GENERATING ELECTRICITY FOR HOMES FROM SOLAR POWER
WHAT IS SOLAR ENERGY
Electromagnetic energy transmitted from the sun 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 power radiated from the sun reaches 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 average value of this constant is approximately 1361-1366 watts per square meter or 431-433 Btu/hr per square foot (see sun's energy diagram). Sun's radiation can be used to produce heat and electricity. Electricity produced from sunlight is casually called solar power.
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GENERATORS OF SOLAR ELECTRICITY
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 photon of light strikes the top of the semiconductor's wafer, it can penetrate through a p-n junction and free an electron, which can cross the junction into the n-type region. Free electrons are 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 are connected in series and parallel and mounted together on a support structure to form photovoltaic panels (or modules).
| Home |
| Solar energy |
| Home PV power |
| Solar systems |
| Off grid |
| Grid tied |
| Grid tie backup |
| PV panels |
| PV cells |
| Cost |
| Backup generators |
TIPS ON INSTALLATION OF A HOME PV-POWERED ELECTRIC SYSTEM
Photovoltaic arrays for residential solar electric 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.
Part of 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. When choosing PV panels consider their cost per watt and efficiency. Today's commercially available modules for residential use deliver efficiency anywhere from 6 to 20%. This means that peak output of a panel at noon with a proper tilt can be 60 to 200 watt per square meter, or from 6 to 18.6 watt per square foot. Additional reflective devices, such as troughs or mirror panels can be used to concentrate more sun's energy, but they are normally used only in commercial installations. A DC voltage from the PV array is then converted to AC voltage by an DC-AC inverter. Such inverter normally incorporates an SMPS converter that regulates the output by pulse width modulation. All inverters for renewable energy application should be compliant to UL standard 1741.
The wiring of the entire system has to be done according to National Electrical Code®
(NEC) and
local regulations.
Per NEC, a DC disconnect switch should be installed near the place
where the cables from PV arrays enter the house. In
addition, it is desirable to have a fuse or a circuit breaker for each
string of series-connected modules to prevent their damage from the
reverse currents from other parallel strings.PV cells do not store energy- at night or on a rainy day you would not get electricity from them. 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 energy generated from your panels 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 battery bank and the inverter. You don't need a battery bank if you plan to use the system just to save on your energy cost rather than as a prime source or 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, allowing the system not only to supply electricity to your loads but to feed any excess of PV-made energy to the utility grid. This can help you defray the energy cost. A grid interactive inverter includes automatic AC line disconnect switch that prevent voltage backfeeding into unenergized utility lines. For stand-alone loads that are not connected to your house wiring and for off-grid applications you can use less expensive portable inverters. You can use them for example for PV powered attic fans. However if you are connected to the grid, you should never hook up a portable [i.e. non grid tie] inverter into your house wiring without a transfer switch. For more information and wiring diagrams see: Solar Powered Systems.
At present, the main disadvantages of residential PV generators are their high initial cost and a relatively low efficiency. However, for remote locations such a system may be a better solution than extending electric grid lines, which would typically cost more than $20,000 per mile.