ac and dc coupling in solar battery system

What is AC and DC Coupling in Solar Battery System | Detailed Guide 2024

The Function of the Solar Battery System is to store the rendered energy from the sun using solar panels. Ever since the evolution of solar batteries, they have been playing a vital role in future use. The concept of energy storage requires a solar battery system that can be categorized into 2 types: AC and DC coupling.

The residential installations are expected to grow by 30%, as the prices would become much more affordable and an increase in energy independence benefits can be observed

One such concept that comes into major play is the types of solar battery systems. Often people get confused in understanding off-grid solar battery systems like DC-coupled systems and AC-coupled systems. While others find it hard to understand Grid-tie battery systems like AC-coupled and DC-coupled Hybrid Systems.

Also, at certain points, people are confused about which type of off-grid system should they choose, whether AC-coupled or DC-coupled.

By the year 2024, the global installation market is said to mature by surpassing 100GWh and marking an important milestone. Also, the CAGR is expected to remain above 20% for the global installations.

Furthermore, It would be intriguing to delve into the comparison of AC and DC coupled off-grid systems. These batteries ease variations in the solar energy flows which influences the change in the amount of sunlight that shines on the photovoltaic (PV) panels.

What is AC and DC Coupling?

The AC Coupling allows the transfer of electrical energy by connecting the solar panels and the batteries to the electricity grid. Whereas, DC Coupling allows the straight flow of energy to the batteries without any unnecessary conversions.

  • AC (Alternating Current) is the flow of current at a fast speed.
  • DC (Direct Current) is the flow of current in a single path.

For instance, guitar amplifiers use AC coupling to separate the signal from strings, leading to the cancellation of unwanted noises.
The DC electricity stored in the battery is converted into AC for supplying power to your houses. Moreover, this inverter charges the solar battery directly with the DC electricity. For example, small-scale systems such as caravans, power banks, robot arms, mobile phones, and many more…

Evolution of Solar batteries:-

Earlier before the evolution of solar batteries, in 250 BC, the Baghdad batteries were the most early demonstration to capture electrical energy, which was a meaningful stepping stone towards battery technology.

Several centuries later, in 1800, Alessandro Volta’s invention of the first battery changed our entire lives. An alternative combination of copper and zinc discs was applied to the basement for the modern batteries.

Multiple improvements have paved the route to develop more safer, efficient storage and cost-effective batteries.

In 1859, Frenchman Gaston Planté, a French scientist invented the first rechargeable lead acid battery.

The first silicon solar battery was developed in 1954 by three researchers at Bell Labs namely Daryl Chapin, Calvin Fuller, and Gerald Pearson. This invention with a 6% efficiency, was capable of converting the sun’s radiation into electrical energy, thereby cementing the foundation for solar battery systems.

Furthermore, the rise of lithium-ion batteries in the 1970s offered a superior performance with their high density compared to their predecessors. In 1985, the prototype of a Lithium-ion solar battery was developed by Akira Yoshino.

The advancement of Lithium-ion batteries for solar applications powered scientific collaborations and uncompromising innovations.

4 Types of Solar Battery Systems:-

Here are the most important 4 types of solar battery systems for off-grid power systems that will help you choose the best ones according to your requirements.

1. DC Coupled Systems: Off-Grid

The DC-coupled systems have been in use for many decades in off-grid solar installations. They provide a direct and secured connection to solar panels, batteries, and solar charge controllers or Solar regulators using Direct Current (DC). A battery inverter is used in this system to supply AC power to household appliances.

For instance, Micro Systems like caravans, boats and RVs, Streetlights, and electric vehicles, require 1 or 2 panels for charging a 12-volt capacity battery connected to PWM-type solar controllers.

Beyond tiny homes, Larger Systems like industrial and remote areas require 3 solar panels connected sequentially for above 3 kW.

MPPT solar charge controllers, for larger systems, are available in sizes up to 200 A with an efficiency of more than 30% and typically operate at a higher string voltage of up to 450 Volts DC.

Compared to grid-tie solar string inverters, the voltage is low between 200 to 800 volts DC. However, the installation of solar panels might increase the complexity which can result in the combining strings of panels parallelly with the fuses.

ADVANTAGES:

  • Chargeable with a higher efficiency of 99% using MPPT.
  • Omits the energy-draining AC conversion process.
  • Enhanced power harvested by solar panels.
  • Additional panels and controllers can be added.
  • Reasonable price.
  • Easy-to-install and maintain setup.
  • Ideal for generating DC appliances and loads.

DISADVANTAGES:

  • Enhanced Complexity setting up larger systems that require multiple strings in parallel.
  • Appliance Adaptability is complex.
  • It is not DC-friendly and you might require DC to AC inverters for specific devices like toasters or coffee makers.
  • Optimal performance cannot be expected as some solar charge controllers.
  • It cannot be compatible with managed lithium battery systems like LG Chem RESU or BYD B-Box.

2. AC Coupled Systems: Off-Grid

The larger-scale off-grid systems most often use the AC Coupled system. With an efficiency of 90 to 94%, they are slightly less than the DC-coupled systems (98%) but more effective at intensifying high AC loads during the day.

Furthermore, it can be expanded with numerous solar inverters to construct microgrids.

AC Coupled Systems can be used in various off-grid applications such as electric vehicles, water pumping and sanitation, agricultural operations, and research stations.

ADVANTAGES:

  • Cost Effective
  • Eliminating the need for off-grid inverters.
  • Lower Installation cost for larger systems above 6kW.
  • Appliance Compatibility.
  • Has dual MMPT inputs for 3kW.
  • Multiple solar inverters can be used in diverse locations.
  • Allows additional combinations with solar panels, batteries, or inverters.

DISADVANTAGES:

  • A low efficiency is observed when charging a battery system- 86 to 92% approximately.
  • Some batteries may not be compatible and do not meet the requirements of AC AC-coupled system.
  • Despite being cost-effective for larger systems, an expensive solar inverter with premium quality is required.
  • The conversion from AC to DC and vice-versa results in a slight efficiency loss.
  • Makes the solar energy inverted at least three times before being operated for home appliances.

3. AC Coupled Battery System: Grid Connected

The AC-coupled battery system is specifically designed for houses provided with a grid connection. The solar panels capture the sunlight energy and convert it into DC electricity.
It consists of a Lithium Battery, a battery management system to store excess solar energy, and a grid-tie inverter that converts Direct current to Alternating current.

A few examples of AC Coupled Battery systems include Tesla Powerwall 2, SonnenBatterie, and Enphase Energy.

Also, its longevity and safety are ensured with the help of a charge controller that regulates the charging current to the battery.

ADVANTAGES:

  • Enhanced Solar energy utilization.
  • Excess solar energy can be stored.
  • Makes less reliable on grid systems.
  • Simple Installation.
  • Provides additional charging options for your batteries.
  • Grid Stabilization during peak demand.
  • Secured, Strong, and Stable grid by Injecting solar energy.

DISADVANTAGES:

  • Doesn’t provide a backup supply.
  • Shorter life span
  • Insufficient capacity.
  • Lower efficiency rate.
  • Installation consultancy is required for the complex design.

4. DC Coupled Hybrid Systems: Grid Connected

The working procedure of a DC Coupled Hybrid system connected to a grid involves capturing sunlight and directly converting it into DC.

Further, the DC output of the solar panels is optimized with the MPPT charge controller for efficient charging.

A Grid-Tie or Hybrid inverter converts the DC electricity from the solar panels into AC electricity, that can be compatible with grid and home appliances.

The main purpose of the battery bank is to store the excess solar energy. Especially, modern Hybrid inverters integrate a high voltage MPPT controller and battery inverter into a single device.

Depending on the availability and system demand, the transfer switch automatically swaps between the grid, battery, and solar power sources.

DC-coupled systems are versatile by extending beyond the basic purpose of supplying power to lights and fans.

ADVANTAGES:

  • Higher Efficiency.
  • DC-coupled systems reduce energy.
  • Eliminates the requirement of double conversion.
  • High-voltage batteries with small-sized cables reduce the failures.
  • Highly compatible with the increased number of lithium-ion batteries.
  • Easy to Install and Simple to Handle.
  • Includes additional solar panels or batteries.
  • Amplifies the solar energy.
  • Cost Efficient than AC Coupling due to the MPPT controllers.
  • Compact battery options.

DISADVANTAGES:

  • Lack of Backup
  • Failed backup power supply of DC DC-coupled system.
  • Delayed backup of 3 to 5 seconds during blackout.
  • Limited Grid-Tie inverter options
  • Low surge rating due to the transformerless inverters.
  • The lack of generator controls.
  • Grid tie inverters are incompatible with the DC-coupled system.
  • High Safety Risks.
  • Improper installation can result in high safety risk.

AC vs DC Coupled: Off-Grid Systems

The interesting debate of AC vs DC Coupled for an off-grid system makes it difficult for any individual to make the best choice. Comparing 2 different scenarios makes it easier to proceed with the best choice.

SCENARIO 1: Why Should you prefer AC-coupled rather than DC-coupled for an off-grid system?

AC coupled for an off-grid system requires a few components for installation. Especially if you have an inverter for your existing solar system, the investment cost is further minimized. Notably, the installation cost for the larger system above 6kW and above is lower because:

  1. Numerous MPP trackers
  2. Considerable string voltage up to 1000 volts
  3. Greater spans up to 10kWp in a single phase

A simple installation can be appealing to provide a plug-and-play situation for off-grid needs.

Mainly, powering high loads such as ACs, Fridges, and water pumps proves the AC-coupled system is much more efficient than the DC-coupled.

SCENARIO 2: Why Should you prefer DC-coupled rather than AC-coupled for an off-grid system?

In an off-grid system, the minimum losses in the double conversion system from AC to DC and vice-versa prioritize the importance of DC-coupled rather than AC-coupled.

As mentioned earlier, DC-coupled systems are cost-effective for small to moderate-sized systems. Consequently, the flexibility and scalability of solar controllers provide you the option to include additional panels.
Moreover, DC charge controllers regulate voltage and current for effective battery charging, thereby extending the battery’s lifespan and assuring its peak performance.

Though the initial cost might be higher compared to the AC-coupled system, it is completely effective for long-term plans. Also, the replacement costs are reduced by placing additional solar panels and batteries to achieve the desired output.

Which one Should you prefer for an off-grid system:-

AC-coupled or DC-coupled?

AC coupled is the perfect choice to be preferred over DC coupled for your off-grid journey because of its simplicity and budget-friendly performance.

However, the ideal choice between AC-coupled and DC-coupled off-grid systems could depend upon other factors like your budget, infrastructure, plans, and long-term goals.

[Video Credit – SolarEdgePV]

Combined AC and DC Coupling

Certain off-grid inverters that include the combined configurations of both AC and DC Coupling include:

  • Selectronic SP PRO
  • SMA Sunny Island
  • Victron Multiplus

Integrating the system of AC and DC Coupling for off-grid can simplify your maintenance process. Eventually, a DC battery charging is provided in case of an AC Shutdown.

Working Process:

The working process involves maximizing the energy by a DC solar Charge controller to optimize the current and voltage. Capturing maximum solar energy results in efficient battery storage.

An inverter powers your appliances to convert excess DC into AC, thereby powering your appliances and having the option to change batteries when required.

For instance, if the inverter shuts down suddenly, the DC-coupled solar charge controllers remain active by attracting power from the PV array, nevertheless with the low battery voltage.

These charge controllers prevent further discharge and prioritize the battery’s life by initiating a controlled recharge cycle.

Once the battery reaches a sufficient level, the inverter is automatically switched on, only if the system is configured with the automatic restart settings of the inverter.

Both AC and DC Coupled system have their unique pros and cons. You need to select any of these based on your demands with a straightforward answer below:

AC Coupled System:

  • Tight Budget
  • A simply designed system
  • Short term needs
  • Having a full setup with an inverter
  • Energy requirements for primary household things such as Air Conditioners, Refrigerators, etc.

DC Coupled System:

  • Long Term plans for system expansion
  • Energy efficiency and quality are your top priority
  • Ready to invest an initial huge budget
  • Maximizing energy savings

A Hybrid model with the integration of AC and DC Coupling can offer the best to the world for a specific.

The combination of AC and DC Coupling follows a simple concept:

(AC+ DC) Coupled System = Cost Efficient+ Energy Efficient

NOTE: The different types of Solar battery systems must be installed by any expert electrical or solar professional. Incorrect installation can result in severe damage and injury.

Conclusion

Overall, AC-coupled systems are a traditional arrangement to the grid-connected systems, that utilize inverters to instantly convert the DC output of solar panels to AC power, commonly utilized by most domestic applications.

On the other side, DC-coupled systems omit the inverter to optimize the battery storage by transmitting the DC output of solar panels straight to a charge controller.

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Ray is an avid reader and writer with over 25 years of experience serving various domestic and multinational private and public energy companies in the USA.

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