Home Power Magazine - Solar Electric Basics.pdf
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HP 104 pages72-78.indd
Issue 104
December 2004
January 2005
Eco-Friendly
Building Products
p. 34
Big City Solar
Living Green in NYC
p. 14
Ultimate Beginner’s Guide
to Solar-Electric Systems
p. 72
Cool Savings
Choose an Energy-Efficient Fridge
p. 66
homepower.com
Solar Hot Tubbing
An Easy Do-It-Yourself Project
p. 28
$6.95 US $10.95 CAN
12
Stream Dreams
Measure Your Site’s Hydroelectric Potential
p. 42
7
2033678082
6
Display until February 1
TOP 10
Solar-Electric Systems
Scott Russell
©2004 Scott Russell
Perhaps what the home-scale renewable energy (RE) world needs most are ways
to introduce people to RE technologies and the gizmos that make it possible. After
all, even the best ideas aren’t embraced until they are explained in simple terms.
So whether
you
are the rookie who wants to understand how solar-electric
systems work, or that better describes your spouse, friend, or prospective customer,
this article explains the guts and bolts of the three most common options in solar-
electric systems: grid-intertied, grid-intertied with battery backup, and off-grid
(stand-alone).
Understanding the basic components of an RE system and how they function
is not an overwhelming task. Here are some brief descriptions of the common
equipment used in grid-intertied and off-grid solar-electric systems. Systems
vary—not all equipment is necessary for every system type. In the diagrams, the
numbers in red correspond to the components needed.
Grid-Intertied Solar-Electric System
Also known as on-grid, grid-tied, or utility-
interactive (UI), grid-intertied solar-electric systems
generate solar electricity and route it to the electric
utility grid, offsetting a home’s or business’s
electrical consumption and, in some instances, even
turning the electric meter backwards. Living with a
grid-connected solar-electric system is no different
than living with grid power, except that some or all
of the electricity you use comes from the sun.
In many states, the utility credits a homeowner’s
account for excess solar electricity produced. This
amount can then be applied to other months
when the system produces less or in months when
electrical consumption is greater. This arrangement
is called net metering or net billing. The specific
terms of net metering laws and regulations vary
from state to state and utility to utility. Consult
your local electricity provider or state regulatory
agency for their guidelines.
1
Photovoltaic
Array
Array DC
Disconnect
3
Array Mounting
Rack
2
72
home power 104
/ december 2004 & january 2005
solar
simple
1
Solar-Electric Panels
AKA: solar-electric modules, photovoltaic (PV) panels
PV panels are a solar-electric system’s
defining component, where sunlight is used
to make direct current (DC) electricity. Behind
a PV panel’s shimmering facade, wafers of
semiconductor material work their magic,
using light (photons) to generate electricity—
what’s known as the
photovoltaic effect
.
Other components in your system enable
the electricity from your solar-electric panels
to safely power your electric loads like lights,
computers, and refrigerators.
You can use the rated output to help determine
how many panels you’ll need to meet your
electrical needs. Multiple modules combined
together are called an array.
Although rigid panels are the most common
form of solar electricity collector, PV
technology also has been integrated into
roofing shingles and tiles, and even peel-
and-stick laminates (for metal standing-seam
roofs).
PV panels are assigned a rating in watts based
on the maximum power they can produce
under ideal sun and temperature conditions.
PV modules are very durable and long-
lasting—most carry 25-year warranties. They
can withstand severe weather, including
extreme heat, cold, and hail stones.
2
Array Mounting Rack
AKA: mounts, racks
Mounting racks provide a secure platform on which to
anchor your PV panels, keeping them fixed in place and
oriented correctly. Panels can be mounted using one of
three approaches: 1) on a rooftop; 2) atop a steel pole set in
concrete; or 3) at ground level. The specific pieces, parts, and
materials of your mounting device will vary considerably
depending on which mounting method you choose.
during the summer and lower in the winter. Adjustable
mounting racks enable you to set the angle of your PV
panels seasonally, keeping them aimed more directly at
the sun. Adjusting the tilt angle increases the system’s
annual energy production by a few percent. The tilt of roof-
mounted arrays is rarely changed—adjusting the angle is
inconvenient and sometimes dangerous, due to the array’s
location.
Usually, arrays in urban or suburban areas are mounted
on a home’s south-facing roof, parallel to the roof’s slope.
This approach is sometimes considered most aesthetically
pleasing, and may be required by local regulators or
homeowner’s associations. In areas with a lot of space,
pole- or ground-mounted arrays are another choice.
Changing the tilt angle of pole- or ground-mounted arrays
can be done quickly and safely. Pole-mounted PV arrays
also can incorporate tracking devices that allow the array
to automatically follow 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 percent.
Mounting racks may incorporate other features, such
as seasonal adjustability. The sun is higher in the sky
To household
loads
To household
loads
8
10
Kilowatt-Hour
Meter
To/From
Utility Grid
Inverter
9
AC Breaker Panel
www.
homepower
.com
73
solar
simple
3
Array DC Disconnect
AKA: PV disconnect
4
Charge Controller
AKA: controller, regulator
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.
The disconnect enclosure
houses an electrical switch
rated for use in DC circuits.
It also may integrate either
circuit breakers or fuses, if
needed.
A charge controller’s primary function is to protect
your battery bank from overcharging. It does this
by monitoring the battery bank—when the bank is
fully charged, the controller interrupts the flow of
electricity from the PV panels. Batteries are expensive
and pretty particular about how they like to be treated.
To maximize their life span, you’ll definitely want to
avoid overcharging or undercharging them.
Most modern charge controllers incorporate maxi-
mum power point tracking (MPPT), which optimizes
the PV array’s output,
increasing the energy it
produces. Some battery-
based charge controllers
also include a low-voltage
disconnect that prevents
over discharging, which
can perma nently damage
the battery bank.
Grid-Intertied
Solar-Electric System with Battery Backup
Without a battery bank or generator backup for your grid-
intertied system, when a blackout occurs, your household
will be in the dark, too. To keep some or all of your electric
needs (or “loads”) like lights, a refrigerator, a well pump, or
computer running even when utility power outages occur,
many homeowners choose to install a grid-intertied system
with battery backup. Incorporating batteries into the system
requires more components, is more expensive, and lowers
the system’s overall efficiency. But for many homeowners
who regularly experience utility outages or have critical
electrical loads, having a backup energy source is priceless.
1
Photovoltaic
Array
4
Charge
Controller
System Meter
6
Array DC
Disconnect
3
Battery Bank
Array
Mounting
Rack
5
2
74
home power 104
/ december 2004 & january 2005
solar
simple
5
Battery Bank
AKA: storage battery
6
System Meter
AKA: battery monitor, amp-hour meter
Your PV panels will produce electricity whenever
the sun shines on them. If your system is off-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 one to three cloudy days. Grid-
intertied systems also can include battery banks to
provide emergency backup power during blackouts—
perfect for keeping critical electric loads operating
until grid power is restored.
System meters measure and display several different
aspects of your solar-electric system’s performance
and status, tracking how full your battery bank is;
how much electricity your solar panels are producing
or have produced; and
how much electricity is
in use. Operating your
solar-electric system
without metering is
like running your car
without any gauges—
although possible to
do, it’s always better
to know how much
fuel is in the tank.
Although similar to ordinary car batteries, the
batteries used in solar-electric systems are specialized
for the type of charging and discharging they’ll need
to endure. Lead-acid batteries are the most common
battery used in solar-electric systems. Flooded lead-
acid batteries are usually the least expensive, but
require adding distilled water occasionally to replenish
water lost during the
normal charging
process. Sealed
absorbent glass mat
(AGM) batteries are
maintenance free
and designed for
grid-tied systems
where the batteries
are typically kept at
a full state of charge.
Gel-cell batteries can
be a good choice
to use in unheated
spaces due to their
freeze-resistant
qualities.
7
Main DC Disconnect
AKA: battery/inverter disconnect
In battery-based systems,
a disconnect between the
batteries and inverter is
required. This disconnect 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 electrical fires.
To household
loads
To household
8
Inverter
10
7
To/From
Utility Grid
Main DC
Disconnect
9
Kilowatt-Hour
Meter
AC Breaker Panel
www.
homepower
.com
75
Kilowatt-Hour
AC Breaker Panel
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