YOUR GUIDE TO SOLAR POWER FOR HOME USE
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 (see solar energy diagram). Electricity produced from sunlight is casually called solar power.
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HOW TO GENERATE SOLAR ELECTRICITY
The sun's radiated 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). The characteristics of the PV panels are basically similar to those of the cells, only scaled up in voltage and/or current based on the number of cells used and their connection.
A silicon wafer-based solar power cell.
INSTALLATION OF A HOME SOLAR-POWERED ELECTRIC SYSTEM
Photovoltaic arrays for residential solar 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 panels, ideally they should face true south if you are in the northern hemisphere, and true north if you in the southern hemisphere. The tilt should be close to your latitude.
A large part of the sun's radiation is absorbed and scattered by the earth's atmosphere. By the time sunlight reaches Earth's surface, its average 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 power output of a solar panel at noon with an optimum tilt can be anywhere from 60 to 200 watt per square meter, or from 5.6 to 18.6 watt per square foot. Additional reflective devices, such as troughs or mirror panels, can be used to concentrate more of the sun's energy, but they are normally used only 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 PV modules consider their cost per watt and efficiency. An average retail price of the panels is currently $4/W before incentives, with the lowest numbers being under $2/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 has 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 comply with national electric code 70
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 during outages in grid-tie installations you need to store the excess of the generated energy in a large battery bank. This battery bank is normally connected to the PV array via a charger (charge controller). There should be another DC disconnect switch between the batteries and 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.
Grid connected 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 supply electricity to your appliances but to feed any excess of PV-made energy to the utility grid 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 loads that are not connected to your house wiring (such as PV powered attic fans) and for off-grid applications you can use less expensive portable inverters. However if you are connected to the grid, you should never hook up a non grid tie source into your house wiring. For more information and wiring diagrams see: Solar Powered Systems.
At present, the main disadvantages of converting to a PV system are its high initial price and relatively low efficiency. However, for remote homes a solar power generator may be a better solution than extending electric grid lines, which would typically run for more than $20,000 per mile.