Download 7. Solar Electric System Components

7. Solar Electric System Components
The components of a solar electric system are:
 solar cells
 solar modules
 inverters
 racks and mounting
systems
 roof attachments
 wiring and
interconnection
components
 monitoring system
 battery systems
 generators
Solar cells make up the building blocks of a PV system. Multiple cells are usually
combined into a complete panel that also includes a frame for the system,
electrical interconnections and mounting hardware. Although photovoltaic solar
modules are the most conspicuous and expensive component, a complete
system also includes an inverter to transform the DC output from the cells into
usable AC current (also called an inverter), wiring to connect the components,
and collector racks so that the collectors can be securely mounted to a roof or on
the ground at the appropriate angle. Solar cells, as well as these "Balance of
System" components are described in the following sections.
7.1 Solar Cells
Photovoltaic cells convert light
energy into electricity at the atomic
level. Although first discovered in
1839, the process of producing
electric current in a solid material
with the aid of sunlight wasn't truly
understood for more than a hundred
years. Throughout the second half of
the 20th century, the principals
underlying the photovoltaic effect
have been determined and the
manufacturing processes have been
more fully refined. As a result, the
cost of these devices has put them
into the mainstream of modern
energy producers. This was caused
in part by advances in the
technology - in which PV conversion efficiencies have improved considerably -
an in part by improvements in manufacturing all the other components in a
complete system.
The conversion efficiency of a PV cell is defined as the ratio of the sunlight
energy that hits the cell divided by the electrical energy that is produced by the
cell. This is very important when discussing PV devices, because by affordably
improving this efficiency, PV energy becomes competitive with fossil fuel
sources. For comparison, the earliest PV devices converted about 1%-2% of
sunlight energy into electric energy. Today's PV devices convert 7%-17% of light
energy into electric energy. Moreover, today's mass produced panel systems are
substantially less expensive than earlier systems.
The "photovoltaic effect" is the basic physical process through which a PV cell
converts sunlight into electricity. Sunlight is composed of photons, or particles of
solar energy. These photons contain various
amounts of energy corresponding to the
different wavelengths of the solar spectrum.
When photons strike a PV cell, they may be
reflected or absorbed, or they may pass right
through. Only the absorbed photons generate
electricity. When this happens, the energy of
the photon is transferred to an electron in an
atom of the cell (which is actually a semiconductor). With its newfound energy,
the electron is able to escape from its normal position associated with that atom
to become part of the current in an electrical circuit. By leaving this position, the
electron causes a "hole" to form. Special electrical properties of the PV cell—a
built-in electric field—provide the voltage needed to drive the current through an
external load (such as a light bulb).
The most important parts of a solar cell are the semiconductor layers, because
this is where the electron current is created.
There are a number of different materials
suitable for making these semi-conducting
layers, and each has benefits and drawbacks.
Unfortunately, there is no one ideal material for
all types of cells and applications. In addition to
the semi conducting materials, solar cells
consist of a top metallic grid or other electrical
contact to collect electrons from the
semiconductor and transfer them to the
external load, and a back contact layer to
complete the electrical circuit. Then, on top of
the complete cell is typically a glass cover or other type of transparent
encapsulated to seal the cell and keep weather out, and an antireflective coating
to keep the cell from reflecting the light back away from the cell.
7.1.1 p-Types, n-Types, and the Electric Field
To induce the electric field within a PV cell, two separate semiconductors are
sandwiched together. The "p" and "n" types of semiconductors correspond to
"positive" and "negative" because of their abundance of holes or electrons (the
extra electrons make an "n" type because an electron actually has a negative
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 the p-type and n-type semiconductors are
sandwiched together, the excess electrons in the n-type
material flow to the p-type, and the holes thereby
vacated during this process flow to the n-type. (The
concept of a hole moving is somewhat like looking at a
bubble in a liquid. Although it's the liquid that is actually
moving, it's easier to describe the motion of the bubble
as it moves in the opposite direction.) Through this
electron and hole flow, the two semiconductors act as a
battery, creating an electric field at the surface where
they meet (known as the "junction"). It's this field that
causes the electrons to jump from the semiconductor out
toward the surface and make them available for the
electrical circuit. At this same time, the holes move in the
opposite direction, toward the positive surface, where
they await incoming electrons.
7.1.2 Making n and p Material
The most common way of making p-type or n-type
silicon material is to add an element that has an extra
electron or is lacking an electron. In silicon, we use a
process called "doping."
We'll use silicon as an example because crystalline
silicon was the semiconductor material used in the
earliest successful PV devices, it's still the most
widely used PV material, and, although other PV
materials and designs exploit the PV effect in slightly
different ways, knowing how the effect works in
crystalline silicon gives us a basic understanding of
how it works in all devices.
As depicted in this simplified diagram, silicon has 14
electrons. The four electrons that orbit the nucleus in
the outermost, or "valence," energy level are given to, accepted from, or shared
with other atoms.
7.1.3 An Atomic Description of Silicon
All matter is composed of atoms. Atoms, in turn, are composed of positively
charged protons, negatively charged electrons, and neutral neutrons. The
protons and neutrons, which are of approximately equal size, comprise the closepacked central "nucleus" of the atom, where almost all of the mass of the atom is
located. The much lighter electrons orbit the nucleus at very high velocities.
Although the atom is built from oppositely charged particles, its overall charge is
neutral because it contains an equal number of positive protons and negative
electrons.
The electrons orbit the nucleus at different distances, depending on their energy
level; an electron with less energy orbits close to the nucleus, whereas one of
greater energy orbits farther away. The electrons farthest from the nucleus
interact with those of neighbouring atoms to determine the way solid structures
are formed.
The silicon atom has 14 electrons, but their natural orbital arrangement allows
only the outer four of these to be given to, accepted from, or shared with other
atoms. These outer four electrons, called "valence" electrons, play an important
role in the photovoltaic effect.
Large numbers of silicon atoms, through
their valence electrons, can bond together
to form a crystal. In a crystalline solid,
each silicon atom normally shares one of
its four valence electrons in a "covalent"
bond with each of four neighbouring
silicon atoms. The solid, then, consists of
basic units of five silicon atoms: the
original atom plus the four other atoms
with which it shares its valence electrons.
In the basic unit of crystalline silicon solid,
a silicon atom shares each of its four
valence electrons with each of four
neighbouring atoms.
The solid silicon crystal, then, is composed of a regular series of units of five
silicon atoms. This regular, fixed arrangement of silicon atoms is known as the
"crystal lattice."
7.1.4 Absorption and Conduction
In a PV cell, photons are absorbed in the p layer. It's very important to "tune" this
layer to the properties of the incoming photons to absorb as many as possible
and thereby free as many electrons as possible. Another challenge is to keep the
electrons from meeting up with holes and "recombining" with them before they
can escape the cell. To do this, we design the material so that the electrons are
freed as close to the junction as possible, so that the electric field can help send
them through the "conduction" layer (the n layer) and out into the electric circuit.
By maximizing all these characteristics, we improve the conversion efficiency* of
the PV cell.
To make an efficient solar cell, we try to maximize absorption, minimize reflection
and recombination, and thereby maximize conduction.
*The conversion efficiency of a PV cell is the proportion of sunlight energy that
the cell converts to electrical energy. This is very important when discussing PV
devices, because improving this efficiency is vital to making PV energy
competitive with more traditional sources of energy (e.g., fossil fuels). Naturally, if
one efficient solar panel can provide as much energy as two less-efficient panels,
then the cost of that energy (not to mention the space required) will be reduced.
For comparison, the earliest PV devices converted about 1%-2% of sunlight
energy into electric energy. Today's PV devices convert 7%-17% of light energy
into electric energy. Of course, the other side of the equation is the money it
costs to manufacture the PV devices. This has been improved over the years as
well. In fact, today's PV systems produce electricity at a fraction of the cost of
early PV systems.
Reprinted from U.S. Department of Energy Photovoltaic Program
7.2 Solar Modules
Solar modules are composed of multiple solar cells, along with the necessary
internal wiring, aluminium and glass framework, and external electrical
connections. Modules range
in power from about 50 watts
up to 165 watts (any bigger
than this and the modules
become difficult to handle by
one person). Although
modules are usually installed
on top of a roof or on an
external structure, new
designs include the
photovoltaic cells as part of
traditional building materials - such as shingles and rolled out roofing. The array shown in the photo at right is
composed of 24 individual 100 watt PV modules.
Modules from Kyocera, Sharp, BP Solar and PowerLight shown below all have
25+ year lifetimes, lightweight roof loading and comparable power outputs per
dollar. Additional costs for the higher efficiency modules offered by most
manufacturers are a good trade off when available un shaded roof space is
limited. In our experience, modules from the established manufacturers are a
reliable and sound investment when properly installed and sized to the building's
power needs.
Kyocera KC-167 "Deep Blue" Module
Sharp NE-Q5E2U 165 watt module
Sharp NT-S5EU1 185 watt module
PowerLight Powerguard Tile
Building Integrated PV
Several companies offer photovoltaic
cells that have been integrated with
traditional shingles. Theoretically,
these can be a good choice for new
construction and major renovations
since they slightly offset costs for other building materials -- although they are still
more expensive than traditional panels. They are quite appealing from an
architectural standpoint, and are usually most compatible with slate or masonry
roofs. However, careful consideration must be paid to roof loading (approximately
750 pounds per square foot), PV shingle interconnection reliability (typically
below the shingles) and output degradation due to high temperature operation.
Solar cells can also be integrated with metal seam roofing,
either as part of pre-fabricated panels or field-applied
rolled out adhesive cell material. Interconnections among modules and other
system components are usually done at the peak of the roof or under the eaves.
Solar modules that are integrated with skylights and window walls are also
available.
UniSolar 128 watt roof modules
7.3 Inverters
Inverters (also referred to as power conditioners) process the electricity produced
by a PV system to make it suitable for meeting the specific demands of the load.
It is extremely important to match the capabilities of these devices with the
characteristics of the equipment that will be using the power. Inverters typically
have to perform these functions:
 Modify the current and voltage provided by the PV panels to maximize
power output
 Convert DC power from the panels to AC power usable by most business
and household devices
 Match the converted AC power to the voltage and frequency of the utility's
electrical network
 Safeguard the utility network system and its personnel from possible harm
during repairs
 Prevent damage to the PV array and other components during unusual
operating conditions.
Major design considerations for
inverters include their capacity,
voltage rating and battery
capability. The SMA 2500U
inverter shown to the right is a
new design that operates at a
relatively high input voltage (up
to 600 VDC). This design
allows the installer to string all
the PV modules in series,
thereby significantly reducing
wiring runs. In addition, the high
operating voltage minimizes
transmission losses. The result
is a system that operates at a
very favourable dollar per watt
basis. As shown to the right, multiple inverters can be combined (these two
inverters provide a total of 5,000 watts AC output).
Inverters are often installed in the control centre along with switches, fuses, and
other electrical components. These components must be able to withstand
expected temperature extremes in both operating and non-operating states.
Certified electrical service boxes must be used. Since high temperatures and
dust will shorten the life of unprotected electronic equipment, inverters should
generally mounted in dry, dust free, ventilated areas. Air inlets and filters for
inverters that use ventilation fans for cooling should be cleaned regularly. New
inverter models that use large external heat sinks instead of fans and are
constructed with sealed enclosures can be mounted in exposed locations.
Although the SunnyBoy inverter shown above is the most popular and reliable
inverter for small systems, once system capacity
gets much about 15kw larger capacity inverters
begin to make good economic and engineering
sense. Trace Technologies makes a series of
commercial inverters that range in size from 15kw
up to 225kw. These inverters operate at 208 volts
for compatibility with industrial power systems,
have active cooling fans to handle high outputs,
and built-in remote communications capabilities.
7.5 Racks and Mounting Systems
PV manufacturers typically put a great deal of engineering effort into the modules
and inverters. However, since racks and mounting systems are frequently
assembled on site, it is easy to overlook these critical components. There are
common issues involved in any installation, whether the array is fixed or tracking,
mounted at ground level, on a pole or on a building.
First, the PV array must be solidly and securely mounted so that it will last for
20+ years and withstand all kinds of weather, wind loads and corrosion. Many
module manufacturers and distributors sell mounting hardware specifically
designed for their modules. Use of this hardware can reduce on site assembly
costs and simplify system installation.
Customized array mounting structures can be expensive. Design and
construction of these customized racks must consider the characteristics of
various mounting materials:
Aluminium - lightweight, strong, resistant to
corrosion and moderately expensive.
Aluminium angle is an easy material to work
with, holes can be drilled with commonly
available tools, and the material is compatible
with many PV module frames. Aluminium is not
easy to weld. Overall, the best combination of
price, durability and weight.
Angle Iron - easy to work with but corrodes
rapidly. Galvanizing will slow corrosion but
mounting brackets and bolts will still rust,
particularly in a wet environment. The material
Tilt Up Aluminium Racks
is readily available and brackets can be welded easily.
Stainless Steel - expensive and difficult to work with but will last for decades.
May be a good investment in salt spray environments.
Wood - inexpensive, available, and easy to work with but will not withstand the
weather for many years--even if treated with preservative. Attaching modules to
a wooden frame requires battens or clips to hold them in place.
For some sites it may be
necessary to install the
racks at a reverse tilt.
Typically this happens when
the best module location is
on a north-facing roof or
hillside. In order to optimise
system performance we
install a rack that is tilted up
sufficiently so that it is
actually facing south.
Changing the tilt angle of an
array to account for seasonal changes in sun altitude is generally not required.
For mid-latitude locations, a tilt angle change every three months is estimated to
increase energy production about 5 percent on an annual basis. For most
applications, the additional labour and the added complexity of the array mount
does not justify the small increase in energy produced.
If tracking of the flat-plate array is desired, the recommended trackers are singleaxis (following the daily motion of the sun from east to west) units that require
little control or power. In high wind areas a powered tracker may be preferred.
Pole mounted trackers that support 4 to 12 PV modules are available and often
used for small stand-alone systems, particularly water pumping applications. The
tracker manufacturer typically provides all the array mounting hardware and
instructions for securely installing the tracker.
The foundation for the array should be designed to meet the wind load
requirements of the region. Wind load depends on the size of the array,
elevation, surrounding obstructions and tilt angle. The amount and type of
foundation for pole-mounted systems depends on the size of the array being
supported. Concrete with heavy-duty galvanized support poles are
recommended. The foundation and frame should be designed to withstand the
worst-case wind expected in the area.
In general, roof mounting of PV modules is more complex than either ground
mounting or pole mounting. Roof mounts are more difficult to install and maintain,
particularly if the roof orientation and angle are not compatible with the optimum
solar array tilt angle. Penetrating the roof seal is inevitable and leaks may occur if
the system is not installed properly. Also, it is important to achieve a firm and
secure attachment of the array mounting brackets to the roof. Attaching the
mounting brackets to the rafters will provide the best foundation, but this may be
difficult because module size and rafter spacing are usually not compatible. If
there is access to the underside of the roof, 2 x 6-inch blocks can be inserted
between the rafters and the attachment made to the blocks. Attaching the array
to the plywood sheathing of the roof may result in roof damage, particularly if
high winds are likely. The brittle nature of tile and slate roofs makes these
installations more complex than composite shingle or built up roofs -- special
mounting hardware is necessary for a secure and leak-proof installation.
If a roof mount is required, it is important to allow a clear air flow path up the roof
under the array. The array will operate at a lower temperature and produce more
energy if it stands off the roof at least 3 inches. Unless specially designed for this
purpose, flush mounting PV modules to the roof of a building is not
recommended. The modules are more difficult to test and replace, and the
performance of the array is decreased because of the higher operating
temperatures.
7.6 Roof Attachments
Most PV systems are installed on roofs. Although this technique is usually
reliable and inconspicuous, leaks can occur over the lifetime of the system -even under the best of circumstances. To minimize the possibility of leakage, we
will not install a PV system on a roof that is old or is already leaking. Moreover,
we have developed a variety of preferred roofing techniques that are suitable for
a variety of different roof materials.
Unlike solar hot water systems, which are quite heavy and stable on a roof, PV
modules are light and subject to substantial wind loading. Therefore, our biggest
concern is ensuring that the roof attachment does not fatigue or weaken as a
result of wind loading over its lifespan. Attaching mounting points to the
underlying sheathing (instead of rafters) or through a brittle roof material is
unlikely to resist water penetration over the lifespan of the rest of the roof. Here is
an overview of the practices we use to provide the best long term value for your
system.
Composition Roof - Since composition shingles
are flexible and compressible, attachment through
the shingle itself is effective. Aluminium angles are
securely attached to the underlying rafters with
stainless steel lag screws. This assembly is then
sealed using a butyl rubber caulking material.
Wood Shake Roof - Wood shakes can crack and
leak if they are drilled or compressed. We therefore
remove several shakes to locate the underlying
rafters. Metal standoffs are then installed to these
rafters with stainless steel lag screws. The standoffs
are then sealed and flashed using a butyl rubber
caulking material. Shakes are replaced around the
standoffs.
Masonry or Rigid Tile Roof - Masonry shakes can crack and leak if they are
drilled or compressed. We therefore remove several shingles to locate the
underlying rafters. Metal standoffs are then installed to these rafters with
stainless steel lag screws. The standoffs are then sealed and flashed using a
butyl rubber caulking material. Shingles are replaced around the standoffs.
Flat Roof - Since water can pool on flat roofs it is
particularly important that the roof be in good
condition. For these installations, Aluminium angles
are securely attached to the underlying rafters with
stainless steel lag screws. This assembly is then
sealed using a butyl rubber caulking material.
7.7 Wiring and Interconnection
Components
It's easy to overlook the wiring and
interconnection components required for PV
systems. But there is probably no field-installed
equipment that is more important for long term
system operation. All wiring and electrical
components should follow the guidelines of the
National Electrical Code, and should be properly approved by the manufacturer
and appropriate rating agency (usually UL).
PV Wires - Module interconnect wires must be sized for the expected current, as
well as attached securely to junction boxes and racks. Loosening connections as
a result of constant rooftop heat cycling is a common reason for poor system
performance.
Ground Wires - All exposed equipment, including modules, inverters and switch
boxes, should be properly grounded. Lightening protection should be provided
depending on local code requirements.
Roof Penetration - Every time
you poke a hole in a roof for
rack mounts or wiring you
create the potential for leaks.
Flashing should surround large
roof penetrations, and butyl
rubber caulking should be used
for smaller penetrations and
rack mounts.
Rooftop Conduit - Your roof is
a very hostile environment. Except for short cable runs we usually use conduit to
protect wiring from high temperatures and UV radiation. Connections are always
made in a junction box according to the National Electric Code.
Paralleled Inverters - Multiple inverters can be combined to provide additional
PV output. This picture shows a
1500 watt and 2500 watt inverter
wired together for a 4000 system.
PV Output Meter- For performance
monitoring purposes it is extremely
useful to have an independent, nonvolatile record of the gross output
provided by the PV system. The
separate PV meter shown in this
picture logs the output of these two
inverters.
AC Disconnect - Many utilities
require a separate AC disconnect to
isolate the PV system from utility
power. Most PV systems also include a DC disconnect to isolate the array output
from the inverter. These inverter models incorporate the DC disconnect within the
inverter enclosure.
7.8 Monitoring System
Only some companies routinely install remote monitoring systems for their techsavvy customers. For example, the SunnyBoy Control shown here allows you to
monitor the output of one or more inverters remotely using direct wiring or power
line carrier communications. System performance can also be displayed on
Windows-compatible
PCs.
Another U.S.
company, Akeena
Solar has created a
real-time web based
monitoring system so
that customers can
see the performance
of their system from
any web browser.
Click here if you would
like to see how you
can remotely monitor your own solar electric system.
Because the dollar value of energy savings is so high, virtually all of their
commercial systems include remote monitoring. Xantrex PV Series inverters
have built-in remote communications capabilities, and modular SunnyBoy
systems are well suited to power line carrier networks.
7.9 Battery Systems
PV systems with battery storage are being used by businesses and residences
all over the world to power a wide range of building equipment, lighting,
appliances, computers and communications equipment. In general, these
systems are best for applications in which backup power must be instantly
available without interruption (for example, to power computers). They are also
good for remote applications in which utility power is not available and a
generator is not desirable.
However, batteries are an additional
expense, require maintenance, only last
5-10 years, decrease system efficiency
and result in a more complicated
system. If you have access to grid
power, by taking advantage of Net
Metering you can simply sell your
excess power back to the utility during
the day and draw utility power at night at 100% efficiency. Many of our
customers opt for a grid-tie system and
purchase a standby generator with a
properly installed manual transfer switch. That way they don't incur the additional
expenses related to batteries, and have backup power available for as long as
they have fuel.
PV systems with batteries can be designed to power DC or AC equipment. An
inverter is necessary for applications in which the DC power from a battery must
be converted to AC. Although a small amount of energy is lost in converting DC
to AC (typical inverter efficiencies are in the range of 90 to 95%), an inverter
makes PV-generated electricity behave like utility power to operate everyday AC
appliances, lights, and electrical equipment. Please note that you will need a
special type of inverter if you want a battery backup system. For safety reason
most grid-tied inverters are designed to shutdown completely if there is a power
failure.
PV systems with batteries operate by connecting the PV modules to a battery,
and the battery, in turn, to the load. During daylight hours, the PV modules
charge the battery. The battery supplies power to the load whenever needed. A
simple electrical device called a charge controller keeps the batteries charged
properly and helps prolong their life by protecting them from overcharging or from
being completely drained.
Batteries make PV systems useful in more situations, but also require
maintenance. The batteries used in PV systems are similar to car batteries, but
are designed for deep cycling use in which a larger percentage of the capacity of
the battery is used each night (and then fully charged up each day). Batteries
designed for PV projects pose the same risks and demand the same caution in
handling and storage as automotive batteries. The fluid in unsealed batteries is
highly corrosive, levels should be checked periodically, batteries must be
appropriately ventilated, and batteries should be protected from extremely cold
weather. In practice we have found that when properly maintained batteries last
for about 5-8 years, after which their capacity is significantly diminished.
A solar generating system with batteries supplies electricity when it is needed.
The amount of electricity that can be used after sunset or on cloudy days is
determined by the output of the PV modules and the storage capacity of the
battery bank. Including more modules and batteries increases system cost, so
energy requirements (both in terms of peak loads and the average duration of the
loads) are carefully studied to determine optimum system size. A well-designed
system balances cost and convenience to meet the needs of the particular
application, and can be expanded if those needs change. Nevertheless, battery
backup PV systems are typically more expensive and less expensive, and have
higher maintenance costs than simpler grid-tied systems.
7.10 Generators
For a given level of power output, generators are usually the least expensive
option for backup power production. In the vast majority of residences we
evaluate, generators provide the most reliable and cost effective source of
extended backup power.
The simplest solution is
to install a relatively
inexpensive generator in
conjunction with a
manual transfer switch.
The transfer switch
directs the source of
power for critical loads
from the utility (which is
presumably down) to the
generator - without back
feeding the electric grid.
During a power outage the transfer switch is operated and the generator is
started, thereby providing power to the critical loads in the house. Total costs for
this type of installation (in the U.S.) are typically in the $3,000 to $7,000 range.
More durable diesel powered generators, such as a
10,000 watt auto-start unit, can be purchased for
about $10,000. This generator has higher quality
power output and a much longer, maintenance
reduced life. Since they don't require complicated
carburetion, propane generators are also good
solutions for remote backup systems. With these higher price range generators
one can install an automatic transfer switch and auto-start capabilities so that the
generator automatically begins to supply your electrical loads in the event of a
power outage. Note that, with most generator systems it will take several minutes
for the generator to come online and provide adequate power.