Off-Grid Solar System

Introduction


All solar power systems work on the same basic principles. Solar panels first convert solar energy or sunlight into DC power using what is known as the photovoltaic (PV) effect. An off-grid solar energy system is not connected to the utility grid, whereas an on-grid solar energy system is connected to the utility grid. Your choice of an off-grid system or on-grid system will determine your access to electricity, what equipment is needed for excess production, what happens when the grid goes down, and how you’re billed for electricity. In this article, we will be looking into the main components used in off-grid systems.

Figure 1 : Off-Grid Solar System Connection

Off-Grid system types - AC or DC coupled


Off-grid systems are built using either AC or DC coupled power sources. AC-coupled generation sources include common solar inverters while DC-coupled sources include MPPT solar charge controllers. Whether a system is AC or DC coupled is generally based on the size of the system. Most small-scale systems are DC coupled and use efficient MPPT solar charge controllers. Larger off-grid systems can be either AC or DC coupled depending on the type of off-grid inverter-charger used, and compatibility with different solar inverters (AC) or solar charge controllers (DC).

Components of Off-Grid Solar Systems:

Off-Grid Battery Banks


Off-grid batteries should charge efficiently so as not to waste valuable energy generated by the solar array or a diesel generator. Off-grid batteries need high power capability to support the high surge draw, discharge and charge current of off-grid inverter chargers. Access to off-grid sites is not always easy, so a maintenance-free battery is highly desirable. Temperature changes must be considered as this will affect the lifetime of the battery.

Figure 2: Battery Bank

State of Charge (SoC) is the level of charge of an electric battery relative to its capacity. The units of SoC are percentage points 0% = empty; 100% = full. SoC is normally used when discussing the current state of a battery in use. Off-grid solar battery systems will be deeply cycled and regularly operated in a partial state of charge (PSOC) condition. So, SoC monitoring is critical for off-grid users.

Lead Acid Battery vs. lithium-Ion Battery in Off-Grid


This all boils down to the number of cycles a battery has and its depth of discharge, how many times the battery can be drained, and how much power can be used. Lead-acid batteries degrade more with every cycle. Where a lithium battery may come with a 10,000-cycle guarantee. Lead-acid batteries are lower in cost for the same voltage and capacity but do not last for many cycles whereas, for lithium-ion batteries with higher discharge cycles, the initial cost is low. Despite having higher upfront costs, lithium-ion batteries are usually more valuable than lead-acid options. lead-acid batteries may be the better decision is in a scenario with an off-grid solar installation that isn’t used very frequently due to lower usage rates a lead-acid battery would be a good solution instead of lithium-ion batteries.

Solar Charge Controller


It regulates the charging of batteries and prevents them from over-charging and further damage. Simple charge controllers stop charging a battery when they exceed a set high voltage level and re-enable charging when battery voltage drops back below that level. Charge controllers may also monitor battery temperature to prevent overheating. Some charge controller systems also display data, transmit data to remote displays over time.

Figure 3: A solar Charge Controller

The most common DC-coupled systems use solar charge controllers (also known as solar regulators) to charge a battery directly from solar, plus a battery inverter to supply AC power to the household appliances. For microsystems, such as those used in caravans/boats or huts, the simple PWM (Pulse Width Modulation) type solar controllers are a very low-cost way to connect 1 or 2 solar panels to charge a 12-volt battery. For larger systems, MPPT (Maximum Power Point Tracking) solar charge controllers are used. Unlike the simple PWM controllers, MPPT systems can operate at much higher string voltages.

Off-Grid Solar Inverters


Grid-tied inverters are simpler and easier to wire since there are usually only two main components but an off-grid inverter needs a battery bank to function. In the case of an off-grid system, the solar panels feed DC power to the batteries. Then the inverter takes that power and converts it into AC power for your home. This works essentially like a miniature power grid. One main difference between off-grid solar inverts and on-grid is they don’t have to match the frequency of the utility power grid compared to a grid-tied inverter.

Hybrid inverters


A hybrid inverter, otherwise known as a battery-based inverter, combines two separate components–a solar inverter and a battery inverter–into a single piece of equipment. it can function as both an inverter for electricity from your solar panels and a solar battery. One of the biggest benefits of a hybrid inverter is that it combines the functionality of two separate pieces of equipment into one. This can mean an easier installation process for your solar installer. Other than this a hybrid inverter allows for centralized monitoring of both battery and solar panels.

AC and DC Disconnects


The purpose of these disconnects is to make sure you can shut off incoming power from the solar panels. DC disconnects are places between solar panels and inverters. AC disconnects are placed after the inverter.
Figure 4: AC and DC Disconnect Switch

AC disconnects are typically mounted on the exterior wall of the customer’s home near the electric meter. The necessity of these disconnects arises from the fact that in case of an emergency like fire or extreme weather conditions to protect the installations from getting damaged and even during maintenance, it is required to shut off the power completely for safety purposes. AC/DC disconnects are just one piece of the BOS (balance of system) components you’ll need for a successful solar installation.

Off-grid systems are more complicated, thanks to additional components like the charge controller, battery monitor, and additional AC and DC circuit breakers. All of these things tend to make off-grid systems more difficult to wire and install.

AFSHIN BAKHTIARI

AE Solar Senior Engineer



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