A power play article two weeks ago discussed using the sun’s energy to preheat water for a home or business. It is the fastest way to reduce energy bills – using the sun for the least initial investment – and requires the least amount of maintenance. But there are other ways to use the sun’s energy to reduce your energy bills.
Last winter’s energy shortages and skyrocketing prices forced many Californians to take action and implement a great many conservation measures, including installing compact fluorescent lamps and purchasing energy-efficient appliances. (Practical information on energy efficiency can be obtained from the City’s energy Web site at: www.ci.berkeley.ca. us/ENERGY, and from Solstice, [http://solstice.crest.org/efficiency/index.shtml] a non-profit organization dedicated to energy efficiency and renewable energy.) It just so happens that conservation measures are also the first step that should be taken before designing and installing a solar electric generating system.
Solar electric, or photovoltaic (PV) systems, use the sun’s light to generate electricity. Simply put, sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy. As these photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. As the absorbed solar photons strike the cell, electrons are knocked from one side of a silicon atom to another across a boundary. This establishes a voltage difference between the two sides. If there is an external circuit connected to the cell, then the cell acts like an electron pump, sending the current out along the circuit back to the other side of the PV cell.
The amount of electricity generated by this cell depends on the amount of light it receives, the size of the cell, and the efficiency by which the solar photons are drawn across the boundary. More information on how PV cells work can be obtained from the Department of Energy’s Web site at http://www.eren.doe. gov/pv/pvmenu.cgi?site=pv&idx=1&body=aboutpv.html and from public libraries.
Most new PV panels are rated between 10 and 14 percent efficient, compared to early solar panels manufactured in the 1970s, which were only 7 percent efficient.
Solar panels may be roof-mounted or installed in any other shade-free area and connected to a building’s electrical system. They may also be integrated into building materials, such as metal roofing, roof shingles, metal siding, or even window glass.
Efficiency varies with each material and its location, but if properly located and maintained, solar systems can be reliable for year-round power production. The payback period can be from 18 to 25 years, depending on the wattage of the panels and whether or not batteries are used, the cost of electricity, and other factors.
Systems can be configured in a variety of ways. Simple systems may consist of panels, wiring and a DC (direct current) motor that may perform tasks such as pumping water when there is sufficient sunlight. A basic system used in a home or business will have panels and wiring, an inverter to convert current from DC to AC (alternating current, as used in homes), and a controller to monitor the flow of current to prevent overcharging. These systems are connected to the building’s main electrical system, and will slow or reverse the direction if the electrical meter by sending the excess current back into the electrical grid. Note that a “Net Metering” contract between the homeowner and the local utility must be signed before this kind of system can be connected.
Net Metering agreements allow residents to store their electricity generated during peak daytime use, and retrieve it at night or during rainy weather. Agreements usually run for one year at a time. At the end of the year, if the PV system’s owners have used more electricity than the system has generated, they only pay for the amount used beyond the amount generated. If less electricity is used than is generated by the system, then the system owners are not compensated for it. Therefore it is important to size the system appropriately to the home or business. Too large a system also means that owners are paying for more equipment than they need, making the payback period longer.
By adding a series of batteries to this system, the owner will then have power during an emergency or power outage, as long as the system is not damaged. The number and size of batteries will dictate how much power will be available during nighttime use.
The California Energy Commission provides a free online guide to solar energy at www.energy.ca.gov/reports/2001-09-04_500-01-020.PDF. This is a basic guide to PV system design and installation, and provides detailed information on equipment wiring, voltage drop calculations and much more. Installing PV systems is complicated and should only be done by a qualified, licensed installer.
For more information on solar energy, net metering, equipment rebates and energy conservation, visit the city’s energy office Web site at www.ci.berkeley.ca. us/ENERGY
Alice Pierre works for the city’s energy office. This column runs in the Daily Planet as a public service the first and third Tuesday of the month.