Photovoltaic Solar Systems

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Photovoltaic Solar Systems

• Dr. William J. Makofske
• August 2004

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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

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Cross Section of Solar Cell
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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

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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.

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Cells, Modules, Arrays
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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

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The Photovoltaic Array with its other electrical
components
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PV was developed for the space program in the
1960s
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PV Price and Quantity Manufactured Relationship
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The PV Market

• Solar Calculators
• REMOTE POWER
• Lighting
• Buoys
• Communications
• Signs
• Water Pumping
• Mountain Cabins

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Photovoltaic Array for Lighting
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Telecommunications Tower
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Remote Water Pumping in Utah
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Recreation Vehicle Outfitted with Solar Panels
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Solar Lanterns for Landscaping
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A Solar Driven Band
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The Market Expands

• As prices dropped, PV began to be used for
  stand-alone home power. If you didnt 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.

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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.

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Net Metering can be done with or without a
battery backup
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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 grid-independent systems.

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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.

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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

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

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Photovoltaic Array on Roof and as an Overhang
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½ KW PV System Installed along Roof Ridge
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California Home PV Installation
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PV on House
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2.4 KW System under Installation in New Hampshire
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PV Installed at Roofline on Building at Frost
Valley, NY
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PV Panels on Tile Roofs in Arizona
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PV on Roof in California
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Totally Inadequate Roof?

• If it is impossible or you dont 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.

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Pole Mount for Solarex Modules
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Pole Mounted PV
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Pole Mounted PV
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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.

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Solar Roofing Shingles
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Roof Integrated Photovoltaics in Misawi, Japan
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Roof Integrated PV in Japan
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Roof Integrated PV in Maine
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Roof Integrated Photovoltaic System in Colorado
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Roof Integrated PV (objects below chimney are
solar hot water collectors)
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PV Installation in Planned Community in Germany
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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.

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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 earths
  surface facing the sun will intercept about 1
  kw-hr of solar energy.
• What you collect depends upon surface orientation
  and collector efficiency

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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.

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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 (EPxt)
• 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

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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 like having 140 Sherpa
  climbing Mt Everest continually. We might want to
  consider reducing this number before adding PV to
  our roof.

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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.

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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

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Larger Scale PV

• Of course you dont 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.

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Rooftop Installation at Mauna Lani Resort, Hawaii
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Details of Roof Installation for Mauna Lani
Resort, Hawaii
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Solar CarportNavy Installation San Diego,
California
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BP Installation on their Gas Station
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Large 57 KW Rural Installation
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Solar Added to Flat Roofs(can upgrade the
insulation as well)
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59 KW Installation of 5600 ft2 in Greenpoint,
Brooklyn
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The Greenpoint, NY Building
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FALA Factory Roof InstallationFarmingdale, LI,
NYNote the number of other roofs
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Solar Cells Installed in Building Facade
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The sun is the primary energy source for almost
all energy flows on the planet. Its time we
started using it.
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