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Efficient Lighting for the Independently Powered Home
Incandescent vs. Quartz-Halogen vs. Fluorescent Light DC vs. ac Power
Three types of electric lights are used for indoor illumination. Each one has its place in the energy- efficient home. The efficiency and economy of your lighting depends on your choice of the right light for each application AND on the way the light is installed. Wise choices in lighting design can reduce energy requirements by 40-80%! Every $100 spent on high efficiency lighting can save $300 or more in system cost (for a typical photovoltaic system). There are two ways to power an alternative energy lighting system, LOW VOLTAGE DC from your battery bank (12 or 24 volts) and 120 VOLTS ac from your inverter or generator. (Readers who are not remote from the utility lines should follow our suggestions for efficient ac lighting.)
Three Types of Electric Lights
(1) INCANDESCENT (the common light bulb): Electric current passes through a thin tungsten metal filament causing it to heat white hot and emit light. The absence of oxygen in the glass bulb prevents rapid oxidation (burning) of the filament. The tungsten evaporates gradually, causing thin spots on the filament (while clouding the glass) leading to reduced efficiency &eventual failure.
(2) QUARTZ-HALOGEN (also called Quartz-Iodide, Tungsten- Halogen): An improvement on the incandescent bulb, works on the same principle except the tungsten filament is run at a higher temperature resulting in brighter, whiter light and higher efficiency. Ordinarily, this would result in short bulb life, so the bulb is (1) filled with "halogen" gas which slows the rate of evaporation of the tungsten (2) made smaller so the glass temperature is much hotter (this helps prevent tungsten from condensing on the bulb) and (3) made of a special "quartz" glass to tolerate the high temperature. Quartz-Halogen light is a very bright white (less red component) which aids the eye in perceiving detail. The most common applications are vehicle headlights, projectors, and spot-lights for displaying art work and merchandise.
(3) FLUORESCENT: Electric current flows thru a gas-filled glass tube, generating ultraviolet light (invisible). The tube is coated on the inside with a phosphorescent material which absorbs the ultraviolet and glows white. Very little heat is generated and efficiency is high. All fluorescent tubes require over 100 volts to operate, so low voltage fluorescents use a transistorized "ballast" to step up the voltage. LOW VOLTAGE DC FLUORESCENTS USE THE SAME TUBES AS 120 vac LIGHTS.
Incandescents, quartz-halogen and fluorescent lights differ in five major ways: (1) efficiency (2) life expectancy (3) installed cost (4) light quality and (5) light dispersion. Consider each separately.
(1) EFFICIENCY: Quartz-Halogen bulbs average 30% higher efficiency than incandescents. (Higher efficiency claims are based on comparison with worst-case incandescents.)
Fluorescent lights average 3 times the efficiency of low voltage incandescents (5 times compared with 120V incandescents!). We are assuming high quality fluorescents. (Some cheap ones are dim and less efficient in comparison).
Efficiency may vary widely even within the same class of light. For instance, low voltage (12 or 24V) incandescents are more efficient than 120 volt (common household) bulbs. This is because the low voltage bulb has a shorter, thicker filament (to pass higher current)
20 Home Power #9 • February/ March1989
so it is physically stronger, allowing a higher operating temperature. Just the shift from 120 volt incandescent bulbs to 12/24V bulbs (inexpensive RV and automotive bulbs) can reduce energy usage by an average 40%! Within the same voltage, incandescents vary. Long life and rough service bulbs run a cooler filament and have the lowest efficiency.
(2) LIFE EXPECTANCY: Incandescents have the shortest life, typically 1,000 hours (about a year of every-evening use.) Quartz- Halogen bulbs last longer--about 3,000 hours. Unlike incandescents, quartz-halogen bulbs do not blacken over time. They retain peak efficiency until the end. High quality DC fluorescents last longer yet--up to 10,000 hours, which can be 10 years of living room use!
(3) INSTALLED COST: Most fluorescent lights come with their own fixtures, ready to screw right to the wall or ceiling. The installed cost of a quartz-halogen or incandescent bulb must include the cost of a light fixture. Quartz-halogen bulbs cost 3 to 10 times as much as incandescents. However, their superior performance make them popular in renewable energy homes. Good fluorescents also cost 3 to 10 times aa much as incandescents (when you count the cost of incandescent fixtures). But, their cost is easily justified by radical gains in efficiency and life expectancy.
WIRING COST (FOR DC CIRCUITS): 1/3 the power requirement means wire may be two sizes smaller. Smaller wire costs less and requires less labor to install. Undersized wire causes voltage drop and reduced light output. For fluorescent lights, a voltage drop of 10% will cause a 10% drop in light output. But, in incandescent or quartz-halogen light circuits, BEWARE! Light output will drop by 25% because lower filament temperature causes further reduction in efficiency! Where wire runs are long (or existing wire is small) fluorescents may be clearly economical even for lights that are seldom used--their INSTALLED cost is less.
A 12 volt DC home using incandescent lights must use AVERAGE #10 wire, which is stiff and awkward to work with. The smaller #12 and #14 wire used in conventional ac homes can cause 12V lights to burn dimly. High efficiency lighting allows use of these smaller wire sizes, at least for some of the wiring in a 12V home, but NOTE: A 24 volt system requires one quarter the wire size of 12V, so conventional home wiring can handle nearly all 24V lighting. See wire size charts in most alternative energy catalogs and reference books for specifics. (24V systems may also run 12V lights and appliances using a "Battery Equalizer"--See HP#6.)
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