AC or DC coupling simply refers to the way solar panels are coupled or linked to a battery system.
For mostly off-grid areas and remote power systems, DC coupled systems have extensively been used to provide energy solutions but over the last five years the revolution of hybrid inverter technology has led to the development of AC coupled energy storage systems which is rapidly gaining extensive support and attention despite DC coupled systems still being the most efficient and simplest way to couple a solar system with storage capabilities.
Battery technologies have evolved over the recent years with many lithium battery types emerging as manufacturers explore different ways to add or couple batteries to new or existing solar systems. Both DC and AC battery systems have gained solid grounds as top manufacturers continue to support the duo. With the many complex varieties of battery storage systems now available, here we explain the advantages and disadvantages of each type.
DC COUPLED SYSTEM
DC coupled systems have been extensively applied in off-grid solar systems and small power capacities applications for many years. They simply require a solar charge controller to regulate or control charging process from the PV array to the battery system; and a battery inverter to convert DC power to AC power for AC loads to be able to work.
For larger systems, an MPPT solar charge controller are usually employed as they are more efficient and can operate at much higher string voltages, typically up to 150 volts.
For micro systems, the usage of PWM type solar charge controllers are cost effective compared to MPPT.
- DC Coupled systems are very efficient with its battery charging efficiency typically ranging from 90% – 99% using MPPTs.
- For micro systems, DC coupling systems offer a very low cost setup.
- Ideal for micro systems requiring only 1 to 2 panels.
- Modular concepts – Additional panels and controllers can be easily added if required
- Minimal system losses for powering DC loads.
- If an electricity service provider restricts or limits the capacity of grid-tie solar allowed (ie. 5kW max), additional solar may be added by DC coupling a battery system.
- More complex to setup systems above 3kW as often multiple strings are required in parallel, plus string fusing.
- Can become expensive for systems above 5kW as multiple higher voltage solar charge controllers are required.
- Lower efficiency (approx 90%) when powering large AC loads during the day due to the conversion from DC-DC-AC.
- Some solar controllers are not compatible with modern lithium battery management systems (BMS).
AC COUPLED SYSTEM
AC coupled systems use a common solar inverter coupled to a multi-mode inverter or battery inverter/charger to manage the battery. Although simple to setup and very powerful they are slightly less efficient at charging than DC coupled systems (90-94%). However these systems are very efficient for powering AC loads during the day and are able to be expanded with multiple solar inverters to form micro-grids.
Most modern off-grid homes use AC coupled systems due to the advanced multi-mode inverter/chargers, generator controls and energy management features. Also since solar inverters operate with high DC voltages (up to 600V or higher), very large systems can be installed. AC coupling is also well suited to medium-large 3-phase commercial systems.
- Higher efficiency when used to power AC appliances during the day such as air-conditioning or pool pumps, approx 95%.
- Generally lower installation cost for larger systems above 3kW.
- Can use multiple solar inverters in multiple locations (AC coupled micro-grids)
- Most string solar inverters above 3kW have dual MPPT inputs so panels can be installed at different orientations and tilt angles.
- Advanced AC coupled systems can use a combination of AC and DC coupling (Note: this can be problematic with some lithium batteries)
- Slightly lower efficiency when charging a battery system – approx 92%
- Quality Solar inverters can be expensive for small systems.
- Lower efficiency when powering direct DC loads during the day.
Both DC and AC couplings systems have their unique advantages and disadvantages and their integration may be dependent on factors as such
- Grid reliability
- Load (AC or DC) type
- Financial budget
- Size of system