collector/ concentrator that will facilitate coupling of light energy into a fiber optic transport network. Chapter 12 assembles the constituent components into central modules. Chapter 5 surveys the light transport media, in particular fiber optics and Prism Optical Light Guide, for suitability to building lighting applications. The exact method of solar couplature is not introduced. Sample energy efficiency comparisons, cost and payback scenarios, implementation issues and concepts for room emitters are included in chapter 13. Related concepts for a transparent concentrating solar collector for use as a window or skylight, and a solar concentrating wall are disclosed in the conclusory chapter. Material included in this thesis has been patented by MIT. The usage of such material for any commercial means requires a licensing agreement.