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ECONOMICS OF SOLAR HOMES

Approximately 400,000 families in the developing world [last I heard, there are over 40,000 off-the-grid households in the USA. The more current numbers] are already using small, household solar electric systems to power fluorescent lights, radio-cassette players, 12 volt black-and-white TVs, and other small appliances. These families, living mostly in remote rural areas, already constitute the largest group of domestic users of PVs [photovoltaics] in the world. For them, there is no other affordable or immediately available source of electric power. These systems have been sold mostly by small entrepreneurs applying their working knowledge of this proven technology to serve rural families who need small amounts of power for electric lights, radios and TVs.

PV is measured in units of "peak watts"(Wp). A peak watt figure refers to the power output of the module under "peak sun" conditions, considered to be 1000 Watts per square meter. "Sun hours," or "insolation," refers to how many hours of peak sun, on average, exist in different countries. North America averages 3 to 4 peak sun hours per day in summer while eqatorial regions can reach above 6 peak sunlight hours. [For those of us in North America, one critical question is winter peak, as the people in Maine and Quebec recently found out.]

Solar Home System:A standard small SHS can operate several lights, a black-and-white television, a radio or cassette player, and a small fan. A 35 Wp SHS provides enough power for four hours of lighting from four 7W compact fluorescent lights (CFLs) each evening [I use 15W compact florescents for my bedroom reading], as well as several hours of television. "System Size" (20, 35, or 50Wp) determines the number of "light-hours" or "TV-hours" available.

Solar Home Systems are 12-volt direct-current (DC) stand-alone systems which use PV to electrify small rural homes. Each SHS includes a PV module, a battery, a charge controller, wiring, compact fluorescent lights (CFLs), and outlets for other appliances. Descriptions of the components follow:

Module:Solar modules for an SHS range between 20-60 Wp. They are mounted on a rooftop or atop a pole [I betcha they'd attach to a south facing window just fine]. Both crystalline and thin-film technologies are appropriate for an SHS, with price, weight, long-term guarantees and degradation being the determining factors.

Battery:An electrochemical storage battery is used to store the electricity converted by the solar module. During the day, electricity from the module charges the storage battery. During the evening, the battery is discharged to power lights and other applications. Batteries are typically 12-volt lead-acid batteries, ranging in capacity from 20-100 Amp-Hours (Ah). Batteries are typically sized to provide several days of electricity or "autonomy", in the event that overcast weather prevents recharging.

Deep-cycle batteries are best for an SHS, as they are designed to operate over larger ranges of charge levels. While car batteries are only designed to be discharged 15% of their maximum charge, deep-cycle batteries can be discharged to 70-80% without incurring damage. Both deep-cycle and automotive batteries are typically used, as they are readily available throughout the developing world. Car batteries have a 3-5 year lifetime; deep-cycle, both sealed and unsealed, can last 7-10 years.

Charge Controller:A charge controller is utilized to control the flow of electricity between the module, battery, and the loads. It prevents battery damage by ensuring that the battery is operating within its normal charge levels. If the charge level in the battery falls below a certain level, a "low voltage disconnect (LVD)" will cut the current to the loads, to prevent further discharge. Likewise, it will also cut the current from the module in cases of overcharging. Indicator lights on the controller display the relative state of charge of the battery. [I want a universal charge controller, multitester control panel that charges all the different kinds of batteries in the house.]

Lights:Compact fluorescent lightbulbs as well as fluorescent tube lights are used for lighting. An SHS normally includes two to six lights. By utilizing efficient fluorescent lighting, an SHS can provide substantially higher lighting levels than would be possible with incandescent lighting. A 9 watt CFL provides equivalent illumination to a 60 watt incandescent bulb. Compact fluorescent lights have a 5 year lifetime; tubes have much shorter lives, but are cheaper and are more readily available in most developing countries.

CONTINUED ON NEXT PAGE -->Written by: Solar Electric Light Fund


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