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Photovoltaic System Components

PV Modules
Solar cells, composed of semi-conductor materials such as silicon, are the basic building block of PV technologies. An individual PV cell typically produces between 1 and 2 Watts, hardly enough to meet household needs. To increase the power output, cells are commonly connected to form larger PV modules. Modules, which are also sometimes called panels, range in power output from about 10 Watts to 300 Watts for residential and business power applications. A typical PV module consists of a protective weatherproof enclosure for the semi-conductor materials and the electric wiring needed to connect the module with the rest of the system. By connecting modules together, system designers can create PV arrays that have power outputs of 15,000 Watts (15kW) or more. In Vermont, 15kW is the current size limit for PV systems to be eligible for net metering. Modules or arrays, by themselves, do not constitute a PV system. We must also have structures on which to put them and point them toward the sun, and components that take the direct-current (dc) electricity produced by the modules or arrays and condition the electricity so it can be used in the specific application. These structures and components are referred to as the balance of system (BOS).

Inverters
Because of their electrical properties PV cells produce direct rather than alternating current (AC). Direct current (DC) is electric current that flows in a single direction. Many devices, such as those that run on batteries, use direct current. In contrast, alternating current reverses its flow direction at regular intervals. AC power is the type of electricity provided by utilities and is required to run most common household appliances and electronic devices. Inverters are used to convert DC to AC, and to provide other power conditioning and safety related functions. Although a small amount of energy is lost in converting DC to AC, inverters make PV-generated electricity behave like utility power that can be sold back to the utility or used to operate everyday ac equipment such as appliances, lights, and computers. For most net metering applications inverters will range in size from 100 Watts to 4kW. The inverter must be carefully selected to insure proper operation with other system components. Small inverters may be mounted right on the back of a PV module. Larger inverters (>1kW) are often wall mounted in a basement or garage. Large inverters typically contain built-in battery chargers. This allows the inverter to operate as a battery charger when power is available from another AC source such as the utility grid or a generator. There are two classes of inverters. Sine wave inverters supply clean, utility-grade power. Sine wave inverters are required for use in grid-tied systems. Modified sine wave inverters supply a "stepped" sine wave output. This power is not as "clean" as pure sine wave inverters, and is not considered utility grade power. However, if properly selected, modified sine wave inverters can operate well in most stand-alone applications and are less expensive than pure sine wave inverters.

Batteries
Batteries are an essential component for off-grid or emergency backup power systems. Several batteries linked together comprise a battery "bank", which collects and stores energy produced by the PV array for periods when the sun does not shine. Several factors can be used to help determine the size of the battery bank. These include the electric load, the duration of required reserve power, and the availability of a source of backup power (grid or generator). A good quality, lead-acid battery bank will last from 500 to 1,000 charge-discharge cycles depending on depth of discharge and attention to maintenance considerations. Other types of batteries are available such as Nickel Cadmium. These batteries are longer lasting, but quite a bit more expensive than lead acid batteries. A battery box is needed to enclose the battery bank. The box contains potential acid spills, keeps out unfamiliar persons, and keeps objects from falling on the batteries possibly damaging or shorting battery terminals. The battery box must also provide adequate ventilation of explosive hydrogen gas (produced during battery charging) to the outside. This includes careful attention to charge and discharge levels, periodic watering (except in the case of gel-cell batteries), and inspection of cables and connections for tightness and corrosion.

Charge Controller
A charge controller regulates the amount of energy flowing from the PV array to the batteries. This is essential to avoid the damaging situation of overcharging the batteries.

Mounting Hardware
PV panels are most commonly roof mounted, although ground and wall mounts are possible. In any case, a PV array needs to be securely mounted to a solid wind and vandal resistant structure. Mounting hardware needs to be weather resistant, and suitable to meet expected loads. Specialized mounting kits are available from most manufacturers and installers. Proper grounding and lighting protection must also be considered during installation and mounting. Just as a utility power connection to a home needs proper protection devices, so does a PV system.

Metering
Grid connected households will most commonly use a single meter set up, whereby the utility meter registers the net difference between the household's load and PV system output. When the PV output is greater than the house's consumption, the meter will spin backwards. If the sun is shining, and the household load is greater than the PV output, then the meter will spin forward (e.g. in the normal manner) but more slowly than it would if there was no contribution of solar electricity. When the sun is not shining, the utility meter operates as usual in a non-solar house. The single meter set up is attractive because there is no additional cost for PV metering. However, it is difficult to know how much solar electricity is actually generated each month. Many PV owners are interested in more specific information on the output of their PV system. Many systems offer options for direct system metering, using either a computer hook-up, or other meter installation. Off-grid homes, or systems with battery back-ups often also have a meter system to monitor battery voltage, charge and discharge levels, battery reserve capacity, power used, and historical battery data. These meters can be set up for automatic monitoring by the system installer. A good battery monitor is a very useful diagnostic and customer service tool.

Generator
Some off-grid home owners opt to install a generator to supplement the PV system during cloudy periods, or for when high-power equipment such as washing machines, water pumps or power tools are being used.

Wiring Fuses, Disconnect Box
Connecting PV panels to the household requires properly sized wiring, installed according to code standards. All systems also require fuses for protection of people and equipment. Interconnection requirements in Vermont include a utility accessible, lockable, load break rated, visible break disconnect switch for all grid connected PV installations.


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