GENERATING ELECTRICITY FOR HOMES FROM SOLAR POWER
WHAT IS SOLAR POWER
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 sun radiation 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). Sunlight can be used to produce heat and electricity. Electricity produced from sunlight is casually called solar power.
Sponsored Ads
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, 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 can be 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 |
| Facts |
| Pros and Cons |
| 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 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. 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 the peak output of a panel at noon with an optimum 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 of the 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 an inverter normally incorporates an SMPS converter that regulates the output by pulse width modulation. The inverters for renewable energy applications should comply to UL standard 1741. The system wiring should comply with national 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, 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 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 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. For more information and wiring diagrams see: Solar Powered Systems.
At present, the main disadvantages of residential PV systems are their high initial cost and 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.