Title: Photovoltaics
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Permanent Link: http://ufdc.ufl.edu/CA01300661/00001
 Material Information
Title: Photovoltaics
Physical Description: Book
Language: English
Creator: Virgin Islands Energy Office
Publisher: Virgin Islands Energy Office
Publication Date: 2007
 Subjects
Subject: Caribbean   ( lcsh )
Spatial Coverage: North America -- United States Virgin Islands
Caribbean
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Bibliographic ID: CA01300661
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

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DEPARTMENT OF PLANNING AND NATURAL RESOURCES -
VIRGIN ISLANDS ENERGY OFFICE '
45 ESTATE MARS HILL
FREDERIKSTED, VIRGIN ISLANDS 00840 '
^ W S TELEPHONE 340 773-1082 STX FAX 340 772-0063 '
340 774-3320 STT FAX 340 714-9531
www.vienergy.org


PHOTOVOLTAICS


Photovoltaics (PV) or solar cells as
they are often referred to, are
semiconductor devices that convert
sunlight into direct current (DC)
electricity. Groups of PV cells are
electrically configured into modules
and arrays, which can be used to
charge batteries, operate motors, and, -:- 'p
to power any number of electrical
loads. With the appropriate power conversion equipment, PV systems
can produce alternating current (AC) compatible with any conventional
appliances, and operate in parallel with and interconnected to the utility
grid.

HISTORY OF PHOTOVOLTAICS

The first conventional photovoltaic cells were produced in the late
1950s, and throughout the 1960s were principally used to provide
electrical power for earth-orbiting satellites. In the 1970s, improvements
in manufacturing, performance and quality of PV modules helped to
reduce costs and opened up a number of opportunities for powering
remote terrestrial applications, including battery charging for
navigational aids, signals, telecommunications equipment and other
critical, low power needs.

In the 1980s, photovoltaics became a popular power source for
consumer electronic devices, including calculators, watches, radios,
lanterns and other small battery charging applications. Following the
energy crises of the 1970s, significant efforts also began to develop PV
power systems for residential and commercial uses both for stand-alone,
remote power as well as for utility-connected applications. During the






same period, international applications for PV systems to power rural
health clinics, refrigeration, water pumping, telecommunications, and
off-grid households increased dramatically, and remain a major portion
of the present world market for PV products. Today, the industry's
production of PV modules is growing at approximately 25 percent
annually, and major programs in the U.S., Japan and Europe are rapidly
accelerating the implementation of PV systems on buildings and
interconnection to utility networks.

HOW PV CELLS WORK

A typical silicon PV cell is composed of a thin wafer consisting of an
ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker
layer of boron-doped (P-type) silicon. An electrical field is created near
the top surface of the cell where these two materials are in contact,
called the P-N junction. When sunlight strikes the surface of a PV cell,
this electrical field provides momentum and direction to light-stimulated
electrons, resulting in a
flow of current when
the solar cell is Electrical load
connected to an (- sun
electrical load.
T C c DPhotovoltaic cell
Phosphorous-doped (N-type)
Boron-doped (P-.,typ) S imon Mv -r 3 :m
S Mp %co n -2ye 0- M; m


Figure 1. Diagram of photovoltaic cell.

Regardless of size, a typical silicon PV cell produces about 0.5 0.6 volt
DC under open-circuit, no-load conditions. The current (and power)
output of a PV cell depends on its efficiency and size (surface area), and
is proportional the intensity of sunlight striking the surface of the cell.
For example, under peak sunlight conditions a typical commercial PV
cell with a surface area of 160 cm^2 (-25 in^2) will produce about 2
watts peak power. If the sunlight intensity were 40 percent of peak, this
cell would produce about 0.8 watts.






PV CELLS, MODULES, & ARRAYS


\ Photovoltaic cells are connected
electrically in series and/or parallel
circuits to produce higher voltages,
currents and power levels.
Photovoltaic modules consist of PV
cell circuits sealed in an
environmentally protective laminate,
and are the fundamental building
block of PV systems. Photovoltaic panels include one or more PV
modules assembled as a pre-wired, field-installable unit. A photovoltaic
array is the complete power-generating unit, consisting of any number of
PV modules and panels.



cell module



*_- -__-L g]^ J -L^ *L-L.-j.j_ ----- -- ---***




Figure 2. Photovoltaic cells, modules, panels and arrays.

The performance of PV modules and arrays are generally rated
according to their maximum DC power output (watts) under Standard
Test Conditions (STC). Standard Test Conditions are defined by a
module (cell) operating temperature of 25o C (77 F), and incident solar
irradiance level of 1000 W/m2 and under Air Mass 1.5 spectral
distribution. Since these conditions are not always typical of how PV
modules and arrays operate in the field, actual performance is usually 85
to 90 percent of the STC rating.

Today's photovoltaic modules are extremely safe and reliable products,
with minimal failure rates and projected service lifetimes of 20 to 30
years. Most major manufacturers offer warranties of twenty or more
years for maintaining a high percentage of initial rated power output.






When selecting PV modules, look for the product listing (UL),
qualification testing and warranty information in the module
manufacturer's specifications.

HOW A PV SYSTEM WORKS
Simply put, PV systems are like any other electrical power generating
systems, just the equipment used is different than that used for
conventional electromechanical generating systems. However, the
principles of operation and interfacing with other electrical systems
remain the same, and are guided by a well-established body of electrical
codes and standards. Although a PV array produces power when
exposed to sunlight, a number of other components are required to
properly conduct, control, convert, distribute, and store the energy
produced by the array.

Depending on the functional and operational requirements of the system,
the specific components required, and may include major components
such as a DC-AC power inverter, battery bank, system and battery
controller, auxiliary energy sources and sometimes the specified
electrical load (appliances). In addition, an assortment of balance of
system (BOS) hardware, including wiring, overcurrent, surge protection
and disconnect devices, and other power processing equipment. Figure 3
show a basic diagram of a photovoltaic system and the relationship of
individual components.

energy ne
Inverson & use
ener\ condloning d
source

energy
/ /l --p B -- I dlmobullhon
converson



Figure 3. Major photovoltaic system components.


Why Are Batteries Used in Some PV Systems?







Batteries are often used in PV systems for the purpose of storing energy
produced by the PV array during the day, and to supply it to electrical
loads as needed (during the night and periods of cloudy weather). Other
reasons batteries are used in PV systems are to operate the PV array near
its maximum power point, to power electrical loads at stable voltages,
and to supply surge currents to electrical loads and inverters. In most
cases, a battery charge controller is used in these systems to protect the
battery from overcharge and overdischarge.

TYPES OF PV SYSTEMS

How Are Photovoltaic Systems Classified?

Photovoltaic power systems are generally classified according to their
functional and operational requirements, their component
configurations, and how the equipment is connected to other power
sources and electrical loads. The two principle classifications are grid-
connected or utility-interactive systems and stand-alone systems.
Photovoltaic systems can be designed to provide DC and/or AC power
service, can operate interconnected with or independent of the utility
grid, and can be connected with other energy sources and energy storage
systems. 1.7.1 Grid-Connected (Utility-Interactive) PV Systems.

Grid-connected or utility-interactive PV systems are designed to
operate in parallel with and interconnected with the electric utility grid.
The primary component in grid-connected PV systems is the inverter, or
power conditioning unit (PCU). The PCU converts the DC power
produced by the PV array into AC power consistent with the voltage and
power quality requirements of the utility grid, and automatically stops
supplying power to the grid when the utility grid is not energized. A bi-
directional interface is made between the PV system AC output circuits
and the electric utility network, typically at an on-site distribution panel
or service entrance. This allows the AC power produced by the PV
system to either supply on-site electrical loads, or to back feed the grid
when the PV system output is greater than the on-site load demand. At
night and during other periods when the electrical loads are greater than
the PV system output, the balance of power required by the loads is
received from the electric utility This safety feature is required in all
grid-connected PV systems, and ensures that the PV system will not






continue to operate and feed back onto the utility grid when the grid is
down for service or repair.

AC Loads



PV Array Inverter/Power Distribution
Conditioner Panel


Electric
Utility

Figure 4. Diagram of grid-connected photovoltaic system.


Stand-Alone Photovoltaic Systems


Stand-alone PV systems are designed to operate independent of the
electric utility grid, and are generally designed and sized to supply
certain DC and/or AC electrical loads. These types of systems may be
powered by a PV array only, or may use wind, an engine-generator or
utility power as an auxiliary power source in what is called a PV-hybrid
system. The simplest type of stand-alone PV system is a direct-coupled
system, where the DC output of a PV module or array is directly
connected to a DC load (Figure 5). Since there is no electrical energy
storage (batteries) in direct-coupled systems, the load only operates
during sunlight hours, making these designs suitable for common
applications such as ventilation fans, water pumps, and small circulation
pumps for solar thermal water heating systems. Matching the impedance
of the electrical load to the maximum power output of the PV array is a
critical part of designing well-performing direct-coupled system. For
certain loads such as positive-displacement water pumps, a type of
electronic DC-DC converter, called a maximum power point tracker
(MPPT) is used between the array and load to help better utilize the
available array maximum power output.






PV Array %-- DC Load



Figure 5. Direct-coupled PV system.

In many stand-alone PV systems, batteries are used for energy storage.
Figure 6 shows a diagram of a typical stand-alone PV system powering
DC and AC loads. Figure 7 shows how a typical PV hybrid system
might be configured.


Figure 6. Diagram of stand-alone PV system with battery storage
powering DC and AC loads.


Figure 7. Diagram of photovoltaic hybrid system.






Pros and Cons of PV


Photovoltaic systems have a number of merits and unique advantages
over conventional power-generating technologies. PV systems can be
designed for a variety of applications and operational requirements, and
can be used for either centralized or distributed power generation. PV
systems have no moving parts, are modular, easily expandable and even
transportable in some cases. Energy independence and environmental
compatibility are two attractive features of PV systems. The fuel
(sunlight) is free, and no noise or pollution is created from operating PV
systems. In general, PV systems that are well designed and properly
installed require minimal maintenance and have long service lifetimes.

At present, the high cost of PV modules and equipment (as compared to
conventional energy sources) is the primary limiting factor for the
technology. Consequently, the economic value of PV systems is realized
over many years. In some cases, the surface area requirements for PV
arrays may be a limiting factor. Due to the diffuse nature of sunlight and
the existing sunlight to electrical energy conversion efficiencies of
photovoltaic devices, surface area requirements for PV array
installations are on the order of 8 to 12 m^2 (86 to 129 ft^2) per kilowatt
of installed peak array capacity.

Q. Can photovoltaic systems operate normally in grid-connected mode,
and still operate critical loads when utility service is disrupted?

A. Yes, however battery storage must be used. This type of system is
extremely popular for homeowners and small businesses where critical
backup power supply is required for critical loads such as refrigeration,
water pumps, lighting and other necessities. Under normal
circumstances, the system operates in grid-connected mode, serving the
on-site loads or sending excess power back onto the grid while keeping
the battery fully charged. In the event the grid becomes de-energized,
control circuitry in the inverter opens the connection with the utility
through a bus transfer mechanism, and operates the inverter from the
battery to supply power to the dedicated loads only. In this
configuration, the critical loads must be supplied from a dedicated sub
panel. Figure 8 shows how a PV system might be configured to operate
normally in grid-connected mode and also power critical loads from a






battery bank when the grid is de-energized.


Figure 8. Diagram of grid-connected critical power supply system.




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