NEWSLETTER

Shedding Light on Photovoltaics

By Joe I. Moreland, PE (GA PE#26321)

Despite a slowly recovering economy, the U.S. solar energy market soared to $6 billion in 2010, up 67% from the previous year, according to a report by the Solar Energy Industries Association. Much of the non-residential growth has been driven by increased demand for sustainability and renewable energy along with improved economic feasibility fueled by federal, state, and utility rebate and incentive programs. This growth trend is expected to continue with costs continuing to decline as manufacturers increase production to meet demand and improve manufacturing technologies. While photovoltaics might still be a foreign subject for you, as it is for many design professionals, the need to be prepared for this exploding market is evident. So if you'll pardon the pun―let's shed some light on the subject, shall we?

Photovoltaic System Basics

Photovoltaic (PV) systems employ solar cells made from semiconductor materials that convert solar radiation into electricity. Multiple solar cells are electrically connected and factory-assembled to create photovoltaic modules. Multiple photovoltaic modules are then electrically connected in the field to create photovoltaic arrays.

Crystalline silicon modules are the most commonly used type for both residential and commercial applications. Thin-film modules are gaining market share in some commercial applications because they generally have a lower cost per watt of power output. Because thin film modules generally have lower efficiencies, they typically require more surface area than crystalline silicon modules to produce the same power output. Consequently, thin film might not be a suitable substitute for crystalline silicon where size of the array is limited―which is often the case with roof-mounted arrays. Thin film modules are therefore more commonly used for utility scale ground-mounted arrays, where size of the array is not as much of a concern.

Building-integrated photovoltaics (BIPV) incorporate solar modules but are used to replace conventional building materials in roofing and façades. Roofing systems include flexible thin-film modules integrated with the roofing membrane, standing seam panels, or shingles. Façade systems include curtain wall products, spandrel panels, and glazing.

PV systems may be grid-tied (with interconnection to utility grid) or off-grid (stand-alone without interconnection to utility grid). In the case of grid-tied systems, the energy produced by the PV system is metered to offset electricity consumption from the utility grid (in some cases the meter even runs backwards). This configuration requires a utility interconnection agreement between the facility owner and the utility company.

Depending on the system configuration, several other components are usually required to complete the connection to the facility electrical system or load. Combiner boxes connect groups of series-connected modules called strings. Inverters are used to connect the photovoltaic DC system to AC loads. PV systems may also incorporate batteries, charge controllers, and engine generators to provide backup power. Monitoring systems may be provided to collect and report system data.

Photovoltaics and LEED

Facility owners seeking LEED certification can receive points based on the percentage of the facility's annual energy use produced by on-site renewable energy sources such as photovoltaics. For new construction and major renovation projects, LEED awards credits ranging from 1 to 7 points, based on renewable energy production of 1 to 13 percent. At first glance this seems like an easy way to earn a lot of points, but in reality the square footage necessary to attain even a modest level can be substantial. This means the number of attainable points is usually limited unless there is an available area large enough for the array. Buildings more than just a few stories tall, which typically have a smaller available collection area along with a larger electrical load, might not be able to qualify for any points at all.

Photovoltaics and the NEC

Requirements for solar photovoltaic (PV) systems are covered in Article 690 of NFPA 70 (NEC). Because the photovoltaic DC systems in commercial applications can operate at voltages up to 600 V and may be energized any time the array is exposed to light, there are many specific requirements for wiring, grounding, disconnection, and identification to ensure a safe installation. The 2005 edition of the NEC was revised to allow the use of ungrounded photovoltaic DC systems, which have been in use in Europe and other parts of the world for years. However, the majority of U.S. installations continue to utilize grounded DC systems. The 2011 edition of the NEC included many revisions and additions to Article 690, most notably a new requirement for a listed DC arc-fault circuit interrupter for PV systems. However, listed products meeting this requirement are not yet available, and the applicable standard (UL1699B) is still under development.

Photovoltaics and BSD SpecLink

BSD's existing Section 26 3100 - Photovoltaic Collectors has been completely revised, including extensive new master note content to assist in editing. Part 1 includes new articles "Administrative Requirements" and "Quality Assurance," as well as extensive new reference standard, submittal, and warranty content. Part 2 has been expanded to include both thin-film and crystalline silicon modules, featuring basis of design products from SolarWorld Americas, as well as the new article "Balance of System Components" for products such as module mounting systems, combiner boxes, inverters, charge controllers, and monitoring systems. Part 3 has been expanded to include specific requirements for wiring, grounding and bonding, and identification of photovoltaic systems, as well as new articles "Field Quality Control," "System Startup," "Cleaning," "Commissioning," "Closeout Activities," and "Maintenance."

Conclusion

If your projects include photovoltaic systems, we expect you'll find our revised Section 26 3100 - Photovoltaic Collectors to be an essential improvement or addition to your specifications. Even if you just learned all that you know about photovoltaics from this article, at least you can't say you're completely in the dark on photovoltaics anymore (last pun, I promise). And when you do tackle that first PV system design, as that likelihood continues to grow along with the photovoltaic industry, you'll be better prepared for the challenge with BSD SpecLink.