Overview
Most solar cells are made from single-crystal silicon and have been very expensive for generating electricity, but have found application in space satellites and remote areas where low-cost conventional power sources have been unavailable. Recent efforts to mass produce the manufacturing costs have made them slightly less expensive as well as utilities and government agencies subsidizing the installation costs to homeowners and businesses. Solar arrays are commonly referred to as a Photovoltaic, or PV, array.
The conversion of sunlight into electrical energy in a solar cell involves three major processes: absorption of the sunlight in the semiconductor material; generation and separation of free positive and negative charges to different regions of the solar cell, creating a voltage in the solar cell; and transfer of these separated charges through electrical terminals in the form of electric current. Solar panels generate Direct Current (DC) so it must be converted to Alternating Current (AC) to be used in standard residential electrical systems.
History
The development of solar cell technology begins in 1839 with the research of French physicist Antoine-César Becquerel. Becquerel observed the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution when he saw a voltage develop when light fell upon the electrode.
According to Encyclopedia Britannica the first genuine solar cell was built around 1883 by Charles Fritts, who used junctions formed by coating selenium (a semiconductor) with an extremely thin layer of gold. Early solar cells, however, had energy conversion efficiencies of less than one percent. In 1941, the silicon solar cell was invented by Russell Ohl. In 1954, three American researchers, Gerald Pearson, Calvin Fuller and Daryl Chapin, designed a silicon solar cell capable of six percent energy conversion efficiency with direct sunlight. The three inventors created an array of several strips of silicon (each about the size of a razorblade), placed them in sunlight, captured the free electrons and turned them into electrical current. They created the first solar panels. Bell Laboratories in New York announced the prototype manufacture of a new solar battery. Bell had funded the research. The first public service trial of the Bell Solar Battery began with a telephone carrier system in Americus, Georgia on October 4 1955.
How To Use PV At Home
Most of us are familiar with the solar cell on calculators, outdoor accent lighting, and even watches. To generate enough electricity to power something other than a light bulb, you will need a large solar array, a charge controller, also known as a regulator, deep cycle batteries, an inverter to convert the power from DC to AC, and assorted hardware, wire and connectors.
It is easiest to buy kits that you can install on the roof or on grade. They usually come in modules that can be connected together to generate more electricity. You may need the help of a qualified contractor or electrician to connect the PV system to your electrical system.
If you install the panels on your roof, you will need to get a permit from you local building department. If you belong to a home owners association, you should review the by-laws to determine where, or if, you can install a PV array. If not, put pressure on the board to try to have the by-laws changed. Your local utility has rules and procedures that must be followed to connect any generator to the grid safely and legally. These rules are generally based on national standards with PV systems must comply with. Check with your dealer to help you with the documentation and procedures. You system should probably be an on-grid system so that you can have both utility and solar power connected to your home.
Location
One of the biggest ways to prepare for the new solar energy system is to determine where you could position them to produce the most electricity. They should go in the place that has the most direct sunlight which would be the south-facing part of your roof. Use caution when installing panels on your roof. Make sure that the structure can adequately support the arrays and mounting frames. Even though the weight of the arrays may be relatively small, you must also consider the wind load. In windy locations you will need to add sufficient bracing and tie-downs to keep your array attached to the roof.
A general rule of thumb is to set the angle of the array at the latitude of your location for spring and fall, and the latitude minus 15 degrees in the summer, since the Sun is higher over head, and the latitude plus 15 degrees in the winter when the Sun is lower in the sky. You can also buy trackers for the system to track the Sun. They come in either 1 or 2-axis systems which will either track the Sun’s path through the course of the day, or the path of the Sun during the day and season. Two-axis trackers continually adjust for both azimuth and elevation, thereby positioning the array directly perpendicular to the Sun at all hours of the day.
Removing Obstructions
Once you have figured out the location of the solar cells, your next step in preparation is to remove any obstructions that would inhibit direct sunlight. If there are branches that overhang and obscure part of the sun during the day, you should trim the branches back, or even remove the entire tree.
Figure out Wattage
Before you can buy any type of solar panels, you will have to know how much electricity your home currently uses. This will determine not only the size and number of panels, but also the overall cost of the solar electric system.
The array size you need depends on your average electrical usage, climate, roof angle, shading problems and many other factors. To approximate the array size you need, multiply your average daily electrical demand in kilowatt-hours by 0.25. The result is the approximate size of solar array, in kilowatts, needed to meet your electrical demand. Look on your electric bill to find out how many kilowatt hours you used during the month and divide it by the number of days in the billing period; it is not always 30 days.
Then figure out how much sun your location will receive go to the National Renewable Energy Laboratory Website listed in Resources to find the Insolation map for the U.S. It is in kilowatt-hour per square meter per day. Convert the size of your array, which will be in square feet, to square meters. One square foot equals 0.0929 square meters, so a 120 square foot array would be 11.14 square meters and would receive about 45 kWh each day in the Northeast. You must multiply this number by your system efficiency to determine the actual electricity produced.
Maintenance
The solar array is practically maintenance free, but they do require cleaning. This is best done with warm water and a mild dish detergent. The glass of the panels should be cleaned in the morning before they get too hot. This can cause streaking or cracking if cold water is sprayed onto the hot surface. Cleaning is not just for appearance, the panels must be free of dust and debris to operate at their rated capacity. Again, use caution if your system is installed on a roof. Periodically check the anchoring points for leaks; you don’t want all of the money you saved spent on fixing your roof and ceilings.
The batteries can last up to 10 years before they need to be replaced. You should count on replacement every 5 to 10 years. If you install a tracking system, this will also need to be replaced at some point, and may or may not be maintenance free. You should figure if the cost of a tracker is worth the additional increase in system efficiency. Changing the tilt angle from latitude to an optimal setting of latitude plus or minus 15 degrees generally adds 5% to 8% usable power to your solar system, while a two-axis tracker will improve your gain by 10% or 15%.
Resources
National Renewable Energy Laboratory
The Regional Per-Capita Solar Electric Footprint for the United States
http://www.nrel.gov/docs/fy08osti/42463.pdf
Insolation Maps
http://www.nrel.gov/gis/solar.html
