The Fundamentals of Photovoltaic Systems

Jan. 1, 2012
Knowing the basics of photovoltaic (PV) equipment and components gives electrical contractors a competitive edge when it comes to securing new work

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.

Combiner and junction boxes — Today, PV modules are nearly universally manufactured with conductors pre-installed on the back of the unit with quick-connect plugs on the end. These plugs allow the PV installer to quickly make the series connections without additional tools. This wiring method is allowed per the NEC (Sec. 690.31). Once the conductors leave the vicinity of the array, either through a building, along the exterior of the building, or through a trench in ground-mounted arrays, these source circuit conductors are most often transitioned into a different wire type, such as THWN-2. This transition happens in either a PV combiner box or a junction box. In a combiner, the source circuits are placed in parallel and very often include overcurrent protection. A junction box is used solely as a location to make the transition from the outdoor-rated conductors to the THWN-2 conductors.

The use of a combiner or junction box is a design decision that is made based on the specifics of the PV array location, proximity to the power electronics, and the preferences of the designer and installer. The NEC has requirements for placing disconnecting means in proximity to fused combiner boxes, so this can affect the combiner versus junction box design decision.

For smaller residential installations, there may only be one or two PV source circuits to deal with, and the box chosen is selected for its ability to transition into a raceway and route the conductors down off the roof through the interior of the building. In large commercial applications, there can be hundreds of circuits located within the array that require dozens of boxes to be strategically located and specified to minimize conductor size and overall length.

Power electronics — The output of the PV array will be connected to some form of power electronics. In systems that employ battery backup, both utility-interactive and stand-alone, the array will connect to a charge controller before going into the batteries. These battery-based systems will also use one or more inverters to then invert the energy stored in the batteries to AC for use by the loads. For grid-direct PV systems, the array will go directly into an inverter that will invert the DC energy from the array into AC for interconnection with the electric utility. An example wiring diagram for grid-direct systems is shown in Fig. 3 (click here to see Fig. 3).

Charge controllers — Charge controllers are only necessary for systems that use batteries. The primary purpose of a charge controller is to keep the battery bank from becoming overcharged, which could damage the batteries. Today’s charge controllers also have multiple auxiliary features that can be used to do things like engage relays and power up loads when the battery bank reaches meets certain conditions. One of the most common applications is a relay that sends power to a vent fan when the batteries are charging so the buildup of hydrogen gas does not occur.

In stand-alone systems, when the battery bank has been fully recharged, it is the charge controller’s job to isolate the PV array from the battery bank by opening the circuit and stopping all current flow from the PV array. For PV systems that use battery storage as a form of backup to the electric utility, the charge controllers are now manufactured with the ability to “talk” to the inverters connected to the other side of the battery bank and make sure the PV array is able to send as much energy back into the electric utility when possible.

Inverters — The inverters used in PV systems primarily are used to invert DC energy into AC that can be used within the building to run loads or be pushed back onto the grid. The inverters used in stand-alone and utility-interactive battery backup applications are very similar in their appearance and function, whereas the inverters used in grid-direct systems are very different from the battery-based units.

Battery-based inverters are modular units, meaning multiple small units can be connected together to create a single large power source. The most commonly used battery-based inverters are actually inverter/chargers, meaning they have the ability to take the energy from the battery bank to run loads as well as connect to an AC source and recharge the batteries as necessary. A stand-alone inverter will commonly have a traditional generator connected to it to provide battery charging as needed. The generator is not a requirement in these systems, but it gives users greater flexibility in their use of energy.

A utility-interactive, battery-based inverter will connect to the electric utility instead of a generator for its AC source of power, allowing the user to send energy back to the grid and receive credits for that energy when the PV array is able to produce more energy than is being consumed. The major difference between a stand-alone inverter and a battery-based backup inverter is the ability to push current back toward the AC source. In stand-alone systems, pushing current back into the generator can quickly damage the generator and serves no good purpose. In utility interactive systems, pushing current toward the grid is a desirable situation and results in a financial benefit to the system owner.

Grid-direct inverters come in a wide variety of power output sizes — from hundreds of watts (micro-inverters) to kilowatts (string-inverters) to thousands of kilowatts (central inverters). These inverters are simply a way to invert the DC energy produced by the PV array and connect it directly to the utility grid. Multiple grid-direct inverters can be combined to increase the overall power output as required. It is important to remember that these systems offer no form of energy backup, so when the electric utility goes down, the inverter will shut down and not come back online until the grid is back up.

The individual components listed here should give you a good idea of the major components used with most PV systems. Additional components required for a complete installation will be covered in future articles.

Mayfield is a principal with Renewable Energy Associates, Corvallis, Ore. He can be reached at: [email protected].

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