This document gives an introduction to off-grid solar PV systems. It talks about the components used in an off-grid solar PV system and the advantages/disadvantages of an off-grid solar PV system.
This document also briefly describes the various types of off-grid solar PV systems that are popular and in use today.
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An Introduction to Off-Grid Solar PV Systems
An off-grid solar PV system is a solar PV system that, as its name suggests, works off the grid. So
in other words, it is not connected to the grid.
The main use of off-grid solar PV systems is in places where the grid isn’t available like the rural
parts of India. They are also very useful in applications like street lights, street signages, traffic lights
where the grid might be close by but it is more convenient to have a standalone or off-grid solar PV
system.
The figure above shows what an off-grid solar PV system looks like.
Solar PV modules generate DC electricity.
The solar charge controller takes the DC electricity generated by the solar PV modules
and stores it in the batteries. Typically, the solar charge controllers also do one other job
which is called Maximum Power Point Tracking, or MPPT in short. More information on
maximum power point tracking can be obtained in the solar charge controller FAQs
section.
The batteries convert the electricity (or electric energy) into chemical energy and store it
in them. Normally, batteries used along with inverters are called into action (or come
into play) only when there is grid failure. However, in case of off-grid solar PV
applications, these batteries are called into action every single day. Also, they are
discharged quite a bit, by as much as 80%, which is the maximum that you can discharge
them without impacting their lifetime adversely. This is done so that the batteries are in
a fully discharged condition when the sun rises again so that all the electricity generated
by the solar PV modules can be stored in them again.
That is why the batteries used in solar applications are deep discharge batteries, or
batteries that can withstand daily deep discharges.
The inverter converts the chemical energy stored in the batteries into electricity (or
electric energy). The input to the inverter (energy stored in the batteries) is DC whereas
its output is AC. This off-grid solar inverter is very much like a normal inverter, but is
called a solar inverter because it is used in a solar application.
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The loads are household appliances like fluorescent lamp, tube light, fan, refrigerator,
TV, computer, pumps, among others.
The main advantage of an off-grid solar PV system is that it can provide electricity to remote
places where the grid is not available, especially in a sun-rich country like India where most places
get good sunshine for 8 to 10 hours a day and for at least 300 days in a year.
The main disadvantages of off-grid solar PV systems are:
Cost: Off-grid solar PV systems are quite costly. Solar PV modules are quite costly and
contribute a god percentage of the overall system cost. Additionally, off-grid solar PV
systems also require batteries, and that too deep cycle batteries, which are quite costly
too. That increases the system cost as well.
Maintenance: Since off-grid solar PV systems need batteries, it automatically means that
they need to be maintained properly. This mainly involves adding distilled water to the
batteries (or “topping up” as it is popularly called) if they are of the flooded lead acid
type, which they are in many cases. Although flooded lead acid batteries have advanced
quite a bit in the recent years and don’t require frequent topping up, it still needs to be
done and can be cumbersome if the place of installation is not easily reachable.
Recurring expenses: Off-grid solar PV systems need batteries, and even if you maintain
them well, they will wear out in anywhere from 300 to 1000 cycles. (In most cases, these
are typical cycle numbers. Batteries that last longer are prohibitively expensive in which
case it is what the first disadvantage talks about.) So off-grid solar PV systems
necessarily have recurring expenses associated with them.
There are many types of off-grid solar PV systems:
1. Solar power packs: They can vary from small to large size. The small ones have only a DC
output and are used to charge mobile phones and run other DC applications. The larger
ones have an inverter as well and are used to AC appliances.
2. Solar home lighting systems: These can vary from small to large sizes. The small ones
can be used to power 1 lamp while the larger ones can be used to power multiple lamps.
Typically, solar home lighting systems have only a DC output. So the lights that they
power should necessarily be DC powered.
3. Solar lanterns: These can have the solar PV module and battery integrated into the
product, or have the battery integrated into the product but have a separate solar PV
module in case of higher capacity lanterns. Typically, solar lanterns allow the users to
select the luminosity level; lanterns run for fewer hours on higher luminosity levels and
for longer hours on lower luminosity levels. Solar lanterns are always DC powered.
4. Solar street lights: Solar street lights comprise of pole, luminaire, solar PV module, solar
charge controller, and battery. The luminaire is always DC powered. It almost always has
an IP65 environmental rating since it is exposed to the elements. In many cases, the
charge controller is embedded inside it. In some cases, even the battery is embedded
inside it. If that is the case, then the batteries are invariably Li-ion batteries since they
have to be compact.
5. Solar signages (or signboards): Solar signages or signboards are very much like solar
street lights and have the same components as compared to solar street lights.
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6. Solar pumps: Solar pumps have solar PV modules and a three-phase inverter which
drives the pumps. Solar pumps do not have batteries since there is no need to store the
energy. Whenever the solar PV modules produce electricity, it is used to pump water
from the well to the ground level (actually in storage tanks that are slightly above the
ground level). And since solar pumping systems do not have batteries, they don’t have
solar charge controllers as well.