Types of Photovoltaic Solar Electric Systems

When you're thinking of a photovoltaic solar electric system of your own, whether you're having it installed or intend to build it yourself, it's important to know what types are available and how each may, or may not, fit your needs.

What's it Called?

The phrase "photovoltaic solar electric system" is perhaps the most descriptive name given to this technology, but it is often referenced by alternate names such as, "solar photovoltaic electric system," "photovoltaic electric system," "solar electric system," and "photovoltaic system".  They all mean the same thing.  For simplicity, I'll refer to it as yet an even more abbreviated name, a "PV system."

The Common Denominator

There are many PV system configurations possible, but most have two important things in common:

1. They use photovoltaic solar cells (usually arranged in a panel of many cells) to produce DC (direct current) electricity.
2. They convert that DC electricity to the AC (alternating current) electricity that your house and all it's appliances are designed to use.

Beyond that, PV systems have many variations in solar cells, solar panel types, array configurations, wiring, capacities, and how and when power is stored and/or converted to the needed form.  Ideally, each system is customized to the exact needs of it's owner, it's users, and the environmental conditions involved.

Two Trends in Fixed PV Systems

For fixed PV systems (those that are non-portable; permanent installations), two classifications are common: Off-Grid Systems and Grid-Tied Systems.  Each have characteristic advantages and disadvantages that must be weighed when considering a PV system, but the latter is becoming more popular for economic reasons.

Off-Grid PV Systems

When PV systems are mentioned, this is often the type of system people think of.  It reflects the intent of early PV systems and is regarded as the ultimate goal by many PV system dreamers; to generate all their power locally, completely disconnecting from "the grid" (the power company).

Off-Grid PV System Configuration

An off-grid PV system consists of four major components:

1. Solar panel array - generates DC electricity when exposed to solar radiation
2. Charge controller - safely charges the battery array and prevents reverse-current flow back to solar panel array at night
3. Battery array - stores the produced energy until needed
4. Inverter - converts DC from battery array to AC (typically 240 volts) for household electrical system

The last component, the inverter, connects to your existing house mains (which is first disconnected from the power company's supply).  Optionally, the inverter can sometimes be used as an alternate charging system for the battery array by attaching a fuel-powered generator during excessively cloudy times.

Advantages

• Clean energy production
• No grid connection; no electricity bill, no required coordination with power company
• Provides power even during grid-based blackouts, day or night
• Initial cost may be lower than a new grid connection if your house is in a remote location

Disadvantages

• Expensive to build; requires the most components
• Costly to maintain; batteries eventually fail and need replacing
• Must be over-sized to provide all required power, day or night, all year around
• Once the batteries are drained, there's no more power (at night or with extended cloudy conditions)
• If batteries reach a full charge, any extra power produced that day is thrown away
• Sizable real-estate needed (floor or wall space) for adequate battery array
• May need a generator to provide supplemental power during lowest sun exposure times of the year

Historically, off-grid PV systems comprised the bulk of all new PV system installations; around 98% before the year 2000.  However, due to their initial cost, maintenance expenses, and size requirements, the volume of these installations has always been quite low.

Additionally, taking conventional-built homes off the grid isn't usually practical since they are not built to high energy efficient standards, and are furnished with power-hungry appliances.  Installing a sufficient off-grid system to an existing house means performing a whole-house makeover simultaneously.  This makes off-grid PV systems most attractive to homeowners and builders in remote locations where access to grid-based power is difficult, expensive, or impossible.

Though off-grid PV systems continue to be installed, a new type of system has emerged, surpassing them in both volume cost effectiveness: the grid-tied PV system.

Grid-Tied PV Systems

This is an alternative to the traditional off-grid PV system, and one that has become quite attractive in recent years.  This system replaces the costly battery array and charge controller of the off-grid PV system with the grid.  That's right, the power storage mechanism in grid-tied systems is the actual power company's grid.

Grid-Tied PV System Configuration

A grid-tied PV system consists of just two major components:

1. Solar panel array - generates DC electricity when exposed to solar radiation
2. Grid-Tie Inverter - converts DC from solar panel array to AC (typically 240 volts) for household electrical system and the grid

As indicated above, when compared to the off-grid PV system, the grid-tied PV system uses the same solar panel array, but removes the charge controller and battery array; the two most costly components over the life of the traditional system. Also, the inverter is changed to a special "grid-tie inverter," that connects to your existing house mains while leaving your house's connection to the power company intact.

Advantages

• Clean energy production
• Lower cost and easier to build (compared to off-grid); requires fewer components
• No cost to maintain; fewer components to fail, none needing regular replacement
• No need to over-size the system; grid will supply extra power as needed, day or night, all year around
• Less real-estate needed for components
• Highest energy production efficiency; no battery-related losses and all power produced is available for use
• Net-metered grid power; pay for grid power used / get credit for power produced

Disadvantages

• Requires grid connection 
• No power during a grid blackout; for safety reasons
• Requires coordination and cooperation with the power company 
• Power during the night, or the lowest sun exposure times, comes from the grid

The volume of new grid-tied PV system installations swiftly overcame that of off-grid PV systems around 2005; over 90% are grid-tied PV systems as of 2012.

Grid-tied PV systems are great for conventional-built homes, allowing the owner to invest in long-term savings now, witness the difference in their electric bill quickly, and ease into reducing their household's power waste over time.  Since most homes experience very infrequent and short-term grid blackouts, the lack of local power storage is rarely an issue and the savings with such a configuration is more readily apparent.

The Cost Factor

The cost factor of a PV system can be a real inhibitor.  For many, it prevents exploring the possibilities and ultimately installing a system. For others, it limits the size (maximum power capability) of the system they choose to install.

But it may not be as big an obstacle as you might think.  As governments realize the long-term benefits of having a distributed power grid formed by many small and interconnected PV systems, more financial opportunities become available to entice installations.  For example, up until the year 2016, the United States Federal Government awards new PV system installations with a tax break equal to 30% of the system cost, including labor.  Also, some states offer additional rebates for systems depending on their configuration and the amount of load they will remove from the grid.

In a future post, I will discuss the costs in more detail, how rebates are awarded, and use my own installation as an example.