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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 20-

30%

• 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 poweroutputs 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 yourproperty, it was cheaper to have a PV

system (including batteries and a backup

generator) than to connect to the grid. Astechnology 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 usingmore 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 mayproduce 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 houseschoose 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 grid-

independent 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 electricalenergy. 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 200amp-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 roofsthat do not have steep slopes can work

fairly well if you are doing net metering

since the summer sun is so much higher andmore 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 arrayssomewhere near the house as long as the

solar exposure is good. Pole mounted solar

arrays also have the potential to rotate tofollow the sun over the day. This provides a

30% or more boost to the performance.



Pole Mount for Solarex Modules



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 thecost of the roof itself, and offers a more

aesthetic design. The new roof can be

shingled or look like metal roofing. A fewexamples 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 hoursis 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

ConsumptionA 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 anaverage 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

Consumption1 kW = 1000 watts = 1.34 hp (presumably

the maximum sustained output of a horse)

1 kW-hr = 3413 Btu is the consumption of a1 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 Sherpa-

week. 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 likehaving 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 areasare 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 areaAssume 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 1000watts/m2. So the maximum output with

100% efficiency is P = I x A = 1000 w/m2 x

0.8 m2 = 800 wattsThe 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 whowant to reduce their cost of electricity by

reducing their peak power consumption, or

who want to emphasize their greenness aspart 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 F R f I ll i



FALA Factory Roof Installation

Farmingdale, LI, NYNote the number of other roofs



Solar Cells Installed in Building

Facade

T e sun s t e pr mary energy



p y gysource for almost all energy

flows on the planet. It’s time westarted using it.

photo voltaic solar systems

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