Photovoltaic Information (Solar Electric Cells)

PV System Components

The other stuff you need to really have a functional system.

Mounting a PV Array
PV arrays for stand-alone systems are installed in many unique and innovative ways. However, there are common issues involved in any installation, whether the array is fixed or tracking, mounted at ground level, or on a pole or building.

The objective is a solidly mounted PV array that will last for many years and withstand all kinds of weather. Regardless of whether you buy or build the mounting structure make sure it is anchored and the modules are restrained. Many module manufacturers and distributors sell mounting hardware specifically designed for their modules. This hardware is intended for multiple applications and different mounting techniques and considerations like wind loading have been included in the design. Using this mounting hardware is the simplest and often the most cost effective. Customized array mounting structures can be expensive. Consider the characteristics of various mounting materials:

-Aluminum - lightweight, strong, and resistant to corrosion. Aluminum angle is an easy material to work with, holes can be drilled with commonly available tools, and the material is compatible with many PV module frames. Aluminum is not easy to weld.

-Angle Iron - easy to work with but corrodes rapidly. Galvanizing will slow corrosion but mounting brackets and bolts will still rust, particularly in a wet environment. The material is readily available and brackets can be welded easily.

-Stainless Steel - expensive and difficult to work with but will last for decades. May be a good investment in salt spray environments.

The foundation for the array should be designed to meet the wind load requirements of the region. Wind load depends on the size of the array and the tilt angle. Ask a local contractor or your module distributor how to anchor your array to withstand the wind expected in your area.

Changing the tilt angle of an array to account for seasonal changes in sun altitude is not required. For mid-latitude locations, a tilt angle change every three months is estimated to increase energy production about 5 percent on an annual basis. For most applications, the additional labor and the added complexity of the array mount does not justify the small increase in energy produced.

In general, roof mounting of PV modules is more complex than either ground mounting or pole mounting. Roof mounts are more difficult to install and maintain, particularly if the roof orientation and angle are not compatible with the optimum solar array tilt angle. Penetrating the roof seal is inevitable and leaks may occur. Also, it is important to achieve a firm and secure attachment of the array mounting brackets to the roof. Attaching the mounting brackets to the rafters will provide the best foundation. Attaching the array to the plywood sheathing of the roof may result in roof damage, particularly if high winds are likely.

If a roof mount is required, be sure to allow a clear air flow path up the roof under the array. The array will operate cooler and produce more energy if it stands off the roof at least 3 inches. Flush mounting PV modules to the roof of a building is not recommended. The modules are more difficult to test and replace, and the performance of the array is decreased because of the higher operating temperatures.

Batteries
Batteries must be protected from the elements. If freezing temperatures are expected, the batteries can be buried below the frost line in a water-tight enclosure or in a building where the temperature will remain above freezing. If the batteries are buried, a well-drained location should be selected and a drainhole provided in the battery enclosure. Batteries should not be set directly on concrete surfaces as self discharge will be increased, particularly if the surface gets damp. Adequate venting must be provided to minimize explosion hazard if open-cell batteries are used. Any battery should be stored in a location where access is limited to knowledgeable personnel. Never allow unsupervised children or pets near batteries.

Commercial battery enclosures may be available but are usually expensive. For small systems, a heavy-duty plastic tub may serve as an inexpensive alternative. Be sure it will withstand direct sunlight if the batteries are to be installed outdoors and above ground.

Control Center
Electronic controllers, converters, or inverters are often installed in the control center along with switches, fuses, and other BOS. Electronic components must be able to withstand expected temperature extremes in both operating and non-operating states. Any printed circuit boards in these units should be coated or sealed to protect the electronics from humidity and dust. Certified electrical service boxes should be used. Consult any electrical supply company to get advice about the type of box needed for a specific application.

High temperatures will shorten the life of electronic equipment. Try to mount the boxes in a shaded area and/or provide air circulation, particularly for inverters. Dust can be a problem in a well-vented enclosure. Some boxes have filters at the air access points. Filters require regular cleaning. Screen the inlets of the electrical boxes to prevent spiders, wasps, and other insects from setting up residence. Finding wasps in the electrical box may not affect performance, but it will certainly make maintenance more exciting.

In a photovoltaic system, the term 'balance of system' refers to all of the system components except the PV modules. These components frequently account for half of the system cost and, unfortunately, most of the system maintenance. The components consist of structures, enclosures, wiring, switch gear, fuses, ground fault detectors, charge controllers, batteries, and inverters. Of these, ground fault detectors, charge controllers, batteries, and inverters are the components that could benefit most from developments in both technology and basic understanding. A diagram of a typical hybrid system, illustrating some of these components, is shown below.

Some of the design considerations associated with using components are:

batteries--type, depth of discharge, rate of charge, lifetime (in PV applications)
charge controllers--algorithm configuration, charge current
stand alone inverters--load compatibility, power rating, power quality, battery health
grid-tied inverters--islanding, susceptibility to line disturbances, RFI
hybrid inverters--load compatibility, generator compatibility, battery health

Grounding
A good ground will provide a well-defined, low-resistance path from the stand-alone PV system to earth ground. This path is expected to carry fault current if system malfunctions occur so the ground wire must be as large as the largest conductor in the system. Two types of grounding are needed in PV systems--system ground and equipment ground. For the system ground, one of the current carrying conductors, usually the negative, is grounded at a single point. This establishes the maximum voltage with respect to ground and also serves to discharge surge currents induced by lightning. Any exposed metal that might be touched by personnel should be grounded. This includes equipment boxes and array frames. This will limit the risk of electrical shock should a ground fault occur.

A low-resistance earth ground requires good contact between the ground rod and earth. Subterranean water lowers the resistivity of the contact. If the system is in an area with rocky soil, a good ground may be difficult to achieve. Consult a local electrician for suggestions.

A PV array can attract lightning, especially if located at a high elevation relative to the surrounding terrain. In particular, water pumping systems may draw lightning because of the excellent ground path provided by the well casing. Current surges can be caused by a direct lightning hit or by electromagnetic coupling of energy into the PV system's conductors. There is little that can be done to protect the PV system equipment from a direct lightning strike. Surges caused by near strikes occur more frequently and the severity of possible damage depends on the distance from the strike to the array. Commercially available surge protection devices (movistors and silicon oxide varistors) are reasonably priced and their use is recommended. They are normally installed in the array output and at the dc input to any electronic device. If an inverter is used, surge protection devices should be installed at the ac output as well as the dc input. Installing the wiring in grounded, buried metallic conduit will decrease susceptibility to lightning.