Solar Energy may seem puzzling to some people that
are not familiar with it. To help those of you that
are being exposed to solar and wind power for the
first time, we have compiled a few of the most frequently
asked questions (with their answers) that we hear
everyday. We hope this FAQ file is helpful to you.
How do solar cells generate electricity?
Will solar work in my location?
How much will a system cost for my
2000 square foot home?
Can I use all of my normal 120/240
VAC appliances?
What components do I need for a grid-tie
system?
What components do I need?
What type of solar module mounting
structure should I use?
Where should I mount the solar modules
and what direction should I face them?
Should I set my system's battery
bank up at 12, 24 or 48 VDC?
Should I wire my home for AC or
DC loads?
Can I use PV to heat water or
for space heating?
How
do solar cells generate electricity?
Photovoltaics
or PV for short can be thought of as a direct current
(DC) generator powered by the sun. When light photons
of sufficient energy strike a solar cell, they knock
electrons free in the silicon crystal structure forcing
them through an external circuit (battery or direct
DC load), and then returning them to the other side
of the solar cell to start the process all over again.
The voltage output from a single crystalline solar
cell is about 0.5V with an amperage output that is
directly proportional to cell's surface area (approximately
7A for a 6 inch square multicrystalline solar cell).
Typically 30-36 cells are wired in series (+ to -)
in each solar module. This produces a solar module
with a 12V nominal output (~17V at peak power) that
can then be wired in series and/or parallel with other
solar modules to form a complete solar array to charge
a 12, 24 or 48 volt battery bank.
Will solar work in my
location?
Solar is universal and will work virtually anywhere,
however some locations are better than others are.
Irradiance is a measure of the sun's power available
at the surface of the earth and it averages about
1000 watts per square meter. With typical crystalline
solar cell efficiencies around 14-16%, that means
we can expect to generate about 140-160W per square
meter of solar cells placed in full sun. Insolation
is a measure of the available energy from the sun
and is expressed in terms of "full sun hours"
(i.e. 4 full sun hours = 4 hours of sunlight at an
irradiance level of 1000 watts per square meter).
Obviously different parts of the world receive more
sunlight from others, so they will have more "full
sun hours" per day. The solar insolation zone
map on the right will give you a general idea of the
"full sun hours per day" for your location.
How much will a system
cost for my 2000 square foot home?
Unfortunately
there is no per square foot "average" since
the cost of a system actually depends on your daily
energy usage and how many full sun hours you receive
per day; And if you have other sources of electricity.
To accurately size a system to meet your needs, we
need to know how much energy you use per day. If your
home is connected to the utility grid, simply look
at your monthly electric bill. Using this information,
your authorised Energia solar Dealer can design a
system to meet you needs.
Can
I use all of my normal 120/240 VAC appliances?
Maybe.
Many older homes were not designed or built with energy
efficiency in mind. When you purchase and install
a renewable energy system for your home, you become
your own power company so every kWh of energy you
use means more equipment (and hence more money) is
required to meet your energy needs. Any appliances
that operate at 240 VAC (such as electric water heaters,
cook-stoves, furnaces and air conditioners) are impractical
loads to run on solar. You should consider using alternatives
such as LP or natural gas for water/space heating
or cooking, evaporative cooling instead of compressor
based AC units and passive solar design in your new
home construction if possible. Refrigeration and lighting
are typically the largest 120 VAC energy consumers
in a home (after electric heating loads) and these
two areas should be looked at very carefully in terms
of getting the most energy efficient units available.
Great strides have been made in the past 5 years towards
improving the efficiency of electric refrigerators/freezers.
Compact fluorescent lights use a quarter to a third
of the power of an incandescent light for the same
lumen output and they last ten times longer. These
fluorescent lights are now readily available at your
local hardware or discount store. The rule of thumb
in the renewable energy industry is that for every
dollar you spend replacing your inefficient appliances,
you will save three dollars in the cost of a renewable
energy system to run them. So you can see that energy
conservation is crucial and can really pay off when
considering a renewable energy system.
What
components do I need for a grid-tie system?
Grid-tie
systems are inherently simpler than either grid-tie
with battery back-up or stand-alone solar systems.
In fact, other than safety disconnects, mounting structures
and wiring a grid-tie system is just solar modules
and a grid-tie inverter! Today's sophisticated grid-tie
inverters incorporate most of the components needed
to convert the direct current form the modules to
alternating current, track the maximum power point
of the modules to operate the system at peak efficiencies
and terminate the grid connection if grid power is
interrupted form the utility.
What
components do I need?
There
are many components that make up a complete solar
system, but the 4 main items are: solar modules, charge
controller(s), batteries and inverter(s). The solar
modules are physically mounted on a mount structure
(see question 7) and the DC power they produce is
wired through a charge controller before it goes on
to the battery bank where it is stored. The two main
functions of a charge controller are to prevent the
battery from being overcharged and eliminate any reverse
current flow from the batteries back to the solar
modules at night. The battery bank stores the energy
produced by the solar array during the day for use
at anytime of day or night. Batteries come in many
sizes and grades. The inverter takes the DC energy
stored in the battery bank and inverts it to 120 VAC
to run your AC appliances.
What
type of solar module mounting structure should I use?
There are four basic types of mount structures: roof/ground,
top-of-pole, side-of-pole and tracking mounts, each
having their own pros and cons. For example roof mount
structures typically keep the wire run distances between
the solar array and battery bank to a minimum, which
is good. But they also require roof penetrations in
multiple locations (a potential source of leakage)
and they require an expensive ground fault protection.
On the other hand, ground mounted solar arrays require
fairly precise foundation setup, are more susceptible
to theft/vandalism and excessive snow accumulation
at the bottom of the array. Next are top-of-pole mounts
which are relatively easy to install (you sink a 2-6
inch diameter SCH40 steel pole up to 4-6 feet in the
ground with concrete). Make sure that the pole is
plumb and mount the solar modules and rack on top
of the pole. Top-of-pole mounts reduce the risk of
theft/vandalism (as compared to a ground mount). They
are also a better choice for cold climates because
snow slides off easily. Side of pole mounts are easy
to install, but are typically used for small numbers
of solar modules (1-4) for remote lighting systems
where there already is an existing pole to attach
them to. Last but not least are the trackers, which
increase the daily number of full sun hours and are
used for solar water pumping applications. Trackers
are extremely effective in the summer time when water
is needed the most. Typical home energy usage peaks
in the winter when a tracker mount makes very little
difference as compared to any type of fixed mount
(roof, ground or top-of-pole). In this situation,
having more modules on a less expensive fixed mount
will serve you better in the winter than fewer modules
on a tracker.
Where should I mount the
solar modules and what direction should I face them?
If your site is in the Northern Hemisphere you need
to aim your solar modules to the true south direction
(the reverse is true for locations in the Southern
Hemisphere) to maximize your daily energy output.
For many locations there is quite a difference between
magnetic south and true south, so please consult the
declination map below before you setup your mount
structure. The solar modules should be tilted up from
horizontal to get a better angle at the sun and help
keep the modules clean by shedding rain or snow. For
best year round power output with the least amount
of maintenance, you should set the solar array facing
true south at a tilt angle equal to your latitude
with respect to the horizontal position. If you plan
to adjust your solar array tilt angle seasonally,
a good rule of thumb to go by is latitude minus 15°
in the summer, latitude in the spring/fall and latitude
plus 15° in the winter. Most mount structures
provide for a seasonal adjustment of the tilt angle
from horizontal to 65°. To determine if your proposed
array site will be shaded at any time of the day or
year you should consider using the Solar Pathfinder.
Should
I set my system's battery bank up at 12, 24 or 48
VDC?
The
PV industry really began with the 12V recreational
vehicle market. These systems were typically small
(1-2 solar modules) and had all 12 VDC loads. As the
solar industry matured and entered the home market,
systems became much larger (16+ solar modules) and
no longer used DC loads exclusively. Most home systems
today are 24 or 48 VDC since the higher system voltage
gives you a lot more flexibility as to how far away
you can place your solar modules from the battery
bank as compared to a 12V system. For a given power
output, a higher system voltage reduces your amperage
flow (but not your power) which allows you to use
a smaller and less expensive gauge wire for your solar
to battery and battery to inverter wire runs. Of course,
if you already have a lot of 12VDC loads, that may
be your deciding factor as to what voltage you set
your system up at. Most grid-tied systems operate
at 48 volts or higher.
Should
I wire my home for AC or DC loads?
It
depends on the size of the system and what type of
loads you want to run. DC appliances are usually more
efficient than AC since you don't have to worry about
the loss through the inverter, but DC loads are typically
more expensive and harder to find than their AC counterparts.
Small cabin and RV systems are typically wired DC
while most home systems are wired for AC loads exclusively.
With improvements in inverter efficiency and reliability
in the last 5 years, AC is the way to go for a home
system. Another advantage AC has over DC is that the
voltage drop for a 120VAC circuit is much less than
a 12VDC circuit carrying the same power, which allows
you to use smaller gauge wire.
Can
I use PV to heat water or for space heating?
No.
Photovoltaics converts the sun's energy into DC electricity
at a relatively low efficiency level (14-16%), so
trying to operate a high power electric heating element
from PV would be very inefficient and expensive. Solar
thermal (or passive solar) is the direct heating of
air or water from the heat of the sun and is much
more efficient for heating applications than photovoltaics.
