A GUIDE TO SOLAR POWER FOR YOUR HOME
THE BASIC FACTS
Electromagnetic radiation transmitted from the sun can be used to produce heat, electricity and a chemical reaction. The energy carried by sunlight is casually referred to as solar energy. Note that technically speaking, power is energy per unit time, although both terms are often used interchangeably. Only a very small portion of total sun's radiation is intercepted by the Earth. Numerically, this amount is characterized by solar constant- the average amount of sun irradiance that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays. The value of this constant is approximately 1361-1366 watts per square meter or 431-433 Btu/hr per square foot. Electricity produced from sunlight is casually called solar power.
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HOW TO GENERATE SOLAR ELECTRICITY
The sun's radiant energy can be converted directly to electricity in photovoltaic (PV) cells. A PV cell is normally made of specially treated semiconductor with p-n junctions. When a photon of light strikes the top of the semiconductor's wafer, it can penetrate through a p-n junction and free an electron. Free electrons can cross the junction into the n-type region and then held in this region, being unable to return into the p-region. This creates a voltage of about 0.50-0.65 volts under open circuit (no-load) condition. When connected to a load, a typical commercial PV cell can produce from 0.5 to 5.0 watt.
Multiple cells can be connected in series and parallel and mounted together on a support structure to form photovoltaic panels (or modules).
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INSTALLATION OF A HOME PV-POWERED ELECTRIC SYSTEM
Photovoltaic arrays for residential PV generators are usually attached to the roof of the house. Normally they are installed on racks, which have to be bolted into the roof joists. For the longest length of time of direct sunlight on the PV cells, ideally they should face true south if you are in the northern hemisphere, and true north if you in the southern hemisphere. The right tilt should be slighly lower than your latitude.
A large part of the sun's irradiance is absorbed and scattered by the earth's atmosphere. By the time sunlight reaches Earth's surface, its peak energy density drops to about 1,000 watts per square meter at noon on a bright day. Today's commercially available modules for residential use deliver efficiency anywhere from 6 to 20%. This means that the peak output of a PV module at noon with an optimum tilt can be anywhere from 60 to 200 W/sq.m, or from 5.6 to 18.6 watt per sq.foot. Additional reflective devices, such as troughs or mirrors, can be used to concentrate more of the sun's energy. Concentrated solar power is more efficient, but it is used primarily in commercial installations. The buyers are often asking about the cost of solar panels per square foot. This is not a very useful characteristic: depending on their efficiency, different models will yield different output with the same surface area. When choosing solar panels for the home consider their cost per watt and efficiency. An average retail price of the panels is currently $2.60 per watt before incentives, with the lowest numbers being around $1.00/W.
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A PV array produces DC voltage, which is then converted to AC by an DC-AC inverter. Such an inverter normally operates as an SMPS converter that regulates the output by pulse width modulation. All inverters for renewable energy applications should meet UL standard 1741. The system wiring should also comply with National Electric Code NFPA70® (particularly Article 690) and local codes.PV cells do not store energy- at night or on a rainy day you would not produce any solar electricity. To provide continuous power in off-the-grid homes or emergency backup in grid-tie installations you need to store the excess of the generated energy in a large battery bank. Another reason batteries are used is to supply surge currents to electrical loads. When choosing a battery, look for a deep cycle type for more storage capacity. A deep cycle battery is typically designed to allow repeated 80% depth of discharge. The storage bank is normally connected to the PV array via a charger (charge controller). There should be another DC disconnect switch before the inverter. You don't need batteries if you are on grid and you plan to use the system just to save on your energy cost rather than for emergency backup.
Utility-interactive photovoltaic systems must use special grid-tied inverters. These devices are synchronizing their voltage, frequency and phase with utility AC voltage. This allows the system not only to energize your appliances but to feed any excess of PV-made electricity back to utility and help you defray the energy cost. A grid interactive inverter also includes an automatic AC line disconnect switch that prevents voltage backfeeding into unenergized utility lines. For stand-alone PV powered loads that are not connected to your house wiring and for off-grid applications you can use less expensive portable inverters. However if you are connected to the AC line, you should never hook up any non-approved voltage source directly into your home wiring. For more information and wiring diagrams see facts on solar powered home generator.
At present, the main disadvantages of a PV system are its high initial price and relatively low efficiency. However, for remote locations a solar generator is a better solution than extending electric lines, which would typically run for $20,000 to as much as $75,000 per mile.