SOLAR ELECTRIC
Solar electric System Basics
edited by joseph
McCabe, P.e.
Photovoltaic (PV) technology produces electricity from sunlight, using solid-state materials with no
grid at other times, like at night. This process is called
net-metering.
moving parts. It’s a mature technology, first invented by
In an off-grid system, DC power flows from the
Alexandre-Edmond Becquerel in 1839 and initially com-
modules through a charge controller (also called a
mercialized at Bell Labs in the 1950s.
For residential application, PV falls into two main
This article is adapted from
categories. First is grid-tied, where the home generates
the Solar Energy Resource
its own electricity but can also draw power from the util-
Guide 2008, published by
ity company at night. The second is off-grid, where the
the NorCal Solar Energy As-
home is located too far from an electrical utility cable and
sociation ( norcalsolar.org),
the home must generate its own power, storing energy in
a chapter of the American
batteries for use at night.
Solar Energy Society. It was
A basic home PV system consists of PV cells con-
edited and updated by Joe
nected and packaged together in weather-protected
McCabe, P. E. (energyideas@
modules, which are fastened side-by-side on a racking
gmail.com), an ASES Fellow
system to form an array. The PV modules produce direct
and Lifetime ASES member.
current (DC), which in a grid-tied system flows to a grid-
interactive inverter. An inverter changes DC voltage to
the alternating current (AC) for the household electric
circuit powering wall outlets and all AC appliances.
Excess power from the inverter may flow out of the
house through the utility company’s electric meter,
into the city-wide grid. When this happens, the meter
may run backward, and the utility will credit the out-
flowing electricity against electricity purchased from the
regulator), which is an electronic device that produces a smooth flow of current at the desired voltage. From the charge controller, the power can go to a set of storage bat- teries and then on to the inverter, as needed. Today’s commercially available PV panels come in three versions: Single-crystal (or monocrystalline) modules are currently the most efficient — that is, 1 square meter produces the most electric power. They must be mounted in a rigid frame. Multicrystalline (or polycrystalline) modules are made of cells cut from multiple crystals, grown together in an ingot. They are slightly less efficient than single- crystal. Thin-film modules are made by depositing thin layers of materials on glass, metal or plastic substrates. They’re considerably less efficient so you may need more space to generate the same amount of power, but they’re less expensive and, depending on the substrate, can be very robust and flexible. One practical use is to glue a flexible thin-film module directly to a metal roof.
What are PV cells made of, and how? Crystalline PV cells use
silicon, a little bit of boron and phosphorus along with anti-reflection
materials and a screen printing of electrically conductive grid lines on
the top and a coating of aluminum on the bottom to collect the electrons. The cells are made by liquefying the silicon (derived from pure
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 sawed into very thin wafers, to produce
the individual cells.
Thin-film modules are made from very thin layers deposited on an
electrical conducting surface. These materials may originate as silane
dan wiLLiamS
gas for amorphous silicon, cadmium and tellurium for Cd Te, or
copper, indium, gallium and selenium for CIGS. The deposition tech-
niques may include sputtering, co-evaporation in a vacuum, electro-
deposition, sintering or other techniques. Many variations of thin-film
If the roof is shaded or faces the wrong direction,
consider a ground-mount array.
materials are being investigated for low-cost manufacturing and higher
solar-to-electrical efficiencies.
