There are a wide variety of PV system types and structural applications. In most cases, the basic requirements for successful PV systems are that they be durable, waterproof, fire-restrictive, and aesthetically acceptable. The most common system applications involve the use of flat plate arrays either as stand-alone entities or as integrated building components. These may be fixed arrays or use single- or dual-axis tracking.

An ever-increasing number of structural applications are being installed, including many that are both aesthetically and functionally integrated into building structures. For example, a newly developed PV technology allows application of PV cells as roofing material. Roof tiles interconnect and can be installed over an existing roof or as a new roof. These may be designed as exchangeable PV shingles, which are flat panels that can be easily laid on and taken off a roof, or pre-fabricated roof panels that are roof-integrated modules composed of a panel that takes the place of roof sheathing and solar roof tiles. Thin-film technologies offer great application flexibility and can serve as windows, atriums, and awnings.

Three basic PV system application options are:

• Stand-alone systems that are not connected to the grid. These may use a battery for storage and may power dc devices directly or ac devices through an inverter.

• Grid-connected systems that provide ac power and are tied directly to the utility grid, or tied into the grid indirectly through the facility's internal electrical distribution system.

• Hybrid systems that may be tied into a dc or ac circuit along with other power producing devices, such as Diesel engine or wind power systems.

With both stand-alone and grid-connected systems, PVs are sometimes integrated with interruptible power supply (UPS) systems. These systems feature a PV module connected to a battery and, for some systems, to the electric grid. The battery stores power for use when the sun is not shining or during a utility outage. A charge controller regulates the flow of current to and from the battery subsystem to protect the battery from overcharge and over discharge. While this is similar in some respects to standard UPS and/or Diesel back-up systems, the PV has the advantage of being able to run as long as the sun shines.

In states with utility regulation that allows or requires net metering, a grid-connected PV system serves as an unobtrusive power plant that can provide power both to the facility and into the grid. Power fed into the grid allows the customer to either receive payment from the utility at the wholesale rate, or simply reduces consumption, lowering the bill at the retail rate. The latter situation typically applies as long as the customer is still a net consumer, not a producer of electricity for a given billing period.

Options for applying PV systems range from residential systems and small systems that power discrete equipment in remote locations, to larger utility power plant applications of several MW and larger. A 6.5 MW PV array has recently been retired after 20 years of operation at Carissa Plains, California. By PV system standards, this was quite large, but very small relative to the capacity of most conventional power plants. However, the potential for larger and larger central PV power plants certainly exists.

Relatively large-scale utility PV power plants consisting of many arrays installed together can prove useful to utilities. They can be built much faster than conventional power plants, which can be important under conditions of capacity shortages. In California, for example, the ability to add capacity quickly, with the added benefit of avoiding permitting and emissions trading to install new capacity, is of great importance and can make the more costly PV system a good tactical choice. For example, a 750 kW PV system currently being installed at a Naval facility in San Diego was developed and designed in just a few months and is expected to be constructed and operational in less than six months.

Figure 14-49 shows a traditional PV system array. This is a 500 kW system that has been in operation since 1993. Note the build-up of the array, with 3 rows of 12 PV cells (totaling 36) in each panel, which is connected to a series of panels, making up modules that are lined up in rows of 8. Figure 14-50 shows a grid-connected 300 kW facility located near Austin, Texas. Notice the conventional power plant in the rear. Figure 14-51 shows a 2 MW PV power plant operated by Sacramento Municipal Utility District. The 1,600 modules are spread across an 8,094 m2 field in this very sunny region. Behind the PV system is a nuclear power plant. The utility has opted for nuclear plant decomissioning and has invested in expanded decentralized generation and/or in using renewable resources. This PV system has been expanded over time, with new arrays being added periodically.

Fig. 14-49 Traditional 500 kW PV System Array. Source: Terry O'Rourke, DoE/NREL
Fig. 14-50 Grid-Connected 300 kW Facility Sited Near Conventional Power Plant. Source: 3 M Corporation, DoE/NREL

Fig. 14-52 Roof-Mounted PV System at a Furniture Factory in Massachusetts. Source: Bill Eager, DoE/NREL

Fig. 14-51 2 MW PV Power Plant Operated by Sacramento Municipal Utility District, Sited Near Decommissioned Nuclear Power Plant. Source: DoE/NREL

In addition to the many standalone power station type applications, many PV systems are installed directly on a facility roof or integrated into the building construction. Figure 14-52 shows a roof-mounted PV system at a furniture factory in Massachusetts. Figure 14-53 shows a 372 panel system that is integrated into the roof of a downtown office building in Boston, Massachusetts. Figure 1454 shows the installation of PV roof shingles. These newly developed triple-junction amorphous silicon shingles mount directly on the roof structure and take the place of asphalt shingles. Each shingle produces 17 Watt under full design conditions.

Figure 14-55 shows the final construction of a PV system that was installed at the Olympic swimming com-

Fig. 14-53 372 Panel PV System Integrated into Roof in a Downtown Boston Office Building. Source: Roman Piaskoski, DoE/NREL

plex in Atlanta, Georgia. A 340 kW rooftop PV array was installed on the main structure and a custom arched glass PV canopy was designed for the entrance to the complex. The rooftop PV system employs 2,832 Solarex 120-W PV modules mounted above the steel roof deck to allow for the free flow of cooling air below the array. A central Kenetech 300 kW dc-to-ac inverter feeds three-phase power into the campus utility grid. The total array has an area of about 27,900 ft2 (3,000 m2). The entry canopy

Fig. 14-54 Installation of Newly Developed Triple-Junction Amorphous Silicon PV Roof Shingles. Source: Warren Gretz, DoE/NREL

features special large-area 250 kW Solarex PV modules that have a clear back skin to allow light transmission between the individual crystalline cells. Each PV module has its own integrated dc-to-ac micro inverter, developed by Advanced Energy Systems to deliver 60 Hz ac power directly to the building complex. Figure 14-56 shows an artist's rendition of a 48-story skyscraper that uses thin-film PV panels to replace traditional glass cladding material. The PV curtain wall extends from the 35th to the 48th floor on the south and east walls of the building.

While currently not cost-effective in most conventional markets, PVs have for some time proven useful and cost-effective in certain niche markets and for some

Fig. 14-56 Artist's rendition of Skyscraper Using Thin-Film PV Panels in Place of Traditional Glass Cladding Material. Source: Kiss + Cathcart - Architects, DoE/NREL

specific applications. PVs have been effectively applied in numerous remote applications where utility interconnection was not feasible or too costly. In some cases, these applications were utility-sponsored where facilities in their service territory had isolated, low-energy, or low-revenue loads, or where line expansion was not feasible.

PVs have also been successfully applied for applications such as communications, warning signals, sectionalizing switches, cathodic protection, lighting, monitoring, and battery charging:

Fig. 14-55 Final Construction of PV System at the Olympic Swimming Complex in Atlanta, Georgia. Source: Solar Design Associates, Inc., DoE/NREL

• PV system battery charging has been applied at an electric vehicle recharging station in Southern Florida. The system is grid-connected and can sell power when vehicle-charging load is below system output.

• PV systems have been applied for certain sectionalizing switches on the Public Service Company of New Mexico's transmission line. Even though conventional power was available, the cost of transformers, surge arrestors, switches, and rectifiers to make the dc required for switch operation was greater than the cost of the installed PV system.

• Cathodic protection of pipelines is an economical and effective application of PVs to solve corrosion problems. The solution is an electric current from a PV source to counteract the natural corrosive currents generated around buried metallic devices.

PV systems have commonly been combined with other technologies at isolated sites in establishing a village energy concept by which multiple systems are combined to produce the optimal blend of life-cycle energy costs and reliability. This includes use with conventional Diesel engine power systems or, in some cases, with other renewable systems, such as WECS to eliminate the need for fuel or electricity acquisition and delivery. In one interesting application at a Tokyo Japan, water re-use promotion center, a desalination system uses both PVs and solar thermal applications for the membrane distillation process. The system produces fresh water by passing sea water through a membrane module. Heat to warm the seawater is provided by a solar collector and the pumps are powered by PV panels so that the system can be installed at sites without any other electricity source or fuel supply.

PVs may soon be used to generate hydrogen through the electrolysis of water. This hydrogen can be stored and ultimately used for normal combustion (as a cleaner source than fossil fuels) or to generate power in a fuel cell.

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