Photovoltaic (Solar Electric) Systems
Making Electricity When the Sun Shines
Bird's eye view
Rebates and tax credits make these systems more affordable
Photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) cells make electricity directly from sunlight, reducing or eliminating the use of power derived from fossil fuels. The cost of PV panels has dropped dramatically in recent years, and utility rebates and government tax credits now make PV electricity cheaper than grid power in many regions of the U.S.
Crystalline PV modules
A traditional PV module is a glass-and-polymer sandwich encapsulating thin wafers of crystalline silicon. The most efficient commercially available crystalline PV cells are roughly 20% efficient, meaning the cells convert about 20% of the total solar energy striking them into electricity.
Thin-film (amorphous) PV modules. Newer thin-film technology combines an amorphous silicon film with a flexible base material. Amorphous PV films are often integrated into roofing tiles or applied to metal roofing as a peel-and-stick membrane. Some thin-film PV manufacturers claim that their products are less obtrusive when installed than crystalline PV arrays. But amorphous PV modules have efficiencies of only about 10%, so a larger roof area is required for a thin-film array than a crystalline array with the same electrical rating. Amorphous arrays are more susceptible to efficiency losses at high temperatures than crystalline arrays.
Do photovoltaic systems make economic sense?
The cost of PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.-generated electricity has reached grid-parity in areas of the U.S. with high electricity rates. With lifespans of more than 20 or 30 years, you are essentially buying decades of electricity all at once. The solar exposure, size of the array of panels, local utility prices, and behavior of the homeowner are all part of the economic equation. If loan terms (or available cash), electricity rates, and renewable energy incentives all fall into place, a PV array can generate a positive cash flow. Local utility companies will have the necessary information and sometimes the incentive programs when considering adding photovoltaics to your project.
Photovoltaic modules should be pointed close to due south. If the angle off the array is fixed, the angle should equal the home’s latitude. For example, PV modules should be installed at a 32° angle if a house is at 32° North latitude and tilted to 28° for a house at 28° North. The mounting angle and orientation of roof-integrated PV modules such as PV roofing slates matches the orientation and slope of the roof on which they are installed. The designer of a new home may wish to plan the orientation and slope of a home’s south-facing roof with PV in mind. Panels that are not integrated into the roof can reduce design constraints and sometimes address other concerns such as providing shade.
Panels mounted on roof, or on the ground
Mounting options. A PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. array can be mounted on a south-facing roof or can be ground-mounted. Small ground-mounted arrays can be mounted on a single pole, and in some cases can include a tracking device. Either type of array can be expanded in the future or added during a remodel. It is usually easier to adjust the angle of a pole-mounted array than a roof-mounted array.
The most common type of roof-mounted array is a collection of individual modules, each with an aluminum frame and glass facing. These modules are usually mounted on aluminum rails secured to legs or posts attached to the roof framing. Each post or leg requires flashing.
Roof-integrated PV. Those with higher budgets can choose from a variety of roof-integrated PV modules, including PV roofing slates and PV shingles. Most of these products are designed for integration with concrete tile roofing, one of the most popular roofing choices in southern California. Other products can be integrated with asphalt shingle roofs or metal roofing.
Inverters. All grid-connected PV systems require an inverter, which is an electronic device that converts the DC power produced by the PV array to AC power (either 120 volts or 240 volts). Grid-tied inverters synchronize the sine wave of their AC output with the sine wave produced by the grid. All grid-tied inverters have a safety feature that disconnects the output of the PV array whenever the grid experiences a power outage. Because of this feature, most grid-tied PV systems are unable to provide power during blackouts.
Check Home Power magazine
The National Electrical Code covers photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. systems in Article 690.
John Wiles writes a monthly column, "Code Corner," on code requirements for photovoltaic systems. The column appears in Home Power, a useful magazine published in Ashland, Oregon (www.homepower.com).
ABOUT PHOTOVOLTAIC SYSTEMS
A photovoltaic (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) module is usually smaller than a piece of plywood. The most common type contains a collection of individual PV cells made from silicon, each producing about 0.5 volt. The cells are protected by a layer of low-iron tempered glass in an aluminum frame.
When sunlight strikes a cell, it causes the silicon to shed electrons, creating DC current.
Balance of system. An inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements. changes the DC into alternating current (AC), which runs appliances, lights and other electrical equipment.
Most installations in the U.S. are grid-tied, meaning the house can draw power from either the utility grid or its PV array. When the PV cells aren't producing enough power, the grid makes up the difference. When the array makes more than the house consumes, the electricity flows into the grid.
Off-grid systems, which require a battery bank to store power, are more expensive and more complicated.
Measuring power output. PV modules are rated by the amount of electricity they’re capable of producing, but actual performance may be less.
The rated capacity is based on ideal circumstances, with the sun providing 1000 watts per square meter. Local weather conditions and site latitude can diminish or (more rarely) enhance PV module performance.
Solar potential in the U.S. varies widely, from 7 kWh per square meter per day in parts of the Southwest to about 4 kWh per square meter in parts of the upper Midwest, New England and the Pacific Northwest. Where sunlight potential is lower, PV modules produce less electricity.
The National Renewable Energy Laboratory produces a solar map with details for photovoltaic potential around the U.S.
Cold days are best. The best PV weather is a very cold day when the atmosphere is crystal clear and the ground is covered with clean snow. A blue sky dotted with fluffy cumulus clouds to provide reflected sunlight is ideal.
Power output from a solar array also is influenced by the system’s efficiency (there are wiring losses as well as inverter losses) and the array’s temperature. Output drops as temperature rises, so thin-film cells attached directly to a roof deck are likely to generate slightly less electricity than panels with an air space between the panel and roof surface.
Demand and installations are growing. Europe, Japan and the U.S. take the lead in installed photovoltaic capacity. Worldwide, demand is growing sharply.
Rules of thumb:
- Polycrystalline PV arrays have a peak rating of 10 to 12 watts per square foot, while amorphous PV arrays have a peak rating of 5 to 6 watts per square foot.
- Solar electric potential varies by climate, from an average of 0.029 kWh per square foot per day in Seattle to an average of 0.049 kWh per square foot per day in Phoenix.
- A 1-kW PV system will generate an average of 970 kWh per year in Seattle and 1,617 kWh per year in Phoenix. (A useful free online tool for estimating the output of a PV array in different U.S. locations is PVWatts.)
- Most buildings have a roof that is too small to accommodate a PV array sized to supply all of the building’s electricity.
ABOUT THE COST OF PV
The average cost of installed PV equipment varies from region to region; the range is now between $3.50 and $5 per peak wattUnit of rated power output, for example from a photovoltaic (PV) module in full sunlight, as distinct from its output at any given moment, which may be lower. (before government tax incentives and state and utility rebates).
Total system costs depend on what percentage of total household power the panels are designed to collect, as well as solar potential and other factors on the building site. Most residential PV arrays are in the 1 kW to 2 kW range, but that is on the increase.
Net-zero houses need lots of PV. Successful net-zero projects require large PV arrays — generally in the range of 5 kW to 9 kW.
In Arizona, a 5-kW system costing $20,000 (before rebates and incentives) would be large enough to supply all the power for a small, energy-conserving house. In Seattle, the same system would meet a lower percentage of household electrical needs.
Incentives help. A variety of incentives, including rebates and tax credits, are available from local utilities, some state governments, and the federal government. The cost of a PV array to a homeowner depends on available incentives.
BuildingScience.com: PV Primer
North Carolina State University maintains an extensive database of state and utility company incentives for renewable energy.
- Daniel Morrison / Fine Homebuilding
- Rob Wotzak
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