The Fundamentals of Photovoltaic Systems

Jan 1, 2012 9:00 AM, By Ryan Mayfield, Renewable Energy Associates

The basics of photovoltaic (PV) equipment and components

Over the past several years, the solar photovoltaic (PV) industry has enjoyed substantial growth. According to the Solar Energy Industries Association (SEIA) and GTM Research, cumulative grid-connected PV in the United States has now reached 3.1 GW — 10 times the size of the country’s solar capacity in 2005. While some projections call for this segment to hit a speed bump this year — considering the uncertain effects of the U.S. Treasury 1603 tax grant program possibly expiring at the end of the year (unless Congress extends it) coupled with the U.S. government’s reaction to a call for import duties on Chinese-manufactured PV cells and modules — most analysts anticipate the solar industry will continue on its expansion path. That’s good news for electrical contractors.

A typical PV array mounted on a residential roof. Individual modules are attached to a racking system and wired together to form an array.

Thanks to popular demand, these systems are no longer installed exclusively by specialty solar contractors — a situation that opens up ongoing revenue streams for electrical contractors, who are being asked to broaden their scope of work and provide PV installation services. This article will introduce you to the major system types and most commonly used components within various types of PV systems. Armed with this fundamental information, you should come away with a basic understanding of how these different systems operate and how the associated components are involved in their design. In future articles, we’ll focus on the exact design and installation parameters for PV systems as a whole.

System Configurations

The first important point to establish is the difference between the most common types of PV systems. There are applications for PV systems all the way from small electronics to solar attic fans to electric utility power plants. However, the majority of installations most contractors come across involve residential and commercial applications.

When it comes to residential projects, PV systems can be broadly categorized by the presence, or lack thereof, of the electric utility grid. These systems are classified as either stand-alone or utility-interactive.

Stand-alone systems are those that do not have access to the electric utility grid, often referred to as off-grid systems. Typically found in remote locations, they generally use batteries to store energy produced by the PV array. Stand-alone systems normally use a traditional generator to charge batteries during times of the year when solar resources are minimal or when the array is unable to keep up with energy demands of the household. Generally more complex, these systems require more design and installation time than grid-direct systems.

Utility-interactive systems are those that do exactly that: interact with the electric utility power system. The power electronics used in these systems, namely the inverters, monitor the grid and react automatically to the conditions the grid presents. Utility-interactive systems can be broken down into two subcategories — grid-direct or backup systems. In commercial PV applications, the systems are nearly universally utility-interactive, grid-direct systems.

Tax incentives and electric utility rebate programs have made utility-interactive systems the largest market for PV to date. These systems are able to interconnect with and work in parallel with the electric utility grid. In a situation where the PV array is producing more power than what is being consumed on site, the inverter is able to push that power back onto the electric utility line, essentially running the customer’s meter backward and allowing the customer to sell energy to the electric utility. Then, as the sun goes down and more loads are turned on, the meter begins to run forward again, causing the customer to buy back energy from the electric utility. When the sun is up but the loads within the building are consuming more than the solar system is producing, the electric utility makes up the difference — but the meter is moving slower than it would if the PV array wasn’t present.

In utility-interactive, grid-direct systems, as shown in Fig. 1 (click here to see Fig. 1), there is no energy storage, and the PV modules are connected to an inverter, which inverts the DC power produced by the PV modules into AC power that matches the grid power. These systems have the advantage of higher efficiencies and are more simplistic in their design/installation compared to their backup counterparts. Because the grid-direct systems do not employ a method of storing energy, when the electric utility grid fails, they automatically shut down until the grid is back up. On the other hand, utility-interactive, backup systems, as shown in Fig. 2 (click here to see Fig. 2), do employ energy storage, typically in the form of a battery bank. These systems can operate specific loads that have been isolated from the main distribution panel and are connected to a backup load center. It is common for these systems to only run a few loads off the backup system, not the whole house. If a whole house backup is required, doing so with a battery bank and inverter system can be difficult.

All utility-interactive systems use a safety feature known as “anti-islanding” to prevent the solar array from remaining connected to the electric utility when the grid is down. The inverters used in utility interactive systems are required by UL1741 listing to disconnect themselves from the grid when the voltage or frequency of the electric utility falls out of a specific range. The inverters will automatically reconnect to the electric utility system only after the voltage and frequency values are within the acceptable range for five continuous minutes. This is a safety feature to protect electric utility line workers from being injured by PV systems that are pushing power back on to otherwise de-energized power lines.

System Components

Now that you have a better feel for the major types of systems being designed and installed today, let’s go over the major system components used in these systems. This list will not be all inclusive, but rather a good primer on the major components and the roles they play in the entire system.

Modules — The heart of the PV system is the module, an individual unit that contains a number of PV cells, typically encapsulated behind a sheet of glass with an aluminum frame protecting the edges. A number of PV modules are wired together to produce a desired amount of voltage and current that matches the requirements of the power electronics connected on the other end of the PV array. In grid-direct PV systems, a number of modules are typically connected in series to create a string of modules that can operate at elevated voltages (i.e., up to 600VDC in some situations). Battery-based systems, either stand-alone or utility-interactive, will generally operate at lower voltage levels but can still exceed 150VDC within the string. These PV strings are then connected in parallel to create the PV array.

PV modules are offered in a variety of technologies; however, crystalline silicone modules are the most commonly used. Thin film modules, such as amorphous crystalline or cadmium telluride, are another category of PV modules you may come across.

Racking — Traditional PV modules require a support structure to hold the array in place. There are modules known as building integrated photovoltaics (BIPV) that actually replace the outer shell of a building, but these systems are not as common as PV modules mounted to the exterior of a building or ones mounted at ground level. PV modules can be installed in a variety of locations, and there are racking systems for every conceivable location. The racking systems generally fall into one of three categories: roof mount, ground mount, or top-of-pole mount. Within each of these general classifications, there are multiple subcategories.

Regardless of the mounting classification, racking systems are commonly made of extruded aluminum rails that support the PV modules at four points along the module’s aluminum frame. The exact form the rack’s extrusion takes on is a function of the racking manufacturer’s engineering and final location of the array. Common applications include flush-to-roof racking systems that hold the array parallel to and in close proximity (i.e., within 6 inches) to the roof surface, as shown in the Photo. Another racking type is a ballasted rack, which does not penetrate the roof membrane or ground, and uses weights, such as concrete ballast blocks, to hold the array in place. PV arrays can also be mounted close to the ground on piers that support the racking system, which directly support the modules.

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