Solar Photovoltaic Systems


The heart of a photovoltaic system is the solar module.  Many photovoltaic cells are wired together by the manufacturer to produce a solar
module. When installed at a site, solar modules are wired together in series to form strings. Strings of modules are connected in parallel to
form an array.

Module Types – Rigid flat framed modules are currently most common and most of these are composed of silicon. Silicon cells have atomic
structures that are single-crystalline (a.k.a. mono-crystalline), poly-crystalline (a.k.a. multi-crystalline) or amorphous (a.k.a. thin film silicon).
Other cell materials used in solar modules are cadmium telluride (CdTe, commonly pronounced “CadTel”) and copper indium diselenide
(CIS). Some modules are manufactured using combinations of these materials. An example is a thin film of amorphous silicon deposited onto
a substrate of single-crystalline silicon

Rated Power – Grid-connected residential PV systems use modules with rated power output ranging from 100-300 watts. Modules as small
as 10 watts are used for other applications. Rated power is the maximum power the panel can produce with 1,000 watts of sunlight per
square meter at a module temperature of 25oC or 77oF in still air. Actual conditions will rarely match rated conditions and so actual power
output will almost always be less.

PV System Voltage – Modern systems without batteries are typically wired to provide from 235V to 600V.  In battery-based systems, the
trend is also toward use of higher array voltages, although many charge controllers still require lower voltages of 12V, 24V or 48V to match
the voltage of the battery string.

Using Manufacturer’s Product Information to Compare Modules – Since module costs and efficiencies continue to change as
technology and manufacturing methods improve, it is difficult to provide general recommendations that will be true into the future regarding,
for example, which type of module is cheapest or the best overall choice. It is best to make comparisons based on current information
provided by manufacturers, combined with the specific requirements of your application.

Two figures that are useful in comparing modules are the modules’ price per watt and the rated power output per area (or efficiency). When
looking through a manufacturer’s catalog of solar modules, you will often find the rated power, the overall dimensions of the module, and its
price.  Find the cost per watt by dividing the module’s price by its rated output in watts. Find the watts per area, by dividing its rated output
by its area.

Module Cost per Watt – As a general rule, thin film modules have lower costs than crystalline silicon modules for modules of similar powers

Module Efficiency (Watts per Area) – Modules with higher efficiency will have a higher ratio of watts to area. The higher the efficiency,
the smaller the area (i.e. fewer modules) will be required to achieve the same power output of an array. Installation and racking

costs will be less with more efficient modules, but this must be weighed against the higher cost of the modules. Amorphous silicon, thin film
CdTe and CIS modules have rated efficiencies that are lower than crystalline silicon modules, but improvements in efficiency continue.

Amorphous Silicon in Cloudy Climates – Of importance to amorphous silicon modules have higher efficiency than crystalline silicon
under overcast conditions. In cloudy weather, all types of amorphous silicon modules tend to perform better than crystalline silicon, with
multi-junction (i.e. double- and triple-junction) amorphous silicon modules performing as much as 15 percent better. In Britain, which has a
similar climate to ours, multi-junction amorphous silicon modules have been shown to produce more power over the course of the year than
crystalline silicon modules.

Note that because the power ratings of modules are determined under high light, their rated efficiency (or rated watts per area), will not
reflect performance in overcast weather.

Poly-crystalline or Single-crystalline Silicon? – The power output of single-crystalline and poly-crystalline modules of the same area is
quite similar.5   Both types of crystalline silicon are very durable and have stable power output over time. Therefore, do not be too
concerned about the distinction between single-crystalline and poly-crystalline silicon in selecting a module.

On the other hand, higher module efficiencies can be achieved by some combination products that have recently appeared on the market,
such as amorphous silicon deposited on a single-crystalline substrate.  These high efficiency modules may be a good choice particularly if
the area available for the installation is limited.

Silicon Modules versus Other Module Types? – The power output of CdTe modules has been less stable than silicon modules,6
although improvements are being made. For the time being, as for modules of any type, check manufacturer’s warranties. A warranty
guaranteeing high power output over 20 to 25 years is an indication of the longevity of the cell material.
severe weather, including extreme heat, cold and hailstorms.  Reflecting this longevity, most silicon modules carry 20- or 25-year
manufacturer warranties.

Array Mounting Racks
Arrays are most commonly mounted on roofs or on steel poles set in concrete. In certain applications, they may be mounted at ground level
or on building walls. Solar modules can also be mounted to serve as part or all of a shade structure such as a patio cover. On roof-mounted
systems, the PV array is typically mounted on fixed racks, parallel to the roof for aesthetic reasons and stood off several inches above the
roof surface to allow airflow that will keep them as cool as practical.

Adjustability – The tilt of sloped rooftop arrays is usually not changed, since this is inconvenient in many cases and sometimes
dangerous.  However, many mounting racks are adjustable, allowing resetting of the angle of the PV modules seasonally.

Tracking – Pole-mounted PV arrays can incorporate tracking devices that allow the array to automatically follow the sun.  Tracked PV
arrays can increase the system’s daily energy output by 25 percent to 40 percent. Despite the increased power output, tracking systems
usually are not justified by the increased cost and complexity of the system.

General Installation Notes – Proper roof mounting can be labor intensive, depending largely on the type of roof and how the mounting
brackets are installed and sealed. It is best to follow the recommendations of the roofing contractor, racking system suppliers and module
manufacturers. Module manufacturers will provide details of support requirements for their modules.  A good racking supplier will provide
code-compliant engineering specifications with their product. As a general rule forbidding purposes, however, it is typical to have one
support bracket for every 100 watts of PV modules.

Particular attention must be given to securing the array directly to the structural members of the roof and to weather sealing of roof
penetrations. All details regarding attaching the mounting brackets to the roof and sealing around them are best approved and carried out
by the roofing contractor so that the roof warranty will not be voided.

Asphalt Composition Roofs – For asphalt composition roofs, all mounts need to be secured to the roof with stainless steel lag bolts,
bolted into the rafters. Mount types include support posts and L-brackets. Support posts are preferred because they are designed to give a
good seal on boots. Support posts are best mounted after the roof decking is applied and before the roof material is installed. Support posts
and roof jacks may be installed by either the roofing contractor or the crew in charge of laying out the array mounting system.  The roofing
contractor then flashes around the posts as they install the roof.

It is very common to install mounts after the roof is installed, drilling through the asphalt composition roofing to install the bolts.  Sealant is
then applied around the bolts without flashing. As well, the top layer of roofing should be carefully lifted back to inject sealant

under the roofing. While this is much less labor intensive than when flashed, unless performed by the roofing contractor, this method may
void the warranty on the roof.

Metal Roofs – There are several types of standing seam metal roof products, including vertical seam, horizontal seam and delta seam
products. Currently, special clamps, referred to as S-5 clamps, are available to attach arrays without any penetrations to vertical and
horizontal seam roofs and certain other standing seam roof profiles. These clamps make installation of the solar array a relatively easy
matter compared to any other roof type.  In contrast, clamps for delta seam metal roofs are not available.  For these roofs, it is necessary to
cut into the roofing, install boots around the mounting posts, and then seal the penetration. This being undesirable and labor intensive, it is
best to clearly specify in advance a vertical or horizontal seam metal roof or other roof type compatible with S-5 clamps.

Other Roof Types – While it is possible to install a PV array on shake, tile and slate roofs, these roof types pose certain problems. Contact
the racking system supplier for information on products and installation methods for these roof types.  Work directly with the roofing
contractor before ordering the racking system. Also look for roof- integrated modules that can be used with tile or slate roofs.

Roof Vents and Fans – We suggest installing roof vents, plumbing vents, and fans on the north side of the roof to avoid interference with
the solar array. This will also reduce the potential for inadvertent shading of the array.

Grounding Equipment
Grounding equipment provides a well-defined, low-resistance path from your system to the ground to protect your system from current
surges from lightning strikes or equipment malfunctions.  Grounding also stabilizes voltages and provides a common reference point.  The
grounding harness is usually located on the roof.

Check with the AHJ – Grounding can be a particularly problematic issue. Be sure to check with the Authority Having Jurisdiction (AHJ) –
typically the building department’s electrical inspector – concerning local code requirements.

Equipment Grounding – Equipment grounding provides protection from shock caused by a ground fault. A ground fault occurs when a
current-carrying conductor comes into contact with the frame or chassis of an appliance or electrical box.  All system components and any
exposed metal, including equipment boxes, receptacles, appliance frames and PV mounting equipment, should be grounded.

System Grounding – System grounding requires taking one conductor from a two-wire system and connecting it to ground. In a DC
system, this means bonding the negative conductor to ground at one single point in the system. This must be accomplished inside the
inverter, not at the PV array.

Combiner Box
Wires from individual PV modules or strings are run to the combiner box, typically located on the roof.  These wires may be single conductor
pigtails with connectors that are pre-wired onto the PV modules.  The output of the combiner box is one larger two- wire conductor in
conduit. A combiner box typically includes a safety fuse or breaker for each string and may include a surge protector.

Surge Protection
Surge protectors help to protect your system from power surges that may occur if the PV system or nearby power lines are struck by
lightning. A power surge is an increase in voltage significantly above the design voltage.

Meters and Instrumentation
Essentially two types of meters are used in PV systems:

•        Utility Kilowatt-hour Meter
•        System Meter

Utility Kilowatt-Hour Meter – The utility kilowatt-hour meter measures energy delivered to or from the grid.  On homes with solar electric
systems, utilities typically install bidirectional meters with a digital display that keeps separate track of energy in both directions. Some
utilities will allow you to use a conventional meter that can spin in reverse. In this case, the utility meter spins forward when you are drawing
electricity from the grid and backwards when your system is feeding or “pushing” electricity onto the grid.

System Meter – The system meter measures and displays system performance and status. Monitored points may include power production
by modules, electricity used, and battery

charge. It is possible to operate a system without a system meter, though meters are strongly recommended.  Modern charge controllers
incorporate system monitoring functions and so a separate system meter may not be necessary.

Inverter
Inverters take care of four basic tasks of power conditioning:

•        Converting the DC power coming from the PV modules or battery bank to AC power
•        Ensuring that the frequency of the AC cycles is 60 cycles per second
•        Reducing voltage fluctuations
•        Ensuring that the shape of the AC wave is appropriate for the application, i.e. a pure sine wave for grid-connected systems

Criteria for Selecting a Grid-Connected Inverter – The following factors should be considered for a grid-connected inverter:

•        A UL1741 listing of the inverter for use in a grid-interactive application
•        The voltage of the incoming DC current from the solar array or battery bank.
•        The DC power window of the PV array
•        Characteristics indicating the quality of the inverter, such as high efficiency and good frequency voltage reg.
•        Additional inverter features such as meters, indicator lights, and integral safety disconnects
•        Manufacturer warranty, which is typically 5-10 years
•        Maximum Power Point Tracking (MPPT) capability, which maximizes power output

Most grid-connected inverters can be installed outdoors, while most off-grid inverters are not weatherproof. There are essentially two types
of grid-interactive inverters: those designed for use with batteries and those designed for a system without batteries.

Power Quality – Inverters for grid-connected systems produce better than utility-quality power.  For grid-connection, the inverter must have
the words “Utility-Interactive” printed directly on the listing label.

Voltage Input – The inverter’s DC voltage input window must match the nominal voltage of the solar array, usually 235V to 600V for
systems without batteries and 12, 24 or 48 volts for battery-based systems.

AC Power Output – Grid-connected systems are sized according to the power output of the PV array, rather than the load requirements of
the building. This is because any power requirements above what a grid-connected PV system can provide is automatically drawn from the
grid.

Surge Capacity – The starting surge of equipment such as motors is not a consideration in sizing grid-connected inverters. When starting,
a motor may draw as much as seven times its rated wattage. For grid-connected systems, this start-up surge is automatically drawn from the
grid.

Frequency and Voltage Regulation – Better quality inverters will produce near constant output voltage and frequency.

Efficiency – Modern inverters commonly used in residential and small commercial systems have peak efficiencies of 92 percent to 94
percent, as rated by their manufacturers.  Actual field conditions usually result in overall efficiencies of about 88 percent to 92 percent.
Inverters for battery-based systems have slightly lower efficiencies.

Integral Safety Disconnects – The AC disconnect in most inverter models may not meet requirements of the electric utility (see section
“Disconnects”). Therefore, a separate exterior AC disconnect may be required even if one is included in the inverter. All inverters that are
UL listed for grid-connection include both DC disconnects (PV input) and AC disconnects (inverter output).  In better inverters, the inverter
section can be removed separately from the DC and AC disconnects, facilitating repair.

Maximum Power Point Tracking (MPPT) – Modern non-battery based inverters include maximum power point tracking. MPPT
automatically adjusts system voltage such that the PV array operates at its maximum power point. For battery-based systems, this feature
has recently been incorporated into better charge controllers.

Inverter-Chargers – For battery-based systems, inverters are available with a factory- integrated charge controller, referred to as inverter-
chargers. Be sure to select an inverter-charger that is rated for grid-connection, however. In the event of a grid power outage, use of an
inverter-charger that is not set up for grid-connection would result in overcharging and damaging the batteries, known as “cooking the
batteries.”

Automatic Load Shedding – For battery-based systems, the inverter can automatically shed any unnecessary loads in the event of a
utility power outage. Solar loads, i.e. the loads that will be kept powered up during the outage, are connected to a separate electrical sub-
panel. A battery-based system must be designed to power these critical loads.

Warranty – Inverters typically carry warranties of 5 years, although the industry is moving toward a 10-year warranty. The transformer and
solid state components of an inverter are both susceptible to overheating and damage from power spikes, reducing its life.  Transformer
less inverters, long available in Europe, are beginning to move into the U.S.market.

To Consider When Researching Inverters – Many references on sizing and selecting inverters have been developed for off-grid
systems, but may not clearly state that they are

specific to off-grid systems.  Sizing and selecting grid-connected inverters entails different considerations and is easier, since the system
does not have to provide 100 percent of the energy requirements. In particular, peak energy demand and surge capacity do not need to be
considered for grid-connected systems.

Disconnects
Automatic and manual safety disconnects protect the wiring and components from power surges and other equipment malfunctions. They
also ensure the system can be safely shut down and system components can be removed for maintenance and repair.  For grid- connected
systems, safety disconnects ensure that the generating equipment is isolated from the grid, which is important for the safety of utility
personnel. In general, a disconnect is needed for each source of power or energy storage device in the system.

For each of the functions listed below, it is not always necessary to provide a separate disconnect. For example, if an inverter is located
outdoors, a single DC disconnect can serve the function of both the array DC disconnect and the inverter DC disconnect.

Before omitting a separate disconnect, however, consider if this will ever result in an unsafe condition when performing maintenance on any
component. Also consider the convenience of the disconnects location. An inconveniently located disconnect may lead to the tendency to
leave the power on during maintenance, resulting in a safety hazard.

Array DC Disconnect – The array DC disconnect, also called the PV disconnect, is used to safely interrupt the flow of electricity from the
PV array for maintenance or troubleshooting.  The array DC disconnect may also have integrated circuit breakers or fuses to protect
against power surges.

Inverter DC Disconnect – Along with the inverter AC disconnect, the inverter DC disconnect is used to safely disconnect the inverter from
the rest of the system. In many cases, the inverter DC disconnect will also serve as the array DC disconnect.

Inverter AC Disconnect – The inverter AC disconnect disconnects the PV system from both the building’s electrical wiring and the grid.
Frequently, the AC disconnect is installed inside the building’s main electrical panel. However, if the inverter is not located near the electrical
panel, an additional AC disconnect should be installed near the inverter.

Exterior AC Disconnect – Utilities commonly require an exterior AC disconnect that is lockable, has visible blades and is mounted next to
the utility meter so that it is accessible to utility personnel.  An AC disconnect located inside the electrical panel or integral to the inverter
would not satisfy these requirements. One alternative that is as acceptable to some utilities as an accessible AC disconnect is the removal
of the meter itself, but this is not the norm.  Prior to purchasing equipment, consult the electric utility to determine their requirements for
interconnection.

Battery DC Disconnect – In a battery-based system, the battery DC disconnect is used to safely disconnect the battery bank from the rest
of the system.

Battery Bank
Batteries store direct current electrical energy for later use. This energy storage comes at a cost, however, since batteries reduce the
efficiency and output of the PV system, typically by about 10 percent for lead-acid batteries. Batteries also increase the complexity and cost
of the system.

Types of batteries commonly used in PV systems are:

•        Lead-acid batteries
-        Flooded (a.k.a. Liquid vented)
-        Sealed (a.k.a. Valve-Regulated Lead Acid)
o        Absorbent glass mat
o        Gel cell
•        Alkaline batteries
-        Nickel-cadmium
-        Nickel-iron

Lead-Acid Batteries – Lead-acid batteries are most common in PV systems in general and sealed lead acid batteries are most commonly
used in grid-connected systems.
Sealed batteries are spill-proof and do not require periodic maintenance. Flooded lead- acid batteries are usually the least expensive but
require adding distilled water at least monthly to replenish water lost during the normal charging process.

There are two types of sealed lead acid batteries: sealed absorbent glass mat (AGM) and gel cell. AGM lead-acid batteries have become
the industry standard, as they are maintenance free and particularly suited for grid-tied systems where batteries are typically kept at a full
state of charge. Gel-cell batteries, designed for freeze-resistance, are generally a poor choice because any overcharging will permanently
damage the battery.

Alkaline Batteries – Because of their relatively high cost, alkaline batteries are only recommended where extremely cold temperatures (-50
oF or less) are anticipated or for certain commercial or industrial applications requiring their advantages over lead-acid batteries. These
advantages include tolerance of freezing or high temperatures, low maintenance requirements, and the ability to be fully discharged or over-
charged without harm.

Sizing Battery Banks – For grid-connected systems, batteries are usually sized for relatively short time periods with 8 hours being typical.  
Size may vary, however, depending on the particular needs of a facility and the length of power outages expected. For comparison, battery
banks for off-grid systems are usually sized for one to three cloudy days.


Interaction with Solar Modules – The solar array must have a higher voltage than the battery bank in order to fully charge the batteries.
For systems with battery back-up, pay particular attention to the rated voltage of the module, also called the maximum power point (Vmpp),
in the electrical specifications.  It is important that the voltage is high enough relative to the voltage of a fully charged battery. For example,
rated voltages between 16.5V and 17.5V are typical for a 12V system using liquid lead-acid batteries. Higher voltages may be required for
long wiring distances between the modules and the charge controller and battery bank.

Charge Controller
A charge controller, sometimes referred to as a photovoltaic controller or battery charger, is only necessary in systems with battery back-
up.  The primary function of a charge controller is to prevent overcharging of the batteries. Most also include a low- voltage disconnect that
prevents over-discharging batteries. In addition, charge controllers prevent charge from draining back to solar modules at night. Some
modern charge controllers incorporate maximum power point tracking, which optimizes the PV array’s output, increasing the energy it
produces.

Types of Charge Controllers – There are essentially two types of controllers: shunt and series. A shunt controller bypasses current
around fully charged batteries and through a power transistor or resistance heater where excess power is converted into heat. Shunt
controllers are simple and inexpensive, but are only designed for very small systems.

Series controllers stop the flow of current by opening the circuit between the battery and the PV array. Series controllers may be single-
stage or pulse type. Single-stage controllers are small and inexpensive and have a greater load-handling capacity than shunt-type
controllers. Pulse controllers and a type of shunt controller referred to as a multi-stage controller (e.g., three-stage controller) have routines
that optimize battery charging rates to extend battery life.

Most charge controllers are now three-stage controllers. These chargers have dramatically improved battery life.

Selection – Charge controllers are selected based on:

•        PV array voltage – The controller’s DC voltage input must match the nominal voltage of the solar array.
•        PV array current – The controller must be sized to handle the maximum current produced by the PV array.

Interaction with Inverter – Since the majority of charge controllers have been installed in off-grid systems, their default settings may not be
appropriate for a grid-connected system.  The charge controller must be set up such that it does not interfere with the proper operation of
the inverter. In particular, the controller must be set up such that

charging the batteries from the PV array takes precedence over charging from the grid. For more information, contact the manufacturer.

Interaction with Batteries – The charge controller must be selected to deliver the charging current appropriate for the type of batteries
used in the system. For example, on a 12V system, flooded lead-acid batteries have a voltage of 14.6V to 15.0V when fully charged, while
sealed lead-acid batteries are fully charged at 14.1 V. Refer to the battery manufacturer for the charging requirements of particular
batteries.
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