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Q1: How do solar cells generate electricity?
Q2: Will solar work in my location?
Q3: How much will a system cost for my 2000 square
foot home?
Q4: Can I use all of my normal 220 VAC appliances?
Q5: What components do I need for a grid-tie system?
Q6: What components do I need?
Q7: What type of solar module mounting structure
should I use?
Q8: Where should I mount the solar modules and
what direction should I face them?
Q9: Should I set my system's battery bank up at
12, 24 or 48 VDC?
Q10: Should I wire my home for AC or DC loads?
Q11: Can I use PV to heat water or for space
heating?
Q1:
How do solar cells generate electricity?
A1: 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 15 cm 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.
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Q2:
Will solar work in my location?
A2: Solar is universal and will work virtually anywhere,
however some locations are better than others. 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. In South
Africa a good general number to work with is 5.5 full sun
hours per day.
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Q3:
How much will a system cost for my 2000 square foot home?
A3: Unfortunately there is no per square meter "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. All domestic users in South
Africa are charged on Kilo Watt hour usage between 20c to 40c
per kilo watt hour.
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Q4:
Can I use all of my normal 120/240 VAC appliances?
A4: 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 220 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 220 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 and Led lights use
a quarter to a third of the power of an incandescent light for
the same lumen output and they last ten to 1 hundred times longer.
The rule of thumb in the renewable energy industry is that for
every Rand you spend replacing your inefficient appliances,
you will save fiveteen Rands 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.
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Q5:
What components do I need for a grid-tie system?
A5: 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.
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Q6:
What components do I need?
A6: 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 220 VAC to run your AC appliances.
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Q7:
What type of solar module mounting structure should I use?
A7: 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 (GFP).
On the other hand, ground mounted solar arrays require fairly
precise foundation setup, are more susceptible to theft or vandalism
and 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). 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. In general in South
Africa we do not recomment trackers as they add an extra maintenance
dimension.
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Q8:
Where should I mount the solar modules and what direction
should I face them?
A8: If your site is in the Southern Hemisphere you need to
aim your solar modules to the true north direction (the reverse
is true for locations in the Northern 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 north 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/autumn and latitude plus 15°
in the winter. Most mount structures provide for a seasonal
adjustment of the tilt angle from horizontal to 65°.
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Q9:
Should I set my system's battery bank up at 12, 24 or 48 VDC?
A9: The PV industry really began with the 12V 4x4 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 48-500 volts.
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Q10:
Should I wire my home for AC or DC loads?
A10: 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 lodges 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 220VAC circuit is much less than a 12VDC
circuit carrying the same power, which allows you to use smaller
gauge wire.
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Q11:
Can I use PV to heat water or for space heating?
A11: 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.
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