Off-grid (isolated) photovoltaic system: everything you need to know
Image: Ariel Celeste Photograph – Shutterstock.

One of the many advantages of solar power is that it is everywhere and can be used thanks to an off-grid photovoltaic system.

Whether you live in a big city or in a farm / remote location with no access to the grid, PV technology can be used to generate your electricity.

What differs in each case is the type of system to be installed, Off-grid or On-Grid photovoltaic system.

Difference between photovoltaic systems.

There are two types of photovoltaic systems, depending on their connection to the public electricity distribution network:

1 – Isolated photovoltaic system, also known as an off-grid photovoltaic system.

of them – Grid-connected photovoltaic system, also referred to as on-grid photovoltaic system.

At the beginning, between the 50s and 70s of the last century, off-grid photovoltaic systems were the only ones to exist because their goal was to bring electricity where the distribution networks did not reach.

The great examples of the use of off-grid photovoltaic systems are the artificial satellites (telecommunications, meteorological …) which orbit our planet, and all are electrically powered by solar panels, which are one of the components of the photovoltaic system. installed on the satellite.

Photovoltaic solar panels have been used for terrestrial applications, primarily in telecommunications.

One of the first documented uses of solar panels was to power a “rural telephone” installed in the American city of Americus, Georgia in 1955.

New possibilities.

The concept of on-grid and off-grid photovoltaics only emerged after the use of inverter technology to directly connect photovoltaic solar panels to the grid, without the energy passing through a battery bank; which would eliminate one of the most expensive (and least durable) components of an isolated PV system (off-grid PV system), which is the battery.

It was then that the use of photovoltaic solar energy surged, and since the end of the 90s, mainly with the incentives of governments for the adoption of this technology, the mass production of the main components (modules , inverters, charger controllers, batteries).

Off-grid photovoltaic system
Image: Photograph by Meg Wallace – Shutterstock.

Components of the off-grid photovoltaic system.

Essentially, an off-grid photovoltaic system has the following structure:

1 – Photovoltaic solar panels: responsible for capturing solar radiation and converting it into electricity.

of them – Batteries: responsible for storing converted electrical energy, allowing its use at any time, even at night.

3 – Charge controller: device responsible for managing the battery charge.

4 – Investor: it is the component in charge of transforming the direct current generated by the solar panels and stored in the batteries into alternating current, allowing the use of electrical energy in equipment intended to operate connected to the electrical network. When the equipment operates only on direct current (as is the case with most devices used in telecommunications), it is not necessary to have an inverter in the isolated PV system.

Use of off-grid photovoltaic systems

Today, the use of off-grid photovoltaic systems is mainly to “bring energy to places not connected to the electricity distribution network”, or to keep certain equipment off-grid, in order to continue operating even in the event of a loss. shortage of electricity in the public grid.

Rural houses, well calculating the installation costs; and the costs of powering the grid (including poles, cables, transformer, etc.), it may be preferable to install an off-grid solar PV system.

Urban households who want to be self-sufficient, people who do not want to depend on power companies or fluctuations in electricity prices.

Sizing of the off-grid photovoltaic system

An off-grid photovoltaic system can be designed to supply any type of load, any power value and any amount of energy.

Of course, the more powerful the equipment to be powered and the longer it is used, the more energy it consumes and the larger the off-grid PV system must be to meet your energy needs.

And the more powerful an off-grid system, the greater the investment we will have to make to install it.

Therefore, it is much more common to power small loads, such as lighting, telecommunications, and small household uses.

Lately we are seeing many such systems in stand-alone irrigation systems, a suitable solution for many.

Requirements for sizing the components of an off-grid photovoltaic system

When designing an off-grid photovoltaic system, two main factors should be taken into account: the electrical energy required to power the electrical equipment to be used and the solar radiation available at the site where the system is installed.

It is very common for newbies to solar PV energy to ask, “What can you ‘feed’ with two plates and a battery?”

When, in fact, the question should be: “how many plates and how many batteries will be needed for this requirement?”

The first component to be sized with batteries, and the following factors should be considered:

  1. All the energy to power the energy consuming equipment must be in the batteries; The function of the photovoltaic solar panel is to replace the energy consumed by the batteries (recharge the batteries).
  2. The batteries must provide the energy necessary for the periods of full use of the equipment, i.e. there must be energy to serve all the equipment for a full day . It is obvious that the time of use of the equipment must be checked, as in the case of a set of luminaires, it is necessary to analyze (or estimate) how long they will remain on, considering a 24 hour period.
  3. The batteries must have the capacity to store at least two full periods of operation, that is, they must store energy for two days of operation, even in cloudy or rainy weather.

Calculation of the number of photovoltaic modules

After sizing the battery system of the off-grid photovoltaic system, the necessary set of solar panels is sized, which must be able to replace the energy consumed by the electrical equipment during operation, and provide an “extra” energy, which is able to compensate for any loss and / or to compensate for a possible day with low solar radiation (cloudy or rainy).

The amount of solar panels to be used in the photovoltaic system depends not only on the amount of energy to be generated daily, but also on the availability of the solar resource on the site, i.e. the amount of solar radiation medium in the site. place of installation of the off-grid photovoltaic system.

In places where there is greater amounts of solar radiation (in annual and monthly averages), the photovoltaic system (the number of plates) is less than in places with less solar radiation.

Charge controller

Once the battery system and the number of PV panels required by the off-grid system have been defined, the sizing and selection of the battery charge controller is performed.

This component being responsible for controlling the electrical energy which comes from the solar panels and goes to the battery bank, its sizing is done according to the electrical characteristics of the photovoltaic panels and the set of batteries.

Depending on the power of these two components, the engineer can choose a simpler device (which simply works with the photovoltaic arrangement of the battery bank), or a more sophisticated device, able to manage the voltage levels (voltage). and electric current (amperage) sent to the batteries.

More sophistication means higher cost, but also higher performance.

It is possible to divide the photovoltaic system between several charge controllers and to connect them all to the same bank of batteries. This is common practice, especially in large off-grid PV systems.

When it is necessary to supply machines operating with alternating current, an inverter is used, sized according to the power of the equipment to be supplied.

The power of the inverter must ALWAYS be greater than the power of the loads to be supplied.

But it is possible to divide the equipments between several inverters and to feed them all with the same bank of batteries, which is done in large off-grid photovoltaic systems.