sunlight. This eliminates excess electrical wires, especially for PV systems
functioning at less than 48V. The combiner box source circuits should be
mapped out in advance of module mounting and then confirmed at comple-
tion during the commissioning. Be sure to include a string map with the
as-built drawings. Every grouping of modules should have the same
source-circuit voltage.
Tracking Mounts
Tracking mounts effectively increase annual array output by
up to 20 percent. They are especially effective in geographic
areas and climates without many cloudy days. For latitudes
above 40 degrees, use double-axis tracking. Both axes traverse
a large scope of azimuth angles from sunup to sundown.
Single-axis tracking mounts work best in locations below 40
degrees latitude.
Another factor in tracking mounts worth considering is
the wind load for the PV site location. Windy areas in parts of
the US are not conducive to tracking mounts. Large wind
loads make the PV arrays less effective. Arrays are more likely to experience
damage and additional maintenance in windy areas.
Tracking mounts need regular maintenance. You need to consider this
ongoing cost as well as the initial cost when debating tracking systems or fixed
systems. Tracking systems require periodic maintenance if they are to deliver
more energy than a fixed system.
Adapting the Electrical System Design
There are three types of PV systems—grid-tied to the utility company, stand-
alone, and hybrid grid-tied with battery backup systems. PV systems also may
incorporate other power sources, such as wind, water, gas, or diesel. The various
PV systems are different mainly in their electrical systems.
It is the installer’s job to ensure that the PV system, whichever system is used,
is assembled in a manner to achieve optimum output. That means performance
and not mediocrity. All PV systems have the following components:
PV modules
Combiner boxes that house breakers and fuses (with some exceptions)
Overcurrent protection
Disconnects
Grounding
Component wiring
DC load center
TECH TIPS
Check the National Renewable
Energy Laboratory (NREL) for
information on the site location’s solar
irradiation. That data will help figure
out the best tracking option—
single-axis, double-axis, or no tracking
mount at all.
CHAPTER 7
PV Technology—Cells, Panels, Arrays, Balance of System, and Inverters 157
PV Module
The two most common PV cell types are monocrystalline silicon cells and multicrys-
talline silicon cells. These PV cells are made of the crystalline silicon PV cells encased
between two glass layers or a glass layer and a back sheet usually made of a material
such as Tedlar. The cells have anti-reflective coatings that trap sunlight. This improves
performance. A module will contain anywhere from 36 to 96 PV cells and can pro-
duce a variety of voltages, although larger panels will have additional cells.
Most PV modules today are between 100 to 300 watts or more. A PV module
of 120 watts is usually about 11 square feet and weighs 35 pounds. A 320-watt
module can be 17.6 square feet and weigh 31 pounds. Some modules weigh over
100 pounds. This indicates a broad range of sizes, weights, wattages, efficiencies,
and energy density.
During peak sun hours, it is possible to obtain about 70 percent of the sun-
light hitting the earth. Irradiation, or intensity of sunlight, is measured in watts
per square meter (W/m
2
). Full sun can achieve up to 1,250 W/m
2
. PV modules
mounted to track the sun can increase the daily energy from a PV module by as
much as 20 percent to 40 percent in summer months. In cloudy areas, the
enhancement from a tracking array is less effective.
PV modules are active even before they are connected together and to the rest
of the PV power system. Even when the system shuts down, the modules are still
collecting solar energy and producing current. When exposed to sunlight, PV
modules always should be treated as “hot.” The electric current can arc between
the open wire ends on modules or anything else that will complete the circuit.
Shorting PV modules normally does not damage them. One way to test for
bad modules is gauging the short-circuit current of a PV module. Potential short
circuits may not blow any fuses. However, the circuits might produce a very
intense DC arc between the wires. Temperature can reach as high as 10,000
degrees Celsius (18,000 degrees Fahrenheit). Short circuits that persist for several
seconds to as long as a few minutes can destroy PV modules. Review data mate-
rials from manufacturers for information on relevant warnings.
PV modules are characterized by the many voltage and current characteris-
tics. These include:
Open-circuit voltage (Voc)
Short-circuit current (Isc)
Maximum power voltage (Vmp)
Maximum power current (Imp)
The I-V curve is dynamic, not static. It is important to grasp the impact of
right sizing versus oversizing. This is true PV science at its best, not simply
opinion, and will make you a much better installer or designer when you apply
this principle properly.
158 ADVANCED PHOTOVOLTAIC INSTALLATIONS
Design and install for the MPPT window for better performance and quicker
system payback.
A PV module’s open-circuit voltage occurs when the module is disengaged
electrically: when the switch is open, and there is no energy flow.
Voltages are cumulative when modules are connected in a series.
Currents are cumulative when modules are connected in parallel.
For a given temperature and sunlight intensity, the maximum power voltage
and maximum power current output occurs at a given instantaneous point, which
is called the Maximum Power Point (MPP).
PV module performance depends on the current and voltage output. Both
are highly correlated with temperature and weather. Voltage has a greater depen-
dency on temperature in crystalline silicon cells than with other technologies.
Voltage may diminish by about 0.4 percent per degree Celsius. This is different for
each cell type. Evaluate voltage based on temperature coefficients provided in the
manufacturer’s data sheets. These values are found at standard test conditions
(STC) and permit you to compare modules.
FIGURE 7-1 This figure shows the voltage and current characteristics of PV strings. Scatter chart of calculated
hourly-average performance values for 3.36-kW array in Albuquerque, N.M., over a one-year period. The “window”
superimposed shows the voltage and power constraints for the inverter used with the system. The fraction of the
cumulative annual energy available from the array is also shown as a function of the array power level.
Courtesy of Sandia Laboratory
The I-V Curve
Isc (Short-Circuit
Current)
Voc
(Open-Circuit
Voltage)
Maximum Power Point
Vmp and Imp
Volts
0.0
0
0.5
1.0
1.5
2.0
2.5
3.0
5 10 15 20 25
Amps
Vmp
CHAPTER 7
PV Technology—Cells, Panels, Arrays, Balance of System, and Inverters 159
NEC Article 690.7 outlines PV system maximum voltages. Always use the
provided PV module manufacturer’s data to access maximum voltage.
Wire, Fuse, Circuit Breaker, and Disconnect Sizing
The size of the conductors used in a PV system affects the system’s capacity,
output, and safety. Ampacity and voltage drop are two of the main criteria to con-
sider when selecting wires. The maximum short-circuit current from a PV
module determines the wire ampacity required for the PV system.
Temperature and Conduit Fill Corrections for Ampacity of Conductors
PV modules are installed outdoors in often harsh conditions. All PV BOS compo-
nents should be rated for use across the various site temperatures. The BOS com-
ponents should also be selected for other site conditions. For example, conduit
exposed to sunlight directly should be appraised for maximum and minimum
temperatures for the location, as per the NEC. Raising the conduit off the roof
will allow the conductors to benefit from cooler temperatures, which also is
addressed in the Code.
The ampacity—the current-carrying capacity, measured in amperes—of the
conductors must always be considered and sized for the site. In NEC Article
310.16, there is a correction table that examines ampacity requirements. NEC
Article 310.16 discusses temperatures at which various wire sizes are operational.
For example, conduit between the combiner box and DC disconnect may have
four or more conductors that carry current. 2011 NEC Article 310.15(B)(3)(a)
states that a further adjustment of 80 percent is needed for conduit fill of four to
six conductors.
Most PV panels use a #10 AWG copper cable and some form of
quick-connect terminal. When you factor in the temperature adjustment for
cable in the highest of temperatures, you probably will not exceed that #10 AWG
size to where you would need a #8 AWG conductor. Even if the tables and mul-
tipliers required #12, it would be a poor design practice to reduce the size of the
conductor and raise the resistance, especially on the DC side of the system. The
cost difference on the copper will be more than paid for by the consistency in
DC energy flow over time.
Once the string cables have been assembled, labeled, and terminated in the
combiner box, you should address the fusing, conductor, and conduit to the DC
disconnect, and inverter. Lastly, you should confirm that it has been adjusted ade-
quately for the site conditions.
There are different rules for overcurrent protection on a PV system for the
AC side of the inverter and the DC side.
Non-grounded conductors must be protected by circuit breakers or fuses.
The fuse must be rated at 125 percent of the continuous current.
On the AC side, overcurrent protection requires a multiplier of 125 percent.
160 ADVANCED PHOTOVOLTAIC INSTALLATIONS
However, because on the DC side “currents shall be considered to be
continuous,” 690.8 Circuit Sizing and Current, (A) Calculation of Maximum
Circuit Current, states:
The maximum current for the specific circuit shall be calculated in accor-
dance with 690.8(A)(1) through (A)(4).
Informational Note: Where the requirements of 690.8(A)(1) and (B)(1) are
both applied, the resulting multiplication factor is 156 percent.
Please familiarize yourself with this section. It must be addressed in your
system design and installation for good safety reasons, which will become clear as
your experience grows.
Using the NEC, you will establish the proper conductor and conduit sizing to
meet the minimum legal standards. Most designers and installers will stop there.
Designing for high temperatures requires thinking beyond the Code and
looking at options that most designers and installers neglect. There is a lot of lab
and field data that indicates the temperatures on panels, conduit, and a variety of
combiner boxes, disconnects, and other BOS components operate at higher tem-
peratures than most people are designing for in almost every environment.
For improved performance, you need to increase the conductor size for
reduced resistance from the wire and temperatures in the conduit. You also will
increase the size of the conduit for ease of pulling and greater ability for the con-
ductors to radiate heat, which will keep them a little cooler.
At this stage, start examining wire types. Copper wire will fulfill the ampacity
requirements. Changing wire types can change the temperature correction factors.
Insulation as an adjustment factor can change the ampacity. In most cases, the
module terminals are rated at 90 degrees Celsius, and the fuse terminals 60 to 70
degrees Celsius. Do not exceed the module and fuse maximum temperature
ratings.
Terminals in a box on the roof exposed to direct sunlight
would have to be rated for a 75 degree Celsius minimum in
order to not overheat on a hot, sunny day, since the tempera-
tures could reach 95 degrees Celsius, as with conduit. The
combiner or pass-through box may be at a much higher tem-
perature than 95 degrees Celsius, and so may require addi-
tional evaluation.
Once the string cables are assembled, they need to be
labeled and terminated in the combiner box. Address the fusing. Attach the con-
ductor and conduit to the DC disconnect and inverter. Adjust the mechanisms for
the site conditions.
There are different rules for overcurrent protection for the devices that pro-
tect the PV system in cases of short-circuit or power surges for the AC and DC
side of the inverter.
NOTE
Remember that in hot locations,
it is possible to exceed the 90 degree
Celsius conditions. Always design for
the actual environmental conditions.
CHAPTER 7
PV Technology—Cells, Panels, Arrays, Balance of System, and Inverters 161
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.