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Photovoltaic (PV) modules make
electricity from sunlight, and are marvelously simple, effective, and durable.
They sit in the sun and, with no moving parts, can run your appliances, charge
your batteries, or make energy for the utility grid. It’s difficult to find a
product that combines the longevity and productivity of PV modules.
When you buy them, you’re buying 40-plus years of electricity for a one-time
cost.
A PV array is the energy collector—the
solar “generator.” To use the energy from the array, you also need other
components which make up a solar-electric system, and you need to design the whole
system for the purpose desired. This article explains the basic components and
configurations for the four most common system options in solar electricity:
·
PV-DIRECT
·
STAND-ALONE (OFF-GRID)
·
GRID-TIED WITH BATTERY BACKUP
·
BATTERYLESS GRID-TIED
Specific systems will vary—not all equipment
is necessary for every system type. In the diagrams, the numbers in red
correspond to the major components needed.
1. PV MODULES
(AKA: solar-electric modules)
PV modules are a solar-electric system’s
defining component, where sunlight is used to make direct current (DC)
electricity. Behind a PV module’s shimmering face, semiconductor materials work
their magic, using light (photons) to move electrons in a circuit—what’s known
as the photovoltaic effect.
PV modules are rated in watts, based on the
maximum power they can produce under ideal sun and temperature conditions. You
can use the rated output (along with a figure representing your local solar
resource and an efficiency factor) to determine how many modules it will take
to meet your electrical needs. Multiple modules combined together are called an
array. Although framed modules are most common, PV technology also has been
integrated into roofing shingles and tiles, and even peel-and-stick laminates
for standing-seam metal roofs.
PV modules are very durable and
long-lasting—most carry 25-year warranties. They can withstand severe weather,
including extreme heat, cold, and hail.
2. DC-TO-DC
(AKA: distributed power harvesters, power boxes, module maximizers)
A new component that’s showing up on some
batteryless grid-tied PV systems is DC-to-DC
3. ARRAY MOUNTING SYSTEM
(AKA: mounts, racks)
Mounts provide a secure platform on which to
anchor your PV modules, keeping them in place and oriented correctly. Modules
are generally mounted on a rooftop, atop a steel pole set in concrete, or at
ground level. The specific pieces, parts, and materials of your mounting system
will vary considerably depending on which method you choose.
Usually, arrays in urban or suburban areas are
mounted on a south-facing roof (although east- and west-facing roofs can also
be used), parallel to the roof’s slope. This approach is sometimes considered
most aesthetically pleasing, and may be a local requirement. In areas with a
lot of space or if your roof is not ideal because of orientation or shading,
pole- or ground-mounted arrays are options.
Pole-mounted PV arrays can incorporate
tracking, automatically following the sun across the sky from east to west each
day. Tracked PV arrays can increase the system’s daily energy output by 25% to
40%, but come with more cost, complexity, maintenance, and potential failure
than fixed arrays.
4. COMBINER BOX
(AKA: series string combiner)
The array combiner box is used to wire and
combine parallel strings of PV modules. These are most commonly found in
off-grid systems, although larger on-grid systems will have combiner boxes as
well. Coming into the input side of a combiner box will be the positive and
negative wire for individual module strings, each with its own terminal. Each
positive terminal is internally connected to a series circuit breaker (or fuse)
for that string. The output of each breaker/fuse is connected together on a
common bus bar to which a positive output wire is connected. The strings’
negative wires are simply connected to a common bus bar along with the negative
output wire. Some batteryless grid-tied inverters integrate a combiner box on
the input side of the inverter, eliminating a separate combiner box. And some
grid-tied systems only have a few PV module strings (3 or less), and do not
need a combiner box at all.
5. DC DISCONNECT
The DC disconnect is used to safely interrupt
the flow of electricity from the PV array. It’s an essential component when
system maintenance or troubleshooting is required, and may be mandated by local
inspectors. The disconnect enclosure (sometimes a part of the inverter
package), houses an electrical switch rated for use in DC circuits. It also may
integrate either circuit breakers or fuses, if needed.
6. CHARGE CONTROLLER
(AKA: controller, regulator)
A charge controller’s primary function is to
protect the battery bank from over‑
charging. As a battery becomes charged, the controller moderates the flow of electricity from the PV modules. Batteries are expensive and need careful treatment. To maximize their life, avoid overcharging or undercharging them. Most modern charge controllers incorporate maximum power point tracking (MPPT), which optimizes the PV array’s output to maximize energy production. Some battery-based charge controllers also include a low-voltage disconnect for the DC loads to help prevent over-discharging, which can permanently damage the battery bank.
charging. As a battery becomes charged, the controller moderates the flow of electricity from the PV modules. Batteries are expensive and need careful treatment. To maximize their life, avoid overcharging or undercharging them. Most modern charge controllers incorporate maximum power point tracking (MPPT), which optimizes the PV array’s output to maximize energy production. Some battery-based charge controllers also include a low-voltage disconnect for the DC loads to help prevent over-discharging, which can permanently damage the battery bank.
7. BATTERY BANK
(AKA: storage battery)
PV modules produce electricity only when the
sun shines on them. If your system is designed to provide energy without the
utility grid, you’ll need a battery bank—a group of batteries wired together—to
store energy so you can have electricity at night or on cloudy days. For
off-grid systems, battery banks are typically sized to keep household
electricity running for up to three cloudy days. Grid-tied systems also can
include battery banks, which provide emergency backup power during grid outages
to keep critical electric loads operating until grid power is restored.
Although similar to car batteries, the deep
cycle batteries used in solar-electric systems are specialized for the type of
charging and discharging they’ll need to endure. Flooded lead-acid batteries
are most commonly used in solar-electric systems, are the least expensive, but
require adding distilled water occasionally to replenish water lost during the
charging process. Sealed batteries, absorbed glass mat (AGM) and gel-cell, do
not require adding water and often used for grid-tied systems where the battery
bank is usually small (as compared to off-grid banks), and the batteries are
typically kept at a full state of charge.
8. BATTERY BANK TO CHARGE
CONTROLLER DISCONNECT
Because all electrical components may need to
be serviced periodically, it is necessary, and required by the National
Electric Code(NEC)
to place disconnects between all sources of power and the other components.
Because of this, a disconnect (usually a circuit breaker to also protect the
wire) is placed between the battery bank and charge controller, which enables
isolating the charge controller from the battery bank for servicing.
9. SYSTEM METER
(AKA: battery monitor, amp-hour meter)
System meters measure and display several
different aspects of a PV system’s performance and status—tracking how full
your battery bank is; how much electricity your solar-electric array is
producing or has produced; and how much electricity is being used. Web-based
monitoring is offered in some metering packages and is extremely handy to keep
tabs and potentially troubleshoot the system. Operating your solar-electric
system without metering is like running your car without any gauges—although
it’s possible to do, it’s always better to know how much fuel is in the tank.
10. BATTERY
TO INVERTER DISCONNECT
(AKA: main DC disconnect)
In battery-based systems, a disconnect between
the batteries and inverter is typically a large, DC-rated breaker mounted in a
sheet-metal enclosure. This breaker allows the inverter to be quickly
disconnected from the batteries for service, and protects the
inverter-to-battery wiring against too-high current.
11. INVERTER
(AKA: DC-TO-AC
Inverters transform the DC electricity
produced by the PV modules or from the batteries into the alternating current
(AC) electricity commonly used for lights, pumps, and other electrical
appliances. Grid-tied inverters synchronize the electricity they produce with
the grid’s AC electricity, allowing the system to feed any unused solar-made
electricity to the utility grid.
Most grid-tied inverters are designed to
operate without batteries, either tying to one or more strings (series
grouping) of modules, or using a “microinverter” for each module. Similar to
systems using DC-to-DC
Battery-based inverters for off-grid or
grid-tied use often include a battery charger, which is capable of charging a
battery bank from either the grid or a backup generator during cloudy weather.
Most batteryless inverters can be installed outdoors, but most battery-based
inverters are not weatherproof and should be mounted indoors, close to the
battery bank.
12. INVERTER AC
DISCONNECT
Utilities usually require an AC disconnect
between the inverter and the grid. Some grid-tied inverters have integrated AC
disconnects, but these may or may not meet local requirements, calling for a
separate PV system AC disconnect box, usually located near the utility kWh
meter. In battery-based systems an AC disconnect is also required between the
inverter, the AC breaker panel and any other AC power source. It is usually
incorporated into an inverter bypass breaker assembly, allowing the AC loads to
be fed by either the inverter, or if power from the inverter is unavailable, by
another AC power source such as a backup generator.
13. PV PRODUCTION
MONITORING
An additional meter to measure solar
production is useful for tracking system performance, and is needed for
production-based (per kWh) incentives. This can be a dedicated kWh meter that
counts the kWh coming out of the inverter, or can be a full revenue-grade or
Web-based data monitoring package.
14. AC BREAKER PANEL
(AKA: mains panel, AC load center, breaker box, fuse box)
The AC breaker panel is where a building’s
electrical wiring connects to the source of the electricity, whether that’s the
grid or a solar-electric system. This wall-mounted panel or box is usually
installed in a utility room, basement, garage, or on the building’s exterior.
It contains a number of labeled circuit breakers that route electricity to the
various rooms or household circuits. These breakers allow electricity to be
disconnected for servicing, and also protect the building’s wiring against
overcurrent, which may cause electrical fires.
Just like other electrical circuits, an
inverter’s electrical output needs to be routed through an AC circuit breaker.
This breaker is usually mounted inside the building’s mains panel, which
enables the inverter to be turned off and isolated if servicing is necessary,
and also safeguards the circuit’s electrical wiring.
15. KILOWATT-HOUR METER
(AKA: kWh meter, utility meter)
Most homes with a grid-tied solar-electric
system will have AC electricity coming from and going to the grid. A
bidirectional kWh meter can cumulatively track the flow in both directions. The
utility company often provides these special meters at no cost.
16. BACKUP GENERATOR
(AKA: gas guzzler, the racket)
Off-grid PV systems can be sized to provide
electricity during cloudy periods when the sun doesn’t shine. But sizing a
system to cover a worst-case scenario, like several cloudy weeks during the
winter, can result in a very large, expensive system that will rarely get used
to its capacity. To spare your pocketbook, size the system moderately, but
include a backup generator to get through those occasional sunless stretches.
Generators are also used to provide battery equalizing charging—occasional,
high-voltage, prolonged charging that brings the weaker battery cells up to the
charge level of the stronger cells.
Engine generators can be fueled with
biodiesel, petroleum diesel, gasoline, or propane. These generators produce AC
electricity that a battery charger (either stand-alone or incorporated into an
inverter) converts to direct current, which is stored in batteries. Like most
internal combustion engines, generators tend to be loud and polluting, and
require maintenance. A well-designed PV system will require running a generator
only 50 to 200 hours a year.
SOLAR-ELECTRIC SYSTEMS
DEMYSTIFIED
As you can see, the anatomy of a
solar-electric system isn’t that complicated. All of the parts have a purpose,
and once you understand the individual tasks that each part performs, the whole
system makes more sense. Now you’re ready to look at the system articles and
schematics in Home Power without your eyes glazing over, and
you’ll have a clearer understanding of what is going on. To solidify your
understanding, your next task could be to examine a solar-electric system in
person, going on a local solar tour, or getting on the solar grapevine to visit
folks ahead of you on the solar curve.
As written in HomePower Magazine...
Justine Sanchez is Technical Editor at Home Power, a Solar Energy International
instructor, a NABCEP-certified PV installer, and is certified by ISPQ as a PV
Affiliated Master Trainer.
Home Power Senior Editor Ian Woofenden has been living with solar-electric
systems since the early 1980s. His systems include a wide range of
applications, including solar flashlights, vent fans, hybrid wind-PV systems
for home and shop, a PV-powered waterslide, electric fence chargers, an iPhone
backup charger, and more.
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