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Solar Installation Guidebook - Solar PV Basics
This section of the guidebook provides an overview of solar photovoltaic (PV) technologies and systems. The reader is encouraged to review the Additional Resources listed at the end of this section to delve deeper into many of the topics presented here as well as the glossary (Appendix A) and Frequently Asked Questions (Appendix B).
How Does a PV System Work?

PV systems convert sunlight directly into electricity. PV systems allow homeowners and businesses to generate some or all of their daily electrical energy demand either on their own roof or somewhere on their property.

The majority of solar PV systems are ―grid-tied.‖ This means they remain directly connected to the power grid at all times and do not require battery storage. Grid-tied PV systems will generate electrical power to supply part of a building’s energy usage during the day and provide the greatest benefit during crucial times when the price and demand for electricity is the highest. Figure 2.1 depicts an illustration of a solar PV system interconnected to the grid.

A solar PV system can provide power to a home or business, reducing the amount of power required from the utility; when the solar PV system power generation exceeds the power needs, then the surplus power automatically back feeds into the grid. This arrangement is called ―net metering‖ for which PECO has a special tariff and will install a special utility meter that will essentially record the ―net‖ power coming in from the utility and the surplus power flowing out from the solar PV system.

A solar PV system will not operate during a power outage unless it has battery backup. It ceases to operate during outages as a safety feature for utility personnel who might be working on electric lines trying to restore power (a PV system would energize electric lines that the utility assumes is not energized, and create a shock hazard to personnel).

PV systems can also include battery backup or uninterruptible power supply (UPS) systems that can operate selected circuits in a building for hours or days during a utility outage.

The basic building block of PV technology is the solar "cell". Multiple PV cells are connected to form a PV ―module,‖ the smallest PV component sold commercially. A PV system connected or "tied" to the utility grid has these components:

  • PV Array: A PV Array is made up of PV modules, which are collections of PV cells. The most common PV module is 5-to-25 square feet in size and weighs about 3-4 lbs/ft2. Modules range in power output from about 10 watts to 300 watts (although higher wattages are available for utility-scale PV applications), with the power density ranging from about 5- to-18 watts per square foot.
  • DC to AC Inverter: This is the device that takes the DC power from the PV array and converts it into standard ac power used by the household appliances.
  • Balance of System equipment (BOS): BOS includes mounting systems and wiring systems used to integrate the solar modules into the structural and electrical systems of the building. The wiring systems include disconnects for the DC and AC sides of the inverter, ground-fault protection, and overcurrent protection devices, junction boxes and possibly circuit combiner boxes. (See Figure 2-2).
  • Metering: While meters indicate home energy usage, metering for a solar installation is used to record and display total electricity generation by the solar PV system and may provide indication of system performance.
  • Batteries (optional) can provide energy storage or backup power in case of a power interruption or outage on the grid. (This guidebook does not cover solar PV systems with battery backup because of their increased complexity compared to grid-tied PV systems, and because they account for less than 5% of all the solar PV systems installed.)

Applications for PV systems are constantly expanding with new uses being identified all the time. In addition to offsetting loads for homeowners as described previously, PV systems also serve facilities such as commercial, educational, industrial, and government buildings. PV technologies are rapidly becoming installed at the utility-scale supplying power for utilities and retail electric providers in multi-megawatt capacities.

PV Technologies

Today’s PV systems come in a range of efficiencies and configurations. PV systems with modules that are mounted on top of existing roofing are still the most common, but building integrated photovoltaic (BIPV) systems are gaining in popularity (See Figure 2-3). In a BIPV system, the modules do double duty—they generate electricity AND can replace traditional building materials such as roof shingles and windows. Table 2.1 presents a comparison of the main PV technologies commercially available today that are suited for this region, and Table 2.2 gives a comparison of the area needed to achieve the same output for PV modules of different efficiencies.

New PV products are being introduced to the market at a rapid rate. The most common products available today are:

  • Flat plate collectors (see Figure 1-1)
  • Flexible PV laminates integrated into roofing products (membrane, metal roofs, shingles, etc.) or other surfaces (see Figure 2-4)
  • Cylindrical cells (see Figure 2-5)
  • Flat plate collectors that also serve as skylights or windows allowing dispersed amounts of sunlight into a building (see Figure 2-6)
  • AC Modules (Flat plate modules with a DC to AC inverter mounted directly to the back of the module) (see Figure 2-7)

Installation Methods

Common PV array mounting methods for residential systems include:

  • Integral mounting (rooftop) (Figure 2-8)
  • Standoff mounting (rooftop) (Figure 2-9)
  • Rack mounting (rooftop or ground) (Figure 2-10)
  • Ballasted mounting (rooftop or ground) (Figure 2-11)
  • Pole mounting (ground) (Figure 2-12)

Large-scale flat roof commercial projects are often accomplished with fully engineered and certified systems, and some have no roof penetrations. For projects that require no roof penetrations, mounting hardware is either ballasted, interlocking or some combination of the two in order to withstand design wind speeds for a particular area. Nonpenetrating ballasted systems require adequate roof structural integrity in order to withstand the additional weight of the ballast (see Figure 2-11). These interlocking systems are often limited to the maximum angle that the PV array can be tilted at in order to withstand design wind speeds. Non-penetrating mounting hardware can be installed on standing seam metal roofs with roof clips.

Mounting hardware can also be mechanically attached to the roof and underlying structural members. A structural analysis is highly recommended and often required for commercial systems.

Other Considerations
Often it is desirable that the array be mounted at the ground level either on a pole or a rack. In public places, ground-mounted arrays are more susceptible to vandalism than pole or roof - mounted systems. Philadelphia has recently upgraded to the 2008 National Electric Code (NEC), which requires that all wiring on a solar PV array, either mounted on the ground or on a pole, must be protected. Therefore fencing material will be required around the backs of the ground or pole mounted PV arrays, or that these arrays must be positioned at least 6 feet above the ground (see Figure 2-12). The addition of a fence, if placed in front of the PV array, may require a greater footprint so that the fence does not shade the array.

Site Assessment Overview

One of the first steps that must occur when considering a PV system for a property is a site assessment. A well-conducted site assessment will determine the viability of a solar PV installation for a particular property and can always be used to identify fatal flaws that may cause problems during project implementation and for the property owner. In addition to the steps listed below, a project developer should determine if there are any competing uses for the proposed or adjoining space that might hinder the implementation of the project. For example, it may be possible to determine if there will be any new construction planned that might shade the proposed site in the future. Or a land area might be better used for housing development, urban agriculture, public open space, etc. These issues should be explored as part of a site assessment.

The primary goals of a site assessment are to:

  • Determine whether the array would be shaded during critical times.
  • Determine the location of the array (should be free from vandalism or able to be protected from vandalism without compromising energy production)
  • Determine the mounting method for the array
  • Determine where the Balance-of-System (BOS) components will be located
  • Determine how the PV system will interface with the existing electrical system
  • Identify issues that could jeopardize the viability of a project or result in increased design and installation complexity and implementation cost such as insufficient electric service (based on local electric code or utility requirements—see Section 4.3 PECO Interconnection) or insufficient structural support requiring costly upgrades
  • Identify any concerns of the building owner that may impact the design such as aesthetic concerns or financing options.

Shading calculations can be performed by hand by taking accurate measurements, noting surrounding objects and their position relative to a potential area, and using a sun path chart for the specific area. Several tools are also available to assist installers with easily identifying areas that will be shaded during crucial times of the day and throughout the year. The most commonly used tools include the Solmetric Sun Eye (Figure 2-13), the Solar Pathfinder (Figures 2-14 and 2-15) and the Acme Solar Site Evaluation Tool (Wiley Electronics).