Photovoltaic Systems: All About Grid-Connected PV Systems

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Grid-connected systems are so named because they are connected directly to the electrical grid — the vast network of electric wires that spans the nation and crisscrosses your neighborhood.

The following is an excerpt from Solar Electricity Basics by Dan Chiras (New Society Publishers, 2010). Richly illustrated and clearly written, Solar Electricity Basics is an indispensible primer for homeowners or small business owners looking to tap the power of the sun for electricity. Chiras, an expert in residential renewable energy and a MOTHER EARTH NEWS contributing editor, discusses the theoretical, practical and economic aspects of residential solar installations, including thorough yet easily understandable information about inverters, batteries and controllers, permits, system installation and maintenance, and much more. This excerpt is from Chapter 5, “Solar Electric Systems — What Are Your Options?”

PV systems fall into three categories: (1) grid-connected, (2) grid-connected with battery backup and (3) off-grid. The information here will help you decide whether a grid-connected PV system suits your needs, lifestyle and pocketbook.

Grid-Connected PV Systems

Grid-connected PV systems are the most popular solar electric system on the market today. Grid-connected systems are so named because they are connected directly to the electrical grid — the vast network of electric wires that spans the nation and crisscrosses your neighborhood. These systems are sometimes referred to as “battery-less grid-connected” or “battery-less utility-tied” systems.

grid-connected system consists of five main components: (1) a PV array, (2) an inverter, (3) the main service panel or breaker box, (4) safety disconnects and (5) meters.

To understand how a battery-less grid-connected system works, let’s begin with the PV array. The PV array produces DC electricity. It flows through wires to the inverter, which converts the DC electricity to AC electricity. (For more on AC and DC electricity, see “AC vs. DC Electricity” later in this article.)

The inverter doesn’t just convert the DC electricity to AC; it converts it to grid-compatible AC — that is, 60 cycles per second, 120-volt (or 240-volt) electricity. Because the inverter produces electricity in sync with the grid, inverters in these systems are often referred to as “synchronous” inverters.

The 120-volt or 240-volt AC produced by the inverter flows to the main service panel, aka the breaker box. From there, it flows to active loads (electrical devices that are operating). If the PV system is producing more electricity than is needed to meet these demands — which is often the case on sunny days — the excess automatically flows on to the grid.

As shown in the schematic, surplus electricity travels from the main service panel through the utility’s electric meter, typically mounted on the outside of the house. It then flows through the wires that connect to the utility lines. From here, it travels along the power lines running by your home or business, where it is consumed in neighboring homes and businesses. After the electricity is fed to the grid, the utility treats it as if it were its own. End users pay the utility directly for the electricity you generate.

In most locations, an electric meter monitors the contribution of small-scale producers to the grid. The meter also keeps track of electricity the utility supplies to these homes or businesses when their PV systems aren’t producing enough to meet their demands, or when the PV system is not operating (for example, at night).

In addition to the utility electric meter (or meters — some utilities require two or more) that monitors the flow of electricity to and from the local utility grid, grid-connected solar electric systems also contain two safety disconnects. Safety disconnects are manually operated switches that enable service personnel to disconnect key points in the system to prevent electrical shock when servicing the system.

As shown in the schematic, the first disconnect is located between the solar array and the inverter. This is a DC disconnect. The manual disconnect allows the operator to terminate the flow of DC electricity from the array to the inverter in case the inverter needs to be serviced. These systems also require an AC disconnect switch. Shown in the schematic, this disconnect must be mounted outside the home or business. It must be readily accessible to utility workers, and it must contain a switch that can be locked in the open position by utility workers so no electricity flows to or from the grid. This disconnect is required so workers can isolate PV systems from the electrical grid and work on electrical lines without fear of shock if, for example, a line in your area goes down in an ice storm.

For many years, lockable AC disconnects were considered critical for the safety of utility personnel. Although utility-company-accessible, lockable, visible AC disconnects are required by many utilities, large California utilities with thousands of solar and wind electric systems now online and Colorado’s main electric utility have dropped this requirement. They’ve found that AC disconnects are not needed because grid-compatible (synchronous) inverters automatically shut off when the utility power goes down. Properly installed PV systems will not back feed to a dead grid. Period.

The Pros and Cons of Grid-Connected PV Systems

Battery-less grid-connected systems represent the majority of all new solar electric systems in the United States. They’re the least expensive of all systems and require the least maintenance, primarily because they contain no batteries. In essence, the electrical grid becomes your battery bank. Although popular, they do have some disadvantages, summarized on this chart. You may want to take a moment to study them.

The biggest downside of battery-less grid-connected PV systems is that they are vulnerable to grid failure. That is, when the grid goes down, so does the PV system. A home or business cannot use the output of a battery-less photovoltaic system when the grid is not operational. Even if the sun is shining, battery-less grid-tied PV systems shut down if the grid experiences a problem — for instance, if a line breaks in an ice storm or lightning strikes a transformer 2 miles from your home or business, resulting in a power outage. Even though the sun is shining, you’ll get no power from your system.

If power outages are a recurring problem in your area and you want to avoid service disruptions, you may want to consider installing an uninterruptible power supply (UPS) on critical equipment such as computers or medical equipment. A UPS has a battery pack and an inverter. If the utility power goes out, the UPS will supply power until its battery gets low. Or, you may want to consider installing a standby generator that switches on automatically if grid power goes down.

You may also want to consider installing a grid-connected system with battery backup. In this case, batteries provide backup power to a home or business if the grid goes down.

AC vs. DC Electricity

Electricity comes in two basic forms: direct current and alternating current. Direct current electricity consists of electrons that flow only in one direction through an electrical circuit. It’s the kind of electricity produced by a flashlight battery or the batteries in portable devices like cell phones and laptop computers. It’s also the kind of electricity produced by photovoltaic modules.

Most other sources of electricity — including wind turbines and conventional power plants — produce alternating current electricity. Like DC electricity, AC electricity consists of the flow of electrons through a circuit. However, in alternating current, the electrons flow back and forth. That is, they change (alternate) direction in very rapid succession, hence the name. Each change in the direction of flow (from left to right and back again) is called a “cycle.”

In North America, electric utilities produce electricity that cycles back and forth 60 times per second. It’s referred to as 60-cycle-per-second — or 60 Hertz (Hz) — AC. In Asia and Europe, the utilities produce 50-cycle-per-second AC.

Reprinted with permission from Solar Electricity Basics, published by New Society Publishers, 2010.