2014 will be a year of further growth and even boom for global photovoltaic (PV) energy storage systems, primarily due to their wide applicability and compatibility. Due to the inherent instability and limited timeframe of PV power generation, commercial systems remain the optimal choice for traditional PV systems: peak sunshine hours typically coincide with peak electricity consumption in commercial buildings, enabling effective self-consumption and maximizing the utilization of solar power. However, distributed rooftop systems have also developed rapidly in recent years, with penetration rates in some areas even far exceeding those of commercial systems. This raises two unavoidable issues: First, when the system is operating at full power at midday, if there are insufficient loads to absorb the electricity, this power will be directly fed into the local grid. If several households on a street have installed solar systems, the phase voltage of the street's grid can easily exceed the standard range at midday. At this time, some inverters may fail to start, and even some user appliances may disconnect from the grid for self-protection, causing unexpected outages. Second, the global environment is currently characterized by continuous reductions in feed-in tariffs. In some Australian states, local grid companies are even allowed to set their own prices, making it less economical to feed surplus electricity into the grid, especially in situations where large amounts of electricity are lost when no one is home. The concept of energy storage systems has been proposed as a solution and is gaining popularity due to its satisfactory compatibility with stand-alone, microgrid, and grid-connected systems.
Energy storage photovoltaic (PV) systems refer to PV arrays paired with batteries to alter the power transmission and discharge time of traditional PV systems to the load. Due to the introduction of energy storage systems, electricity that the load cannot absorb during peak periods can be stored in the battery bank and used to compensate for insufficient PV power generation. Energy storage systems can effectively improve the system's power supply time and the rationality of power supply. Common system structures can be divided into three categories:
1. Independent energy storage system
2. Grid-connected energy storage system
3. Energy storage equipped with generator system
Compared to the first two types of systems, systems with generators are gradually being phased out due to their fuel requirements, higher noise levels, and lower efficiency. Except in special regions and under specific conditions, few energy storage systems currently choose to be paired with generators. This system will not be discussed in this paper. For stand-alone and grid-connected energy storage systems, the most mainstream topologies are currently "DC Coupling" and "AC Coupling." This paper will analyze and compare the advantages and disadvantages of these two topologies and their applicability in practical situations.
A DC Coupling topology typically includes the following components: photovoltaic modules, a regulator (or charge controller), a battery bank, and an inverter. The definition of the inverter requires further analysis. In DC Coupling, the inverter can be understood as a battery inverter, which differs significantly from the commonly understood grid-connected inverter. Firstly, grid-connected inverters usually have built-in MPPT (Multi-Level Testing), while battery inverters are mismatched, primarily due to the different discharge characteristics of photovoltaic modules and batteries. Secondly, grid-connected inverters do not allow AC-to-DC conversion to charge the modules, but the battery inverter in a grid-connected energy storage system is bidirectional; the battery can discharge through the inverter, and the grid can also charge the battery through the inverter. Finally, and most importantly, grid-connected inverters continuously and stably output power from the photovoltaic system. However, battery inverters, due to the discharge characteristics of batteries, have different discharge power levels, commonly including "continuous power supply," "60-minute discharge power," "1-minute discharge power," and "30-second discharge power." This is because, in the instant of a sudden grid outage, the battery needs to release a considerable amount of power to compensate for the power demand. Therefore, a typical 3kW battery inverter can have an instantaneous rated power of 7kW to 7.5kW. In general, the discharge of batteries and their coordination with the inverter are much more complex than those of ordinary photovoltaic grid-connected inverters, which we will explain further in future articles. Independent energy storage systems without generators have a unique system structure where the battery inverter discharges in a unidirectional manner. When the battery reaches the set SOC (State of Charge), the charging controller will disconnect the photovoltaic system from the battery bank. Similarly, when the battery discharges too deeply, exceeding the set DOD (Depth of Discharge), the inverter will stop supplying power and disconnect the battery from the load.
Let's first compare the advantages and disadvantages of stand-alone energy storage systems and grid-connected energy storage systems using the DC Coupling topology. How does this topology work? In fact, the core operating principle of both systems is the same: the battery is responsible for the main power supply, while photovoltaics only plays a charging role. This is key to understanding the DC Coupling topology. Simply put, when the photovoltaic system is running, it can charge the battery through a regulator with its built-in MPPT (Maximum Power Point Test). When electrical loads demand power, the battery will release electricity in ampere-hours (Ah), with the specific current depending on the discharge time—the so-called "high current for short periods, low current for long periods" discharge principle. The advantage of grid-connected energy storage systems lies in maximizing the utilization of photovoltaic power generation while ensuring the health of the battery capacity. When the energy storage system is connected to a reliable grid system, if the battery is fully charged and the photovoltaic system can still generate electricity under no-load conditions, the charging controller will communicate with the battery inverter to start supplying power to the grid. This is equivalent to photovoltaic power generation being fed into the grid, effectively improving the system's solar energy utilization rate. When load demand exceeds the system's actual power generation, the grid begins supplying power to the load while simultaneously charging the battery via a bidirectional inverter. In essence, for a reliable grid, the required battery storage capacity for a grid-connected energy storage system is theoretically zero; however, this is both an advantage and a disadvantage. Compared to stand-alone energy storage systems, grid-connected systems often suffer from design flaws due to their high flexibility. As a grid-connected system, users inevitably need to purchase electricity from the grid at certain times. Inadequate design considerations, such as requiring users to purchase electricity from the grid during peak pricing periods to charge the battery and then using the battery to power the load during off-peak periods, are neither economical nor practical. Secondly, the advantages of grid-connected energy storage systems lie in their stable grid supply. Designing an energy storage system without assessing local grid stability can easily lead to overuse of the battery or even permanent damage. These design details will be discussed in detail in the next section. Furthermore, in remote areas where it is uneconomical or impractical to run power from the grid, the independence of stand-alone energy storage systems becomes apparent.
AC Coupling topology typically comprises two parts: a photovoltaic (PV) power supply system and a battery power supply system. The PV system consists of a PV array and a grid-connected inverter; the battery system consists of a battery bank and a bidirectional inverter. The operating principle of AC Coupling topology is very similar to that of microinverter design topology, i.e., several AC sources connected in parallel. In stand-alone energy storage system applications, the bidirectional inverter internally simulates grid signals for the grid-connected inverter to support the operation of the PV power supply system. When the PV system is not needed, the bidirectional inverter will change the reference information to activate the inverter's anti-islanding protection to disconnect the connection. The disadvantage of this control method is that it reduces the lifespan of the relay switches in the grid-connected inverter. Also, if the communication between the grid-connected inverter and the bidirectional inverter fails, overcharging or over-consumption can easily occur. Grid-connected systems still have significant advantages in this topology; when grid connection is possible, excess and insufficient power can be fed into or drawn from the grid. Since the "AC source parallel connection" topology is relatively familiar, it will not be elaborated further here.
Finally, we will compare the advantages and disadvantages of DC Coupling and AC Coupling topologies, mainly from the perspectives of system reliability and feasibility.
In my personal opinion, both DC coupling and current mainstream system technologies have room for improvement in reliability, primarily in communication. DC coupling requires communication between the charging controller and the inverter, which presents two main problems. First, the charging controller uses its own shunt to measure and calculate the battery's state of charge, while the inverter also uses its own shunt to calculate the battery's state of charge. However, the charging controller and inverter are often from different manufacturers, resulting in differences in shunt accuracy, core processor calculation methods, and calculation errors. This can lead to discrepancies in the logical decision regarding whether the battery should be charged. Similarly, AC coupling requires communication between the grid-connected inverter and the battery inverter. Any communication failure during this process can easily lead to overcharging and pose a fire hazard. Since batteries involve chemical reactions, any fire will be a chemical fire, and its severity should not be underestimated. The second point concerns inverter error communication. To my knowledge, most bidirectional inverters, except for a few top-tier brands that have error reporting functions, either lack alarm communication errors or, by default, manufacturers assume that machine communication errors do not exist. Ultimately, the issue boils down to topology. Communication involves two sides; if the bidirectional inverter's communication function is normal, but the grid-connected inverter's communication fails, a communication failure will still occur. Currently, a common solution in Australian microgrid systems is to purchase both the bidirectional and grid-connected inverters from the same manufacturer during the procurement phase. For example, SMA and Selectronic both offer AC Coupling energy storage system packages, and both utilize battery safety status sensors for monitoring.
Both topologies are excellent in terms of feasibility. In short, they completely overturn the traditional power generation periods of solar systems and users' electricity consumption habits. For traditional systems, if users need to use high-power appliances, it's best to use them during peak solar hours, which is not feasible for most residential systems. The introduction of energy storage systems changes the concept from "shifting load" to "shifting power" and provides UPS power supply options. Occasional power outages are normal in microgrid systems, and the duration is uncertain. UPS can ensure the safety and stability of machines that must run 24 hours a day. Currently, almost all government agencies, banks, and hospitals in Australia have installed fairly mature systems like this. However, safety is indeed a challenge hindering the widespread adoption of energy storage systems, especially in densely populated areas of my country. Imagine placing a dozen or twenty batteries of unknown quality on the roof or inside a residential building, and a fire breaks out for various reasons—a chemical reaction that even water might not be able to extinguish—the consequences are unimaginable. Australian standards AS 4089 and AS 62040 have specific requirements and regulations for the installation and site selection of battery storage facilities. However, there have been many cases in recent years where battery storage fires have destroyed entire villas.
Because the concept of energy storage systems is relatively new, the design requirements are also higher than those of traditional photovoltaic systems. In the next article, we will focus on introducing the design considerations and methods.
