Solar Power

Basics of Solar PV

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BASICS OF SOLAR ENERGY

Sunlight—solar energy—can be used to generate electricity, provide hot water,
and to heat, cool, and light buildings.
Sunlight is a limitless resource that is never going to stop shining. In just one minute there is
enough solar energy reaching the earth from the sun to meet the worlds total energy
requirements for a whole year!
Special solar cells called Photovoltaic Cells or ‘PV’ Cells are able to convert sunlight into
electricity.
Solar electricity systems capture energy from the sun to produce electricity that can be used to
power household appliances, heating and lighting.

Photovoltaic panels only require light in order to produce electricity. PV panels are commonly
used to provide small amounts of electricity and are often found providing power for
calculators, phone chargers, watches, road signs and alarm systems.
By using large PV panels or by linking a number of panels together, greater amounts of
electricity can be produced in order to power household appliances, heating and lighting.
These panels are commonly mounted on top, or integrated into, the roof of your home.
There are many different types of PV panels and PV arrays available in a variety of shapes,
sizes and colours. In addition to basic panels they are available as solar tiles that look similar
to roof tiles and even transparent sheets that can be installed on conservatory roofs.
PV panels and PV arrays are made to withstand the outside elements and because they have
no moving parts they are usually guaranteed to last for 20 to 25 years and often have a life
expectancy of more than 50 years.


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Photovoltaic (solar cell) systems convert sunlight directly into electricity.
A solar or PV cell consists of semiconducting material that absorbs the sunlight.
The solar energy knocks electrons loose from their atoms, allowing the electrons
to flow through the material to produce electricity. PV cells are typically
combined into modules that hold about 40 cells. About 10 of these modules are
mounted in PV arrays. PV arrays can be used to generate electricity for a single
building or, in large numbers, for a power plant. A power plant can also use a
concentrating solar power system, which uses the sun's heat to generate
electricity. The sunlight is collected and focused with mirrors to create a high-
intensity heat source. This heat source produces steam or mechanical power to
run a generator that creates electricity.
Solar water heating systems for buildings have two main parts: a solar
collector and a storage tank. Typically, a flat-plate collector—a thin, flat,
rectangular box with a transparent cover—is mounted on the roof, facing the
sun. The sun heats an absorber plate in the collector, which, in turn, heats the
fluid running through tubes within the collector. To move the heated fluid
between the collector and the storage tank, a system either uses a pump or
gravity, as water has a tendency to naturally circulate as it is heated. Systems
that use fluids other than water in the collector's tubes usually heat the water by
passing it through a coil of tubing in the tank.
Many large commercial buildings can use solar collectors to provide more
than just hot water. Solar process heating systems can be used to heat these
buildings. A solar ventilation system can be used in cold climates to preheat air
as it enters a building. And the heat from a solar collector can even be used to
provide energy for cooling a building.
A solar collector is not always needed when using sunlight to heat a
building. Some buildings can be designed for passive solar heating. These
buildings usually have large, south-facing windows. Materials that absorb and
store the sun's heat can be built into the sunlit floors and walls. The floors and
walls will then heat up during the day and slowly release heat at night—a
process called direct gain. Many of the passive solar heating design features also
provide daylighting. Daylighting is simply the use of natural sunlight to brighten
up a building's interior.

SOLAR TECHNOLOGIES
1.0 Photovoltaics
Photovoltaic solar cells, which directly convert sunlight into electricity,
are made of semiconducting materials. The simplest photovoltaic cells power
watches and calculators and the like, while more complex systems can light
houses and provide power to the electrical grid.
Have you ever wondered how electricity is produced by a photovoltaic—
what we often call a PV or solar electric—system? We'll help you understand by

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covering the basics of PV technology, which includes the underlying physics,
how various PV devices are designed and become fully functional systems, and
what's happening today in PV research and development.
The Solar Energy Technologies Program of the U.S. Department of
Energy (DOE) and its partners are adding to our fundamental knowledge and
expertise in this area while improving the technologies that put the abundant
energy of sunlight to work for us.
To help you delve further into this fascinating topic, we've compiled
additional information sources at the bottom of many of these pages that will
direct you to other pages within our own Web site, as well as to other helpful
Web sites. While perusing this material, you may wonder what a specific term
means. If so, visit our solar glossary for a comprehensive listing of renewable
energy and electrical terms.

1.1 PV Physics
What do we mean by photovoltaics? First used in about 1890, the word
has two parts: photo, derived from the Greek word for light, and volt, relating to
electricity pioneer Alessandro Volta. So, photovoltaics could literally be
translated as light-electricity. And that's what photovoltaic (PV) materials and
devices do—they convert light energy into electrical energy (Photoelectric
Effect), as French physicist Edmond Becquerel discovered as early as 1839.
Commonly known as solar cells, individual PV cells are electricity-
producing devices made of semiconductor materials. PV cells come in many
sizes and shapes—from smaller than a postage stamp to several inches across.
They are often connected together to form PV modules that may be up to several
feet long and a few feet wide. Modules, in turn, can be combined and connected
to form PV arrays of different sizes and power output.
The size of an array depends on several factors, such as the amount of sunlight
available in a particular location and the needs of the consumer. The modules of
the array make up the major part of a PV system, which can also include
electrical connections, mounting hardware, power-conditioning equipment, and
batteries that store solar energy for use when the sun isn't shining.
Did you know that PV systems are already an important part of our lives?
Simple PV systems provide power for many small consumer items, such as
calculators and wristwatches. More complicated systems provide power for
communications satellites, water pumps, and the lights, appliances, and
machines in some people's homes and workplaces. Many road and traffic signs
along highways are now powered by PV. In many cases, PV power is the least
expensive form of electricity for performing these tasks.
1.1.1 The Photoelectric Effect
In 1839, Edmond Becquerel discovered the process of using sunlight to
produce an electric current in a solid material. But it took more than another
century to truly understand this process. Scientists eventually learned that the

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photoelectric or photovoltaic (PV) effect caused certain materials to convert
light energy into electrical energy at the atomic level.


To broaden their scientific perspective on a new generation of silicon PV devices, the U.S.
Department of Energy's National Renewable Energy Laboratory has pioneered a new class of
materials — microcrystalline silicon alloys — which may have application in the
photovoltaic and microelectronics industries. To date, the scientists have deposited and
characterized 50 microcrystalline silicon films.

The photoelectric effect is the basic physical process by which a PV cell
converts sunlight into electricity. When light shines on a PV cell, it may be
reflected, absorbed, or pass right through. But only the absorbed light generates
electricity.
The energy of the absorbed light is transferred to electrons in the atoms of
the PV cell. With their newfound energy, these electrons escape from their
normal positions in the atoms of the semiconductor PV material and become
part of the electrical flow, or current, in an electrical circuit. A special electrical
property of the PV cell—what we call a "built-in electric field"—provides the
force, or voltage, needed to drive the current through an external "load," such as
a light bulb.




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To induce the built-in electric field within a PV cell, two layers of
somewhat differing semiconductor materials are placed in contact with one
another. One layer is an "n-type" semiconductor with an abundance of electrons,
which have a negative electrical charge. The other layer is a "p-type"
semiconductor with an abundance of "holes," which have a positive electrical
charge.
Although both materials are electrically neutral, n-type silicon has excess
electrons and p-type silicon has excess holes. Sandwiching these together
creates a p/n junction at their interface, thereby creating an electric field.
When n- and p-type silicon come into contact, excess electrons move from the
n-type side to the p-type side. The result is a buildup of positive charge along the
n-type side of the interface and a buildup of negative charge along the p-type
side.
Because of the flow of electrons and holes, the two semiconductors
behave like a battery, creating an electric field at the surface where they meet—
what we call the p/n junction. The electrical field causes the electrons to move
from the semiconductor toward the negative surface, where they become
available to the electrical circuit. At the same time, the holes move in the
opposite direction, toward the positive surface, where they await incoming
electrons.
How do we make the p-type ("positive") and n-type ("negative") silicon
materials that will eventually become the photovoltaic (PV) cells that produce
solar electricity? Most commonly, we add an element to the silicon that either
has an extra electron or lacks an electron. This process of adding another
element is called doping.

How a PV cell works
PV cells are made from semi-conducting materials, most commonly this is silicon. When
exposed to light PV cells create electricity by making use of the photovoltaic effect.
Sunlight is made up of tiny packets of energy called photons. When PV cells are exposed to
light they are able to absorb photons. These photons excite the atoms in the semi-conducting
materials and cause them to move to the bottom of the PV cell where they exit through the
connecting wire. This flow of electrons is known as electricity.

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Diagram Solar PV 2.
They are referred to as cells because like power cells, more commonly known as batteries,
they produce Direct Current (DC) electricity.
A single PV cell only produces a small amount of electricity, so by putting a number of these
cells together into a panel, larger and more useful amounts of electricity can be produced.

1.1.2 Light and the PV Cell
We've looked at how to construct a solar cell using crystalline silicon.
And we've used this basic type of cell to explain the photoelectric effect, which
is the phenomenon operating at the heart of a solar cell. Here, we want to take a
look at sunlight, the energy source actually used by solar cells. A brief
discussion of several terms will help us better understand aspects of light's
interaction with solar cells.

a) Wavelength, Frequency, and Energy
The energy from the sun is vital to life on Earth. It determines the Earth's
surface temperature and supplies virtually all the energy that drives natural
global systems and cycles. Some other stars are enormous sources of energy in
the form of X-rays and radio signals, but our sun releases the majority of its
energy as visible light. However, visible light represents only a fraction of the
total spectrum of radiation. Specifically, infrared and ultraviolet rays are also
significant parts of the solar spectrum.