Solar PV system design | Centre for Science and Environment

Solar PV system design

A system based on a 20Wp module can supply two or three 6 W lamps for about four hours per day. At the other end of the range, an 80 Wp system can power four 8 W lamps and a black and white television set.

Components of a solar PV system

There are three basic configurations widely adopted for the solar PV systems:

  • The PV array supply power to the electrical load directly. This is the simplest configuration. No storage is involved and the system functions only when sunlight is available. Costs of such a system are comparatively low.

  • The most common configuration is a PV array that powers the load and charges a storage battery, allowing electricity to be used at night and during periods of low insolation.

  • The PV-hybrid system relies on an auxiliary source of power e.g., a fossil fuel generator to complement the power generation from the PV array. This configuration still requires some battery storage to avoid short-term fluctuations. It is particularly suitable for applications that are critical or in regions with large variations in sunlight conditions through the year.

A PV system is made up of different components. These include PV modules (groups of PV cells), which are commonly called PV panels; one or more batteries; a charge regulator or controller for a stand-alone system; an inverter for a utility-grid-connected system and when alternating current (AV) rather than direct current (DC) is required; wiring; and mounting hardware or a framework.

Solar panels or the Photovoltaic array
The basic building blocks of a PV system-consist of semiconductor materials. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms. This phenomenon is called the "photoelectric effect". These free electrons then travel into a circuit built into the solar cell to form electrical current.

Do not assume that a PV panel rated at 170 watts of power will actually give you that amount. It will probably be closer to 150 watts per panel. Because of the difference, care must be used when sizing the system for your electrical needs. Otherwise, you may find that you are generating less power than you need.

Photovoltaic panels are rated by their theoretical power output in watts. Theoretically, a square meter of sunlight generates 1,000 watts of energy. Due to the inefficiency of current PV solar panel technology, the typical PV module output will be about 18% of that. To make things more confusing, the manufacturers rated output of a module will not be what you actually get.

Charge Controller
The function of the charge controller, or control, is that of a voltage regulator, to prevent overcharging or overtaxing of the storage batteries. A photovoltaic control will sense when the batteries are fully charged and reduce or eliminate additional energy being directed to the batteries. Similarly, it will sense when the batteries are close to being depleted, and prevent further current discharge.

The main function of the inverter is to convert the electricity into a form that can be used by household lights and appliances.

Storage Batteries
As photovoltaic cells only produce electricity when they are illuminated, there is an option of using that electricity directly during daytime. However, if you want to store energy for use overnight or on overcast days some sort of battery is required.

Additional components
In addition to these some more equipment is required to be installed with the system.

Safety equipment
Safety features protect stand-alone small renewable energy systems from being damaged or harming people.

Here are the major safety features your system will need:

Safety disconnects
Automatic and manual safety disconnects protect the wiring and components of your small renewable energy system from power surges and other equipment malfunctions. They also ensure that your system can be shut down safely for maintenance and repair.

Grounding equipment

This equipment provides a well-defined, low-resistance path from your system to the ground to protect your system against current surges from lightening strikes or equipment malfunctions. You will have to ground your photovoltaic unit itself and your balance-of-system equipment. Be sure to include any exposed metal (such as equipment boxes) that might be touched by you or a service provider.

Surge protection
These devices also help protect your system in the event that it is struck by lightening.

System Meter
System meters measure and display several different aspects of your solar-electric system’s performance and status, tracking how full your battery bank is; how much electricity your solar panels are producing or have produced; and how much electricity is in use. Operating your solar-electric system without metering is like running your car without any gauges, although possible to do, it’s always better to know how much fuel is in the tank.

A local electrician or your installer should be able to provide you with more information on the safety features required for your particular situation.

Other design parameters to consider

How much electricity will a PV system produce?

It ranges from 4 to 50 kilowatts. On average, a 10-kilowatt PV system will generate approximately 16,500 kilowatt-hours of electricity per year, or a little more than the total amount of electricity used in a typical home.

Will the PV systems produce electricity all the time?

No. Depending on the season, they will ordinarily generate power from 8 am to 6 pm, reaching maximum output between noon and 1 pm.

Can PV systems produce power on cloudy days?

Yes. PV modules generate electricity when the weather is cloudy, although their output is diminished. On a dark, overcast day, a PV system might receive only 5 to 10 percent of the usual amount of sunlight it needs, so power output would decrease proportionately.

Do PV systems work well in the cold?

PV modules actually generate more power at lower temperatures. The reason is that like most other electronic devices, they operate more efficiently when it's cooler. PV systems generate less energy in the winter than in the summer, but this is due to the combination of fewer daylight hours and lower sun angles, not to cooler temperatures.

How long do PV modules last?

PV modules are designed to withstand any potential rigors of the environment, including arctic cold, desert heat, tropical humidity, winds of more than 125 miles per hour, and one-inch hail at terminal velocity. In spite of this very durable construction, though, the glass may break under an extremely strong impact.

A PV system that is designed, installed, and maintained well will operate for more than 20 years. The basic PV module (interconnected, enclosed panel of PV cells) has no moving parts and can last more than 30 years. The best way to ensure and extend the life and effectiveness of your PV system is by having it installed and maintained properly.

Experience has shown that most problems occur because of poor or sloppy system installation. Failed connections, insufficient wire size, components not rated for dc application, and so on, are the main culprits. The next most common cause of problems is the failure of the electronic parts in the balance of systems (BOS): the controller, inverter, and protection components. Batteries fail quickly if they're used outside their operating specification. For most applications (uses), batteries should be fully recharged shortly after use. In many PV systems, batteries are discharged and recharged slowly, perhaps over a period of days or weeks. Some batteries quickly fail under these conditions. Be sure the batteries specified for your system are appropriate for the application.

Site selection

A well-designed PV system needs clear and unobstructed access to the sun's rays for most or all of the day, throughout the year.

Usually, the best location for a PV system is a south-facing roof, but roofs that face east or west may also be acceptable. Flat roofs also work well for solar electric systems, because PV modules can be mounted flat on the roof facing the sky or bolted on frames tilted toward the south at an optimal angle.

If a rooftop can't be used, your solar modules can also be placed on the ground, either on a fixed mount or a “tracking” mount that follows the sun to orient the PV modules.

Space required for a PV system

The amount of space that a PV system needs depends on the size of the system you purchase. Some residential systems require as little as 50 square feet (for a small ‘starter’
system), but others could need as much as 1,000 square feet.

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