Photovoltaic (PV) electrical power systems are becoming increasingly common in residential and commercial building applications. The typical PV system encountered by an HVAC technician during a service visit is connected to the building’s electrical system in a manner that’s not directly associated with the HVAC system (or any specific building electrical load). However, as the technology becomes more prevalent, the best HVAC technicians will know how to proceed when they encounter a building or residence equipped with a PV system.
The alternating-current (AC) connection from a PV system will usually be through a backfed circuit breaker in an existing load center or through a direct connection to the utility service entrance wiring.
For a typical system, the array of PV modules is usually configured into circuits of 200 to 600 volt direct-current (DC) connected to a large single inverter that converts the PV’s DC energy to AC energy compatible with the utility grid. Photos 1 and 2 show examples of typical PV systems of this type which may be mounted on a roof (possibly out of sight), or on the ground, either near the building or some distance away.
When these PV systems are installed in accordance with the requirements of the National Electrical Code (NEC), they are essentially transparent to the user and provide power quality that is equal to or exceeds that of utility generation. PV generation rarely affects any on-site electrical equipment because components have been tested and evaluated to meet stringent standards established by Underwriters Laboratories (UL) (Standard 1741), IEEE (Standard 1547) and the Federal Communications Commission (FCC).
For these systems, the local utility and/or the local jurisdiction will generally require a well-marked, readily accessible AC PV disconnect for the AC output of the PV system (see photo 3).
During HVAC service, if there is any question that the PV system is affecting the performance of the HVAC system, opening this disconnect will immediately isolate the PV system and cause it to shut down. In this condition, all power to the building is provided by the utility grid. Upon re-closing the disconnect, the PV system will resume proper operation (following a code-required five to seven minute start-up delay).
The Dawn of a New Era
Today, there is a new and, so far, unique application in which a PV power unit is installed in a direct AC connection to the outdoor unit of the HVAC system. Lennox Industries is pioneering this type of PV/HVAC system in both residential and commercial applications with the Dave Lennox Signature Collection SunSource Solar Ready systems (see photos 4 and 5).
These systems make use of what is known as an AC PV module. To create AC PV, each PV module is provided with a dedicated micro-inverter permanently mounted to the back that produces power at 240 VAC or 208 VAC, depending on the system design.
The output of the SunSource PV system (at 240 V or 208 VAC) is wired directly to the outdoor HVAC unit and in most cases will have a visible, well-marked, readily-accessible AC PV disconnect in this circuit. On these units, the PV AC output connection to the outdoor unit is through a small, dedicated circuit breaker panel integrated into the outdoor unit (see photo 6). There is one circuit breaker for the PV circuit output (15 or 20 amps), and a second, larger, circuit breaker (or fuses) protecting the outdoor unit itself. This arrangement allows the system to meet the requirements established by the National Electrical Code.
Energy from the PV system flows to the HVAC unit. When this energy exceeds the outdoor unit requirements, the excess energy is fed through the HVAC feeder to the load center in the building. From there, it’s available to power other loads in the building or can be fed back to the utility.
As before, when there’s any suspicion that the PV system is affecting HVAC operation (very unlikely), the PV system may be turned off by opening the external disconnect or opening the breaker associated with the PV system in the outdoor unit. Note: always disconnect both sources of power from the HVAC unit before removing access panels and attempting repairs.
Care must always be observed around any PV system encountered while performing HVAC service. Most PV systems have DC circuits that produce between 200 and 600 volts when energized. These can pose a potential danger any time there’s sunlight on the PV modules. In the case of SunSource HVAC units, only appropriately trained HVAC technicians (such as those who install these systems) should ever conduct repair, modification, or rewiring, of the dedicated PV components associated with them. Even properly trained technicians should not attempt to make changes to the wiring of the HVAC feeder or to the rating or location (in the load center) of the HVAC outdoor unit circuit breaker. These and a few other elements of the HVAC PV system must be retained as installed in order to maintain compliance with the requirements of the National Electrical Code.
Andrew Rosenthal is the director of and John Wiles is a senior research engineer at Southwest Technology Development Institute, College of Engineering, New Mexico State University. They can be reached at [email protected] or [email protected]. For recent articles and white papers about PV codes and standards, visit http://bit.ly/PVcodes.
It’s Not Shocking That Safety Comes First with PV Systems
A photovoltaic system, in most jurisdictions, is required to be permitted, installed, and inspected as any other electrical system and is installed by electricians licensed to make these installations. The same usually applies to the HVAC AC feeder to the outdoor unit. It’s likely that most HVAC contractors will not have the experience, licenses, or training to handle the PV parts of the system, and if the PV system is worked on by untrained persons, significant hazards can result for both the technician and the homeowner. —John Wiles