AC vs DC Coupling : Pros and Cons
Author: Darvin Tocmo Date Posted:5 December 2017
Solar industry has shown a much broader interest in combining energy storage with solar installations.For utility-scale solar-plus-storage projects, one of the most critical considerations is choosing between AC and DC coupling architectures.
What is the difference between AC and DC battery storage, and what are the relative advantages and disadvantages of each?
Below is the full breakdown of advantages and disadvantages of each type.
What are AC and DC electricity?
Direct current (DC) electricity is what solar panels produce and what batteries hold in storage, while alternating current (AC) electricity is the type used on the grid and in most household devices. A device called an inverter is required to convert the DC electricity from solar panels into appliance-friendly AC.
What is AC-Coupled Storage?
AC coupled storage is the connection of a battery system to a solar system via AC (alternating current) electricity. Energy is generated in the form of DC electricity which is then turned into AC by the solar inverter. By contrast, DC coupled storage uses what is commonly referred to as a hybrid inverter. This is where both the solar and battery are connected to the same inverter. However, as we’ll see below there are many reasons why AC-coupled storage using high voltage (HV) batteries offer a number of advantages in terms of cost, flexibility, and risk compared to DC coupling.
One of biggest advantages of AC-coupled storage is that it turns ANY new or existing solar system, into a true “battery ready” system. Since batteries and solar cannot be simply connected together, grid-connected battery storage will always be required to connect to an inverter. AC coupled storage means that the inverter to which the battery is connected is separate to the solar inverter. It can be a much simpler installation in many cases by using two separate inverters for battery and solar Even though there are 2 separate inverters, compared to a number of hybrid inverter solutions it can still represent few components overall.
*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.
What is DC energy storage system?
DC storage is connected to the output of the solar PV panels and stores DC power directly from the panels. Generally speaking, DC-coupled storage systems are not capable of storing grid power.
The larger more efficient MPPT type solar charge controllers are much more powerful. However, There are still limited as most cannot operate above 150 Volts DC. Due to the voltage limitation, only 3 panels can be linked in series usually. This means for larger solar systems above 1kW it is more complex and involves combining strings of panels in parallel. There are higher voltage solar controllers, but they are generally very expensive and don't have multiple (MPPT) inputs. At this size, it is usually more cost-effective to use an AC coupled system.
DC storage is lower cost. As it leverages existing PV inverter – there is no need to install a specialized, storage-ready inverter.
*Much more complex to set up larger systems above 3kW as multiple strings are required in parallel, plus string fusing.
*More expensive to set up for systems above 5kW as multiple higher voltage solar charge controllers are required.
*Lower efficiency when powering large AC loads during the day due to the conversion from DC-DC-AC. *Many solar controllers are not compatible with modern lithium batteries battery management systems (BMS).