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Solar Electricity

System Components

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Photovoltaic (PV) Materials

Photovoltaic (PV) materials are the electricity producing component of a solar electric system. PV materials are made of solar “cells.” When the sun’s light energy (not heat energy) hits and is absorbed by the cells, electrons are released and flow as electricity. The greater the amount and intensity of the sunlight, the more electricity generated. PV materials generate direct current (DC) electricity. Commercially available PV materials include crystalline silicon panels and thin-film materials that are both made in various sizes with various wattages of electrical output.

On partly cloudy days, PV materials will produce about 80 percent of their capacity. Extremely overcast days may reduce electricity output to 30 percent of capacity. PV materials are relatively unaffected by severe weather and temperatures, although like most electronic devices, they operate more efficiently at cooler temperatures. Because they are typically a dark color and face the sun at an angle, snow slides off or melts quickly. PV materials are designed to resist hail damage (one namebrand panel is tested to withstand one-inch hail at 51 mph). They typically come with a 25-year power output warranty, but most will produce electricity 30-plus years.

Crystalline Silicon

Crystalline silicon flat-plate panels range in size and electrical output. They can be used for a variety of applications. Those typically placed on home rooftops range from about two-to-three feet wide by four-to-five feet long with a three-inch thickness. Electrical output ranges from 175 to 250 watts.

Thin-Film PV

Thin-film PV materials are flexible and versatile for a variety of applications. They are made by spreading silicon and other materials in a very thin layer (human hair thickness) directly onto base materials. This makes them ideal for building-integrated products such as roof shingles, tiles, building facades, windows, and skylight glazing.

A third generation of new solar materials includes lightweight foil-based panels, solar inks and dyes, and conductive plastics. Researchers continue to investigate how to make all PV materials more efficient at converting sunlight into electricity.

If your building does not have a south-facing roof or surface (or you cannot use PV materials as structure), panels can be ground-mounted or pole-mounted in a yard or field. Pole-mounted panels can be in a fixed south-facing position or placed on tracking devices. Like sunflowers, tracking devices follow the sun’s skypath. A single-axis tracker follows the sun from east to west. A dualaxis tracker follows the sun from east to west and adjusts for seasonal sun angles. Trackers increase system cost, but can increase power production by 20 to 30 percent. For the more hands-on homeowner or building manager, adjustable
rooftop mounting structures are available for making seasonal sun angle adjustments.

PV Material/Panel Performance

Manufacturers will provide a minimum warranted power rating (in watts) that may be called peak power or peak tolerance rating, etc. Many panels are tested under either Standard Test Conditions (STC) or PVUSA Test Conditions (PTC). The main difference is the testing temperatures. A PTC rating is deemed a more realistic rating. If the panels you are considering have a STC rating, actual performance may be 85-90 percent of stated wattage output. Be sure to also compare efficiency ratings.

System Performance

If you are sizing your own complete system, you can use the rated wattage output (referred to as nameplate DC rating) to estimate the number of panels you will need to meet your targeted electrical load. Actual output of electricity will depend on factors such as roof orientation, tilt angle, and overall system efficiencies. Because there are inefficiencies in the remaining components, multiply the PV panel nameplate DC rating by 77 percent (a conservative de-rate factor used in NREL’s PV Watts on-line tool) for an estimate of the amount of electricity that will actually reach your electrical load. For example: a 230 watt DC Nameplate rating x .77 = approximately 177 watts of actual electrical power will reach your electrical load.

Balance-of-System (BOS)

Balance-of-System (BOS) is a term that refers to the remaining components that accompany PV panels. BOS includes an inverter, meter(s), safety equipment (disconnect switches, etc.), batteries, and a charge controller. It also includes conduit, cables, and combiner boxes.

Inverter

An Inverter converts and conditions electricity. All PV materials/panels produce DC electricity, which can be used for DC-powered appliances, and camping and boating-related equipment, etc. Most appliances, electronics and machinery require alternating current (AC) electricity, and an inverter converts the PV-generated DC into AC electricity. Inverters also “condition” the PV-generated electricity to match the qualities of the utility grid-produced electricity in order to properly power the electrical load. Contact your utility company to ask if it requires a specific Underwriter’s Laboratories (UL)-certified inverter.

Inverter/Charger

For systems with batteries, a combined inverter/charger is used. It converts PV-generated DC (stored in the batteries) to AC and it also allows batteries to be charged by the utility grid or an off-grid system’s back-up generator. It converts the utility’s or generator’s AC electricity to DC for battery storage.

All solar electric system components must be matched to work together as a system. If you plan on adding more PV panels at a later date, size your inverter for the future system. It will be less expensive than upgrading to a larger inverter and the accompanying equipment changes that would also be required. Inverters should be accessible, weather-protected, and kept out of direct sun. Inverters can be up to 98 percent efficient and last up to 20 years. Warranties are typically for 10 years.

Some installers will connect microinverters to each individual PV panel instead of installing one larger inverter. Microinverters work well where there might be potential panel shading. They can make system expansion easier and less expensive.

Meters

Meters track the amount and “direction” of electrical flow in grid-tied systems (off-grid systems often have meters to track battery charge levels, etc.). When the sun is shining, the PV system generates electricity. If your building or machinery does not use all of the electricity being generated at any one time, it is fed into the utility’s grid. When this occurs, you are credited at either a retail or wholesale rate from the utility. The retail rate is the rate you pay for electricity from the utility. The wholesale rate is a lower rate the utility pays for electricity it buys on the market.

How is grid-tied system electricity tracked? Typically, a special Net Meter is provided and installed by the utility company once a grid-tied system installation is completed. This meter spins forward (clockwise) when you are using electricity from the grid and spins backward (counterclockwise) when you are generating excess PV-generated electricity that is fed into the grid. If at the end of a year’s billing period you used more electricity than your PV system generated, you pay the utility company. If your PV system generated more than you used, you receive a utility credit. Contact your utility company to determine if it allows connection to the grid. If it does, ask for current interconnection and net metering requirements.

Safety Equipment

Safety equipment protects owners, utility workers, and system equipment. Safety equipment includes AC and DC disconnect switches, grounding equipment, and surge protection. This equipment is very important for protecting people and system components from power surges, lightning strikes, ground faults, and equipment malfunctions. Automatic and manual disconnect switches are recommended. Disconnect switches shut down the system so it can be worked on safely whether for routine maintenance or repairs. Switches also prevent the system from sending power to the grid and endangering utility workers while they conduct repairs.

Charge Controller

A charge controller regulates battery charging. When batteries are part of a solar electric system, a charge controller, also called a regulator, is required. It is connected between the PV panels and the batteries. A charge controller regulates and optimizes electrical flow from the panels to the batteries, keeps batteries fully charged, and prevents battery overcharging. It also prevents batteries from being excessively discharged, which can damage or ruin them. Charge controllers must be properly matched to the overall solar electric system for proper function. Charge controllers can be up to 98 percent efficient and are typically warrantied for up to five years. Inverters and charge controllers can be combined into one piece of equipment.

Batteries

Batteries store electricity. Off-grid buildings require batteries as part of the solar electric system. Electricity is stored and used from the battery bank, which is sized to provide electricity for the full electrical load for two or three days. Grid-tied buildings with battery back-up typically have a small battery bank used to store electricity for use during utility power outages. Batteries can lower the overall efficiency of a solar electric system because they only release a percentage (80-95 percent) of the electricity that is fed into them. Batteries need periodic maintenance, and have safety considerations. They may last from seven to ten-plus years before requiring replacement. Lifespan depends on factors such as number of discharges and the temperature where they are stored.

Whether installing a solar electric system to power a building or pump water, make sure to purchase quality, certified components.

Component and System Certifications

Organizations that test and certify system components:

References

Frietas, Christopher. (2009/2010 Dec./Jan.). Inverter Basics. Home Power, 134, 88-94.

McCabe, J. (ed.). (2010, Fall/Winter). All About Photovoltaic Systems. Solar Today: Getting Started (Bonus Issue), 16-19.

National Renewable Energy Laboratory (produced) for U.S. Department of Energy. (2002, Sept.). Battery Power for Your Residential Solar Electric System. DOE/GO-102002-1608.

National Renewable Energy Laboratory (produced) for U.S. Department of Energy. (2009, January). Own Your Power! A Consumer Guide to Solar Electricity for the Home. DOE/GO-102009-2656.

Sanchez, Justine and Brad Burritt. (2009, Feb./Mar.). Charge Controller Buyer’s Guide. Home Power, 129, 72-77.

Sanchez, Justine. (2009/2010, Dec./Jan.). 2010 PV Module Guide. Home Power, 134, 50-61.

Seward, Aaron. (2011, February). Thin Is (Almost ) In. Architect, Retrieved March 16, 2011, from http://www.architectmagazine.com/solar-power/thin-is-almost-in.aspx?printerfriendly=true

Solar Energy Industries Association. (2010, March). Photovoltaic Solar Technology: Creating Electricity from Sunlight. Retrieved March 21, 2011, from http://www.seia.org/galleries/FactSheets/Factsheet_PV.pdf

U.S. Dept. of Energy. (2011, Feb.). Connecting Your System to the Electricity Grid. Retrieved February 16, 2011, from http://www.energysavers.gov/your_home/electricity/index.cfm/mytopic=10520

U.S. Dept. of Energy. (2011, Feb.). Meters and Instrumentation for Grid-Connected Systems. Retrieved February 16, 2011, from http://www.energysavers.gov/your_home/electricity/index.cfm/mytopic=10560

U.S. Dept. of Energy. (2011, Feb.). Power Conditioning Equipment for Grid-Connected Systems. Retrieved February 16, 2011, from http://www.energysavers.gov/your_home/electricity/index.cfm/mytopic=10540

U.S. Dept. of Energy. (2011, Feb.). Types of Solar Cells. Retrieved March 11, 2011, from http://www.energysavers.gov/your_home/electricity/index.cfm/mytopic=10791