Off-Grid Photovoltaic Power Generation System
The off-grid photovoltaic power generation system consists of a photovoltaic array, solar controller, inverter, battery bank, and load. The photovoltaic array converts solar energy into electrical energy, charges the battery bank through the controller, and then supplies power to the load via the inverter. Because there is a battery added between the photovoltaic array and the inverter, there are many changes in current direction and equipment selection.
Schematic Diagram of Off-Grid Power Generation System
The current flows into the battery and then back out, causing some loss and reducing the battery's lifespan. Is there a function in the inverter that allows the current to be used directly by the load without passing through the battery? This process can indeed be achieved, but it is not done by the inverter; rather, it is automatically managed by the circuit.
From the perspective of circuit theory, at any given moment, current can only flow in one direction. This means that, at any given time, the battery is either charging or discharging; it cannot do both simultaneously. Therefore, when solar power exceeds load power, the battery is in a charging state, and all the energy for the load comes from the photovoltaic array. Conversely, when solar power is less than load power, the battery discharges, and all the photovoltaic generation is supplied directly to the load without passing through the battery.
The maximum charging current of the inverter itself.
The size of the photovoltaic modules.
The maximum charging current allowed by the battery.
For example, if the module power is 5.4 kW, the controller efficiency is 0.96, and the battery voltage is 48V, then the maximum charging current is:
$$ \text{Max Charging Current}=\frac{5400 \times 0.96}{48}=108A $$
Charging from the grid is generally calculated according to the inverter's maximum charging current. If the maximum charging current of the inverter is 100A, it will limit the current to 100A. Now, looking at the battery's maximum charging current, ordinary lead-acid batteries usually have a charging current of about 0.2C. This means for a 12V 200AH battery, the maximum charging current is:
$$ 200 \times 0.2=40A $$
Therefore, three batteries need to be connected in parallel to meet the 100A current requirement. There are also lithium batteries available in versions capable of 48V 100A, which can be selected.
The maximum discharging current of the inverter itself.
The load size.
The maximum discharging current allowed by the battery.
$$ \text{Discharging Current}=\frac{\text{Load Power}}{\text{Battery Voltage} \times \text{Inverter Efficiency}} $$
For example, if the load power is 3kW, battery voltage is 48V, and inverter efficiency is 0.96, then the maximum discharging current would be calculated as:
$$ \text{Max Discharging Current}=\frac{3000}{48 \times 0.96}=60A $$
It is important to note that the charging and discharging capacities of batteries may differ. For some lead-carbon batteries, the discharging current can reach 1C. In normal operation of the light storage system, if there is sunlight, the current of the battery may not follow the calculations above; it will be lower since both the photovoltaic array and the battery may supply power to the load simultaneously.
Off-grid inverters typically have overload capability. For example, a 3 kW off-grid inverter can support the start of a 1 kW motor, with a maximum starting instantaneous power of up to 6 kW. Some believe this instantaneous power must be provided externally, but actually, no matter if it is photovoltaic or battery, neither can provide this nanosecond-level energy; it is provided by the inverter itself. The inverter contains storage components-capacitors and inductors-that can deliver instantaneous power.
Both charging and discharging of the battery use the same cable, so during the design phase, the actual charging and discharging currents must be considered, choosing the largest one. For instance, if you have a 5 kW inverter paired with a 4 kW array, powering a 3 kW load with a 48V 600AH battery, the maximum charging current of the inverter is 120A, the maximum charging current of the photovoltaic array is 80A, and the maximum discharging current of the battery at full load is 65A.
If the inverter does not support simultaneous charging from photovoltaic and grid power, the cable should be chosen for 80A using 16 square mm cable. If both photovoltaic and grid can charge simultaneously, the current can reach 120A, in which case a 25 square mm cable should be used.
When the power output of the photovoltaic system is about equal to or slightly larger than the load power, the photovoltaic current can directly supply the load without passing through the battery, resulting in the highest efficiency for the off-grid system. When the photovoltaic generation and the load usage do not occur at the same time (e.g., photovoltaic generates power during the day while the load uses electricity at night), the photovoltaic generation must first go into the battery before being supplied to the load, leading to lower system efficiency. Battery cables should be designed according to the maximum charging and discharging current of the battery. The same inverter may have different current requirements depending on the application, necessitating individualized calculations.
