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 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 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 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 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.

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 CarportNavy 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 InstallationFarmingdale, 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.