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Photovoltaics (PV) convert sunlight directly into electricity. Photons in sunlight interact with the outermost electrons of an atom. Photons striking the atoms of a
semiconducting solar cell free it's electrons, creating an electric current. The Photovoltaic effect was first discovered in the 19th century, and was used by Bell Labs
in 1954 to develop the first PV solar cell. PV found its first applications in space, providing electricity to satellites. These early PV cells were produced in small
quantities from exotic materials. While early cells were inefficient, converting less than 1% of the incident sunlight into electricity, they quickly increased to 6%
when researchers experimented with crystalline silicon, the principal component of sand. Current conversion efficiencies have surpassed 30% in the laboratory, and
15% in large-scale production.
Two main types of silicon cells vie for market share: crystalline and thin-film. Crystalline silicon cells are produced by slowly extracting large crystals from a liquid
silicon bath. These crystals are sliced into 1/100th-of-an-inch thick slices, or "wafers", which are processed into solar cells that are then connected and laminated into
solar "modules." While this production process yields highly efficient (10-15%) cells, the production process is expensive. Thin-film silicon cells are produced by
depositing vaporized silicon directly onto a glass or stainless steel substrate. While the efficiencies achieved are lower than with crystalline silicon, the production
process is less expensive. Modules from crystalline cells have a lifetime of over twenty years. Thin-film modules will last at least ten years. Other PV technologies,
such as Gallium-Arsenide or Cadmium Telluride, are also being used. These types are highly efficient, but more expensive at the present time.
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.
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 lamps each evening, 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, fluorescent lights, 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. 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.
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.
Wiring & Mounting:
An SHS also contains additional materials for mounting and connections. Metal frames are included to attach the PV Modules to a pole or roof. SHS components are
connected by wires and contain switches for the lights. In some cases, wiring is housed inside conduit attached to interior walls.
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A study for the U.S. government calculated that the gasoline equivalent of the energy saved over the lifetime of one 24-watt compact fluorescent bulb is sufficient to drive a Prius from New York to San Francisco.
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