Transcript e8 / PPA Solar PV Design Implementation O&M

e8 / PPA Solar PV Design Implementation O&M
2-4. Solar Panels
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Marshall Islands March 31-April 11, 2008
Marshall Islands March 31-April 11, 2008
e8 / PPA Solar PV Design Implementation O&M
Solar Panel Power Rating
• Rated in Watts Peak (Wp). This is the number of Watts
power that the panel should produce if it is exposed to
1000 W/m2 of sunlight coming straight into the clean
panel while there is a cell temperature of 25ºC and an
air mass of 1.5. Power is delivered to the load at the
maximum power point of the panel (the optimum
loading)
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Marshall Islands March 31-April 11, 2008
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Rating Measurement Standards
• 1000 Watts per square meter of solar energy
•
• Energy is falling at right angles onto the panel
• Cell temperature of 25 degrees Celsius
• Air Mass of 1.5 (sun angle about 46° from vertical)
• Clean panel surface
• Load taken from panel at maximum power point
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Actual Panel Output
• Solar energy almost never is great enough to provide
1000 W/m2 of solar radiation. Typically 800-900 W/m2 is
the highest seen on clear days at noon.
• In the tropics, solar cells are 50ºC to over 60ºC. Higher
cell temperatures result in lowered output of 10% to
15% over rated values
• Panels rarely face directly toward the sun, surface
reflections increase and output decreases as a result
• There is often a mismatch between the load and the
panel resulting in a few percent reduction from the
rated value.
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Marshall Islands March 31-April 11, 2008
e8 / PPA Solar PV Design Implementation O&M
Panel Types
•
Single Crystal construction. Each cell is a single crystal of silicon.
Oldest design, highest efficiency. Round cells initially but may be
cut square. Panel made up of many (usually 36) cells connected in
series. Very reliable.
•
Polycrystralline construction. Each cell includes several large
crystals of silicon. Cells can be any shape. Almost as high
efficiency as single cells. Panel made up of many (usually 36) cells
connected in series. Excellent reliability.
•
Thin film construction. Silicon or other PV material is put in a very
thin layer onto metal or plastic. Mass production is relatively easy
and theoretically can be cheaper than crystal based panels.
Efficiency low to medium. Reliability varies from poor to good.
Sometimes called “amorphous” panels.
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Marshall Islands March 31-April 11, 2008
Panel appearances
Single crystal cell
Polycrystalline
Cells
Thin Film panel
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Panel construction
• Top layer is glass or in some cheap panels, clear
plastic.
• The middle layer is the active PV material. In the case
of crystalline cells, 36 individual cells are connected in
series to make a panel for charging a 12V battery. Each
cell produces about 0.5 to 0.6 volts. The area of the cell
determines the Amperes it can produce.
• Backing. Typically a special plastic called Tedlar though
sometimes glass. Thin film panels may have a backing
that is ceramic or metal as well as possibly glass or
plastic.
• Cells are embedded in a clear plastic material between
the top layer and the bottom layer. This is called the
encapsulant.
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Marshall Islands March 31-April 11, 2008
Panel construction
Typical solar PV
panel construction
cross section
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Marshall Islands March 31-April 11, 2008
Terminology
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Recommended specifications
• Panels
– Must have an output adequate to bring the battery
to full charge several times a week
– Screw type wire terminals with lock washers
– 36 cells or more
– Meet international standards for construction and
are certified by testing at an international test center
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Key Characteristics
• Number of cells (must be at least 36 for SHS) or it will
not be very effective in charging a 12V battery
• Voc = the open circuit voltage which is the voltage
across the terminals with no load attached
• Isc = Short circuit current which is the Amperes
measured directly across the terminals with no load
attached
• Impp= Current delivered at the maximum power
conditions under standard conditions
• Vmpp= Voltage delivered at the maximum power
conditions under standard conditions
Note that Impp X Vmpp = Wp
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Marshall Islands March 31-April 11, 2008
Voltage-Ampere Relationship (IV
Curve)
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Solar intensity affects mainly current
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Connecting panels for increased
voltage and amperage
• Connecting panels in series (+ terminal of one panel
connected to – terminal of the next) results in adding
the voltage of the series connected panels
• Connecting panels in parallel (+ terminal of one panel
connected to + terminal of the other and – terminal of
one panel connected to – terminal of the other) results
in adding the amperes produced by each panel
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Marshall Islands March 31-April 11, 2008
Sun’s movements over the year
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Proper Orientation
• Facing toward the Equator (South in the North Pacific)
• Tilted about the same number of degrees as the
latitude of the site
• Never tilt less than 5 degrees because fast water runoff
is necessary for cleaning. 10 degrees is better.
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Shading
• Output from panels in the shade is a small fraction of
the output from a panel in the sun
• Even shading a few cells on the panel will greatly
reduce the output from the panel
• No shade should be on the panel from 0900 to 1500
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Be careful of shading due to changing
sun position
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Heat effects
• Every three degrees C (5.4F) of temperature rise in a
PV cell lowers the output of a PV panel by 1%.
• Keep panels as cool as possible to prevent power loss
due to overheating
• NEVER mount solar panels flush on any surface, if at
all possible provide at 6 inches (150mm) or more of
ventilation space underneath panels, especially on
metal roofs and never less than 2.5 inches (60mm).
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Diodes
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Use of Diodes
• Bypass Diode - Used to take shaded portions of a
panel or array out of the circuit by bypassing cells that
are producing electricity at a much lower rate than
others. Many panels come with by-pass diodes already
installed.
• Blocking Diode – used to prevent back flow of electricity
from the battery to the panel or to isolate the panel
connections from the battery so a short at the panel
does not short the battery. Often not needed
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Use of Blocking and Bypass Diodes
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Increasing Array Voltage
• So two panels
intended to charge
a 12V battery can
be connected in
series to charge a
24V battery. Series
connection will
work well only if
the panels have
the same ampere
rating.
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Increasing Array Current
To increase
the amperes
available,
connect the
panels in
parallel. As
long as the
two panels
have the same
voltage it will
work ok
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Panel Mounting Rules
• Face south and tilt to latitude but no less than 10
degrees
• Must have ventilating air passing underneath the panel
• No shade any time of the year between 0900 and 1500
• Mounting must use marine grade stainless steel
fasteners that isolate aluminum panel frames from the
roof. No aluminum can be allowed to touch a metal
roof.
• Mounting arrangement must be strong enough to
survive storms yet simple enough to allow access to
panel connections without major dismantling of the
array.
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Adequate Panel Size
• Panel size needs to be fitted to the requirement of the
load that will be powered.
• Modest oversizing of the panel can dramatically
increase system reliability, increase battery life and
lower maintenance costs
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