HOW PHOTOVOLTAIC ENERGY WORKS
Photovoltaic, or PV, energy is the direct conversion of sunlight into electrical energy. The photovoltaic effect was discovered in 1839 by French scientist Edmund Becquerel and the first practical PV cell was developed at Bell Laboratories in 1954. PV energy was first used in space in 1958 with the launch of the Vanguard I satellite. Today it is used for a variety of applications around the world. Other types of solar energy are solar thermal systems for water heating or space heating, and concentrating solar power systems, which concentrate heat from the sun to drive turbines which generate electricity.
A silicon PV cell is a semiconductor composed of a positive, or P, layer and a negative, or N, layer, and a junction between them called the P-N junction. The N layer is composed of silicon doped with (impregnated with impurities of) phosphorous atoms, which contain five electrons in the outer electron shell. One of the electrons in the outer shell is not strongly bonded to the atom, and can be easily moved to other atoms. The P layer is composed of silicon doped with boron atoms, which have three electrons in the outer shell. This outer shell has a strong attraction for a fourth electron, essentially an electron “hole”. When the N and P layers are placed in contact, electrons from the N layer phosphorous atoms migrate into the outer shell of boron atoms in the P layer, creating an electric field at the P-N junction between the layers. This electric field acts as a diode, allowing electrons to flow from the P layer to the N layer, but not the opposite direction.
When a photon of light strikes a cell with enough energy, it knocks one electron from a “hole” in the P layer across the P-N junction to the N layer. This disrupts the electrical neutrality, and if an electrical circuit is provided, the electron flows through the circuit with an electromotive force of 0.5 volt back to the P layer. The greater the amount of light, the greater is the amount of electron flow, or current. By wiring PV cells in series, higher voltages can be achieved.
A PV module consists of many solar cells connected in series and encapsulated with a glass front, plastic backing and aluminum frame. Several modules mounted together compose a panel, and a group of panels or modules connected electrically is called an array.
Monocrystalline PV module cells are formed from individual silicon crystals sliced from an ingot. Polycrystalline PV cells are composed of multiple silicon crystals which are formed by pouring molten silicon into molds. Amorphous PV modules are produced from semi-conducting materials other than silicon crystals formed into flexible sheets. These modules are also called thin-film.
Electricity from PV modules flows as direct current (DC). Battery chargers and certain appliances can be powered with direct current electricity, but most household and commercial uses require alternating current (AC). An inverter is a piece of equipment that converts the DC current from the array into AC current to be used for common household or commercial applications.
Several financing options are available for PV systems. The main types are listed below:
Cash Purchase – Customers pay cash and receive all of the economic benefit of the PV system including the 30% Investment Tax Credit and any other incentives. Commercial property owners may depreciate the equipment. Following the payback period, the energy is virtually free.
Loan – Loan types include mortgage refinance, home equity or business line of credit and unsecured loans. The customer receives all of the economic benefit except the loan interest paid to the lender.
Solar Lease – The customer pays little or no down payment and leases the solar equipment from a third party owner. The lease payments are lower than current electric bills, and may be fixed or may have an escalator. The lessor is responsible for maintenance or repair on the system. The lessor enjoys the Investment Tax Credit and commercial depreciation benefits. The customer exports excess energy to the utility grid and buys energy from the utility when their consumption is greater than system production.
Power Purchase Agreement – The PPA is similar to a lease, but the monthly payments are based on the amount of power produced rather than a fixed monthly lease payment. The rate per kilowatt hour may be fixed or have an escalator.
Several mounting options are available for PV systems.
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Interconnection with the Utility Grid
PV systems are usually, but not always, connected to the utility grid. The agreement with the utility company includes a Net Energy Metering agreement, and the terms are generally subject to state laws.
Grid-Interactive – The most common type of system is the Grid-Interactive, or Grid-Tie. The system is connected to the utility grid through the premise utility service panel. When more power is being produced by the PV system than is being consumed on the premises, the meter “spins backward,” and the customer receives a credit from the utility company for the energy exported. At night, on cloudy days, or when energy consumption is high, energy flows from the grid into the service panel. The electrical code requires that the connection between the PV system and the service panel be automatically disconnected during a utility power outage to prevent electrocution of utility repairmen. While disconnected, the PV system generally does not produce electricity. The exception to this rule is that a few recently developed inverters can provide a small amount of PV power at one receptacle on the inverter during outages. Grid-interactive is the simplest and most economical type of system for customers who are able to obtain utility service.
Stand-Alone – For customers who are located distant from utility service lines, or who choose to be disconnected from the utility grid, stand-alone systems provide power directly from the sun during daylight and store energy in batteries for later use when the sun is not shining. The PV system must be sized to meet the energy demand in the winter when the least solar energy is available, so in the summer there is generally an excess of solar energy which is lost rather than exported to the utility grid for credit. Stand-alone systems are commonly used for residences in remote locations and special off-grid applications like communication sites, well pumps and sign lighting.
Grid-Interactive with Battery Backup – A hybrid between grid-interactive and stand-alone, a battery backup system can operate from batteries independently from the grid during power outages. While the utility grid is up, the system can export excess power to the grid for credit or import power as needed. The system is more expensive than a simple grid-interactive system because of the additional equipment needed.
Solar Benefits Everyone
Solar energy benefits the system owner, the nation and future generations around the world.
Good for the Environment
Think Globally, Act Locally
With solar energy, you can become part of the solution. A solar system displaces power plant fossil fuel usage with free energy from the sun. A typical 5 kilowatt PV system may generate about 7.6 megawatt hours per year, replacing 3.9 tons of coal or 56,000 cubic feet of natural gas and eliminating the greenhouse gas emissions which would have resulted.
Prior to the Industrial Revolution, in 1769, the concentration of CO2 in the atmosphere was 280 ppm. The level reached 350 ppm in 1987. It reached 400 ppm in 2014 and it is increasing at an accelerating rate. Follow this link to view the current CO2 level. http://co2now.org/
Reducing fossil fuel consumption also reduces water consumption and the environmental impacts of coal mining and natural gas extraction.
Good for Your Finances
Why Would I Not Want to Harvest Free Solar Power?
If your site and your electricity usage are a good match for a PV system, it will generate a good return on investment and will continue harvesting free energy from the sun for years after the payback period. A PV system generally has an expected life and warranty period of 25 years except for one inverter replacement. The 30% federal income tax credit and steadily declining solar equipment costs make solar energy costs competitive with utility provided energy. A PV system on your roof also increases the resale value of your home due to expected future energy savings. Studies show that the increase in home resale value is comparable to, and possibly greater than, the cost of the PV system. http://emp.lbl.gov/sites/all/files/lbnl-4476e.pdf
And, a PV system protects you against future electric rate inflation.
Good for America
Solar Is the Future
Solar energy reduces our dependence on fossil fuels and energy imports from foreign countries, increasing national security and improving the economy. Reducing fossil fuel consumption also conserves energy resources for our children and grandchildren.
In addition, solar energy creates new jobs. Employment in the solar sector is growing many times faster than jobs in the economy overall, and the jobs pay well. Follow this link for current information on solar employment. http://thesolarfoundation.org/
Plus It’s Fun
Your Own Power Plant
You will enjoy your solar system. Show it off to your neighbors. Everyone will be interested in your clean, silent power plant that keeps operating year after year with no moving parts to wear out. You’ll find yourself frequently checking to see how much energy you have produced, and you will increase your awareness of energy conservation techniques.