Understanding Solar Photovoltaic (PV) Power Generation

Solar photovoltaic (PV) power generation is the process of converting energy from the sun into electricity using solar panels. Solar panels, also called PV panels, are combined into arrays in a PV system. PV systems can also be installed in grid-connected or off-grid (stand-alone) configurations. The basic components of these two configurations of PV systems include solar panels, combiner boxes, inverters, optimizers, and disconnects. Grid-connected PV systems also may include meters, batteries, charge controllers, and battery disconnects. There are several advantages and disadvantages to solar PV power generation (see Table 1).

 

Solar Photovoltaic (PV) Power Generation

Advantages	Disadvantages
•Sunlight is free and readily available in many areas of the country.	•PV systems have a high initial investment.
•PV systems do not produce toxic gas emissions, greenhouse gases, or noise.	•PV systems require large surface areas for electricity generation.
•PV systems do not have moving parts.	•The amount of sunlight can vary.
•PV systems reduce dependence on oil.	•PV systems require excess storage of energy or access to other sources, like the utility grid, when systems cannot provide full capacity.
•PV systems have the ability to generate electricity in remote locations that are not linked to a grid.
•Grid-connected PV systems can reduce electric bills.

Table 1. There are advantages and disadvantages to solar PV power generation.

 

Grid-Connected PV Systems

PV systems are most commonly in the grid-connected configuration because it is easier to design and typically less expensive compared to off-grid PV systems, which rely on batteries. Grid-connected PV systems allow homeowners to consume less power from the grid and supply unused or excess power back to the utility grid (see Figure 2). The application of the system will determine the system configuration and size. For example, residential grid-connected PV systems are rated less than 20 kW, commercial systems are rated from 20 kW to 1MW, and utility energy-storage systems are rated at more than 1MW.

 

Figure 2. A common configuration for a PV system is a grid-connected PV system without battery backup.

 

Off-Grid (Stand-Alone) PV Systems

Off-grid (stand-alone) PV systems use arrays of solar panels to charge banks of rechargeable batteries during the day for use at night when energy from the sun is not available. The reasons for using an off-grid PV system include reduced energy costs and power outages, production of clean energy, and energy independence. Off-grid PV systems include battery banks, inverters, charge controllers, battery disconnects, and optional generators.

 

Solar Panels

Solar panels used in PV systems are assemblies of solar cells, typically composed of silicon and commonly mounted in a rigid flat frame. Solar panels are wired together in series to form strings, and strings of solar panels are wired in parallel to form arrays. Solar panels are rated by the amount of DC that they produce. Solar panels should be inspected periodically to remove dirt, debris, or snow, as well as to check electrical connections.

Since photovoltaics are adversely affected by shade, any shadow can significantly reduce the power output of a solar panel. The performance of a solar panel will vary, but in most cases, guaranteed power output life expectancy is between 10 years and 25 years. Solar panel power output is measured in watts. Power output ratings range from 200 W to 350 W under ideal sunlight and temperature conditions.

 

Solar Arrays Construction and Mounting

When solar arrays are installed on a property, they must be mounted at an angle to best receive sunlight. Typical solar array mounts include roof, freestanding, and directional tracking mounts (see Figure 4). Roof-mounted solar arrays can blend in with the architecture of a dwelling and will save yard space.

 

Figure 4. Typical solar array mounts include roof, freestanding, and directional tracking mounts on the roof or on the ground. Image courtesy of Greensarawak

 

Roof-mounted solar arrays attach to the roof rafters and are engineered to handle the same forces and climate conditions as the rooftop. Composition shingles are considered the easiest roofing on which to mount solar arrays, while slate and tile roofing materials are often considered the most difficult. The main drawback of roof-mounted solar arrays is that they require access for maintenance.

Freestanding solar arrays can be set at heights that allow convenient maintenance. However, freestanding solar arrays usually require a lot of space. Also, freestanding solar arrays should not be mounted on the ground in areas that receive a lot of snow.

Solar array mounts can also be either fixed or tracking. Fixed solar arrays, which are often roof-mounted or freestanding, are preset for height and angle and do not move with the sun. Directional tracking solar arrays move with the sun from east to west and adjust their angle to maintain the maximum exposure as the sun moves. Directional tracking solar arrays can increase the daily energy output of a PV system from 25% to 40%. However, despite the increased power output, directional tracking arrays may not justify the increased cost due to the complexity of the mounting system.

 

PV Combiner Boxes

A PV combiner box receives the output of several solar panel strings and consolidates this output into one main power feed that connects to an inverter. PV combiner boxes are normally installed close to solar panels and before inverters. PV combiner boxes can include overcurrent protection, surge protection, pre-wired fuse holders, and preconfigured connectors for ease of installation to the inverter. The use of pre-wired connectors saves running wires to the inverter. PV combiner boxes should be inspected periodically for leaks or loose connections. 

PV combiner boxes are not required for every PV system installation. For example, when there are only two or three strings of solar panels, a combiner box may not be required. In these cases, the strings of solar panels are connected directly to the inverter.

 

PV Inverters

An inverter is a device that receives DC power and converts it to AC power. PV inverters serve three basic functions: they convert DC power from the PV panels to AC power, they ensure that the AC frequency produced remains at 60 cycles per second, and they minimize voltage fluctuations. The most common PV inverters are micro-inverters, string inverters, and power optimizers (See Figure 5).

 

Figure 5. Microinverters are connected to each solar panel, which are connected in parallel, and convert DC directly to AC. String inverters are used with multiple solar panels connected in series. Power optimizers are installed on each solar panel, which are connected in parallel. Image courtesy of Letsgosolar

 

A microinverter is a device that converts DC power to AC power and is mounted directly to individual solar panels. Because the DC to AC conversion happens at each solar panel, the microinverters maximize the potential output of a system. For example, if one solar panel is shaded by a tree, it will not affect the output of any other solar panels. Microinverters also eliminate the need for potentially hazardous high-voltage DC wiring.

A string inverter is a device that converts DC power to AC power from several solar panels that are connected in series. However, in a series configuration, if one of the solar panels stops producing electricity, even due to temporary shading, it can decrease the performance of the whole system. String inverters are in the high-voltage range (600 V to 1000 V) and are used with large PV systems with no shading concerns. Usually, only one string inverter is needed for a residential application.

A power optimizer (maximizer) is a hybrid microinverter system that conditions the DC power before sending it to a centralized inverter instead of converting the DC power from the solar panels directly into AC power. Power optimizers, like microinverters, still perform well when one or more panels are shaded or when panels are installed facing different directions. Power optimizer systems tend to cost more than string inverter systems but less than microinverter systems.

 

PV Disconnects

Automatic and manual safety disconnects protect the wiring and components of PV systems from power surges and other equipment malfunctions. Disconnects ensure that the PV system can be safely shut down and system components can be removed for maintenance or repair. With grid-connected PV systems, safety disconnects ensure that the generating equipment is isolated from the grid for the safety of utility personnel. A disconnect is needed for each source of power or energy storage device in the PV system. An AC disconnect is typically installed inside the home before the main electrical panel. Utilities commonly require an exterior AC disconnect that is lockable and mounted next to the utility meter so that it is accessible to utility personnel.