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Basics on
Designing a Solar System
How much
Roof Space do you need?
100 sq feet of solar PV panels produce approximately 1 kilowatt of power.
Under ideal conditions this means that you
could get a measured 1000 watts out
of this 100 sq feet of panels. To achieve this, however, the weather would have to be really cold
and the sun would have to be
perpendicular to the panels. This 100 sq feet of
panels, or 1 kilowatt, would produce usually
between 4.5 to 5 kilowatts of electricity every sunny day. You can
determine what number to multiply by on a solar insolation chart. It
would tell you how many hours of sun your site would get on an average
for a year.
These
numbers , on the solar insolation charts are quite conservative and if the
panels are facing south and set at about the angle of your latitude (for
instance, 38 degrees for the San Francisco Bay Area) this
amount of electricity should be
attainable.
Sometimes it is a better idea to have the panels at the best angle to
use summer sun, or even better, have the angles of the panels adjustable
for seasonal sun. (See our notes under
PV Modules Orientation.
)
If your electric company puts your solar system on a yearly contract,
you can use the excess generation of the longer hours of the summer
months and you can take it off your winter usage.
String Sizing: Series and Parallel
Series = + to -
Parallel = + to + and - to -
With a solar PV system it is important to size the strings of panels for
the correct voltage of the inverter.
When you connect panels in series, you add the voltages for each of the
panels together but the current on the string
stays the same as one panel, no matter how many panels you use. This is
the way to make a very efficient
system. It would give you high voltages and low current
If you were connecting up the
panels in parallel you would have the
same voltages but the current would be added for each of the panels.
You would end up with a system with
low voltages and extremely high currents. This would require very heavy
gauge wire and this method would not
give you as efficient a system. This is why solar designers try and
get the highest voltage possible, with the
least amount of current.
Temperature and Panel Output
Another thing to be aware of is that cold panels put out
more power than hot panels. When a
panel gets hot the semiconductor junctions have more resistance so the
panel has a lower output. On the
spec sheet of a panel there will be number for the computation of output
on a temperature basis.
How Much Electricity Do You Use?
When designing a solar system it is important to know how much
electricity or gas you are using and the simplest way is to look on your electric bill. It is easiest to figure out the
numbers in kilowatt hours per day or therms
per day.
If you are going to be off the grid, you can calculate all the different
appliances and lights you would use and multiply this by the hours in use. This can give you
a fairly accurate number for the amount of
electricity you would need to generate in kilowatts per day. See our
System Loads worksheet to calculate
the loads on your system.
If you are
looking at how much gas you are
using, one therm equals 100,000 btus.
Can you reduce electrical Needs?
Before you come to a conclusion as to how much electricity you will need,
you should first think about how
you could reduce your electricity usage without changing your lifestyle.
The easiest way to do this is
usually with more efficient air conditioning and refrigeration and more
efficient lighting.
LED Lighting
LED lighting has really improved over the last year. You can now get a
3-watt LED light that would give you the equivalent light of a 20 watt halogen light.
Be careful when you buy LED lights not to get the kind that have multitudes of regular
LEDs wired together, as these don't give
off enough lumens and the small LEDs have a lens that is preset for a
very close distance.
The best LED lights, with the most lumens, are
ones that are surface mounted with the Cree or SMD LEDs. These LEDs can
sometimes be focused with a lens and they have a very long life expectancy
(11 years). They also produce no RF or heat and don't contain mercury.
LEDs
come in two basic colors, not including the more decorative colored LEDs. For white
light, you can get 5600 Kelvin which would equal what is considered to be
natural light. The light would have a blue hue to it. 3600 Kelvin
would be equivalent to tungsten. Some people seem to prefer the 5600K
while others show a distinct preference for the 3600K, You get more lumens out of the 5600K lights as
the tungsten color is produced with yellow/orange filters. These lights
can be with the Edison base to screw into a regular socket or MR16, like
halogen lights, which
is used in many track lighting systems.
Solar Hot Water
Solar hot water systems are very efficient, low cost and perhaps give
the best return for your energy efficiency spending. We used to sell
flat plate collectors, but when
the evacuated tubes became
available we were quickly became converts.
The evacuated tubes produce about
40% more hot water than a
similar sized flat plate panel. The tubes also produce equally as well
in very cold weather. This is because
of the vacuum that exists between the inner and outer tube. A flat
plate panel could lose a lot of its heat to
the ambient air through the front glass. Not so with an Evacuated
Tube System.
With the evacuated tubes a 30-tube system produces about 40,000 btus
every sunny day and about 1/2
that amount on a cloudy day. 40,000 btus can heat 80 gallons of water
up 60 degrees, so depending on the
temperature of your incoming water, a 30-tube system can heat up 80
gallons very hot.
We think the evacuated tube systems will eventually become very popular for air conditioning.
Right now absorption air
conditioning systems are available but cost about $7,000 per ton of air
conditioning. (The ton number is
an air conditioning number that indicates the amount of AC it would take
to freeze a ton of water.) This price
should be coming way down when these systems become more popular and you
can just buy one off the
shelf.
So when designing a solar hot water system it may be a good idea to
design for the future installation of a
solar air conditioning system.
Another idea is to design the system with
a heat dump. A pool or hot water
heater is perfect for a heat dump and sometimes this is necessary as the
evacuated tubes produce so
much hot water.
One of our favorite uses of the evacuated tubes is to run
a kickspace heater. These heaters usually have a 1/2 inch input and output and hot water
circulates through the system. When
the water gets hot enough, a small fan pushes air through a small
radiator, such as the types used for
under the counter installations with hydronic heating systems where
baseboard heaters do not fit. The kickspace heater can also provide a
dump load if the air could be
redirected outdoors when the weather is hot. In the winter months the
air could be used for space heating.
Kickspace heaters can be small and produce 2000 to 3000 btus but they
can also be big enough to produce 7000 to
8000 btus and one or more kickspace heaters can be combined with the
evacuated tubes.
We usually don't recommend putting more than 90 evacuated tubes in series as the
water would get too hot. However, you can parallel plumb systems if you
do need more hot water. The tubes
are ideal to put into radiant floor
heating systems and if they are installed in the concrete floor, the
concrete can act as a heat sink to release the heat
into the house throughout the night. | |
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