solar electricity
Seth maSIa
all about Photovoltaic Systems
edited by
Photovoltaic (PV) technology produces electric- ity from sunlight, using solid-state materials with no moving parts. It’s a mature technology, first invented by alexandre-edmond becquerel in 1839 and initially commercialized at bell
labs in the 1950s.
for residential applications, PV falls into two main cat-
egories. first, there is grid-tied, where the home generates
its own electricity but can also draw power from the utility
What are PV cells made of, and how are they
made? >> crystalline PV cells use silicon, a little bit of
boron and phosphorus. anti-reflection materials (the blue
color element) and a screen printing of electrically conductive
grid lines go on the top, and the bottom receives a coating
of aluminum to collect the electrons. the cells are made by
liquefying the silicon (derived from simple sand) at high temperatures and then slowly cooling the material in a way that
makes large crystals. For single crystals, a cylindrical boule is
very slowly pulled from the molten silicon. Polycrystals are
cooled in a block formed by quartz glass, making grains of
crystals as large as possible. the solid materials are turned
into very thin wafers, to produce the cells.
Thin-film PV modules are made from very thin layers deposited on an electrical conducting surface. these materials may
originate as silane gas for amorphous silicon, cadmium and
tellurium for Cdte thin film or copper, indium, gallium and
selenium for CIgS thin film. the deposition techniques may
include sputtering, co-evaporation in a vacuum, electrodeposition, sintering or other techniques. many variations of
thin-film materials are being investigated for low-cost manufacturing and higher solar-to-electrical efficiencies.
Electric power is measured in watts. >> a kilowatt
(k W) is 1,000 watts, and a megawatt (m W) is 1 million watts.
you buy electricity in kilowatt-hours (k Wh), which is energy
(as opposed to power). For instance, if you run a 100-watt
light bulb for an hour, you’ve used 100 watt-hours of energy.
If you run it for 10 hours, you’ve used 1 k Wh, for which the
average household would be billed 11 cents (at $0.11/k Wh).
a 1 k W Pv array can produce 1 k Wh per hour in direct sun. If
sunlight falls directly on it for five hours a day, it may produce
5 k Wh that day.
company at night. The second is off-grid, where the home
is located too far from an electrical utility cable and must
generate its own power, storing energy in batteries for use
at night.
Tied to the Grid
a basic home PV system consists of PV cells connected
and packaged together in weather-protected modules,
which are fastened side by side on a racking system to form
an array. The PV modules produce direct current (DC),
which, in a grid-tied system, flows to a grid-interactive
inverter. The inverter changes DC voltage to the alternating
current (aC) that powers the household electric system:
wall and ceiling lamps, outlets and appliances.
excess power from the inverter may flow out of the
house, through the utility company’s electric meter and into
the city-wide grid. When this happens, the homeowner’s
meter may run backward, and the utility will credit the outflowing electricity against electricity purchased from the
grid at night. This process is called net metering.
In an off-grid system, DC power flows from the
