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Photovoltaic Solar Systems

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Photovoltaic Solar Systems
Dr. William J. Makofske
August 2004

What is a solar cell?
• Solid state device that converts incident
solar energy directly into electrical energy
• Efficiencies from a few percent up to 2030%
• No moving parts
• No noise
• Lifetimes of 20-30 years or more

Cross Section of Solar Cell

How Does It Work?
• The junction of dissimilar materials (n and p type
silicon) creates a voltage
• Energy from sunlight knocks out electrons,
creating a electron and a hole in the junction
• Connecting both sides to an external circuit
causes current to flow
• In essence, sunlight on a solar cell creates a small
battery with voltages typically 0.5 v. DC

Combining Solar Cells
• Solar cells can be electrically connected in
series (voltages add) or in parallel (currents
add) to give any desired voltage and current (or
power) output since P = I x V
• Photovoltaic cells are typically sold in modules
(or panels) of 12 volts with power outputs of 50
to 100+ watts. These are then combined into
arrays to give the desired power or watts.

Cells, Modules, Arrays

Rest of System Components
While a major component and cost of a PV system is
the array, several other components are typically
needed. These include:
• The inverter – DC to AC electricity
• DC and AC safety switches
• Batteries (optional depending on design)
• Monitor – (optional but a good idea)
• Ordinary electrical meters work as net meters

The Photovoltaic Array with its
other electrical components

PV was developed for the space
program in the 1960’s

PV Price and Quantity
Manufactured Relationship

The PV Market
Solar Calculators

REMOTE POWER







Lighting
Buoys
Communications
Signs
Water Pumping
Mountain Cabins

Photovoltaic Array for Lighting

Telecommunications Tower

Remote Water Pumping in Utah

Recreation Vehicle Outfitted with
Solar Panels

Solar Lanterns for Landscaping

A Solar Driven Band

The Market Expands
• As prices dropped, PV began to be used for
stand-alone home power. If you didn’t have
an existing electrical line close to your
property, it was cheaper to have a PV
system (including batteries and a backup
generator) than to connect to the grid. As
technology advanced, grid-connected PV
with net metering became possible.

NET METERING
In net metering, when the PV system produces
excess electricity, it is sent to the grid system,
turning the meter backwards. If you are using
more power than is being produced, or it is at
night, the electricity is received from the grid
system and the meter turns forwards. Depending
on PV size and electrical consumption, you may
produce more or less than you actually use.
Individual houses may become power producers.

Net Metering can be done with or
without a battery backup

BATTERIES
• Batteries can be used to provide long-term
or short-term electrical supply in case of
grid failure. Many grid-connected houses
choose to have a small electrical battery
system to provide loads with power for half
a day in case of outage. Larger number of
batteries are typically used for remote gridindependent systems.

Battery Sizing I
If your load is 10 kw-hr per day, and you want to
battery to provide 2.5 days of storage, then it
needs to store 25 kw-hr of extractable electrical
energy. Since deep cycle batteries can be
discharged up to 80% of capacity without harm,
you need a battery with a storage of 25/0.8 =
31.25 kw-hr. A typical battery at 12 volts and 200
amp-hour capacity stores 2.4 kw-hr of electrical
energy.

Battery Sizing II
The relationship between energy in kw-hr and
battery capacity is
E(kw-hr) =capacity(amp-hr) x voltage/1000
E = 200 amp-hr x 12 volts/1000= 2.4 kw-hr
So for 31.25 kw-hr of storage we need
31.25 kw-hr/2.4 kw-hr/battery = 13 batteries
If we are happy with one half day, we need only 2 or 3
batteries

2 KW PV on Roof with battery
storage. Solar hot water
collectors and tank

PV On Homes
• PV can be added to existing roofs. While south
tilted exposure is best, flat roofs do very well.
Even east or west facing roofs that do not have
steep slopes can work fairly well if you are
doing net metering since the summer sun is so
much higher and more intense than the winter
sun. The exact performance of any PV system
in any orientation is easily predictable.

Photovoltaic Array on Roof and
as an Overhang

½ KW PV System Installed
along Roof Ridge

California Home PV Installation

PV on House

2.4 KW System under
Installation in New Hampshire

PV Installed at Roofline on
Building at Frost Valley, NY

PV Panels on Tile Roofs in
Arizona

PV on Roof in California

Totally Inadequate Roof?
• If it is impossible or you don’t want to put a
PV system on your existing roof, it is
possible to pole mount the arrays
somewhere near the house as long as the
solar exposure is good. Pole mounted solar
arrays also have the potential to rotate to
follow the sun over the day. This provides a
30% or more boost to the performance.

Pole Mount for Solarex Modules

Pole Mounted PV

Pole Mounted PV

Roof Integrated PV
• If you are doing new construction or a
reroofing job, it is possible to make the roof
itself a solar PV collector. This saves the
cost of the roof itself, and offers a more
aesthetic design. The new roof can be
shingled or look like metal roofing. A few
examples follow.

Solar Roofing Shingles

Roof Integrated Photovoltaics in
Misawi, Japan

Roof Integrated PV in Japan

Roof Integrated PV in Maine

Roof Integrated Photovoltaic
System in Colorado

Roof Integrated PV
(objects below chimney are solar hot water collectors)

PV Installation in Planned
Community in Germany

Sizing a PV System
Solar modules are typically sold by the peak watt. That
means that when the sun is at its peak intensity (clear day
around midday) of 1000 watts per m2, a solar module rates
at say 100 Wp (peak watts) would put out 100 watts of
power. The climate data at a given site summarizes the
solar intensity data in terms of peak sun hours, the
effective number of hours that the sun is at that peak
intensity on an average day. If the average peak sun hours
is 4.1, it also means that a kw of PV panels will provide
4.1 kw-hr a day.

Thinking About Solar Energy
• When the sky is clear and it is around
midday, the solar intensity is about 1000
watts per m2 or 1 kw/m2
• In one hour, 1 square meter of the earth’s
surface facing the sun will intercept about 1
kw-hr of solar energy.
• What you collect depends upon surface
orientation and collector efficiency

Sizing a PV System to
Consumption
A PV system can be sized to provide part or all of
your electrical consumption. If you wanted to
produce 3600 kw-hr a year at a site that had an
average of 4.1 peak sun hours per day,
PV Size in KWp =

3600 kw-hr

4.1 kw-hr/day x 365 days/yr x 0.9 x0.98
= 2.7 KWp
Note: the 0.9 is the inverter efficiency and the
0.98 represents the loss in the wiring.

Thinking About Electrical
Consumption
1 kW = 1000 watts = 1.34 hp (presumably the
maximum sustained output of a horse)
1 kW-hr = 3413 Btu is the consumption of a 1
kW device operated for an hour (E=Pxt)
Now think about a Sherpa mountain guide
carrying a 90 lb pack up Mount Everest,
about 29,000 ft or 8,839 meters high, over a
week, the typical time for such a trip

The Sherpa-Week
Since we know that the energy in lifting is given by mgh or
40.8 kg x 9.8 m/s2 x 8839m = 3,539,000 joules or about 1
kw-hr, we can say that roughly 1 kw-hr = 1 Sherpaweek. Typical U.S. household consumption is 600 kw-hr
per month or 20 kw-hr per day, or every day it is like hiring
20 Sherpa to carry the 90 lb packs up Mt. Everest. At the
end of the week, we have 140 Sherpa climbing the slopes so
the equivalent power that we consume is like having 140
Sherpa climbing Mt Everest continually. We might want to
consider reducing this number before adding PV to our roof.

How Much Area Is Needed?
The actual area that you need depends on the
efficiency of the solar cells that you use. Typical
polycrystalline silicon with around 12% efficiency
will require about 100 ft2 of area to provide a peak
kilowatt. Less efficient amorphous silicon may
need 200 ft2 to provide the same output. Modules
are sold in terms of peak wattage and their areas
are given so you can easily determine the total
roof area that is needed for a given size array.

Find the efficiency of a solar cell module
given its power rating and its area
Assume it is a 100 Wp module and its area
is 0.8 m2. Remember that the peak power
rating is based on an intensity of 1000
watts/m2. So the maximum output with
100% efficiency is P = I x A = 1000 w/m2 x
0.8 m2 = 800 watts
The actual efficiency = Pactual peak/Pmaximum peak

= 100 watts/800 watts = 0.125 or 12.5%

Larger Scale PV
• Of course you don’t have to stop with home
based PV systems. They make equally good
sense for businesses and corporations who
want to reduce their cost of electricity by
reducing their peak power consumption, or
who want to emphasize their greenness as
part of their image, or who need to operate
in a grid failure.

Rooftop Installation at Mauna
Lani Resort, Hawaii

Details of Roof Installation for
Mauna Lani Resort, Hawaii

Solar Carport
Navy Installation – San Diego, California

BP Installation on their Gas
Station

Large 57 KW Rural Installation

Solar Added to Flat Roofs
(can upgrade the insulation as well)

59 KW Installation of 5600 ft2
in Greenpoint, Brooklyn

The Greenpoint, NY Building

FALA Factory Roof Installation
Farmingdale, LI, NY
Note the number of other roofs

Solar Cells Installed in Building
Facade

The sun is the primary energy
source for almost all energy
flows on the planet. It’s time we
started using it.

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