Lesson 5
Photovoltaics (PV) System Basics
- Overview
- Types of PV systems
- PV System Components
- Grid-Connected PV System Components
- System Components
- Questions
- Answers
Overview
Photovoltaic (PV) systems convert sunlight to electric current. You are already familiar with some simple PV applications in today’s society, such as calculators and wristwatches. More complicated systems provide power for communications satellites, water pumps, and the lights, appliances, and machines in homes and workplaces. Many road and traffic signs along highways are now powered by PV.
PV systems produce some electric current any time the sun is shining, but more power is produced when the sunlight is more intense and strikes the PV modules directly (as when rays of sunlight are perpendicular to the PV modules). While solar thermal systems use heat from the sun to heat water or air, PV does not use the sun's heat to make electricity. Instead, electrons freed by the interaction of sunlight with semiconductor materials in PV cells create an electric current. PV modules are much less tolerant of shading than are solar water-heating panels. When siting a PV system, it is most important to minimize any shading of the PV modules.
PV allows you to produce electricity—without noise or air pollution—from a clean, renewable resource. A PV system never runs out of fuel, and it won't increase U.S. oil imports. Many PV system components are manufactured right here in the United States. These characteristics could make PV technology the U.S. energy source of choice for the 21st century.
Types of PV Systems
Net-Metered Systems
Lesson 7 will focus on installation of grid-connected/net-metered PV systems. PV systems interconnected to the utility called grid-connected or net-metered systems can be metered in such a way as to allow the customer-generator to get credit for electric energy produced by the PV system. The PV system is connected at the customer’s breaker panel, and if the power generated is greater than the load, the power runs in reverse through the meter, and runs it backwards. The Pennsylvania Public Utility Commission adopted net-metering standards that govern how small alternative electric generators—such as PV systems—connect to the electric distribution system and how they are compensated for generation they provide into the electric utility distribution system. The net-metered customer is to be reimbursed (by the electric distribution company) at the full retail rate for each kilowatt-hour produced by the customer during a billing period and at the end of the billing period, the customer will be compensated if they generated more than they used during the period. In other words, the electric utility meter on the building can backup whenever the PV system produces more electricity than is being consumed and if at the end of the billing period the building still has generated more than it consumed, the distribution company will pay for the excess. Net metering laws are generally in place in order to encourage renewable energy generation. Before the homeowner buys or installs any generation equipment to be net metered, they should call their electric utility service provider, and find out from them all of the utility requirements and rules for installing and interconnecting a generator.
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Net-metered PV systems use the existing utility grid as storage. Image: DOE |
Stand-alone Systems
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| PV provides electric energy to this remote home. Image: NREL/PIX 07630 |
Rafters are usually 16 inches or 24 inches center to center. If you cannot attach the collector mounting hardware to the rafter itself, you must install a spanner block between the rafters and mount the collector hardware to the sleeper. Do not rely on the roof sheathing to support the solar collectors. Be sure that the collector mounting hardware is securely attached to the framing members.
In remote areas where existing utility lines are a considerable distance away, PV is often the least expensive way to provide electricity to a building. The expense of installing the power line can be at least $25,000.00 per mile and can be as much as $60,000.00 per mile. A remote solar electric system can be less expensive than the line extension.
Off-grid systems have the same components as grid-connected systems, except that they do not need a grid-tie inverter, and they do need storage batteries. Also, off-grid systems may have additional components such as an auxiliary generator, or even a wind turbine.
PV System Components
Specific PV system components may include a DC-AC power inverter, battery bank, system and battery controller, auxiliary energy sources, and sometimes the specified electrical load (appliances). In addition, an assortment of balance-of-system hardware, including wiring, over-current, surge protection and disconnect devices, and other power processing equipment may be included. The following diagram illustrates the relationship of individual components.
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| Diagram: Florida Solar Energy Center |
Grid-Connected PV System Components
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| This schematic shows example components of a general grid-connected system. |
PV modules are mounted on mounting racks and are attached to a structure or may be mounted on a pole. A number of modules assembled together is called an array. Individual modules produce electric current and voltage that depends upon the specific module. The electric output wires of the modules are wired together in a combiner box in order to get the voltage and current required by the inverter. The array output can be disconnected by a DC disconnect switch. In order for the system to be disconnected from the grid by utility workers, a utility accessible AC disconnect switch is installed on the inverter output. The inverter may have two connections to the breaker panel.
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Pole-mounted PV system array. |
System Components: Solar Cells
Semiconductor
The primary material used to convert sunlight to electricity is called a semiconductor. There are two basic types of semiconductors: p-type and n-type. The p-type semiconductor material has an abundance of “holes” with a positive electrical charge, while the n-type semiconductor material has an abundance of electrons with a negative electrical charge. When these two semiconductors come into contact with each other, a p/n junction is created at the interface. At this junction, excess electrons move from the n-type side to the p-type side, resulting in a positive charge along the n-type side and a negative charge along the p-type side. This creates an electric field much like a battery with one side having a positive charge and the other a negative charge.
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System Components: Arrays
Modules are commonly connected in series and parallel strings to form what is called an array. The output of an array can be designed to meet almost any electric requirement, large or small. The picture below shows an array that has a peak DC rating of 4.5 kilowatts.
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| Image: National Center for Appropriate Technology, Butte, MT |
System Components: Mounting Structures
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| Image: DOE | Image: Spa Hot Springs Motel Tracking Solar Arrays |
Generally, solar modules do not have the structure needed to withstand wind loading, and so must be mounted on a mounting structure. Mounting structures are usually made of steel or aluminum and may be attached to the roof of the home in a fashion similar to that for solar water-heating panels (see Lesson 4). Mounting structures may be fixed mount, may allow the array to be tilted seasonally, or may, on pole mounts, be able to track the sun.
System Components: Combiner Box
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System Components: Inverters
PV cells, modules, and arrays produce direct current (DC). Electric loads that are not connected to the utility grid can use the PV-generated power if they are designed to operate on direct current. Using a charge controller, PV-generated power can charge a bank of storage batteries which can power DC loads when the sun is not shining on the array. Most appliances and equipment found in the home are designed to operate on alternating current (AC) which is generated by electric utility companies. The device that converts DC to AC for use in the home is called an inverter. In stand-alone or grid-connected PV system installations, inverters are commonly used to power household appliances, tools and other equipment. These inverters do not need the utilities voltage and frequency reference to produce AC with electrical characteristics much like utility-generated AC. Inverters that are connected to the utility grid produce AC that is identical to the power produced by the utility. These inverters sense the utility’s generated voltage and wave form characteristics and produce AC of the same form.
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In the picture to the right, the inverter is the large, white, rectangular device. The white, square box to the right of the inverter and the gray box to its left are disconnect switches—the one on the left disconnects the inverter from the utility and the one on the right is the DC disconnect. In this case, the DC disconnect switch also contains a ground fault interrupter for the PV array. The small box just above the inverter is a monitor that shows the array’s DC voltage and current output. Note the electrical conduit into and out of the disconnect switches.
Questions
- Give three ways that a PV cell and a battery are alike.
- A battery converts chemical energy into electrical energy. What type of energy does a PV cell convert?
- Since each individual PV cell’s electrical output is small, how can the cells be configured to produce the electrical output needed to power a high electric demand?
- Why is an inverter needed for a PV system that is connected to the local utility grid?
- What function does the combiner box perform in a PV system?