In the previous article, I gave you an introduction to on-grid solar PV systems. In this article, I will give you an introduction to off-grid solar PV systems.
As their name suggests, off-grid systems are systems that are “off the grid” or not connected to the grid.
Compared to the typical on-grid system shown in figure 20.1, the off-grid system shown in figure 22.1 has two additional components:
1. Batteries: Batteries are an integral part of all off-grid systems; without them, off-grid systems simply cannot work. Off-grid systems, by definition, are not connected to the grid. Moreover, solar PV is infirmed, which is to say that it is variable. So then the question is: how do you power your loads reliably? That is where batteries come in. They store energy, act as a buffer, and thus enable the off-grid system to power the loads reliably.
2. Solar charge controller: A solar charge controller acts as an interface between the solar PV modules and the batteries. Its main functions are: 1) to limit the rate at which batteries are charged, 2) to limit the rate at which batteries are discharged, 3) to ensure that the batteries are not over-charged, and 4) to ensure that the batteries are not over-discharged. It is imperative to ensure that batteries are not over-charged or over-discharged as it reduces the battery life drastically.
Typically, a solar charge controller also has a DC output which can used to power DC loads directly, if there are any.
There are also a couple of differences in the off-grid system shown in figure 22.1 with respect to the typical on-grid system shown in figure 20.1.
1. Inverter: The inverter in an off-grid system is an off-grid inverter instead of a grid-tie inverter in an on-grid system. An off-grid inverter need not and does not have the capability to synchronize with the grid. However, it is absolutely crucial that it is “sized” properly. It has to be ensured that it will be able to power the maximum load that it will be subjected to, since the grid isn’t there to back it up in case it falls short.
2. Meters: Since an off-grid system is, well, off-grid, it does not have meters like in the on-grid system. However, it doesn’t hurt to measure the energy generated by an off-grid system since it will provide a good indication of whether it is functioning properly or not.
An off-grid system requires all the BoS components that an on-grid system requires.
Sizing of Off-Grid Systems
It is very important to estimate the loads accurately in an off-grid system to ensure that it is sized properly. And there are no shortcuts to doing this; it has to be done the old-fashioned way.
You have to figure out all the loads that will be part of the off-grid system, and how many hours each one of them will operate.
That will enable you to accurately estimate your total energy requirement.
This will help you size your batteries. The inverter has to be sized by taking the maximum load into account.
Using the “effective sun hours” of the place where the system is to be installed, will help you determine the wattage of the PV modules.
The above methodology implicitly assumes that the charging and discharging will be done on a daily basis. However, that may not be the case. For example, assume that you have a farmhouse which is off the grid and therefore powered by an off-grid system. If you visit this farmhouse only on weekends (Saturdays and Sundays), the situation requires the batteries to be charged in seven days but discharged in two days.
Conversely, consider the case of a solar street light which needs to have 3-day autonomy, which means that it should work three days (or rather three nights) on one day’s charge. This situation is the exact opposite of the farmhouse situation; it requires the batteries to be charged in one day and discharged in three days, in the worst-case where the sun comes out one day and doesn’t the next two days.
So how you design the off-grid system depends on what the requirement is, and it has to be determined very carefully and accurately on a case-to-case basis.
Types of Off-Grid Systems
There are many types of off-grid systems:
1. Solar power packs: These systems can vary in size from small to large. The smaller systems power one or more lights, and maybe a fan or two. The larger systems power entire homes/farmhouses with a few kWs of load. If used to power homes/offices, solar power packs are also called solar home lighting systems.
The output can be DC, in which case the loads would have to be DC as well, like say DC LEDs and DC fans. Mobile chargers, or “power banks” as they are popularly called, are also a good example of solar power packs. In most cases, power banks are charged by an AC supply. But theoretically, they can also be charged by solar PV modules, and would be very handy for people who are on the move and away from the grid.
The output can also be AC, and can therefore be used to power home/office appliances which need an AC supply. And for that matter, the output can also be 3-phase AC and used to power motors in farmhouses.
2. Solar lanterns: These are very popular off-grid systems and have a great utility in rural areas, which is where they are mostly used. However, there’s nothing stopping a person from using these lanterns in urban areas; they can be very handy when there are power outages.
3. Solar street lights: These are also very popular off-grid systems, and have been used in rural as well as urban areas. Solar street lights can be standalone or centralized. A standalone solar street light consists of pole, luminaire, solar PV module, the controller, and the battery. A few years back, these were rather “clunky”, especially the battery, which used to be in a box attached to the pole. (In India, these batteries would get stolen from the box, thus rendering the system useless. And I bet you have seen one of these “useless” solar street lights somewhere.) However, with the advent of compact Li-ion batteries – these power our mobiles as well – the street lighting systems have become very compact too. These days, the controller and the battery are housed inside the weather-proof IP65 enclosure which contains the luminaire. If you didn’t know that this was the case, you could find yourself asking, “Where’s the battery? Where’s the controller? How does this thing work? Who’s controlling it?”
4. Solar pumps: Although these systems can be used for pumping pretty much any other liquid, they commonly refer to solar water pumps. The use case for solar pumps is absolutely perfect. When the sun rises and the solar PV module starts generating enough power, the pump starts operating and pumping water from the ground-well to the surface. In many cases, the water is directly let into the field, which is called “flood irrigation”. However, it is recommended to pump the groundwater and store it in a tank just above the ground. This arrangement incurs some additional cost and adds a couple of metres to the “head” (the height for which the water is pumped), but is well worth the investment since it allows the farmer to control when the water is let into the field; the farmer may not want to water the crops during the day.
5. Solar sign-boards: These are relatively less popular but are a good application of solar PV technology. Conceptually, they are very similar to standalone solar street lights.
These systems are a mix of on-grid and off-grid and systems; they work like on-grid systems when the grid is available, and like off-grid systems when it is unavailable. So batteries are a necessary part of these systems as well, and provide a solid reference when there is a power outage.
The term hybrid is also used to describe other types of systems. For example, the system described above – with a DG backup and solar – is called DG-solar hybrid. Systems that have solar and wind are solar-wind hybrid systems.
Yet another example of hybrid systems is telecom towers, which have a DG set backup without exception. The DG sets are sized to take care of the peak loads. Therefore, most of the times, they operate at sub-optimal loads, which means that the DG set operates at lower efficiencies. So what is done these days in many telecom towers is to run the telecom tower and charge the batteries in parallel. When the batteries are fully charged, the DG set is switched off and the telecom tower is run on batteries. Thus the DG set operates for fewer number of hours at higher efficiency, which is better than the running it 24/7 at a lower efficiency. These kinds of systems are called DG-battery hybrids, even when solar and wind are not part of the system.
I hope this article gave you a good understanding of off-grid systems. This was the last article in the mini-series of articles on solar PV systems. I will start a fresh topic from the next article.
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