Roof-mounted PV panels require mounting frames, roof connections and other hardware that adds to the installation cost of the PV system. However, advances in PV materials and manufacturing processes has resulted in building-integrated PV (BIPV) where PV is being integrated into roofing materials, glass and other building materials and surface coatings.
BIPV has the potential to transform a building's entire outer surface into an electric generator. Many electrical contractors mistakenly believe the BIPV trend will mean a loss of market share to other trades that install PV. On the contrary, BIPV spells a tremendous growth opportunity for electrical contractors.
PV panels or building materials represent only a portion of the cost. Once installed, the PV technology must be safely integrated into the power-distribution system, which requires special expertise and includes the installation of high-tech inverters and traditional materials such as conductor and raceway.
The remainder is often referred to as the balance of system (BOS). The electrical contractor may leave the installation of PV panels to other trades and avoid water-infiltration liability and other installation risks. Instead, the contractor may find the design, procurement, installation and ongoing service of the BOS equipment provide the greatest business opportunity. The type and extent of the BOS equipment and installation requirements are often determined by whether the PV system is a stand-alone or grid-connected.
Stand-alone PV systems are sometimes thought of as the simplest type because they are not connected to the utility grid. However, this is not necessarily the case today where the hardware to interconnect a PV installation with the grid is becoming an off-the-shelf item rather than a custom-built array of overcurrent protection and power-control devices. Stand-alone PV systems are common in remote areas, where there is no readily available utility power supply or where particular building loads can be isolated from the traditional utility supply and served solely by the PV system. The BOS for a stand-alone PV system usually requires an energy-storage system such as a battery bank to provide power at night or on overcast days when the PV array cannot generate the power needed. When the load includes sensitive electronic or other critical equipment, an uninterruptible power supply (UPS) may be warranted.
Unless the stand-alone PV system load can operate directly on low-voltage DC, the BOS will require the installation of an inverter to convert the low-voltage DC to 60-hertz AC. The stand-alone PV system will probably also require a battery charge controller, unless it is built into the inverter to protect the battery bank from being overcharged as well as prevent the load from completely discharging the battery bank. Additionally, a stand-alone PV installation will require disconnecting means, overcurrent protection and wiring in accordance with the NEC including Article 690/Solar Photovoltaic Systems.
Installing grid-connected PV systems is often more complex than stand-alone systems because they have a point of common coupling (PCC) with the serving utility and must meet the utility's requirements that usually include power flow control, metering and power quality. Most PV systems today do not generate enough power even at peak times to meet the building demand so there is usually no surplus power to transfer to the utility grid. However, grid-connected PV systems are bidirectional and power can flow both to and from the utility.
If there is a power surplus, it will flow into the utility system displacing some of the power that the utility would otherwise have to generate to meet its load. With the proper metering and interconnection equipment, the utility may purchase the surplus power from the building owner at a predetermined rate or there may be a net-metering arrangement where the customer's monthly energy usage is directly offset by the amount of energy supplied to the utility.
Grid-connected systems usually do not include batteries because the utility service provides the needed energy. However, in areas where there utility supply is unreliable, a backup generator or UPS system may be installed. Like the stand-alone system, the grid-connected system includes disconnecting means, overcurrent protection and wiring required by the NEC including Article 690/Solar Photovoltaic Systems.
This article is the result of a research project investigating the investigating the potential of the emerging PV market for the electrical contracting firm being sponsored by The Electrical Contracting Foundation Inc. The author would like to thank the foundation for its support. EC
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.