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.
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 pr