Photovoltaics

Photovoltaic (PV) systems convert the sun’s energy into direct current (DC) electricity and then alternate current (AC), which is used to run your home’s appliances.  PV technology is both innovative and sustainable with its use of the sun’s abundant source of energy.

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FAQs

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What are Photovoltaics?

Outside Hardware
Photovoltaics (PV) are made up of small panels called solar cells that produce about 1 or 2 watts of power each.  Many combined solar panels make up a module and several connected modules make up an array.  These parts are mounted with hardware, electrical wiring, and a battery to store electricity produced.  

Inside The Cell
The PV cell surface is made of a high light transmitting material like glass.  It also contains a textured anti-reflection coating to ensure light is absorbed, not reflected.  The next layers are called n-type and p-type.  These parts make up an electric field with material, such as silicon, whose atoms have loose electrons and conduct electric current.  The substrate is a thin sheet of supporting material that holds up the rear surface and is stable at high temperatures.

At present, about 90% of all solar cells are silicon based. These cells are typically made using a diffusion process with silicon wafers.  The other 10% of the market is thin film, a generally more expensive method with less efficiency.  The emerging technology in PV is thin film cell that uses a sputtering process that was developed in the semiconductor industry. These silicon-free solar cells could easily gain momentum from the current worldwide shortage of silicon.  These cells are applied to a stainless steel substrate, although they may be applied to a plastic substrate in the future, providing  great flexibility.  The manufacturing cost of thin film PV is going down while the efficiency is going up every year.

How Photovoltaics Work

Sunlight Energy to Electric Energy            
Arrays are mounted at an angle on the ground or roof to capture direct and indirect sunlight.  The process of converting light to electricity can be broken down in these steps:
Atoms in the PV cell absorb light photons.  Photon energy transfers to the electrons.   Electrons energize and break free from their position in the atom.   Free electrons flow together in an electrical current.   
The PV cell’s electric field drives the current through appliances.

Storing and Managing Energy
Batteries are sometimes installed to store excess energy that is usable during the night or on cloudy days.  Conditioners maximize power by balancing the system’s current and voltage, and an inverter converts the DC to AC power.  Charge Controllers protect batteries from overcharging, and Axis Trackers follow the sun’s movement, though most systems are installed fixed in position.

Economics of Photovoltaics

The installed cost or a PV system is likely to be around $8 per watt.  Assuming an increase in energy costs of about 5% per year, the system could pay for itself in about 15 years, or less than 10 years using rebate and tax incentives that are available in certain areas.  If the system is in a remote location and it is displacing utility power and the cost of the utility line extension, the return can be as little as 3 years.  

When the PV system is producing more power than the home uses, the energy can be sent into the utility grid, effectively making the meter run backwards.  This is called Net Metering and is available by law in 35 states.  Check with your state energy office to find out the requirements for net metering and for any available installation incentives.

The expected life of the PV modules is between 20-25 years.  Maintenance costs are fairly low since you can check the system periodically on your own.  Your PV system may only need check-ups on the condition of solar cells, battery, and electrical connections.  The biggest expected maintenance expense is for the inverter, which has an expected life of about 8-10 years.  The inverter is the gizmo that converts the DC power of the cells to the AC power your home and utility uses.  The cost for replacement can be as high as $8,000 though those costs are also coming down.  You should hire a professional to do any major repairs.

The cost of PV has decreased steadily in the last 25 years and the industry continues to grow.  Energy experts predict costs will approach prices comparable to common technological products, opening a huge market of PV, and making the price of power more competitive.

Environmental Impact

PV and solar thermal devices are essentially specialized formations of glass, steel, aluminum, and plastics; their manufacture is comparable to that involved in making household windows, water heaters, or mirrors. The energy payback of a PV system (the point where it produces as much energy as it took to be produced) is about 3 years.

PV devices are essentially "electric glass" - their (usually) silicon substrate is a close relative of window glass, and the processes used to render it electrically reactive are the same as are used in the microchip manufacturing industry.   Some hazardous materials are used to produce PV.  PV systems also contain certain toxic materials such as cadmium and selenium.  However, these toxic materials are minimal compared with conventional energy sources.  Research is being done to determine the best way to recycle PV systems at the end of their lifetime.   Over its life, a typical residential system will avoid 124,300 lbs. of coal or 8,800 gallons of oil or 13.5 illion cubic feet of natural gas used in power production, and reduce NOx emissions by more than 1.5 pounds for each megwatt-hour produced.   

FAQs (Frequently Asked Questions)

• Would a PV system work at my house?
• How do PVs function in inclement weather?
• How do I size my PV system?
• How much energy do PV produce and how long do they last?
• Do I need special permitting?
• What maintenance is required?
• What financial incentives are available?
• How do I sell excess energy back to the utility grid?

Would a PV system work at my house?
PV easily fit on unobstructed roofs of most homes, depending on the size of your system.  Arrays are also mounted on the ground, if your roof can’t accommodate them.  Your roof must be in good condition and unshaded.  Panels are mounted at optimum angle and facing south (or north if you live south of the equator) to capture the greatest amount of sunlight, with variations up to 45 degrees.  If you live in an area with a great deal of insolation (sunshine) the PV will perform better. The map below shows the greatest areas of sunlight exposure in the U.S. (the red area of the map below receives the greatest amount of insolation).   If you live in a less exposed area, ask a local PV supplier for a solar site analysis to see if a system would meet your energy needs.

Remember, solar cells operate on sunlight, not heat.  Photovoltaics are even used in antarctica quickly melting any fallen snow on the glass.

How do Photovoltaics function in inclement weather?
Even during cloudy days, PV produce up to 80% of their maximum energy.  On an extremely overcast day, a system produces up to 25% of its potential energy.  Manufacturers test solar cells for year round weather conditions including high wind, hail, freezing, and thawing.  Some PV produce enough excess power during the summer to supply energy needs in the winter.  Sleet and hail rarely harm the panels and the panel efficiency may actually improve in cooler weather.  See Success Stories below.

How do I size my Photovoltaic system?
The size of your systems depends on several factors.  Talk to a system designer or contractor to determine size factors such as:

• How much electricity, heat, and hot water you use
• The amount of sunshine in your climate
• How much you want to invest

First consider what percent of your home’s energy you’d like the PV to supply.  For systems not connected to the utility grid (stand-alone systems), determine an accurate amount of energy output.  You may also add more modules in the future but must size the inverter and other equipment (such as storage batteries if off grid) for the expansion.

How much energy do Photovoltaics produce and how long do they last?
Electricity production depends on sunlight exposure, and how well PV can convert the sun’s energy.  At average exposure and efficiency, a PV system produces 1,800 kWh a year per an optimum output of 1 kW.  Typical PV residential systems are between 5 and 10 kW.  Compare this to the average energy use of an American home: 1,000 kWh per month.

PV systems should last over 20 years.  Most manufacturers guarantee their products for up to 25 years, with silicon based PV loosing efficiency at less than 1% per year.

Do I need special permitting?
Regulations on the who, what, and how of installation vary with location.  Installing PV usually requires an electrical license, and there may be codes or limits on the following:

• Type of system
• Installer’s level of certification
• Electrical installation
• Building codes
• Connection to a utility grid
• Eligibility for rebates and tax credits

Standards are set by the following groups:

• Home owners’ association
• Local government building department
• Utility company

Talk to officials of your local government or state energy office for details on regulations in your area.

What maintenance is required?
The expected life of the PV modules is between 20-25 years.  Maintenance costs are fairly low since you can check the system periodically on your own.  Your PV system may only need check-ups on the condition of solar cells, battery, and electrical connections.  The biggest expected maintenance expense is for the inverter, which has an expected life of about 8-10 years.  The inverter is the gizmo that converts the DC power of the cells to the AC power your home and utility uses.  The cost for replacement can be as high as $8,000 though those costs are also coming down.  You should hire a professional to do any major repairs.

What financial incentives are available?
Depending on where you live, these incentives may be available:

• Property tax exemptions
• Sales tax exemptions
• Tax credits and deductions
• Renewable Energy Certificates (credits paid to the energy producer)
• Rebates
• Grants
• Loans
• Bonds
• Mortgages

 These incentives may be offered through the following providers:

• Federal government
• State government
• Local government
• Utility

Contact the energy office in your state or province, or your local utility for more information.

How do I sell excess energy back to the utility?
If your PV system is connected to a utility grid (grid-tied system), and produces more energy than your home uses, utility companies buy back the excess power.  This buy back is called Net Metering or a Feed-In-Tariff.  During times of excess energy, such as a sunny, summer day, the inverter converts electricity to the utility’s standards, allowing the company to purchase the energy as it flows backwards through your meter.  Currently, only 39 states allow connection of PV to the grid, and only 36 states require net metering for utility companies.  Ask your utility company about local polices.  A customer may use power from the utility during the night or on a cloudy day.  At the end of the month or year, the electricity bill shows whether the PV system produced more or less energy than the customer used.

Installing a Photovoltaic System
Self Installation
The U.S. Department of Energy advises hiring a contractor instead of installing a PV system yourself.  Professional installment ensures a functional system, a guaranteed warranty, safety, and protection of your home appliances. Search for photovoltaic distributors on this web site.

Finding Contractors
For design, installation, and maintenance, search for a supplier on this web site.  Solar vendors and installers range from large corporations, small businesses, discount warehouses, or mail order companies.     

Choosing a Contractor
When you talk to a professional, ask about their experience, licensing, recent training, and level of certification in PV systems.  Find an installer whose professional relationship with the local inspector is reputable.  Consider liability insurance with your contractor and ask about repair and maintenance.
Ask about products the contractor uses: Do they meet industry standards?  Are warranties available?  Talk to the installer’s business references and former clients about their experience, and the current condition of their PV system.

Success Stories
This Massachusetts home has used 40 square meters of PV since 1980.  Mounted on the south roof, this system generates more electricity than the house uses during the summer.  The excess electricity sold back to the grid during the summer covers almost the entire cost of energy during the winter.

Also in Massachusetts, a 26 unit condominium is opening in Brockton that features a PV system for each unit.  Residents will have their own 18 panel PV system and save $600 per year.  Designers expect each system to generate 4,212 kWh per year, over 60% of the electricity needed for each town home.  This six million dollar project also includes rainwater collection tanks, and energy star ® and LEED® certification.  A $458,300 grant from the Renewable Energy Trust helped finance the cost of panels and installation.